Preionization and gain studies in fluorine based excimer laser gas discharges

Abstract

Fluorine-based excimer gas lasers are powerful sources of coherent radiation in the UV and VUV part of the electro-magnetic spectrum. Due to their short wavelengths and high output power they are widely employed in high resolution material processing like micromachining and in lithography. In this field pattern sizes several times shorter than the used wavelength can be achieved using an immersion technique. However due to the short duration of the laser pulse (typically of few tens of ns for commercially available fluorine-based excimer lasers), the laser pulse makes only a few round-trips inside the laser resonator cavity. As the result, the optical quality of the laser beam of such an excimer laser is poor, leading to non optimal focusing conditions. The purpose of this work was to investigate different methods and techniques to produce excimer lasers, based on fluorine containing gas mixtures, emitting optical pulse lengths of 100 ns or longer. In order to achieve this goal we have studied different gas discharge excitation techniques in a small scale discharge chamber (0.5 – 4 cm discharge gap, 5 cm electrodes diameter) at gas pressures varying from 2 to 5 bar. We have thoroughly investigated a pre - main pulse gas discharge pumping scheme with X-ray preionization and a single pulse excitation scheme with X-ray preionization. As preionization source we investigated two homemade X-ray sources. We have developed a high voltage open barrier discharge device producing a fast electron beam directly in the gas. It was shown that it was possible to generate soft X-ray radiation (10 – 100 keV) directly in gases by means of this beam. With this and with an earlier developed traditional X-ray source the produced electron densities in different gases and gas mixtures have been measured. During our experiments we discovered that, apart from the well known direct electron generation in the gas, a substantial part of the measured preionization electrons were generated indirectly by the Xrays via the photo-electric effect at the electrode. With the single pulse excitation scheme we succesfully ignited a homogeneous discharge and were able to sustain it as a stable, homogeneous pulsed gas discharge in gas mixtures typically containing 5 % of Ar and 0.1 % of F2 at a total gas pressure of 2 bar and an electrode distance of 1 cm. In this laser gas mixture the typical achieved peak power deposition density was 1 – 2 MW cm-3 with a pulse duration (FWHM) of ~ 100 ns. Under these conditions the observed spontaneous emission intensity was 119 kW cm-3 at the ArF* excimer wavelenth (193 nm). The width of this emission signal was ~ 60 ns (FWHM). With a probe laser (λ = 193 nm) the amplification of the probe signal in the excited laser medium was measured. Under the same conditions a net gain was measured of ~ 34±20 % cm-1 with a FWHM of ~ 60 ns. The combination of such high gain and the reasonably long optical pulse duration makes our gas discharge excitation system a promising device for the development of small scale fluorine-based excimer gas lasers. The short resonator length of several cm will result in a higher number of the intra cavity round-trips and thus to a better beam quality compared to the usual excimer lasers