Abstract:
A method of fabricating electrical connections in an integrated MEMS device is disclosed. The method comprises forming a MEMS wafer. Forming a MEMS wafer includes forming one cavity in a first semiconductor layer, bonding the first semiconductor layer to a second semiconductor layer with a dielectric layer disposed between the first semiconductor layer and the second semiconductor layer, and etching at least one via through the second semiconductor layer and the dielectric layer and depositing a conductive material on the second semiconductor layer and filling the at least one via. Forming a MEMS wafer also includes patterning and etching the conductive material to form one standoff and depositing a germanium layer on the conductive material, patterning and etching the germanium layer, and patterning and etching the second semiconductor layer to define one MEMS structure. The method also includes bonding the MEMS wafer to a base substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     Under 35 U.S.C. 120, this application is a Continuation Application and claims priority to U.S. application Ser. No. 14/456,973, filed Aug. 11, 2014, entitled “INTERNAL ELECTRICAL CONTACT FOR ENCLOSED MEMS DEVICES,” which is a Continuation Application and claims priority to U.S. application Ser. No. 14/033,366, filed Sep. 20, 2013, entitled “INTERNAL ELECTRICAL CONTACT FOR ENCLOSED MEMS DEVICES” which is a Divisional Application and claims priority to U.S. patent application Ser. No. 13/754,462, filed on Jan. 30, 2013, entitled “INTERNAL ELECTRICAL CONTACT FOR ENCLOSED MEMS DEVICES,” all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to MEMS devices and more specifically to providing electric contact of the enclosure of the MEMS devices. 
     BACKGROUND 
     MEMS devices are utilized in a variety of environments. In such devices a handle layer is normally required to be electrically grounded to provide an electric shield for low noise performance. The electrical connection to the handle layer is provided by a wire bond. However, the wire bond requires vertical space and increases overall thickness of the MEMS device when packaged. Accordingly, what is desired is a MEMS device and method where the wire bond is not necessary. 
     The MEMS device and method for providing electrical connection to the handle layer should be simple, easily implemented and adaptable to existing environments. The present invention addresses such a need. 
     SUMMARY 
     A method of fabricating electrical connections in an integrated MEMS device is disclosed. The method comprises forming a MEMS wafer. Forming a MEMS wafer includes forming one cavity in a first semiconductor layer, bonding the first semiconductor layer to a second semiconductor layer with a dielectric layer disposed between the first semiconductor layer and the second semiconductor layer, and etching at least one via through the second semiconductor layer and the dielectric layer and depositing a conductive material on the second semiconductor layer and filling the at least one via. Forming a MEMS wafer also includes patterning and etching the conductive material to form one standoff and depositing a germanium layer on the conductive material, patterning and etching the germanium layer, and patterning and etching the second semiconductor layer to define one MEMS structure. The method also includes bonding the MEMS wafer to a base substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a diagram which illustrates a cross-section of the bonded MEMS-base substrate device with an internal direct electric coupling in accordance with a first embodiment. 
         FIGS. 2A-2E  are diagrams which illustrate a series of cross-sections illustrating processing steps to build the electric coupling from handle layer to MEMS device layer ready to bond to a base substrate for the device of  FIG. 1 . 
         FIG. 3  is a diagram which illustrates a cross-section of the bonded MEMS-base substrate device with an internal direct electric coupling in accordance with a second embodiment. 
         FIG. 4  is a diagram which illustrates a cross-section of the bonded MEMS-base substrate device with an internal direct electric coupling in accordance with a third embodiment. 
         FIGS. 5A-5G  are diagrams which illustrate a series of cross-sections illustrating processing steps to build the electric coupling from handle layer to MEMS device layer ready to bond to a base substrate for the device of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention relates generally to MEMS devices and more specifically to electric coupling for enclosed CMOS-MEMS devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
     In the described embodiments Micro-Electro-Mechanical Systems (MEMS) refers to a class of structures or devices fabricated using semiconductor-like processes and exhibiting mechanical characteristics such as the ability to move or deform. MEMS often, but not always, interact with electrical signals. MEMS devices include but are not limited to gyroscopes, accelerometers, magnetometers, pressure sensors, and radio-frequency components. Silicon wafers containing MEMS structures are referred to as MEMS wafers. 
     In the described embodiments, MEMS device may refer to a semiconductor device implemented as a micro-electro-mechanical system. MEMS structure may refer to any feature that may be part of a larger MEMS device. An engineered silicon-on-insulator (ESOI) wafer may refer to a SOI wafer with cavities beneath the silicon device layer or substrate. Handle wafer typically refers to a thicker substrate used as a carrier for the thinner silicon device substrate in a silicon-on-insulator wafer. Handle substrate and handle wafer can be interchanged. 
     In the described embodiments, a cavity may refer to an opening or recession in a substrate wafer and enclosure may refer to a fully enclosed space. 
     To describe the features of the invention in more detail, apparatus and fabrication methods to achieve a direct electric coupling of handle layer, device layer and base substrate of a MEMS device without a metal wire-bond are disclosed. 
       FIG. 1  is a diagram which illustrates a cross-section of the bonded MEMS-base substrate device with an internal direct electric coupling in accordance with a first embodiment. An engineered silicon-on-insulator (ESOI) substrate  120  includes a handle layer  101  with cavities  112  and a device layer  104 , fusion bonded together with a thin dielectric film  103  (such as silicon oxide) in between the device layer  104  and handle layer  101 . An electrical connection between the handle layer  101  and the device layer  104  may be achieved by etching one or more vias  106  through the device layer  104  and the thin dielectric layer  103  into the handle layer  101  and by filling the vias  106  with a conductive material  114 , such as polysilicon, tungsten, titanium, titanium nitride, aluminum, or germanium. The MEMS substrate is considered complete after a germanium (Ge)  109  and standoffs  105  comprising conductive material  114  are formed and MEMS actuator structures are patterned and etched in device layer  104 . Alternately, the standoff can be formed from both the conductive material  114  and a portion of the device layer  104  by partially etching into the device layer during standoff formation. In other embodiments the base substrate can comprise CMOS circuitry. 
     The MEMS to a base substrate integration may be provided by eutectic bonding of germanium  109  of the MEMS substrate with aluminum  107  of a base substrate  102 , where the AlGe bond provides the direct electrical coupling between MEMS substrate (handle  101  and device  104 ) and base substrate  102 . In addition, AlGe bond provides hermetic vacuum seal of the MEMS device. 
       FIGS. 2A-2E  are diagrams which illustrate a series of cross-sections illustrating processing steps to build the electric coupling from handle layer  101  to MEMS device layer  104  ready to bond to a base substrate  102  shown in  FIG. 1 . 
       FIG. 2A  is a diagram which illustrates the cross-section of an ESOI (engineered SOI) substrate with device layer  104  fusion-bonded to a handle silicon layer  101  with cavities  112 . In an embodiment, as shown in  FIG. 2B , vias  106  are patterned on device layer  104  of ESOI substrate and etched through device layer  104 , through thin dielectric layer  103 , and into handle layer  101 . In another embodiment, vias  106  are patterned on device layer  104  of ESOI substrate and etched through device layer  104  and through thin dielectric layer  103  to expose a portion of the surface of handle layer  101 . A conformal deposition of a conductive material  105  is then provided, as shown in  FIG. 2C , to fill via  106  to establish electrical coupling between device layer  104  and handle layer  101 . A germanium layer  109  is then deposited onto the conductive material  105 . The next step shown in  FIG. 2D  is to pattern and etch conductive material  105  and germanium layer  109  to form standoffs  121  from the conductive material  105 , followed by MEMS device layer  104  pattern and etch, as shown in  FIG. 2E  to complete the MEMS substrate processing, ready to bond to a base substrate. Alternately, the standoff  121  can be formed from both the conductive material  105  and a portion of the device layer  104  by partially etching into the device layer during standoff formation. 
       FIG. 3  is a diagram which illustrates a cross-section of the bonded MEMS-base substrate device with an internal direct electric coupling in accordance with a second embodiment. In this embodiment, the electric coupling path is formed from handle layer  201  to MEMS device layer  204 , across dielectric film  203 , and eventually to base substrate Al pad  207  after MEMS to base substrate AlGe eutectic bonding. 
     An ESOI substrate  220  is comprised of a handle layer  201  with cavities  212  and a device layer  204 , fusion bonded together with a thin dielectric layer  203  (such as silicon oxide) in between the device layer  204  and handle layer  201 . The ESOI substrate is completed after device layer thinning. An electrical connection between handle layer  201  and device layer  204  can be achieved by etching at least one via  206  at any locations through device layer  204  and thin dielectric layer  203  into or exposing the surface of handle layer  201  and filling the via  206  by conductive materials, such as polysilicon, tungsten, titanium, titanium nitride, aluminum or germanium. In this embodiment, the remaining conductive materials on device layer  204  could be removed by thinning, polishing or etching-back to expose device layer for standoff formation  205 . Steps of germanium deposition, standoff pattern, germanium etch, device layer  204  pattern, and etch, will be processed to complete the MEMS substrate. 
     The MEMS-base substrate integration is achieved by eutectic bonding of MEMS substrate with germanium pads  209  to base substrate with aluminum pads  207 , where the AlGe bonding provides direct electrical coupling between MEMS substrate (handle  201  and device  204 ) and base substrate  202 . In an embodiment, the standoff  205  forms a ring around the MEMS structure, the AlGe bond provides a hermetic seal for the MEMS structure. Via  206  can be positioned within or outside the seal ring formed by the standoff  205 . 
       FIG. 4  is a diagram illustrating a third embodiment of the electric coupling between handle layer  301 , MEMS device layer  304 , and base substrate  302  using polysilicon for the device layer  304  and AlGe eutectic bonding. The process flow and fabrication method of MEMS substrate using a surface micro-machining process technique are illustrated in  FIGS. 5A-5F .  FIGS. 5A-5F  are diagrams which illustrate a series of cross-sections illustrating processing steps to build the electric coupling from handle layer  301  to device layer  306  ready to bond to a base substrate  302  for the device of  FIG. 4 . Starting from  FIG. 5A , a thin dielectric layer  303  (typically silicon oxide) is deposited on a handle layer  301 . Thereafter the layer  303  is patterned and etched to form vias  312 . A silicon layer  306  ( FIG. 5B ) is deposited onto the handle layer  301  followed by thinning and planarization, (for example grinding or chemical mechanical polishing) to desired device layer thickness.  FIG. 5C  illustrates an embodiment with a second thicker silicon device layer. In this embodiment, an additional silicon wafer  311  can be bonded to the thin polysilicon  312  and thinned down to desired device thickness. The bonding of the additional silicon wafer  311  overcomes thickness limitations from conventional deposition techniques. 
     A Ge layer  309  is then deposited, as shown in  FIG. 5D .  FIG. 5E  is a diagram which illustrates standoff  305  formation by patterning and etching into device layer  306 .  FIG. 5F  is a diagram which illustrates patterning and etching silicon layer  306  to form MEMS structure  304 . The patterning and etching step is followed by etching the silicon oxide to release the device layer  304  as shown in  FIG. 5G . The MEMS substrate is now ready to be integrated with a base substrate. 
     As shown in  FIG. 4 , the MEMS-base substrate integration is achieved by eutectic bonding of MEMS substrate with germanium pads  309  to base substrate with aluminum pads  307 , where the AlGe bonding provides direct electrical coupling between MEMS substrate (handle  301  and device  305 ) and base substrate  302 . In addition, AlGe bonding provides hermetic vacuum seal of the MEMS device. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.