Photodecomposition, ablative

Pulsed ultraviolet laser radiation can lead to clean and precise removal of material at the irradiated site on a polymer surface (Fig. 9.1). This phenomenon was discovered in early 1980s using an excimer laser it was termed ablative photodecomposition and was investigated for several polymers (Table 9.1) [1167,2029, 2045, 2046]. It was also widely reviewed [160, 1296, 1297, 2028, 2030, 2031, 2035, 2036, 2044]. 

The absorption of laser radiation in polymers is governed by Beer s law and the etch depth exhibits a logarithmic dependence on the incident laser fluence above a threshold value [137, 304, 611]. The penetration depth of ultraviolet laser radiation is usually greater than the etch depth, so that the surface left behind after ablation has been exposed to laser radiation [1229]. 

The absorption of radiation by the film is governed by Beer s law, from which an expression has been derived relating the etch depth per pulse (/f) to the logarithm of these fluence [1090, 2032]  

JR = intensity (fluence) threshold for the onset of ablative photodecom- 

The etch depth is independent of the atmosphere in which the experiment is performed. Fluence thresholds for etching and etch depth plots for a variety of polymers have been reported [137, 304, 475, 578, 611, 712, 1298, 2029, 2031, 2032, 2048]. 

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The irradiation of a polymer surface with the high intensity, pulsed, fer-UV radiation of the excimer laser causes spontaneous vaporization of the excited volume. This phenomenon was first described by Srinivasan (1) and called ablative photodecomposition. The attention of many researchers was drawn to the exceptional capabilities of photoablation (2). Etching is confined to the irradiated volume, which can be microscopic or even of submicron dimensions, on heat-sensitive substrates like polymers. In most experimental conditions, there is no macroscopic evidence of thermal damage, even when small volumes are excited with pulses of... 

Ablation rate constant definition, 418 determination, 414,417f Ablative photodecomposition description, 411 See also Photoablation Ace ty la ted m-cresol—novolac copolymers, preparation, 193... 

There are two ashes (KA and EA) on which none of the five PAHs exhibits detectable photoreactivity, and a third ash (IL) on which none of the PAHs examined thus far (excluding BaA) exhibits detect4-able photodecomposition. At the other extreme, all five PAHs undergo appreciable phototransformation when adsorbed on two ashes (TX and AR). In fact, for each, greater photochemical reactivity is observed on the Texas lignite ash than on any other ash (though less reactivity is observed on the TX ash than on alumina, silica, or glass adsorbents). 

An alternative approach to the complicated photoresist systems could be the application of APD (ablative photodecomposition), where a strong absorbance at the irradiation wavelength is one of the conditions for successful ablation. A logical approach to the use of APD as a dry etching technique in microlithography is the development of polymers designed for APD. This is especially true for photolithographic applications that do not require a submicron resolution, such as thin film transistor (TFT) fabrication for liquid crystal displays (LCD) which require a resolution around 1 pm. 

AHMA American Hotel and Motel Association APD ablative photodecomposition... 

Ablative photodecomposition (Rangaswamy Srinivasan) Srinivasan s research on ablative photodecomposition leads to multiple applications, including laser-assisted in situ keratomileusis (LASIK) surgery, which shapes the cornea to correct vision problems. 

As a result of the ablative photodecomposition of polymers by excimer lasers, different stable microstructures on polymer surfaces can be observed and classified into the following categories based on their mode of formation [1587] ... 

Penetration of light. Poly(methyl methacrylate) film doped with a high concentration of pyrene shows well defined ablation. Small curd-like particles are observed for a film with a low dopant concentration. The origin of these particles can be explained by the low density and inhomo-genous distribution of bond scission in the polymer subsurface during ablative photodecomposition with an increase in beam penetration [1417]. 

Phases in the polymer. Semicrystalline polymer films of poly(ethylene terephthalate), poly(ethylene-2,6-naphthalate) and poly(phenylenesulphide), which are stressed biaxially or monoaxially, always show a wavy surface structure (Fig. 9.7) of the order of 1-50 pm after ablative photodecomposition, whereas amorphous films of these polymers produce a smooth surface. It has been suggested that this behaviour is correlated with the amorphous and crystalline areas in the polymer [137, 1302, 1602] or is influenced by the internal stress field from the stretching operation [164]. 

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