Silent discharges for the generation of ultraviolet and vacuum ultraviolet excimer radiation

Excimers, known from electron beam experiments and UV lasers, can also fomi in silent discharges, a discharge type used for large-scale ozone production. In this high pressure non-equilibrium discharge a dielectric barrier leads to the formation of localized short-lived microdischarges. Inside the microdischarge filaments energetic electrons can excite rare gas (RG) atoms which, under appropriate conditions, react to form RG/RG dimers or RG/halogen dimers. With the pure RG dimers Ar2*, Kr2*, Xe2*, their chlorides ArCl*, KrCl*, XeCl* and their fluorides ArF*, KrF*, XeF* a number of narrow-band UV sources can be obtained in the wavelength region between 120 nm and 360 nm. The physical parameters of the microdischarges and the reaction kinetics of excimer formation in silent discharges are discussed. The potential of these new UV sources for industrial UV processing is demonstrated. As an example of an application for such incoherent excimer UV sources structured and large-area metal deposition induced by the 172 nm radiation from a silent discharge in xenon is shown. Patterned thin palladium films could be produced by irradiating a palladium acetate coating through a contact mask. In a second step thicker copper layers were deposited on top of the palladium patterns by an elec@oless plating process. INTRODUCTION Energetic UV photons can initiate a variety of chemical, physical or biological processes. Some examples are photochemical synthesis and degradation, the UV initiated polymerization of coatings and paints, the UV induced deposition of metals and dielectrics.and the disinfection of drinking water. For the industrial application of such processes UV sources of high intensity are required. In most cases the UV spectrum of the source will have to be matched to the intended process. Intense UV radiation is commercially available from medium and high pressure arc discharges in xenon and mercury / rare gas mixtures. These discharges emit a broad spectrum ranging from about 200 nm to the near infrared. For many desired processes selective UV radiation of certain wavelengths is required. Apart from UV lasers only few sources and wavelengths are available. By far the most important UV source is the low pressure glow discharge in mercury /rare gas mixtures emitting the Hg resonance lines at 254 nm and possibly also at 185 nm. Much lower photon fluxes can be obtained from low pressure rare gas resonance line sources, e.g. the krypton line at 124 nm and the xenon line at 147 nm. These VUV sources are mainly used as wavelength and intensity standards. Also the two VUV standards for continuum radiation the deuterium lamp and the argon miniarc, due to their moderate photon fluxes, are mainly used as laboratory tools. The utilization of excimer radiation from gas discharges provides a new approach for the generation of high-intensity narrow-band UV radiation (ref. 1,2). Excimers (excited dimers, trimers) are unstable excited molecular complexes of molecules that under normal conditions do not possess a stable ground state and thus are not known from classical chemistry. Under the influence of short wavelength irradiation or energetic particle bombardment excimers can form in many different gases and vapours. One of the most striking properties of these excimer complexes is that they disintegrate typically within less than a microsecond and, during their decay, give off their binding energy in the form of UV or VUV radiation. So far, the most important technical devices using excimer radiation are the excimer lasers. Commercial excimer lasers are pumped by pulsed gas discharges (ref. 3). Most investigations into excimer kinetics are based on the excitation of different gas mixtures by high energy electron beams (ref. 4-6). Alternative ways of generating excimers rely on the excitation by W V or synchrotron radiation (ref. 7-9), by protons (ref. lo), by aparticles (ref. l l , 12), heavy ions (ref. 13) or even nuclear fragments (ref. 14, 15). In addition, different types of gas discharges reaching from DC glow discharges (ref. 16) over AC discharges (ref. 17), pulsed discharges (ref. 18) to microwave discharges (ref. 23-26) have successfully been used as sources of excimer radiation. An interesting discharge which holds great promise for large-scale industrial generation of excimer UV radiation is the silent discharge. Its potential for high-intensity generation of UV and VUV radiation has evolved only in recent years. SILENT DISCHARGE CHARACTERISTICS Tanaka was probably the first to realize that rare gas excimers can form in silent discharges (ref. 27). Other investigations into the rare gas excimer continua in silent discharges were reported by Volkova et al. (ref. 28) and by Xu (ref. 29). While these experiments were mainly oriented towards providing miniature light sources for spectroscopic purposes newer investigations (ref. 1,2) show the possibility to build large-area high-intensity UV sources for industrial processing.