Abstract:
A microelectromechanical system includes a first wafer, a second wafer including a moveable portion, and a third wafer. The movable portion is movable between the first wafer and the third wafer. The first wafer, the second wafer, and the third wafer are bonded together.

Description:
FIELD OF THE INVENTION 
   This invention relates generally to a microelectromechanical system (MEMS). More particularly, the invention relates to microelectromechanical system having a three-wafer structure. 
   BACKGROUND OF THE INVENTION 
   MEMS devices are a combination of micro mechanical and micro electronic systems that are revolutionizing nearly every product category by bringing together these systems. A MEMS device typically comprises a movable micro mechanical structure and silicon based microelectronics. One type of known MEMS device is a MEMS transducer. Analog Devices™ manufactures a capacitive MEMS transducer used in an air-bag system for crash detection. For example, a crash is detected by monitoring the movement of the mechanical structure in the MEMS transducer using associated microelectronics in the MEMS transducer. The mechanical structure in the transducer comprises a capacitive plate which is relative to another capacitive plate in the transducer. As the mechanical structure moves, a change in capacitance is caused by the displacement of the capacitive plate. This change in capacitance is detected by the microelectronics and used to activate the air bag. 
   The Analog Devices™, MEMS transducers are manufactured from a single wafer. The mechanical structure of these transducers is created by depositing a poly-silicon layer on a silicon wafer, which is typically only a few microns thick. Because of the limited thickness of the mechanical structure, the mechanical structure suffers from performance limitations. For example, because of the minimal thickness of the mechanical structure it is difficult to restrict the movement of the mechanical structure to the desired plane. This results in the microelectronics detecting movement in the desired plane as well as movement out of the desired plane (i.e., crosstalk). Due to crosstalk, resulting from the single-wafer MEMS structure, these transducers have very limited application. 
   Other conventional MEMS devices typically comprise two chips wired together. In these MEMS devices, one chip includes the micro mechanical structure and the other chip includes the micro electronic structure. These two chips are manufactured separately and wired-bonded together. This results in performance degradation and increased costs. For example, stray capacitances are introduced due to the necessity of wire-bonding the two chips together. Also, these two chips must be packaged together as a single device, which increases costs. 
   SUMMARY OF EMBODIMENTS OF THE INVENTION 
   According to an embodiment, a microelectromechanical system (MEMS) device comprises a first wafer, a second wafer, and a third wafer. At least a portion of the second wafer is movably connected between the first wafer and the third wafer. A material is included that bonds the first wafer, the second wafer, and the third wafer together. 
   According to another embodiment, a MEMS device comprises a chip including three wafers connected together in a stacked arrangement. The stacked arrangement comprises a first wafer of the three wafers, a second wafer of the three wafers connected below the first wafer in the stacked arrangement wherein the second wafer includes a movable portion, and a third wafer of the three wafers connected below the second wafer in the stacked arrangement. The three wafers are connected using a bonding material. At least one via in the second wafer is provided that is operable to pass electrical signals through the second wafer. 
   According to another embodiment, a three-wafer MEMS chip comprises mechanical means for moving in response to one of an external force and a force generated internal to the chip. The mechanical means is a portion of a second wafer located between a first wafer and a second wafer. The chip further comprises bonding means for bonding the first wafer, the second wafer and the third wafer to form a single chip, and via means for conducting electrical signals through the second wafer. 
   According to yet another embodiment, a MEMS data storage device comprises a first wafer, a second wafer, and a third wafer, wherein at least a portion of the second wafer is movably connected between the first wafer and the third wafer. The device further comprises material bonding the first wafer, the second wafer, and the third wafer, a storage media storing data, and at least one circuit associated with performing data operations using the storage media. 
   According to yet another embodiment, a MEMS transducer device comprises a first wafer, a second wafer, and a third wafer, wherein at least a portion of the second wafer is movably connected between the first wafer and the third wafer. The transducer further comprises material bonding the first wafer, the second wafer, and the third wafer, and at least one circuit operable to detect movement of the at least a portion of the second wafer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example and not limitation in the accompanying figures in which like numeral references refer to like elements, and wherein: 
       FIG. 1  illustrates a perspective view of a MEMS device, according to an embodiment; 
       FIG. 2  illustrates a cross-section of the MEMS device of  FIG. 1 ; 
       FIG. 3  illustrates a cross-section of a MEMS data storage device, according to an embodiment; 
       FIGS. 4A-4B  illustrate cross-sections of a MEMS transducer device, according to an embodiment; and 
       FIG. 5  illustrates a cross-section of a MEMS transducer device, according to another embodiment. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   According to an embodiment, a MEMS comprises at least three-wafers. The three wafers are sealed together to form a one-chip MEMS device. According to another embodiment, a three-wafer MEMS device comprises a transducer. According to yet another embodiment, a three-wafer MEMS device comprises a storage device. 
     FIG. 1  illustrates a perspective view of a MEMS device  10 , according to an embodiment. The MEMS device  10  includes a middle wafer  40  positioned between an upper wafer  30  and a lower wafer  20 . A material  60  bonds the wafers  20 ,  30  and  40  together to form a single chip. The material  60  also seals the device  10 . A cavity  80  is formed between the upper wafer  30  and lower wafer  20 . The cavity  80  is sealed by the material  60 . The material  60  may comprise a wafer bonding material, or the like. 
   The middle wafer  40  includes a movable portion  50  capable of moving relative to the lower and upper wafers  20  and  30  within the cavity  80 . For example, the middle wafer  40  may be trenched to form the movable portion  50 . Flexures  90  connect the movable portion  50  to the remaining portion of the wafer  40 . The flexures  90  allow the movable portion  50  to move in a desired direction relative to the lower wafer  20  and the upper wafer  30 . For example, the flexures  90  may be designed to allow the movable portion  50  to move in any of the X, Y or Z directions or combination of any of those directions. The flexures  90  may also be formed from the middle wafer  40 . 
   The movable portion  50  moves within the cavity  80  of the MEMS device  10 . The cavity  80  is sealed by the material  60 . The cavity  80 , for example, may include a vacuum or may include a dielectric. Also, a hermetic seal operable to substantially prevent moisture from entering the MEMS device  10  may be created from the material  60  and/or using other materials and seals. 
   The MEMS device  10  is shown with the material  60  being significantly thicker than the wafers  20 - 40  for purposes of illustrating all the features of the MEMS device  10 . It will be apparent to one of ordinary skill in the art that the thickness of the material  60  and the wafers  20 - 40  may have proportions other than shown in FIG.  1 . In one embodiment, the thickness of the material  60  between the middle wafer  40  and the bottom wafer  20  (which is approximately equal to the gap between the wafers), for example, may be approximately 0.1 to 10 microns. Similarly, the thickness of the material  60  between the middle wafer  40  and the top wafer  10 , for example, may be approximately 0.1 to 10 microns. Furthermore, the thickness of a wafer is typically 500-600 microns thick. The middle wafer  40  may have a thickness of approximately 300 microns or less for forming the vias  72 . By reducing the thickness of the middle wafer  40 , the manufacturing process for creating the vias  72  becomes much less difficult. 
   The middle wafer  40  comprises vias  72  which conduct electrical signals through the middle wafer  40 . For example, electrical signals may be transmitted from a circuit  32  on the upper wafer  30  to a circuit  22  on the lower wafer  20  or vice versa through the vias  72 . Also, the vias  72  may be used to transmit signals to a circuit on a surface of the middle wafer  40  from one of the other wafers  20  and  30 . For example, the circuit  22  can transmit signals to the electrodes  70  on an upper surface of the middle wafer  40 , and the circuit  32  may transmit signals to electrodes  52  (shown in  FIG. 2 ) on a lower surface of the middle wafer  40 . The circuits  22  and  32  and the electrodes  52  are shown to illustrate that the vias  72  may be used to transmit signals through the middle wafer  40  to a component on a surface of the movable portion  50  or to a component on the upper wafer  30  or the lower wafer  20 . Furthermore, conductors (not shown), for example, running along the flexures  90 , may be used to connect circuits on the movable portion  50  of the middle wafer  40  to the vias  72 . It will be apparent to one of ordinary skill in the art that in various embodiments, one or more of the circuits  22  and  32  and the electrodes  70  and  52  are optionally used depending on the design of the MEMS device  10  for any particular application. Furthermore, a circuit, as described herein, comprises passive components (e.g., capacitors, inductors, resistors, electrodes, etc.) or active components (e.g., transistors, etc.), or a combination thereof. Electrodes  70  and  52  are shown as being provided on surfaces of the middle wafer  40 , however, a circuit including active and/or passive components may be provided on any of these surfaces. In addition, a circuit may include components on more than one wafer. For example, components of the circuit  22  may also be provided on the upper wafer  30 , and these components may communicate through the vias  72 . 
     FIG. 2  illustrates a cross-section of the MEMS device  10 , shown in  FIG. 1 , taken across the line  2 - 2 ′. The vias  72  and other components of the MEMS device  10  are illustrated in FIG.  2 . In one embodiment, the vias  72  may each include the wafer substrate (e.g., silicon or a polysilicon) surrounded by an insulator. The wafer substrate may be conductive, so it may be used as a conductor for the vias  72  to pass signals through the wafer  40 . An insulator is used for each of the vias  72  to create more than one via in the wafer  40  by isolating the conductors forming the vias  72 . In another embodiment, an insulator may be filled with metal to form a via in the wafer  40 . 
     FIG. 3  illustrates a cross-section of a MEMS data storage device  300 , according to an embodiment of the invention, which incorporates many of the features of the MEMS device  10  shown in  FIGS. 1 and 2 . The MEMS data storage device  300  includes three bonded wafers, i.e., a tip wafer  330 , also referred to as an upper wafer, a rotor wafer  340 , also referred to as the middle wafer, and a stator wafer  320 , also referred to as the lower wafer. The wafers  320 - 340  are bonded and sealed, for example, using the bonding material  360 . The rotor wafer  340 , e.g., approximately 100 microns thick, may be much thinner than the tip wafer  330  and the stator wafer  320 , e.g., approximately 500-600 microns thick for forming the vias  392 . 
   The wafer-to-wafer bonds form an internal cavity  380  sealed at high vacuum. The bonding material  360  seals the cavity  380  to maintain the vacuum in the cavity  380 . The bonding material  360  may comprise ultra-high vacuum (UHV) seals and/or other known materials for maintaining the internal environment of the MEMS data storage device  300 . 
   The MEMS storage  300  further comprises tip emitter electronics  312 , field emitter tips  314 , storage media  322 , and read/write (R/W) electronics  332 . The tip emitter electronics  312  may comprise one or more circuits formed on the tip wafer  330 . The tip emitter electronics  312  are connected to the field emitter tips  314 . The field emitter tips  314 , under the control of the tip emitter electronics  312 , are operable to emit electron beams by drawing electrons off a metal in the field emitter tips  314  with a high electromagnetic field. Each beam may be focused on a specific location of the storage media  322  located on an upper surface of the rotor wafer  340 , across from the field emitter tips  314 . The beams are focused and used to write data bits onto the storage media  322  by heating tiny data spots and altering the data spots physical state or phase. A beam may also be used to determine a data bit state (value) in the storage media  322 . The storage media  322  may include medium recording cells (not shown) for storing bits of data in the MEMS data storage device  300 . U.S. Pat. No. 6,440,820, entitled, “Process Flow for ARS Mover Using Selenidation Wafer Bonding After Processing a Media Side of a Rotor Wafer” by Lee et al. and U.S. Pat. No. 5,557,596, entitled, “Ultra-High Density Storage Device” by Gibson et al. disclose storage devices with emitters, and are hereby incorporated by reference in their entireties. 
   Instead of the field emitter tips  314 , other R/W mechanisms may be used. In one embodiment, optical emitters (e.g., laser emitters, LEDs, etc.) are used. The optical emitters, which also may be represented by  314  (but used instead of the field emitter tips), emit optical beams (i.e., photons). Similarly to the electron beams of the field emitter tips, the optical beams emitted by the laser emitters may be focused and used to write data bits onto the storage media  322  by heating tiny data spots and altering the data spots physical state or phase. A beam may also be used to determine a data bit state (value) in the storage media  322 . In yet another embodiment of a R/W mechanism, micro-cantilevers, which also may be represented by  314 , are used instead of the field emitter tips. The micro-cantilevers may include heated cantilevers or piezoelectric cantilevers for interacting with the storage media  322  to read or write data from the storage media  322 . For each embodiment, the tip emitter electronics  312  may be substituted with other electronics that can be used to control the respective implementation of the R/W mechanism. 
   R/W electronics  332  comprises one or more circuits, which control reading or writing of data bits in the storage media  322 , and to access data bits in the storage media  322  to determine data bit value. The R/W electronics  332  control with nanometer precision the movement of a movable portion  350  of the rotor wafer  340 . The movable portion  350  includes the storage media  322 . The movable portion  350  is moved such that the field emitter tips  314  can focus beams on the storage media  322  to access a specific set of bits. Electrodes  334  are provided on a lower surface of the movable portion  350  and electrodes  336  are provided on an upper surface of the stator wafer  320 , across from the electrodes  334 . 
   The electrodes  334  and  336  are coupled to move the movable portion  350  under control of the R/W electronics  332 . The electrodes  334  and  336  comprise multiple individual electrodes. The individual electrodes may be grouped together to form repeating patterns of electrodes covering much of the surface of the moveable portion  350 . The R/W electronics  332  energizes the electrodes  334  and  336  to one of two voltage states in a pattern. The individual electrodes repeat this pattern across the moveable portion  350 . The position of the moveable portion  350  can be changed by changing the voltage pattern on electrodes  334  and  336  in a particular order. 
   Also, the storage media  322  is connected through electrodes (not shown) to the vias  372  so bit values may be transmitted to the R/W electronics  332 . Also, the R/W electronics  332  are connected through the vias  372  to the tip emitter electronics  312 , such that the R/W electronics  332  may transmit signals to the tip emitter electronics  312  to control reading, writing and accessing bits on the storage media  322 . 
   Flexures  390 , shown in  FIG. 3 , hold the movable portion  350  of the rotor wafer  340  between the field emitter tips  314  and the stator wafer  320  to allow the data bits in the storage media  322  to be moved relative to the field emitter tips  314 , thus allowing each field emitter tip  314  to access multiple data bits after each movement of the storage media  322 . 
   The R/W electronics  332  are shown in  FIG. 3  as provided on the stator wafer  320 . However, one or more circuits of the R/W electronics  332  may be provided on the rotor emitter wafer  330  or the rotor wafer  320 . Similarly, one or more circuits of the tip emitter electronics  312  may be provided on the stator wafer  320  or the rotor wafer  340 . By using the vias  372 , circuits may be in electrical communication even if distributed on multiple wafers in the MEMS data storage device  300 . Furthermore, the MEMS data storage device  300  is provided as a single chip, which is generally cheaper than packaging a storage device comprised of multiple chips. In addition, because a three-wafer structure is used, rather than a single wafer structure, machining of the wafers may be performed (e.g., thinning the rotor wafer  340  to approximately 100 microns) without significantly impacting the integrity of the wafers. The advantages of the three-wafer structure are also applicable to the MEMS device  10  shown in  FIGS. 1 and 2 , and the MEMS transducer device  400  shown in FIG.  4 . 
     FIGS. 4A-B  illustrate a cross-section of a MEMS transducer device  400 , according to an embodiment of the invention, which incorporates many of the features of the MEMS device  10  shown in  FIGS. 1 and 2 . Referring to  FIG. 4A , the MEMS transducer device  400  detects movement of the MEMS transducer device  400  using capacitor plates, or electrodes, to detect movement of a moveable portion  450  of a middle wafer  440 . Flexures  490  allow the moveable portion  450  to move in one or more of the x, y, or z directions in response to an external force, depending on the design of the system. 
   The middle wafer  450  is positioned between an upper wafer  430  and a lower wafer  420  and connected to each with a material  460 . The material  460  functions as a seal to seal a dielectric in a cavity  480 . The seal may also be hermetic to keep moisture out of the cavity  480 . The material  460  may include wafer bonding material as is known in the art. The wafers  420 - 440  are bonded and sealed to form a single chip. 
   The MEMS transducer device  400  includes an electrode  471  on a lower surface of the movable portion  450  of the middle wafer  440 . Electrodes  473  and  475  are located opposite electrodes on an upper surface of the lower wafer  420 . As the movable portion  450  moves, the overlap between the electrode  471  and the electrodes  475  and  473  varies causing a change in capacitance between the electrodes  471  and  473 , 475 . Movement of the MEMS transducer device  400  in the x and/or y direction is detected by detecting the change in capacitance. 
   Equation 1 may be used to calculate a change in capacitance between electrodes, where ε is the dielectric constant.
 
 C =(ε* A )/ d   Equation (1)
 
   A is the overlap between electrodes in the x and y direction and d is the distance between electrodes in the z direction. This equation is also described in U.S. Pat. No. 6,504,385, entitled, “Three-Axis Motion Detector” by Hartwell et al, which is hereby incorporated by reference in its entirety. 
   Movement in the z direction may also be determined using another set of electrodes shown in FIG.  4 B.  FIG. 4B  is rear view of the MEMS transducer device  400  taken at the same cross-section shown in FIG.  4 A. Electrode  472 , located on the movable portion  450 , and electrode  474 , located on the lower wafer  420 , are provided for determining movement in the z-direction. The electrode  472  may have a short length and the electrode  474  may extend the length of the moveable portion  450  such that the overlap between the electrodes  472  and  474  does not change. Therefore, any change in capacitance detected between the electrodes  472  and  474  is substantially the result of movement in the z-direction. The electrodes  473  and  475  shown in  FIG. 4A  are also on the lower wafer  420 . However, the electrodes  473  and  475  are hidden from view by the electrode  474  shown in FIG.  4 B. The electrode  471 , shown in  FIG. 4A  on the movable portion  450 , is only partially hidden by the electrode  472  in the rear view shown in FIG.  4 B. The partially hidden electrode  471  in  FIG. 4B  is shown as shaded. In  FIG. 4A , a portion of the electrode  474  would be visible between the electrodes  473  and  475 . However, this portion of the electrode  474  is not shown in  FIG. 4A  to clearly illustrate the electrodes  473  and  475 . Also, the electrode  472  is hidden behind the electrode  471  in the view shown in FIG.  4 A. 
   The three-wafer structure allows the moveable portion  450  to move a significantly greater distance in the x, y, or z directions than conventional single-wafer capacitive MEMS transducers. Furthermore, the greater distances may allow the MEMS transducer device  400  to be used for different applications and to achieve greater accuracy in known applications. 
   Transducer electronics  422  shown in  FIGS. 4A-B  includes one or more circuits detecting the change in capacitance between the electrodes  471  and  473 , 475  and between the electrodes  472  and  474 . The electrodes  471  and  472  on the moveable portion  450  are connected to the transducer electronics  422  using the vias  492 . For example, conductors, not shown, connect the electrodes  471  and  472  to the vias  492 . The signal from the electrodes is passed through the vias  492  to the transducer electronics  422 . The transducer electronics  422  is also connected to the electrodes  473 - 475  on the lower wafer  420 . Thus, the transducer electronics  422  is operable to detect the change in capacitance between the electrodes. The transducer electronics may comprise one or more circuits for calculating the change in overlap A and/or distance d between the electrodes. Alternatively, the transducer electronics  422  may output the change in capacitance to an external circuit for calculating the change in overlap A and/or distance d. Using equation 1, the distance d may be calculated from the change in capacitance between the electrodes  472  and  474 . Also, if d is known, the overlap A may also be calculated from the change in capacitance detected between the electrodes  471  and  473 , 475  shown in FIG.  4 A. 
   In this embodiment, the upper wafer  430  may comprise a cap wafer that protects the MEMS transducer device  400 . In other embodiments, such as shown in  FIG. 5 , one or more electrodes may be placed on the upper wafer  430 . Also, one or more circuits for the transducer electronics can be provided on the upper wafer  430 . 
     FIG. 5  illustrates a cross-section of another embodiment of a MEMS transducer device. A MEMS transducer device  500  is shown that is similar to the MEMS transducer device  400  of  FIGS. 4A-B . In this embodiment, the electrode  472  is located on the upper surface of the moveable portion  450  and opposite the electrode  474  located on the upper wafer  430 . Transducer electronics  422  are provided in both the upper wafer  430  and the lower wafer  420  for detecting change in capacitance. 
   It will be apparent to one of ordinary skill in the art that more electrodes may be used or the size and shape of the electrodes may be varied for detecting change in capacitance in one or more of the x, y, and z directions. For example, in U.S. Pat. No. 6,504,385 five electrodes and five counter electrodes are used to detect movement in the x, y, and z directions. Also, a less number of electrodes may be used if movement in one or two directions is to be detected. 
   What has been described and illustrated herein are embodiments of the invention along with some of variations. The terms, descriptions and figures used herein are set forth by way of illustration only and are not meant as limitations. Those skilled in the art will recognize that many variations are possible within the spirit and scope of the invention, which is intended to be defined by the following claims—and their equivalents—in which all terms are meant in their broadest reasonable sense unless otherwise indicated.