Photo-ionisation threshold yields for the pulsed laser irradiation of diamond films at 266 nm

Abstract

Laser damage thresholds have been measured for polycrystalline diamond films supported on silicon substrates by monitoring the ion yield following pulsed laser irradiation. A Q-switched frequency quadrupled Nd: YAG laser at 266 nm combined with a time-of-flight mass spectrometer has been employed to investigate the nature of the ions emitted from the focal region and the laser power thresholds for the detection of these specific ions. High purity films of diamond have been deposited onto polished crystalline silicon substrates from a 0.2% methane in hydrogen gas mixture using a 2.45 GHz remote microwave plasma. The crystallinity of the films has been assessed by scanning electron microscopy, diffraction and laser Raman spectroscopy. The relative ion yields of C + 1, H − and C − 2 have been measured at normal incidence as a function of laser power density at 266 nm. The onsets for the detection of C + 1 and C − 2 are indistinguishable within experimental error and the power threshold was determined to be 4–5 GW cm −2. The corresponding onset for H − was slightly higher at 7–8 GW cm −2. The presence of hydrogen was directly observed as H − and also by the occurrence of C n H − m cluster ions with n up to approximately 10. Theoretical fragmentation patterns for C − n were calculated using the natural abundance of the 13C isotope (1.13%) and compared with the observed pattern. The hydrogen content is significantly less than that for hard a-C: H films.