Laser assisted fabrication of titanium salicide for deep sub-quarter micron MOSFETs

Fabrication of titanium salicide using laser thermal processing is investigated. Laser thermal processing uses pulsed excimer laser radiation (XeCl, lambda = 308 nm) to rapidly heat and melt the silicon surface. Laser radiation is absorbed by the titanium layer which subsequently heats and melts the silicon. Titanium diffuses into molten silicon during the melt/regrowth cycle resulting in the fabrication of titanium salicide. Melt depth in silicon and hence the thickness of the salicide is controlled by the laser energy. The process suffers from limited margins. Two dimensional heat flow effects make it challenging to fabricate salicides in the source/drain regions up to the side wall spacer edge without completely melting the gate and destroying its physical integrity. We demonstrate amorphization of the wafer surface as a solution to overcome these challenges. Amorphous silicon has significantly different material properties than crystalline silicon. We exploit these differences to overcome the two dimensional heat diffusion effects and to create enhanced process margins. We first demonstrate this through thermal simulations. We subsequently demonstrate, on un-patterned wafers that amorphization can be used create process margins and stociometrically correct salicide can be fabricated on amorphous silicon. The salicide fabricated is demonstrated to be insensitive to the presence of dopants, have smooth interfaces and its phase transformation characteristics are studied. We also demonstrate, on patterned wafers, that using amorphization the two dimensional heat flow constraints can be overcome. Using differential thermal budget, inherent in laser thermal processing, different thickness salicides can be fabricated on the gate and the source/drain regions. This allows us to fabricate thicker salicide in the gate regions than in the source/drain regions thus overcoming a significant barrier to phase transformation of titanium salicides fabricated using conventional rapid thermal processing where the thickness of the salicide on the gate region is constrained by the depth of the source/drain junctions. We demonstrate successful fabrication and phase transformation of salicides on gate lengths down to 0.07 mum. Finally, we demonstrate through the fabrication of diodes that amorphization for laser thermal processing does not degrade the electrical properties of the device.