In-situ measurements of VUV optical materials for F2 laser

Abstract

F<SUB>2</SUB> laser lithography (wavelength:157 nm) is a candidate of post-ArF excimer laser lithography. In order to test the characteristics of vacuum ultraviolet (VUV) materials for F<SUB>2</SUB> laser lithography, we developed an evaluation system consisting of a 1 kHz F<SUB>2</SUB> laser, an in-situ real-time transmittance measurement unit and an in-situ VUV spectrophotometer. The precision of the real-time transmittance measurement is +/- 0.5%. The precision of the VUV spectrophotometer measurement is +/- 0.5% for scanned wavelengths (140 - 300 nm) and +/- 0.1% for a constant wavelength (at 157.6 nm). Due to F<SUB>2</SUB> laser irradiation cleaning, the transmittance of uncoated calcium fluoride (CaF<SUB>2</SUB>) substrates and of F<SUB>2</SUB> laser coatings at first rapidly and then gradually increased. Thereafter the transmittance remained constant. Results of the real-time transmittance and the VUV spectrophotometer measurement were almost identical. In addition, durability tests of CaF<SUB>2</SUB> substrates and of F<SUB>2</SUB> laser coatings were performed with a 4 kHz F<SUB>2</SUB> laser for more than 10 billion pulses (Bpls). After the initial transmittance increase of CaF<SUB>2</SUB> substrates, no change in transmittance was observed during more than 10 Bpls. In order to maintain the CaF<SUB>2</SUB> substrate transmittance, silicon compounds have to be removed from the purge gas and from the irradiation chamber where optical materials are placed. F<SUB>2</SUB> laser coating quality varied enormously between suppliers.