Patent Publication Number: US-7723741-B2

Title: Spacers for packaged microelectronic imagers and methods of making and using spacers for wafer-level packaging of imagers

Description:
This application is a divisional application of application Ser. No. 10/922,192, filed Aug. 19, 2004, now U.S. Pat. No. 7,223,626 which is hereby incorporated herein by reference in its entirety. 

   TECHNICAL FIELD 
   The methods and devices described below are related to packaging microelectronic imagers having solid state image sensors. More specifically, several embodiments of the invention are related to wafer-level packaging of microelectronic imagers by attaching an imager workpiece on one side of a prefabricated spacer and attaching a cover substrate on an opposing side of the spacer. 
   BACKGROUND 
   Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and Personal Digital Assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts. 
   Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect the delicate components and to provide external electrical contacts. 
     FIG. 1  is a schematic view of a conventional microelectronic imager  1  with a conventional package. The imager  1  includes a die  10 , an interposer substrate  20  attached to the die  10 , and a spacer  30  attached to the interposer substrate  20 . The spacer  30  surrounds the periphery of the die  10  and has an opening  32 . The imager  1  also includes a transparent cover  40  over the die  10 . 
   The die  10  includes an image sensor  12  and a plurality of bond-pads  14  electrically coupled to the image sensor  12 . The interposer substrate  20  is typically a dielectric fixture having a plurality of bond-pads  22 , a plurality of ball-pads  24 , and traces  26  electrically coupling bond-pads  22  to corresponding ball-pads  24 . The ball-pads  24  are arranged in an array for surface mounting the imager  1  to a board or module of another device. The bond-pads  14  on the die  10  are electrically coupled to the bond-pads  22  on the interposer substrate  20  by wire-bonds  28  to provide electrical pathways between the bond-pads  14  and the ball-pads  24 . 
   The imager  1  shown in  FIG. 1  also has an optics unit including a support  50  attached to the transparent cover  40  and a barrel  60  adjustably attached to the support  50 . The support  50  can include internal threads  52 , and the barrel  60  can include external threads  62  engaged with the threads  52 . The optics unit also includes a lens  70  carried by the barrel  60 . 
   One aspect of fabricating the imager  1  is forming the spacer  30  and attaching the cover  40  to the spacer  30 . The spacer  30  can be formed by placing an uncured, flowable epoxy onto the interposer substrate  20 . In a typical application, the interposer substrate  20  has a plurality of separate dies  10 , and the spacer  30  is formed as a grid of uncured epoxy on the interposer substrate  20  in the areas between adjacent dies  10 . After depositing the epoxy, the cover  40  is attached to the spacer  30 . The epoxy is then cured to harden the spacer  30  such that it becomes dimensionally stable after enclosing the die  10  between the cover  40  and the interposer substrate  20 . 
   One problem of forming the spacer  30  by stenciling an uncured epoxy on the interposer substrate is that the stenciling process produces a textured surface on the top surface of the spacer  30 . This can lead to leaks between the spacer  30  and the cover  40  through which moisture or other contaminants can enter into the cavity where the image sensor  12  is located. Another problem of forming the spacer  30  by stenciling an uncured epoxy onto the substrate is that the height of the spacer  30  is limited because the epoxy tends to slump after the stencil is removed. This causes the epoxy to flow laterally and occupy a significant percentage of the real estate on the substrate  20 . Additionally, a significant problem of using an uncured epoxy is that the uncured epoxy outgases during the curing cycle after the cover is mounted to the epoxy. Such outgasing can contaminate the compartment and impair or ruin the performance of the die  10 . 
   Another process for forming the spacer  30  is to dispense a small flow of uncured epoxy via a needle-like tube or nozzle between adjacent dies. This process is undesirable because it is difficult to control the flow of the uncured epoxy at the intersections of the grid. The intersections typically have rounded corners that occupy additional real estate on the interposer substrate. Additionally, as with the stencil printing process, the epoxy is cured after the cover  40  is mounted to the spacer  30  such that it outgases into the image sensor compartment. Therefore, processes that dispense an epoxy using needle-like tubes are also undesirable. 
   U.S. Pat. No. 6,285,064 discloses another process in which a preformed adhesive matrix is fabricated in the shape of a wafer. The adhesive matrix has openings in the pattern of the image sensors, and it is formed separately from the wafer. In operation, the adhesive matrix is attached to the wafer such that the openings are aligned with the microlenses, and a cover glass is then attached to the top of the adhesive matrix. The adhesive matrix is subsequently activated by application of light, pressure and/or heat. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional view of a packaged microelectronic imager in accordance with the prior art. 
       FIG. 2  is an exploded cross-sectional isometric view of an imager assembly having a plurality of microelectronic imagers that have been packaged at the wafer level in accordance with an embodiment of the invention. 
       FIG. 3  is a cross-sectional view taken along line A-A of  FIG. 2  illustrating a portion of a spacer for use in wafer-level packaging of microelectronic imagers in accordance with an embodiment of the invention. 
       FIG. 4  is a cross-sectional view illustrating a plurality of microelectronic imagers that have been packaged with the spacer shown in  FIG. 3  in accordance with an embodiment of the invention. 
       FIG. 5  is a cross-sectional view illustrating a portion of a spacer for use in packaging microelectronic imagers in accordance with another embodiment of the invention. 
       FIGS. 6A-6C  are cross-sectional views illustrating a method of wafer-level packaging of microelectronic imagers in accordance with additional embodiments of the invention. 
       FIGS. 7A and 7B  are isometric views illustrating different embodiments of spacers on imaging workpieces in accordance with the invention. 
   

   DETAILED DESCRIPTION 
   A. Overview 
   The following disclosure describes several embodiments of (1) spacers for use in wafer-level packaging of microelectronic imagers, (2) microelectronic imagers including such spacers, (3) methods for wafer-level packaging of microelectronic imagers, and (4) methods for producing or otherwise providing prefabricated spacers for use in microelectronic imagers. Wafer-level packaging of microelectronic imagers is expected to significantly enhance the efficiency of manufacturing imaging devices because a plurality of imagers can be packaged simultaneously using highly accurate and efficient processes developed for packaging semiconductor devices. Wafer-level packaging of microelectronic imagers is also expected to enhance the quality and performance of such imagers because the semiconductor fabrication processes can reliably produce and assemble the various components with a high degree of precision. As such, several embodiments of wafer-level packaging processes for packaging microelectronic imagers and the imagers packaged using such processes disclosed herein are expected to significantly reduce the cost for assembling microelectronic imagers, increase the performance of imaging devices, produce smaller imagers compared to conventional devices, and produce higher quality imagers. 
   One aspect of the invention is directed toward methods of packaging microelectronic imagers. An embodiment of such a method can include providing an imager workpiece having a plurality of imager dies arranged in a die pattern and providing a cover substrate through which a desired radiation can propagate. The imager dies include image sensors and integrated circuitry coupled to the image sensors. The method further includes providing a spacer having a web that includes an adhesive and has openings arranged to be aligned with the image sensors. For example, the web can be a film having an adhesive coating, or the web itself can be a layer of adhesive. The method continues by assembling the imager workpiece with the cover substrate such that (a) the spacer is between the imager workpiece and the cover substrate, and (b) the openings are aligned with the image sensors. The attached web is not cured after the imager workpiece and the cover substrate have both been adhered to the web. As such, the web does not outgas contaminants into the compartments in which the image sensors are housed. 
   Another embodiment of a method for packaging a plurality of imager dies includes forming a spacer having a web including an adhesive and a plurality of openings arranged in a die pattern corresponding to the pattern of individual imager dies on an imager workpiece. This embodiment further includes (a) adhering the imager workpiece to one side of the spacer with the image sensors being aligned with the openings, and (b) adhering a cover substrate to an opposite side of the spacer such that the image sensors are enclosed in individual compartments within the openings in the web. In this embodiment, the web is not cured after the imager workpiece and the cover substrate have both been adhered to the spacer. 
   Still another embodiment of a method in accordance with the invention is directed toward assembling an imager workpiece having a plurality of imager dies arranged in a die pattern with an optically transmissive cover substrate. The individual imager dies include an image sensor and an integrated circuit operatively coupled to the image sensor. This embodiment comprises prefabricating a spacer having a web with a desired thickness to space the imager workpiece apart from the cover substrate by a desired distance, openings arranged in the die pattern, a substantially flat first side, and a substantially flat second side. The web is in a non-flowable state before enclosing the image sensor between the imager workpiece and the cover substrate. This method further includes sealing (a) the imager workpiece to the first side of the web such that individual image sensors are aligned with a corresponding opening in the web, and (b) sealing the cover substrate to the second side of the web opposite the first side to enclose the image sensors between the imager workpiece and the cover substrate. 
   Another aspect of the invention is directed toward producing a spacer for separating the imager workpiece from the cover substrate by desired distance. This method comprises producing a non-flowable film having a first flat surface and a second flat surface spaced apart from the first flat surface by a thickness at least approximately equal to the desired distance between the imager workpiece and the cover substrate. The method continues by forming holes in the non-flowable film in the die pattern. The method can further include coating the film with an adhesive. 
   Additional aspects of the invention are directed toward microelectronic imager assemblies. In one embodiment, a microelectronic imager assembly comprises an imager workpiece having a plurality of imager dies arranged in a die pattern and a cover substrate. The individual imager dies include an image sensor and an integrated circuit operatively coupled to the image sensor. The cover substrate can be an optically transmissive plate, or it can be transmissive to another type of radiation in an operating spectrum of the image sensors. The imager assembly further includes a spacer. In one embodiment, the spacer has an integral web with a first side adhered to the workpiece, a second side spaced apart from the first side by a prefabricated separation distance and adhered to the cover substrate, and openings arranged in the die pattern and aligned with corresponding image sensors. This embodiment of the web includes a material in a cured, non-flowable state before the imager workpiece and the cover substrate are both adhered to the spacer. In another embodiment, the spacer is a prefabricated web including a plurality of cut-edged openings arranged in the die pattern and aligned with the image sensors. The web further includes a first side adhered to the imager workpiece, a second side adhered to the cover substrate, and a thickness that spaces the image sensors apart from the cover substrate by a desired distance. 
   Specific details of several embodiments of the invention are described below with reference to CMOS imagers to provide a thorough understanding of these embodiments, but other embodiments can use CCD imagers or other types of solid state imaging devices. Several details describing structures or processes that are well known and often associated with other types of microelectronic devices are not set forth in the following description for purposes of brevity. Moreover, although the following disclosure sets forth several embodiments of different aspects of the invention, several other embodiments of the invention can have different configurations or different components than those described in this section. As such, it should be understood that the invention may have other embodiments with additional elements or without several of the elements described below with reference to  FIGS. 2-7B . 
   B. Wafer-Level Packaged Microelectronic Imagers 
     FIG. 2  is an exploded, cross-sectional isometric view of an imager assembly  200  in accordance with an embodiment of the invention. In this embodiment, the imager assembly includes an imager workpiece  202 , a spacer  204 , and a cover substrate  206 . The imager workpiece  202  includes a first substrate  210 , a plurality of imager dies  211  having image sensors  212  arranged in a die pattern on the first substrate  210 , and lanes  214  between the image sensors  212 . The spacer  204  includes a web  220  having a first side  222 , a second side  224 , and a plurality of openings  226 . The first side  222  is spaced apart from the second side  224  by a thickness “T” at least approximately equal to a desired separation distance between the imager workpiece  202  and the cover substrate  206 . The openings  226  are arranged in the die pattern so that the image sensors  212  are aligned with corresponding openings  226 . The cover substrate  206  is a plate composed of a material through which a desired radiation for the image sensor  212  can propagate. The cover substrate  206 , for example, can be quartz, glass, or another type of optically transparent material. The cover substrate  206  is generally a second substrate having the same or similar shape as the first substrate  210 . Additionally, the cover substrate  206  is adhered to the second side  224  of the web  220  to enclose the image sensors  212  in corresponding compartments defined by the openings  226 . 
   The web  220  of the spacer can be a prefabricated unit that is constructed separately from the imager workpiece  202  and the cover substrate  206 , or the web  220  can be constructed on one of the imager workpiece or cover substrate  206 . The web  220  is composed of a material that is not cured after the imager workpiece and the cover substrate have both been adhered to the spacer. As such, the spacer  204  is a dimensionally stable component with precise dimensions. 
   The imager assembly  200  illustrated in  FIG. 2  is expected to provide several advantages compared to conventional imaging assemblies having conventional spacers, as shown above with reference to  FIG. 1 . For example, because the web  220  is not cured after the imager workpiece  202  and the cover substrate  206  have both been adhered to the spacer  204 , the web  220  does not outgas contaminants into the openings  226  after the image sensors  212  have been fully enclosed. Additionally, the spacer  204  can be composed of a substantially incompressible material and the openings  226  can be cut into the web  220  such that the spacer  204  has a controlled thickness and the openings  226  have well-defined “cut-edge” sidewalls  228 . The spacer  204  accordingly provides a dimensionally stable and highly accurate interface between the imager workpiece  202  and the cover substrate  206 . Moreover, the second side  226  of the web  220  can be highly planar such that it provides an extremely good seal with the cover substrate  206  to avoid leaks.  FIGS. 3-7B  illustrate several specific embodiments of spacers that provide several of these advantages and additional benefits as set forth below. 
   C. Embodiments of Spacers for Wafer-Level Packaging of Microelectronic Imagers 
     FIG. 3  is a cross-sectional view illustrating a portion of one embodiment of the spacer  204 . In this embodiment, the web  220  includes a film  310  having a first side  312  and a second side  314 . The web  220  further includes a first adhesive  322  on the first side  312  of the film  310 , and a second adhesive  324  on the second side  314  of the film  310 . The embodiment of the spacer  204  illustrated in  FIG. 3  further includes a first release element  332  over the first adhesive  322  and a second release element  334  over the second adhesive  324 . The first and second release elements  332  and  334  can be peeled away from the first and second adhesives  322  and  324 , respectively, to expose the adhesives before attaching the imager workpiece  202  ( FIG. 2 ) and the cover substrate  206  ( FIG. 2 ) to the spacer  204 . 
   The embodiment of the spacer  204  shown in  FIG. 3  can be fabricated by coating a sheet of the film  310  with the first and second adhesives  322  and  324 , and subsequently applying the release elements  332  and  334  to the first and second adhesives  322  and  324 . In another embodiment, the first and second adhesives  322  and  324  can be applied to the first and second release elements  332  and  334 , respectively, and then the assembly of the adhesives and release elements can be rolled onto a sheet of the film  310 . The spacer  204  can then be completed by forming the openings  226 . The openings  226  are generally cut through the release elements, adhesives and the film to form cut edges  340  along the sidewalls  228 . The holes  226  can be cut using a punch/die stamp, a knife-edged stamp or roller, lasers and/or water jets. 
   The film  310  and the adhesives  322  and  324  can be made from several different materials. In one embodiment, the film  310  is a tape that is either in a precured or post-cured state. Suitable tapes include polyimide films, polyester films, ultra-high molecular weight films, PTFE films, and other suitable materials. Such materials are readily available from Tapes II International located in Santa Ana, Calif. The adhesives can be any suitable adhesives used in the semiconductor packaging industry or elsewhere. 
     FIG. 4  is a cross-sectional view illustrating a portion of the imager assembly  200  after the imager workpiece  202  and the cover substrate  206  have been adhered to the spacer  204 . The imager assembly  200  includes a plurality of individual imagers  400  that have an imaging die  211  aligned with an opening  226  in the spacer  204 . In this embodiment, individual imaging dies  211  include an image sensor  212 , an integrated circuit  410  operatively coupled to the image sensor  212 , and external contacts  420  operatively coupled to the integrated circuit  410 . The external contacts  420  illustrated in  FIG. 4  are through-wafer interconnects having external contacts pad  422  at the backside of the first substrate  210 . 
   The imager assembly  200  is assembled by removing the first release element  332  from the first adhesive  322  and aligning the openings  226  with corresponding image sensors  212 . The first adhesive  322  is then adhered to the imager workpiece  202 . The second release element  334  is subsequently removed from the second adhesive  324 , and the cover substrate  206  is adhered to the second adhesive  324 . This process can be reversed such that the second side  314  of the film  310  is adhered to the cover substrate  206  before the first side  312  of the film  310  is adhered to the imager workpiece  202 . 
   The imager assembly  200  is constructed by assembling the imager workpiece  202  and the cover substrate  206  with the web  310  after the web  310  is in a state that does not require subsequent curing. The web  310 , therefore, is not cured after the imager workpiece  202  and the cover substrate  206  have both been adhered to the spacer  204  and the image sensors  212  have been enclosed in the openings  226 . As such, the web  310  is incompressible and/or in an otherwise non-flowable state when the imager workpiece  202  and the cover substrate  206  are both adhered to the spacer  204 . 
   The imager assembly  200  is expected to provide several benefits compared to conventional processes and devices for spacing the cover apart from the image sensors. First, because the web  310  is not cured after sealing the imager workpiece  202  and the cover substrate  206  to the web  310 , the spacer  204  does not significantly outgas into the openings  226  enclosing the image sensors  212 . This is expected to significantly reduce the contaminants and enhance the quality of the imagers  400 . The spacer  204  is also dimensionally stable when the imager workpiece  202  and the cover substrate  206  are attached to the spacer  204 . This is expected to provide highly accurate spacing between the imager workpiece  202  and the cover substrate  206 . Moreover, cutting the web  310  to form the openings is a relatively inexpensive process. 
     FIG. 5  is a cross-sectional view illustrating another embodiment of the cover  204  that can be used in an imager assembly. In this embodiment, the cover  204  includes a film  510  and a backing  520  that carries the film  510 . The film  510  is an adhesive with a first side  512  and a second side  514  spaced apart from the first side  512  by a distance approximately equal to the desired spacing between the imager workpiece  202  and the cover substrate  206 . The film  510  is either formed on the backing  520  by depositing a layer of adhesive in a flowable state and then curing the adhesive before presenting the spacer  204  to the imager workpiece  202 . Alternatively, the film  510  can be formed in a molding procedure separately from the backing  520  and then attached to the backing  520 . The openings  226  can be formed in the film  510  using a stamping or cutting procedure as described above with reference to  FIG. 3 , or the openings  226  can be molded. 
   The embodiment of the cover  204  illustrated in  FIG. 5  can be assembled with the imager workpiece  202  ( FIG. 2 ) and the cover substrate  206  ( FIG. 2 ) by attaching the first side  512  of the web  510  to the imager workpiece  202  such that the openings  226  are aligned with the image sensors  212  ( FIG. 4 ). The backing  520  is then removed from the second side  514  of the web  510 . The backing can be removed by peeling it from the web  510 . For example, the web  510  can be composed of a UV-activated material that responds to ultraviolet radiation such that the backing  520  can be removed from the second side  514  of the web  510 . The cover substrate  206  is attached to the second side  514  of the web  510  after removing the backing  520 . Alternatively, the film  510  can be attached to the cover substrate  206  first, and then the imager workpiece  202  can be attached to the other side of the film  510 . 
     FIGS. 6A-6C  are cross-sectional views illustrating sequential stages of a method for producing a spacer on an imager assembly in accordance with another embodiment of the invention. Referring to  FIG. 6A , this process includes covering the imager workpiece  202  with a layer of web material  600 . The web material  600  can be a photosensitive dry film adhesive or a liquid adhesive in a flowable state. In one embodiment, the adhesive can include photopatternable polydimethylsiloxane (PDMS) that can be activated by an O 2  plasma. Other embodiments can use liquid or dry adhesives that can be rolled, sprayed or applied to the workpiece using other techniques. After depositing the adhesive  600  onto the imager workpiece  202 , the adhesive  600  is patterned using a mask  605  and an appropriate radiation R to cure the exposed portions of the adhesive  600 . 
     FIG. 6B  illustrates a subsequent stage in the method at which a web  610  is formed from the adhesive  600  by removing the unexposed portions of the adhesive  600  to create a plurality of openings  612  aligned with the image sensors  212 . The web  610  and openings  612  together define a spacer  620 . In the case of PDMS, the exposed upper surface of the web  610  is then activated for adhesion using an O 2  plasma. Referring to  FIG. 6C , the cover substrate  206  is then attached to the activated upper surface of the web  610  to enclose the image sensors  212  in the opening  612 . 
   The method described above with reference to  FIGS. 6A-6C  can have several different embodiments. For example, the method can further include cleaning the image sensors  212  after forming the openings  612  to remove any contaminants generated while forming the openings  612 . In another embodiment, the adhesive  600  can be deposited onto the cover substrate  206  and then the openings  612  can be formed in the adhesive  600  to fabricate the spacer  620  on the cover substrate  206  instead of the imager workpiece  202 . It follows that the bottom surface of the web  610  can be activated using an appropriate plasma, and the imager workpiece  202  can be attached to the activated bottom surface of the web  610 . 
     FIGS. 7A and 7B  illustrate different configurations of the spacers for use in imager assemblies in accordance with additional embodiments of the invention. Referring to  FIG. 7A , any of the spacers described above can be a grid  710   a  having openings  712  aligned with image sensors (not shown in  FIG. 7A ) on the imager workpiece  202 . Alternatively,  FIG. 7B  illustrates a different embodiment in which any of the spacers described above with reference to  FIGS. 2-6C  are defined by frames  710   b  surrounding individual image sensors  212  on the imager workpiece  202 . The frames  710   b  can be formed on a backing as described above with reference to  FIG. 5  or a deposition/pattern process as described in  FIGS. 6A-6B . Additionally, the frames  710   b  can be formed on the cover substrate instead of the imager workpiece  202 . One aspect of the frames  710   b  is that the lanes between the image sensors  212  are not covered by the spacers. This is expected to be advantageous for cutting the imager workpiece  202  because there is less material in the lanes between the image sensors  212 . 
   From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.