Patent Publication Number: US-11393869-B2

Title: Wafer level shim processing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Divisional application of U.S. patent application Ser. No. 16/285,690 filed Feb. 26, 2019, which application is hereby incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Wafer processing can include shims to promote flatness. Conventional die processing can include epoxy application for a die level shim which can leave voids in the bond interface layer. A shim provides a mechanical layer to impart a Z-axis shift and/or add thermal dissipation. 
     For some devices it is highly desirable to have a flat die. For example, sensing devices, such as focal plane arrays, can include die that can have sensors and/or circuitry. Conventional techniques for attempting to achieve flat die, e.g., to remove bow in the die, may include the use of glass beads in an epoxy bondline to control thickness. Weights can be used to flatten the device after epoxy application. However, some known techniques to remove bow in a die may put an assembly, such as focal planes, at risk, such as due to weight contact with optical surface. 
     SUMMARY 
     Embodiments of the invention provide methods and apparatus for processing a wafer using oxide deposited for bonding a shim at the wafer level at room temperature at atmosphere, for example. Embodiments can include bow compensation to reduce bow in the die. Integrating bonding oxide deposition into a sensor chip assembly (SCA) process flow reduces the likelihood of damaging a delicate front side of the imager, for example. In embodiments, an oxide layer for shim attachment is applied before thinning to prevent damage to the wafer surface. It will be appreciated that a wafer providing a photodetector should have a polished and defect-free surface to enhance detector performance. 
     Embodiments can include ion implantation and annealing to bond multiple wafers using oxide-to-oxide bonding for production of silicon-based imaging arrays. In example embodiments, a first wafer is provided as a detector wafer bonded to a second wafer provided as a ROIC (read out integrated circuit) wafer. In embodiments, DBH bonding can be used to attach the first and second wafers. In embodiments, DBH bonding refers to using direct bond hybridization to form a covalent bonding between wafers. 
     After the first and second wafers are bonded, oxide can be deposited to the backside of the ROIC wafer prior to detector thinning, for example. Oxide can be deposited to the backside of the ROIC wafer after which a shim can be bonded to the SCA. In embodiments, the shim is bonded to the assembly using low temperature, e.g., room temperature, lower than the DBH temperatures used to bond the first and second wafers. 
     The illustrative steps enable assembly of a CTE (coefficient of thermal expansion) engineered structure for focal plane array (FPA) detectors, for example. In addition, wafer bow can be modified to promote die flatness in the final assembly. Also, the bonded wafers increase rigidity to allow post bond processing of thin outside wafers. Further, thin film deposition for oxide, for example, can improve SCA/die Z-height control/flatness over adhesives. In embodiments, bonding oxide is deposited by PECVD or PVD have high uniformity (5000±500 Å), whereas epoxy has a large variation (e.g., 10000-20000±50000 Å) across a 200 mm wafer. 
     It will be appreciated that increasing a flatness of an optical surface is desirable to increase performance, resolution, and the like, of a sensing device, such as a device in a FPA. By decreasing bow in the die, the optical plane of the sensor is flattened for enhanced sensor performance. 
     With use of the above illustrative processing, an example process allows for standard wafer thicknesses to enable standard wafer processing equipment (e.g., no handling 1500-3000 mm thick wafers). 
     In one aspect, a method comprises: deploying a circuit assembly having a first wafer bonded to a second wafer with an oxide layer, wherein a first surface of the first wafer is bonded to a first surface of the second wafer; creating a bonding oxide on a second surface of the second wafer; thinning the first wafer after depositing the bonding oxide; annealing the circuit assembly; applying a coating over at least a portion of the first wafer after annealing the circuit assembly; polishing the bonding oxide on the second surface of the second wafer; securing a shim to the bonding oxide on the second surface of the second wafer to reduce bow of the circuit assembly; and removing the coating. 
     A method can further include one or more of the following features: the circuit assembly comprises a sensor circuit assembly with interconnection embedded in the bond interface, the first wafer comprises a detector, the second wafer comprises a read out integrated circuit (ROIC), the circuit assembly provides a sensor for a focal plane array, the circuit assembly does not comprise epoxy, applying photoresist material to the first wafer prior to bonding the shim, applying a non-photosensitive material to the first wafer prior to bonding the shim, removing the photoresist material prior to annealing the circuit assembly, removing the non-photosensitive material prior to annealing the circuit assembly, the bonding oxide bonding oxide has a uniformity of about 5000±500 Å, and/or the shim comprises a material selected from the group consisting of Silicon, AlN, and sapphire. 
     In another aspect, an integrated circuit assembly comprises: a circuit assembly having a first wafer bonded to a second wafer with an oxide layer, wherein a first surface of the first wafer is bonded to a first surface of the second wafer; a bonding oxide on a second surface of the second wafer, wherein a surface of the bonding oxide is polished; and a shim secured to the bonding oxide on the second surface of the second wafer to reduce bow of the circuit assembly. 
     An assembly can further include one or more of the following features: the circuit assembly comprises a sensor circuit assembly with interconnection embedded in the bond interface, the first wafer comprises a detector, the second wafer comprises a read out integrated circuit (ROIC), and/or the circuit assembly does not comprise epoxy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
         FIG. 1  is a flow diagram showing an example sequence of steps for providing an assembly having an oxide bonded shim to reduce wafer bow; 
         FIG. 2  is a schematic representation of an example assembly after bonding first and second layers with an oxide layer; 
         FIG. 3  is a schematic representation of an example assembly after depositing a bonding oxide to a surface of the assembly; 
         FIG. 4  is a schematic representation of an example assembly after thinning the first layer; 
         FIG. 5  is a schematic representation of an example assembly after annealing; 
         FIG. 6  is a schematic representation of an example assembly after applying a coating to a surface of the assembly in at least one region; 
         FIG. 7  is a schematic representation of an example assembly after etching the assembly; 
         FIG. 8  is a schematic representation of an example assembly after applying a photoresist material; 
         FIG. 9  is a schematic representation of an example assembly after polishing the bonding oxide on the surface of the assembly; 
         FIG. 10  is a schematic representation of an example assembly after bonding a shim to the assembly using the oxide layer; and 
         FIG. 11  is a schematic representation of an example assembly after removing the photoresist material. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows example process steps for providing an assembly having a shim in which bonding oxide deposition is integrated into the process for minimizing damage to a front side of the assembly, such as an imager forming part of a focal plane array (FPA). The assembly can include an oxide-bonded shim to reduce bow in the sensor and flatten an optical plane for enhanced sensor performance. 
     In step  10  of  FIG. 1  and referring to  FIG. 2 , a first wafer  102  is bonded to a second wafer  104  with a bonding oxide layer  106  to form an assembly  108 , which can be provided as a sensor chip assembly (SCA). In embodiments, DBH bonding can be used to attach the first and second wafers  102 ,  104  with integrated interconnection layer for electrical connections using embedded metal posts and high temperature annealing in a manner well known in the art. 
     In embodiments, the first wafer  102  corresponds to a detector and the second wafer  104  corresponds to a read out integrated circuit (ROIC). As is known in the art, a ROIC refers to an integrated circuit configured to read data from certain types of detectors, such as infrared sensors. In general, the ROIC accumulates photocurrent from pixels for transferring the respective pixel signals onto output taps for readout. The pixels can form a focal plane array to detect a variety of signals. 
     It is understood that the first and second wafers can be provided with any suitable functionality and features to meet the needs of a particular embodiment. It is understood that a SCA having a first wafer provided as a detector and a second wafer provided as a ROIC is one particular embodiment that should not be construed as limiting with respect to the functionality of the wafers in an assembly. The assembly can have varying thicknesses depending upon the application. An illustrative thickness is about 725 μm. 
     In step  12 , and referring to  FIG. 3 , a bonding oxide  110  for later attachment of a shim is deposited onto the second wafer  104  of the assembly  108 . In embodiments, the assembly  108  is flipped prior to application of the bonding oxide  110  using ion implantation and annealing. The bonding oxide  110  has a thickness selected to achieve certain tuning of wafer bow/flatness characteristics. It will be appreciated that in many applications, such as FPAs, it is desirable to minimize bow of the detectors and/or complete hybrid focal plane structure. 
     In step  14 , referring to  FIG. 4 , the first wafer  102  is processed so that a thickness  115  of the first wafer  102  is reduced to a desired level, such as about 40 μm. For an illustrative visible hybrid CMOS imager the range is about 5-185 μm, depending on the required spectral response in the near infrared spectrum. 
     In embodiments, the bonding oxide layer  110  to later attach a shim is applied prior thinning the first wafer  102 . With this arrangement, the likelihood of damage to the first wafer  102  (i.e., the detector) is reduced as compared with conventional processing techniques in which an attachment mechanism is applied to second wafer  104  (i.e., the ROIC) after wafer thinning (backgrind and CMP) so that the assembly  108  must be flipped and possibly damaged. In the conventional process, the bonding oxide is applied after the SCA completion. This requires the fragile imaging surface of the top device to be place face down in onto chucks and handled with vacuum tooling which can scratch the surface, causing optical defects or circuit damage. 
     In step  16 , referring to  FIG. 5 , the assembly  108  is subject to implant and laser anneal to form layer  116  for the first wafer  102 . This process is used to apply a conductive layer to the backside surface after hybrid processing has completed, where thermal anneal temperatures (&gt;900° C.) cannot be tolerated. 
     In optional step  18 , referring to  FIG. 6 , a coating  120 , such as an optical anti-reflective coating (ARC) is applied to the assembly. In embodiments, the coating  120  has a first portion  120   a  covering a first region of the detector layer of the first wafer  102  and a second portion  120   b  covering a second region of the detector layer. Any damage to the multi-layer ARC films will result in reduced optical performance of the sensor. 
     In step  20 , referring to  FIG. 7 , the assembly  108  is etched to singulate the detector wafer into individual die  122  of the second wafer  104  to form multiple detectors  124 ,  126  on respective die. 
     In step  22 , referring to  FIG. 8 , a photoresist coating  128  is applied to the assembly  108  to protect the detectors  124 ,  126 . The coated assembly  108  can be baked to cure the photoresist material, as needed. 
     In step  24 , referring to  FIG. 9 , the bonding oxide layer  110  that was applied to second wafer  104  for bonding a shim is polished, such as by CMP (chemical mechanical planarization) polishing to produce a surface roughness acceptable for fusion bonding. In embodiments, the assembly  108  is flipped for polishing. 
     In step  26 , referring to  FIG. 10 , a shim  130  is applied to the oxide layer  110  on the surface of the second wafer  104 . In embodiments, the shim  130  can be bonded at or about room temperature. In one embodiment, the shim  130  is manually applied to the assembly  108 . In other embodiments, suitable machines are used to attach the shim  130 . It will be appreciated that the annealing temperature for oxide activation of layer  110  should be less that the annealing temperature used during annealing to DBH bond the first and second wafers  102  and  104  of the assembly  108  in step  10 . For example, the interconnection DBH bond can occur at 300° C., whereas the shim bond can occur at 200° C., so the shim bond will not affect the interconnection. 
     In embodiments, the shim  130  can comprise silicon with a thickness and rigidity to achieve a desired reduction in wafer bow. Examples of shim materials include Silicon (100-3000 μm), AlN (500-3000 μm), and sapphire (500-3000 μm). 
     In step  28 , referring to  FIG. 11 , the photoresist material  128  can be removed from the assembly  108  which can then be annealed in step  30 . In embodiments, annealing of the assembly  108  is performed at a temperature in the order of 150 degrees. The anneal converts the weak van der Waal bond to a very strong covalent bond to increase the wafer bond strength, rendering the bond permanent. 
     In embodiments, by using the shim, pre-existing bow in the die is significantly reduced or eliminated resulting in an ultra-flat optical surface. In addition, the use of bonding oxide for securing the shim to the assembly generates minimal, if any, voids, which may be an issue in epoxy-based shim processing. Application epoxy is generally challenging to deliver thin, uniform layers without voided regions due to the thick viscosity and deposition methods. Any voids in the layer will become a bond void after wafer bonding, leading to poor thermal and mechanical properties. 
     It is understood that embodiments of the invention are applicable to a wide range of devices having die for which flatness is desirable, such as SCAs and FPAs. A sensor chip assembly (SCA) or focal plane array (FPA) refers to an image sensing device having an array of light-sensing pixels at the focal plane of a lens. FPAs may be useful for imaging applications, such as taking pictures or videos, as well as non-imaging applications. Example applications include spectrometry, LIDAR, guidance systems, inspection, wave-front sensing, infrared astronomy, manufacturing inspection, thermal imaging for firefighting, medical imaging, and infrared phenomenology. Some FPAs operate by detecting photons at particular wavelengths and generating an electrical charge, voltage, or resistance in relation to the number of photons detected at each pixel. This charge, voltage, or resistance is then measured, digitized, and used to construct an image of the object, scene, or phenomenon that emitted the photons. 
     In illustrative embodiments, a die can have an example bow of +/−50 microns prior to processing and an example bow of about 2 microns after processing with an example range of about ±5 μms. In an illustrative embodiment, a die having a pre-processing bow of about 50 microns and a post-processing bow of about 2 microns provides a 96% reduction in bow. An example shim will have a bow of less than about 2 microns. 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety. 
     Elements of different embodiments described herein may be combined to form other embodiments not specifically set forth above. Various elements, which are described in the context of a single embodiment, may also be provided separately or in any suitable subcombination. Other embodiments not specifically described herein are also within the scope of the following claims.