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Float-zone refined silicon is produced by establishing a molten zone in a silicon rod, vertically situated and passing the zone through the rod. Induction heating is the typical conventional means for producing the molten zone. Since power levels are very high with large-diameter rods, the efficiency of the heating process is a concern. One of the key parameters is the frequency of the radio frequency current in the induction coil. Silicon rods of 65-mm diameter were zone-refined, at a series of frequencies in the 200 kHz to 3.5 MHz range. It was found that frequencies lower than 500 kHz are not suitable because they produce undesirable surface melting characteristics. On the other hand, higher frequencies tend to increase the susceptibility for arcing. Indeed, the suitable range appears to be between 2 and 3 MHz. The presentation also highlights the major differences between the float-zone and Czochralski processes, and explains the reasons why large-diameter float-zone technology has lagged behind Czochralski technology.
silicon, zone refining, float-zone, crystal growth, induction heating
Engineering Manager, Semiconductor Division, Cincinnati Milacron, Maineville, Ohio
Professor, Washington University, St. Louis, Mo.