Equipment and process for creating a custom sloped etch in a substrate
Equipment and processes for creating a custom sloped etch in a substrate are disclosed. An illustrative process may include the steps of providing a substrate having a surface to be etched, providing a control layer on the surface of the substrate, forming a mask above the control layer, and then selectively etching each of the control layer and substrate at variable rates to form a sloped etch in the substrate.
The present invention relates generally to the field of semiconductor manufacturing and microelectromechanical systems (MEMS). More specifically, the present invention pertains to equipment and processes for creating a custom sloped etch in a substrate.
BACKGROUND OF THE INVENTIONThe creation of custom sloped etches is important in the manufacture of microelectromechanical system (MEMS) devices and other small-scale devices. In the construction of MEMS devices, for example, such custom sloped etches can be useful in helping to reduce the voltage necessary to electrostatically actuate small structures such as beams or diaphragms, or to perform some other desired function. A sloped surface may, for example, allow an electrode that is positioned on the sloped surface to be near one or more electrodes on a beam or diaphragm at one location. The electrode on the sloped surface may then slope away from the beam or diaphragm. This may allow the beam or diaphragm to be initially actuated with a relatively small voltage, and then roll down along the sloped surface to provide the desired displacement.
In certain devices, the absence of such sloped surfaces can increase the voltage necessary to displace actuatable surfaces, and can cause a decrease in actuation speed. In certain cases, the shape of the sloped surface can also limit the amount of travel or displacement of the actuatable surface(s), further reducing the effectiveness of the device. The creation of a sloped surface in a substrate has many other useful applications including, for example, the formation of optical lens, as well as other such device having a desired contour or shape.
To overcome these shortcomings, several processes have been developed to form slope etches within a substrate that are adapted to contour to the size and shape of the actuatable surfaces. In a gray-scale lithography process, for example, an optical mask and a photolithography stepper system can be used to locally modulate the frequency of an ultraviolet (UV) light source, forming a graduated pattern of photo-resist in a photomask layer. Once formed thereon, a dry or wet-etch step containing a single etchant solution capable of selectively etching the substrate material is then used to transfer the graduated pattern of photo-resist to the substrate.
The resolution of many prior art methods prohibit the creation of certain custom sloped etches. In a gray-scale lithography process, for example, the depth at which the slope can be formed within the substrate is often limited to only a few microns, preventing the formation of deep slopes useful in many conventional MEMS devices. Moreover, the ability to vary the steepness of the contoured slope and or shape may be limited by the resolution of the etching method employed, further preventing the formation of certain slopes in the substrate. As a result, there is a need in the art for equipment and processes for creating custom sloped etches in a substrate.
SUMMARY OF THE INVENTIONThe present invention pertains to equipment and processes for creating a custom sloped etch in a substrate. An illustrative process for creating a custom sloped etch may include the steps of providing a substrate having a surface to be etched, providing a control layer on or above the surface of the substrate, providing at least one patterned mask layer onto or above the control layer, and then selectively etching each of the control layer and the substrate surface, at varying and/or controlled rates, to form a sloped etch in the substrate surface. The patterned mask layer can include one or more openings exposing the control layer to etchant contained, for example, in an etch bath or other suitable etching apparatus. The geometry and/or shape of the openings can be modified to alter the depth, steepness, shape, and other various characteristics of the slope, as desired.
The process of selectively etching the control layer to form the sloped etch can be accomplished by immersing the substrate in an etch bath containing one or more etchants adapted to selectively etch each of the substrate and the control layer materials. In certain embodiments, for example, a relatively fast-rate etchant solution of nitric acid (HNO3) can be used to selectively etch the control layer material, whereas a relatively slow-rate etchant solution of hydrofluoric acid (HF) can be used to selectively etch the substrate material. The relative concentrations of the two etchants can be varied throughout the etching process to alter the etch rate of the substrate and/or control layer, allowing the creation of a custom sloped etch having a particular shape or profile. In some cases, the temperature of the etch bath may also be varied and/or controlled throughout the etching process to help alter the etch rate of the substrate and/or control layer.
In another illustrative embodiment of the present invention, a single etchant capable of selectively etching each of the control layer and substrate at different temperatures, and thus at different etch rates, can be used to form a custom sloped etch in a substrate. In certain embodiments, for example, the materials forming the substrate and control layer can be selected to exhibit different etch rates at various temperature ranges. When placed within an etch bath including one or more heaters, for example, the temperature of the etchant can be varied in a manner that alters the etch rate in one material (e.g. the substrate material) more or less relative to the other material (e.g. the control layer material). By adjusting the temperature of the etch bath during the etching process, any number of desired shapes can be formed on the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
The following description should be read with reference to the drawings, in which like elements in different drawings are numbered in like fashion. The drawings, which are not necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. Although examples of construction, dimensions, and materials are illustrated for the various elements, those skilled in the art will recognize that many of the examples provided have suitable alternatives that may be utilized.
Referring now to
As can be further seen in
The control layer 16 should typically include a material different from that used in forming the substrate 12. In certain embodiments, for example, the control layer 16 can include a layer of nickel having a thickness of approximately 1 to 2 μm. Other materials and/or dimensions are also possible, however, depending on the particular slope characteristic desired in the surface 14. As is discussed in further steps below, the various properties of the materials used in forming the substrate 12 and control layer 16 can be used to control the etch rate within the surface 14 of the substrate 12, allowing a custom sloped etch to be formed in the substrate 12.
In certain embodiments, it may be desirable to bimorph the photomask 18 to cause it to curl and/or displace in a direction away from the surface 14 of the substrate 12 during the etching process. The second photomask layer 22 can include a material similar to that of the first photomask layer 20, or can include a material having different mechanical and/or thermal properties than that of the first photomask layer 20. In certain embodiments, for example, the second photomask layer 22 can include a relatively thin (e.g. 5 Å thick) layer of polysilicon applied over the first photomask layer 20 at room temperature. To bimorph the photomask 18, the first photomask layer 20 can be applied to the control layer under compression whereas the second photomask layer 22 can be applied under tension, imparting a residual stress within the photomask 18 that causes it to curl and/or displace in a particular manner as the control layer 16 is being removed.
While the application of a second photomask layer 22 is specifically illustrated in
As can be further seen in
A second etchant 58 adapted to selectively etch the substrate 12 can also be delivered into the etch bath 40 via a second pipe 52. In contrast to the first etchant 54, the second etchant 58 may be a relatively slow rate-etchant configured to etch the substrate 12 at a slower rate than the first etchant 54. In certain embodiments, for example, a diluted solution of hydrofluoric acid (HF) can be utilized to etch the substrate 12 at a rate of approximately 1 to 400 times slower than the etch rate of the first etchant 54. A flow control valve 60 or other suitable flow control means can be used to adjust the flow of second etchant 58 into the etch bath 40.
Based on the relatively weak concentration of the second etchant 58 (e.g. hydrofluoric acid (HF)) contained within the etch bath 40, the etch rate within the control layer 16 is greater than the etch rate within the substrate 12. In certain embodiments, for example, the relatively fast-rate first etchant 54 can be configured to etch the control layer 16 at a rate of about 1 to 10 microns/min, whereas the relatively slow-rate second etchant 58 can be configured to etch the substrate 12 at a rate of about 0.01 to 1.0 microns/min. As shown in
The amount of etching occurring within the substrate 12 can also be made to depend on the characteristics of the photomask 18 used. When bimorph properties are imparted to the photomask layers 20,22, for example, the photomask 18 can be configured to curl upwardly away from the surface 14 of the substrate 12, allowing more etchant to become entrained within the gap 62. The existence of more etchant within the gap 64 tends to accelerate the vertical etch rate of the substrate 12 during the etch, in some cases forming a slope having a greater depth D.
As can be further seen in
A first curved region 70 can be formed in the substrate 12 between times t1 and t2 The first curved region 70 can be formed by varying relative concentrations and/or temperature of first and second etchants 54,58 contained within the etch bath 40. In certain embodiments, for example, the first curved region 68 can be formed by adding an initial amount of HNO3 and HF within the etch bath 40 (at time t=0), and then steadily increasing the amount of HF between times t1 and t2 to gradually increase the vertical etch rate within the substrate 12.
A second curved region 72 can also be formed in the substrate 12 between times t2 and t3. In contrast to the first curved region 70, the second curved region 72 can be formed, for example, by shutting-off the flow of HF into the etch bath 40 and gradually increasing the amount of HNO3 contained within the etch bath to gradually decrease the vertical etch rate within the substrate 12. As can be seen at time t2 in
The characteristics of the sloped etch 68 can further be altered by the selection of etchants used. In certain embodiments, for example, an anisotropic etchant exhibiting crystallinity dependence can be utilized to produce other desired profiles in a crystalline substrate such as silicon, if desired. Other factors such as the concentration of the etchant can also be exploited to create a desired slope in the substrate.
To create a custom sloped etch in the substrate 12, the temperature within the etch bath 76 can be varied at one or more times during the etching process to alter the respective etch rates of the substrate 12 and control layer 16. The steepness of the slope imparted to the substrate 12 will depend on the relative etch rates of the substrate 12 and control layer 16 at various temperatures. In certain embodiments, for example, the etch rate of the control layer 16 can be configured to increase at a greater rate at a particular temperature or temperature range (e.g. at 100° C.). In general, the greater the difference in relative etch rates between the two materials, the more gradual the slope that can be imparted to the substrate 12, all other factors being the same. Thus, by selectively increasing and/or decreasing the temperature within the etch bath 76, a desired sloped etch can be formed in the substrate 12.
In the illustrative embodiment of
In a first step depicted in
Alternatively, the curved surfaces 116 can be formed using single etchant by adjusting the temperature within the etch bath at various times during the etching process to increase and/or decrease the etch rate of the substrate 94 and/or control layer 98. In either case, the photomask layer 120 can be configured to bimorph away from the surface 96 of the substrate 94 during the etching process, if desired.
In the illustrative embodiment of
Having thus described the several embodiments of the present invention, those of skill in the art will readily appreciate that other embodiments may be made and used which fall within the scope of the claims attached hereto. Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size and arrangement of parts without exceeding the scope of the invention.
Claims
1. A method for creating a custom sloped etch in a substrate, comprising the steps of:
- providing a substrate having a surface;
- providing a control layer above the surface of the substrate,
- providing a photomask above the control layer, said photomask defining at least one opening exposing at least a portion of the control layer; and
- selectively etching the control layer and substrate surface to form a sloped etch within the substrate surface.
2. The method of claim 1, wherein said step of selectively etching the control layer and substrate surface comprises the steps of:
- applying a first etchant configured to selectively etch the control layer; and
- applying a second etchant configured to selectively etch the substrate.
3. The method of claim 2, wherein the etch rate of the first etchant is greater than the etch rate of the second etchant.
4. The method of claim 2, further comprising the step of adjusting the relative concentrations of the first and second etchants during said step of selectively etching the control layer and substrate surface.
5. The method of claim 2, wherein said first etchant is a fast-rate etchant solution of nitric acid.
6. The method of claim 2, wherein said second etchant is a slow-rate etchant solution of hydrofluoric acid.
7. The method of claim 1, wherein said step of selectively etching the control layer and substrate surface comprises the steps of:
- placing the substrate within an etch bath containing at least one etchant solution; and
- heating the substrate at one or more intervals to selectively adjust the relative etch rates of the substrate and the control layer.
8. The method of claim 1, wherein said substrate includes quartz.
9. The method of claim 1, wherein said control layer includes nickel.
10. The method of claim 1, wherein said step of forming a photomask above the control layer comprises the steps of:
- providing a first photomask layer above the substrate; and
- providing a second photomask layer on the first photomask layer.
11. The method of claim 10, wherein said first photomask layer includes a compressive layer of silicon nitride.
12. The method of claim 10, wherein said second photomask layer includes a tensile layer of polysilicon.
13. The method of claim 1, wherein said photomask is a bimorph photomask.
14. The method of claim 1, wherein said at least one opening comprises a longitudinal slit.
15. The method of claim 1, wherein said at least one opening comprises a rectangular slot.
16. The method of claim 1, wherein said at least one opening comprises a plurality of openings.
17. The method of claim 1, wherein said sloped etch is an S-shaped sloped etch.
18. The method of claim 1, wherein said sloped etch has a depth of between about 4 to 8 μm.
19. The method of claim 1, further comprising the step of removing the control layer and photomask after said step of selectively etching the control layer and substrate surface.
20. A method for creating a custom sloped etch in a substrate, comprising the steps of:
- providing a substrate having a surface;
- providing a control layer above the surface of the substrate,
- providing a photomask above the control layer, said photomask defining at least one opening exposing at least a portion of the control layer;
- applying a first etchant configured to selectively etch the control layer;
- applying a second etchant configured to selectively etch the substrate; and
- adjusting the relative concentrations of the first and second etchants to form a sloped etch within the substrate surface.
21. The method of claim 20, wherein the etch rate of the first etchant is greater than the etch rate of the second etchant.
22. The method of claim 20, wherein said first etchant is a fast-rate etchant solution of nitric acid.
23. The method of claim 20, wherein said second etchant is a slow-rate etchant solution of hydrofluoric acid.
24. The method of claim 20, wherein said substrate includes quartz.
25. The method of claim 20, wherein said control layer includes nickel.
26. The method of claim 20, wherein said step of applying a photomask above the control layer comprises the steps of:
- providing a first photomask layer above the substrate; and
- providing a second photomask layer on the first photomask layer.
27. The method of claim 26, wherein said first photomask layer includes a compressive layer of silicon nitride.
28. The method of claim 26, wherein said second photomask layer includes a tensile layer of polysilicon.
29. The method of claim 20, wherein said photomask is a bimorph photomask.
30. The method of claim 20, wherein said at least one opening comprises a longitudinal slit.
31. The method of claim 20, wherein said at least one opening comprises a rectangular slot.
32. The method of claim 20, wherein said at least one opening comprises a plurality of openings.
33. The method of claim 20, wherein said sloped etch is an S-shaped sloped etch.
34. The method of claim 20, wherein said sloped etch has a depth of between about 4 to 8 μm.
35. The method of claim 20, further comprising the step of removing the control layer and photomask after said step of selectively etching the control layer and substrate surface.
36. A method for creating a custom sloped etch in a substrate, comprising the steps of:
- providing a substrate having a surface;
- providing a nickel control layer above the surface of the substrate,
- providing a photomask above the control layer, said photomask defining at least one opening exposing at least a portion of the control layer;
- applying a fast-rate etchant solution of nitric acid configured to selectively etch the control layer;
- applying a slow-rate etchant solution of hydrofluoric acid configured to selectively etch the substrate; and
- adjusting the relative concentrations of the first and second etchant solutions to form a sloped etch in the substrate surface.
37. A method for creating a custom sloped etch in a substrate, comprising the steps of:
- providing a substrate having a surface;
- providing a control layer above the surface of the substrate,
- providing a bimorph photomask above the control layer, said photomask defining at least one opening exposing at least a portion of the control layer; and
- selectively etching the control layer and substrate surface to form a sloped etch in the substrate surface.
38. A system for etching a custom sloped etch in a substrate, the system comprising:
- a substrate having a sacrificial control layer and at least one mask layer, said at least one mask layer defining one or more openings exposing the control layer; and
- etching means for selectively etching the substrate and control layer to form a sloped etch in the substrate.
39. The system of claim 38, wherein said etching means includes a first etchant source configured to adjustably etch the control layer at a first etch rate, and a second etchant source configured to adjustably etch the substrate at a second etch rate.
41. The system of claim 40, wherein said first etch rate is greater than said second etch rate.
42. The system of claim 39, wherein said etching means includes an etch bath and at least one heater element.
43. The system of claim 42, further comprising a controller adapted to regulate the temperature of said at least one heater element.
44. The system of claim 42, further comprising a temperature sensor operatively coupled to said controller, said temperature sensor being adapted to monitor the temperature within said etch bath.
45. A system for etching a custom sloped etch in a substrate, the system comprising:
- a substrate having a sacrificial control layer and at least one mask layer, said at least one mask layer defining one or more openings exposing the control layer; and
- an etching apparatus configured to deliver at least one etchant solution into an etch bath containing the substrate, said at least one etchant solution being configured to selectively etch the substrate and control layer at an adjustable rate to form a sloped etch in the substrate.
46. A system for etching a custom sloped etch in a substrate, the system comprising:
- a substrate having a sacrificial control layer and at least one mask layer, said at least one mask layer defining one or more openings exposing the control layer; and
- an etching apparatus configured to deliver a first etchant solution configured to adjustably etch the control layer at a first etch rate, and a second etchant solution configured to adjustably etch the substrate at a second etch rate different than said first etch rate.
Type: Application
Filed: Dec 19, 2003
Publication Date: Jun 23, 2005
Inventors: Dan Youngner (Maple Grove, MN), James Detry (Plymouth, MN), John Starzynski (Brooklyn Park, MN)
Application Number: 10/739,521