Web-dendritic growth. Final report, October 1975-August 1977

This final report gives the results of work performed by the University of South Carolina to develop methods of producing large areas of silicon ribbon by the web-dendritic method. A prototype web-dendritic growth machine, on hand at the beginning of the contract, was assembled and activated. A program for investigating the role of the various machine design parameters on the growth of web-dendritic ribbon was carried out during the duration of the contract. The development of the machine proceeded to the point where ribbons could be reproducibly grown up to lengths of 1 meter, with widths increasing linearly from a minimum, at the initiating seed button, up to 1 cm. at the point of termination of growth. Considerable thermal data was collected and evaluations were made of actual seeding and growth for variations in a large number of parameters affecting heat loss. From this we found for achieving suitable growth that the mechanical system should be very rigid and stable, and the tolerances and specifications of the quartz crucibles must be far tighter than normal quartz tolerances. The widening rates of the ribbons were found to be a function of the temperature gradient rather than the temperature differences alone. A twin spacing in the seed of 3..mu.. - 2..mu.. was found to be unfavorable for growth; whereas spacing of .9..mu.. - 2..mu.. and 8..mu.. - 2..mu.. were favorable. It was found, however, that the spacing of 8..mu.. - 2..mu.. sets an upper limit of 4 cm/min on the maximum achievable rate. Extensive thermal modeling studies were carried out to investigate the effect of furnace design parameters on the temperature distributions in melt and the growth of the dendritic-web ribbon. From this study it was found that the pull rate of the ribbon is strongly dependent on the temperature of the top thermal shield, the spacing between this shield and the melt, and the thickness of the growing web