Microfabrication using germanium-based release masks

• US 6,440,766 B1
• Filed: 07/07/2000
• Issued: 08/27/2002
• Est. Priority Date: 02/16/2000
• Status: Expired due to Term

• Current Assignee: Analog Devices IMI, Inc. (Analog Devices, Inc.)
• Original Assignee: Analog Devices IMI, Inc. (Analog Devices, Inc.)
• Inventors: Clark, William A.
• Primary Examiner(s): Fourson, George
• Assistant Examiner(s): Pham, Thanh V
• Application Number:
  US09/612,563
• Time in Patent Office: 781 Days
• Field of Search: 438/52-53, 438/752, 438/756, 438/757, 438/750, 438/751, 438/FOR.127, 438/FOR.432, 148/DIG.159, 216/2, 216/11
• US Class Current: 438/52
• CPC Class Codes: B81C 1/00246 Monolithic integration, i.e. micromechanical structure and electronic processing unit are integrated on the same substrate
• CPC Class Codes: B81C 2201/053 Depositing a protective layers
• CPC Class Codes: B81C 2203/0735 Post-CMOS, i.e. forming the micromechanical structure after the CMOS circuit
• CPC Class Codes: Y10S 148/159 Strain gauges

Abstract

A method of fabricating MicroElectroMechanical systems. The method includes: providing a substrate in which electrical interconnections and a sacrificial layer have been formed, forming a release mask including germanium, etching exposed sacrificial material, and removing the release mask. The performance of MicroElectroMechanical devices is improved by 1) integrating electronics on the same substrate as the mechanical elements, 2) increasing the proximity of electronics and mechanical elements, 3) increasing the undercut of a release etch to reduce or eliminate etch holes or to allow circuit elements to be undercut, 4) increasing the yield and reliability of the MEMS release processes. In addition to released mechanical structures, the invention also provides a means for forming circuits such as a bandgap reference as a released MEMS element. Forming a bandgap circuit as a released MEMS element may improve reference voltage performance by allowing resistive heating of the circuit region to a constant, elevated temperature independent of the substrate temperature.

Claims

1. A method for forming released mechanical structures on the same substrate as protected regions, the method comprising the steps of:

(a) providing a substrate having protected regions, a first surface, and a sacrificial layer, said sacrificial layer including silicon dioxide;

(b) depositing a protective layer including germanium onto said first surface;

(c) photolithographically defining a release region in said protective layer where a mechanical structure will be released;

(d) removing portions of said protective layer including germanium at said release regions defined in said step (c), leaving remaining portions;

(e) releasing a microstructure by etching at least a portion of the sacrificial layer using an etchant including hydrogen fluoride; and

(f) removing remaining portions of said protective layer including germanium.

2. A method for forming released mechanical structures on the same substrate as interconnections, the method comprising the steps of:

(a) providing a substrate having interconnections, a first surface, and a sacrificial layer, said sacrificial layer including silicon dioxide;

(b) depositing a protective layer including germanium onto said first surface;

(c) defining one or more mechanical structures into said first surface, thereby exposing a portion of said sacrificial layer by

(c1) removing a portion of the protective layer overlying said first surface leaving remaining portions of said protective layer, and

(c2) etching a trench from said first surface to said sacrificial layer;

(d) releasing said mechanical structure from said sacrificial layer by etching at least a portion of said sacrificial layer using an etchant including hydrogen fluoride, thereby forming a structure-released substrate; and

(e) removing remaining portions of said protective layer including germanium.

3. The method of claim 2 wherein the substrate further includes integrated circuits having one or more transistors.

4. The method of claim 2 wherein said layer including germanium includes germanium and silicon in a molar ratio of germanium to silicon greater than 1:

3.

5. The method of claim 2 wherein said step (b) comprises the following steps:

(b1) depositing a first layer of silicon; and

(b2) depositing a first layer of germanium.

6. The method of claim 2 wherein said step (b) comprises the following steps:

(b1) depositing a first layer of silicon;

(b2) depositing a first layer of germanium;

(b3) depositing a second layer of silicon; and

(b4) depositing a second layer of germanium.

7. The method of claim 2 additionally comprising a step, following said step (a), of:

(a1) exposing said substrate to a siliconoxide etchant selected from one of the following list;

a plasma etch containing fluorine ions, a solution containing hydrogen fluoride.

8. The method of claim 2 wherein the thickness of said protective layer including germanium deposited in said step (b) is less than 2 microns.

9. The method of claim 2 wherein said step (e) comprises immersion of said structure-released substrate in a solution including hydrogen peroxide.

10. The method of claim 2 wherein said step (c) comprises the following steps:

(c1) photolithographically defining regions where portions of said substrate are to be removed;

(c2) etching completely though said layer including germanium in said photolithographically defined regions; and

(c3) etching from exposed regions of said first surface through the substrate until the sacrificial layer is reached, thereby defining a mechanical structure.

11. The method of claim 10 wherein said step (c2) and said step (c3) are performed in a single reactive ion etcher, wherein the reactive ion species is generated from a gas comprising atoms selected from one of the following list:

fluorine, chlorine, bromine.

12. The method of claim 2 wherein said step (c) comprises the following steps:

(c1) photolithographically defining a structure region where a mechanical structure will be formed;

(c2) etching said layer including germanium from said structure region wherein the etchant is selected from one of the following list;

a plasma etch containing fluorine, a solution containing hydrogen peroxide;

(c3) photolithographically defining regions where portions of said substrate are to be removed; and

(c4) etching from exposed regions of said first surface through the substrate until the sacrificial layer is reached, thereby defining a mechanical structure.

13. The method of claim 10 wherein said substrate comprises a silicon device layer and a handle substrate, wherein said silicon device layer is bonded to said handle substrate with said sacrificial layer, and said sacrificial layer comprises silicon dioxide.

14. The method of claim 12 wherein said substrate comprises a silicon device layer and a handle substrate, wherein said silicon device layer is bonded to said handle substrate with said sacrificial layer, and said sacrificial layer comprises silicon dioxide.

15. The method of claim 2 wherein said substrate further includes one or more active electrical devices selected from the following list:

MOS transistor, BJT transistor, JFET transistor.

16. The method of claim 2 wherein said step (d) comprises immersion of said substrate in a solution including greater than 2% hydrogen fluoride by weight.

17. The method of claim 2 wherein said step (d) comprises immersion of said substrate in a solution including hydrogen fluoride for a period of time sufficient to laterally etch at least 5 microns of said sacrificial layer.

22. The method of claim 2 wherein the thickness of said layer including germanium deposited in said step (b) is greater than 200 nm.

23. The method of claim 2 wherein said mechanical structures comprise silicon mechanical structures.

18. A method for forming released mechanical structures on the same substrate as interconnections, the method comprising the steps of:

(a) providing a substrate having a first surface, interconnections, a sacrificial layer, and one or more defined mechanical structures affixed to the sacrificial layer;

(b) depositing a mask layer including germanium onto said first surface;

(c) photolithographically defining a release region enclosing at least a portion of a defined mechanical structure;

(d) etching a portion of said mask layer including germanium from said release region to expose said first surface and leave remaining portions of said layer, wherein the etchant is selected from one of the following list;

a plasma etch containing fluorine, a solution containing hydrogen peroxide;

(e) releasing said mechanical structure from said sacrificial layer by etching at least a portion of said sacrificial layer using an etchant including hydrogen fluoride, thereby forming a structure-released substrate; and

(f) removing remaining portions of said layer including germanium.

19. The method of claim 18 wherein said defined mechanical structures are formed by surface micromachining.

20. The method of claim 18 wherein said defined mechanical structures are located within a recessed region below the surface of the substrate.

21. A method for forming released mechanical structures on the same substrate as interconnections, the method comprising the steps of:

(a) providing a substrate having interconnections, a first surface, and a sacrificial layer;

(b) defining one or more mechanical structures (by etching trenches from said first surface to said sacrificial layer) into said first surface, thereby exposing a portion of said sacrificial layer;

(c) selectively depositing a layer including germanium onto said substrate, wherein exposed portions of said sacrificial layer serve as a masking surface for said selective deposition;

(d) releasing said mechanical structure from said sacrificial layer by etching at least a portion of said sacrificial layer using an etchant including hydrogen fluoride, thereby forming a structure-released substrate;

(e) substantially removing remaining portions of said mask layer including germanium.