Abstract:
A hermetically sealed microelectromechanical system (MEMS) package includes a MEMS switch having a movable portion and a stationary portion with an electrical contact thereon. A glass lid is anodically bonded to the MEMS switch to form a sealed cavity over the movable portion of the MEMS switch. The glass lid includes a contact aperture to allow access to the electrical contact on the stationary portion of the MEMS switch. A family of body-implantable hermetically-sealed MEMS packages are provided according to certain aspects and embodiments of the present invention.

Description:
RELATED APPLICATION 
       [0001]    This application is a divisional of U.S. patent application Ser. No. 11/066,820, filed Feb. 25, 2010 entitled “WAFER LEVEL HERMETICALLY SEALED MEMS DEVICE”, herein incorporated by reference in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to hermetic sealing of an implantable medical device, and particularly to a wafer level hermetic seal for an implantable medical device that employs a microelectromechanical system (MEMS). 
         [0003]    Implantable medical devices (IMDs) employ internal electronic circuitry that is hermetically sealed in a biostable package. This internal electronic circuitry is typically implemented as one or more integrated circuits (ICs). More recently, the prospect of implementing switching circuitry as a MEMS system has been considered, as discussed in U.S. application Ser. No. 10/973,117 filed Oct. 26, 2004 for “MEMS Switching Circuit and Method for an Implantable Medical Device” by R. Receveur et al., which is hereby incorporated by reference. However, a MEMS is more difficult to employ in an implantable medical device than a system that employs only ICs, at least in part because of the more stringent packaging requirements to provide a particular environment around the MEMS. In addition, a MEMS package may need to redistribute electrical signals, provide mechanical support, handle power and provide thermal management functions. These considerations present challenges in designing an effective and efficiently manufacturable sealed MEMS package for an IMD. 
         [0004]    Accordingly, there is a need in the art for an improved packaging solution for MEMS devices employed in implantable medical devices. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a hermetically sealed MEMS package and a method of forming that package. The MEMS package includes a MEMS switch having a movable portion and a stationary portion with an electrical contact thereon. A glass lid is anodically bonded to the MEMS switch to form a sealed cavity over the movable portion of the MEMS switch. The glass lid includes a contact aperture to allow access to the electrical contact on the stationary portion of the MEMS switch. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a cross-sectional view of a MEMS package according to an embodiment of the present invention. 
           [0007]      FIG. 2  is a cross-sectional view of an intermediate fabrication condition of the MEMS package of  FIG. 1 , shown after a temporary shadow mask has been removed for selective deposition of contact metallization and switch metallization layers. 
           [0008]      FIG. 3  is a perspective view of a hermetically sealed MEMS package according to an embodiment of the present invention. 
           [0009]      FIG. 4A-C  depict elevational side views in cross-section of the packages depicted in  FIG. 1  and  FIG. 2  and including 15 annotated processing steps to be performed according to the present invention. 
           [0010]      FIG. 5A-C  are flowcharts that include processing steps to be performed according to the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]      FIG. 1  is a cross-sectional view of MEMS packaging  10  according to an exemplary embodiment of the present invention. MEMS packaging  10  includes silicon-on-insulator (SOI) carrier layer  12 , silicon dioxide features  14 , SOI device layer  16  patterned as a MEMS switch (also referred to herein as MEMS switch  16 ) and having movable parts  18  and stationary part  20 , MEMS switch metallization layers  22  and  23 , contact metallization  24 , and glass lid  26  having contact aperture  28  for allowing electrical connection to contact metallization  24 . Glass lid  26  forms sealed interior cavity  30  around movable parts  18  of MEMS switch  16 . 
         [0012]    MEMS switch  16  is illustrated schematically in  FIG. 1  to include movable parts  18  and stationary part  20 . Generally speaking, MEMS switch  16  is closed and opened by switch metallization layers  22  and  23  being forced into and out of contact with one another by electro-mechanical operation of movable parts  18  of MEMS switch  16 . Contact metallization  24  is provided on a top surface of stationary part  20  to receive electrical control signals for operating MEMS switch  16 , and also to receive the electrical signals that are to be transmitted or blocked by MEMS switch  16 , such as a pulse generated by an implantable pulse generator in one particular embodiment. For example, MEMS switch  16  may be implemented as an electrostatic comb actuator, having a movable actuator member and fixed actuator members that are attracted to one another by application of an actuation voltage to contact metallization  24 , resulting in movement of movable parts  18  to selectively open and close MEMS switch  16 . A more thorough description of the configuration and operation of a MEMS switch that would be suitable for implementation as MEMS switch  16  is found in U.S. application Ser. No. 10/973,117 filed Oct. 26, 2004 for “MEMS Switching Circuit and Method for an Implantable Medical Device” by R. Receveur et al., which has been incorporated herein by reference. 
         [0013]    MEMS switch  16  is housed in cavity  30  that includes a controlled, preserved interior atmosphere where movable parts  18  are free to move. This atmosphere is provided by glass lid  26 , which forms hermetically sealed cavity  30  around MEMS switch  16 . Glass lid  26  has standoff portions  32  around movable parts  18  of MEMS switch  16  and a recessed area above movable parts  18  of MEMS switch  16 , so that free movement within the resulting cavity  30  inside of glass lid  26  is possible. In addition, glass lid  26  is formed with contact aperture  28 , which allows electrical contact-to-contact metallization  24  on stationary part  20  of MEMS switch  16 . 
         [0014]    In one embodiment, glass lid  26  is secured to the silicon structure of MEMS switch  16  by anodic (i.e., electrostatic) bonding. A covalent bond is formed between the surface atoms of glass lid  26  and the silicon of MEMS switch  16  when both structures are pressed together under application of a voltage at an elevated temperature, creating a hermetic seal between the structures. In a typical embodiment, glass lid  26  is composed of a material that has a thermal coefficient of expansion (TCE) similar to that of silicon, such as Pyrex or a similar material. The silicon material of MEMS switch  16  may be composed of a fine grain polysilicon layer, a silicon-on-insulator (SOI) layer, or another semiconductive material known in the art. 
         [0015]    An example of an anodic bonding process for securing glass lid  26  to the silicon material of MEMS switch  16  is performed at a temperature in the range of 300-450° C. at a voltage in the range of 200-2000 Volts. The bond width is typically at least 250 micro-meters (μm), and the processing time is typically about 10-20 minutes. A bond strength of greater than 20 mega-Pascals (Mpa) can be achieved by anodically bonding glass lid  26  to the silicon material of MEMS switch  16 , which provides an excellent hermetic seal suitable for use in an implantable medical device. 
         [0016]    An example of a process of fabricating MEMS packaging  10  will now be described. MEMS switch  16  is fabricated on SOI layer  12 , supported by silicon dioxide features  14 , in a manner generally known in the art. After MEMS switch  16  has been formed, a shadow mask technique is used to selectively deposit contact metallization layer  24  and switch metallization layers  22  and  23  while keeping the areas where glass lid  26  is to contact MEMS switch  16  clean, for subsequent anodic bonding. To form the shadow mask, a silicon wafer is etched (such as by potassium hydroxide (KOH) etching) with through holes for formation of metallization regions. The silicon wafer is temporarily attached to portions of the SOI layer forming MEMS switch  16 , such as by wax, and is aligned so that the through holes are located in the areas where contact metallization layer  24  and switch metallization layers  22  and  23  are to be formed. Next, a metal deposition step is performed, with a metallization material such as Titanium-Ruthenium (where Titanium is the adhesion layer for the Ruthenium) being deposited through the through holes of the silicon wafer to form contact metallization layer  24  and switch metallization layers  22  and  23 . Finally, the silicon wafer that forms the shadow mask is detached from MEMS switch  16 , such as by heating the wax that attaches the two structures, leaving contact metallization layer  24  and switch metallization layers  22  and  23  formed on MEMS switch  16 . This stage of the process is illustrated in  FIG. 2 . which is a cross-sectional view of MEMS packaging  10  after the metallization layers have been deposited and the temporary shadow mask has been removed. 
         [0017]    Next, glass lid  26  is pre-etched and aligned in the proper position with respect to MEMS switch  16 , for anodic bonding. In an example of the process, glass lid  26  is pressed toward MEMS switch  16  with 250 kilo-grams (kg) of pressure, at a temperature of 365° C. and with application of a voltage of 230 V. In consideration of these parameters, MEMS switch  16  is designed to have an actuation voltage (that is, the voltage applied in order to close the switch) that is not significantly lower than the anodic bonding voltage (which is 230 V in this example). For example, the actuation voltage may be within 100 V of the anodic bonding voltage. One way to achieve this relatively high actuation voltage is to design the number of comb fingers and the size of the springs of MEMS switch  16  in such a manner that MEMS switch  16  has a stiffness that requires a relatively high voltage (such as about 150 V, for example) to actuate. This design is different than many typical existing MEMS switch designs, which are actuated by a voltage in the range of 10-20 V. In addition, MEMS switch  16  (and specifically contact metallization layer  24  and switch metallization layer  22  and  23 ) is designed so that the anodic bonding temperature (in this example, 365° C.) does not have a negative effect on the metallization. For example, Titanium-Ruthenium metallization layers may be used, as this material remains a good electrical contact material with low electrical resistance at high temperatures such as a 365° C. anodic bonding temperature. 
         [0018]    In a typical process, a plurality of MEMS switches  16  and glass lids  26  are formed simultaneously on a wafer. Thus, once each glass lid  26  is bonded to each MEMS switch  16 , the wafer is diced into individual hermetically sealed switches. As a result, it is relatively simple to manufacture high quantities of hermetically sealed MEMS packaging  10 . 
         [0019]      FIG. 3  is a perspective view of hermetically sealed MEMS packaging  10  according to an embodiment of the present invention.  FIG. 3  shows the configuration of contact apertures  28  in glass lid  26  that allow electrical connection to be made to contact metallization  24  of the MEMS switch sealed inside the package. The ability of this design to contact the MEMS switch through the top of glass lid  26  has a number of advantages. First, this contact scheme has minimal effect on the mechanical stability and hermeticity of the packaging structure, ensuring that the MEMS switch is able to freely move when actuated. Second, this design allows multiple glass lids  26  on a wafer to be diced simultaneously, after bonding to a respective MEMS switch. MEMS packaging  10  is therefore efficient to reliably manufacture. 
         [0020]    Hermetically sealed MEMS packaging  10 , fabricated in the manner described in the examples above, was tested to confirm that the bond strength and hermeticity was acceptable for use in an implantable medical device. Using MIL-STD-883 standard test methods, a shear strength of 114 Newtons ±26 Newtons and a Helium leak rate of better than 2×10 −8  cubic centimeters per second were measured in 31 of 37 samples tested. 16 of the 31 samples were then subjected to thermal stress tests (thermal cycling), and 14 samples survived. The other 15 samples were subjected to mechanical stress tests (shock and vibration), and 13 samples survived. Thus, MEMS packaging samples were obtained that provide excellent bond strength, hermeticity, thermal stress resistance and mechanical stress resistance, at a rate of greater than 70% yield. These results confirm that MEMS packaging  10  is acceptable for use in implantable medical devices. It is expected that further refinement of the quality of the process and material conditions employed could result in further improvement of the yield rate as well. 
         [0021]      FIG. 4A-C  depict elevational side views in cross-section of the packages depicted in  FIG. 1  and  FIG. 2  and  FIG. 5A-C  are flow charts including  15  annotated processing steps to be performed according to the present invention. The three views in  FIG. 4A-C  depict processing of the package according to the invention at discrete moments during fabrication. Processing steps shown in  FIG. 5A  correspond to the processing of the package as depicted in  FIG. 4A . Processing steps shown in  FIG. 5B  correspond to the processing of the package as depicted in  FIG. 4B , and processing steps shown in  FIG. 5C  correspond to the processing of the package as depicted in  FIG. 4C . Also, while only representative shadow mask is depicted herein those of skill in the art readily recognize that many different specific shadow masks can be used according to the instant invention based on a desired end product. 
         [0022]    The present invention provides a hermetically sealed MEMS package suitable for use in an implantable medical device, for example. A glass lid is anodically bonded to a MEMS switch, forming a sealed interior cavity for free movement of the MEMS switch. A contact aperture is provided in the glass lid to allow electrical contact to a metallization layer that is formed on a stationary portion of the MEMS switch. This design provides excellent mechanical stability and hermeticity, and is efficient to manufacture, with all aspects fabricated at the wafer level. 
         [0023]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.