Process for etching trenches in an integrated optical device
The described process allows trenches to be etched in a structure comprising a support substrate and a multilayer, formed on the substrate, for the definition of wave guides of an integrated optical device and comprises a selective plasma attack in the multilayer through a masking structure that leaves uncovered areas of the multilayer corresponding to the trenches to be etched. Such a masking structure is obtained by forming a mask of metallic material on the multilayer that leaves uncovered the areas corresponding to the trenches to be etched and forming a mask of non-metallic material, for example photoresist, on it that leaves uncovered regions comprising at least part of the areas and an edge portion of the mask of metallic material.
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This application claims priority from Italian patent application No. RM2004A000445, filed Sep. 17, 2005, which is incorporated herein by reference.
TECHNICAL FIELDThe present invention refers generally to processes for manufacturing integrated optical devices and, more specifically, to a process for etching trenches in an integrated optical device.
BACKGROUNDIntegrated optical devices are used, as known, in communications networks that use light signals for the transmission of information. The most common optical devices are modulators, attenuators and interferometers.
In a standard manufacturing process of an integrated optical device the subsequent step is the formation of further structures 6 of the integrated optical device. An example of these structures is shown in
The last step of the process is the formation of trenches in the multilayer consisting of the layers of oxide 3, 4 and 5.
A trench can have different functions: for example, it can make a physical insulation, both from the optical point of view and from the thermal point of view, between two optical paths that are adjacent or in any case very close to each other, or else it can thermally insulate one or more optical paths from heat sources present on the device itself or it can be used to make an optical switch. In this case the trench is filled with a medium the characteristics of which, in terms of refraction index and propagation constant, can be modified so as to be selectively equal to or less than those of the cores of the wave guides of the interrupted optical path. In this way it is possible to close or open the path of the light signal.
The trench that one wishes to make typically must cut the entire thickness of the multilayer of oxide that, as stated, comprises three layers: the lower and upper coating layers 3 and 5, both typically having a thickness of about 16 μm, and the core layer 4, on the other hand, having a thickness that can be between 4 μm and 8 μm. In total, therefore, the depth that typically must be reached with the etch is about 36-40 μm. Typically, for these depth values (a few tens of μm) a trench is considered deep; on the other hand, for depths of a few μm a trench is considered not very deep, i.e., shallow.
For the etching of a trench, currently techniques are used that foresee the deposition on the multilayer of a layer of a material resistant to the attack of the oxide, the definition on it of a mask 7, like the one shown in
As stated previously, the attack step of the multilayer through plasma is preceded by a process for the definition of a mask that leaves unprotected the areas in which the trenches are to be etched. The mask typically must keep its profile as unaltered as possible under the action of the plasma and typically must have high selectivity.
If the material constituting the masking layer is such that the profile of the mask during the attack step remains unaltered, the attack shall be substantially vertical and shall allow the desired resolution to be obtained. Here, resolution means the measurement of the fidelity of transfer of the image from the mask to the attacked material. The trenches shall thus have substantially vertical walls, i.e. with inclination of at least 88° with respect to the surface plane of the silicon substrate.
The selectivity is the ratio between the attack speeds on different materials: in this case it is the ratio between the attack speed of the oxide of the multilayer and the attack speed of the masking material. In common processes this ratio can be between 5:1 and 8:1. This means that the attack of the oxide is from 5 to 8 times quicker than the attack of the masking material. Ratios of this magnitude are typical of masks of low selectivity. For greater ratios (even of 10:1, 20:1) a mask is considered to have good selectivity; for ratios of 100:1 a mask can be considered to have infinite selectivity.
In standard processes the masks used are of photosensitive organic polymeric material, commonly known as photoresist. This type of material is not very resistant to the action of the attack through plasma, be it high or low density so that it reacts chemically with the plasma. In particular, during the attack step it deteriorates considerably at the edges that define the unprotected areas to be etched. For example, if in the structure of
As an alternative to organic masks, so-called hard masks can be used, for example masks made from metallic material. A metal mask 10, like the one represented in
In the case in which the area of the multilayer not protected by the mask of metallic material is a very small percentage of the area of the device (roughly less than 25%), a large amount of metallic material is exposed to the attack. It follows from this that numerous metallic particles are gradually removed from the mask and find themselves free in the plasma itself, for which reason they slow down the action of the plasma. This effect, known as “resputtering”, often does not allow the attack to be completed up to the desired depth. Moreover, a large amount of metal exposed directly to the plasma may become electrically charged during the attack, and this can cause further drawbacks from the electrical point of view. The trenches obtained, like the trench 11 shown in
An embodiment of the present invention is a process for etching deep trenches in the multilayer of oxide of an integrated optical device that allows the drawbacks outlined above to be avoided.
BRIEF DESCRIPTION OF THE DRAWINGSThe invention shall become clearer from the following detailed description of two embodiments thereof given as an example with reference to the attached drawings, in which the figures are section views of structures of integrated optical devices in different manufacturing stages. In particular:
A process according to an embodiment of the invention foresees, like in the prior art described with reference to
As shown in
At this point, as shown in
The next operation is an anisotropic attack preferably with high-density plasma. The “resputtering” effect shall be much less pronounced than that of the known process since no metallic part with the exception of the edge close to the area 13 is directly exposed to the attack. As can be seen in
The last two steps of the process, after which the structures represented in
A variant of the process according to an embodiment of the invention allows an asymmetrical trench to be etched into the multilayer, i.e. a trench that has, in the section view of the figures, on one side, a substantially vertical wall and on the opposite side a wall that is inclined, for example by 50°-60°, with respect to the plane of the multilayer. As can be seen in
As can be seen in
The last two steps of the process are the removal, with known techniques, of the mask of photoresist and of the metallic mask, respectively, and they allow the structure of
As can easily be seen, the described processes allow trenches with vertical walls to be etched in the oxide avoiding the drawbacks of the prior art. In particular, these processes can be implemented also proceeding to a low-density plasma attack, thus allowing the use of old-style machines having low costs and greater availability, as explained previously.
An integrated circuit (IC) may incorporate one or more of the structures of
Claims
1. Process for etching trenches in a structure comprising a support substrate and a multilayer, formed on the substrate, comprising a core layer and coating layers for the definition of wave guides of an integrated optical device, the process comprising a selective plasma attack in the multilayer through a masking structure that leaves uncovered areas of the multilayer corresponding to the trenches to be etched and being
- wherein the masking structure is obtained with the following successive operations:
- forming a mask of metallic material on the multilayer that leaves uncovered the areas corresponding to the trenches to be etched and
- forming a mask of non-metallic material that leaves uncovered regions comprising at least part of the areas corresponding to the trenches to be etched and an edge portion of the mask of metallic material.
2. Process for etching trenches according to claim 1, wherein the operation of forming the mask of non-metallic material leaves uncovered regions comprising the areas corresponding to the trenches to be etched and an edge portion of the mask of metallic material.
3. Process for etching trenches according to claim 1, wherein the mask of metallic material leaves uncovered further areas of the multilayer not corresponding to the trenches to be etched.
4. Process for etching trenches according to claim 1, wherein the operation of formation of the mask of non-metallic material is preceded by operations for the formation of other structures of the integrated optical device.
5. Process for etching trenches according to claim 1, wherein the non-metallic material comprises a photoresist.
6. Process for etching trenches according to claim 1, wherein the attack comprises a low-density plasma attack.
7. Process for etching trenches according to claim 1, wherein the attack comprises a high-density plasma attack.
8. A method, comprising:
- forming on a material a metallic first mask that exposes a region of the material;
- forming on the first mask a second mask that exposes a section of the region and a portion of the first mask that is contiguous with the section of the region; and
- forming a trench in the exposed section of the material.
9. The method of claim 8, further comprising:
- forming on a substrate a first optical layer having a first index of refraction;
- forming on the first layer a second optical layer having a second index of refraction; and
- forming on the second layer a third optical layer having a third index of refraction, the first, second, and third layers composing the material.
10. The method of claim 8, further comprising removing the first and second masks after forming the trench.
11. The method of claim 8 wherein forming the first mask comprises forming the first mask only within predetermined distance from a perimeter of the region.
12. The method of claim 8 wherein forming the second mask comprises forming the second mask such that the second mask exposes the entire region of the material and exposes a portion of the first mask that is contiguous with multiple sides of the region.
13. The method of claim 8 wherein forming the second mask comprises forming the second mask such that the second mask exposes a section that is smaller than the entire region of the material and exposes only a portion of the first mask that is contiguous with fewer than all sides of the section.
14. The method of claim 8 wherein forming the second mask comprises forming the second mask such that the second mask exposes a section that is smaller than the entire region of the material and exposes only a portion of the first mask that is contiguous with a single side of the section.
15. The method of claim 8 wherein forming the trench comprises anisotropically etching the exposed section of the material.
16. The method of claim 8 wherein forming the trench comprises plasma etching the exposed section of the material.
17. The method of claim 8 wherein forming the second mask comprises forming the second mask from a non-metallic material.
18. A structure, comprising:
- an optical path having a surface and through which an optical signal can propagate; and
- a trench disposed in the path, having a first side wall that is substantially perpendicular to the surface, and having a bottom that is substantially flat and substantially parallel to the surface.
19. The structure of claim 18 wherein the trench further comprises a second side wall that is substantially perpendicular to the surface of the optical path.
20. The processor of claim 18 wherein the trench further comprises a second side wall that forms an angle other than 90° with the surface of the optical path.
21. The structure of claim 18 wherein the trench further comprises a second side wall that is substantially perpendicular to the surface of the optical path and that is substantially parallel to the first side wall.
22. The structure of claim 18 wherein the optical path comprises:
- a first optical layer having a first index of refraction;
- a second optical layer disposed on the first optical layer and having a second index of refraction; and
- a third optical layer disposed on the second optical layer, having a third index of refraction, and having a surface that forms the surface of optical path.
23. The structure of claim 22 wherein the first index of refraction substantially equals the third index of refraction.
24. The structure of claim 22 wherein:
- the first index of refraction substantially equals the third index of refraction; and
- the second index of refraction is greater than the first and third indices of refraction.
25. The structure of claim 22 wherein the trench is disposed at an end of the optical path.
26. An integrated circuit, comprising:
- an optical path having a surface and through which an optical signal can propagate; and
- a trench disposed in the path, having a first side wall that is substantially perpendicular to the surface, and having a bottom that is substantially flat and substantially parallel to the surface.
27. An electronic system, comprising:
- an integrated circuit having, an optical path having a surface and through which an optical signal can propagate, and a trench disposed in the path, having a first side wall that is substantially perpendicular to the surface, and having a bottom that is substantially flat and substantially parallel to the surface.
Type: Application
Filed: Sep 19, 2005
Publication Date: Mar 30, 2006
Applicant:
Inventors: Pietro Montanini (Milano), Giovanna Germani (Bareggio (Milano)), Ilaria Gelmi (Albiate (Milano)), Marta Mottura (Milano)
Application Number: 11/230,292
International Classification: H01L 21/336 (20060101);