Rapid thermal LMVD has been applied to the deposition of polysiliccn layers. The single process wafer sits in an air—cooled quartz chamber, and is heated by tungsten—halcgen lamps. A nicrowave cavity upstrean of the hot zone may be used to introduce a remote plasma.
Pblysilicon films have been deposited on ocidized suhstrates in the temperature range 590C–860C, and at pressures between 0.lmber and 10mbar. Conditions for the reduction of carbon and oxygen levels down to 5x1017 cm are presented.
An in—situ CF4 plasma clean prior to polysiliccn deposition is shown to dramatically affect grain—size. At 720C and Imbar, plasma-cleaned polysilicon has a grain—size of 35Chm, whereas non—cleaned polysilicon has a grain—size of only 120nm. The CF4 schedule leaves a clean ocide surface with fewer nucleation sites, resulting in larger grains.
Both types of polysilicon were used in the fabrication of TMT'S. Devices with larger grain material pass 10 times as much current as those with smaller grains, show a channel nobility of >20cm v s, and have a threshold voltage of 5V. Thus, direct deposition of polysilicon layers on a plasma—cleaned substrate is an attractive alternative to crystallization from amorphous silicon, for TFT's.