Time-resolved reflectivity measurements of silicon during pulsed excimer laser irradiation

Several improvements have recently been made in our measurement system to allow accurate measurements of the reflectivity (on the nanosecond time scale) and the pulse energy density. The reflectivity of silicon versus time during and after pulsed irradiation with an excimer (KrF) laser is shown for energy densities below and just above the melting threshold. Also shown for comparison is the time profile of the laser pulse. For all energy densities shown, the reflectivity begins to rise when the laser pulse turns on and continues to rise monotonically to approx. 42% before it goes into the high-reflectivity phase (this value corresponds to crystalline silicon at the melting point). The lowest energy density shown does not produce a high-reflectivity phase, while the three intermediate energy densities produce reflectivity jumps less than that of the highest energy density shown, indicating incomplete melting (over the first 200 A for these energy densities)