Abstract:
A method for spalling a layer from an ingot of a semiconductor substrate includes forming a metal layer on the ingot of the semiconductor substrate, wherein a tensile stress in the metal layer is configured to cause a fracture in the ingot; and removing the layer from the ingot at the fracture. A system for spalling a layer from an ingot of a semiconductor substrate includes a metal layer formed on the ingot of the semiconductor substrate, wherein a tensile stress in the metal layer is configured to cause a fracture in the ingot, and wherein the layer is configured to be removed from the ingot at the fracture.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/185,247, filed Jun. 9, 2009. This application is also related to attorney docket numbers YOR920100056US1, YOR920100058US1, YOR920100060US1, and FIS920100006US1, 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]    The present invention is directed to semiconductor substrate fabrication using stress-induced substrate spalling. 
       DESCRIPTION OF RELATED ART 
       [0003]    A large portion of the cost of a semiconductor-based solar cell may be due to the cost of producing a layer of a semiconductor substrate on which to build the solar cell. In addition to the energy costs associated with the separation and purification of the substrate material, there is a significant cost associated with the growth of an ingot of the substrate material. To form a layer of the substrate, the substrate ingot may be cut using a saw to separate the layer from the ingot. In the process of cutting, a portion of the semiconductor substrate material may be lost due to the saw kerf. 
       SUMMARY 
       [0004]    In one aspect, a method for spalling a layer from an ingot of a semiconductor substrate includes forming a metal layer on the ingot of the semiconductor substrate, wherein a tensile stress in the metal layer is configured to cause a fracture in the ingot; and removing the layer from the ingot at the fracture. 
         [0005]    In one aspect, a system for spalling a layer from an ingot of a semiconductor substrate includes a metal layer formed on the ingot of the semiconductor substrate, wherein a tensile stress in the metal layer is configured to cause a fracture in the ingot, and wherein the layer is configured to be removed from the ingot at the fracture. 
         [0006]    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 
         [0007]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0008]      FIG. 1  illustrates an embodiment of method for spalling for an ingot of a semiconductor substrate. 
           [0009]      FIG. 2  illustrates an embodiment of an ingot of a semiconductor substrate with a seed layer. 
           [0010]      FIG. 3  illustrates an embodiment of an ingot of a semiconductor substrate with an adhesion layer. 
           [0011]      FIG. 4  illustrates an embodiment of a system for forming a stressed metal layer on an ingot of a semiconductor substrate. 
           [0012]      FIG. 5  illustrates an embodiment of an ingot of a semiconductor substrate with a stressed metal layer. 
           [0013]      FIG. 6  illustrates an embodiment of a spalled layer of an ingot of a semiconductor substrate. 
           [0014]      FIG. 7  illustrates a top view of an embodiment of a spalled layer of an ingot of a semiconductor substrate. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Embodiments of systems and methods for spalling for a semiconductor substrate are provided, with exemplary embodiments being discussed below in detail. 
         [0016]    A layer of tensile stressed metal or metal alloy may be formed on a surface of an ingot of a semiconductor material to induce a fracture in the ingot by a process referred to as spalling. A layer of the semiconductor substrate having controlled thickness may be separated from the ingot at the fracture without kerf loss. The stressed metal layer may be formed by electroplating or electroless plating. Spalling may be used to cost-effectively form layers of semiconductor substrate for use in any semiconductor fabrication application, such as relatively thin semiconductor substrate wafers for photovoltaic (PV) cells, or relatively thick semiconductor-on-insulator for mixed-signal, radiofrequency (RF), or microelectromechanical (MEMS) applications. 
         [0017]      FIG. 1  illustrates an embodiment of a method  100  for spalling for an ingot of a semiconductor substrate.  FIG. 1  is discussed with reference to  FIGS. 2-7 . The semiconductor material comprising the ingot may comprise germanium (Ge), or single- or poly-crystalline silicon (Si) in some embodiments, and may be n-type or p-type. For an n-type semiconductor material, block  101  is optional. In block  101 , a surface of an ingot  201  of a semiconductor material that is to be spalled is pre-treated by forming a seed layer  202  on the surface of the ingot, as is shown in  FIG. 2 . The seed layer  202  is necessary for an ingot  201  of p-type semiconductor material (in which holes are the majority carriers), as direct electroplating on p-type material is difficult due to the surface depletion layer that may be formed when a p-type ingot  201  is subjected to a negative bias with respect to the electroplating solution. The seed layer  202  may comprise a single layer or multiple layers, and may comprise any appropriate material. The seed layer  202  may comprise palladium (Pd) in some embodiments, which may be applied to ingot  201  via immersion in a bath comprising a Pd solution. In other embodiments, in which the ingot  201  comprises Si, formation of the seed layer  202  may comprise forming a layer of titanium (Ti) on ingot  201 , and forming a silver (Ag) layer over the Ti layer. The Ti and the Ag layers may each be less than about 20 nanometers (nm) thick. Ti may form a good adhesive bond to Si at low temperature, and the Ag surface resists oxidation during electroplating. The seed layer  202  may be formed by any appropriate method, including but not limited to electroless plating, evaporation, sputtering, chemical surface preparation, physical vapor deposition (PVD), or chemical vapor deposition (CVD). The seed layer  202  may be annealed after formation in some embodiments. 
         [0018]    In block  102 , an adhesion layer  301  of a metal is formed on the ingot  201 . For embodiments comprising a p-type ingot  201 , the adhesion layer  301  is optional, and formed over the seed layer  202  as is shown in  FIG. 3 . For embodiments comprising an n-type ingot  201 , the adhesion layer is formed directly on the ingot  201 , and there is no seed layer  202 . The adhesion layer  301  may comprise a metal, including but not limited to nickel (Ni), and may be formed by electroplating or by any other appropriate process. The adhesion layer  301  may be less than 100 nm thick in some embodiments. Formation of the adhesion layer  301  may be followed by annealing to promote adhesion between the metal adhesion layer  301 , the seed layer  202  (for p-type semiconductor material), and semiconductor ingot  201 . Annealing causes the adhesion layer  301  to react with the semiconductor material  201 . Annealing may be performed at a relatively low temperature, below 500° C. in some embodiments. Inductive heating may be used for annealing process in some embodiments, allowing heating of the metal adhesion layer  301  without heating the ingot  201 . 
         [0019]    In block  103 , electroplating (or electrochemical plating) is performed by immersing the surface of ingot  201  comprising adhesion layer  301  in a plating bath  401 , and applying a negative bias  402  with respect to plating bath  401  to the ingot  201 , as is shown in  FIG. 4 . The plating bath  401  may comprise any chemical solution capable of depositing a stressed metal layer  501  (as shown in  FIG. 5 ) on the ingot  201  either autocatalytically (electroless) or upon application of external bias  402 . In an exemplary embodiment, plating bath  401  comprises a 300 gram/liter (g/l) aqueous solution of NiCl 2  with 25 g/l of boric acid. The plating bath temperature may be between 0° C. and 100° C. in some embodiments, and between 10° C. and 60° C. in some exemplary embodiments. The plating current flowing in ingot  201  during electroplating may vary; however, the plating current may be about 50 mA/cm 2  in some embodiments, yielding a deposition rate of about 1 micron/min. Prior to electroplating, if any oxide layers have formed on adhesion layer  301 , these oxide layers may be removed chemically. For example, a diluted HCl solution may be used to remove oxide layers from an adhesion layer  301  comprising Ni. 
         [0020]    Electroplating causes stressed metal layer  501  to form on adhesion layer  301 , as is shown in  FIG. 5 .  FIG. 5  shows an embodiment of an ingot  201  comprising p-type semiconductor material, with a seed layer  202 . If the ingot  201  comprises n-type semiconductor material, seed layer  202  is not present. The stressed metal layer  501  may be between 1 and 50 microns thick in some embodiments, and in between 4 and 15 microns thick in some exemplary embodiments. The tensile stress contained in metal layer  501  may be greater than about 100 megapascals (MPa) in some embodiments. 
         [0021]    In block  104 , semiconductor layer  601  is separated from ingot  201  via spalling at fracture  603 , as is shown in  FIG. 6 .  FIG. 6  shows an embodiment of an ingot  201  comprising p-type semiconductor material, having a seed layer  202 . If the ingot  201  comprises n-type semiconductor material, seed layer  202  is not present. Spalling may be used in conjunction with an ingot  201  having any crystallographic orientation; however, fracture  603  may be improved in terms of roughness and thickness uniformity if fracture  603  is oriented along the natural cleavage plane of the material comprising ingot  201  (&lt;111&gt; for Si and Ge). 
         [0022]    Spalling may be either controlled or spontaneous. In controlled spalling (as shown in  FIG. 6 ), a handle layer  602  is applied to the metal layer  501 , and is used to induce fracture in the ingot  201  to remove the semiconductor layer  601  from the ingot  201  along fracture  603 . The handle layer  602  may comprise a flexible adhesive, which may be water-soluble in some embodiments. Use of a rigid material for the handle layer  602  may render the spalling mode of fracture unworkable. Therefore, the handle layer  602  may further comprise a material having a radius of curvature of less than 5 meters in some embodiments, and less than 1 meter in some exemplary embodiments. In spontaneous spalling, the stress contained in the stressed metal layer  501  causes semiconductor layer  601  and the stressed metal layer  501  to spontaneously separate themselves from the ingot  201  at a fracture, without the use of a handle layer  602 . Controlled spalling may be made to become spontaneous spalling upon heating of the stressed metal  501 . Heating tends to increase the tensile stress in the stressed metal  501 , and can initiate spontaneous spalling. Heating may be performed in any appropriate manner, including but not limited to a lamp, laser, resistive, or inductive heating. 
         [0023]      FIG. 7  illustrates a top view of an embodiment of a semiconductor layer  601  on a handle layer  602 . The handle layer  602  may be removed, and stressed metal layer  501 , adhesion layer  301 , and seed layer  202  (in the case of a p-type ingot  201 ) may be etched off, depending on the application for which semiconductor layer  601  is to be used. Semiconductor layer  601  may have any desired thickness, and be used in any desired application. Semiconductor layer  601  may comprise single- or poly-crystalline silicon in some embodiments. 
         [0024]    In block  105 , blocks  101 - 104  may be repeated using ingot  201 . Because there is no kerf loss, layers of the ingot  201  may removed from the ingot  201  with relatively little waste, maximizing the number of layers of a semiconductor material that may be formed from a single ingot. 
         [0025]    The technical effects and benefits of exemplary embodiments include reduction of waste in semiconductor fabrication. 
         [0026]    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. 
         [0027]    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.