On the Pulsed Laser Ablation of Metals and Semiconductors

This dissertation covers pulsed laser ablation of Al, Si, Ti, Ge, and InSb, with pulse durations from tens of picosecond to hundreds of microseconds, fluences from ones of J/cm2 to over 10,000 J/cm2, and in ambient air and vacuum. A set of non-dimensional scaling factors was created to interpret the data relative to the laser and material parameters, and it was found that pulse durations shorter than a critical timescale formed craters much larger than the thermal diffusion length, and longer pulse durations created holes much shallower than the thermal diffusion length. Low transverse order Gaussian beams with a pulse duration of 28 ps were used to ablate the materials in air and vacuum. It was found that plateauing of crater depth with increasing fluence was due to melt-reflow into the crater and recondensation of evaporated material. Long pulse mid-infrared ablation of aluminum and titanium in air was found to produce aluminum monoxide and titanium monoxide, respectively. The spectra were similar to those observed in ablation studies using shorter wavelength and higher irradiance lasers, despite the much weak shock front in the current study

Plasma Spectroscopy of Titanium Monoxide for Characterization of Laser Ablation

Ablation of titanium wafers in air is accomplished with 60 µs pulsed, 2.94 µm laser radiation. Titanium monoxide spectra are measured in the wavelength range of 500 nm to 750 nm, and molecular signatures include bands of the C3 Δ → X3 Δ α, B3 Π → X3 Δ γ\u27, and A3 Φ → X3 Δ γ transitions. The spatially and temporally averaged spectra appear to be in qualitative agreement with previous temporally resolved studies that employed shorter wavelengths and shorter pulse durations than utilized in this work. The background signals in the current study are possibly due to particulate content in the plume. A chemical kinetic model of the plume is being developed that will be coupled to a diatomic emission model in order to extract a molecular temperature from the observed spectra