Large Area Photochemical Dry Etching of Polymers with Incoherent Excimer UV Radiation
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LARGE AREA PHOTOCHEMICAL DRY ETCHING OF POLYMERS WITH INCOHERENT EXCIMER UV RADIATION Hilmar Esrom, Jun-Ying Zhang and Ulrich Kogelschatz * Asea Brown Boveri AG, Corporate Research, W-6900 Heidelberg, GERMANY and *) CH-5405 Baden, SWITZERLAND ABSTRACT Photochemical dry etching and surface modification of different polymers, e.g., poly-methylmethacrylate (PMMA), polyimid (PI) and poly-ethylene terephthalate (PET) was achieved with an incoherent excimer UV source. Decomposition and etch rates of PMMA were determined as a function of UV intensity and exposure time at different wavelengths X = 172 nm (Xe2*), X = 222 nm (KrCI*) and X = 308 nm (XeCI*). The morphology of the exposed area of PMMA was investigated with SEM. The etching of the polymers can be explained as a result of extensive photooxidation. The results are compared with data obtained from mercury lamp and excimer laser experiments. INTRODUCTION Thin films, foils and casings of polymers find intensive use in electronic, electrotechnical and optical applications. For example, polyimides and fluoropolymers offer desirable properties for microcircuit fabrication [1]. PMMA is widely used as photoresist in lithographic processing [2]. Applications of metallized plastics in the automotive industry and in the fields of electromagnetic shielding and packaging technologies are spreading. Controlled modification of polymer surface properties by interaction of energetic particle beams, photons and plasmas is a field of considerable scientific and technical importance. It can be used to improve the adhesion properties of coatings as well as wettability and printability of polymers by changing the morphology and chemical properties of the surface. Selective processing of polymeric materials by ultraviolet (UV) irradiation with excimer lasers forms a growing field of applied research [3]. Recently, new incoherent UV excimer sources [4-5] were shown to be very useful in large area surface processing, e.g. the metallization of plastic materials [6-81. In this paper, we report on decomposition and dry etching of polymers (PMMA, PI, PET) by using incoherent excimer UV radiation at X = 172 nm (Xe2°), X = 222 nm (KrClI) and X = 308 nm (XeCI°). EXPERIMENTAL In our experiments we used incoherent excimer radiation from a dielectric barrier discharge (silent discharge) operating in pure xenon, gas mixtures of krypton/chlorine and xenon/chlorine providing intense narrow band radiation at X= 172 nm (Xe2°), X = 222 nm (KrClI) and X = 308 nm (XeCI'), respectively.The discharge was initiated in an annular gap between coaxial quartz tubes [5]. The outer quartz tube had a diameter of 3 cm and an active length of 15 cm defined by the length of the grounded wire mesh acting as transparent outer electrode. The radial width of the annular discharge gap was about 0.8 cm. The excimer-UVsources were operated at frequencies of about 20 kHz and voltage amplitudes up to 20 kV. More details about this special excimer UV source can be found in the Mat. Res. Soc. Symp. Proc. Vol. 236. 01992 Materials
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