GB878763A - Improvements in and relating to a process and apparatus for the production of highly-pure silicon - Google Patents

Abstract

<PICT:0878763/III/1> <PICT:0878763/III/2> Highly pure silicon is prepared by thermal decomposition of monosilane, in a chamber containing a body of highly pure silicon by heating the silicon body by thermal radiation so as to reduce its electrical resistivity, and thereafter heating the silicon body by induction heating in the presence of monosilane supplied to the chamber, the heating by thermal radiation being terminated when the resistivity of the silicon body has been reduced to a value such that induction heating can maintain the silicon body above the decomposition temperature of monosilane, so that silicon is deposited on the silicon body. A solution of ammonium bromide in liquid NH3 is placed in flask 1 in bath 2 maintained at -50 DEG C. Magnesium silicide, prepared by heating Mg, and Si in H2 is tipped into the solution from tubes 3 to form SiH4. NH3 is condensed in tubes 6 by solid CO2 and acetone mixture 7, and the SiH4 passes through heat exchanger 8, where it is cooled to -100 DEG C. by gaseous N2 cooled by liquid N2, to remove residual NH3, and is condensed in boiler 12 by liquid N2. The boiler 12 is then surrounded by bath 13 at -130 DEG C. after H2 has been evacuated through 24, and the SiH4 is evaporated into vacuum jacketted fractionating column 14, packed with quartz spirals, and condenser 15 cooled with liquid N2. Evaporated N2 is passed down over column 14. Pure SiH4 is withdrawn through quartz wool filter 19 immersed in a bath at -78 DEG C. and quartz filter disc 21 to enter quartz decomposition chamber 35 (Fig. 2) through jet 34 directed at liquid silicon globule 36 on seed crystal 37. The globule is maintained liquid by water-cooled copper inductor loop 40, after the seed has been preheated by focusing a lamp by elliptical or parabolic mirrors. Alternatively, the seed may be preheated by placing an annulus of molybdenum, tungsten, or graphite round the seed coupled to inductor 40, and when the temperature is suitable, 500-1000 DEG C., removing the annulus. Gases are withdrawn through water-cooled cones 41 and outlet 42. The SiH4 is decomposed at the surface of the globule 36, while the seed is slowly rotated, and withdrawn from the chamber by holder 39 to build up a continuous rod of silicon. Specifications 878,764 and 878,765 are referred to. Reference has been directed by the Comptroller to Specification 826,575.