GB594783A - Improvements in the preparation of silicon ingots - Google Patents

Abstract

Silicon ingots are prepared by allowing a melt to cool in a silica crucible that has thin walls, the crucible and contents being heat treated at temperatures above the solidification temperature of silicon for a time sufficient to render the walls plastic at the solidification temperature and to devitrify partially the interior wall surface of the crucible, whereby expansion of the silicon on cooling deforms the walls of the crucible, and contraction of the ingot on further cooling causes it to break away from the walls, whereby interior strains in the ingot are substantially avoided. Fig. 1 shows an induction furnace comprising a silica tube 1 surrounded by a high frequency coil 2, and containing a bed of aluminium oxide grain 3 on which is disposed a graphite shield 4 coated internally, if desired, with beryllium oxide, in which is placed silica crucible 5. Heat is developed in the shield 4 by induction. A heat-retaining aluminium oxide shield 6 is provided. The top of the furnace is closed by a cap 12 through which passes a metal rod 15 mounted in a flexible rubber sleeve 16. The rod terminates in a silica tube 18, and serves to transfer the powdered charge from hopper 23 to the crucible during the melting operation. A cooling coil 9, and a pipe 10, terminating in silica tube 11 and pipe 14 for the passage of an inert gas such as helium, are provided. A window 13 is formed in cap 12 for the observation of the melt by pyrometer 19. The charge is heated up to 1,600 DEG C., held at that temperature for <PICT:0594783/III/1> twenty minutes, and then for an hour at 1,450-1,550 DEG C., during which time the crucible walls acquire plasticity and some degree of devitrification. The furnace tube is then slowly withdrawn from coil 2 so that solidification proceeds downwardly. The shape assumed by the ingot, together with an indication of crystal structure, are shown in Fig. 3. To facilitate separation of the cooling melt from the crucible; this may be heated to 1,600 DEG C. prior to the introduction of the charge, or its inner surface may be coated with a hydrolized solution of ethyl silicate. Specification 590,458 is referred to.