NIST PERFORMS FIRST MEASUREMENTS OF THE EFFECT OF STRESS ON THE REFRACTIVE INDEX OF MATERIALS IN THE VACUUM ULTRAVIOLET.

The semiconductor industry is presently developing a new technology known as 157 nm lithography for producing faster integrated circuits by further shrinking circuit critical feature sizes down to at least 70 nm. This technology is based on illuminating circuit patterns that have been created on photomasks with 157 nm excimer laser radiation, and imaging these patterns onto silicon wafers using high-precision optical systems known as steppers. The focusing characteristics of the optics depend on the index of refraction of the lens materials, first measured to high accuracy at the wavelength of 157 am at NIST. The index properties, however, are strongly distorted by stress, which is unintentionally grown into these optical materials during the fabrication process. The materials are also subject to unavoidable external stresses applied through the lens mountings and gravity. The high tolerances required for these optics necessitate accurate knowledge of the effects of stress on the index of refraction, characte rized by the stress-optical coefficients.

NIST recently completed the first measurements of the stress-optical coefficients in the ultraviolet near 157 am. These meas-urements were carried out on calcium fluoride and barium fluoride using a combination of a unique ultra-violet polarimetry system and a unique ultraviolet Twyman-Green interferometer that was developed at NIST. Lithography system manufacturers around the world are now using these values in the optical design of their 157 am lithography systems. The NEST ultraviolet polarimetry and interferometry systems are being exploited to characterize the index inhomogeneity and residual stress-birefringence of materials at 157 nm, measurements important for evaluating the suitability of materials for 157 nm lithography optics.