Temporal profile of optical transmission probe for pulsed-laser heating of amorphous silicon films

Pulsed laser irradiation is employed over a wide spectrum of materials processing applications, including surface hardening, alloying, curing, synthesis of compound and semiconductor films. In semiconductor systems,* it is used to anneal ion-implantation surface damage, recrystallize amorphous and polycrystalline films, and enhance dopant diffusion. Recent studies2 have shown that one of the most effective ways of removing submicron-sized particles from solid surfaces is achieved with the deposition of a liquid film on a substrate surface and the application of an ultraviolet (UV) excimer laser pulse on the surface. One of the main issues in improving this process is the control of the induced transient temperature field. Timeresolved optical transmission and reflection measurements have been reported for the investigation of the irradiation of crystalline silicon (c-Si) on sapphire structures at the picosecond3 and the nanosecond,415 time scales. This work presents an optical transmission probing technique for the transient, in situ monitoring of the temperature field in pulsed excimer laser irradiation of thin amorphous silicon (a-Si) films. The sample is a 0.2 pm-thick amorphous silicon film deposited by electron beam evaporation of crystalline silicon in vacuum onto a 250 pm-thick fused quartz substrate. The substrate temperature is kept at 140 “C and the deposition rate at 10 A/s. The uniformity of the thickness of the a-Si layer is monitored by surface profilometry. The sample is irradiated by a KrF (il=O.248 ,um) excimer laser beam. The laser beam fluence F is determined by measuring the pulse energy using an energy meter. An infrared probing diode laser beam (/2=0.752 ,um) is incident normal onto the sample surface. The transmitted signal is captured by a fast photodiode and a digitizing oscilloscope. The optical transmission measurement technique is based on the variation of the material optical properties with temperature. It has been reported’ that the optical