Femtosecond laser irradiation of metal and thermal oxide layers on silicon: studies utilising cross-sectional transmission electron microscopy

We present the results of 800 and 400 nm wavelength, femtosecond laser pulse irradiation of a sample consisting of a metal film on thermally-grown oxide on silicon. On selected sites, cross-sectional transmission electron microscopy was performed to provide information on sub-surface changes not observable with surface scanning electron microscopy. A range of pulse energies in single-pulse irradiation exists for which the metal film was removed but the oxide was not appreciably thinned. For a sufficiently high pulse energy within this range, substantial defects were observed in the underlying silicon. Five infrared pulses of a relatively high fluence created significant defects, as well as producing polycrystalline material on top of the original oxide and metal. We discuss various factors which may play a role in the formation of the observed features.