Patent ID: 7972942

Claim:
A method of reducing metal impurities of an upgraded metallurgical grade (UMG) silicon (Si) wafer by using an epitaxial silicon film, the method comprising: (a) obtaining a UMG Si substrate as a gettering material, wherein said UMG Si substrate has a purity not greater than 99.999% (5N) and a thickness not greater than 200 micrometers (μm); (b) placing said UMG Si substrate on a graphite susceptor into a reaction chamber of an atmospheric pressure chemical vapor deposition (APCVD) equipment and raising a temperature of said graphite susceptor in said reaction chamber of said APCVD equipment from a room temperature until at least 1100 Celsius degrees (° C.) by radiofrequency coils circled around said reaction chamber of said APCVD equipment; (c) obtaining a gas of dicholosilane (Si 2 H 2 Cl 2 ) as a raw gas for epitaxial Si and a hydrogen gas as a carrying gas of said Si 2 H 2 Cl 2 gas to obtain an epitaxial Si thin film on a first surface of said UMG Si substrate with a second surface of said UMG Si substrate facing said graphite susceptor, wherein, on raising said temperature in said reaction chamber to a temperature above 1100° C., some impurity in said UMG Si substrate is driven away from original sink positions, some impurity in said UMG Si substrate is dissolved, and both are released to new sink positions on said epitaxial Si thin film; wherein step (c) comprises steps of: (c1) obtaining a concentration gradient through a relatively high metal impurity concentration in said UMG Si substrate and a very low, even close to zero, metal impurity concentration of said epitaxial Si thin film to obtain a first diffusion drive of impurity in said UMG Si substrate to diffuse metal impurities in said UMG Si substrate into said epitaxial Si thin film to reduce a metal impurity concentration of said UMG Si substrate; (c2) obtaining a temperature gradient through a relatively high temperature of said UMG Si substrate and a relatively low temperature of said epitaxial Si thin film to obtain a second diffusion drive of impurity in said UMG Si substrate to diffuse metal impurities of said UMG Si substrate into said epitaxial Si thin film to reduce a metal impurity concentration of said UMG Si substrate, wherein said UMG Si substrate obtains said relatively high temperature by directly contacting with said heated-up graphite susceptor; and wherein said epitaxial Si thin film obtains said relatively low temperature by directly perpendicularly facing said raw gas and said carrying gas; and (c3) obtaining sink positions of metal impurities through imperfection formed by lattice matching of said UMG Si substrate and said epitaxial silicon/Si thin film; and (d) stopping providing said raw gas of Si 2 H 2 Cl 2 ; keeping providing said carrying gas of H 2 ; and switching off power to rapidly reduce temperature after finishing diffusing metal impurities of said UMG Si substrate.