SBIR-STTR Award

The traditional methods used today to produce all of the silicon crystals for the semiconductor industry result in large rod-shaped ingots which are subsequently cut into thin wafers for device fabrication. It would be advantageous if silicon crystals could be produced directly in sheet form in order to eliminate waste of expensive crystalline silicon. A variety of innovative silicon sheet production techniques has been explored in attempts to reduce the cost of semiconductor devices. one of these methods, the vertical edge-supported pulling (ESP) process, in which the silicon ribbon pulled from the melt is supported by carbon of quartz filaments, has shown great promise.However, the production speeds achieved to date are severely restricted by the rate of heat dissipation from the very narrow sheet/melt interface. Theoretically, it is possible to produce large sheet crystals of silicon at extremely high production rates if the sheets could be grown horizontally. To date, however, an overriding mechanical difficulty, i.e., the walls of the crucible containing the molten silicon, remains a seemingly impenetrable barrier to the practical realization of horizontal crystal production. Our research has shown that it is now possible to confine large, high purity silicon melts in an inductively heated, water-cooled cold crucible while maintaining the liquid surface well over 1 cm above the rim of the container. Moreover, since the solid silicon feed-stock is introduced through the bottom of the container, continuous operation appears to be feasible. This program will endeavor to integrate the operational features of the inductively-heated cold crucible with the ESP process in order to demonstrate the feasibility of the horizontal edge supported pulling (HESP) method for the production of silicon sheet crystals.The potential applications as described by the company:Continuous silicon crystal production has been a long sought goal in the semiconductor industry. The cold crucible system combined with a horizontal edge-supported pulling (HESP) provides the first practical combination of existing technologies which has the promise of continuous economical silicon sheet production.

The Phase ll program will continue to explore the feasibility and scale-up of the horizontal growth of silicon sheet crystals using the edge-supported pulling (ESP) process frorr. silicon melts contained in an RF-coupled cold crucible. Theoretical evaluation of the basic Horizontal Ribbon Growth (HRG) process has shown conclusively the enormous potential of this crystal growth method; i.e., it would be possible to produce large, flat sheet crystals of silicon at extremely high growth rates. To date, however, the potential of the HRG process has not been demonstrated fully in a practical sense. Problems involving control of meltaheet temperature and melt level have not beer, resolved, and the overrriding difficulty (i.e., the crucible wall) remains a seemingly impenetrable barrier to horizontal crystal growth. Our research has shown that it is possible to confine large, high-purity silicon melts in a cold crucible while maintaining the liquid surface well over I cm above the rim of the container with a high degree of stability. Moreover, since the solid feed material is introduced continuously in the bottom of the cold crucible, precise control of the melt level can be maintained during crystal growth. The vertical edge-supported pulling process provides exceptionally stable sheet growth conditions. However, the sheet growth rates achieved to date are severely restricted by the rate of heat dissipation from the narrow sheet/melt interface. Moreover, because the filaments in the ESP process must pass through holes in the bottom of the melt container, shallow melt levels must be used and melt replenishment during growth has yet to be achieved. The Phase I portion of this program focused on determining the feasibility of adapting the edge-supported technique to horizontal ribbon growth utilizing the cold crucible, and the results achieved were most encouraging.Anticipated Results/Potential Commercial Applications as described by the awardee: Continuous silicon sheet/ribbon crystal production has been a long-sought goal, not only for the low-cost fabrication of photovoltaic devices, but for the manufacture of more sophisticated semiconductor devices as well. The cold crucible system combined with a horizontal edge-supported pulling process provides the first practical combination of existing technologies which has the promise of continuous, economical, high-quality silicon sheet production.