Laser Lift-off Process

Because of the much longer pulse duration in the case of the KrF laser, a
higher pulse energy of typically 600 mJ/cm2 is necessary to heat the GaN
above the sublimation threshold, whereas pulse energies of 300 mJ/cm2 are
sufficient in the case of the Nd:YAG laser.
Since optical thermalization and relaxation processes in GaN occur on a much
faster timescale of ps or a few ns, the temperature profiles generated by the
different laser pulses can be calculated quite easily in a one-dimensional
model based on the temporal shape of the laser pulse, the known absorption
coefficients of GaN, and the thermal properties ( heat capacitance and
conductivity) of GaN and sappire.
67
Photograph of a 275 μm thick free-standing GaN
film, after removal from the 2” sapphire substrate
The missing pieces at the wafer border actually had broken off right after the
HVPE growth.
68
26