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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Provisional Application Serial No. 60/183,502 filed Feb. 16, 2000 entitled “MICROFABRICATION USING GERMANIUM-BASED RELEASE MASKS.” 
    
    
     IDENTIFICATION OF GOVERNMENT INTEREST 
     This invention was made with government support under contract number DAA01-99-C-R215 awarded by the Defense Advanced Research Projects Administration (DARPA). The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to microfabricated devices, and more particularly to devices fabricated using silicon oxide or similar material as a sacrificial layer. This invention improves release of mechanical devices formed on the same substrate as integrated circuitry using conventional microfabrication techniques. 
     2. Description of the Related Art 
     MicroElectroMechanical Systems (MEMS) combine mechanical structures and microelectronic circuits to create integrated devices. MEMS have many useful applications such as microsensors and microactuators. Examples of microsensors include inertial instruments such as accelerometers and gyroscopes, detectors for gasses such as carbon-monoxide, and pressure sensors. Examples of microactuators include optical mirrors used for video displays, optical switches, or multiplexors, and disk-drive head actuators used for increasing track density. 
     Many MEMS fabrication methods and techniques are known to those skilled in the art and are documented in publications such as Education in Microelectronics and MEMS by Payne, et al., the IEEE Journal of MicroElectroMechanical Systems, and numerous national and international conferences including: IEEE Solid-State Sensor and Actuator Workshop, IEEE International Conference on Solid-State Sensors and Actuators, and IEEE International Conference on Micro Electro Mechanical Systems. Many of the mechanical elements in MEMS devices are formed using layers of structural and sacrificial materials. A layer of sacrificial material typically supports structural materials during manufacture. The sacrificial layers are generally removed near the end of the manufacturing process to release the mechanical structures from surrounding materials. This is accomplished using a release etch which is typically isotropic and highly selective to the sacrificial material, leaving the structural material largely unaffected. Often a long release etch is required to undercut structural materials for a distance many times greater than the thickness of the sacrificial material. In many cases, etch holes are included in the structural material in an attempt to minimize the required undercutting and thereby shorten the release etch. Minimization of required undercutting can result in closely spaced etch holes that significantly constrain the mechanical design of a structure and reduce the performance of the MEMS device. 
     In many cases, MEMS sacrificial materials such as silicon oxides are also used in integrated circuits. When circuits are integrated with structures that require a release etch, the integrated circuits must be protected from the release etch. When silicon oxides are used as a sacrificial material, hydrofluoric acid based solutions or vapors may be used as the release etchant. Hydrofluoric based release etches include hydrogen fluoride vapor, hydrofluoric acid, buffered hydrofluoric acid, and various pad etchants. These release etchants can damage unprotected integrated circuitry, particularly when significant undercut is required. 
     Depositing a release mask may provide protection for integrated circuits and other sensitive elements. The release mask must not only be acid resistant but also impermeable. Since integrated circuits typically include aluminum interconnect, the release mask must also be deposited and removed at temperatures less than 450° C. using compatible materials. A variety of organic coatings have been employed as release masks but have proven inadequate for heavily undercutting release etches. Films deposited using chemical vapor deposition (CVD), low pressure CVD (LPCVD), plasma enhanced CVD (PECVD) have also been considered as a release mask. For example, amorphous silicon can be deposited over aluminum metallization using LPCVD or PECVD and can be an effective release mask, however, removal of amorphous silicon without damaging other materials can prove problematic. Other PECVD materials may also be deposited at lower temperatures but typically have poor conformality and are typically low-quality materials and therefore unsuitable as a release mask. 
     An effective release mask may improve performance, cost, manufacturability, and reliability of MEMS devices. A durable release mask may enable more undercutting during release etches thereby increasing the maximum etch-hole spacing, or even eliminating etch holes that constrain structure design. Large, more elaborate structures may also be designed, including MEMS systems comprising structures, electrical interconnection, and circuitry. The release etch may also be performed using a more aggressive etchant saving fabrication time and expense. Furthermore, the performance of devices such as sensors and actuators may benefit from combining mechanical structures with circuits integrated in the same substrate. Such MEMS devices are often improved as the distance diminishes between mechanical elements and the release-etch-sensitive elements. Increased proximity can reduce parasitic resistance and capacitance associated with electrical interconnect. Reduced parasitic resistance and capacitance yields higher electrical performance and hence a better performing MEMS device. Increased proximity of the different MEMS elements also lowers fabrication costs by increasing the number of batch fabricated components per substrate thereby reducing the cost per component. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a method of fabricating MEMS systems. The method includes: providing a substrate in which integrated circuits and a sacrificial layer have been formed, forming a release mask including germanium, etching exposed sacrificial material, and removing the release mask. The release mask provides protection for materials that may be adversely affected by the release etch. The mask may be removed without affecting other materials. This invention may potentially be used with a variety of MEMS processes, see for example: U.S. PPA Serial No. 60/127,973, Filed Apr. 6, 1999; U.S. patent application Ser. No. 09/322,381, filed May 28, 1999; Montague et al., U.S. Pat. No. 5,798,283 issued Aug. 25, 1998; Kung, et al., U.S. Pat. No. 5,504,026 issued Apr. 2, 1996; Sherman, et al., U.S. Pat. No. 5,847,280 issued Dec. 8, 1998; Tsang et al., U.S. Pat. No. 5,326,726, issued Jul. 5, 1994; Spangler et al., U.S. Pat. No. 5,343,064, issued Aug. 30, 1994; Bashir et al., U.S. Pat. No. 5,747,353, issued May 5, 1998; Zhang et al, U.S. Pat. No. 5,506,175 issued Apr. 9, 1996; Diem et al., U.S. Pat. No. 5,576,250, issued Nov. 19, 1996. 
     Implementations of the invention may include the following. Circuit elements may be formed in the substrate including active devices such as transistors. Electrical interconnection may be formed among circuit elements and structural elements. Electrical circuits and electrical interconnection may involve materials that may be adversely affected by a release etch. The structural elements may comprise materials that are not adversely affected by a release etch. The circuit and/or structural elements may be supported by or connected to sacrificial materials that may be removed by a release etch. A temporary release mask may be deposited and defined to allow a release etch to remove sacrificial materials where desirable while leaving other materials undisturbed. The temporary release mask may be removed using an etch process that will not damage materials used in the MEMS device including electrical circuits, interconnection, and structural elements. Sacrificial materials may include silicon oxides either grown on silicon or deposited on the substrate using CVD, LPCVD, or PECVD. Structural materials may include, for example, single crystal silicon, polycrystalline silicon (polysilicon), or silicon nitride deposited using CVD, LPCVD, or PECVD. The release etch may be performed using any of many known silicon-oxide etching chemistries including hydrofluoric acid (HF) or buffered hydrofluoric acid (BHF or BOE). The temporary release mask may be an amorphous or a polycrystalline germanium/silicon mixture deposited using CVD, LPCVD, or PECVD. When the release mask is largely germanium, the temporary release mask removal may be accomplished with an oxidizing etch including water, a hydrogen peroxide solution, or an oxygen plasma. 
     Advantages of the invention include the following. Performance of devices fabricated in accordance with the invention are improved due to the proximity of interface circuitry built into the same substrate as the microstructures. Proximity of circuitry and microstructures also reduces the cost of manufacture. Etch hole spacing can be increased, providing added flexibility of mechanical structure design. When etch holes can be fully eliminated, depending on the particular MEMS technology, mechanical devices may be suspended from a limited number of locations thereby reducing the effect of substrate stress from, for example, forces applied to a package. 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. Addition of a robust temporary release mask allows for greater undercut, more aggressive release-etch chemistries, more robust manufacturing methods, and higher yields. Furthermore, the invention is compatible with existing microfabrication techniques and is compatible with established integrated circuit fabrication processes. Additional benefits are derived from the use of germanium as the temporary release mask. Germanium can be impermeable to and is not attacked by many release etchants. Germanium may be deposited conformally at temperatures low enough to ensure that circuit components such as transistors and metallization are largely unaffected. Conformal deposition ensures the release mask is effective over topology on which the mask is deposited. Germanium is also easily removed without causing damage to materials commonly found in semiconductor processing making germanium an ideal release mask for integrated MEMS. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a substrate containing a region in which a MEMS device will be formed and a region containing electronics. 
     FIG. 2 is a cross-sectional view of a substrate after the deposition of a temporary release mask. 
     FIG. 3 is a cross-sectional view of a substrate after patterning of a temporary release mask. 
     FIG. 4 is a cross-sectional view of a substrate after a structural etch has been performed in the MEMS region exposed through the temporary release mask. 
     FIG. 5 is a cross-sectional view after the completion of the release etch that removed the sacrificial material underlying the MEMS device. 
     FIG. 6 is a cross-sectional view of the MEMS device after the removal of the temporary release mask. 
     FIG. 7 is a plot which relates the etch rate of germanium/silicon in an oxidizing solution as a function of the mole fraction of germanium. 
     FIG. 8 is a cross-sectional view illustrating a release mask patterned using the MEMS structural etch. 
     FIG. 9 is a cross-sectional view of a substrate containing a region of electronics and an unreleased MEMS device. 
     FIG. 10 is a cross-sectional view of a substrate containing a MEMS device and electronics coated with a temporary release mask material. 
     FIG. 11 is a cross-sectional view of a temporary release mask protecting a region of a substrate and exposing a MEMS device for a release etch. 
     FIG. 12 is a cross-sectional view of a released MEMS structure adjacent to a region protected by a temporary release mask. 
     FIG. 13 is a cross-sectional view of a released MEMS structure adjacent to a region of electronics. 
     FIG. 14 is a cross-sectional view of a substrate containing a region in which a MEMS device will be formed and a region containing electronics. 
     FIG. 15 is a cross-sectional view of a substrate with electronics coated with a material to be used as a nucleation layer for subsequent material deposition. 
     FIG. 16 is a cross-sectional view of a substrate in which a structural etch that, in part, defines a MEMS device has been performed. 
     FIG. 17 is a cross-sectional view of a substrate after the selective deposition of a temporary release mask. 
     FIG. 18 is a cross-sectional view of a substrate after completion of a release etch. 
     FIG. 19 is a cross-sectional view of a substrate after the removal of a temporary release mask. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Fabrication of devices in accordance with the present invention comprises three basic steps: deposition of the temporary release mask, a release etch to remove exposed sacrificial material, and removal of the temporary release mask. The present invention makes use of three regions: the protected region that may include electronics and/or electrical interconnect, the exposed region that may include the MEMS structural elements and sacrificial material, and the release seal that separates the exposed and protected regions. 
     Referring to FIG. 1, a first embodiment of the present invention begins with starting material comprising a substrate  100 . The substrate  100  may comprise a substantially conventional bonded wafer structure having a number of layers including: handle layer  103 , sacrificial material  102 , device layer  101 , dielectric isolation trenches  104  formed through device layer  101 , and a protected region  105  that may include electronics and/or electrical interconnect. While different materials may be used, handle layer  103  may be single crystal silicon; sacrificial layer  102  may include silicon dioxide; and the device layer  101  may be single crystal silicon. The protected region  105  may comprise a variety of materials that may include: single crystal silicon, polycrystalline silicon, silicon dioxide, aluminum, copper, tungsten, silicon nitride, spin-on-glass, or silicon carbide. Several methods of forming substrate  100  are described in prior art (refer to U.S. patent application Ser. No. 09/322,381 filed May 28, 1999 and “An SOI Based, Fully Integrated Fabrication Process for High-Aspect-Ratio Microelectromechanical Systems,” Doctoral Thesis, Brosnihan, U.C. Berkeley, Fall 1998). 
     As illustrated in FIG. 2, temporary release mask  110  is deposited on substrate  100 . Temporary release mask  110  includes germanium, and may include silicon, deposited using CVD, LPCVD, PECVD or sputter processes. Removal of native oxide prior to deposition will improve the deposition and the efficacy of the temporary release mask  110 . Native oxide may be removed with a brief oxide etch using many known chemistries including solutions containing hydrofluoric acid or a fluorine based plasma. LPCVD deposition of silicon/germanium films may be accomplished by alternating layers of silicon and germanium. Amorphous silicon may be deposited using an LPCVD furnace held at 350° C. and 300 mTorr while flowing 200 sccm of disilane for 20 minutes resulting in a film thickness of 5 nm to 10 nm. Deposition of a thin amorphous silicon layer typically precedes germanium deposition for improved mask quality. 
     Polycrystalline germanium may be deposited in an LPCVD furnace held at 350° C. and 600 mTorr while flowing 88 sccm of germane, for 30 minutes resulting in a film thickness of approximately 200 nm. Additionaly, silane and/or disilane may be added to the germane to increase the silicon content of the silicon/germanium film. Dopant gasses such as phosphene or diborane may also be used in the deposition. The use of alternating layers of silicon and silicon/germanium or germanium may be used to reduce the formation of pinholes that can violate the integrity of release mask  110 . Provided that the germanium and silicon depositions interdiffuse resulting in a film with high germanium concentration, the temporary release mask may be removed using an oxidizing solution as evidenced by FIG. 7, a plot of germanium etch rates in a hydrogen peroxide solution taken from “Post-CMOS Integration of Germanium Microstructures” by A. E. Franke, et al., published in the 12 th  IEEE International Conference on MEMS. Germanium-rich silicon/germanium may be etched using many known oxidizing chemistries including hydrogen peroxide solution heated to 90° C. or an oxygen plasma. 
     Referring to FIG. 3, definition of temporary release mask  110  is accomplished by removing material from regions  106  that will be exposed to a release etch. Removal of material from regions  106  may be accomplished using lithographic techniques coupled with a wet etch process or plasma etch process. Temporary release mask  110  is defined to cover protected region  105 , cover release seal region  107 , and expose region  106 . Release seal  107  may include regions where the temporary release mask  110  makes intimate contact with materials that are not adversely affected by the release etch such as device layer  101  and isolation trenches  104 . 
     In the present embodiment of the invention, definition of the MEMS structure is performed in exposed region  106  as shown in FIG.  4 . Once this structural etch is complete, underlying sacrificial silicon oxide  102  is exposed. A release etch may now be performed removing regions of sacrificial silicon oxide  102  as illustrated in FIG.  5 . The release etchant may be one of many known chemistries including hydrofluoric acid, hydrogen fluoride vapor, or buffered oxide etchant. Following the release etch, temporary release mask  110  is removed using an oxidizing etch leaving behind the MEMS device shown in FIG.  6 . These etches result in MEMS elements that are released from handle layer  103 . Fully and partially released MEMS elements may include structural elements  108  and protected regions  105 . When the released region includes protected circuits, improved circuits such as a thermally controlled bandgap voltage reference may be realized (refer to Reay, et al., “A micromachined low-power temperature-regulated bandgap voltage reference,” IEEE Journal of Solid-State Circuits, December 1995) 
     In an alternate embodiment of the invention, a separate step for defining temporary release mask  110 , illustrated in FIG. 3, is eliminated. Referring to FIG. 8, the MEMS structural etch also defines exposed regions  106   a  and  106   b.  When the temporary release mask  110   a  is largely comprised of germanium and the device layer  101   a  comprises silicon, the same plasma or reactive ion etch used to etch the device layer  101  a may often be used to etch the release mask  110   a  owing to the chemical similarity between materials. Alternatively, a separate etch process may be employed to remove temporary release mask  110   a  in the exposed regions  106   a  and  106   b  prior to the structure etch. 
     In another embodiment of the invention, starting material may comprise a substrate  200 , a protected region  205 , and a region in which MEMS devices are formed  206  as illustrated in FIG.  9 . While different materials may be used, the substrate may comprise single crystal silicon, polycrystalline silicon, or epitaxial single crystal silicon. Protected region  205  may comprise a variety of materials including: single crystal silicon, polycrystalline silicon, silicon oxide, aluminum, copper, tungsten, silicon nitride, spin-on-glass, or silicon carbide. The MEMS device area may contain sacrificial material  202  such as silicon oxide. Additionally the MEMS device area may include structural materials  203  such as single crystal silicon, polycrystalline silicon, amorphous silicon, silicon nitride, silicon carbide, and high silicon content silicon/germanium. Examples of similar starting materials are described in prior art (refer to Tsang et al., U.S. Pat. No. 5,326,726 issued Jul. 5, 1994; Montague et al., U.S. Pat. No. 5,798,283 issued Aug. 25, 1998; Kung, et al., U.S. Pat. No. 5,504,026 issued Apr. 2, 1996; and Sherman, et al., U.S. Pat. No. 5,847,280 issued Dec. 8, 1999). Referring to FIG. 10, a brief oxide etch to remove residual or native oxide may be performed before depositing temporary release mask  210  on substrate  200 . Temporary release mask  210  includes germanium and may include silicon. To preserve the integrity of release seal  207 , there is preferably no silicon oxide exposed at the edges of temporary release mask  210  in the region of the release seal  207 . Referring to FIG. 11, temporary release mask  210  is defined to expose MEMS region  206  while remaining over protected region  205  and release seal  207 . As illustrated in FIG. 12, a release etch may be used to remove sacrificial silicon oxide  202  thereby releasing structural elements  203  from the substrate. Temporary release mask  210  may now be removed as shown in FIG.  13 . 
     Yet another embodiment of the invention is illustrated in FIG.  14  through FIG.  19 . Starting material may comprise substrate  300  shown in FIG. 14 that may be exposed to a brief oxide etch to ensure that no undesired silicon oxide remains exposed. Substrate  300  may be coated with nucleation layer  309  that may include amorphous silicon or polycrystalline silicon/germanium as illustrated in FIG.  15 . Alternatively, if portions of the substrate to be protected by deposited germanium comprise a surface layer upon which germanium may be directly deposited, formation of nucleation layer  309  may be skipped. Referring to FIG. 16, a MEMS structure definition exposes sacrificial material  302  through trenches  306   a  and  306   b.  A brief oxide etch may be performed to ensure that no silicon oxide remains on non-sacrificial surfaces. As shown in FIG. 17, germanium-rich, temporary release mask  310  is selectively deposited. Because temporary release mask  310  comprises germanium-rich polycrystalline silicon/germanium or poly-germanium, deposition is selective and temporary release mask  310  does not nucleate and grow on the sacrificial silicon oxide (see: Tsu-Jae King, Saraswat, K. C., “Deposition and properties of low-pressure chemical-vapor deposited polycrystalline silicon-germanium films,”  Journal of the Electrochemical Society,  August 1994, pp. 2235-41.) When nucleation layer  309  and temporary release mask  310  materials are largely comprised of silicon and germanium and when the germanium content is sufficiently high and interdiffusion occurs, nucleation layer  309  and temporary release mask  310  become a layer of silicon/germanium that is sufficiently germanium-rich for removal by an oxidizing etch. As shown in FIG. 18, a release etch may be performed to remove sacrificial material  302  that is exposed through regions  306   a  and  306   b.  Temporary release mask  310  is then removed as illustrated in FIG.  19 . When temporary release mask  310  is silicon/germanium, the material may be removed with an oxidizing etch. 
     The foregoing description, for the purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for the purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. For instance, different gasses, times, and temperatures may be used to deposit silicon or germanium films. Additional cleaning steps may be added before or after deposition of the thin films, or between thin-film layer depositions. The starting material may comprise various exposed materials to be protected, including exposed metal bondpads comprising aluminum, aluminum/silicon, or aluminum with a thin barrier layer of a material such as titanium nitride, or platinum. Furthermore, the invention can be used in conjunction with a variety of MEMS processes for improved yield, manufacturability, and performance, for example: U.S. PPA Serial No. 60/127,973, Filed Apr. 6, 1999; U.S. patent application Ser. No. 09/322,381, filed May 28, 1999; Montague et al., U.S. Pat. No. 5,798,283 issued Aug. 25, 1998; Kung, et al., U.S. Pat. No. 5,504,026 issued Apr. 2, 1996; Sherman, et al., U.S. Pat. No. 5,847,280 issued Dec. 8, 1998; Tsang et al., U.S. Pat. No. 5,326,726, issued Jul. 5, 1994; Spangler et al., U.S. Pat. No. 5,343,064, issued Aug. 30, 1994; Bashir et al., U.S. Pat. No. 5,747,353, issued May 5, 1998; Zhang et al, U.S. Pat. No. 5,506,175 issued Apr. 9, 1996; Diem et al., U.S. Pat. No. 5,576,250, issued Nov. 19, 1996.