Patent Publication Number: US-2012025335-A1

Title: Microelectromechanical systems (mems) package

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part under 37 C.F.R. § 1.53(b) of and claims priority under 35 U.S.C.§120 from commonly owned U.S. patent application Ser. No. 12/844,857 entitled “MEMS Transducer Device having Stress Mitigation Structure and Method of Fabricating the Same” filed on Jul. 28, 2010 to Timothy LeClair, et al. The disclosure of this application is specifically incorporated herein by reference. 
    
    
     BACKGROUND 
     Transducers generally convert electrical signals to mechanical signals or vibrations, and/or mechanical signals or vibrations to electrical signals. Acoustic transducers, in particular, convert electrical signals to acoustic signals (sound waves) in a transmit mode (e.g., a speaker application), and/or convert received acoustic waves to electrical signals in a receive mode (e.g., a microphone application). Transducers, such as ultrasonic transducers, are provided in a wide variety of electronic applications, including filters. As the need to reduce the size of many components continues, the demand for reduced-size transducers continues to increase, as well. This has led to comparatively small transducers, which may be micromachined according to various technologies, such as micro-electromechanical systems (MEMS) technology. 
     Various types of MEMS transducers, such as piezoelectric ultrasonic transducers (PMUTs), include a resonator stack, having a layer of piezoelectric material between two conductive plates (electrodes), formed on a thin membrane. The membrane may be formed on a substrate over a cavity passing through the substrate. Typically, the substrate is formed of a material compatible with semiconductor processes, such as silicon (Si). The transducers may be packaged by polishing the back side of the transducer substrate and mounting the polished transducer substrate directly onto a package substrate. For example, when the transducer is to be included in a lead frame package, the transducer substrate is typically mounted on a metal package substrate. 
     In known packaging, a coefficient of thermal expansion (CTE) of the transducer is significantly different from the CTE of the package in which it is mounted. Generally, CTE indicates the rate or proportion of change of a material or structure with respect to changes in temperature. The difference between the transducer and package CTEs results in varying responses to changes in temperature, both during packaging processes and during operation, which impose physical stress on the transducer. In other words, the source of parametric shifts in MEMS bending mode and/or thickness mode transducers due to die mounting and operating temperature variation, for example, is mismatch of thermal properties between the materials of the transducer and the package. The stress is most pronounced between the transducer substrate and the package substrate to which the transducer substrate is attached, due to the intimate physical contact and significant CTE mismatch of the respective materials. 
     After the MEMS transducer is packaged, the package is aligned to and mounted on a system-level printed circuit board. In known MEMS packaging, the alignment process adds complexity to the fabrication process and often does not provide suitable alignment of the MEMS transducer. 
     What is needed is a MEMS package that overcomes at least the shortcomings of known MEMS packages described above. 
     SUMMARY 
     In a representative embodiment, a micro-electromechanical systems (MEMS) transducer device is mounted to a substrate. The MEMS transducer device comprises: a package substrate having a first coefficient of thermal expansion (CTE); and a transducer substrate comprising a transducer, the transducer substrate being disposed over the package substrate, wherein the transducer substrate has a second CTE that is substantially the same as the first CTE. 
     In another representative embodiment, a micro-electromechanical systems (MEMS) transducer device comprises: a package substrate having a first coefficient of thermal expansion (CTE); and a transducer substrate comprising a transducer, the transducer substrate being disposed over the package substrate, wherein the transducer substrate has a second CTE that is substantially the same as the first CTE. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The example embodiments are best understood from the following detailed description when read with the accompanying drawing figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion. Wherever applicable and practical, like reference numerals refer to like elements. 
         FIGS. 1A and 1B  are isometric exploded views of a MEMS package, according to a representative embodiment. 
         FIGS. 2A-2B  are isometric exploded views of a MEMS package, according to a representative embodiment. 
         FIG. 3  is a cross-sectional view of a MEMS package mounted to a substrate, according to a representative embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation and not limitation, representative embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. However, it will be apparent to one having ordinary skill in the art having had the benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatuses and methods may be omitted so as to not obscure the description of the representative embodiments. Such methods and apparatuses are clearly within the scope of the present teachings. 
     Generally, it is understood that the drawings and the various elements depicted therein are not drawn to scale. Further, relative terms, such as “above,” “below,” “top,” “bottom,” “upper,” “lower,” “left,” “right,” “vertical” and “horizontal,” are used to describe the various elements&#39; relationships to one another, as illustrated in the accompanying drawings. It is understood that these relative terms are intended to encompass different orientations of the device and/or elements in addition to the orientation depicted in the drawings. For example, if the device were inverted with respect to the view in the drawings, an element described as “above” another element, for example, would now be “below” that element. Likewise, if the device were rotated  90  degrees with respect to the view in the drawings, an element described as “vertical,” for example, would now be “horizontal.” 
     According to various embodiments, a transducer device, such as a MEMS ultrasonic transducer or a PMUT, comprises a package substrate having a first coefficient of thermal expansion (CTE); a transducer comprising an active area disposed over a transducer substrate, the transducer substrate having a second CTE that is substantially the same as the first CTE; and an opening in the package substrate configured to receive and to transmit mechanical waves from the transducer. 
       FIG. 1A  is an isometric exploded view of a MEMS transducer device  100 , according to a representative embodiment. The MEMS transducer device  100  comprises a package substrate  101 , a transducer substrate  102 , a cover  103  and a screen  104 . As described more fully below, an opening  105  in the package substrate  101  is configured to receive and to transmit mechanical waves (e.g., ultrasonic waves) to and from a plurality of transducers  106  provided over the transducer substrate  102 . 
     In the presently illustrated embodiment, there are three (3) transducers  106 . It is emphasized that this is merely illustrative, and that more or fewer transducers  106  may be provided over the transducer substrate  102 . The transducers  106  may be ultrasonic MEMS transducers, for example, although it is understood that other types of transducers may be incorporated without departing from the scope of the present teachings. The transducers  106  are shown as annular resonators, where the cross-section is taken across the center. The transducers  106  may be substantially circular in shape, for example, although other shapes are contemplated including, but not limited to ovals, squares, rectangles, or the like, without departing from the scope of the present teachings. 
     The transducer substrate  102  comprises silicon (Si), or silicon-germanium (SiGe), or silicon-on-insulator (SOD, or gallium arsenide (GaAs), or indium phosphide (InP), or sapphire, or alumina, or doped SiO 2  (e.g., borosilicate glass (BSG) or Pyrex®). Among other considerations, the material selected for the transducer substrate  102  is useful for integrating electrical connections and electronics, thus reducing size and cost. In representative embodiments, the package substrate  101  may be alumina, sapphire, or a comparatively high density ceramic material within the purview of one of ordinary skill in the art having had the benefit of review of the present disclosure. The material selected for the package substrate  101  is selected to provide a CTE that substantially matches to the CTE of the transducer substrate  102 . In particular, the CTE of the package substrate  101  is selected to be as close to the CTE of the transducer substrate  102  as possible, while taking into account other desired material properties such as ease of fabrication of useful features thereon (e.g., metallization, contacts, openings), ease of integrating electrical connections and electronics, reliability and cost. For example, in a representative embodiment, the transducer substrate  102  is silicon (Si), which has a CTE of approximately 3.0 ppm/° C., and the package substrate  101  is alumina, which has a CTE of approximately 6.0 ppm/° C. 
     In representative embodiments, the transducers  106  may be PMUTs fabricated using MEMS technology. Further details of the components and configurations of the transducers  106  may be found in commonly owned U.S. patent application Ser. No. 12/844,857 entitled “MEMS Transducer Device having Stress Mitigation Structure and Method of Fabricating the Same” filed on Jul. 28, 2010 to Timothy LeClair, et al. The disclosure of this application is specifically incorporated herein by reference. Generally, examples of methods, materials and structures for fabricating transducers 106 are described in commonly owned U.S. Pat. Nos. 5,587,620, 5,873,153, 6,384,697 and 7,275,292 to Ruby, et al.; commonly owned U.S. Pat. No. 6,828,713 to Bradley; in commonly owned U.S. Patent Application Pub. Nos. 2008/0122320 and 2008/0122317 to Fazzio, et al; in commonly owned U.S. Patent Application Pub. No. 2007/0205850 to Jamneala, et al; in commonly owned U.S. Patent Application Pub. No. 2008/0258842 to Ruby, et al; in commonly owned U.S. Patent Application Pub. No. 2006/0103492 to Feng, et al.; and in commonly owned U.S. patent application Ser. no. 12/495,443 to Martin, et al. The disclosures of these commonly owned patents, patent application publications and patent applications are specifically incorporated herein by reference. 
     When the transducers  106  are PMUTs, for example, the translation is made through a piezoelectric material (not shown). In various alternative embodiments, the MEMS transducer device  100  may be any type of micromachined transducer with a membrane having stress as a significant parameter, such as a capacitive micro-machined ultrasonic transducer (CMUT), in which case the translation is made through a capacitance variation. It is understood that other types and arrangements of transducers may be incorporated, without departing from the scope of the present teachings. 
     Cover  103  is provided over the package substrate  101  and surrounds the transducer substrate  102 . Among other functions, the cover  103  provides protection from debris from contacting the transducers  106 . In a representative embodiment, the cover  103  comprises plastic, aluminum, steel, copper, brass or other suitable material. As described more fully herein, the cover is sized to fit through an opening in a circuit board (not shown in  FIG. 1A ) or other substrate to which the MEMS transducer device  100  is mounted. 
     Openings  108  are provided in the package substrate  101  and located to receive a respective post  109  (only one post  109  can be seen in  FIG. 1A ) for securing the cover  103  over the transducer substrate  102  and to the package substrate  101 . The openings  108  may extend through the package substrate  101  and are formed by laser drilling or other known techniques. A suitable adhesive may be used to secure the posts  109  to the package substrate  101 . 
     Screen  104  is provided over opening  105  in the package substrate  101 . The screen  104  comprises a plurality of holes  107  so that mechanical waves emitted from or incident on the transducers  106  can traverse the screen  104  without significant interference or impedance. The screen  104  protects the transducers  106  from debris or other objects that can deleteriously impact the performance of the transducers  106 . Illustratively, the screen  104  comprises the same material as the package substrate  101 . The holes  107  are machined into a blank substrate by a known laser drilling method. Illustratively, the holes  107  have a diameter of 0.015 in (15 mils). Generally, when using laser drilling techniques to form the holes  107 , the diameter of the holes  107  is substantially the same as the thickness of the package substrate  101 . 
     In a representative embodiment, electrical connections to the transducers  106  are made with wirebonds  110  that connect contacts  111  on the transducer substrate  102  to contacts  112  on the package substrate  101 . As described more fully below, the contacts  112  provide electrical connections to electrical circuitry and components useful in the transmission and reception of signals by the transducers  106 . The contacts  111 ,  112  are provided over the transducer substrate  102  and the package substrate  101 , respectively, by known metallization techniques. Illustratively, the contacts comprise a suitable conductive material such as gold (Au), copper (Cu) or aluminum (Al) or a suitable conductive alloy such as gold-tin alloy. Notably, the contacts  112  are partially covered by the cover  103 , as can be appreciated from a review of  FIG. 1A . 
     Connection pads  113  are provided over the package substrate  101 , and vias  114  are provided through the package substrate. As described more fully below, the connection pads  113  contact connection pads on a circuit substrate (not shown in  FIG. 1A ) for securing (e.g., bonding) the MEMS transducer device  100  thereto. In an embodiment, the cover  103  is disposed at least partially over the connection pads  113  and vias  114 . Illustratively, one or both of the connection pads  113  are connected by the vias  114  to a ground plane (described as a shield  117  below) to ensure properly grounding and to avoid “floating” grounds. 
       FIG. 1B  is an isometric exploded view of MEMS transducer device  100 , according to a representative embodiment. As  FIG. 1B  presents a different perspective of the MEMS transducer device  100  described above, details of many common aspects provided in the description of  FIG. 1A  are not repeated. 
     Transducer substrate  102  comprises cavities  115  aligned with respective transducers  106  (not visible in  FIG. 1B ). The cavities  115  provide a path for mechanical waves to and from the transducers  106 . The cavities  115  are also aligned over the opening  105  in the package substrate  101  and the screen  104 . The cavities  115  have a comparatively high-aspect ratio, and are formed by a known method such as dry reactive ion etching (DRIE), the so-called “Bosch method.” Many of the commonly-owned references incorporated by reference above provide details of the fabrication of the cavities  115  and are not generally repeated herein. 
     Cover  103  comprises a cavity  116  into which the transducer substrate  102  is disposed. As such, once assembled, the cover  103  encloses the transducer substrate  102 . The cover  103  provides protection of the transducers  106  from debris and moisture. Furthermore, the depth of the cavity  116  is selected to provide an acoustic backplane for the transducers  106 . Beneficially, the acoustic backplane fosters frequency stabilization of mechanical waves emanating from the transducers  106 . 
     In a representative embodiment, a shield  117  is provided over a first side  118  of the package substrate  101  and opposing a second side  119  of the package substrate  101  over which the transducer substrate  102  is disposed. The shield  117  illustratively comprises a metal or metal alloy and is printed on the package substrate  101  by a known technique. The shield  117  provides a ground plane and prevents stray electromagnetic signals (e.g., RF signals) from adversely interfering with the operation of the transducers  106 . 
       FIG. 2A  is an isometric exploded view of a MEMS transducer device  200 , according to a representative embodiment. The MEMS transducer device  200  comprises package substrate  101 , transducer substrate  102  and cover  103  as described in connection with the representative embodiments of  FIG. 1A . The MEMS transducer device  200  also comprises an integral screen  201 . The integral screen  201  comprises a plurality of holes  202  that extend through a thickness of the package substrate from a first side  203  to a second side  204 . 
     In the presently illustrated embodiment, there are three (3) transducers  106 . It is emphasized that this is merely illustrative, and that more or fewer transducers  106  may be provided over the transducer substrate  102 . The transducers  106  may be ultrasonic MEMS transducers, for example, although it is understood that other types of transducers may be incorporated without departing from the scope of the present teachings. The transducers  106  are shown as annular resonators, where the cross-section is taken across the center. The transducers  106  may be substantially circular in shape, for example, although other shapes are contemplated including, but not limited to ovals, squares, rectangles, or the like, without departing from the scope of the present teachings. 
     The transducer substrate  102  comprises silicon (Si), silicon-germanium (SiGe), silicon-on-insulator (SOD, gallium arsenide (GaAs), indium phosphide (InP), glass, sapphire, alumina, doped SiO 2  (e.g., borosilicate glass (BSG) or Pyrex®). Among other considerations, the material selected for the transducer substrate  102  is useful for integrating electrical connections and electronics, thus reducing size and cost. The material selected for the package substrate  101  is selected to provide a CTE that substantially matches the CTE of the transducer substrate  102 . In particular, the CTE of the package substrate  101  is selected to be as close to the CTE of the transducer substrate  102  as possible, while taking into account other desired material properties such as ease of fabrication of useful features thereon (e.g., metallization, contacts, openings), ease of integrating electrical connections and electronics, reliability and cost. For example, in a representative embodiment, the transducer substrate  102  is silicon (Si), which has a CTE of approximately 3.0 ppm/° C., and the package substrate  101  is alumina, which has a CTE of approximately 6.0 ppm/° C. 
     In representative embodiments, the transducers  106  may be PMUTs fabricated using MEMS technology. Further details of the components and configurations of the transducers  106  may be found in the commonly owned U.S. Patents, U.S. Patent Application Publications and U.S. Patent Applications incorporated by reference herein. 
     When the transducers  106  are PMUTs, for example, the translation is made through a piezoelectric material (not shown). In various alternative embodiments, the MEMS transducer device  100  may be any type of micromachined transducer with a membrane having stress as a significant parameter, such as a capacitive micro-machined ultrasonic transducer (CMUT), in which case the translation is made through a capacitance variation. It is understood that other types and arrangements of transducers may be incorporated, without departing from the scope of the present teachings. 
     Cover  103  is provided over the package substrate  101  and surrounds the transducer substrate  102 . Among other functions, the cover  103  provides protection from debris from contacting the transducers  106 . In a representative embodiment, the cover  103  comprises plastic aluminum, steel, copper, brass or other suitable material. As described more fully herein, the cover is sized to fit through an opening in a circuit board (not shown in  FIG. 1A ) or other substrate to which the MEMS transducer device  100  is mounted. 
     Openings  108  are provided in the package substrate  101  and are located to receive respective post  109  (only one post  109  can be seen in  FIG. 2A ) for securing the cover  103  over the transducer substrate  102  and to the package substrate  101 . The openings  108  may extend through the package substrate  101  from first side  203  to second side  204  and are formed by laser drilling or other known techniques. A suitable adhesive may be used to secure the posts  109  to the package substrate  101 . 
     The transducer substrate  102  is disposed over integral screen  201 . Integral screen  201  comprises a holes  202  extending from first side  203  to second side  204  so that mechanical waves emitted from or incident on the transducers  106  can traverse the integral screen  201  without significant interference or impedance. Integral screen  201  protects the transducers  106  from debris or other objects that can deleteriously impact the performance of the transducers  106 . Illustratively, the screen  104  comprises the same material as the package substrate  101 . The holes  202  are machined into package substrate  101  by a known laser drilling method. Illustratively, the holes  202  have a diameter of 0.015 in (15 mils). Generally, when using laser drilling techniques to form the holes  107 , the diameter of the holes  107  is substantially the same as the thickness of the package substrate  101 . The holes  202  have the same diameter as holes  107  in screen  104  described above in connection with the representative embodiments of  FIGS. 1A-1B . 
     In a representative embodiment, electrical connections to the transducers  106  are made with wirebonds  110  that connect contacts  111  on the transducer substrate  102  to contacts  112  on the package substrate  101 . As described more fully below, the contacts  112  provide electrical connections to electrical circuitry and components useful in the transmission and reception of signals by the transducers  106 . The contacts  111 ,  112  are provided over the transducer substrate  102  and the package substrate  101 , respectively, by known metallization techniques. Illustratively, the contacts comprise a suitable conductive material such as gold (Au), copper (Cu) or aluminum (Al) or a suitable conductive alloy such as gold-tin alloy. Notably, the contacts  112  are partially covered by the cover  103  as can be appreciated from a review of  FIG. 1A . 
     Connection pads  113  are provided over the package substrate  101 , and vias  114  are provided through the package substrate. As described more fully below, the connection pads  113  contact connections pads on a circuit substrate (not shown in  FIG. 2A ) for securing the MEMS transducer device  100  thereto. In an embodiment, the cover  103  is disposed at least partially over the connection pads  113  and vias  114 . Illustratively, one or both of the connection pads  113  are connected by the vias  114  to a shield  117  to ensure properly grounding and to avoid “floating” grounds. 
       FIG. 2B  is an isometric exploded view of MEMS transducer device  200 , according to a representative embodiment. As  FIG. 2B  presents a different perspective of the MEMS transducer device  200  described above, details of many common aspects provided in the description of  FIG. 2A  are not repeated. 
     Transducer substrate  102  comprises cavities  115  aligned with respective transducers  106  (not visible in  FIG. 2B ). The cavities  115  provide a path for mechanical waves to and from the transducers  106 . The cavities  115  are also aligned over the opening  105  in the package substrate  101  and the screen  104 . The cavities  115  have a comparatively high-aspect ratio, and are formed by a known method such as dry reactive ion etching (DRIE), the so-called “Bosch method.” Many of the commonly-owned references incorporated above provide details of the fabrication of the cavities  115  and are not generally repeated herein. 
     Cover  103  comprises a cavity  116  into which the transducer substrate  102  is disposed. As such, once assembled, the cover  103  encloses the transducer substrate  102 . The cover  103  provides protection of the transducers  106  from debris and moisture. Furthermore, the depth of the cavity  116  is selected to provide an acoustic backplane for the transducers  106 . Beneficially, the acoustic backplane fosters frequency stabilization of mechanical waves emanating from the transducers  106 . 
     In a representative embodiment, shield  117  is provided over the first side  203  of the package substrate  101 . The shield  117  illustratively comprises a metal or metal alloy and is printed on the package substrate  101  by a known technique. The shield  117  provides a ground plane and prevents stray electromagnetic signals (e.g., RF signals) from adversely interfering with the operation of the transducers  106 . 
       FIG. 3  is a cross sectional view of MEMS transducer device  100  mounted in a substrate  301  in accordance with a representative embodiment. As will be appreciated by one of ordinary skill in the art, MEMS transducer device  200  depicted in  FIGS. 2A-2B  could be mounted in substrate  301  by techniques described presently. 
     The substrate  301  has an opening having a width “w” as depicted in  FIG. 3 . The circuit traces  302  are electrically connected to contacts  112  of the package substrate  101  so that electrical signals can be transmitted to and from the transducers  106 . The contacts  112  are normally soldered to the circuit traces on the substrate  301 . The connection pads  113  are also soldered to the substrate  301  to mechanically fasten the MEMS transducer device  100  to the package substrate  101 . 
     In a representative embodiment, the substrate  301  is a circuit board (e.g., FR4) having circuit traces  302  disposed over a first side  303  of the substrate  301 . Additionally, electronic components (not shown) and electrical circuitry (not shown) useful in the transmission and reception of signals by the transducers  106  is provided over the first side  303  or over a second side  304  of the substrate  301 , or both. In operation, mechanical waves can be transmitted from the transducers  106  through the screen  104  disposed along a second side  303  of the substrate  301 . Likewise, mechanical waves can be received by the transducers  106  after traveling through the screen  104 . 
     The width “w” of the opening is selected to allow the cover  103  to pass through the opening, but not wide enough for the package substrate  101  to pass through the opening. Moreover, the contacts  112  are located to ensure alignment with circuit traces  302  as needed. The contacts  112  and connection pads  113  allow for surface mounting of the MEMS transducer device  100  with all electrical and mechanical connections to the end application PCB board. Beneficially, no interconnect leads are required to mount the MEMS transducer device  100 , thereby foregoing costly lead forming processes (so-called “trim and form”) during fabrication. Moreover, because the MEMS transducer device  100  is surface mountable, the MEMS transducer device  100  is readily adapted to high volume pick/place (e.g., robot) assembly used to assemble “mass-reflowable” electronic products. Beneficially, the reflowed solder will wet both electrical traces on the substrate  301  and on the package substrate  101  to form electrical connections as required. 
     In alternative embodiments in which a plurality of transducer substrates  102  are provided over a common package substrate  101 , the width “w” of the opening would be wide enough for the common cover or the individual covers to pass through the opening in the substrate  301 , but not wide enough for the common package substrate  101  to pass through the opening. Accordingly, the MEMS transducer device  100  is self-aligned to the substrate  301  and surface mounted thereto. 
     The various components, materials, structures and parameters are included by way of illustration and example only and not in any limiting sense. In view of this disclosure, those skilled in the art can implement the present teachings in determining their own applications and needed components, materials, structures and equipment to implement these applications, while remaining within the scope of the appended claims.