Methods and apparati for making thin semiconductor bodies from molten material

Abstract

A pressure differential is applied across a mold sheet and a semiconductor (e.g. silicon) wafer is formed thereon. Relaxation of the pressure differential allows release of the wafer. The mold sheet may be cooler than the melt. Heat is extracted almost exclusively through the thickness of the forming wafer. The liquid and solid interface is substantially parallel to the mold sheet. The temperature of the solidifying body is substantially uniform across its width, resulting in low stresses and dislocation density and higher crystallographic quality. The mold sheet must allow flow of gas through it. The melt can be introduced to the sheet by: full area contact with the top of a melt; traversing a partial area contact of melt with the mold sheet, whether horizontal or vertical, or in between; and by dipping the mold into a melt. The grain size can be controlled by many means.

1 . A method for fabricating a semi-conductor body, the method comprising the steps of:
 a. providing a molten semi-conductor material, having a surface;   b. providing a porous mold, comprising a forming surface;   c. providing a differential pressure regime such that pressure at least a portion of the forming surface is less than pressure at the molten material surface;   d. contacting the forming surface to the molten material for a contact duration such that, for at least a portion of the contact duration:
 i. the differential pressure regime is provided and; 
 ii. at least a portion of the forming face is at a temperature below a melting point of the semi-conductor material, 
   such that a body of semi-conductor material, solidifies upon the forming surface;   e. causing motion of the forming surface relative to the molten semi-conductor material, with the solidified body upon the forming surface; and   f. reducing the degree of the differential pressure regime, thereby contributing to the solidified body detaching from the forming surface.   
     
     
         2 . The method of  claim 1 , the step of providing a differential pressure regime comprising, providing at the molten material surface, atmospheric pressure, and providing at the forming surface a pressure that is less than the atmospheric pressure. 
     
     
         3 . The method of  claim 1 , the mold comprising a single forming surface, which contacts a surface of the molten semi-conductor material. 
     
     
         4 . The method of  claim 1 , further comprising, before the step of contacting the forming surface to the molten material, the step of tilting the forming surface relative to the surface of the molten material, so that only a portion of the forming surface makes initial contact with the surface of the molten material. 
     
     
         5 . The method of  claim 1 , further comprising, after the step of contacting the forming surface to the molten material, the step of tilting the forming surface relative to a gravitational field, so that molten material flows off from the forming surface. 
     
     
         6 . The method of  claim 1 , further comprising, before the step of contacting the forming surface to the molten material, the step of orienting the forming surface relative to the surface of the molten material, so that only a portion of the forming surface makes initial contact with the molten material. 
     
     
         7 . The method of  claim 1 , further comprising, during or after the step of causing motion of the forming surface relative to the molten material, the step of spinning the forming surface about an axis having a component that is perpendicular to the forming surface, so that molten material flows off from the forming surface. 
     
     
         8 . The method of  claim 1 , further comprising the step of providing a functional material between the forming surface and the molten material, the functional material chosen to contribute to a function chosen from the group of:
 a. suppressing nucleation of grain growth;   b. preventing passage of impurities from the mold to the solidified semiconductor body;   c. enhancing release of the solidified semiconductor body from the forming surface; and   d. encouraging nucleation of grain growth at specific locations of the solidified semiconductor body.   
     
     
         9 . The method of  claim 1 , further comprising the steps of:
 a. before the step of causing relative motion, the step of providing on the solidified semi-conductor material, which is designated a sacrificial solidified body, functional material comprising an oxide;   b. melting the solidified sacrificial solidified semi-conductor body, thereby leaving a functional body upon the molten material; and   c. contacting the forming surface to the functional body upon the molten material for a second contact duration such that, for at least a portion of the second contact duration:
 i. a differential pressure regime is provided and; 
 ii. at least a portion of the forming surface is at a temperature below a melting point of the semi-conductor material, 
   such that a body of semi-conductor material, having a free face, solidifies upon the forming surface.   
     
     
         10 . The method of  claim 8 , the step of providing a functional material comprising providing the functional material to the forming surface. 
     
     
         11 . The method of  claim 8 , the step of providing a functional material comprising providing the functional material to the surface of the molten material. 
     
     
         12 . The method of  claim 8 , the functional material being selected from the group consisting of: silicon oxide, silicon dioxide (silica), silicon carbide, silicon nitride, silicon, silicon oxynitride, silicon oxycarbide, and boron nitride. 
     
     
         13 . The method of  claim 1 , further comprising providing a preferential nucleation agent at an interface between the forming surface and the molten material, before the step of contacting the forming surface to the molten material. 
     
     
         14 . The method of  claim 1 , further comprising the step of applying a meniscus control element to detach adhering molten material from the solidified body. 
     
     
         15 . The method of  claim 1 , the forming surface comprising a substantially untextured surface. 
     
     
         16 . The method of  claim 1 , the forming surface comprising a textured surface. 
     
     
         17 . The method of  claim 1 , the porous mold comprising a material selected from the group consisting of: graphite, silicon carbide, silicon nitride, silica, silicon oxynitride silicon oxycarbide and boron nitride. 
     
     
         18 . The method of  claim 1 , the porous mold comprising a body of sintered powder. 
     
     
         19 . The method of  claim 1 , the porous mold comprising a graphite body. 
     
     
         20 . The method of  claim 1 , the porous mold comprising a body of originally solid silicon that has been processed to be porous. 
     
     
         21 . The method of  claim 20 , the porous mold further comprising at least one outer surface layer of a silicon oxide. 
     
     
         22 . The method of  claim 1 , further comprising the step of controlling nucleation of grain growth at selected locations of the forming surface. 
     
     
         23 . The method of  claim 1 , further comprising the step of controlling directionality of growth of grains at selected locations of the forming surface. 
     
     
         24 . The method of  claim 23 , the step of controlling directionality of growth of grains selected from the group consisting of:
 a. providing a mold with a spatially varied thickness;   b. providing a pressure differential that varies spatially with respect to the forming surface;   c. providing a mold with spatially varied thermal insulation;   d. providing a forming surface with a spatially varied texture;   e. providing a mold with a spatially varied thermal diffusivity;   f. providing area-specific temperature profile at the forming face; and   g. providing a crystal seed at a location upon the forming surface that first contacts the molten material.   
     
     
         25 . The method of  claim 1 , the mold comprising a compound plenum, having at least two chambers, wherein the step of providing a differential pressure regime comprises providing two different differential pressure regimes, such that pressure at least two different portions of the forming face is less than that of an atmosphere at the molten material surface, and further comprising during the contacting step, the step of moving one chamber relative to the other, to change the relative location of the two different pressure regimes. 
     
     
         26 . The method of  claim 1 , further wherein:
 a. the step of providing molten material comprises providing molten material in a container, the container having at least one wall, such that a meniscus of the molten material exists with a convex curvature facing away from the container, having an uppermost part that is above the wall; and   b. the step of contacting the forming surface to the molten material comprises passing the forming face against the convex meniscus.   
     
     
         27 . The method of  claim 1 , the step of providing a differential pressure regime comprising:
 a. providing a first differential pressure adjacent a first region of the mold surface; and   b. providing a second, different differential pressure at a plurality of discrete locations of the mold surface.   
     
     
         28 . The method of  claim 1 , further comprising the step of suppressing oscillatory motion of the surface of the molten material. 
     
     
         29 . The method of  claim 1 , the step of providing molten material comprising providing molten material in a vessel, such that the molten material has a depth of less than approximately five mm, and preferably less than approximately three mm. 
     
     
         30 . The method of  claim 1 , the step of contacting comprising contacting the forming surface to the surface of the molten material, such that each portion of the forming surface contacts the molten material for approximately the same duration. 
     
     
         31 . The method of  claim 1 , further wherein the mold has a limited heat capacity, such that the temperature of the forming surface rises substantially to a temperature approximately equal to that of the molten material, such that thereafter, no additional molten material solidifies.