Method for etched cavity devices
View Patent ↗MEMS devices ( 40 ) using etched cavities ( 42 ) are desirably formed using multiple etching steps. Preliminary cavities ( 20 ) formed by locally anisotropic etching to nearly the final depth have irregular ( 46 ) sidewalls ( 44 ) and steep and/or inconsistent sidewall ( 44 ) to bottom ( 54 ) intersection angles ( 48 ). This leads to less than desired cavity diaphragm ( 26 ) burst strengths. Final cavities ( 42 ) with smooth sidewalls ( 50 ), smaller and consistent sidewall ( 50 ) to bottom ( 54 ) intersection angles ( 58 ), and having more than doubled cavity diaphragm ( 26 ) burst strengths are obtained by treating the preliminary cavities ( 20 ) with TMAH etchant, preferably relatively dilute TMAH etchant. In a preferred embodiment, a cleaning step is performed between the etching step and the TMAH treatment step to remove any anisotropic etching by-products present on the preliminary cavities' ( 20 ) initial sidewalls ( 44 ). The multi-step cavity etching procedure is especially useful for forming robust MEMS pressure sensors, but is applicable to any type of MEMS device.
1. A method for producing a MEMS device comprising:
for a substrate having first and second principal surfaces with a desired MEMS element location proximate the first surface, locally anisotropically etching in the substrate a preliminary cavity extending toward the first surface and underlying the desired MEMS element location; and
exposing the preliminary cavity to tetra-methyl-ammonium hydroxide (TMAH) etchant;
wherein the substrate comprises a body of semiconductor material, wherein the body of semiconductor material is locally anisotropically etched to produce the preliminary cavity that is subsequently exposed to the TMAH etchant, and wherein the preliminary cavity has preliminary sidewalls extending substantially through or entirely through the body of semiconductor material.
2. The method of claim 1 , further comprising prior to the etching step, providing a protective layer over at least the MEMS element location.
3. The method of claim 1 , further comprising prior to the etching step, providing a mask layer having an etch window therein on the second surface opposite the desired MEMS element location, and thereafter performing the etching step through the etch window.
4. The method of claim 1 , wherein the etching step comprises reactive ion etching.
5. The method of claim 1 , further comprising prior to the exposing step, cleaning the preliminary cavity of anisotropic cavity etching byproducts.
6. The method of claim 1 , wherein following the exposing step, a subsequent cavity is formed having a sidewall portion making an angle with a bottom of the subsequent cavity in the range of about 20 to less than 90 degrees.
7. The method of claim 1 , wherein the TMAH etchant comprises TMAH concentrations in the range of about 2 to 10 weight percent TMAH in water.
8. The method of claim 1 , wherein the substrate further comprises a composite layer overlying the body of semiconductor material, wherein exposing comprises contacting the preliminary cavity with the TMAH etchant to produce a final cavity extending through the body of semiconductor material and exposing a central portion of the composite layer, and wherein the exposed central portion of the composite layer acts as a pressure sensor diaphragm of the MEMS device.
9. The method of claim 8 , wherein the final cavity includes a sidewall transition portion forming an angle with the exposed central portion of the composite layer between about 50 and about 60 degrees.
10. A method for etching a cavity in a substrate including a pressure sensor diaphragm and a body of semiconductor material, the body of semiconductor material having a first principal surface over which the pressure sensor diaphragm is formed and having a second principal surface opposite the first principal surface, the method comprising:
anisotropically locally etching a preliminary cavity in the body of semiconductor material extending from the second principal surface toward the first principal surface and underlying the pressure sensor diaphragm; and
exposing the preliminary cavity to a tetra-methyl-ammonium hydroxide (TMAH) etchant thereby forming a final cavity having a smoother final sidewall, the final cavity extending from the first principal surface to the second principal surface to expose at least a portion of the pressure sensor diaphragm through the body of semiconductor material.
11. The method of claim 10 , wherein the smoother final sidewall has a portion making an angle with a surface of the pressure sensor diaphragm in the range of about 20 to less than 90 degrees.
12. The method of claim 10 , wherein the TMAH etchant comprises TMAH concentrations in the range of about 2 to 10 weight percent TMAH in water.
13. The method of claim 10 , wherein the etching step comprises reactive ion etching.
14. The method of claim 10 , further comprising prior to the exposing step, cleaning a preliminary sidewall of the preliminary cavity of anisotropic cavity etching byproducts.
15. The method of claim 14 , wherein the cleaning step comprises using a water-hydrofluoric acid mixture having a concentration ratio in the range of about H 2 O:HF=500:1 to H 2 O:HF=25:1.
16. A method for producing a pressure sensor device comprising:
for a substrate having a substantially silicon body, a first principal surface, a second principal surface spaced from the first principal surface, and a pressure sensor element location proximate the first principal surface, locally anisotropically etching a preliminary cavity extending at least partly through the silicon body extending from the second principal surface toward the first principal surface, wherein the preliminary cavity underlies the pressure sensor element location and has a preliminary sidewall; and
exposing the preliminary sidewall of the preliminary cavity to tetra-methyl-ammonium hydroxide (TMAH) etchant to provide a final cavity extending through the substantially silicon body to expose a portion of the substrate underlying the pressure sensor element location and acting as a pressure sensor diaphragm of the pressure sensor device.
17. The method of claim 16 , wherein the final cavity has a final sidewall having a portion making an angle with a bottom of the final cavity in the range of about 20 to less than 90 degrees.
18. The method of claim 16 , further comprising prior to the etching step, providing a mask layer on the first principal surface having an etch window therein opposite the pressure sensor element location, and thereafter etching the preliminary cavity through the etch window.
19. The method of claim 16 , wherein the substrate further comprises a dielectric layer between the body and the pressure sensor element location, and wherein the dielectric layer forms a bottom of the final cavity.
20. The method of claim 19 , wherein the substrate further comprises a further semiconductor region overlying the dielectric layer and wherein the pressure sensor element location includes a portion of the further semiconductor region.