Evaluation of characteristics of VUV optical materials irradiated by F 2 laser

Abstract

The 157 nm molecular fluorine laser is regarded as the next generation light source for semiconductor exposure technology in the vacuum ultraviolet (VUV) region. Research for high performance F<SUB>2</SUB> laser optical materials is therefore indispensable. In this paper, we describe methods and results of evaluating optical materials used in the 157 nm region. We have developed an in-situ VUV evaluation system, which can measure the transmittance in the deep ultraviolet (DUV) and the VUV region directly after laser irradiation and the temporal transmittance during 157 nm-laser irradiation without airborne contamination. The system consists of a 2 kHz F<SUB>2</SUB> laser, an in-situ VUV irradiation system and a specialized VUV spectrophotometer. Laser irradiation and measurements were carried out under high purity nitrogen gas. During the first phase of F<SUB>2</SUB> laser irradiation (0 approximately 0.7 million pulses), a rapid transmittance increase (87% yields 89%) of calcium fluoride (CaF<SUB>2</SUB>) substrates was observed and this change took almost place within one minute after starting the irradiation. It is assumed that this effect is due to surface cleaning by the F<SUB>2</SUB> laser beam. Surface polishing has been excluded because the sample surface roughness measured with an atomic force microscope (AFM) showed no difference before and after irradiation. After an irradiation of 0.1 million pulses, the fast initial increase of the transmittance slowed down and finally reached about 89%. The slower increase might be correlated with a reduced chemical bonding of hydroxyl groups on the surface, because the transmittance change at 157 nm was in good agreement with the measured VUV transmittance below 170 nm. This is corresponds with the hydroxyl absorption band below 170 nm. The transmittance and reflectance of high reflection coated substrates were examined as well. Obvious damage and a huge reflectivity loss (82.4% yields 47.4%) were observed after 1.5 billion irradiation pulses. The information obtained during this work is very useful in devising optical F<SUB>2</SUB> laser components.