Abstract:
A method includes bonding a first side of a metal shim to a silicon shim, removing metal from the metal shim to form a plurality of cleared metal lanes in accordance with a pattern, bonding a readout integrated circuit having a plurality of saw lanes in accordance with the pattern to a second side of the metal shim to form a wafer assembly wherein the plurality of saw lanes is aligned with the plurality of cleared metal lanes, and dicing the wafer assembly.

Description:
TECHNICAL FIELD 
       [0001]    The exemplary and non-limiting embodiments of this invention relate generally to a method and apparatus for reducing thermal mismatches in wafer assemblies. 
       BACKGROUND 
       [0002]    With reference to  FIG. 1 , there are illustrated the main components of an infrared sensor chip assembly  10  (SCA) as follows. A detector  11  (such as a CdZnTe detector  11 ) is coupled to a readout IC (ROIC)  13  via one or more indium interconnects (bumps)  15 . The ROIC  13  is coupled to a platform  21  which is coupled to a coldfinger  25  via an endcap  23 . The indium interconnects  15  are used to join the detector  11  to the ROIC  13 . These indium interconnects  15  take the signals generated by the detector  11  pixels (not shown) and transfer them to the signal processing circuits contained in the ROIC  13 . The hybridized detector  11 /ROIC  13  may then be attached directly to the platform  21 , such as by being adhesively attached. 
         [0003]    During operation, SCA  10  is typically cooled to cryogenic temperatures (e.g., 70 K). As the SCA  10  is cooled to such temperatures, problems can arise when different components of the SCA  10  contract at different rates. For example, if the detector  11  is formed of CdZnTe, it will likely contract approximately four times faster than the silicon ROIC  13  to which it is coupled. This differential in contraction strains the indium interconnects  15 , particularly at the corners of the detector  11 . As the SCA  10  is cycled from ambient to operating temperatures, the indium interconnects  15  can experience substantial fatigue. If the strains experienced by the indium interconnects  15  are excessive, they may fail due to fatigue, usually starting at the corners of the detector  11  where the displacement differential is typically most severe. 
         [0004]    One attempt to address this problem is the production of Glued REAdout to Platform (GREATOP) SCAs  10 . As used herein “GREATOP” refers to the configuration of SCAs formed, generally, in accordance with the disclosures of U.S. Pat. Nos. 5,672,545 and 5,308,980. GREATOP SCAs  10  utilize a balanced silicon readout/titanium shim/silicon shim wafer assembly  27  to force the contraction of the ROIC  13 , formed of silicon, to match that of the detector  11 , which may be formed of CdZnTe. 
         [0005]    With reference to  FIG. 2 , there is illustrated a cross section of a GREATOP SCA  10  showing the material expansion mismatches. The titanium shim  17  of the wafer assembly  27  has a thermal contraction rate that is approximately seven times greater than that of silicon. There is illustrated a vertical baseline indicating the alignment of components prior to contraction. As is evident from the horizontal, left pointing arrows, the displacements of the components experienced by unconstrained contraction vary considerably. The relatively high titanium contraction rate forces the silicon ROIC  13  to contract faster than it would by itself. 
         [0006]    With reference to  FIG. 3 , there is illustrated a cross section of a GREATOP SCA  10  wherein the thickness of the titanium shim  17  is chosen so that the contraction of the silicon ROIC  13  approximately matches that of the CdZnTe detector  11 . As noted above, this arrangement can greatly reduce, or theoretically eliminate, indium interconnect fatigue failure. The wafer assembly  27  may be further balanced by incorporating a silicon shim  19  below the titanium shim  17  to create an axial symmetry which prevents the GREATOP-SCA  10  structure from bowing or warping. 
         [0007]    The thermal mismatch between a detector  11  formed of InSb and an ROIC  13  is comparable to that of a detector  11  formed of CdZnTe. Historically, RVS InSb detectors  11  have not utilized a thermal shim concept because the indium interconnects  15  are wicked with adhesive during production. The adhesive wicked in between the detector  11  and ROIC  13  prevents de-hybridization at cooldown. While de-hybridization is not a problem for SDA, the thermal mismatch between the InSb detector  11  and Silicon ROIC  13  causes a substantial amount of detector cracking (up to a 15% yield loss) at initial cooldown. 
         [0008]    In order to implement the GREATOP concept on existing products utilizing InSb detectors  11 , the entire height of the wafer assembly  27  is designed to be approximately 0.0185 inches, that is, the height of current SDA silicon ROICs  13 . In response to this constraint, a wafer assembly  27  structure has been devised of the materials illustrated in  FIG. 4 . With reference to  FIG. 5 , there is illustrated an InSb detector  11  showing the maximum principal stress in the InSb detector  11  when it is cooled from 300K to 77K on a leadless chip carrier (LCC). As shown, the maximum principal stress ranges to a high of ˜5,300 psi. 
         [0009]    With respect to  FIG. 6 , there is illustrated the field stress in an InSb detector  11  formed according to the structure illustrated in  FIG. 4  and incorporating a reduced thickness titanium shim  17  when it is cooled from 300K to 77K on a leadless chip carrier (LCC). As shown, reducing the thickness of the titanium shim  17  reduces the maximal principal stress to approximately 0 psi. Unfortunately, constructing a sandwich  27  with a reduced titanium shim  17  thickness yields various manufacturing challenges. One significant challenge is maintaining detector flatness while bonding and curing the very thin wafer assembly  27 . 
       SUMMARY 
       [0010]    The foregoing and other problems are overcome, and other advantages are realized, in accordance with the exemplary embodiments of these teachings. 
         [0011]    In accordance with an exemplary embodiment of the invention, a method includes bonding a first side of a metal shim to a silicon shim, removing metal from the metal shim to form a plurality of cleared metal lanes in accordance with a pattern, bonding a readout integrated circuit having a plurality of saw lanes in accordance with the pattern to a second side of the metal shim to form a wafer assembly wherein the plurality of saw lanes is aligned with the plurality of cleared metal lanes, and dicing the wafer assembly. 
         [0012]    In accordance with an exemplary embodiment of the invention, a wafer assembly includes a metal shim having a first side comprising a plurality of cleared metal lanes arranged in accordance with a pattern and a second side, a silicon shim coupled to the second side of the metal shim, and a readout integrated circuit having a plurality of saw lanes arranged in accordance with the pattern coupled to the first side of the metal shim to form a wafer assembly wherein the plurality of saw lanes is aligned with the plurality of cleared metal lanes. 
         [0013]    In accordance with an exemplary embodiment of the invention, an apparatus includes an infrared sensor assembly, a wafer assembly coupled to the infrared sensor assembly including, a 17-7 stainless steel shim having a first side and a second side, a silicon shim coupled to the second side of the metal shim, and a readout integrated circuit coupled to the first side of the metal shim. 
         [0014]    In accordance with an exemplary embodiment of the invention, an apparatus includes an element for detecting infrared radiation, and an element for coupling the detecting element to a wafer assembly including a 17-7 stainless steel shim having a first side and a second side, a silicon shim coupled to the second side of the metal shim, and a readout integrated circuit coupled to the first side of the metal shim. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The foregoing and other aspects of the exemplary embodiments of this invention are made more evident in the following Detailed Description, when read in conjunction with the attached Drawing Figure, wherein: 
           [0016]      FIG. 1  is a diagram of a sensor chip assembly (SCA) known in the art. 
           [0017]      FIG. 2  is a cross-section diagram of the SCA of  FIG. 1 . 
           [0018]      FIG. 3  is a cross section diagram of the SCA of  FIG. 1  showing the effects of cooling. 
           [0019]      FIG. 4  is an illustration of the materials used to form the SCA of  FIG. 1 . 
           [0020]      FIG. 5  is a perspective view of an InSb detector known in the art showing the maximum principal press generated when cooled from 300K to 77K. 
           [0021]      FIG. 6  is a perspective view of an InSb detector showing field stress when cooled from 300K to 77K. 
           [0022]      FIG. 7  is a perspective view of an ANSYS® half-symmetry model of a SCA incorporating a Wafer Coefficient of Thermal Expansion Matching (WACTEM) wafer assembly according to an exemplary embodiment of the invention. 
           [0023]      FIG. 8  is a perspective view of an ANSYS® half-symmetry model of a SCA incorporating a WACTEM wafer assembly according to an exemplary embodiment of the invention showing the exemplary component materials. 
           [0024]      FIG. 9  is a table of material specifications for the exemplary embodiment of a WACTEM wafer assembly of  FIG. 8 . 
           [0025]      FIG. 10  is a perspective view of the maximum principal stress on a InSb detector surface. 
           [0026]      FIG. 11  is an illustration of a stress line across the InSb detector surface of  FIG. 10 . 
           [0027]      FIG. 12  is an X-Y plot of the maximum principal stress along the stress line of  FIG. 11 . 
           [0028]      FIG. 13  is a perspective view of a contour plot of maximum principal stress of a SCA incorporating an exemplary embodiment of a WACTEM wafer assembly of the invention. 
           [0029]      FIG. 14  is an X-Y plot of the maximum principal stress along a detector surface plot path of the SCA of  FIG. 13 . 
           [0030]      FIG. 15  is a cross-section view of 2D ANSYS® model of the SCA of  FIG. 13 . 
           [0031]      FIG. 16  is an X-Y plot of the maximum principal stress along a detector surface plot path of the SCA of  FIG. 15 . 
           [0032]      FIG. 17  is a cross-section view of 2D ANSYS® model of a SCA according to an exemplary embodiment of the invention showing a stainless steel undercut. 
           [0033]      FIG. 18  is a cross-section view of a WACTEM wafer assembly according to an exemplary embodiment of the invention. 
           [0034]      FIG. 19  is a flow chart of a method according to an exemplary embodiment of the invention. 
           [0035]      FIG. 20  is an X-Y plot of the maximum principal stress along a detector surface plot path of an SCA coupled to a WACTEM wafer assembly according to an exemplary embodiment of the invention. 
           [0036]      FIG. 21  is an X-Y plot of the maximum principal stress along a detector surface plot path of an SCA coupled to a WACTEM wafer assembly according to an exemplary embodiment of the invention. 
           [0037]      FIG. 22  is an X-Y plot of the maximum principal stress along a detector surface plot path of an SCA. 
           [0038]      FIG. 23  is an X-Y plot of the maximum principal stress along a detector surface plot path of an SCA incorporating a WACTEM wafer assembly according to an exemplary embodiment of the invention. 
           [0039]      FIG. 24  is an exploded view of a WACTEM wafer assembly according to an exemplary embodiment of the invention prior to bonding. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Exemplary and non-limiting embodiments of the invention disclose an infrared sensor chip assembly (SCA)  70 , specifically a GREATOP SCA  70 , wherein the Wafer Coefficient of Thermal Expansion Matching (WACTEM) wafer assembly  77  is formed of a metal containing, more preferably an iron-containing, and even more preferably a stainless steel-containing shim  81  coupled, on alternating sides, to both the silicon ROIC  73  and a silicon shim  79 . As discussed more fully below, the use of the stainless steel shim  81  provides greater ease of manufacture compared to typical titanium shims  17 , as well as improved physical characteristics. 
         [0041]    With reference to  FIG. 18 , there is illustrated an exemplary embodiment of a WACTEM wafer assembly  77  of the invention. WACTEM wafer assembly  77  is formed of three substantially planar layers. Specifically, a first layer is formed of the silicon ROIC  73  with a thickness of, by example, approximately 150 μm. One side of silicon ROIC  73  is coupled to a side of stainless steel shim  81  via an approximately 5 μm thickness epoxy layer. Stainless steel shim  81  in this non-limiting example is approximately 150 μm in thickness. A blank silicon wafer, silicon shim  79 , is bonded to another side of the stainless steel shim  81  to form the WACTEM wafer assembly  77 . As with the silicon ROIC  73 , the silicon shim  79  is coupled to the stainless steel shim  81  via an epoxy layer of approximately 5 μm in thickness. As described, the exemplary embodiment of the WACTEM wafer assembly  77  has an approximate thickness of 460 μm. Slight variations of the approximate component thicknesses described above yields an overall thickness of the WACTEM wafer assembly  77  of between approximately 460 μm and 470 μm. 
         [0042]    As discussed more fully below, exemplary embodiments of the WACTEM wafer assembly  77  result in a composite WACTEM wafer assembly  77  with a coefficient of thermal expansion (CTE) of approximately 5×10 −6  m/m/° K. In addition, having an overall thickness of approximately 470 μm results in a WACTEM wafer assembly  77  having a composite structure that can be utilized in existing SCA  10  designs, does not require dewar redesign, is compatible with current processes, and exhibits die-flatness as required for flip-chip hybridization. Furthermore, the WACTEM wafer assembly  77  is compatible with currently used processes and parameters including, but not limited to, maximum temperature constraints and wafer singulation via dicing. In addition, the WACTEM wafer assembly  77  exhibits die flatness as may be required for flip-chip hybridization. 
         [0043]    With reference to  FIG. 19 , there is illustrated a flow chart of an exemplary method for fabricating a WACTEM wafer assembly  77  wafer according to exemplary embodiments of the invention. At step A, a stainless steel preform, forming the stainless steel shim  81 , is bonded to silicon shim  79 . The bonding can be achieved through the use of an epoxy  76 ′. 
         [0044]    At step B, metal is removed from the stainless steel shim  81  in accordance with a saw lane pattern. Exemplary processes for removing the metal include, but are not limited to, photolithography and chemical etch processes. At step C, the saw lanes of the ROIC  73  are aligned with the cleared metal lanes on the stainless steel shim  81  and the ROIC  73  is bonded to the stainless steel shim  81 , such as by epoxy  76 . 
         [0045]    At step D, individual dies, comprised of single WACTEM wafer assembly  77  structures are formed using a standard dicing process. Bonding at the wafer level and then dicing the balanced WACTEM wafer assembly  77  mitigates problems related to detector  11  flatness that can arise with the use of a titanium shim  17 . In addition, titanium cannot be etched using the type of process described above. It is noted that the thermal contraction rate of 17-7 stainless steel at 8.61 E-6/K is but slightly higher than the value of 7.54E-6/K for titanium over the same temperature range. 
         [0046]    With reference to  FIG. 24  there is illustrated an exploded view of a WACTEM wafer assembly  77 , prior to bonding, according to an exemplary embodiment of the invention showing the cleared metal lanes  241  on the stainless steel shim  81  arranged in accordance with a pattern, for example a grid pattern. Note that the saw lanes  243  of the ROIC  73  are formed in accordance with the same pattern and are aligned with the cleared metal lanes  241  on the stainless steel shim  81  prior to bonding. 
         [0047]    With reference to  FIG. 7 , there is illustrated an image of a half-symmetry three-dimensional ANSYS® finite element model used in a study of an exemplary embodiment of a stainless steel SCA  70  formed of a WACTEM wafer assembly  77  according to an exemplary embodiment of the invention. The results of the study are depicted in the following figures as described below. With reference to  FIG. 8 , there is illustrated a detailed image of the model mash of the stainless steel SCA  70  according to an exemplary embodiment of the invention. The illustrated model consists of 83,000 type 186 20-noded solid elements. As illustrated, detector  71  is bonded to the WACTEM wafer assembly  77  via a backfill adhesive. The WACTEM wafer assembly  77  is formed of a central stainless steel shim  81  bonded on one side to silicon ROIC  73  and on another side to silicon shim  79 . The material properties used in the model are shown in  FIG. 9 . 
         [0048]    In order to observe the improved material characteristics of the WACTEM wafer assembly  77  with respect to the computer models described above, such as in  FIG. 5 , an ANSYS® model of the detector  11  of  FIG. 5  was constructed. With reference to  FIG. 10 , there is illustrated the maximum principled stress distribution in the InSb detector  11  of  FIG. 5  calculated using an ANSYS 3-D model wherein the model is similarly subjected to a cooling from 300 to 77K As is evident, the calculated maximum principled stress distribution in the InSb detector  11  is very similar to the stress calculated previously. To obtain more quantitative information on the stress distribution on the top of the detector  11 , the stress along a specific line  111  on the top of the detector  11 , as shown in  FIG. 11 , is graphed. With reference to  FIG. 12 , there is illustrated a X-Y plot of the maximum principal stress along the line  111  shown in  FIG. 11 . The plot of  FIG. 12  indicates that the field stress on the surface of the SDA detector is approximately 5,400 psi with a peak stress of approximately 6,000 psi. This result is broadly consistent with the results illustrated above in reference to  FIG. 5 . 
         [0049]    With reference to  FIG. 13 , there is illustrated a contour plot of the maximum principal stress on the detector  71  surface bonded to an exemplary embodiment of the WACTEM wafer assembly  77  of the invention. The maximum stress on the detector  71  is approximately 300 psi and the field stress is less than 10 psi. With reference to  FIG. 14 , there is shown a more quantitative look at the stress along a diagonal line of the detector  71  corresponding to the line shown in  FIG. 11 . The field stress in  FIG. 14  shows a maximum field stress of just over 10 psi and a peak stress of 82 psi at the edge of the detector  71 . 
         [0050]    With reference to  FIG. 15 , there is illustrated a high densify two-dimensional axisymmetric finite element analysis model of a cut through a 3-D model of the exemplary stainless steel SCA  70  shown in  FIG. 13  along the same diagonal used for plotting X-Y results as shown in  FIG. 14 . For the high density 2D axisymmetric model, 16,000 8-noded plane-82 type elements were used. The 2D model yielded a stress distribution along the surface of the detector  71  of approximately 0 psi in the field with a peak stress of 788 psi at the edge of the detector as illustrated with reference to  FIG. 16 . 
         [0051]    The WACTEM wafer assembly  77  according to exemplary embodiments of the invention provides clearance between the stainless steel shim and the saw lines of approximately 0.002″ or greater on either side of each saw line. The clearance is required if the full assembly is to be diced using a diamond blade. With reference to  FIG. 17 , there is illustrated a 0.002″ undercut of the stainless steel shim  81  formed relative to the adjacent silicon ROIC  73  and silicon shim  79 . In the exemplary embodiment shown, the resulting gap is shown filled with adhesive  171 . 
         [0052]    As evidenced by the disclosure above, exemplary embodiments of the WACTEM wafer assembly  77  reduce thermal stress in InSb detectors to insubstantial levels during cool-down by matching the CTE of the composite ROIC structure with InSb material. As a result, there is realized a reduction in the rate of “infant mortality” cracking. Exemplary embodiments of the invention allow one to employ commercially available processes to assemble the WACTEM wafer assembly  77 . In addition, by leaving the saw lanes devoid of metal, standard dicing procedures can be employed to singulate and dice the WACTEM wafer assemblies  77 . The resulting WACTEM wafer assemblies  77  exhibit improved thermal cycle reliability for InSb focal plane assemblies or any other type of detector having a CTE in excess of 2×10 −6  m/m/° K as well as improved reflow hybridization capability. 
         [0053]    The foregoing teachings have been described in the context of various dimensions, material types, wavelengths and the like, it can be appreciated that these are exemplary of the preferred embodiments, and are not intended to be read in a limiting matter upon these teachings. For example, while described in various exemplary embodiments with reference to 17-7 stainless steel, any stainless steel can be used that exhibits acceptable hardness and CTE properties for a particular composite structure. Thus, while these teachings have been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of these teachings.