This invention relates to a process for producing semiconductor grade silicon and more particularly to the production of trichlorosilane as a part of that process.
In the semiconductor industry relatively large quantities of high purity monocrystalline silicon are required for the production of semiconductor devices and integrated circuits.
One of the most commonly used methods for the growth of monocrystalline silicon is the Czochralski method in which a charge of hyperpure polysilicon is placed in a high purity quartz crucible set inside a graphite susceptor. The susceptor, crucible, and charge are placed in a controlled inert atmosphere inside a quartz cylinder. RF induction coils are placed outside the quartz cylinder and around the graphite susceptor. RF power is supplied to the coils and the charge is heated until it is completely molten and the temperature has been stabilized just above the melting point of the cylinder. After stabilization of the temperature, a seed crystal is dipped into the melt and the seed is allowed to melt back a short distance to remove any surface imperfections which may have resulted from its preparation. The seed is then rotated and very slowly withdrawn from the melt. With the temperature of the melt properly maintained, the seed grows into a single oriented crystal.
The hyperpure polysilicon can be produced by hydrogen reduction of trichlorosilane at a temperature of about 1100.degree.C. During the process, a large proportion of the trichlorosilane is not reduced to silicon but is converted to silicon tetrachloride in accordance with the approximate reaction: 3SiHCl.sub.3 + H.sub.2 .fwdarw. Si + SiHCl.sub.3 + SiCl.sub.4 + 2HCl + H.sub.2. Thus, only 1/3 of the trichlorosilane converts to silicon while 1/3 remains unreacted and another 1/3 converts to silicon tetrachloride. The residual gases can be separated to remove the trichlorosilane from the residue and returned as part of the feed for the reaction. However, the silicon tetrachloride has been essentially a waste product except to the extent that it is used in the epitaxial deposition of silicon upon silicon slices, as set forth in the article, "Analysis of the Hydrogen Reduction of Silicon Tetrachloride Process on the Basis of a Quasi-Equilibrium Model", T. O. Sedgewick, Journal of the Electrochemical Society, December, 1964, pages 1381-1383.
The Si--Cl--H system is a complex multireaction system, essentially reversible, in which partial pressures of several species occur. These species are for example, H.sub.2, HCl, SiCl.sub.4, SiHCl.sub.3, SiCl.sub.2, SiH.sub.2 Cl.sub.2, SiH.sub.3 Cl, SiCl, SiH.sub.4 and Si in some equilibrium which is a function of pressure, silicon/chlorine/hydrogen ratio, and temperature. Thus, for example, the reaction SiCl.sub.4 + 2H.sub.2 .revreaction. Si + 4HCl is reversible to either deposit an epitaxial layer of silicon on a silicon slice or to etch a silicon slice by removing silicon therefrom. The reversibility of this reaction is normally used in a epitaxial reactor, wherein the silicon slice may be etched by a higher concentration of HCl in the vapors to produce a clean surface on the silicon wafer and then the concentration of SiCl.sub.4 is increased to affect deposition of monocrystalline silicon on the cleaned slice.
The products of the reaction are also controlled by the presence of certain catalysts such as copper, nickel or silicon. Thus, for example, in U.S. Pat. No. 2,595,620, Wagner et al, silicon tetrachloride is partially converted to trichlorosilane in the presence of a catalytic metal. Similarly, chlorosilanes are prepared in U.S. Pat. No. 2,657,114, Wagner, by reacting a metal chloride with silicon, wherein the metal of the chloride is a catalytic metal. Another process for obtaining trichlorosilane from silicon in the presence of a catalyst is disclosed in U.S. Pat. No. 2,943,918, Pauls. In the production of silicon for semiconductor purposes, the presence of such catalytic metals is undesirable since the metals can be dopants affecting the electrical characteristics of the semiconductor. Thus, for semiconductor purposes, a reaction in which no catalytic metal appears is highly desirable.