Interactive effects in excimer laser photoablation

Abstract

Laser irradiation of surfaces leads to the rapid ejection of atomic and larger species. An ablative process based on (classical thermal) vaporization is often contrasted to photochemical etching, i.e., material removal owing to bond breaking. Collisional sputtering, electrical breakdown (plasma formation), and mechanical disintegration are also possible. This talk will summarize interactions which occur when removing <<10 monolayers in ≤10 ns and how these interactions affect the etch product, i.e., plume material. Our experiments primarily utilize laser-induced fluorescence, LIF, to measure the density and energy of specific species in the plume. Energy information is particularly useful as one then has information about the surface temperature. The energy is derived both from densities, and rotational/vibrational state distributions (of diatomic radicals), see Fig. 1. At low fluences both C2 and Cu (from the resp. substrates) are vaporization products. The electronic energy distributions are also being followed currently; this includes simultaneously following Cu° atoms, Cu+ (metastable) ions and a few Cu2 radicals above laser-etched copper metal. Initial generation of the Cu+ ions appears to be a photon→vapor interaction. With higher fluences (>4 × 108W/cm2) plasma effects become dominant. The overall conclusion is that while direct thermal effects are present in nsec laser ablation, other interactions can be quantitatively followed by laser probing techniques, primarily LIF. Specific examples (sapphire) exist for lasers producing photochemical ablation when the excimer beam interacts with sensitive sites. Also, secondary interactions between photons or radicals and other radicals, ions and electrons are common, and indeed, amenable to quantitative evaluation and understanding. One example of this is that plasma physics concepts and equations explain many of the effects seen at slightly higher fluences.