Patent Publication Number: US-2012043628-A1

Title: Packaged device including a well for containing a die

Description:
BACKGROUND 
     Performance of mechanical devices, such as transducers, may be affected by stress inherent within the device, as well as stress coupled or transferred to the device from the package that houses the device. This is particularly true of comparatively small devices, such as transducers manufactured using micro-electromechanical systems (MEMS) technology. For example, stress may be caused when a die, formed of silicon (Si), for example, has a coefficient of thermal expansion (CTE) different from the CTE of the package in which it is mounted. CTE indicates the rate or proportion of change of a material or structure with respect to changes in temperature. The difference between the die and package CTEs results in varying responses to changes in temperature, both during packaging processes and during operation. Even at the macro-scale, the placement of the MEMS device into a larger system may induce further stress in the MEMS device. 
     One conventional technique to package a die includes a plastic over-mold of a metal lead frame. In this configuration, the die is mounted onto the lead frame using a cured non-conductive adhesive and electrical connections are provided via bonding wires. The outer case of the package is then formed by placing the lead frame in a mold and injecting plastic to completely surround the die. For a MEMS device that requires an air cavity for operation, such as an acoustic transducer, a similar technique is used where plastic is molded around a lead frame without the die attached, leaving metal pads exposed for electrical contact and an opening in the package to enable subsequent inserting, attaching and wirebonding the die. An example of this technique is described by LECLAIR et al. in U.S. patent application Ser. No. 12/609,176, filed Oct. 30, 2009, the contents of which are hereby incorporated by reference. 
       FIG. 1  is a cross-sectional diagram of conventional packaged device  100 . Referring to  FIG. 1 , die  110  is mounted to package  130 , which includes lead frame  134  and plastic over-mold or plastic portion  136 . Bonding wire  105  provides electrical connection between the lead frame  134  and a contact (not shown) on the die  110 . The die  110  is physically connected to the lead frame  134  using a die attach adhesive  115 . Typically, the die attach adhesive  115  is first dispensed on the lead frame  134 , and then the die  110  is positioned and set into the die attach adhesive  115 . The packaged device  100  is then cured at an elevated temperature. The cured die attach adhesive  115  does not prevent package induced stresses from coupling to the die  110 . 
     SUMMARY 
     In a representative embodiment, a packaged device includes a package defining a well having a well top, a die positioned in the well of the package, and a retaining substrate attached to the package over the well top. The retaining substrate holds the die in direct contact with a portion of the package exposed at a well bottom opposite the well top. 
     In another representative embodiment, a packaged device includes a lead frame, a plastic portion molded on the lead frame and defining a well, and a die positioned in the well of the package. A bottom surface of the die directly contacts a top portion of the lead frame exposed at a bottom of the well. The packaged device further includes an adhesive dispensed between at least one sidewall of the well and a corresponding at least one side the die. The adhesive holds the die in direct contact with the top portion of the lead frame exposed at the bottom of the well. 
     In another representative embodiment, a device package includes a lead frame, a plastic portion molded on the lead frame, and a well formed through a first surface of the plastic portion and exposing a portion of the lead frame, where the well contains a micro electro-mechanical system (MEMS) transducer device. The device package further includes a retaining substrate attached to the well, the retaining substrate holding the MEMS transducer device in direct contact with the exposed portion of the lead frame, and a pressure port formed through the lead frame and a second surface of the plastic portion opposite the well. The MEMS transducer device includes a membrane and a back-etched portion substantially aligned with the pressure port and an opening formed through the retaining substrate. 
    
    
     
       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. 
         FIG. 1  is a cross-sectional diagram illustrating a conventional packaged device including a die. 
         FIG. 2A  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, according to a representative embodiment. 
         FIG. 2B  is a top plan view illustrating the packaged device of  FIG. 2A , according to a representative embodiment. 
         FIG. 3A  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, according to a representative embodiment. 
         FIG. 3B  is a top plan view illustrating the packaged device of  FIG. 2A , according to a representative embodiment. 
         FIG. 4  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, according to a representative embodiment. 
         FIG. 5  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, according to another representative embodiment. 
         FIGS. 6A and 6B  are cross-sectional diagrams illustrating a packaged device including a well for containing a die, without a retaining substrate, according to a representative embodiment. 
       The  FIGS. 7A-7F  are top plan views illustrating an open end of a well formed in a package housing a die, according to representative embodiments. 
         FIG. 8A  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, without a retaining substrate, according to a representative embodiment. 
         FIG. 8B  is a top plan view illustrating the packaged device of  FIG. 8A , according to a representative embodiment. 
         FIG. 9A  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, without a retaining substrate, according to a representative embodiment. 
         FIG. 9B  is a top plan view illustrating the packaged device of  FIG. 9A , according to a representative embodiment. 
         FIG. 10  is a cross-sectional diagram illustrating a die consisting of a MEMS transducer, 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.” 
     Packaging configurations are described herein that utilize low stress mounting techniques by placing a die, such as a MEMS transducer, in a well formed in the package. In various embodiments, the die is not physically attached, via adhesive or other means, to the package or to the well formed in the package. Rather, the die is held in position by a retaining substrate covering a top portion or opening of the well containing the die. In various alternative embodiments, rather than being held in position by a retaining substrate, the die is physically attached only to portions (e.g., one or more sidewalls) of the well formed in the package, but not to a top surface of the package in which the well is formed or to a lead frame of the package. Accordingly, stress transferred by the package to the die is reduced, controlled or repositioned. 
       FIG. 2A  is a cross-sectional diagram and  FIG. 2B  is a top plan view illustrating a package housing a die in a package well, according to a representative embodiment. More particularly,  FIG. 2A  is a cross-section of the package taken along line A-A′ shown in  FIG. 2B . 
     Referring to  FIGS. 2A and 2B , packaged device  200  includes die  210  housed within package  230 , which includes lead frame  234  and plastic over-mold or plastic portion  236 . The plastic portion  236  defines a well  231 , which contains the die  210 . The well  231  may be formed, for example, during a molding operation using a transfer mold to define the shape of the plastic portion  236 . A retaining substrate  220  covers the open end or well top of the well  231  in order to hold the die  210  in position within the well  231 . The retaining substrate  220  is attached to the package  230  at the well top of the well  231  via adhesive  227  or other attachment technique. The well  231 , the die  210  and the retaining substrate  220  may be located within a larger package cavity  201  formed in the package  230 . 
     The retaining substrate  220  may be formed of any suitable material, such as metal, plastic, ceramic and/or semiconductor materials, such as Si. For example, the retaining substrate  220  may be formed of the same material as the package  230 , discussed below. In the depicted embodiment, the retaining substrate  220  optionally includes a cut-out or opening  221  substantially centered over the die  210 , which exposes a portion of a top surface of the die  210 . The opening  221  enables access by representative bonding wire  205  to provide electrical connections between the lead frame  234  and contacts (not shown) on the die  210 . Further, the opening  221  may be needed to enable proper operation of the die  210 , for example, when the die  210  is a MEMS transducer, discussed below with reference to  FIG. 10 . Alternative configurations may include no opening  221 , or different sizes and/or shapes of opening  221 , without departing from the scope of the present teachings. For example, the opening  221  may be circular (as shown), square, rectangular, or any geometry suitable to meet desired specifications, as would be apparent to one of ordinary skill in the art. 
     A bottom surface of the die  210  is in direct contact with the lead frame  234  in the package  230  at the closed end or well bottom  235  of the well  231 . More particularly, in the depicted embodiment, the die  210  abuts the top surface of the lead frame  234  to the extent it is exposed at the well bottom  235 . In alternative configurations, the die  210  may contact the top surface of the plastic portion  236  (in addition to or instead of the top surface of the lead frame  234 ) exposed at the well bottom  235 , without departing from the scope of the present teachings. Also, in the depicted embodiment, the die  210  is positioned over optional pressure port  238 , formed through the lead frame  234  and the bottom portion of the plastic portion  236 . 
     The die  210  being in direct contact with the top surface of the lead frame  234  means that there is no intervening layer of adhesive, solder, epoxy or other bonding material holding the die  210  to the lead frame  234 . Rather, the die  210  is secured in place by the mechanical confines of the well  210  and the retaining substrate  220 . Notably,  FIG. 2A  shows representative gaps between the sides of the die  210  and the sidewalls of the well  231 , and between the top surface of the die  210  and the bottom surface of the retaining substrate  220 . These gaps, which are exaggerated for clarity, indicate that the die  210  has limited freedom of movement within well  231  to avoid creation of stress points and otherwise to prevent or reduce stress induced on the die  210  by fluctuations or other changes in the package  230  caused, for example, by thermal expansion, vibration, bending, or the like. 
     The lead frame  234  of the package  230  may be formed from an electrically conductive material, such as various metals and metal alloys, including copper, nickel, aluminum, brass, copper/zinc alloys, and the like, or a combination thereof, for example. The material may be etched or stamped to form separate conductors, terminal leads, and the like, e.g., depending on application specific design requirements of various implementations, as would be apparent to one skilled in the art. The plastic portion  236  may be formed from a non-conductive material, such as various plastics or polymers, including liquid crystal polymer (LCP), polybutylene terephthalate (PBT), polypropylene (PP), polyphthalamide (PPA), and the like, for example. In various embodiments, the plastic portion  236  may include an integrated acoustic horn (not shown), formed over the pressure port  238  using transfer molding or other molding techniques, to support different environmental and operating conditions. Formation of a molded package and integrated acoustic horn is described, for example, by LECLAIR et al. in U.S. patent application Ser. No. 12/609,176, filed Oct. 30, 2009, the contents of which are hereby incorporated by reference. 
     In various embodiments, the die  210  may be a transducer device, such as an ultrasonic MEMS acoustic transducer or piezoelectric MEMS ultrasonic transducer (PMUT), for example, although other types of dies, including various types of semiconductor devices, may be incorporated without departing from the scope of the present teachings. The die  210  may include various materials, such as Si, that differ from the material(s) of the package  230  (and thus have differing CTEs, for example). 
       FIG. 10  is a cross-sectional diagram illustrating an example of die  210 , according to a representative embodiment. In particular,  FIG. 10  depicts MEMS transducer  1010 , which includes transducer substrate  1011 , membrane  1020  and resonator or resonator stack  1030 , where the membrane  1020  and the resonator stack  1030  form an active transducer, e.g., over cavity  1015  formed through a backside of the transducer substrate  1011 . In the depicted embodiment, the membrane  1020  is formed of a single layer of membrane material, although the membrane  1020  may have multiple layers without departing from the scope of the present teachings. 
     The resonator stack  1030  includes first electrode  1031  disposed over a portion of the membrane  1020 , and piezoelectric layer  1035  and second electrode  1032  stacked on the first electrode  1031 . The piezoelectric layer  1035  may be formed from aluminum nitride, lead zirconate titanate (PZT), or other film compatible with semiconductor processes. The first and second electrodes  1031 ,  1032  may be formed from a metal compatible with semiconductor processes, such as molybdenum, tungsten, aluminum or a combination thereof. 
     The resonator stack  1030  is shown as an annular resonator, where the cross-section is taken across the center. The annular resonator stack  1030  may be substantially circular in shape, for example, although it may be formed in different shapes, such as ovals, squares, rectangles, or the like, without departing from the scope of the present teachings. Further, in various embodiments, the resonator stack  1030  need not have an annular shape, but may simply be a solid resonator stack on the substrate  1011 . The resonator stack  1030  is substantially centered over the cavity  1015 , enabling mechanical movement of the membrane  1020  and/or the resonator stack  1030 . 
     The transducer substrate  1011  may be formed of various types of materials compatible with semiconductor processes, such as Si, gallium arsenide (GaAs), indium phosphide (InP), glass, sapphire, alumina, or the like, which is useful for integrating connections and electronics, thus reducing size and cost. The cavity  1015  formed through the transducer substrate  1011  may be substantially the same shape as the resonator stack  1030 , e.g., circular, although it may have any of a variety of sizes and shapes, such as oval, square, rectangular, or the like, without departing from the scope of the present teachings. The cavity  1015  may be obtained by back side etching the bottom surface of the transducer substrate  1011 , which may include a dry etch process, such as a Bosch process, for example, although various alternative techniques may be incorporated. Formation of the transducer substrate  1011  and the resonator stack  1030  (on a membrane) is described, for example, by MARTIN et al. in U.S. patent application Ser. No. 12/495,443, which is hereby incorporated by reference. 
       FIG. 3A  is a cross-sectional diagram and  FIG. 3B  is a top plan view illustrating a package housing a die in a package well, according to a representative embodiment. More particularly,  FIG. 3A  is a cross-section of the package taken along line B-B′ shown in  FIG. 3B . 
     Referring to  FIGS. 3A and 3B , packaged device  300  includes die  210  housed within package  330 , which includes lead frame  334  and plastic over-mold or plastic portion  336 . The plastic portion  336  defines a well  331 , which contains the die  210 . A retaining substrate  320  covers the open end or well top of the well  331  in order to hold the die  210  in position within the well  331 . As discussed above, the bottom surface of the die  210  is in direct contact with a top surface of the lead frame  334  exposed at the closed end or well bottom  335  of the well  331 , in that there is no adhesive, solder, epoxy or other bonding material securing the die  210  directly to the top surface of the lead frame  334 . Also, in the depicted embodiment, the die  210  is positioned over (optional) pressure port  338  formed through the lead frame  334  and the plastic portion  336 . 
     The packaged device  300 , including the associated components and materials, of  FIGS. 3A and 3B  is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that retaining substrate  320  is larger than the retaining substrate  220 , and thus fits substantially the entire package cavity  301 . Accordingly, the retaining substrate  320  may be used for self-alignment purposes during assembly. Also, the retaining substrate  320  provides more area for bonding, e.g., through adhesive attachment, of the retaining substrate  320  to the package  330 . For example, in order to support the entire retaining substrate  320 , the plastic portion  336  may be built up throughout the package cavity  301 , rather than just at the location of the well  331 . Accordingly, the plastic portion  336  includes additional openings to expose desired portions of the lead frame  334 , indicated by representative lead frame openings  332 ,  333  and  334 . Thus, in various embodiments, the retaining substrate  320  is attached to the package  330  via adhesive  327  applied throughout the package cavity  301 , and not just surrounding the well  331 . 
     The retaining substrate  320  includes opening  321  substantially centered over the die  210 , which exposes a portion of a top surface of the die  210 , as discussed above with reference to the opening  221 . Also, the retaining substrate  320  includes additional representative openings  322 ,  323  and  324  corresponding to the lead frame openings  332 ,  333  and  334  to expose the portions of the lead frame  334  for wirebonding or other purposes. Of course, alternative configurations may include no openings  321 - 324 , or different numbers, sizes and/or shapes of openings  321 - 324 , without departing from the scope of the present teachings. 
     Various embodiments incorporate alternative means for securing a die within a package well using a retaining substrate, in order to enhance contact between the die and the package (e.g., the lead frame), while maintaining sufficient freedom of movement of the die to avoid creation of stress points and otherwise to prevent or reduce stress induced by the package onto the die.  FIGS. 4 and 5  provide examples of various embodiments, discussed below. For simplicity of explanation,  FIGS. 4 and 5  show only portions of the packaged devices surrounding the corresponding package wells, in which the dies are housed, respectively. 
       FIG. 4  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, according to a representative embodiment. More particularly, packaged device  400  includes die  210  housed within package  430 , which includes lead frame  434  and plastic over-mold or plastic portion  436 . 
     The plastic portion  436  defines a well  431 , which contains the die  210 . A retaining substrate  420  covers the open end or well top of the well  431  in order to hold the die  210  in position within the well  431 . As discussed above, the bottom surface of the die  210  is in direct contact with a top surface of the lead frame  434  at the closed end or well bottom  435  of the well  431 , in that there is no adhesive, solder, epoxy or other bonding material securing the die  210  directly to the top surface of the lead frame  434 . Also, in the depicted embodiment, the die  210  is positioned over (optional) pressure port  438  formed through the lead frame  434  and the plastic portion  436 . 
     The packaged device  400  of  FIG. 4 , including the associated components and materials, is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that retaining substrate  420  includes a projection over the die  210  that creates notch  420   a , which has a shape complementary to the top edges of the die  210 . The retaining substrate  420  thus provides additional positioning control through alignment of the die  210  within the notch  420   a . For example, manufacturing processes may provide greater dimensional precision with respect to forming the notch  420   a  and opening  421  in the retaining substrate  420 , than with respect to forming the well  431  in the plastic portion  436 . In this case, incorporation of the notch  420   a  better controls positioning within desired specifications, while still maintaining some freedom of movement of the die  210 . The opening  421  may be formed in the retaining substrate  420 , if needed, to expose a portion of a top surface of the die  210 , as discussed above with reference to the opening  221 . The retaining substrate  420  is attached to the plastic portion  436  at the open end of the well  431  via adhesive  427 . 
       FIG. 5  is a cross-sectional diagram illustrating a packaged device including a well for containing a die, according to another representative embodiment. More particularly, packaged device  500  includes die  210  housed within package  530 , which includes lead frame  534  and plastic over-mold or plastic portion  536 . 
     The plastic portion  536  defines a well  531 , which contains the die  210 . A retaining substrate  520  covers the open end or well top of the well  531  in order to hold the die  210  in position within the well  531 . As discussed above, the bottom surface of the die  210  is in direct contact with a top surface of the lead frame  534  at the closed end or well bottom  535  of the well  531 , in that there is no adhesive, solder, epoxy or other bonding material securing the die  210  directly to the top surface of the lead frame  534 . Also, in the depicted embodiment, the die  210  is positioned over (optional) pressure port  538  formed through the lead frame  534  and the plastic portion  536 . 
     The packaged device  500  of  FIG. 5 , including the associated components and materials, is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that retaining substrate  520  includes one or more stand-offs, indicated by representative stand-offs  520   a  and  520   b , which extend from a bottom surface of the retaining substrate  520  to contact the top surface of the die  210 . The stand-offs  520   a  and  520   b  may be separate extensions or pillars that contact the die  210  at corresponding discrete locations. Alternatively, the stand-offs  520   a  and  520   b  may form a single, annular extension that contacts the dies along its entire circumference. Of course, the number, shapes and arrangement of the stand-offs  520   a  and  520   b  may vary without departing from the scope of the present teachings. 
     In an embodiment, the stand-offs  520   a  and  520   b  are formed of the same material as the retaining substrate  520 , and thus may be integral with the retaining substrate  520 . However, the stand-offs  520   a  and  520   b  may be formed of any suitable material and/or may be formed separately from the retaining substrate  520 . For example, in an alternative embodiment, the stand-offs  520   a  and  520   b  are formed of a compressible material, such as foam (e.g., weather stripping), rubber, specially designed plastic or metal spring structures, and plastic or retaining cantilevers, or the like, attached to the retaining substrate  520 . The compressible material applies constant pressure to the die  210  toward the top surface of the lead frame  534  exposed at the well bottom  535  to hold the die  210  in position, but does not allow for the transference of stress from the package  530  to the die  210 . 
     The retaining substrate  520  thus provides additional positioning control by exerting downward pressure on the die  210 , or otherwise further confining movement of the die  210  in a direction parallel to the side walls of the well  531 , by virtue of the stand-offs  520   a  and  520   b . For example, the stand-offs  520   a  and  520   b  may compensate for added thickness resulting from application of the adhesive  527  between the retaining substrate  520  and the plastic portion  536 . An opening  521  may be formed in the retaining substrate  520 , if needed, to expose a portion of the top surface of the die  210 , as discussed above with reference to the opening  221 . 
     Various alternative embodiments enable securing a die within a package well without use of a retaining substrate, while reducing and/or controlling stress induced by the package onto the die.  FIGS. 6A-6B ,  7 A- 7 F,  8 A- 8 B and  9 A- 9 B provide examples of various embodiments, discussed below. For simplicity of explanation,  FIGS. 6A-6B ,  7 A- 7 F,  8 A- 8 B and  9 A- 9 B show only portions of the package devices surrounding the corresponding package wells, in which the dies are housed, respectively. 
       FIGS. 6A and 6B  are cross-sectional diagrams illustrating a packaged device including a well for containing a die, without a retaining substrate, according to a representative embodiment. More particularly, packaged device  600  includes die  210  housed within package  630 , which includes lead frame  634  and plastic over-mold or plastic portion  636 . 
     The plastic portion  636  defines a well  631 , shown in  FIG. 6A , which contains the die  210 . As discussed above, the bottom surface of the die  210  is in direct contact with a top surface of the lead frame  634  at the closed end or well bottom  635  of the well  631 , in that there is no adhesive, solder, epoxy or other bonding material securing the die  210  directly to the top surface of the lead frame  634 . Also, in the depicted embodiment, the die  210  is positioned over (optional) pressure port  638  formed through the lead frame  634  and the bottom surface of the plastic portion  636 . 
     The packaged device  600  of  FIGS. 6A and 6B , including the associated components and materials, is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that there is no retaining substrate covering the open end of the well  631 . Thus, the well  631  is employed to align the die  210  in the package  630 . In order to hold the die  210  in position within the well  631 , adhesive  627  is dispensed at the open end of the well  631  over gaps between the sides of the die  210  and corresponding sidewalls of the well  631 , as shown in  FIG. 6A . The adhesive  627  is uncured when initially dispensed, and therefore descends into the gaps between the sides of the die  210  and the sidewalls of the well  631 , indicated by descended adhesive  627   a  shown in  FIG. 6B . When subsequently cured, the adhesive  627 ,  627   a  holds the die  210  in position within the well  631  and in contact with the package  630 , thus restricting movement of the die  210  without the bottom surface of the die  210  being physically attached or bonded to the lead frame  634 . In the depicted embodiment, since there is no retaining substrate, the top surface of the die  210  is exposed, e.g., enabling connection of the bonding wires (not shown in  FIGS. 6A and 6B ). 
     Without the retaining substrate, the adhesive may be selectively applied in various locations in the well to help control transfer of stress from the package to the die. The  FIGS. 7A-7F  are top plan views illustrating an open end of a well formed in a package housing a die, according to representative embodiments. More particularly,  FIGS. 7A-7F  show examples of alternative adhesive dispensing geometries enabled by the well structure. 
     Referring to  FIGS. 7A-7F , packaged device  700  includes die  210  positioned within well  731 , which is formed in plastic portion  736  of package  730 . The packaged device  700 , including the associated components and materials, is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that there is no retaining substrate covering the open end of the well  731 , and thus adhesive is applied to hold the die  210  in position. 
     In  FIG. 7A , adhesive  727   a  is applied substantially uniformly around the perimeter of the die  210  within the well  731 , which is similar to the application of adhesive  627   a  discussed above with reference to  FIG. 6B . In other words, the adhesive  727   a  is dispensed between the sides of the die  210  and the sidewalls of the well  731 . This substantially uniform application of the adhesive  727   a  results in relatively even distribution of stress induced by the package  730  on the die  210 . Thus, the die  210  maintains substantially the same relative position with respect to the package  730  and/or the well  731 . 
     In  FIG. 7B , adhesive  727   b  is applied only on one edge of the die  210 . Therefore, the adhesive  727   b  is dispensed between only one side of the die  210  and one sidewall of the well  731 . The die  210  is therefore connected by the adhesive  727   b  on only one of its four sides, as well as along one top edge to the extent the adhesive  727   b  overlaps the top surfaces of the die  210  and the well  731  (indicated by the oval shape of the adhesive  727   b ). This non-uniform application of the adhesive  727   b  results in stress induced by the package  730  on the die  210  having an asymmetric stress profile. This allows the die  210  to expand or shrink unconstrained in a single direction, while the orthogonal direction is constrained by the adhesive. 
     In  FIG. 7C , adhesive  727   c  is applied to each of the four corners of the die  210 . Therefore, the adhesive  727   c  is dispensed between the corners of the die  210  and corresponding corners of the sidewalls of the well  731 . The die  210  is therefore connected by the adhesive  727   c  along each edge running parallel to the sidewalls of the well  731 , as well as on the corners of the top surface of the die  210  to the extent the adhesive  727   c  overlaps the top surfaces of the die  210  and the well  731  (indicated by the circular shapes of the adhesive  727   c ). Because the adhesive  727   c  is applied at only the four corners, stress induced by the package  730  has less effect on the die  210  due to a relatively weak coupling. 
       FIGS. 7D-7F  are variations of the arrangement depicted in  FIG. 7C , where adhesive is applied to fewer than all four corners of the die  210 , resulting in non-uniform distribution of stress induced by the package  730  to the die  210 . The various configurations allow for expansion or contraction due to CTE to propagate unconstrained in vertical axis, diagonal axis, or all axes, respectively. In  FIG. 7D , adhesive  727   d  is applied to two adjacent corners of the die  210 . Therefore, the adhesive  727   d  is dispensed between these corners and the corresponding corners of the sidewalls of the well  731 . In  FIG. 7E , adhesive  727   e  is applied to two opposite corners of the die  210 . Therefore, the adhesive  727   e  is dispensed between these corners and the corresponding corners of the sidewalls of the well  731 . In  FIG. 7F , adhesive  727   f  is applied to only one corner of the die  210 . Therefore, the adhesive  727   f  is dispensed between only this corner and the corresponding corner of the sidewalls of the well  731 . These non-uniform applications of the adhesive  727   d ,  727   e  and  727   f  result in stress induced by the package  730  on the die  210  having a skewing, tilting or other non-uniform effect on the relative position of the die  210  with respect to the package  730  and/or the well  731 . 
     In other embodiments, the well itself may have features that direct and contain adhesive in predetermined locations. That is, the sidewalls of the well formed in the package may include cambers with notches, protrusions or other features that help align and secure the die within the well, and/or to reduce stress induced from the package to the die. 
     For example,  FIG. 8A  is a cross-sectional diagram and  FIG. 8B  is a top plan view illustrating a packaged device including a well having adhesive containment features for containing a die, without a retaining substrate, according to a representative embodiment. More particularly,  FIG. 8A  is a cross-section of the package taken along line C-C′ shown in  FIG. 8B . 
     Referring to  FIGS. 8A and 8B , packaged device  800  includes die  210  housed within package  830 , which includes lead frame  834  and plastic over-mold or plastic portion  836 . Also, the die  210  may be positioned over (optional) pressure port  838  formed through the lead frame  834  and the plastic portion  836 . The plastic portion  836  defines a well  831 , which contains the die  210 . As discussed above, the bottom surface of the die  210  is in direct contact with a top surface of the lead frame  834  of the package  830  at the closed end or well bottom  835  of the well  831 . The packaged device  800  of  FIGS. 8A and 8B , including the associated components and materials, is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that there is no retaining substrate covering the open end of the well  831  and the well  831  includes containment features. 
     In the depicted embodiment, protrusions  829  extend from each of the sidewalls of the well  831 , creating pockets  828  at each corner of the die  210  to contain the adhesive  827 . The protrusions  839  physically align the die  210  within the well  831  of the package  830 . The adhesive  827  is dispensed in the pockets  828 , as shown in  FIG. 8B , in order to hold the die  210  in position within the well  831  and in contact with the package  830 , without being physically attached or bonded to the lead frame  834 . In the depicted embodiment, since there is no retaining substrate, the top surface of the die  210  is exposed, e.g., enabling connection of the bonding wires (not shown in  FIGS. 8A and 8B ). 
     Similarly,  FIG. 9A  is a cross-sectional diagram and  FIG. 9B  is a top plan view illustrating a packaged device including a well having adhesive containment features for containing a die, without a retaining substrate, according to a representative embodiment. More particularly,  FIG. 9A  is a cross-section of the package taken along line D-D′ shown in  FIG. 9B . 
     Referring to  FIGS. 9A and 9B , packaged device  900  includes die  210  housed within package  930 , which includes lead frame  934  and plastic over-mold or plastic portion  936 . Also, the die  210  may be positioned over (optional) pressure port  938  formed through the lead frame  934  and the bottom surface of the plastic portion  936 . The plastic portion  936  defines a well  931 , which contains the die  210 . As discussed above, the bottom surface of the die  210  is in direct contact with a top surface of the lead frame  934  of the package  930  at the closed end or well bottom  935  of the well  931 . The packaged device  900  of  FIGS. 9A and 9B , including the associated components and materials, is similar to the packaged device  200  of  FIGS. 2A and 2B , discussed above, except that there is no retaining substrate covering the open end of the well  931  and the well  931  includes containment features. 
     In the depicted embodiment, notches  939  are formed at each of the corners of the well  931 . The adhesive  927  is dispensed within the notches  929  and along the sidewalls of the well  931 , e.g., as discussed above with reference to  FIGS. 6A and 6B , in order to hold the die  210  in position within the well  931  and in contact with the package  930 , without being physically attached or bonded to the top surface of the lead frame  934 . In the depicted embodiment, the notches  939  are relatively shallow in comparison to the depth of the well  931  and substantially circular, although the notches  929  may have different sizes and/or shapes without departing from the scope of the present teachings. Also, since there is no retaining substrate, the top surface of the die  210  is exposed, e.g., enabling connection of the bonding wires (not shown in  FIGS. 9A and 9B ). 
     The packaged device having a well for containing a die may be fabricated and assembled according to various techniques, e.g., compatible with semiconductor processes. For example, referring again to  FIGS. 2A and 2B , the lead frame  234  may be etched to provide a desired pattern of conductors, terminal leads and other features, e.g., as shown in  FIG. 2B . The etching may include chemical etching using photolithography, for example, although various alternative techniques may be incorporated. The etched lead frame  234  may be plated for wirebonding, for example, using an optimized plating material, such as nickel and/or gold, to permit gold or aluminum bonding wire attachment. The bonding wires, indicated by bonding wire  205 , provide electrical interconnections from the die  210  to the lead frame  234 . 
     A molding operation is performed on the lead frame  234  to form plastic portion  236  on and around the lead frame  234 . The molding operation may include placing the lead frame  234  in a transfer mold previously formed to define the shape of the plastic portion  236 , including formation of the well  231 . A polymer, e.g., LCP, PBT, PP, or PPA, is then transfer molded, for example, to encapsulate the lead frame  234  and simultaneously to form the well  231 . The polymer is typically a solid at room temperature, and melted prior to transfer to the mold. The shape of the well, including various protrusions or notches, e.g., as discussed above with reference to  FIGS. 8A-8B  and  9 A- 9 B, is defined by the shape of the machined transfer mold. The cooled (after melting) mold plastic will assume the desired shape within the transfer mold. Accordingly, the plastic portion  236  and the well  231  are integrally formed to surround the lead frame  234  during the molding operation. 
     The die  210  is then inserted into the well  231 , such that a bottom surface of the die  210  is in direct contact with the top surface of the lead frame  234  at the closed end or well bottom  235  of the well  231 , as discussed above. Adhesive  227  is applied to the open end or well top of the well  231 , and the retaining substrate  220  is placed on the adhesive  227  to attach the retaining substrate  220  to the plastic portion  236  via the open end of the well  231 . The adhesive  227  may then be cured at an elevated temperature. In alternative embodiments, the retaining substrate  220  may be attached using various techniques, such as epoxy bonding, soldering, ultrasonic welding, and the like. The retaining substrate  220  may be previously fabricated to include the opening  221 , which is substantially centered over the die  210 . 
     Wirebonding may then be performed, during which representative bonding wire  205  is connected between pads (not shown) on the die  210  and the conductor pattern of the lead frame  234  via lead terminals (not shown). The pads on the die  210  may be top pads, for example, electrically connected to the top electrodes of an acoustic transducer (e.g., as shown in  FIG. 10 ) of the die  210 . The die  210  may be previously fabricated for attachment to the lead frame  234 . A lid (not shown) may be attached over the package cavity  201 . The lid may be previously formed, for example, using a molding process similar to the transfer molding process of the plastic portion  236 , described above. In an embodiment, the lid may be mechanically attached to the plastic portion  236  by press fitting, for example, or using an epoxy adhesive, for example, creating a hermetically sealed environment. 
     According to various embodiments, a well formed in a package precisely positions a die within the package, while reducing and controlling stress induced on the die from the package. For example, the well enables the die to be securely held in place without rigid adhesive connecting the die to a surface of the package lead frame. Also, the package well enables placement of adhesive, e.g., on all or portions of sidewalls of the well, to control manner in which stress is transferred to the die. 
     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.