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 thermoset polymer, the handler comprising a material that is transparent in a wavelength range of about 193 nanometers (nm) to about 400 nm; 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to Ser. Nos. 12/788,832 and 12/788,843, 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 
     This disclosure relates generally to the field of integrated circuit fabrication. 
     DESCRIPTION OF RELATED ART 
     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. 
     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 
     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 thermoset polymer, the handler comprising a material that is transparent in a wavelength range of about 193 nanometers (nm) to about 400 nm; 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. 
     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 thermoset polymer, the handler comprising a material that is transparent in a wavelength range of about 193 nanometers (nm) to about 400 nm; and a laser, the laser configured to ablate the adhesive through the handler, wherein a wavelength of the laser is selected based on the transparency of the handler material. 
     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 
       Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
         FIG. 1  illustrates an embodiment of a method for attachment and release of a handler to a wafer using laser ablation. 
         FIG. 2  illustrates an embodiment of a handler attached to a wafer using an adhesive. 
         FIG. 3  illustrates an embodiment of the device of  FIG. 2  during laser ablation. 
         FIG. 4  illustrates an embodiment of the device of  FIG. 2  during laser ablation. 
         FIG. 5  illustrates an embodiment of the device of  FIG. 2  during laser ablation. 
         FIG. 6  illustrates an embodiment of the device of  FIG. 2  during laser ablation. 
         FIG. 7  illustrates an embodiment of the device of  FIG. 2  during laser ablation. 
         FIG. 8  illustrates an embodiment of the device of  FIG. 2  during laser ablation. 
         FIG. 9  illustrates an embodiment of the device of  FIG. 2  after laser ablation and release of the handler from the wafer. 
     
    
    
     DETAILED DESCRIPTION 
     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. and 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. 
       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 350° C. to 400° C. in some preferred embodiments. 
     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 3D or 4D IC chips, and may comprise electrical contacts and vias in some embodiments. The wafer  203  may comprise 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. 
     Handler  201  provides mechanical support to wafer  203  during CMOS processing. Handler  201  may comprise a relatively rigid material that is transparent in the range of about 193 nanometers (nm) to about 400 nm in some embodiments, and in some preferred embodiments in the range of about 248 nm to about 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, or a layer of an additional polymer material. 
     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  before laser ablation (discussed below with respect to block  102 ). In some embodiments, a chemical may be applied to the edge  204  to disintegrate the adhesive  202  located at the edge  204  of the device  200 . 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 . 
     In block  102 , as shown in  FIGS. 3-8 , the adhesive  202  is ablated by a laser, as illustrated by lasers  301 ,  401 ,  501 ,  601 ,  701 , and  801  of  FIGS. 3-8 . Energy from the laser passes through handler  201  and is absorbed by adhesive  202 , causing the adhesive  202  to carbonize or vaporize, allowing release of handler  201  from wafer  203 . The laser 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 351 nm in some preferred embodiments. In embodiments in which handler  201  comprises quartz, the laser may have a wavelength of about 193 nm. The laser may be applied as a flood exposure to the surface of handler  201 , or may be a focused beam having, for example, a line or square shape. The laser may be perpendicular to the surface of handler  201 , or may be applied at an angle to reduce edge diffraction of the laser energy in handler  201 . The laser may also be pulsed along the surface of handler  201  in order to avoid damage to wafer  203  while ablating adhesive  202 . The pulse duration and pulse repetition rate of the laser may be varied as appropriate. Ablation may be performed at room temperature in some embodiments, or in other embodiments, the device  200  may be heated (up to about 400° C. in some embodiments) during ablation in order to promote the release of handler  201 . Depending on the laser wavelength and the handler material, the transmission of the energy from the laser through handler  201  may be greater than 80%, and the light absorption depth in the adhesive  202  may be less than 1 μm. 
       FIGS. 3-8  illustrate various embodiments of laser ablation as performed in block  102 .  FIG. 3  illustrates a top view of a focused line laser  301  ablating adhesive  202  through handler  201 . Focused line laser  301  may be pulsed across handler  201  in the direction indicated by arrow  302  to cover the entire surface of handler  201 .  FIG. 4  illustrates an embodiment of a top view of a square beam laser  401  ablating adhesive  202  through handler  201 . Square beam laser  401  may be pulsed across handler  201  in the direction indicated by arrow  402  to eventually cover the entire surface of handler  201 . The pulse duration and pulse repetition rate of lasers  301  and  401  may be varied as appropriate.  FIGS. 3-4  are shown for illustrative purposes only; the laser used to ablate adhesive  202  in block  201  may be any appropriate shape. 
     Edge diffraction of the laser energy may occur near the edge  204  of device  200 ; therefore, the laser may be angled with respect to handler  201 , as shown in  FIGS. 5-8 , to reduce edge diffraction issues in the handler  201  during ablation.  FIG. 5  illustrates a cross section of an embodiment of a system for laser ablation in which hander  201 , adhesive  202 , and wafer  203  are supported at an angle by support  502  during ablation by vertical laser  501 .  FIG. 6  illustrates a cross section of an embodiment of a system for laser ablation in which laser  601  is angled with respect to handler  201 , and hander  201 , adhesive  202 , and wafer  203  are flat.  FIG. 7  illustrates a cross section of an embodiment of a system for laser ablation in which hander  201 , adhesive  202 , and wafer  203  are supported at an angle on a rotary stage  702  during ablation by vertical laser  701 . Rotary stage  702  may move as needed in any appropriate direction during ablation by vertical laser  701  in order to ablate adhesive  202  across the entire surface of handler  201 .  FIG. 8  illustrates a cross section of an embodiment of a system for laser ablation in which hander  201 , adhesive  202 , and wafer  203  are supported at an angle on a rotary stage  802  during ablation by angled laser  801 . Rotary stage  802  may move as needed in any appropriate direction during ablation by angled laser  801  in order to ablate adhesive  202  across the entire surface of handler  201 .  FIGS. 5-8  are shown for illustrative purposes only; the laser may be at any appropriate angle to handler  201 , and the hander  201 , adhesive  202 , and wafer  203  may be in any appropriate configuration with respect to the laser. 
     In block  103 , the handler  201  is released from wafer  203 , as shown in  FIG. 9 . Block  103  may include a wet soak of wafer  203  to remove any ablated adhesive  901  that may remain on wafer  203  after release of handler  201 . Block  103  may be performed at room temperature in some embodiments, or in other embodiments, the structure  200  may be heated up to 400° C. during release to promote release of handler  201  from wafer  203 . 
     The technical effects and benefits of exemplary embodiments include attachment and release of a handler to an IC wafer without damage to the IC wafer and with good throughput. 
     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.