Abstract:
A method for releasing a handler from a wafer, the wafer comprising an integrated circuit (IC) includes attaching the handler to the wafer using an adhesive comprising a polymer; performing edge processing to remove an excess portion of the adhesive from an edge of the handler and wafer; ablating the adhesive through the handler using a laser, wherein a wavelength of the laser is selected based on the transparency of the handler material; and separating the handler from the wafer. A system for releasing a handler from a wafer, the wafer comprising an IC includes a handler attached to a wafer using an adhesive comprising a polymer; an edge processing module, the edge processing module configured to remove an excess portion of the adhesive from the edge of the handler and wafer; and a laser, the laser configured to ablate the adhesive through the handler.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is related to attorney docket numbers YOR920100151US1 and YOR920100152US1, each assigned to International Business Machines Corporation (IBM) and filed on the same day as the instant application, all of which are herein incorporated by reference in their entirety. 
       FIELD 
       [0002]    This disclosure relates generally to the field of integrated circuit fabrication. 
       DESCRIPTION OF RELATED ART 
       [0003]    Multiple integrated circuit (IC) products, which may be referred to as chips or dies, may be formed on a larger semiconductor substrate, referred to as a wafer. The IC fabrication process may comprise fabrication of multiple complementary metal-oxide-semiconductor (CMOS) devices on the wafer. If the wafer is relatively thin, an adhesive may be used to attach the wafer to a rigid handler, so that the handler may provide mechanical support for the wafer during the CMOS fabrication processes. However, the CMOS fabrication process may include chemical processing and/or high temperature processing, which may reach temperatures up to 400° C., which may cause the adhesive to break down. 
         [0004]    The wafer may be released from the handler after CMOS fabrication is completed. Some methods of handler release include use of a temperature-sensitive adhesive or a chemically dissolvable adhesive. However, a temperature-sensitive adhesive may only adhere the handler to the chip at temperatures under about 300° C. or lower. A chemically dissolvable adhesive may also only be appropriate for relatively low-temperature processing, and may require use of a specialized handler material to allow the release chemicals to diffuse through the handler in order to dissolve the adhesive. Wax sliding is another release method, but it is also limited to relatively low temperature processing, and may require a special sliding mechanism. 
       SUMMARY 
       [0005]    In one aspect, a method for releasing a handler from a wafer, the wafer comprising an integrated circuit (IC) includes attaching the handler to the wafer using an adhesive comprising a polymer; performing edge processing to remove an excess portion of the adhesive from an edge of the handler and wafer; ablating the adhesive through the handler using a laser, wherein a wavelength of the laser is selected based on the transparency of the handler material; and separating the handler from the wafer. 
         [0006]    In one aspect, a system for releasing a handler from a wafer, the wafer comprising an IC includes a handler attached to a wafer using an adhesive comprising a polymer; an edge processing module, the edge processing module configured to remove an excess portion of the adhesive from the edge of the handler and wafer; and a laser, the laser configured to ablate the adhesive through the handler. 
         [0007]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0008]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0009]      FIG. 1  illustrates an embodiment of a method for attachment and release of a handler to a wafer using laser ablation. 
           [0010]      FIG. 2  illustrates an embodiment of a handler attached to a wafer using an adhesive. 
           [0011]      FIG. 3  illustrates an embodiment of the device of  FIG. 2  after C4 processing. 
           [0012]      FIG. 4  illustrates an embodiment of the device of  FIG. 3  after underfilling and attachment to a pedestal. 
           [0013]      FIG. 5  illustrates an embodiment of the device of  FIG. 3  during laser ablation. 
           [0014]      FIG. 6  illustrates an embodiment of the device of  FIG. 5  after handler release. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Embodiments of systems and methods for laser ablation of adhesive for IC fabrication are provided, with exemplary embodiments being discussed below in detail. Appropriate selection of a handler material, an adhesive, and a laser having a specified wavelength allows for attachment of a handler to a CMOS wafer using the adhesive, and subsequent release of the handler from the wafer using laser ablation without contamination of or damage to the wafer. The adhesive may comprise a polymer, including but not limited to a thermoset polymer and/or a polyimide-based polymer, that may withstand temperatures up to about 400° C. as well as any chemical process that may occur during the fabrication process. The release process may be relatively fast, allowing good throughput for the overall IC fabrication process. 
         [0016]      FIG. 1  illustrates an embodiment of a method for attachment and release of a handler to a wafer using laser ablation.  FIG. 1  is discussed with reference to  FIGS. 2-9 . In block  101 , a handler  201  is attached to a wafer  203  using an adhesive  202 , as shown in  FIG. 2 . The wafer  203  may comprise a thinned substrate, and may be between about 700 and 800 microns (μm) thick in some embodiments. The adhesive  202  may comprise a polymer, including but not limited to a thermoset polymer and/or a polyimide-based polymer, and may withstand high-temperature processing at over 280° C. in some embodiments, and in the range of about 350° C. to about 400° C. in some preferred embodiments. The adhesive  202  may be applied to wafer  203  by spin application, and then the handler may be placed on the adhesive  202  in some embodiments. 
         [0017]    After attachment of handler  201 , processing, which may include CMOS fabrication processing, may be performed on wafer  203 . The processed wafer  203  may comprise any appropriate CMOS devices, including silicon-based 3-dimensional (3D) or 4-dimensional (4D) IC chips, and may comprise electrical contacts and/or vias in some embodiments. The wafer  203  may comprise a solar cell, including but not limited to a thin film solar cell, a solar cell comprising a copper-indium-gallanide-selenium (CIGS) based thin film, a silicon solar cell, or a glass substrate based solar cell. 
         [0018]    Handler  201  provides mechanical support to wafer  203  during processing. Handler  201  may comprise a relatively rigid material that is transparent in the range 193 nanometers (nm) to 400 nm in some embodiments, and in some preferred embodiments in the range of 248 nm to 351 nm, including but not limited to quartz, glass, diamond, or sapphire. Handler  201  may be selected such that the coefficient of thermal expansion (CTE) of handler  201  is closely matched to the CTE of the material comprising wafer  203 . Handler  201  may comprise electrical vias with connections that mate to any electrical contacts on wafer  203 , allowing device  200  comprising handler  201  to be used in a test apparatus before release of handler  201  in some embodiments. Handler  201  may further comprise one or more additional layers of optical energy absorbing material located at the interface between adhesive  202  and handler  201 , including but not limited to one or more layers of thin sputtered metal a thin layer, or a layer of an additional polymer material. 
         [0019]    The adhesive  202  may be thicker at the edge  204  of the device  200  than at the middle, which may optionally require special processing of the adhesive at the edge  204  of device  200  in block  102  before laser ablation (discussed below with respect to block  103 ). In some embodiments, a chemical may be applied to the edge  204  to dissolve or weaken the adhesive  202  located at the edge  204  of the device  200 . The chemical may be applied by immersion of device  200  in the chemical for an appropriate amount of time in some embodiments. In some embodiments, a focused ion beam or a high-energy electron beam may be applied to edge  204  to remove adhesive  202  located at the edge  204 . In some embodiments the device  200  may be exposed to a vacuum or atmospheric pressure plasma environment whereby plasma disintegrates adhesive  202  located around the edge  204  of device  200 . In some embodiments, the device  200  may be exposed to a super-critical solvent environment containing chemicals selected to degrade the adhesive  202 , allowing for penetration of the solvents around the edge  204  of device  200 . 
         [0020]    The edge  204  may also be removed using an edge deletion technique, which may comprise dicing, or any appropriate abrasive or mechanical cutting of the edge of device  200  at lines  205  to remove the portions having relatively thick adhesive  202  at edge  204 . The removed portion may be selected so as not to remove or damage the active portion (containing, for example, any CMOS devices) of the wafer  203 . Dicing using repetitive tangential patterns on the outer diameter of wafer  203  may be used to remove the inactive area of wafer  203 . Lines  205  are shown for illustrative purposes only; the edge deletion may remove any appropriate portion of device  200 . 
         [0021]    The CMOS processing of block  101  may further comprise fabrication of a C4 layer on wafer  203 , as is shown in  FIG. 3 . The C4 layer may comprise a plurality of electrical contacts  301 . The C4 layer may have a non-uniform distribution on wafer  203 , which may cause damage to the wafer  203  during laser ablation (discussed below with respect to block  103 ). Therefore, in block  102 , as shown in  FIG. 4 , the wafer  203  may be underfilled using underfill material  401  on the side of wafer  203  comprising C4 contacts  301  in order to offer a uniform surface during the laser ablation process, resulting in separation of handler  201  from the wafer  203  without damage. The underfill material  401  may be selected so as to not damage functionality of wafer  203 , and may comprise any appropriate type of flux, epoxy, or gel, or water. Pedestal  402  may then be placed under underfill material  401 . Pedestal  402  may comprise a ceramic in some embodiments. C4 contacts  301  are shown for illustrative purposes only; wafer  203  may comprise any appropriate distribution of C4 contacts. 
         [0022]    In block  103 , as shown in  FIG. 5 , the adhesive  202  is ablated by a laser  501 , which may comprise ultraviolet (UV) light having a wavelength between about 193 nm to about 400 nm in some embodiments, and between about 248 nm to about 351 nm in some preferred embodiments. In embodiments in which handler  201  comprises quartz, laser  301  may have a wavelength of about 193 nm. Energy from laser  501  passes through handler  201  and is absorbed by adhesive  202 , causing the adhesive  202  to carbonize or vaporize, allowing release of handler  201 . Depending on the laser wavelength and the handler material, transmission of the energy from the laser  501  through handler  201  may be greater than 80%, and the light absorption depth in the adhesive  202  may be less than 1 μm. Laser  501  may be directed at an angle to handler  201  in some embodiments; the angle may be selected to reduce or eliminate edge diffraction of the laser  501  as it travels through the handler  201 . While C4 contacts  301 , underfill material  401 , and pedestal  402  are not shown in  FIG. 5 , device  500  may comprise C4 contacts  301 , underfill material  401 , and pedestal  402  in some embodiments. In other embodiments, the device comprising wafer  203 , handler  201 , and adhesive  202  may be mounted on a rotary stage (embodiments of which are shown, for example, in  FIG. 7 , element  702 , and  FIG. 8 , element  802  of Attorney Docket Number YOR920100153US1) that may move as needed in any appropriate direction during ablation by laser  501  in order to ablate adhesive  202  across the entire surface of handler  201 . 
         [0023]    Over-ablation by laser  501  may cause damage to wafer  203 . In embodiments in which laser  501  comprises a pulsed laser source, not all areas of handler  201  may separate evenly from wafer  203  due, for example, to uneven application of the laser energy across the surface of hander  201 . Multiple passes of laser  501  over areas that do not separate with a single pass may be required, which may damage wafer  203 . Further, wafer  203  may be damaged by the diving board effect. In the diving board effect, one portion of wafer  203  is released from handler  201 , and the release progresses from the released portion to another portion of the wafer  203  that is still attached to handler  201 , which may cause a top layer of the wafer  203  to pivot on the unreleased portion of the wafer substrate due to acoustic bounce The substrate of wafer  203  may be damaged at the fulcrum point of the pivot. Various techniques which are discussed below, including detection of acoustic signature and visual ablation detection, may be employed during laser ablation in order to avoid damage to wafer  203  from over-ablation. 
         [0024]    The acoustic signature of the portion of wafer  203  under ablation may be detected, and the fluence of laser  501  or the stage rotation and/or speed may be modulated in order to encourage release of handler  201  without damage to wafer  203  based on the acoustic signature, as a change in the acoustic signature may indicate separation of handler  201  from the wafer  203 . An acoustic or vibration sensor may be integrated into a rotary stage or a pedestal (such as pedestal  402  of  FIG. 4 ) that wafer  203  is mounted on during ablation, and the sensor may be configured to detect the any changes in the acoustic signature of the portion of the wafer  203  under ablation. The acoustic or vibration sensor may then send a signal to an attenuator to adjust the movement of the rotary stage and/or stop the laser  501  based on any detected change in the acoustic signature, preventing damage to wafer  203  due to over-ablation. 
         [0025]    The adhesive  202  may also change color when the adhesive  202  is fully ablated (i.e., carbonized or vaporized) by laser  501 . A visual image of the adhesive  202  under ablation by laser  501  may be captured in order to identify any color changes that occur in adhesive  202 . A detected change in the color of adhesive  202  may be used to determine if there is a need to stop laser ablation, or to increase any laser parameters or perform an additional pass of laser  501  over the surface of handler  201  if adhesive  202  is not fully ablated. 
         [0026]    In block  104 , as shown in  FIG. 6 , the handler  201  is released from wafer  203 . Release of handler  201  may result in a portion of ablated adhesive  401  remaining on wafer  203  in some embodiments. Any remaining ablated adhesive  401  may be removed, using, for example, a wet soak, resulting in wafer  203 . 
         [0027]    The technical effects and benefits of exemplary embodiments include attachment and release of a handler to a wafer while avoiding damage to the wafer. 
         [0028]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0029]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.