Method for making a microstructure by surface micromachining
Various methods for forming surface micromachined microstructures are disclosed. One aspect relates to executing surface micromachining operation to structurally reinforce at least one structural layer in a microstructure. Another aspect relates to executing the surface micromachining operation to form a plurality of at least generally laterally extending etch release channels within a sacrificial material to facilitate the release of the corresponding microstructure.
The present invention generally relates to making a microstructure by surface micromachining. One aspect relates to making a structurally reinforced microstructure to provide a flatter profile on one or more of its structural layers. Another aspect relates to providing a plurality of at least generally laterally extending etch release channels to facilitate the release of the microstructure from the substrate.
BACKGROUND OF THE INVENTIONThere are a number of microfabrication technologies that have been utilized for making microstructures (e.g., micromechanical devices, microelectromechanical devices) by what may be characterized as micromachining, including LIGA (Lithographie, Galvonoformung, Abformung), SLIGA (sacrificial LIGA), bulk micromachining, surface micromachining, micro electrodischarge machining (EDM), laser micromachining, 3-D stereolithography, and other techniques. Bulk micromachining has been utilized for making relatively simple micromechanical structures. Bulk micromachining generally entails cutting or machining a bulk substrate using an appropriate etchant (e.g., using liquid crystal-plane selective etchants; using deep reactive ion etching techniques). Another micromachining technique that allows for the formation of significantly more complex microstructures is surface micromachining. Surface micromachining generally entails depositing alternate layers of structural material and sacrificial material using an appropriate substrate which functions as the foundation for the resulting microstructure. Various patterning operations may be executed on one or more of these layers before the next layer is deposited so as to define the desired microstructure. After the microstructure has been defined in this general manner, the various sacrificial layers are removed by exposing the microstructure and the various sacrificial layers to one or more etchants. This is commonly called “releasing” the microstructure from the substrate, typically to allow at least some degree of relative movement between the microstructure and the substrate. Although the etchant may be biased to the sacrificial material, it may have some effect on the structural material over time as well. Therefore, it is generally desirable to reduce the time required to release the microstructure to reduce the potential for damage to its structure.
Microstructures are getting a significant amount of attention in the field of optical switches. Microstructure-based optical switches include one or more mirror microstructures. Access to the sacrificial material that underlies the support layer that defines a given mirror microstructure is commonly realized by forming a plurality of small etch release holes down through the entire thickness or vertical extent of the mirror microstructure (e.g., vertically extending/disposed etch release holes). The presence of these small holes on the upper surface of the mirror microstructure has an obvious detrimental effect on its optical performance capabilities. Another factor that may have an effect on the optical performance capabilities of such a mirror microstructure is its overall flatness, which may be related to the rigidity of the mirror microstructure. “Flatness” may be defined in relation to a radius of curvature of an upper surface of the mirror microstructure. This upper surface may be generally convex or generally concave. Known surface micromachined mirror microstructures have a radius of curvature of no more than about 0.65 meters.
BRIEF SUMMARY OF THE INVENTIONThe present invention is generally embodied in a method for making a microstructure by surface micromachining. In this method, at least one and more typically a plurality of at least generally laterally extending etch release channels or conduits are formed within a sacrificial material. This sacrificial material is used to fabricate the microstructure on an appropriate substrate. At least some of this sacrificial material is removed when the microstructure is released from the substrate (and thereby encompassing the situation where all of this sacrificial material is removed).
A first aspect of the present invention is embodied in a method for making a microstructure by surface micromachining that includes forming a first structural layer over a sacrificial material. “Over” includes being deposited directly on the substrate or being deposited on an intermediate layer that is disposed between the subject sacrificial layer and the substrate. “On” in contrast means that there is an interfacing relation. In any case, a plurality of hollow etch release pipes, channels, conduits, or the like extend at least generally laterally through/within this sacrificial material. Lateral or the like, as used herein, means that the etch release conduits are disposed or oriented in a direction which is at least generally parallel with the substrate. Although the etch release conduits will typically extend laterally at a constant elevation relative to the substrate, such need not necessarily be the case. Ultimately, at least some of the sacrificial material is removed at least in part by allowing an etchant to flow through any and all of these hollow etch release conduits (and thereby encompassing the situation where all of this sacrificial layer is removed).
Various refinements exist of the features noted in relation to the first aspect of the present invention. Further features may also be incorporated in the first aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. In one embodiment, at least one end of at least one etch release conduit may be disposed at least generally at the same radial position as a perimeter of the first structural layer. Hereafter, “at least generally at the same radial position” in relation to any end of any etch release conduit or structure used to define the same means within 50 μm of the radial position of the perimeter of the relevant structural layer in one embodiment, more preferably within 25 μm of the radial position of the perimeter of the relevant structural layer in another embodiment, and even more preferably at the same radial position as the perimeter of the relevant structural layer. As such, the etchant does not have to etch in from the perimeter “too far” before encountering an open end of one or more etch release conduits associated with the first aspect. Reducing the time required for the etchant to reach the etch release conduits should at least to a point reduce the overall time for accomplishing the release of the microstructure from the substrate.
Various layouts of the plurality of etch release conduits in the noted lateral dimension may be utilized in relation to the first aspect. In one embodiment, each of the plurality of etch release conduits may be disposed in non-intersecting relation, in another embodiment the plurality of etch release conduits are disposed in at least substantially parallel relation, and in yet another embodiment at least some of the etch release conduits intersect. The plurality of etch release conduits may extend at least generally toward (and thereby including to) a common point, such as one that corresponds with a center of the first structural layer in the lateral dimension. All etch release conduits need not extend the same distance toward this common point, although such may be the case. Some of the plurality of etch release conduits may extend at least generally toward (and thereby including to) a first common point (e.g., one that corresponds with a center of the first structural layer in the lateral dimension), while some of the plurality of etch release conduits may extend toward a different common point. The plurality of etch release conduits may also extend in the lateral dimension in a variety of configurations. In one embodiment, the plurality of etch release conduits are at least generally axially extending, while in another embodiment the plurality of etch release conduits are non-linear (e.g., in a sinusoidal configuration that is at least generally parallel with the substrate).
In one embodiment of the first aspect, the laterally extending etch release conduits are completely defined before the stack that includes the microstructure being fabricated by the methodology of the first aspect is exposed to a release etchant for removing the first sacrificial layer. One particularly desirable application for the first aspect is in the formation of a mirror microstructure or multiple mirror microstructures for a surface micromachined optical system that has at least some degree of movement relative to the first substrate. Because of the presence of the plurality of laterally extending etch release conduits, there is no need to have a plurality of etch release apertures or holes that extend entirely down through the entire vertical extent of the first structural layer to remove the sacrificial material that directly underlies this first structural layer. That is, there is no need for a plurality of vertically disposed etch release apertures, with “vertical” being at least generally opposite “lateral.” As such, the upper surface of the first structural layer retains a very smooth surface that lacks any such vertically disposed etch release apertures, which makes the microstructure that is made by the methodology of the first aspect particularly suited for use in optical applications. This is true whether the upper surface of the first structural layer is the actual optical surface for the mirror microstructure, or whether a film or other layer is deposited on the first structural layer to provide more desirable optical properties/characteristics.
The sacrificial material that directly underlies the first structural layer used by the first aspect may actually be defined by the sequential deposition of multiple sacrificial layers, one on top of the other. That is, a lower portion of this sacrificial material may be formed as one layer, and an upper portion of this sacrificial material may be subsequently formed in overlying relation. Consider a case where a first sacrificial layer is formed over the first substrate, and where a first intermediate layer is formed on this first sacrificial layer. The first intermediate layer may be patterned to define a first subassembly (e.g., a plurality of strips). Portions of the first sacrificial layer are exposed by a patterning of the first intermediate layer to define the first subassembly. An upper portion of the first sacrificial layer (i.e., something less than the entirety of the first sacrificial layer) may be etched an amount after this patterning such that at least part of the first sacrificial layer that underlies at least part of the first subassembly is removed (e.g., using a timed etch). The first subassembly may be disposed directly on the first sacrificial layer, directly on a structural layer, or directly on both the first sacrificial layer and a structural layer (e.g., for the case where there is both a sacrificial material and a structural material at the same level within a stack which contains the microstructure being fabricated by the methodology of the subject first aspect).
The “gap” that now exists between the first subassembly and that portion of the etched first sacrificial layer that is directly beneath the first subassembly may be characterized as an undercut. A second sacrificial layer may be formed on at least the first sacrificial layer. One could characterize this as “backfilling.” Notwithstanding this characterization of the backfilled sacrificial material as a “second sacrificial layer”, the first and second sacrificial layers may in fact be indistinguishable from each other and may in effect define a continuous structure. Nonetheless, the sacrificial material that has been characterized as the second sacrificial layer will not fill the entire extent of each of the undercuts. This failure to fill the undercuts defines the plurality of etch release conduits that are associated with the first aspect of the present invention. One could visualize that the first subassembly acts as an umbrella of sorts that prevents the material that has been characterized as the second sacrificial layer from totally filling the noted undercuts that are protected by the first subassembly. Typically the second sacrificial layer will also cover the first subassembly. In this case and possibly in other instances during the fabrication of the microstructure associated with the first aspect, it may be desirable to planarize an upper surface of a given layer before depositing the next layer thereon. One appropriate technique for providing this planarization function is chemical mechanical polishing.
The first subassembly may be a reinforcing structure for the first structural layer, in which case the first subassembly would exist in the microstructure that is defined by the methodology of the first aspect. Reinforcement may be provided by structurally interconnecting the first structural layer with the first subassembly through the above-noted second sacrificial layer which may be deposited on the first subassembly in addition to the first sacrificial layer as noted. Further reinforcement of the first structural layer may be accomplished by structurally interconnecting the first subassembly with a structural layer that underlies the first sacrificial layer. In both cases, the actual reinforcement structure could be in the form of a plurality of posts or columns that are disposed in spaced relation, in the form of a plurality of at least generally laterally extending ribs or rails, or in the form of a grid-like reinforcement structure.
The first subassembly need not remain in the microstructure that may be defined by the methodology of the first aspect. That is, the first subassembly need not be part of the final microstructure that is ultimately fabricated by the methodology of the first aspect. In this case, the only purpose of the first subassembly would be to at least assist in the formation of the plurality of etch release conduits that are associated with the first aspect of the present invention. This “temporary” first subassembly may be in the form of a plurality of rails that are formed on or in a sacrificial material that is used in the methodology of the first aspect. Removal of the first subassembly from the final microstructure being made by the subject first aspect may be desirable in order to retain a low mass for this microstructure (e.g., in an upper structural layer of such a microstructure).
One way in which the first subassembly may be removed or alleviated from the final microstructure being made by the first aspect of the present invention is to form the first subassembly from a material that would be etched away along with the various sacrificial layers, although possibly at a different rate. Appropriate materials for the first subassembly in this case include silicon nitride, poly-silicon-germanium, or any other material that is soluble in the release etch or other etchant that will not have an adverse effect on any of the structural layers that may be included in the microstructure being made by the methodology of the subject first aspect. Having the first subassembly be of a reduced thickness may also contribute to the first subassembly being removed along with the sacrificial material within a desired time when releasing the microstructure made by the methodology of the first aspect. In one embodiment where the first subassembly is formed from silicon nitride and in the form of a plurality of strips, the first subassembly has a thickness or vertical extent of typically less than about 1,500 Å for this purpose.
Another option for creating the plurality of at least generally laterally extending etch release channels in accordance with the first aspect entails forming a first intermediate layer that will underlie the first sacrificial layer. This first intermediate layer may be patterned to define a plurality of at least generally laterally extending strips that are disposed in non-intersecting relation over at least a portion of their length. These strips are spaced relatively close to each other such that when the first sacrificial layer is deposited on the first intermediate layer, the sacrificial material is unable to entirely fill the space between the adjacent strips. More specifically, an upper portion of the space between adjacent strips will “close off” during the deposition of the material that defines the first sacrificial layer before a lower portion of the space between the adjacent strips has had a chance to be filled with the material that defines the first sacrificial layer. Each “unfilled” void between adjacent pairs of strips defines one of the plurality of etch release channels referenced in relation to the first aspect. The manner in which the voids are formed may be characterized as “keyholing.” Keyholing in relation to the first aspect is a result a relatively close spacing between adjacent strips in relation to their thickness or vertical extent. In one embodiment, a ratio of the height of these strips to the spacing between adjacent strips is at least about 1:1.
Further options exist for creating the plurality of at least generally laterally extending etch release channels in accordance with the first aspect. One way is to use multiple, different etchants. A first etchant that is not selective to the first sacrificial layer may be used to form the plurality of at least generally laterally extending etch release channels. A second, different etchant that is selective to the first sacrificial layer may thereafter be directed through the plurality of at least generally laterally extending etch release channels or conduits (again created/defined by the first etchant) to remove the first sacrificial layer. The first etchant may be selective to a material that forms a plurality of at least generally laterally extending etch release rails that are embedded or encased within the first sacrificial layer or at least in a sacrificial material. Any appropriate layout may be utilized for these plurality of at least generally laterally extending etch release rails, including a plurality of separate and discrete etch release rails, a network or grid of interconnected etch release rails, or some combination thereof.
The intermediate structure in the fabrication of the microstructure in accordance with the first aspect may be characterized as a stack, and includes the various layers that are sequentially deposited on the substrate, and thereafter possibly patterned. This stack includes an exterior surface that is opposite the first substrate. Access to at least one of the etch release rails in the above-noted two etchant example may be provided by a first runner that extends from this exterior surface of the stack and at least generally toward the first substrate to a level such that it may structurally interconnect with at least one of the plurality of etch release rails. The same material that defines the etch release rails may define this first runner, such that the first etchant will first remove the first runner, and then each etch release rail that is structurally interconnected therewith (either directly or indirectly). Multiple first runners may be provided for accessing the plurality of etch release rails, multiple etch release rails may be accessed by a single first runner, or some combination thereof.
A second aspect of the present invention is embodied in a method for making a microstructure in which a plurality of at least generally laterally extending etch release channels or conduits are formed within a sacrificial material that is used to build/assemble the microstructure on an appropriate substrate, but which is at least in part removed when the microstructure is released from this substrate. These etch release channels do not exist until the release of the microstructure is initiated in the second aspect. In this regard, the method of the second aspect includes forming a first intermediate layer on top of a first sacrificial layer or possibly on top of the substrate. This first intermediate layer is patterned to define a plurality of at least generally laterally extending first strips that sit on top of the first sacrificial layer. A second sacrificial layer is thereafter deposited on that portion of the first sacrificial layer which was exposed by the patterning of the first intermediate layer so as to be disposed at least alongside the first strips. Although not fundamentally required by the second aspect, the second sacrificial layer may also be disposed on top of the first strips as well. In any case, a first structural layer is formed on top of the second sacrificial layer. Both the first and second sacrificial layers are removed at least in part using an appropriate etchant. Generally, those portions of the second sacrificial layer that interface with or are disposed adjacent to the first strips etch at a greater rate than other portions of the second sacrificial layer which effectively defines a plurality of at least generally laterally extending etch release pipes, channels, conduits or the like. A plurality of hollow and at least generally laterally extending etch release channels or conduits (e.g., disposed at least generally parallel with an upper surface of the first substrate) are thereby formed in the second sacrificial layer by this differential etch rate. Although these etch release conduits will typically be disposed at a constant elevation relative to the substrate, such need not necessarily be the case. Ultimately, at least part of the second sacrificial layer, as well as the first sacrificial layer, are removed at least in part by allowing an etchant to flow through any and all of the noted conduits after the formation of the same in the releasing operation (and thereby encompassing the situation where all of the first and second sacrificial layers are removed, as well as the first intermediate layer if the same is not a structural material as discussed below).
Various refinements exist of the features noted in relation to the second aspect of the present invention. Further features may also be incorporated in the second aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. The noted differential etch rate is believed to be based upon the density of that portion of the second sacrificial layer in proximity to the first strips being less than a density of the remainder of the second sacrificial layer, as well as a density of the first sacrificial layer as well for that matter. The differential density is due to the sticking characteristics of the depositing atoms or molecules, which depends on the characteristics of the deposition technique. For example, the density of add-on molecules in a plasma-enhanced chemical vapor deposition (PECVD) system depends on directional orientation of the surface to the plasma body. In this way, atoms or molecules striking the surface have different energy available to them to aid in their positioning in low-energy (i.e. high-density) positions on the surface.
In one embodiment of the second aspect, at least one end of at least one first strip may be disposed at least generally at the same radial position as a perimeter of the first structural layer. Since the first strips effectively define the etch release conduits, at least one end of at least one etch release conduit may be disposed at this same radial position as well. As such, the etchant does not have to etch in from the perimeter “too far” before encountering a region that will have a higher etch rate, and that again defines a corresponding etch release conduit. The parameters mentioned above in relation to the first aspect regarding this feature are equally applicable to this second aspect.
Various layouts of the plurality of first strips (and thereby the etch release conduits) in the noted lateral dimension may be utilized in accordance with the second aspect. In one embodiment, each of the plurality of at least generally laterally first strips are further disposed in non-intersecting relation, in another embodiment each of the plurality of first strips are disposed in at least substantially parallel relation, and in yet another embodiment at least some of the first strips intersect. The plurality of first strips may extend at least generally toward (and thereby including to) a common point, such as one that corresponds with a center of the first structural layer in the lateral dimension. All of the first strips need not extend the same distance toward this common point, although such may be the case. Some of the plurality of strips may extend at least generally toward (and thereby including to) a first common point (e.g., one that corresponds with a center of the first structural layer in the lateral dimension), while some of the plurality of first strips may extend toward a different common point. The plurality of first strips utilized by the second aspect may also extend in the lateral dimension in a variety of configurations. In one embodiment, the plurality of first strips are at least generally axially extending, while in another embodiment the plurality of first strips are non-linear (e.g., in a sinusoidal configuration within a plane that is parallel with the substrate).
One particularly desirable application for the second aspect is in the formation of a mirror microstructure or multiple mirror microstructures for a surface micromachined optical system that has at least some degree of movement relative to the first substrate. Because of the different etch rates that result from the way in which the microstructure is made by the methodology of the second aspect, there is no need to have a plurality of vertically disposed etch release apertures that extend down entirely through the second structural layer to release the mirror microstructure from the first substrate by the removal of the underlying first and second sacrificial layers. As such, the upper surface of the second structural layer retains a very smooth surface that lacks any such vertically disposed etch release apertures, which makes the microstructure that is made by the methodology of the second aspect particularly suited for use in optical applications. This is true whether the upper surface of the second structural layer is the actual optical surface for the mirror microstructure, or whether a film or other layer is deposited on the second structural layer to provide more desirable optical properties/characteristics.
Typically the second sacrificial layer will also cover the first strips (again, formed from the first intermediate layer) such that they are effectively embedded between the first and second sacrificial layers. In this case, it may be desirable to planarize an upper surface of the second sacrificial layer before depositing the first structural layer thereon. It may also be desirable to planarize an upper surface of other layers within the microstructure made in accordance with the methodology of the second aspect as well before depositing another layer thereon. One appropriate technique for executing this planarization function is chemical mechanical polishing.
The first strips may be a reinforcing structure for the first structural layer and would then exist in the microstructure that is made by the methodology of the second aspect. Reinforcement may be provided by structurally interconnecting the first structural layer with the first strips through the above-noted second sacrificial layer which may be deposited on the first strips in addition to the first sacrificial layer as noted. Further reinforcement of the first structural layer may be accomplished by structurally interconnecting the first strips with a structural layer that underlies the first sacrificial layer and thereby the first structural layer. In both cases, the actual reinforcing structure could be in the form of a plurality of posts or columns that are disposed in spaced relation, in the form of a plurality of at least generally laterally extending ribs or rails, or in the form of a grid-like reinforcement structure.
A third aspect of the present invention is embodied in a method for making a surface micromachined microstructure. A first sacrificial layer is formed over a first substrate in a manner so as to define a plurality of at least generally laterally extending low density regions therein. A first structural layer is thereafter formed over the first sacrificial layer. The release of the first structural layer from the first substrate is affected by removing the first sacrificial layer with an appropriate etchant. The etching rate within the low density regions of the first sacrificial layer is greater than in other regions of the first sacrificial layer.
Various refinements exist of the features noted in relation to the third aspect of the present invention. Further features may also be incorporated in the third aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. The higher etch rate in the low density regions of the first sacrificial layer may define a plurality of at least generally laterally extending etch release pipes, channels, conduits, or the like, which in turn should reduce the time required to completely release the microstructure from the first substrate. Although the low density regions will typically be disposed at a constant elevation relative to the substrate, such need not be the case.
In one embodiment of the third aspect, at least one end of at least one low density region may be disposed at least generally at the same radial position as a perimeter of the first structural layer. Since the low density regions effectively define the etch release conduits, at least one end of at least one etch release conduit may be disposed at this same radial position as well. As such, the etchant does not have to etch in from the perimeter “too far” before encountering a low density region for definition of an etch release conduit(s). The parameters mentioned above in relation to the first aspect regarding this feature are equally applicable to this third aspect.
Various layouts of the plurality of low density regions, and thereby the etch release conduits, in the noted lateral dimension may be utilized in accordance with the third aspect. In one embodiment, each of the plurality of at least generally laterally extending low density regions are disposed in non-intersecting relation, in another embodiment each of the plurality of low density regions are disposed in at least substantially parallel relation, and in yet another embodiment at least some of the low density regions intersect. The plurality of low density regions may extend at least generally toward (and thereby including to) a common point, such as one that corresponds with a center of the first structural layer in the lateral dimension. All of the low density regions need not extend the same distance toward this common point, although such may be the case. Some of the plurality of low density regions may extend at least generally toward (and thereby including to) a first common point (e.g., one that corresponds with a center of the first structural layer in the lateral dimension), while some of the plurality of low density regions may extend toward a different common point. The plurality of low density regions utilized by the third aspect may also extend in the lateral dimension in a variety of configurations. In one embodiment, the plurality of low density regions are at least generally axially extending, while in another embodiment the plurality of low density regions are non-linear (e.g., in a sinusoidal configuration within a plane that is at least generally parallel with the substrate).
One way in which the low density regions associated with the third aspect may be formed is by forming a second sacrificial layer over the first substrate, and then patterning the same to define a plurality of at least generally laterally extending etch release conduit apertures. Each of these etch release conduit apertures is defined by first and second sidewalls that are disposed in spaced relation to each other. The first sacrificial layer is formed such that the material of the first sacrificial layer is deposited within these etch release conduit apertures. The first sacrificial layer may be deposited on the top of the second sacrificial layer as well. In any case, the low density regions associated with the third aspect will thereby exist along the first and second sidewalls of each of the etch release conduit apertures.
One advantage of the above-noted method for defining the low density regions in accordance with the third aspect, and thereby for defining a plurality of etch release channels, is that a layout of the low density regions may define a network or grid-like structure or such that a plurality of these low density regions cross and/or are interconnected in some manner. For instance, the patterning of the second sacrificial layer could define a repeating pattern of interconnected “diamonds,” a honeycomb or honeycomb-like structure, or the like. This ability to define a network could further enhance the distribution of the etchant during the release of the first structural layer from the first substrate. Another advantage of this particular method for defining the low density regions, and thereby for defining a plurality of etch release channels, is that the same does not require the use of any structural layer or material for the formation thereof. Therefore, this particular embodiment of the third aspect could be used to enhance the release of a simple, single structural layer in a microstructure.
The above-noted methodology for defining the low density regions in accordance with the third aspect may also be utilized where structural reinforcement of the first structural layer is desired. Structural reinforcement of the first structural layer may be realized by having an appropriate reinforcement structure cantilever downwardly from a lower surface of the first structural layer. Another way to structurally reinforce the first structural layer is to structurally interconnect the first structural layer with an underlying structural layer. The only limitation on the use of any such reinforcement structure is that it should not extend downwardly through any of the noted low density regions so as to cut off any etch release channels. This may be done in variety of manners. Consider the case where the second sacrificial layer is patterned in the form of a honeycomb. A plurality of columns or posts could extend downwardly from the first structural layer through the “closed cell” portions of the honeycomb without intersecting with any of the low density regions which define the profile of the honeycomb (in plan view).
Notwithstanding the advantages of the above-noted method for forming the low density regions in accordance with the third aspect, these low density regions may also be defined in the manner discussed above in relation to the second aspect. Therefore, those features discussed above in relation to the second aspect may be used in this third aspect as well.
A fourth aspect of the present invention is embodied in a method for making a surface micromachined microstructure. A first sacrificial layer is formed over a first substrate. A first structural layer is formed over the first sacrificial layer. The release of the first structural layer from the first substrate is affected by what may be characterized as a two step etch. In this regard, a first etchant may be used to form a plurality of at least generally laterally extending etch release channels or conduits within the first sacrificial layer or so as to otherwise be embedded within a sacrificial material. A second, different etchant that is selective to the first sacrificial layer may thereafter be directed through the plurality of at least generally laterally extending etch release channels (again created/defined by the first etchant) to remove the first sacrificial layer.
Various refinements exist of the features noted in relation to the fourth aspect of the present invention. Further features may also be incorporated in the fourth aspect of the present invention as well. These refinements and additional features may exist individually or in any combination. The first etchant may be selective to a material that forms a plurality of at least generally laterally extending etch release rails that are embedded or encased within the first sacrificial layer or at least in a sacrificial material. Any appropriate layout may be utilized for these plurality of at least generally laterally extending etch release rails, including a plurality of separate and discrete etch release rails, a network or grid of interconnected etch release rails, or some combination thereof.
The intermediate structure in the fabrication of the microstructure in accordance with the fourth aspect may be characterized as a stack, and includes the various layers that are sequentially deposited on the substrate, and thereafter possibly patterned. This stack includes an exterior surface that is opposite the first substrate. Access to at least one of the noted etch release rails may be provided by a first runner that extends from this exterior surface of the stack and at least generally toward the substrate to a level such that it may interconnect with at least one of the plurality of etch release rails. The same material that defines the etch release rails may define this first runner, such that the first etchant will first remove the first runner, and then each etch release rail interconnected therewith. Multiple first runners may be provided for accessing the plurality of etch release rails, multiple etch release rails may be accessed by a single first runner, or some combination thereof.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIGS. 12A-M are sequential views of one embodiment for making one embodiment of a microstructure for a surface micromachined system.
FIGS. 13A-M are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system.
FIGS. 14A-F are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system.
FIGS. 15A-G are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system.
FIGS. 16A-C are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system.
FIGS. 17A-G are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system.
FIGS. 19A-F are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system, and which uses the same technique for forming a plurality of etch release conduits as the method of FIGS. 17A-G.
FIGS. 20A-D are sequential views of another embodiment for making one embodiment of a microstructure for a surface micromachined system.
DETAILED DESCRIPTION OF THE INVENTIONThe present invention will now be described in relation to the accompanying drawings which at least assist in illustrating its various pertinent features. Surface micromachined microstructures and methods of making the same are the general focus of the present invention. Various surface micromachined microstructures and surface micromachining techniques are disclosed in U.S. Pat. Nos. 5,783,340, issued Jul. 21, 1998, and entitled “METHOD FOR PHOTOLITHOGRAPHIC DEFINITION OF RECESSED FEATURES ON A SEMICONDUCTOR WAFER UTILIZING AUTO-FOCUSING ALIGNMENT”; U.S. Pat. No. 5,798,283, issued Aug. 25, 1998, and entitled “METHOD FOR INTEGRATING MICROELECTROMECHANICAL DEVICES WITH ELECTRONIC CIRCUITRY; U.S. Pat. No. 5,804,084, issued Sep. 8, 1998, and entitled “USE OF CHEMICAL MECHANICAL POLISHING IN MICROMACHINING”; U.S. Pat. No. 5,867,302, issued Feb. 2, 1999, and entitled “BISTABLE MICROELECTROMECHANICAL ACTUATOR”; and U.S. Pat. No. 6,082,208, issued Jul. 4, 2000, and entitled “METHOD FOR FABRICATING FIVE-LEVEL MICROELECTROMECHANICAL STRUCTURES AND MICROELECTROMECHANICAL TRANSMISSION FORMED, the entire disclosures of which are incorporated by reference in their entirety herein.
The term “sacrificial layer” as used herein means any layer or portion thereof of any surface micromachined microstructure that is used to fabricate the microstructure, but which does not exist in the final configuration. Exemplary materials for the sacrificial layers described herein include undoped silicon dioxide or silicon oxide, and doped silicon dioxide or silicon oxide (“doped” indicating that additional elemental materials are added to the film during or after deposition). The term “structural layer” as used herein means any other layer or portion thereof of a surface micromachined microstructure other than a sacrificial layer and a substrate on which the microstructure is being fabricated. Exemplary materials for the structural layers described herein include doped or undoped polysilicon and doped or undoped silicon. Exemplary materials for the substrates described herein include silicon. The various layers described herein may be formed/deposited by techniques such as chemical vapor deposition (CVD) and including low-pressure CVD (LPCVD), atmospheric-pressure CVD (APCVD), and plasma-enhanced CVD (PECVD), thermal oxidation processes, and physical vapor deposition (PVD) and including evaporative PVD and sputtering PVD, as examples.
In more general terms, surface micromachining can be done with any suitable system of a substrate, sacrificial film(s) or layer(s) and structural film(s) or layer(s). Many substrate materials may be used in surface micromachining operations, although the tendency is to use silicon wafers because of their ubiquitous presence and availability. The substrate is essentially a foundation on which the microstructures are fabricated. This foundation material must be stable to the processes that are being used to define the microstructure(s) and cannot adversely affect the processing of the sacrificial/structural films that are being used to define the microstructure(s). With regard to the sacrificial and structural films, the primary differentiating factor is a selectivity difference between the sacrificial and structural films to the desired/required release etchant(s). This selectivity ratio is preferably several hundred to one or much greater, with an infinite selectivity ratio being preferred. Examples of such a sacrificial film/structural film system include: various silicon oxides/various forms of silicon; poly germanium/poly germanium-silicon; various polymeric films/various metal films (e.g., photoresist/aluminum); various metals/various metals (e.g., aluminum/nickel); polysilicon/silicon carbide; silicone dioxide/polysilicon (i.e., using a different release etchant like potassium hydroxide, for example). Examples of release etchants for silicon dioxide and silicon oxide sacrificial materials are typically hydrofluoric (HF) acid based (e.g., undiluted or concentrated HF acid, which is actually 49 wt % HF acid and 51 wt % water; concentrated HF acid with water; buffered HF acid (HF acid and ammonium fluoride)).
Only those portions of a surface micromachined microstructure that are relevant to the present invention will be described herein. There may and typically will be other layers that are included in a given surface micromachined microstructure, as well as in any system that includes such microstructures. For instance and in the case where the surface micromachined microstructures described herein are utilized as a movable mirror microstructure in a surface micromachined optical system, a dielectric isolation layer will typically be formed directly on an upper surface of the substrate on which such a surface micromachined optical system is to be fabricated, and a structural layer will be formed directly on an upper surface of the dielectric isolation layer. This particular structural layer is typically patterned and utilized for establishing various electrical interconnections for the surface micromachined optical system which is thereafter fabricated thereon.
One embodiment of at least a portion of a surface micromachined optical system 2 is presented in
A lower electrode 10 is disposed below the mirror microstructure 6 in spaced relation and is interconnected with the substrate as well. An electrical contact 12a is interconnected with the lower electrode 10, while an electrical contact 12b is interconnected with the mirror microstructure 6. Appropriate voltages may be applied to both of the electrical contacts 12a, 12b to move the mirror microstructure 6 toward or away from the lower electrode 10 (and thereby the substrate) into a desired position to provide an optical function. This movement is typically at least generally perpendicular relative to the lower electrode 10 and the substrate. The mirror microstructure 6 is commonly referred to as a piston mirror based upon the described motion.
Another embodiment of at least a portion of a surface micromachined optical system 16 is presented in FIGS. 1B-C. The surface micromachined optical system 16 is fabricated on a substrate (not shown) and includes at least one microstructure in the form of a mirror microstructure 20. The surface micromachined optical system 16 may and typically will include multiple mirror microstructures 20 that are disposed/arranged in the form of an array (not shown), although there may be applications where only a single mirror microstructure 20 is required. The mirror microstructure 20 is interconnected with the substrate by a pair of suspension springs 22. An imaginary line that extends between the springs 22 defines an axis about which the mirror microstructure 20 may pivot. One end of each spring 22 is interconnected with the mirror microstructure 20, while its opposite end is interconnected with a structural support 18 that is in turn interconnected with the substrate.
A pair of lower electrodes 24 are disposed below the mirror microstructure 20 and are interconnected with the substrate as well. One electrode 24a is associated with one side of the above-noted pivot axis, while the electrode 24b is associated with the opposite side of the above-noted pivot axis. There is one electrical contact 26a electrically interconnected with each lower electrode 24, while there is an electrical contact 26b electrically interconnected with the mirror microstructure 20. Appropriate voltages may be applied to appropriate ones of the contacts 26 to pivot the mirror microstructure 20 about its pivot axis in the desired direction and amount into a desired position to provide an optical function.
Details regarding a particular configuration of a mirror microstructure in a surface micromachined optical system, such as for the mirror microstructures 6 and 20 in the surface micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C, respectively, are presented in
An upper surface 46 of the second structural layer 38 is or may include an optically reflective layer or film. That is, the materials that are used to define the second structural layer 38 may provide the desired/required optical properties/characteristics for the mirror microstructure 30. More typically a separate layer or film 48 (
Depending upon the method of manufacture, a plurality of small etch release holes (not shown) may be formed through the entire vertical extent of the first and/or second structural layer 34, 38 to allow for the removal of any sacrificial layer(s) that is disposed between the first structural layer 34 and the second structural layer 38 of the mirror microstructure 30, and that is disposed between the first structural layer 34 and the substrate 54, respectively, when the mirror microstructure 30 is released from the substrate 54 (i.e., during the fabrication of the mirror microstructure 30). For instance, using the general principles of the manufacturing technique represented in FIGS. 12A-M to define the microstructure 30 would require such etch release holes. It should be appreciated that having etch release holes that extend entirely through the second structural layer 38, and which are thereby exposed on its upper surface 46, may have an adverse effect on its optical performance capabilities. Certain degradations in optical performance may be acceptable in some instances. However, the mirror microstructure 30 also may be made without having any etch release holes that extend down through the second structural layer 38, including in accordance with the methodology represented in
Various desirable characteristics of the types of reinforced mirror microstructures described herein are addressed following the discussion of various other embodiments. Certain parameters are used in this summarization. One such parameter is the diameter of the uppermost structural layer in the mirror microstructure, and that is represented by the dimension “dSL” (“SL” being an acronym for “structural layer”). “Diameter” does not of course limit this uppermost structural layer to having a circular configuration (in plan view), but is a simply the distance of a straight cord or line that extends laterally from one location on a perimeter of this uppermost structural layer, through a center of this uppermost structural layer, and to another location on the perimeter of this uppermost structural layer. The dimension dSL for the case of the mirror microstructure 30 thereby represents the diameter of the second structural layer 38 (the distance from one location on a perimeter 44 of the second structural layer 38, through a center 42 of the second structural layer 38, and to another location on this perimeter 44).
Another parameter that is used in the summarization of the desirable characteristics of the mirror microstructures disclosed herein is the distance from the center (in the lateral dimension) of the uppermost structural layer in the mirror microstructure to that reinforcing structure (by engaging a lower surface of this uppermost structural layer) which is closest to the center of this uppermost structural layer. This is represented by the dimension “dRS” (“RS” being an acronym for “reinforcing structure”). The dimension dRS for the case of the mirror microstructure 30 represents the distance from the center 42 of the second structural layer 38 to that column 50 which is closest to the center 42. A final parameter that is used in the noted summarization is the radius of curvature of the uppermost structural layer in the mirror microstructure (i.e., the amount which this uppermost structural layer is “cupped” or “bulged”). This is represented by the dimension RC. The dimension RC for the case of the mirror microstructure 30 thereby defines the radius of curvature of the second structural layer 38.
Another configuration of a mirror microstructure for a surface micromachined optical system, such as for the mirror microstructures 6 and 20 in the surface micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented in
The first columns 74 may be disposed in either equally spaced relation or the spacing between adjacent columns 74 may vary in at least some manner. The same applies to the columns 90. The plurality of first columns 74 may be offset in relation to the plurality of second columns 90 or such that no first column 74 is axially aligned (in the vertical direction) with any second column 90 as shown. However, other relative positionings between the plurality of first columns 74 and plurality of second columns 90 may be utilized as well, including where one or more of the plurality of first columns 74 is at least partially aligned with a second column 90.
An upper surface 102 of the third structural layer 94 is or includes an optically reflective layer or film. That is, the materials that are used to define the third structural layer 94 may provide the desired/required optical properties/characteristics for the mirror microstructure 58. More typically a separate layer or film will be deposited on the third structural layer 94 to realize the desired/required optical properties/characteristics. Those materials discussed above in relation to the mirror microstructure 30 for this purpose may be utilized by the mirror microstructure 58 and in the general manner illustrated in
Depending upon the method of fabrication, a plurality of small release holes (not shown) may be formed through the entire vertical extent of one or more of the first structural layer 62, the second structural layer 78, and the third structural layer 94 to allow for the removal of any underlying and adjacently disposed sacrificial layer(s), or when the mirror microstructure 58 is released from the substrate 60 (i.e., during the fabrication of the mirror microstructure 58). For instance, using the general principles of the manufacturing technique represented in FIGS. 12A-M to define the mirror microstructure 58 would require such etch release holes. It should be appreciated that having etch release holes that extend entirely through the third structural layer 94, and which are thereby exposed on its upper surface 102, may have an effect on its optical performance capabilities. Certain degradations in optical performance may be acceptable in some instances. However, the mirror microstructure 58 also may be made without having any etch release holes that extend down through the third structural layer 94, including in accordance with the methodology represented in
Certain parameters are identified on
Another configuration of a mirror microstructure for a surface micromachined optical 10 system, such as for the mirror microstructures 6 and 20 in the surface micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented in
The rails 118 further extend at least generally laterally from one location on or at least generally proximate to a perimeter 116 of the microstructure 106 (e.g., within 50 μm of the perimeter 116, more preferably within 25 μm of this perimeter 116) to another location on or at least generally proximate to this perimeter 116 (e.g., within 50 82 m of the perimeter 116, more preferably within 25 μm of this perimeter 116). In the illustrated embodiment, the rails 118 are of an axial or linear configuration in the lateral dimension, and further are disposed in at least generally parallel and equally spaced relation.
An upper surface 126 of the second structural layer 122 is or includes an optically reflective layer or film. That is, the materials that are used to define the second structural layer 126 may provide the desired/required optical properties/characteristics for the mirror microstructure 106. More typically a separate layer or film will be deposited on the second structural layer 122 to realize the desired/required optical properties/characteristics. Those materials discussed above in relation to the mirror microstructure 30 for this purpose may be utilized by the mirror microstructure 106 and in the general manner illustrated in
There are a number of significant advantages in relation to the design utilized by the mirror microstructure 106 of
Another function provided by the plurality of rails 118 is structural reinforcement of the second structural layer 122. That is, the plurality of rails 118 structurally interconnect the second structural layer 122 with the first structural layer 110. The structural reinforcement function of the rails 118 would not be adversely affected, and may in fact improve, by having at least some of the rails 118 be disposed in intersecting relation (e.g., see FIGS. 9A-B).
Certain parameters are identified on one or more of
Another configuration of a mirror microstructure for a surface micromachined optical system, such as for the mirror microstructures 6 and 20 in the surface micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented in
The rails 154 extend at least generally laterally from one location on or at least generally proximate to a perimeter 138 of the third structural layer 158 (e.g., within 50 μm of the perimeter 138, more preferably within 25 μm of this perimeter 138) to another location on or least generally proximate to this perimeter 138 (e.g., within 50 μm of the perimeter 138, more preferably within 25 μm of this perimeter 138). In the illustrated embodiment, the plurality of rails 154 are of an axial or linear configuration in the lateral dimension, and further are disposed in at least generally parallel and equally spaced relation. The plurality of rails 142 are also of an axial or linear configuration in the lateral dimension, and further are disposed in at least generally parallel and equally spaced relation.
An upper surface 162 of the third structural layer 158 is or includes an optically reflective layer or film. That is, the materials that are used to define the third structural layer 158 may provide the desired/required optical properties/characteristics for the mirror microstructure 130. More typically a separate layer or film will be deposited on the third structural layer 158 to realize the desired/required optical properties/characteristics. Those materials discussed above in relation to the mirror microstructure 30 for this purpose may be utilized by the mirror microstructure 130 and in the general manner illustrated in
There are a number of significant advantages in relation to the design utilized by the mirror microstructure 130. First is in relation to its optical characteristics. The upper surface 162 of the third support layer 158 of the mirror microstructure 130 need not and preferably does not include any vertically disposed etch release holes in order to allow for the removal of any sacrificial material from between the third structural layer 158 and the second structural layer 146 when the microstructure 130 is released from the substrate 132, which significantly enhances the optical capabilities of the mirror microstructure 130. That is, preferably the upper surface 162 of the mirror microstructure 130 is continuous and devoid of any indentations, holes, or the like (depressions or indentations that may develop on the upper surface 162 of the third support layer 158 during the manufacture the microstructure 130 may be addressed by a planarization operation). Exactly how the mirror microstructure 130 may alleviate the need for any vertically disposed etch release holes in the third structural layer 158 will be discussed in more detail below in relation to the manufacturing methodologies represented in FIGS. 12A-M, 13A-M, 15A-G, and 17A-G. Suffice it to say for present purposes that the rails 154 may at least assist in the definition of a plurality of laterally extending etch release pipes, channels, or conduits in a sacrificial layer(s) that is disposed between the second structural layer 146 and the third structural layer 158 during the manufacture of the mirror microstructure 130. As such, the rails 154 may be characterized as etch release rails as noted above. Again and in this case, how the rails 154 are oriented relative to each other, or stated another way the layout of the rails 154, may have an effect on their ability to create this plurality of etch release channels or conduits within any sacrificial layer that is disposed between the second structural layer 146 and the third structural layer 158 during the manufacture of the mirror microstructure 130 by surface micromachining techniques. The general design considerations for etch release rails again are summarized below following the description of the various embodiments of microstructures that include such etch release rails.
In the event that it would be desirable to avoid the use of etch release holes in the second structural layer 146 to allow for the removal of any sacrificial layer(s) that is disposed between the second structural layer 146 and the first structural layer 134, the characteristics noted above in relation to the rails 154 could be utilized by the rails 142 as well. Etch release holes would likely be required in the first structural layer 134 in order to allow for the removal of any sacrificial layer(s) that is disposed between the first structural layer 134 and the substrate 132 during the release of the microstructure 130 from the substrate 132.
Another function provided by the plurality of first rails 142 and second rails 154 is structural reinforcement of the mirror microstructure 130, and more particularly the third structural layer 158. It is believed that enhanced structural reinforcement is realized by having the plurality of first rails 142 disposed in a different orientation within the lateral dimension than the plurality of second rails 154. In the illustrated embodiment, the plurality of first rails 142 are disposed at least substantially perpendicular to the plurality of second rails 154 in the lateral dimension. The structural reinforcement function of the rails 142 and 154 would not be adversely affected, and may fact improve, by having at least some of the rails 142 be disposed in intersecting in relation and/or by having at least some of the rails 154 be disposed in intersecting relation (e.g., see FIGS. 9A-B).
Certain parameters are identified on one or more of
Another configuration of a mirror microstructure for a surface micromachined optical system, such as for the mirror microstructures 6 and 20 in the surface micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented in FIGS. 9A-B in the form of a two-layered mirror microstructure 166. The mirror microstructure 166 is fabricated on a substrate 168 by surface micromachining techniques. Components of the mirror microstructure 166 include a first structural layer or support 170 that is spaced vertically upward relative to the substrate 168 (e.g., disposed at a higher elevation relative to the substrate 168), a second structural layer or support 172 that is spaced vertically upward relative to the first structural layer 170 (e.g., disposed at a higher elevation relative to the substrate 168), a plurality of at least generally laterally extending first rails 174 that are disposed in spaced relation, and a plurality of at least generally laterally extending second rails 178 that are also disposed in spaced relation. Both the first rails 174 and the second rails 178 extend between and fixedly interconnect the first structural layer 170 and the second structural layer 172 to structurally reinforce the microstructure 166 and, more particularly, the second structural layer 172. In the illustrated embodiment, the plurality of first rails 174 are disposed in at least substantially parallel and equally spaced relation, as are the plurality of second rails 178. However, the plurality of first rails 174 are not disposed in the same orientation as the plurality of second rails 178 in the lateral dimension. In the illustrated embodiment, the first rails 174 are disposed at least substantially perpendicular to the second rails 178. As such, the first structural layer 170 and second structural layer 172, as well as the interconnecting rails 174, 178, may be moved simultaneously if acted upon by any interconnected actuator to provide a desired/required optical function.
The plurality of first rails 174 and the plurality of second rails 178 will not form the type of etch release channels or conduits in any sacrificial layer that may exist between the first structural layer 170 and the second structural layer 172 during the manufacture of the mirror microstructure 166 using surface micromachining in comparison to the mirror microstructures 106 and 130 discussed above. As such, a plurality of small vertically disposed etch release holes (not shown) will typically extend through the entire vertical extent of the second structural layer 172 to allow for the removal of any sacrificial layer(s) that is disposed between the first structural layer 170 and the second structural layer 172 of the mirror microstructure 166 used in the assembly thereof (a plurality of small vertically disposed etch release holes (not shown) will also typically extend through the entire vertical extent of the first structural layer 170 to allow for the removal of any sacrificial layer(s) that is disposed between the first structural layer 170 and the substrate 168 of the mirror microstructure 166 to release the microstructure 166 from the substrate 168). This again may have an adverse impact on the optical performance capabilities of an upper surface 176 of the second structural layer 172. However, the microstructure 166 still has desirable structural reinforcement characteristics. In this regard, the plurality of first rails 174 and the plurality of second rails 178 may be viewed as defining a grid 177 having a plurality of closed cells 179 (i.e., having a closed boundary). Any appropriate configuration may be used to define the perimeter of these closed cells 179 (e.g., a honeycomb, cylindrical), and each such closed cell 179 need not be of the same configuration.
An upper surface 176 of the second structural layer 172 is or includes an optically reflective layer or film of the type discussed above in relation to the mirror microstructure 30. Due to the likely existence of the plurality of vertically disposed etch release holes in the second structural layer 172, the upper surface 176 will at least have a plurality of dimples or the like which may have an effect on its optical performance capabilities. As such, the principal advantage of the mirror microstructure 166 is the structural reinforcement of the second structural layer 172 that is provided by the plurality of rails 174, 178 that structurally interconnect the second structural layer 172 with the first structural layer 170.
Certain parameters are identified on one or more of FIGS. 9A-B and that are addressed in the above-noted summarization of desirable characteristics that follows below. The dimension “dSL” for the case of the microstructure 166 represents the diameter of the second structural layer 172 (e.g., the distance of a straight line that extends from one location on a perimeter 173 of the second structural layer 172, through a center 171 of the second structural layer 172, and to another location on this perimeter 173) that is being structurally reinforced collectively by the plurality of rails 174, the rails, 178, and the first structural layer 170. The dimension “dRS” for the case of the microstructure 166 represents the distance from the center 171 of the second structural layer 172 to that portion of the grid 177 that is closest to the center 171. Finally, RC for the case of the microstructure 166 represents the radius of curvature of the second structural layer 172.
Another configuration of a mirror microstructure for a surface micromachined optical system, such as for the mirror microstructures 6 and 20 in the surface micromachined optical systems 2 and 16 of FIGS. 1A and 1B-C, respectively, is presented in FIGS. 10A-C in the form of a reinforced, single-layer mirror microstructure 384. The mirror microstructure 384 is fabricated on a substrate 386 by surface micromachining techniques and is separated therefrom by a space 392. Other structural components could be disposed within this space 392, although any such structures would typically be spaced from the mirror microstructure 384 at least when providing its optical function. Components of the mirror microstructure 384 include a structural layer or support 404 that is spaced vertically upward relative to the substrate 386 (e.g., disposed at a higher elevation relative to the substrate 386); a plurality of at least generally laterally extending rails 400 that are fixedly interconnected with the structural layer 404, that extend toward but not to the substrate 386, and that are disposed in a first orientation; and a plurality of at least generally laterally extending rails 396 that are fixedly interconnected with the lower extreme of the rails 400 at a plurality of discrete locations, that extend toward but not to the substrate 386, and that are disposed in a second orientation that is different than the first orientation. In the illustrated embodiment, the rails 400 are disposed at least generally perpendicular to the rails 396, although other relative orientations or relative angular positions could be utilized. Therefore, the structural layer 404, as well as the rails 400 and 396, may be moved simultaneously if acted upon by any interconnected actuator to provide a desired/required optical function.
At least the rails 400, and possibly the rails 396, extend at least generally laterally from one radial location corresponding with or at least generally proximate to a perimeter 390 of the microstructure 384 (e.g., within 50 μm of the perimeter 390, more preferably within 25 ∞m of this perimeter 390) to another radial location corresponding with or at least generally proximate to this perimeter 390 (e.g., within 50 μm of the perimeter 390, more preferably within 25 μm of this perimeter 390). In the illustrated embodiment, the rails 400 are of an axial or linear configuration in the lateral dimension, and are further disposed in at least generally parallel and equally spaced relation, while the rails 396 are also of an axial or linear configuration in the lateral dimension, and are further disposed in at least generally parallel and equally spaced relation.
An upper surface 406 of the structural layer 404 is or includes an optically reflective layer or film. That is, the materials that are used to define the structural layer 404 may provide the desired/required optical properties/characteristics for the mirror microstructure 384. More typically a separate layer or film will be deposited on the structural layer 404 to realize the desired/required optical properties/characteristics. Those materials discussed above in relation to the mirror microstructure 30 for this purpose may be utilized by the mirror microstructure 384 and in the general manner illustrated in
There are a number of significant advantages in relation to the design utilized by the mirror microstructure 384. First is in relation to its optical characteristics. The upper surface 406 of the support layer 404 of the mirror microstructure 384 need not and preferably does not include any vertically disposed etch release holes which extend entirely through the structural layer 404 in order to allow for the removal of any sacrificial material from between the structural layer 404 and the substrate 386 when the microstructure 384 is released from the substrate 386, which significantly enhances the optical performance capabilities of the mirror microstructure 384. That is, preferably the upper surface 406 of the mirror microstructure 384 is continuous and devoid of any indentations, etch release holes, or the like (depressions or indentations that may develop on the upper surface 406 of the structural layer 404 from the manufacture of the mirror microstructure 384 may be addressed by a planarization operation). Exactly how the mirror microstructure 384 may alleviate the need for the etch release holes in the structural layer 404 will be discussed in more detail below in relation to the manufacturing methodologies represented in FIGS. 12A-M, 13A-M, 15A-G, and 17A-G. Suffice it to say for present purposes that the rails 396, the rails 400, or both may at least assist in the definition of a plurality of at least generally extending etch release pipes, channels, or conduits in a sacrificial layer(s) to allow for a more expedient removal of the same and without the need for any conventional etch release holes that extend downwardly entirely through the structural layer 404. As such, the plurality of rails 396 and 404 may both be characterized as etch release rails as noted above. Again and in this case, how the rails 400 are oriented relative to each other and how the rails 396 are oriented relative to each other, or stated another way the layout of the rails 396 and 404, may have an effect on their respective abilities to these etch release conduits within any sacrificial layer that is disposed between the structural layer 404 and the substrate 386 during the manufacture of the mirror microstructure 384 by surface micromachining techniques. The general design considerations for etch release rails again are summarized below following the description of the various embodiments of microstructures that include such etch release rails.
Another function provided by the plurality of rails 400 and the plurality of rails 396 is structural reinforcement of the microstructure 384 and, more particularly the structural layer 404. In the illustrated embodiment, the rails 400 provide enhanced stiffness in effectively one direction and the plurality of rails 396 provide enhanced stiffness in effectively one direction as well, but different from that associated with the rails 400. It is believed that enhanced structural reinforcement is realized by having the plurality of rails 400 disposed in a different orientation within the lateral dimension than the plurality of rails 396. In the illustrated embodiment, the plurality of rails 400 are disposed at least substantially perpendicular to the plurality of rails 396 in the lateral dimension. The structural reinforcement function of the rails 400 and 396 would not be adversely affected, and may in fact improve, by having at least some of the rails 400 be disposed in intersecting in relation and/or by having at least some of the rails 396 be disposed in intersecting relation (e.g., see FIGS. 9A-B).
Certain parameters are identified on one or more of FIGS. 10A-C in relation to the mirror microstructure 384 and that are addressed in the above-noted summarization of the desirable characteristics that follows below. The dimension “dSL” for the case of the microstructure 384 represents the diameter of the structural layer 404 (e.g., the distance of a straight line that extends from one location on the perimeter 390 of the structural layer 404, through a center 388 of the structural layer 404, and to another location on this perimeter 390) that is being structurally reinforced collectively by the plurality of rails 400 and the plurality of rails 396. The dimension “dRS” for the case of the microstructure 384 represents the distance from the center 388 of the structural layer 404 to that rail 400 that is closest to the center 388. Finally, RC for the case of the mirror microstructure 384 represents the radius of curvature of the structural layer 404.
Other layouts of rails that provide/allow for the formation of the etch release conduits in a sacrificial layer(s) (i.e., etch release rails), are within the scope of the present invention. Representative examples of other etch release rail layouts that may be appropriate for forming etch release channels and that may be used in any of the above-described embodiments of mirror microstructures are presented in FIGS. 11A-F.
Various layouts of etch release rails have been described above. However, it should be appreciated that these layouts are merely representative of the various ways in which etch release rails may be patterned to define a plurality of etch release conduits. Any layout of etch release rails may be utilized that will allow for the removal of one or more sacrificial layers in a desired manner by providing or allowing for the formation of a plurality of at least generally laterally extending etch release conduits or channels within one or more sacrificial layers through which an appropriate etchant may flow during the release of the corresponding microstructure from its substrate. What etch release rail layouts are appropriate for this purpose is based upon a number of factors. Generally, the etchant will proceed at least generally perpendicularly away from its corresponding etch release conduit. At least one and more typically a pair of etch release conduits will extend at least generally along the lateral extent of each etch release rail. A targeted total etch release time will be established or specified. The maximum total etch release time for the types of microstructures described herein is preferably that which does not significantly damage any of the structural layers of the corresponding microstructure by exposure of the same to the etchant. The etch rate will also be known and for design purposes may be assumed to be constant. Therefore, a determination may then be made as to how far from a given etch release conduit the etchant will proceed in the specified total etch release time to create what may be characterized as a projected etch release void. So long as the projected etch release voids between adjacent etch release rails abut or more preferably overlap to a degree, or stated another way such that the entirety of the space between adjacent etch release rails is defined by at least one and more typically a plurality of projected etch release voids, the proposed layout of the etch release rails will be appropriate for affecting the release.
Another way to characterize an appropriate layout of etch release rails is in relation to a maximum desired spacing between adjacentmost etch release rails. Each etch release rail should be positioned such that the maximum space between a given etch release rail and an adjacentmost etch release rail is no more than about twice the linear distance that an etchant will proceed through a sacrificial layer in a specified total etch release time. In one embodiment, this maximum spacing is less than about 100 microns, and in another embodiment is within a range of about 50-75 microns.
Another important factor in relation to the various mirror microstructures discussed above, as well as in relation to the methods and mirror microstructures to be discussed below, is the surface topography of the uppermost structural layer of these mirror microstructures. This is particularly an issue when reinforcing structures extend or depend from the lower surface of this uppermost structural layer. These types of structures are generally formed by first forming a plurality of apertures within a layer of sacrificial material, and then depositing a structural material over this layer of sacrificial material (discussed in more detail below). That portion of the structural material that is deposited within the apertures formed in the layer of sacrificial material defines the reinforcing structures. The surface topography that is desired for the upper surface of the uppermost structural layer of any of the mirror microstructures described herein, as deposited, or stated another way before being planarized, is one where the maximum distance between any peak and valley on the upper surface of this uppermost structural layer is less than the maximum thickness of this uppermost structural layer. This type of surface topography allows the upper surface of the uppermost structural layer to thereafter be planarized a reasonable amount to yield the desired degree of optical flatness.
There are a number of ways in which the desired surface topography can be realized for the case where reinforcing structures extend from the lower surface of the uppermost structural layer of the mirror microstructure. One is to form this uppermost structural layer for the mirror microstructure to a sufficient thickness such that appropriate planarization techniques (e.g., chemical mechanical polishing) may be utilized to reduce the surface roughness of this surface to a desired level without an undesirable amount of thinning of this uppermost structural layer at any location. A sufficient thickness for the uppermost structural layer in this scenario is where the thickness of the uppermost structural layer is thicker than the underlying layer of sacrificial material by an amount such that after being planarized (e.g., by chemical mechanical polishing) to realize the desired optical surface, the uppermost structural lay will still have sufficient mechanical integrity. Another option for the case when the thickness of the uppermost structural layer of the mirror microstructure is comparable to the thickness of the underlying layer of sacrificial material is to control the width of the apertures in this layer of sacrificial material that again are used to define the reinforcing structures that extend or depend from the lower surface of the uppermost structural layer of the mirror microstructure.
The mirror microstructures described herein that structurally reinforce the uppermost structural layer, that have one or more structures extending or depending from the lower surface of the uppermost structural layer for purposes of providing etch release rails, or both, have a desired surface topography on the uppermost structural layer. This may be provided or realized by the sizing of the structures that extend or depend from the lower surface of the uppermost structural layer. Generally, the maximum width of any individual structure that extends or depends from the lower surface of the uppermost structural layer (e.g., the width of a rail, the diameter of a post or column) should be less than twice the thickness of the uppermost structural layer to provide the desired surface topography based solely on the selection of reinforcement structure width. This “thickness of the uppermost structural layer” is that thickness that is disposed above the depending reinforcing structure, or stated another way the thickness at a location that is between any adjacent depending reinforcing structures. This provides a desirable surface topography for the uppermost structural layer, namely one that may be planarized (e.g., by chemical mechanical polishing) a reasonable amount to yield a desired degree of optical flatness while allowing the uppermost structural layer to retain sufficient mechanical integrity. These same principles may be applied to any underlying structural layer of the mirror microstructure as well to improve the surface topography. It should be noted that the maximum width of the reinforcing structures that extend or depend from the lower surface of the uppermost structural layer becomes less important as the ratio of the thickness of the uppermost structural layer to the thickness of the underlying sacrificial layer increases. Therefore, some combination of structural layer thickness and reinforcement structure width may provide the noted desired surface topography as well.
The various reinforcing structures discussed above may be used at any level within a mirror microstructure. That is, even though a particular reinforcing structure may have only been described herein in relation to a two-layered structure does not mean that the same could not be used to structurally reinforce a single structural layer of a mirror microstructure (i.e., so as to cantilever from the same) or to structurally interconnect adjacent but spaced structural layers in a mirror microstructure having three or more spaced structural layers. Moreover, any combination of the above-noted reinforcing structures may be used in any combination in a microstructure having three or more spaced structural layers (e.g., to interconnect any two adjacent but spaced structural layers), or may cantilever from a lower surface of a structural layer to structurally reinforce the same in the general manner, for instance, of the microstructure 302 to be discussed below in relation to the methodology of FIGS. 15A-G.
The above-noted reinforcing structures may also utilize any appropriate vertical and/or horizontal cross-sectional profiles/configurations, and further may extend in the lateral dimension in any appropriate manner (e.g., axially, sinusoidally, meandering, “zig-zagging”). Although the above-noted reinforcing structures have been illustrated as being at least generally perpendicular to an interconnected structural layer(s), such need not be the case. Moreover, all reinforcing structures need not necessarily be disposed in the same vertical orientation (i.e., the same may be disposed at one or more different angles relative to vertical).
It should be appreciated that the mirror microstructures 30, 58, 106, 130, 166, 384, and 384′ discussed above may be incorporated into any surface micromachined system and at any elevation within such a system, and that these microstructures 30, 58, 106, 130, 166, 384, and 384′ may be appropriate for applications other than as a mirror. Characterizing the various structural layers in these microstructures 30, 58, 106, 130, 166, 384, and 384′ as “first,” “second” and the like also does not necessarily mean that these are the first, second, or the like structural layers that are deposited over the associated substrate, although such may be the case. It should also be appreciated that the characterization of the various structural layers of these microstructures 30, 58, 106, 130, 166, 384, and 384′ as “first,” “second,” and the like also does not mean that the same must be “adjacent” structural layers in any surface micromachined system that includes these microstructures 30, 58, 106, 130, 166, 384, and 384′. There may be one or more intermediate structural layers that are deposited over the associated substrate at an elevation that is between what has been characterized as “first” and “second” structural layers or the like, although any such structural layers will have been removed from the area occupied by the microstructures 30, 58, 106, 130, 166, 384, and 384′ (e.g., there may be one or more of structural layers that are “off to the side” or laterally disposed relative to a given microstructure for various purposes). It should also be appreciated that the various structural layers of the noted microstructures 30, 58, 106, 130, 166, 384, and 384′ may also be defined by one or more structural layers, such as involving multiple and spaced in time depositions.
One key advantage of the microstructures 30, 58, 106, 130, 166, 384, and 384′ discussed above is their structurally reinforced nature. Some of these reinforcement alternatives may require etch release holes through one or more of the various structural layers depending upon the particular manufacturing technique that is employed, which may degrade the performance of the microstructures 30, 58, 106, 130, 166, 384, and 384′ in a given application (e.g., when functioning as a mirror in an optical system, and where etch release holes are required for the uppermost structural layer because of the reinforcement structure that was utilized). A certain amount of degradation may be acceptable based upon the enhanced structural rigidity realized by these microstructures, and in certain applications the existence of the noted etch release holes may be irrelevant or at least of reduced significance. However, certain of the reinforcement structures utilized by the above-noted microstructures may actually enhance the release of the microstructure from the associated substrate and altogether alleviate the need for vertically disposed etch release holes in one or more structural layers. These techniques will be discussed in more detail below in relation to
The above-described structurally reinforced mirror microstructures will typically have a minimum surface area of about 2,000 μm2 with a minimum lateral dimension of about 50 μm for the optically functional surface of the mirror microstructure (e.g., in the case of a circular mirror microstructure, this minimum lateral dimension would be its diameter; in the case of a square mirror microstructure, this minimum lateral dimension would be the length of any of its four sides; in the case of a rectangular mirror microstructure, this minimum lateral dimension would be the length of the shortest of its four sides). Moreover, the above-described structurally reinforced mirror microstructures will utilize a structural layer with the optically functional surface that has a maximum film thickness of about 10 μm in one embodiment, and more typically about 6 μm in another embodiment. This “maximum film thickness” does not include the thickness of any reinforcement structure that extends or depends from a lower surface of the relevant structural layer.
The actual amount by which the above-noted microstructures are structurally reinforced is affected by one or more characteristics of the reinforcing structures that are used. There may be two extremes in relation to the number or density of reinforcing structures that are used. The first is having a high density for the individual reinforcing structures, which is limited only by the design rules and minimum dimensions of the process technology, as well as the ability to insure that the etchant for the release can adequately access the sacrificial films for their removal. The second extreme is to go to a very sparse or low density for the reinforcing structures, which gets to the point of doing limited reinforcing or stiffening of the associated structural layer(s). The benefit of the former is to provide a maximum effect of reinforcement or stiffening of the associated structural layer(s) or microstructure, but at the expense of density and therefore total mass of the microstructure (i.e., in the case where the microstructure is a mirror, the microstructure may be very stiff, but too massive to allow for the use or realization of rapid switching speeds). Therefore, the optimum spacing and size of the reinforcing structures will oftentimes be an engineering compromise between stiffness and mass. If total mass does not matter, but stiffness is at a premium, then using a high density for the reinforcing structures, as allowed by the design and processed rules, would be optimum for the particular application. However, if the microstructure is a mirror or some other structure which must switch or otherwise move at a relatively fast rate (e.g., in sub-millisecond times), then a calculation of total mass versus available actuation forces will likely determine the appropriate density for the reinforcing structures. Mass may also be an issue for those microstructures that are moved in relation to the physical size of any associated actuator, the amount of voltage required to accomplish the desired movement, or both.
There are a number of ways in which the microstructures 30, 58, 106, 130, 166, 384, and 384′ may be at least generally characterized. One or more of the microstructures 30, 58, 106, 130, 166, 384, and 384′ may be characterized as including: 1) at least two separate and distinct (i.e., not interconnected and disposed in spaced relation) reinforcing structures (e.g., at least two separate and discrete columns or posts; at least two separate and discrete rails); 2) at least two separate and distinct reinforcing structures that are disposed at different and radially spaced locations or positions in the lateral dimension, or at least two different portions of what may be characterized as a single reinforcing structure (e.g., the “grid” defined by the rails 174 and rails 178 in the microstructure 166 of FIGS. 9A-B) being disposed at different and radially spaced locations or positions in this lateral dimension; 3) having a ratio of dRS/dSL that is no more than about 0.5, and thereby including a ratio of “0.”, for instance where dRS is “0” (i.e., where there is a reinforcing structure at the center of the structural layer being structurally reinforced); or 4) or any combination thereof. Visualization of the second noted characterization may be enhanced by a reference to the
Another way of characterizing the microstructures 30, 58, 106, 130, 166, 384, and 384′ is in relation to a radius of curvature RC of the uppermost structural layer (e.g., the amount by which the uppermost structural layer is “bowed” or “dished”). The radius of curvature RC may have its center on either side of the uppermost structural layer in the microstructures 30, 58, 106, 130, 166, 384, and 384′. That is, the upper surface of the uppermost structural layer in the microstructures 30, 58, 106, 130, 166, 384, and 384′ may be generally concave or generally convex. In one embodiment, the uppermost structural layer of the microstructures 30, 58, 106, 130, 166, 384, and 384′ has a radius of curvature RC that is at least about 1 meter, and in another embodiment that is at least about 2 meters. The reinforcement configuration used by the microstructure 166 in a three-layered mirror microstructure has been fabricated with a radius of curvature RC that is about 14 meters. It should be noted that increasing the stiffness of a microstructure does not in and of itself mean that the radius of curvature of an uppermost structural layer of this microstructure will in turn be increased. That is, the case of a constant stress gradient through a plate or a beam leads to the same radius of curvature independent of thickness to first order. This then indicates that simple stiffening by adding thickness to a plate or a beam does not, in and of itself, necessarily lead to a flatter structure with a greater radius of curvature. However, the complex method of reinforcing microstructures in the manner disclosed herein can lead to significant internal stress compensation between and within the individual structural layers of these reinforced microstructures, such that greater flatness (i.e., a larger radius of curvature for the uppermost structural layer) can be realized in addition to achieving greater stiffness.
Another characterization that can be made in relation to the microstructures 30, 58, 106, 130, 166, 384, and 384′ is the effect of the various reinforcing structures/layouts on overall structural stiffness of the microstructures 30, 58, 106, 130, 166 384, and 384′, which can be characterized by moment of inertia. In very general terms, the moment of inertia for a simple rectangular cross-section is I=bh3/12, where h is the plate thickness and b the width. This illustrates the general idea or concept that structural stiffness is a cubic function of the corresponding thickness. Thus, going from a 2.25 ∞m thick single structural layer to an approximately 11.0 μm multi-layered, structurally reinforced microstructure of the type contemplated by the microstructures 30, 58, 106, 130, 166, 384, and 384′ implies an approximate increase of stiffness by a factor of 113/2.253=117, or roughly two orders of magnitude (with the orders by 10-based). This is important when there is a need for a structure to not deform out of plane, or in the case of a mirror microstructure that is coated with a reflective gold layer to not be deformed by the stress in the gold layer. In other words, because of thermal mismatch for example, the gold can be in relative tension to the underlying structural layer (e.g., polysilicon), and thus would have the tendency to cause the underlying structural layer to curl into a cup shape. The dramatic increase in stiffness by reinforcement will keep the mirror microstructure much flatter. Also, during temperature cycles, the gold-coated plate will not change its RC as much (i.e., it will be much more mechanically stable). The overall complexity of the geometry and the deposition and anneal precludes a simple ‘estimate’ of the resulting RC especially at the current levels being obtained. Empirical determination is less time consuming and more accurate.
Investigations are still being undertaken in relation to evaluating the structural reinforcement of the microstructures 30, 58, 106, 130, 166, 384, and 384′. Generally, it is believed that a structurally reinforced three-layered microstructure will be more rigid than a structurally reinforced two-layered microstructure. In this case, any combination of the above-noted reinforcing structures that structurally interconnect adjacent structural layers in the microstructures 30, 58, 106, 130, 166, 384, and 384′ may be utilized in a three-layered microstructure in accordance with one or more principles of the present invention. However, evaluations are still ongoing in relation to “optimizing” the structural reinforcement of the microstructures 30, 58, 106, 130, 166, 384, and 384′ in the general manner described herein. There may be instances where different combinations of the above-noted reinforcing structures may be more appropriate for when the associated microstructure is used in a given application or in certain conditions.
Various microstructure fabrication methods will now be described. Each of the various fabrication methods to be discussed that utilize reinforcement/etch release rails may also use the above-noted guidelines for maximum reinforcement structure widths to realize the desired surface topography for the uppermost structural layer prior to planarizing the same. Each of the various fabrication methods to be discussed which define etch release conduits should be implemented such that at least one end of at least one etch release conduit is disposed at or at least generally proximate to a perimeter of the microstructure being fabricated (e.g., within 50 microns of this perimeter in one embodiment, and more preferably within 25 microns of this perimeter in another embodiment). This again reduces the amount of time that the release etchant must “etch in” from this perimeter before reaching the etch release conduit(s), and thereby allows the release to be finished within a desired amount of time. Multiple ends of etch release conduits or etch release conduit accesses are preferably disposed at this radial position. It should be noted that sacrificial material is disposed about the perimeter of the microstructures that are fabricated in accordance with the following. Therefore, absent a preformed via or the like, the release etchant must first etch down to the level(s) at which the etch release conduits are disposed. However, the time required for the release etchant to go down to the level(s) of the etch release conduits is not that significant due to the rather minimal vertical distance which these microstructures extend above their corresponding substrate.
One method for making a microstructure is illustrated in FIGS. 12A-M. This methodology may be utilized to make the mirror microstructure 106 of
A second structural layer 202 is deposited on the second sacrificial layer 194 as illustrated in
The second structural layer 202 is then patterned to define a plurality of at least generally laterally extending second reinforcement sections 210 for the reinforcing assembly 208, as illustrated in
A third sacrificial layer 218 is then deposited on the second reinforcement sections 210 that were formed from the second structural layer 202 as illustrated in
The third sacrificial layer 218 is then patterned to define a plurality of interconnect apertures 222 as illustrated in
A third structural layer 226 is deposited on the third sacrificial layer 218 as illustrated in
The upper surface of the third structural layer 226 will typically have a wavy or uneven contour as illustrated in
The microstructure 180 is now ready to be released. “Released” means to remove each of the sacrificial layers of the surface micromachined system and thereby including the first sacrificial layer 186, the second sacrificial layer 194, and the third sacrificial layer 218. An etchant is used to provide the releasing function. The manner in which the microstructure 180 was formed in accordance with the methodology of FIGS. 12A-M reduces the time required for the sacrificial layers 194 and 218 to be totally removed. Ultimately, a plurality of at least generally laterally extending etchant flow pipes, channels, or conduits are formed in the third sacrificial layer 218. Those portions of the third sacrificial layer 218 that are positioned against/near the second reinforcement sections 210 that were formed from the second structural layer 202 are believed to be less dense than the remainder of the third sacrificial layer 218 since the etch rate is greater in proximity to the second reinforcement sections 210 and including along the length thereof. Recall that the third sacrificial layer 218 was deposited after the second reinforcement sections 210 were formed, which creates these low density regions. Low density regions in the third sacrificial layer 218 thereby exist along the entire length of both sides of each second reinforcement 210. Principally these low density regions will exist along a sidewall 212 of each of the second reinforcement section 210 (e.g., the vertically disposed/extending portion of the second reinforcement section 210). The etch rate will be greater in the low density regions of the third sacrificial layer 218 than throughout the remainder of the third sacrificial layer 218. This will effectively form at least two etch release pipes, channels or conduits in the third sacrificial layer 218 along the side of each second reinforcement section 210. The development of the etch release channels during the initial portion of the release etch provides access to interiorly disposed locations within the sacrificial layers for the etchant to complete the release before the etchant has any significant adverse effect on the microstructure 180. Notwithstanding the characterization of the structures 214, 210, and 230 as “reinforcement sections,” it should be appreciated that the entire focus of the methodology of FIGS. 12A-M could in fact be to simply provide a plurality of at least generally laterally extending etch release conduits, to in turn provide a “rapid etch release function” for the microstructure 180. That is, it is not required that the structures 214, 210, and 230 actually structurally reinforce the microstructure 180, although such is preferably the case. Therefore, the structures 210 that provide for the definition of the low density regions in the third sacrificial layer 218, and thereby the etch release conduits, could also be properly characterized as etch release rails or the like.
The microstructure 180 of
The basic principle for forming etch release channels or conduits encompassed by the methodology represented in FIGS. 12A-M is that low density regions of sacrificial material are formed when the sacrificial material is deposited along at least generally vertically disposed surfaces of an etch release rail, and that the same each effectively defines a etch release channel or conduit. These etch release rails may exist at any desired level within the microstructure being fabricated and yet still provide this low density region formation function. Moreover, these etch release rails do not need to be structurally interconnected with the uppermost structural layer of the microstructure being fabricated to provide this low density region formation function. For instance, these etch release rails instead could be anchored to the underlying substrate or another underlying structural layer. In fact, these etch release rails need not remain in the final structure of the microstructure being fabricated at all, but instead may be removed during the release of the microstructure from the substrate.
Another method for making a microstructure is illustrated in FIGS. 13A-M. This methodology may be utilized to make the mirror microstructure 106 of
A second structural layer 254 is deposited on the second sacrificial layer 246 as illustrated in
The second structural layer 254 from
A third sacrificial layer 274 is then deposited on the second reinforcement sections 262 that were formed from the second structural layer 254 and on the second sacrificial layer 246 as illustrated in
The upper surface of the third sacrificial layer 274 may retain a wavy or uneven contour after being deposited. The upper surface of the third sacrificial layer 274 may then be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the third sacrificial layer 274 and as illustrated in
A third structural layer 282 is then deposited on the third sacrificial layer 274 as illustrated in
The upper surface of the third structural layer 282 will typically have a wavy or uneven contour, such as a plurality of laterally disposed and axially extending depressions. Generally, those portions of the third structural layer 282 that are disposed between adjacent third reinforcement sections 286 will be recessed to a degree. Therefore, the upper surface of the third structural layer 282 will typically be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the third structural layer 282 and as illustrated in
The microstructure 232 is now ready to be released. “Released” means to remove each of the sacrificial layers in the system, and thereby including the first sacrificial layer 238, the second sacrificial layer 246, and the third sacrificial layer 274. An etchant is used to provide the releasing function. Predefined flow paths for this etchant are defined in and extend through portions of the second sacrificial layer 246 and the third sacrificial layer 274 in the form of the above-noted etch release channels 270. The existence of the etch release channels 270 provides access to interiorly disposed locations within the sacrificial layers 246, 274 for the etchant to complete the release before the etchant has any significant adverse effect on the microstructure 232. Notwithstanding the characterization of the structures 258, 262, and 286 as “reinforcement sections,” it should be appreciated that the entire focus of the methodology of FIGS. 13A-M could in fact be to simply provide a plurality of at least generally laterally extending etch release conduits, to in turn provide a “rapid etch release function” for the microstructure 232. That is, it is not required that the structures 258, 262, and 286 actually structurally reinforce the microstructure 232, although such is preferably the case. Therefore, the structures 258 and/or 262 that provide for the definition of the etch release conduits 270 could also be properly characterized as etch release rails or the like.
The basic principle for forming etch release channels or conduits encompassed by the methodology represented in FIGS. 13A-M is that one or more undercuts may be formed under an etch release rail in such a manner that the subsequent deposition of a sacrificial material will not entirely fill these undercuts, thereby leaving a void that defines an etch release channel or conduit. These etch release rails may exist at various levels within the microstructure being fabricated and yet still allow for the formation of etch release channels or conduits in this same general manner. Moreover, these etch release rails do not need to be structurally interconnected with the uppermost structural layer of the microstructure being fabricated to allow for the formation of etch release channels or conduits in this same general manner. For instance, these etch release rails instead could be anchored to the underlying substrate or another underlying structural layer. In fact, these etch release rails need not remain in the final structure of the microstructure being fabricated at all, but instead may be removed during the release of the microstructure from the substrate as in the case of the methodology of FIGS. 14A-F.
Another method for making another embodiment of a microstructure 358 is illustrated in FIGS. 14A-F.
At this time, part of an upper portion of the first sacrificial layer 360 is removed as illustrated in
A second sacrificial layer 372 is then deposited on the strips 368 and on the first sacrificial layer 360 as illustrated in
The upper surface of the second sacrificial layer 372 may retain a wavy or uneven contour after being deposited (not shown). The upper surface of the second sacrificial layer 372 may then be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the third sacrificial layer 372 and as illustrated in
The microstructure 358 is now ready to be released. “Released” means to remove each of the sacrificial layers in the system, and thereby including the first sacrificial layer 360 and the second sacrificial layer 372. The plurality of strips 368 are also removed in this release. Etchants are used to provide the releasing function. Predefined flow paths for this etchant are defined in and extend through portions of the second sacrificial layer 372 and the first sacrificial layer 360 in the form of the above-noted etch release channels 380. The existence of the etch release channels 380 provides access to interiorly disposed locations within the sacrificial layers 360, 372 for the etchant to complete the release before the etchant has any significant adverse effect on the microstructure 358.
The manner in which the etch release channels 380 are formed is similar to the methodology of FIGS. 13A-M. The primary difference is that the methodology of FIGS. 14A-F does not define any reinforcing structure for its microstructure 358, whereas the methodology of FIGS. 13A-M does define a reinforcing assembly 256 for its microstructure 232. A related difference is that the strips 368 in the methodology of FIGS. 14A-F are removed during the release etch or in a post-release etch, whereas the reinforcing assembly 256 in the methodology of FIGS. 13A-M is not removed during the release.
Another method for making another embodiment of a reinforced microstructure 302 is illustrated in FIGS. 15A-G. In addition to being able to form a desired reinforcing structure, the methodology of FIGS. 15A-G further provides a desired manner for releasing the microstructure 302 at the end of processing by forming a plurality of at least generally laterally extending etch release channels in one or more of its sacrificial layers to facilitate the removal thereof when releasing the microstructure 302. Any of the layouts noted above for etch release rails may be used to form these etch release channels, but in the manner set forth in relation to FIGS. 15A-G.
A second sacrificial layer 316 is deposited on the first structural layer 308 as illustrated in
The upper surface of the second sacrificial layer 316 may retain a wavy or uneven contour. The upper surface of the second sacrificial layer 316 may then be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the second sacrificial layer 316 and as illustrated in
A second structural layer 328 is deposited on the second sacrificial layer 316 as illustrated in
The upper surface of the second structural layer 328 will typically have a wavy or uneven contour or at least a plurality of laterally disposed and preferably axially extending depressions. Generally, those portions of the second structural layer 328 that are disposed between adjacent upper reinforcement sections 332 will be recessed to a degree. Therefore, the upper surface of the second structural layer 328 will typically be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the second structural layer 328 and as illustrated in
The microstructure 302 is now ready to be released. “Released” means to remove each sacrificial layer and thereby including the first sacrificial layer 304 and the second sacrificial layer 316. An etchant is used to provide the releasing function. Predefined flow paths for this etchant are defined in and extend through portions of the first sacrificial layer 304 and the second sacrificial layer 316 in the form of the above-noted etch release channels 320. The existence of the etch release channels 320 provides access to interiorly disposed locations within the sacrificial layers for the etchant to complete the release before the etchant has any significant adverse effect on the microstructure 302.
As opposed to other of the reinforced microstructures disclosed herein, the microstructure 302 is only a single structural layer (second structural layer 328) that is structurally reinforced by a plurality of “cantilevered” structures extending downwardly therefrom, namely the plurality of at least generally laterally extending lower reinforcement sections 312. Notwithstanding the characterization of the structures 312 and 332 as “reinforcement sections,” it should be appreciated that the entire focus of the methodology of FIGS. 15A-G could in fact be to simply provide a plurality of at least generally laterally extending etch release conduits, to in turn provide a “rapid etch release function” for the microstructure 302. That is, it is not required that the structures 312 and 332 actually structurally reinforce the microstructure 302, although such is preferably the case. Therefore, the structures 312 could also be characterized as etch release rails or the like.
The basic principle for forming etch release channels or conduits encompassed by the methodology represented in FIGS. 15A-G is that the deposition of a sacrificial material onto a layer having at least one and more typically a plurality slots having a height that is at least as great as the width produces a keyholing effect at the bottom of the slot, which in turn defines an etch release channel or conduit. The layer with the noted types of slots may exist at various levels within the microstructure being fabricated and yet still allow for the formation of etch release channels or conduits in this same general manner. Moreover, a layer with the noted types of slots does not need to be structurally interconnected with the uppermost structural layer of the microstructure being fabricated to allow for the formation of etch release channels or conduits in this same general manner. For instance, the layer with the noted types of slots instead could be anchored to the underlying substrate or another underlying structural layer. In fact, this layer with the noted types of slots need not remain in the final structure of the microstructure being fabricated at all, but instead may be removed during the release of the microstructure from the substrate.
Another method for making another embodiment of a reinforced microstructure 338 is illustrated in FIGS. 16A-C.
The apertures 344 effectively function as a mold cavity and may be in any desired shape for the resulting reinforcing structure. For instance, the apertures 344 may be arranged to define a plurality of at least generally laterally extending ribs or rails (e.g., similar to the rails 118 of the mirror microstructure 106 of
A first structural layer 348 is deposited on the first sacrificial layer 340 as illustrated in
The upper surface of the first structural layer 348 may retain a wavy or uneven contour. The upper surface of the first structural layer 348 may then be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the first support layer 348. Thereafter, the first structural layer 348 is released by removing the first sacrificial layer 340. A plurality of small etch release holes (not shown) will extend through the entire vertical extent of the first structural layer 348 to allow for the removal of the first sacrificial layer 340 that is disposed between the first structural layer 348 and the substrate 336. Therefore, the primary benefit of the design of the microstructure 338 is the structural reinforcement of the first structural layer 348 and the existence of a relatively large space between the lower extreme of the reinforcing assembly 352 and the substrate 336.
Another method for making a microstructure is illustrated in FIGS. 17A-G. This methodology may be utilized to make the mirror microstructure 106 of
Multiple layers are first sequentially deposited/formed over an appropriate substrate 448 as illustrated in
A third sacrificial layer 466 is then deposited on the second sacrificial layer 454 as illustrated in
The upper surface of the third sacrificial layer 466 may retain a wavy or uneven contour, as illustrated in
Reinforcement structures may be incorporated into the microstructure using the method of FIGS. 17A-G. In this regard, the second sacrificial layer 454, as well as any overlying portion of the third sacrificial layer 466, may be patterned to define a plurality of interconnect apertures 470 as illustrated in
A second structural layer 474 is deposited on any exposed portions of the second sacrificial layer 454 and the third sacrificial layer 466, as illustrated in
The upper surface of the second structural layer 474 may retain have a wavy or uneven contour. Generally, those portions of the second structural layer 474 that are disposed over the reinforcement sections 478 may be recessed to a degree. Therefore, the upper surface of the second structural layer 474 may be planarized in an appropriate manner, such as by chemical mechanical polishing, to yield a sufficiently flat upper surface for the second structural layer 474 and as illustrated in
The microstructure 476 is now ready to be released. “Released” means to remove each of the sacrificial layers of the surface micromachined system and thereby including the first sacrificial layer 449, the second sacrificial layer 454, and the third sacrificial layer 466. An etchant is used to provide the releasing function. The manner in which the microstructure 476 was formed in accordance with the methodology of FIGS. 17A-G reduces the time required for the sacrificial layers 454 and 466 to be totally removed. Ultimately, a plurality of at least generally laterally extending etchant flow pipes, channels, or conduits are formed in the third sacrificial layer 466. Those portions of the third sacrificial layer 466 that are positioned alongside the sidewalls 462 of the etch release conduit apertures 458 that were formed in the second sacrificial layer 454 are less dense than the remainder of the third sacrificial layer 466. The etch rate will be greater in the low density regions of the third sacrificial layer 466 than throughout the remainder of the third sacrificial layer 466. This will effectively form an etch release pipe, channel or conduit in the third sacrificial layer 466 along each sidewall 462. The development of the etch release channels during the initial portion of the release etch provides access to radially inwardly disposed locations within the sacrificial layers for the etchant to complete the release before the etchant has any significant adverse effect on the microstructure 476.
The methodology represented by FIGS. 17A-G provides a number of advantages. One is that the first sacrificial layer 454 may be patterned to define any appropriate arrangement for the etch release conduit apertures 458 within/throughout a sacrificial layer(s) (and thereby an arrangement for the low density regions which will ultimately define the etch release conduits), including an arrangement where one or more of these etch release conduit apertures 458 intersect. That is, the formation of the etch release conduits is not adversely affected by having the low density regions intersect.
FIGS. 18A-B present one arrangement where the first sacrificial layer 454 has been patterned to define a network 482 or grid of one embodiment of intersecting/interconnected etch release conduit apertures 458. The network 482 of intersecting/interconnected etch release conduit apertures 458 increases the amount of the sacrificial layer that is initially exposed to the release etchant within radially inward locations, and thereby should reduce the total amount of time required to release the microstructure 476 that is being formed. The various etch release conduit apertures 458 may be routed to define a desired network 482 for distribution of the release etchant throughout the first sacrificial layer 454 to not only reduce this total etch release time, but to also allow for use of a desired reinforcing structure. In this regard,
Another advantage associated with the manner in which the etch release conduits are formed in the methodology of FIGS. 17A-G, is that these etch release conduits may be formed without requiring the use of any reinforcement structures. This is illustrated by the sequential view presented in FIGS. 19A-F. Multiple layers are sequentially deposited/formed over an appropriate substrate 488, including a first sacrificial layer 490 as illustrated in
A second sacrificial layer 496 is then deposited on the first sacrificial layer 490, as illustrated in
The upper surface of the second sacrificial layer 496 may retain a wavy or uneven contour, as illustrated in
A first structural layer 498 is then deposited on any exposed portions of the first sacrificial layer 490 and the second sacrificial layer 496, as illustrated in
The microstructure 486 is now ready to be released. “Released” means to remove each of the sacrificial layers of the surface micromachined system and thereby including the first sacrificial layer 490 and the second sacrificial layer 496. An etchant is used to provide the releasing function. The manner in which the microstructure 486 was formed in accordance with the methodology of FIGS. 19A-F reduces the time required for the sacrificial layers 490 and 496 to be totally removed. Ultimately, a plurality of at least generally laterally extending etchant flow pipes, channels, or conduits are formed in the second sacrificial layer 496. Those portions of the second sacrificial layer 496 that are positioned alongside the sidewalls 494 of the etch release conduit apertures 492 that were formed in the first sacrificial layer 490 are less dense than the remainder of the second sacrificial layer 496. The etch rate will be greater in the low density regions of the second sacrificial layer 496 than throughout the remainder of the second sacrificial layer 496. This will effectively form an etch release pipe, channel or conduit in the second sacrificial layer 496 along each sidewall 494. The development of the etch release channels during the initial portion of the release etch provides access to radially inwardly disposed locations within the sacrificial layers for the etchant to complete the release before the etchant has any significant adverse effect on the microstructure 486.
Etch release channels that exist prior to the release of the microstructure 486 may also be formed by the “keyholing” concept discussed above in relation to FIGS. 15A-G. Generally, a relationship between the width of the etch release conduit apertures 492 and the height or vertical extent of these etch release conduit apertures 492 may be selected to allow a plurality of etch release channels to be defined in the lower portion of these apertures 492. When this relationship is selected in the same general manner discussed above in relation to FIGS. 15A-G, the second sacrificial layer 496 will extend within and occupy only a portion of each of the apertures 492 that were previously formed from the first sacrificial layer 490. However, the material that defines the second sacrificial layer 496 will not fill or occupy the entirety of the apertures 492 due to the relative close spacing between the sidewalls 494 that define the etch release conduit apertures 492 in relation to the height or vertical extent of these apertures 492 (i.e., a void will remain in the lower portion of each aperture 492). This again may be characterized as “keyholing.” In any case, the remaining voids in the lower portion of each of the etch release conduit apertures 492 will define a plurality of at least generally laterally extending etch release channels.
Another method for making a microstructure is illustrated by reference to FIGS. 20A-D. The fundamental principles of this methodology may be utilized to make the mirror microstructure 30 of
Multiple layers of at least two different types of materials are sequentially deposited to define a stack 594 on a substrate 582 that is appropriate for surface micromachining. The various deposition and patterning steps that may yield the configuration illustrated in
The etch release conduit fill material 586 may be disposed at one or more levels relative to the substrate 582 within the microstructure 592. An at least generally vertically extending runner 588 of etch release conduit material 586 is disposed laterally beyond the area occupied by the microstructure 592 (i.e., off to the side), and extends at least generally downwardly from an uppermost exterior surface 596 of the stack 594 toward the substrate 582 to the etch release conduit fill material 586 at one or more of the levels within the stack 594. Any appropriate number of runners 588 may be utilized, and each may be of any appropriate configuration.
The etch release conduit fill material 586 at any level within the stack 594 may occupy the entirety of this level under at least one structural layer of the microstructure 592. More typically, a patterning operation will have been done at a given level to define an appropriate layout of etch release rails from the etch release conduit fill material 586.
Regardless of the layout of the etch release rails 598 of etch release conduit fill material 586, the microstructure 592 is released in the same general manner. Initially, the stack 594 is exposed to a first etchant that is selective to the etch release conduit fill material 586. In one embodiment, the etch release conduit fill material 586 is the same material that is used to form the various structural layers that define the microstructure 592. In this case, it is necessary for the various structural layers of the microstructure 592 to be isolated from the etch release conduit fill material 586 by sacrificial material 584. There may be instances where a particular etchant may be sufficiently selective to the etch release conduit fill material 586 so as to not require this isolation of the structural material of the microstructure 592 from the etch release conduit fill material 586.
The first etchant removes the etch release conduit fill material 586 within the runner 588 to define an access 604, as well as any etch release rails 598 of etch release conduit fill material 586 connected therewith. This first etchant preferably does not remove any significant portion of any sacrificial material 584 that encases the etch release rails 598 and/or the runner(s) 588. The resulting void by this removal of material of the etch release rails 598 defines at least one etch release conduit 602 that is at least generally laterally extending and disposed under at least one structural layer of the microstructure 592 (
There is a separate and distinct second etching operation in accordance with the methodology of FIGS. 20A-D. After the first etching operation has been executed to define at least one and more typically a plurality of etch release conduits 602, the stack 594 undergoes a second etching operation. The second etching operation uses a second etchant that is different from the first etchant, and that is selective to the sacrificial material 584. This second etchant flows down through the various accesses 604 that may be associated with the stack 594 and into any etch release conduit 602 fluidly interconnected therewith. The second etchant removes any sacrificial material 584 in contact therewith to release the microstructure 592 from the substrate 582 (
The foregoing description of the present invention has been presented for purposes of illustration and description. Furthermore, the description is not intended to limit the invention to the form disclosed herein. Consequently, variations and modifications commensurate with the above teachings, and skill and knowledge of the relevant art, are within the scope of the present invention. The embodiments described hereinabove are further intended to explain best modes known of practicing the invention and to enable others skilled in the art to utilize the invention in such, or other embodiments and with various modifications required by the particular application(s) or use(s) of the present invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.
Claims
1-66. Canceled
67. A method for making a surface micromachined microstructure, comprising the steps of:
- forming a first sacrificial layer over a first substrate, wherein said first substrate comprises an upper surface that extends in a lateral dimension;
- forming a plurality of conduits that are hollow, embedded, have a closed perimeter that is defined at least in part by said first sacrificial layer, and extend in said lateral dimension;
- forming a first structural layer over said first sacrificial layer and after said forming a first sacrificial layer step, wherein said first structural layer extends in said lateral dimension and comprises a first perimeter that defines a lateral extent of said first structural layer, wherein said plurality of conduits extend in said lateral dimension underneath said first structural layer such that said first structural layer overlies said plurality of conduits; and
- removing said first sacrificial layer after said forming a first structural layer step, wherein said removing step comprises flowing an etchant within at least some of said plurality of conduits, and wherein said forming a plurality of conduits step is completed before said forming a first structural layer step.
Type: Application
Filed: Jun 29, 2004
Publication Date: Mar 17, 2005
Inventors: Jeffry Sniegowski (Tijeras, NM), M. Rodgers (Albuquerque, NM)
Application Number: 10/879,496