Theory for the etching of organic materials by ultraviolet laser pulses

Abstract

A theoretical description of the ultraviolet laser etching process is developed. The threshold for laser ablation is reached when the density of absorbed photons is approximately equal to the density of chromophores in the material. Saturation of the absorption coefficient, absorption by the plume of ablated products, and multiphoton effects are considered. Agreement with all available experimental etch data, including femtosecond ultraviolet laser ablation, is found. The description is based on an analysis of the radiation transport at high intensities and is independent of the question as to whether ultraviolet laser ablation is photochemical or thermal.

Transmission of polyimide during pulsed ultraviolet laser irradiation

“…We now analyze these experimental results using the theoretical framework presented in [1,5]. The features of that theory necessary to the discussion here are shown schematically in Fig.…”

“…For these reasons we have conducted the experiments described here, quantifying the change in absorption for thin films of polyimide (a common photoablation substrate) over a wide range of laser fluences at three distinct ultraviolet wavelengths. This paper thus provides the first quantitative experimental test of the ideas formulated in [1,5], i.e. that proper treatment of the radiation transport to include chromophore saturation and excited-state absorption is extremely important for determination of the etch depth per pulse in organic materials.…”

“…In order to optimize these procedures, as well as to assess possible risks of clinical laser use, the laser/material interaction must be well understood. One aspect of photoablation that has only lately become apparent is that the absorptive properties of the target can change during the intense laser irradiation [1][2][3][4]. Recently, two of the current authors presented a theoretical description of such dynamic optical properties [1,5] showing that these absorption changes could account for the commonly noted substantial discrepancy between experimental data and the etch depth versus laser fluence relationship predicted by Beer's law (i.e., the simple "blow-off" model [6] described below).…”

“…One aspect of photoablation that has only lately become apparent is that the absorptive properties of the target can change during the intense laser irradiation [1][2][3][4]. Recently, two of the current authors presented a theoretical description of such dynamic optical properties [1,5] showing that these absorption changes could account for the commonly noted substantial discrepancy between experimental data and the etch depth versus laser fluence relationship predicted by Beer's law (i.e., the simple "blow-off" model [6] described below).However, other processes such as thermal diffusion could also affect the ablation depth/fluence behavior [7]. A recent theoretical analysis concluded that dynamic target optics were probably more important than thermal diffusion in affecting photoablation [8], yet this issue has not been conclusively resolved.…”

ArF Excimer Laser Annealing of Polycrystalline Silicon Thin Film

“…For photochemical ablation to occur, energy of the photons at that wavelength should overcome the intermolecular bond energies of the target material. The relation between the photon energy of light and laser wavelength is given by (Sauerbrey, 1989),…”