UV-excimer-laser ablation of polymethylmethacrylate at 248 nm: Characterization of incubation sites with Fourier transform IR- and UV-Spectroscopy

Abstract

Abstract. Ultraviolet and Fourier transform infrared (FTIR) spectroscopic experiments with thin films of polymethylmethacrylate (PMMA) are reported. During the incubation with 248 nm excimer laser light or continuous (cw) UV light sources PMMA exhibits a rapidly increasing, broad UV absorption. This is caused by the production of unsaturated species, which are detected in the infrared spectrum of irradiated PMMA films. The spectral data explain the incubation process preceding the ablation of PMMA at 248 nm. Taking advantage of the increased UV absorption, cw light incubated PMMA films can be selectively ablated by standard 308 nm excimer laser pulses. PACS: 33.20, 81.40, 82.30 Surfaces can be structured by ablation with intense UV laser light [-1,2]. A wide variety of polymers with different penetration depths for UV laser light show for certain fluence ranges high removal rates of material on the order of micrometers per pulse, along with little thermal damage to the edges. Thus the surfaces of many polymers have been patterned with a resolution down to less than one micrometer [3]. While this technique is already applied in industry and surgery, many effects ofphotoablation still have to be explained and a detailed understanding of the ablation process remains a challenge.In a wide fluence range polymethylmethacrylate (PMMA) does not start ablation with the first laser pulse, but requires a number of incubation pulses [-4, 5] before a steady ablation regime is reached. During these incubation pulses the material is believed to suffer damage in its chemical structure. The etch curve of PMMA (cf. Fig. 2 of [4]) with 248 nm excimer laser pulses shows for higher fluences a leveling off and reaches a saturation plateau, where a further increase in fluence does not yield a significantly higher etch rate. This effect had been attributed to a shielding of the PMMA surface by the ablation plume I-5,6]. Our model calculations [-4] showed, however, that this alone could not fully explain the effect. Assuming a dynamic absorption coefficient, depending on the irradiation dosis and the intensity of the laser light, the etch rate and the number of incubation pulses could be predicted quantitatively for the ablation of PMMA with excimer laser pulses of 16 ns duration at 248 nm. This paper will discuss the origin of the increase in absorption due to the irradiation dosis.Infrared and UV spectra provide valuable information about the excimer laser induced photochemistry of PMMA. In earlier studies [7][8][9][10][11] other researchers have observed an increasing UV absorption upon irradiation of PMMA with a 253.7 nm mercury lamp or 266 nm laser radiation. The increasing UV absorption was attributed mainly to the formation of carbonyl or unsaturated species [10], which, however, were not detected in the infrared spectrum.In the following we shall describe ultraviolet-and infrared-spectroscopic experiments with PMMA, irradiated with light from a 248 nm (16 ns) excimer laser and various continuous light sources. The light in...

Optical and chemical effects governing femtosecond laser-induced structure formation on polymer surfaces

“…Due to the complex nature of laser-polymer interactions, these novel structures are expected to have properties that are significantly different from the base material because of the defects generated during laser irradiation. These defects might have a physical origin due to a local increase in absorption [19] or a chemical origin due to preferential bond breaking [20].…”

Analysis of UV radiation transport in polymers exhibiting one-photon incubated absorption

“…These chromophores can increase absorption not only at the irradiating wavelength )~o but also at other wavelengths (usually on the long-wavelength side of 2 o), where the unirradiated polymer is essentially transparent. In UV laser experiments on relatively weakly absorbing polymers this steady growth in absorption can explain why there is a delay, in terms of number of pulses, before ablation commences, i.e., a so called incubation effect [1][2][3]. In these systems it has also been demonstrated that "incubated" absorption induced by short wavelength exposure can permit the subsequent ablation of the polymer at a longer laser wavelength and at a significantly lower fluence than tbr the unexposed case [3].…”

The effects of excimer laser irradiation at 248 nm on the surface mass loss and thermal properties of PS, ABS, PA6, and PC polymers

“…[3][4][5] For example, Kueper and Stuke reported production of unsaturated species in irradiated poly(methyl methacrylate) films. 6 The wetting behavior of the irradiated polymer improves as a result, and the increased surface roughness due to ablation can be an important effect in enhancing adhesion. 7,8 Enhancement of polymer surface conductivity due to laser irradiation has also been reported.…”