Fluoropolymers for 157nm Lithography: Optical Properties from VUV Ellipsometry and Absorbance Measurements

With the introduction of 157 nm as the next optical lithography wavelength, the need for new pellicle and photoresist materials optimized for this wavelength has produced much activity in optical characterization of thin film materials. Here we focus on ultratransparent fluoropolymers for 157 nm pellicle applications where absorbances below 0.01/μm are necessary to achieve transmissions above 98%. Transmission-based absorbance/μm measurements performed using VUV spectroscopy are characterized by rapid turn-around time, and are essential during the materials design and screening phase of a new materials development program. Once suitable candidate materials families have been identified for development into 157 nm pellicles, VUV ellipsometry becomes essential to model the film structure, characterize the complex index of refraction, and to tune the pellicle’s etalon design. Comparison of VUV absorbance measurements of fluoropolymer thin films on CaF2 substrates with VUV ellipsometry measurements of the same polymers on silicon substrates demonstrates some of the artifacts in, and helps define the accuracy of transmission based absorbance measurements. Fresnel interference fringes can produce transmission oscillations that can lead to underestimation, or even negative values, of the film absorbance. Film thickness nonuniformity can serve to reduce the Fresnel interference fringes, leading to reduced variation in the apparent 157 nm absorbance for μm thick films. VUV ellipsometry coupled with Fresnel analysis of the thin film/substrate system formally takes into consideration all of these optical artifacts, while at the same time determining the complex index of refraction of the materials. Using VUV ellipsometry and Fresnel analysis, the absorbance values do not show the large apparent oscillations, the film thickness is directly determined in the measurement, and film microstructure is also modeled. We have identified ultratransparent fluoropolymers which have 157 nm absorbances below 0.01/μm. These materials have the appropriate optical properties for use as 157 nm pellicles with greater than 98% transmission. This is an important for the development of 157 nm lithography, since the lack of a 157nm pellicle has been identified as a critical path issue.