Laser sputtering–part IV: Laser interferometry and laser-induced fluorescence studies of insulators

Abstract

Ultra-violet laser etching (sputtering) of solids has been explained in various terms, including thermal, electronic, hydrodynamic, exfoliational, and even collisional processes. To distinguish between these possibilities, two laser techniques are used here to probe the plume of material expanding away from the etched surface of certain insulators due to 248 nm pulses. The first set of experiments measures the free-electron density in the plume using a Michelson interferometer. The results indicate that a plasma is formed for laser fluences greater than some threshold value (which depends on the material). With Al₂O₃, the plasma threshold is, moreover, similar to but slightly higher than the etching threshold, suggesting that collisional sputtering due to fast ions from this plasma does not contribute substantially to the etching. The second set of experiments, using laser-induced fluorescence, measures the number densities and energy distributions of AI and AIO in the plume when etching Al₂O₃. Time-of-flight distributions indicate Boltzmann-like behavior with kT equal to ∼3.5–17eV for A1 and ∼1.1–0.3 eV for AlO. In contrast, the rotational and vibrational energies of A10 correspond to only 0.04–0.16 eV, suggestive of a low surface temperature during etching. These observations are consistent with an electronic desorption mechanism similar to that found with incident fission fragments, electron pulses, or small accelerated dust particles.