Patent Publication Number: US-2011073967-A1

Title: Apparatus and method of forming a mems acoustic transducer with layer transfer processes

Description:
PRIORITY 
     This patent application claims priority from provisional U.S. patent application No. 61/237,982, filed Aug. 28, 2009, entitled, “HIGH PERFORMANCE INTEGRATED MICROPHONE EMPLOYING LAYER TRANSFER TECHNIQUE,” and naming Li Chen and Kuang Yang as inventors, the disclosure of which is incorporated herein, in its entirety, by reference. 
    
    
     TECHNICAL FIELD 
     The invention generally relates to microelectromechanical systems (MEMS) and, more particularly, the invention relates to methods of forming a MEMS acoustic transducer. 
     BACKGROUND ART 
     Condenser microphones, such as MEMS microphones, typically have associated detection circuitry that detects diaphragm deflections and transmits such deflections to other circuitry for further processing. Forming such circuitry on the same die as the microphone, however, generally presents a number of challenges. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the invention, a method of forming a MEMS acoustic transducer forms circuitry and first MEMS microstructure on a first wafer in a first process, and second MEMS microstructure on a second wafer in a second process. The first process is thermally isolated from the second process. The method also layer transfers the second MEMS microstructure onto the first wafer. The first MEMS microstructure and second MEMS microstructure thus form a variable capacitor that communicates with the circuitry on the first wafer. 
     The first MEMS microstructure may have a first capacitive plate, and the second MEMS microstructure may have a second capacitive plate. The first and second capacitive plates thus form the variable capacitor. For example, the first capacitive plate may form a backplate and the second capacitive plate may form a diaphragm. 
     Some embodiments layer transfer by bonding the second wafer to the first wafer, and removing at least one entire layer of the second wafer after bonding. Moreover, the first MEMS microstructure and second MEMS microstructure both may be formed at least in part from a silicon-based material. To that end, the first wafer may include a SOI wafer while the second wafer may include at least one of polysilicon, single crystal silicon, silicon carbide, or silicon germanium. The method also may release at least the second MEMS microstructure after layer transferring the second MEMS microstructure onto the first wafer. 
     In accordance with another embodiment of the invention, a method of forming a MEMS microphone forms circuitry and a semiconductor backplate on a first wafer, and a semiconductor diaphragm on a second wafer. The method then forms a variable capacitor on the first wafer by layer transferring the semiconductor diaphragm onto the first wafer. The variable capacitor includes the backplate and diaphragm to form a MEMS microphone. The capacitor is electrically connected with the circuitry. 
     In accordance with other embodiments of the invention, a method of forming a MEMS microphone forms circuitry and first MEMS microstructure on a first wafer, and a semiconductor film on a second wafer. The method micromachines the film on the second wafer to form second microstructure, and forms a variable capacitor on the first wafer by layer transferring the second MEMS structure onto the first wafer. The variable capacitor includes the first MEMS structure and the second MEMS structure and is electrically connected with the circuitry. 
     In accordance with yet other embodiments of the invention, a MEMS microphone has a backplate formed from single crystal silicon, and circuitry formed on the single crystal backplate. The microphone also has a diaphragm coupled with the backplate that forms a variable capacitor with the backplate. The diaphragm also is formed from single crystal silicon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following “Description of Illustrative Embodiments,” discussed with reference to the drawings summarized immediately below. 
         FIG. 1  schematically shows a perspective view of a MEMS microphone that may be formed in accordance with illustrative embodiments of the invention. 
         FIG. 2  schematically shows a cross-sectional view of the MEMS microphone of  FIG. 1  across line B-B. 
         FIG. 3  schematically shows a cross-sectional view of an alternative embodiment that may be formed in accordance with illustrative embodiments of the invention. 
         FIG. 4  shows a process of fabricating a MEMS microphone in accordance with illustrative embodiments of the invention. 
         FIG. 5  schematically shows a cross-sectional view of a silicon-on-insulator wafer that may be used by the process of  FIG. 4  to form either the backplate or the diaphragm. 
         FIG. 6  schematically shows a cross-sectional view of a die or wafer formed by step  400 A of  FIG. 4 . 
         FIG. 7  schematically shows a cross-sectional view of a die or wafer formed by step  400 B of  FIG. 4 . 
         FIG. 8  schematically shows a cross-sectional view of a die or wafer formed by step  400 B of  FIG. 4  in accordance with alternative embodiments of the invention. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Illustrative embodiments fabricate a MEMS microphone with integrated circuitry (or other circuitry) on a single die. To that end, various embodiments form the variable capacitor microstructure using layer transfer techniques. Accordingly, steps requiring high temperatures, such as those for forming a flexible diaphragm, can be performed away from temperature sensitive circuitry. Details of illustrative embodiments are discussed below. 
       FIG. 1  schematically shows a top, perspective view of a MEMS microphone  10  (also referred to as a “microphone chip  10 ”) that may be fabricated using layer transfer processes; namely, in accordance with illustrative embodiments of the invention.  FIG. 2  schematically shows a cross-sectional view of the same microphone  10  across line B-B of  FIG. 1 . 
     Among other things, the microphone  10  includes a static backplate  12  that supports and forms a variable capacitor  14  with a flexible diaphragm  16 . In illustrative embodiments, the backplate  12  and diaphragm  16  each are formed from single crystal silicon (e.g., the top layer of a silicon-on-insulator wafer, discussed below). Alternatively, the diaphragm  16  is formed from a film of silicon-based material, such as polysilicon, silicon carbide, or silicon germanium. In a similar manner, other types of materials can form the backplate  12 . For example, the backplate  12  can be formed from a relatively low temperature film, such as silicon germanium. If thermal budget is not a primary issue, the backplate  12  can be formed from high temperature materials, such as polysilicon, silicon carbide, or silicon germanium. 
     To facilitate operation, the backplate  12  has a plurality of through-hole apertures (“backplate apertures  18 ”) that lead to a backside cavity  20 . Springs  22  movably connect the diaphragm  16  to a static/stationary portion of the microphone  10 , which includes a substrate that at least in part includes the backplate  12 . The springs  22  effectively form a plurality of openings  24  that permit at least a portion of the audio signal to pass through the microphone  10 . These openings  24  may be any reasonable shape, such as in the shape of a slot, round hole, or some irregular shape. 
     The microphone  10  also includes circuitry  26  that cooperates with the variable capacitor  14  to convert audio signals incident upon the diaphragm  16  into electronic signals. The circuitry  26  is shown in a partial cutaway view on  FIG. 1 , and within the substrate of  FIG. 2 . Among other things, the circuitry  26  may provide a voltage bias for the backplate  12  and diaphragm  16 , and convert the variable capacitance into an electronic signal. In illustrative embodiments, the circuitry  26  is formed primarily from CMOS circuitry, although other types of circuitry may suffice. Metal contact pads  23  on the top surface of the microphone  10  enable electrical access to the circuitry  26  and relevant microstructure. 
       FIG. 3  schematically shows a cross-sectional view of an alternative embodiment of the microphone  10 . Specifically, this embodiment of the microphone  10  positions the diaphragm  16  between the backplate  12  and the backside cavity  20 . As with the embodiments of  FIGS. 1 and 2 , the backplate  12  and/or the diaphragm  16  can be formed from any one of a variety of materials, such as single crystal silicon, polysilicon, silicon carbide, or silicon germanium. 
     Illustrative embodiments first at least in part form critical microstructure, such as the variable capacitor, across two separate wafers (e.g., two silicon-on-insulator wafers, also referred to as “SOI wafers”), and then bond those two wafers together using a low temperature process. Alternately, the process may bond the wafers prior to the complete fabrication of at least one of the wafers (i.e., when some fabrication steps remain for at least one of the wafers). In illustrative embodiments, each wafer is an SOI wafer, although various embodiments are not necessarily limited to SOI wafers. Discussion of SOI wafers thus is for exemplary purposes only. 
     A low temperature bond secures the wafers together; preferably lower than the temperature at which a MEMS structure or circuit elements  26  may be damaged. For example, in various embodiments, the bond may be fabricated under pressure in a bonder at temperatures of between about 200 to 400 degrees Celsius. Accordingly, illustrative embodiments permit the use of more circuitry  26  sensitive to higher temperatures, such as the deposition temperature of polysilicon. 
     To those ends,  FIG. 4  shows a process of forming the MEMS microphone  10  of  FIGS. 1 and 2  in accordance with illustrative embodiments of the invention. It should be noted that for simplicity, this described process is a significantly simplified version of an actual process used to form the MEMS microphone  10 . Accordingly, those skilled in the art would understand that the process may have additional steps not explicitly shown in  FIG. 4 . Moreover, some of the steps may be performed in a different order than that shown, or at substantially the same time. Those skilled in the art should be capable of modifying the process to suit their particular requirements. 
     The process begins at steps  400 A and  400 B by processing, in parallel, two different silicon-on-insulator wafers in separate processes. Specifically,  FIG. 5  schematically shows a cross-sectional view of a silicon-on-insulator wafer  30 , which has a silicon base layer  32  (often referred to as the “handle layer  32 ”) for supporting a top, silicon device layer  34  and insulator layer  36  (e.g., an oxide). As known by those skilled in the art, the insulator layer  36  secures the device layer  34  to the base layer. 
     More particularly, step  400 A forms circuitry  26  and a first plate of the variable capacitor  14  on a first SOI wafer  30 A, while step  400 B forms a second plate of the variable capacitor  14  on a second SOI wafer  30 B. For example, in the embodiment shown in  FIG. 2 , the first plate is the backplate  12  while the second plate is the diaphragm  16 .  FIG. 6  schematically shows the (partially processed) SOI wafer  30 A having the circuitry  26  and the backplate  12 , while  FIG. 7  schematically shows the (partially processed) SOI wafer  30 B having the diaphragm  16 . As shown in  FIG. 6 , the circuitry  26  may be formed about the backplate  12 ; namely, circumferentially outward of the backplate  12 . Although the figures show the circuitry  26  schematically at one spot only, it may be distributed across the wafer  30 A in appropriate locations. 
     It is important to note that these two processes are separate and thus, thermally isolated, i.e., heat produced to form either one of those components does not materially impact the temperature for forming the other component. Accordingly, the diaphragm  16  and springs  22  may be formed from high-temperature processes and still not impact/damage the circuitry  26  on the SOI wafer  30 A formed by step  400 A. 
     As shown in the figures discussed above, steps  400 A and  400 B can be formed in parallel/generally at the same time. Those skilled in the art nevertheless can perform those steps in series, with either step being performed first. 
     The process then continues to step  402  by layer transferring the second plate (the diaphragm  16 ) onto the SOI wafer  30 A having the circuitry  26  and backplate  12 . To that end, step  402  bonds the SOI wafer  30 B having the diaphragm  16  to the SOI wafer  30 A having the backplate  12  and circuitry  26 . 
     More specifically, a low temperature bonding medium  28  secures the SOI wafer  30 B having the diaphragm  16  in a manner that positions the diaphragm  16  adjacent to, but spaced from, the backplate  12  (as shown in  FIG. 2 ). Those skilled in the art can select the appropriate bonding medium  28 , which may include a metal, adhesive, or oxide. It nevertheless should be noted that other bonding media may provide sufficient results. Discussion of specific bonding media thus is illustrative and not intended to limit various embodiments. 
     The thickness of the bonding medium  28  is important in determining the capacitance of the variable capacitor  14 . In the mass production of such microphone systems  10 , the variation in the gap between the backplate  12  and the diaphragm  16  illustratively may be less than about five percent (5%) of the nominal gap. 
     To complete the layer transfer process, step  402  removes portions of the second wafer  30 B. In illustrative embodiments, the removed portions of the second wafer  30 B are those that are farthest from the bonding point of the two wafer  30 A and  30 B (e.g., the outside of the so-called sandwich). For example, entire planes of the wafer  30 B having the diaphragm  16  may be removed (e.g., the handle layer  32  and the at least part of the insulator layer  36  between the handle and device layers  32  and  34 ). Thus, a layer of the second SOI wafer  30 B (i.e., the diaphragm  16 ) effectively has been transferred to the first SOI wafer  30 A. The second SOI wafer  30 B may thus be referred to as a “donor” wafer  30 B. 
     If, as shown in  FIG. 7 , the second wafer  30 B is an SOI wafer with the diaphragm  16  in the top layer, the process may remove layers of the donor wafer  30 B by merely etching away most or all of the insulator layer  36 . This effectively removes/detaches the handle layer  32 , which is not necessary in the final product. If the second wafer  30 B is not an SOI wafer  30 , but has a diaphragm  16  supported by a sacrificial layer between the diaphragm  16  and the surface of some substrate, then the process may remove layers of the donor wafer  30 B in a similar manner; namely, by etching away the sacrificial layer. Alternately, the portions to be removed may be removed by grinding or etching away some of the silicon with an appropriate acid. Some embodiments may remove portions of the donor wafer  30 B by a combination of etching, and grinding, or lapping down the portions to be removed. For example, if the wafer  30 B is an SOI wafer, the handle layer  32  may be removed by grinding, thus exposing the insulator layer  36 . The insulator layer  36  may then be removed by etching. 
     Some embodiments may form the diaphragm  16  on a pre-weakened bulk silicon wafer  30 C that can be easily cleaved to remove unnecessary portions.  FIG. 8  schematically shows one such wafer  30 C. Specifically, the wafer  30 C of  FIG. 8  has hydrogen ions implanted into its interior to form an internal damage plane  38 . This damage plane  38  is generally parallel to the surface that supports (or will support) the diaphragm  16 . Accordingly, after bonding the two wafers  30 A and  30 C, the damaged layer may later be cut or severed to separate the remaining substrate from the layer to be transferred. 
     Such processes, which may be known in the art as “Smart Cut,” are described, for example, in U.S. Pat. No. 5,374,564, or U.S. Pat. No. 5,882,987. As noted above, exemplary processes implant ions, such as hydrogen ions, into the wafer  30 C, to create, for example, a hydrogen-rich layer damage plane  38  in the donor wafer  30 C prior to bonding the two wafers  30 A and  30 C. The ions create a region that makes the wafer  30 C susceptible to fracture. The crystalline silicon may be fractured along the damage plane  38  through an annealing process to leave behind the diaphragm layer  16 . 
     These processes thus leave a layer (e.g., a diaphragm  16 ) of the second wafer  30 B or  30 C bonded to the first wafer, effectively transferring the layer from the donor wafer  30 B or  30 C to the first wafer  30 A. The unused portion of the donor wafer  30 B or  30 C may be reused if it is thick enough to have a transferable layer fabricated on its face. 
     It should be noted that steps  400 A and  400 B may not have fully processed their respective SOI wafers  30 A and  30 B before the layer transfer step  402 . For example, some embodiments fabricate a sacrificial layer, or leave an existing sacrificial layer in place, between the diaphragm  16  and the underlying substrate (e.g., a handle layer  32 ) until after the donor wafer  30 B (or  30 C) donates its capacitive plate to the wafer  30 A having the circuitry  26  and first plate. In this way, the diaphragm  16  is immobilized after it is formed, and remains immobilized while other processing is performed on the device. 
     Alternatively, some embodiments skip one or both of steps  400 A and  400 B. For example, those embodiments may simply transfer an entire layer from the donor wafer  30 B or  30 C to the main wafer  30 A, and then form the microstructure at a later time. 
     Accordingly, after layer transferring the diaphragm  16 , step  404  releases the diaphragm  16  using conventional processes. For example, the process may remove the oxide, polymer, metal, or other sacrificial layer using an appropriate acid or other etchant. The process also may perform some post processing steps, such as polishing surfaces (e.g., through a mechanical grind), releasing additional MEMS structures, or interconnecting circuits. Polishing the surface of the diaphragm  16  that faces the backplate  12  may not be necessary if that surface was acceptably smooth or polished prior to the layer transfer. 
     The process concludes at step  406  by dicing the wafer structure into individual MEMS microphones  10 . At this point, further post-processing steps may be performed before packaging and/or assembly into an end product, such as a computer system or mobile telephone. 
     Illustrative embodiments may package the microphone chip  10  in any of a variety of different types of packages. One important consideration is the susceptibility of the microphone chip  10  to electromagnetic interference (“EMI”). To protect the microphone chip  10  against EMI, illustrative embodiments use packages that effectively form a Faraday cage around the microphone chip  10 . For example, the package may have a base formed from printed circuit board material, such as FR4 or laminate. Alternatively, the base may be formed from leadframe packaging technology, carrier, or ceramic packages. Other embodiments may use wafer level packaging techniques (e.g., using another wafer to cap the variable capacitor and/or other microstructure). For additional examples of microphone packaging, see co-pending U.S. patent application Ser. No. 12/847,682, filed Jul. 30, 2010, entitled “Reduced Footprint Microphone System with Spacer Member Having Through-Hole,” the disclosure of which is incorporated herein, in its entirety, by reference. 
     It should be reiterated that discussion of the embodiments using two SOI wafers  30 A and  30 B is illustrative and not intended to limit all embodiments. For example, as noted above, the donor wafer  30 B may simply be a bulk silicon wafer or other substrate supporting a thin film of material, such as polysilicon, silicon carbide, or silicon germanium. In a similar manner, the wafer  30 A having the circuitry  26  can be something other than an SOI wafer, such as a bulk silicon wafer. 
     Moreover, also as noted above, discussion of the donor wafer  30 B or  30 C providing a diaphragm  16  is for simplicity purposes only. Instead, as shown in  FIG. 3 , the donor wafer  30 B or  30 C may provide the backplate  12 . In fact, some embodiments may form circuitry  26  in the donor wafer  30 B or  30 C. Alternative embodiments, however, do not have circuitry  26  in either of the wafers  30 A and  30 B (or on wafers  30 A and  30 C) and thus, require a separate, off-chip integrated circuit. 
     Accordingly, various embodiments form a micromachined acoustic sensor, or MEMS transducer, specifically implemented as a condenser microphone. By forming the plates of a single capacitor  14  on two separate wafers  30 A and  30 B by two separate processes, this microphone  10  can have on-chip circuitry  26  that is not limited by the thermal requirements of the fabrication process. Removing this limitation of the prior art thus gives the microphone designer more flexibility to use a wider variety of circuitry  26 . Consequently, the final microphone system  10  can have improved overall performance and additional functionality. 
     Although the above discussion discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention.