Rapid thermal annealing is a
term that covers various methods of heating wafers for periods from 100 seconds
down to nanoseconds, allowing repair of damage with minimal diffusion. [2]
In this the heating time is less
than 10-7seconds. A high-energy laser pulse is used to melt the
surface to a depth of less than 1 micron and the surface recrystallizes by
liquid phase epitaxy. Dopant diffusion in the liquid state is very fast so the
final profile is rectangular and extends from the surface to the melt depth. By
adjusting the pulse time and energy, shallow junctions can be obtained but as
it is not possible to preserve other doping profiles or surface film, this
technique is not usually applicable to VLSI.
It occurs when the time is of
the order of between 10-7 and 1 second. Wafer is heated from one
side with a laser, electron beam or flash lamp to give a temperature gradient
across the wafer thickness. Surface is not melted and Solid Phase Epitaxy can
still repair surface damage before any diffusion has time to occur. The main
disadvantage is that the rapid quenching from high temperature leaves many
point defects, which may condense to form dislocations, degrading minority
carrier lifetime of the material.
It covers heating processes
longer than one second including furnace steps. It uses tungsten-halogen lamps
or graphite resistive strips to heat the wafer from one or both sides. This
offers significant advantages for VLSI processing because good activation can
be obtained with less diffusion than with furnace annealing. [2]