Method of wafer-level fabrication of MEMS devices

• US 20110140216A1
• Filed: 12/14/2010
• Published: 06/16/2011
• Est. Priority Date: 12/16/2009
• Status: Active Grant

• Current Assignee: Oakland University
• Original Assignee: Oakland University
• Inventors: Qu, Hongwei
• Granted Patent: US 8,445,324 B2
• Time in Patent Office: Days
• US Class Current: 257/419
• CPC Class Codes: B81B 2201/0235 Accelerometers
• CPC Class Codes: B81B 2203/0136 Comb structures
• CPC Class Codes: B81B 2207/015 the micromechanical device and the control or processing electronics being integrated on the same substrate
• CPC Class Codes: B81C 1/00952 Treatments or methods for avoiding stiction during the manufacturing process not provided for in groups B81C1/00928 - B81C1/00944
• CPC Class Codes: B81C 2203/0714 Forming the micromechanical structure with a CMOS process
• CPC Class Codes: H01L 21/76229 Concurrent filling of a plurality of trenches having a different trench shape or dimension, e.g. rectangular and V-shaped trenches, wide and narrow trenches, shallow and deep trenches

Abstract

The present disclosure relates to a method of fabricating a micromachined CMOS-MEMS integrated device as well as the devices/apparatus resulting from the method. In the disclosed method, the anisotropic etching (e.g., DRIE) for isolation trench formation on a MEMS element is performed on the back side of a silicon wafer, thereby avoiding the trench sidewall contamination and the screen effect of the isolation beams in a plasma etching process. In an embodiment, a layered wafer including a substrate and a composite thin film thereon is subjected to at least one (optionally at least two) back side anisotropic etching step to form an isolation trench (and optionally a substrate membrane). The method overcomes drawbacks of other microfabrication processes, including isolation trench sidewall contamination.

Claims

1. A method of fabricating a micromachined device, the method comprising:

(a) providing a wafer comprising (i) a substrate having (A) a front side surface and (B) a back side surface opposing the front side surface, (ii) a composite thin film on the front side surface of the substrate, the composite thin film comprising one or more etchable portions and one or more etch-resistant portions, and (iii) an etch mask on the back side surface of the substrate, the etch mask defining one or more isolation portions substantially aligned with the etchable portions of the composite thin film;

(b) anisotropically etching the back side of the substrate via the isolation portions of the etch mask, thereby forming one or more first isolation trenches, wherein the first isolation trenches (i) are substantially aligned with the etchable portions of the composite thin film and (ii) extend partially into the substrate;

(c) removing the etch mask;

(d) anisotropically etching the back side of the substrate via the first isolation trenches, thereby extending the first isolation trenches to form one or more second isolation trenches, wherein the second isolation trenches (i) are substantially aligned with the etchable portions of the composite thin film and (ii) extend through the substrate to the composite thin film; and

(e) anisotropically etching the etchable portions of the composite thin film, thereby defining one or more final isolation trenches from the removed etchable portions of the composite thin film and the second isolation trenches.

2. The method of claim 1, wherein the etch mask comprises a patterned photoresist layer.

3. The method of claim 2, wherein in the etch mask further comprises a patterned insulating layer between the back side surface of the substrate and the patterned photoresist layer.

4. The method of claim 3, wherein:

(i) the patterned insulating layer defines a microstructure region on the back side surface of the substrate; and

(ii) step (d) further comprises anisotropically and isotropically etching the back side surface of the substrate in the microstructure region, thereby forming a substrate membrane in the microstructure region.

5. The method of claim 4, wherein:

(i) the substrate membrane has a membrane thickness ranging from about 10 μ

m to about 100 μ

m; and

(ii) the final isolation trenches have a width ranging from about 0.5 μ

m to about 5 μ

m and extend through the substrate membrane.

6. The method of claim 1, wherein:

(i) the etch mask further defines one or more separation portions;

(ii) step (b) further comprises anisotropically etching the back side surface of the substrate via the separation portions of the etch mask, thereby forming one or more first separation trenches, wherein the first separation trenches extend partially into the substrate; and

(iii) step (d) further comprises anisotropically etching the back side surface of the substrate via the first separation trenches, thereby extending the first separation trenches to form one or more second separation trenches, wherein the second separation trenches extend through the substrate to the composite thin film layer on the front side surface of the substrate.

7. The method of claim 1, wherein substrate sidewalls of the one or more final isolation trenches are substantially free of contaminants deposited from the composite thin film.

8. The method of claim 1, wherein substrate sidewalls of the one or more final isolation trenches are substantially free of contaminants comprising metal components of the composite thin film.

9. The method of claim 1, wherein:

(i) the composite thin film layer comprises a complementary metal-oxide-semiconductor (CMOS) circuitry layer stack;

(ii) the etchable portion comprises an insulating material on the front side surface of the substrate; and

(iii) the etch-resistant portion comprises an insulating material on the front side surface of the substrate and one or more metallic layers separated by the insulating material.

10. The method of claim 9, wherein:

(i) the insulating material comprises a material selected from the group consisting of an oxide and a nitride; and

(ii) each metallic layer independently comprises a metal selected from the group consisting of aluminum and copper.

11. The method of claim 3, wherein the insulating layer comprises a material selected from the group consisting of an oxide and a nitride.

12. The method of claim 1, wherein anisotropically etching the substrate comprises performing a deep reactive ion etching (DRIE) process.

13. The method of claim 1, wherein the final isolation trenches have an aspect ratio ranging from about 5:

1 to about 10;

1, the aspect ratio being defined as the height;

width ratio of the final isolation trenches.

14. The method of claim 1, wherein:

(i) the wafer is indexed to permit double-sided alignment of the wafer when etching from the front side of the substrate and from the back side of the substrate; and

(ii) the method further comprises flipping over the wafer after the second back side anisotropic etching of the substrate and before the front side etching of the composite thin film layer.

29. The method of claim 1 or claim 15, wherein:

(i) the wafer is indexed to permit double-sided alignment of the wafer when etching from the front side of the substrate and from the back side of the substrate; and

(ii) the method further comprises flipping over the wafer after the second back side anisotropic etching of the substrate and before the front side etching of the composite thin film layer.

30. A micromachined device formed according to the method of claim 1.

15. A method of fabricating a micromachined device, the method comprising:

(a) providing a wafer comprising (i) a substrate having (A) a front side surface, (B) a back side surface opposing the front side surface, and (C) a substrate thickness between the front side surface and the back side surface, and (ii) a composite thin film having (A) a front side surface and (B) a back side surface on the front side surface of the substrate, the composite thin film comprising one or more etchable portions and one or more etch-resistant portions;

(b) depositing an insulating layer over the back side surface of the substrate;

(c) patterning the insulating layer, thereby forming one or more isolation portions of the insulating layer, wherein the isolation portions of the insulating layer are substantially aligned with the etchable portions of the composite thin film;

(d) depositing a photoresist layer over the back side surface of the substrate and over the isolation portions of the insulating layer;

(e) patterning the photoresist layer, thereby defining one or more isolation portions of the photoresist layer, wherein the isolation portions of the photoresist layer are substantially aligned with the etchable portions of the composite thin film;

(f) etching the isolation portions of the insulating layer via the isolation portions of the photoresist layer, thereby exposing the back side surface of the substrate;

(g) anisotropically etching the back side of the substrate via the isolation portions of the photoresist layer, thereby forming one or more first isolation trenches, wherein the first isolation trenches (i) are substantially aligned with the etchable portions of the composite thin film and (ii) extend partially into the substrate;

(h) removing the photoresist layer;

(i) anisotropically etching the back side of the substrate via the first isolation trenches, thereby extending the first isolation trenches to form one or more corresponding second isolation trenches, wherein the second isolation trenches (i) are substantially aligned with the etchable portions of the composite thin film and (ii) extend through the substrate to, but not through, the composite thin film on the front side surface of the substrate; and

(j) anisotropically etching the etchable portions of the composite thin film from the front side surface of the composite thin film and through the composite thin film to the second isolation trenches, thereby defining one or more final isolation trenches from the removed etchable portions of the composite thin film and the second isolation trenches.

16. The method of claim 15, wherein:

(i) step (c) further comprises forming a microstructure region on the back side surface of the substrate, the microstructure region being defined by remaining portions of the patterned insulating layer and the isolation portions of the insulating layer;

(ii) step (i) further comprises anisotropically and isotropically etching the back side of the substrate in the microstructure region, thereby forming a substrate membrane in the microstructure region; and

(iii) the composite thin film comprises at least one structural isolation etchable portion that does not correspond to a formed second isolation trench in the microstructure region, step (j) further comprises anisotropically etching the structural isolation etchable portion to expose the underlying substrate, and the method further comprises (k) anisotropically etching the substrate from the front side through the etched structural isolation etchable portion to form a structural trench permitting movable device structures and device release.

17. The method of claim 16, wherein:

(i) the substrate membrane has a membrane thickness ranging from about 10 μ

m to about 100 μ

m; and

(ii) the final isolation trenches have a width ranging from about 5 μ

m to about 10 μ

m and extend through the substrate membrane.

18. The method of claim 15, wherein:

(i) step (e) further comprises defining one or more separation portions of the photoresist layer;

(ii) step (g) further comprises anisotropically etching the back side of the substrate via the separation portions of the photoresist layer, thereby forming one or more first separation trenches, wherein the first separation trenches extend partially into the substrate; and

(iii) step (i) further comprises anisotropically etching the back side of the substrate via the first separation trenches, thereby extending the first separation trenches to form one or more second separation trenches, wherein the second separation trenches extend through the substrate to, but not through, the composite thin film layer on the front side surface of the substrate.

19. The method of claim 15, wherein the substrate is a single-crystal substrate.

20. The method of claim 19, wherein the single crystal substrate comprises a material selected from the group consisting of single-crystal silicon, Si/Ge, and GaAs.

21. The method of claim 15, wherein substrate sidewalls of the one or more final isolation trenches are substantially free of contaminants deposited from the composite thin film.

22. The method of claim 15, wherein substrate sidewalls of the one or more final isolation trenches are substantially free of contaminants comprising metal components of the composite thin film.

23. The method of claim 15, wherein:

(i) the composite thin film layer comprises a complementary metal-oxide-semiconductor (CMOS) circuitry layer stack;

(ii) the etchable portion comprises an insulating material on the front side surface of the substrate; and

(iii) the etch-resistant portion comprises an insulating material on the front side surface of the substrate and one or more metallic layers separated by the insulating material.

24. The method of claim 21, wherein:

(i) the insulating material comprises a material selected from the group consisting of an oxide and a nitride; and

(ii) each metallic layer independently comprises a metal selected from the group consisting of aluminum and copper.

25. The method of claim 15, wherein the insulating layer comprises a material selected from the group consisting of an oxide and a nitride.

26. The method of claim 15, wherein etching the isolation portions of the insulating layer comprises performing a reactive ion etching (RIE) process.

27. The method of claim 15, wherein anisotropically etching the substrate comprises performing a deep reactive ion etching (DRIE) process.

28. The method of claim 15, wherein the final isolation trenches have an aspect ratio ranging from about 5:

1 to about 10;

1, the aspect ratio being defined as the height;

width ratio of the final isolation trenches.

31. A micromachined device formed according to the method of claim 15.