Fluorine based excimer lasers such as KrF, ArF and F2 are currently the most powerful sources available in the ultraviolet wavelength range, operating at 248 nm, at 193 nm and at 157 nm, respectively. They are thus of central importance for numerous applications in this range. At these short wavelengths, reaching the laser threshold for an efficient operation, F2-based lasers require to be pumped, in a controlled manner, with very high power densities. This can practically be achieved only via pulsed, transversal glow discharges in high-pressure gas mixtures containing at least Fluorine, Krypton and Argon. However, such discharges are rather difficult to control because internal mechanisms, which are often not fully understood, let such discharges become instable with time. This manifests itself in a so-called filamentation of the discharge, i.e., the formation of thin channels of enhanced current. Eventually, an initially homogeneous discharge will turn into an arc that bridges the electrode gap. Such filamentation or arcing deteriorates the optical homogeneity of the gain medium, resulting in a termination of the laser pulse before the end of the current pump pulse, thereby limiting the maximum laser pulse duration to 20 - 30 ns. As a consequence, the resonator-internal light becomes amplified only through a smaller number of round trips rendering an output radiation consisting mostly of amplified spontaneous emission with a poor spatial coherence. This is a significant disadvantage because in many industrial applications of excimer lasers it is desired to have a diffraction-limited, high-quality laser beam. For instance, in material processing, a high beam quality allows structuring and drilling at high resolution. The desired high beam quality can be obtained only when the gain in the discharge is present and also remains spatially homogeneous for an extended time interval, such as several hundreds of nanoseconds, allowing the resonator-internal light to perform a sufficient number of round trips and thereby suppress amplified spontaneous emission. Realizing a stable, homogeneous discharge for up to hundreds of nanoseconds by elimination or suppression of discharge instabilities is therefore a key issue for providing high spatial quality and powerful UV laser radiation. The objective of this thesis is to identify the mechanisms that lead to discharge instabilities in F2-based excimer laser gas discharges and, also, to eliminate or suppress the growth of discharge instabilities with suitable measures.
|Award date||27 Apr 2007|
|Place of Publication||Enschede|
|Publication status||Published - 27 Apr 2007|