Transmission of polyimide during pulsed ultraviolet laser irradiation

Abstract

Abstract. Experiments that demonstrate quantitatively the importance of laser absorption dynamics for ultraviolet laser ablation of organic materials are presented. Laser pulse transmission measurements have been performed on 0.1 jam spin-coated polyimide films at three ultraviolet wavelengths ( 193 nm, 248 nm, and 355 nm) over the fluence range l0 -3-10 J/cm 2. Target transmission is observed to increase with increasing fluence by a factor of --,5 at 193 nm, and a factor of ~ 10 at 248 nm. In contrast, transmission decreases by approximately one half during 355 nm target irradiation. These results are analyzed theoretically with a two-level model of chromophore absorption. This theory is also applied to reported pulsed UV-laser polyimide ablation data. It is shown that an accurate description of the fluence-dependent film absorption leads to a prediction of the etch depth versus pulse fluence relationship in good agreement with experimental data. PACS : 42.10, 81.60, 82.50 Pulsed ultraviolet-laser ablation has been shown in a multitude of studies to etch organic targets precisely, which in turn has led to widespread application of UV lasers in materials processing, microelectronics, and medicine. 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).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. The absolute magnitude of the optical effect cannot be deduced from the reports cited above because those studies were either qualitative in nature or limited in terms of incident laser fluence. 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.

Laser Application of Polymers

“…One explanation for the decrease of a is chromophore saturation, which is similar to those reported for the ablation of collagen at 193 nm [179] and PI at 193 and 248 nm [132,174,175]. In these studies, the transmission of the laser pulse through thin polymer films was used as a probe for the dynamic target optics, which is also described with the same theoretical model we are using in this study.…”

“…One important parameter in the model is the number of chromophores. In previous studies [132,175] this number was always used as an adjustable parameter in the model. In our opinion it seemed to be more correct to calculate the chromophore number independently.…”

“…The ratio of T H /T L , as suggested by Pettit et al [175], was plotted against the laser fluence. For 248-nm irradiation, only fluence values up to about 800 mJ cm 2 could be used because at higher fluences (e.g., 1.1 J cm 2 ) ablation of the quartz occurs.…”

“…These results were analyzed theoretically using a "two-level" model of chromophore absorption [175]. In this model, excited states of the chromophore are capable of photon absorption.…”

Pulsed laser ablation of dental calculus in the near ultraviolet

“…26 During photoablation, the number density of chromophores, ρ a , decreases as absorbed UV photons break chemical bonds and reduce absorption of subsequent photons. 27,28 Permanent photochemical damage of the chromophores from photobleaching similarly decreases ρ a during laser irradiation. The derivation for the modified blow-off model is found in Ref.…”