Minoru Otani

Development of optical coatings for 157-nm lithography. I. Coating materials

In a basic study to identify low-loss optics for applications in F2 lithography, five potential coating materials (AlF3, Na3AlF6, MgF2, LaF8, and GdF3) and three deposition methods (thermal evaporation by a resistance heater and by electron beam and ion-beam sputtering) were investigated in the vacuum ultraviolet (VUV) region. Samples were supplied as single-layer coatings on CaF2 substrates by four Japanese coating suppliers. Refractive indices (n) and extinction coefficients (k) of these coatings at 157 nm were evaluated; the transmittance and the reflectance were measured by a VUV spectrometer and were compared. As a result, resistance heating thermal evaporation is seen to be the optimal method for achieving low-loss antireflection coatings. The relation among optical constants, microstructures, and stoichiometry is discussed.

Development of optical coatings for 157-nm lithography. II. Reflectance, absorption, and scatter measurement

The total loss that can be suffered by an antireflection (AR) coating consists of reflectance loss, absorption loss, and scatter loss. To separate these losses we developed a calorimetric absorption measurement apparatus and an ellipsoidal Coblentz hemisphere based scatterometer for 157-nm optics. Reflectance, absorption, and scatter of AR coatings were measured with these apparatuses. The AR coating samples were supplied by Japanese vendors. Each AR coating as supplied was coated with the vendors coating design by that vendors coating process. Our measurement apparatuses, methods, and results for these AR coatings are presented here.

Development of low-loss optical coatings for 157-nm lithography

In the F2 laser lithography, it is essential to reduce the loss of the optical coatings deposited on calcium fluoride lenses. In order to make low loss optical coatings, we have developed measurement apparatus, evaluated the coatings with various analyses, and found a correlation with the optical constants. In this paper we describe the optical loss measurement apparatus and the evaluation results analyzed for either single layer coatings or multi-layer anti-reflection coatings.

Measurement of Laser Damage Threshold of 355-nm Antireflection Coating

The laser damage thresholds of various types of antireflection (AR) coatings for an UV (355-nm) laser system were measured. Among them, the coatings consisting of two low-index materials, SiO2 and MgF2, had the highest average thresholds: 2.1 J/cm2 fused silica substrates and 2.6 J/cm2 sapphire substrates. The SiO2/MgF2 coating is hopeful for UV laser component.

Improvement of the lifetime of the optical coatings under high power laser irradiations

To maintain high transmittance for long time use in the ArF and F2 lithography exposure tools is highly requested, which is strongly dependent on the transmittance of the optical coatings. The following experimental fact was reported in the F2 laser durability testing; the transmittance degradation of the optical coatings did not occur under low power laser irradiation of less than 5 mJ/cm2, but a catastrophic degradation of transmittance did under high power laser irradiation over 15 mJ/cm2. We have set a program to investigate the causes of the laser-induced degradation of the optical coatings. Consequently, we have found a relationship between the contaminations of the irradiation atmosphere and the transmittance degradation through various experimental conditions and analyses such as SIMS and GCMS. Finally, we have succeeded in remarkably improving the durability of the antireflection coating (ARC) by optimizing the atmospheric conditions.