Si L-edge structures in the soft x-ray reflectivity of laser-annealed Si surfaces (with Ni overlayers)

Abstract

We have developed a pump-and-probe method to detect structural changes in surfaces under pulsed laser irradiation, by means of changes in the soft X-ray reflectivity. First results of reflectivity measurements of Si surfaces at X-ray energies around the Si L-edge (80–180 eV) are presented and compared to calculated reflectivities. The presence of the native silicon oxide layer is discussed. The measurements were performed with 20 ns X-ray pulses at an angle of incidence of 1.5° with respect to the surface, which was laser annealed at a power density of ~1 × 10 8 W/cm 2. The reflectivity of the annealed surface, measured 30 ns after the anneal pulse, can be described by the product of the reflection by a solid surface and the absorption in the molten Si top layer. We have also performed preliminary measurements on the inter-diffusion process of a 5 Å Ni overlayer into Si under pulsed laser irradiation. The reflection spectrum of this system shows, upon pulsed laser annealing, a clearly visible Si L-edge, indicating that the Ni has inter-diffused into the Si wafer. Apart from the Si edge, a new feature in the reflectivity becomes visible at 83 eV during annealing and remains visible afterwards. The feature was tentatively assigned as originating from Ni-oxide and/or Ni-silicide (Ni 1+, M-edge). Measurements on the angle of incidence dependence of the reflectivity shows that the structures in the reflected signal are best visible at an angle of about 1.0°.