Christine E Heckle

Laser-induced birefringence in fused silica from polarized lasers

Fused silica, when exposed to excimer laser light, exhibits permanent anisotropic birefringence and wavefront changes. These laser-induced changes depend on the silica composition and processing conditions. The optical anisotropy is most clearly observed in samples that are exposed with linear polarization. This polarization-induced effect has been known for several years, but has become much more important with the advent of immersion lithography and its associated very high numerical apertures. High numerical aperture optics require controlled polarization, notably linear polarization, in order to maintain phase contrast at the image. When birefringence and wavefront changes are induced by laser exposure, the image contrast at the wafer deteriorates. We interpret the changes in optical properties in terms of permanent anisotropic strain induced by laser damage, and the associated strain-induced optical effects. This is accomplished using the mathematics of tensors to account for anisotropic strain and optical anisotropy, and using finite element analysis to calculate the strain fields taking the sample and exposure geometries into account. We report the relations between underlying density and strain anisotropy changes and the induced birefringence and wavefront for a given experimental sample geometry. We also report some examples of the different degree of laser damage from silica with different compositions and processing conditions.

Measuring and tailoring CTE within ULE glass

Corning Incorporated is improving material and metrology in order to meet the requirements for both EUVL optics and photomask substrate applications. The EUV optics requirements present a unique challenge to the lens designer. The temperature of each optic in the system can experience a different thermal profile based upon the geometry of the element and the intensity of the beam at each element location. This places a need on the optical material for small variation in the coefficient of thermal expansion (CTE) uniformity and the ability to achieve targeted optimum zero CTE cross-over temperatures. This paper addresses Corning’s ability to target specified CTE values as well as discusses a new metrology tool for measuring CTE variations within the glass. Past data suggested that index variation within the material were related to CTE variations. This correlation was investigated with the results presented here. This preliminary work suggests a new metrology tool with the capability of non-destructively measuring peak to valley (P-V) CTE variations to within 70 parts per trillion per degree Kelvin (ppt/K) at possible spatial frequencies in the micron range on thick or thin samples. This technique is vital for certifying photomasks and will be the foundation needed to reduce CTE variations in photomasks and optics to targeted values of less than 1 ppb/K for future EUVL needs.

Development of Mask Materials for EUVL

Though the Semiconductor market is soft, the technology that drives it continues to march on. Corning has supplied the semiconductor market through two generations of lithography with KrF and ArF grade HPFS Glass; the established excellence will continue with the supply of CaF2 for 157nm and ULE Glass for 13nm. ULE Glass is a low expansion silicate glass that has historically been used for ground and spaced based telescope mirrors such as Gemini and Hubble. Industry experts have now identified ULE Glass as a material of choice for EUVL applications; but with new opportunities come new hurdles, and ULE Glass will need to be improved in order to meet the challenges of EUVL. The purpose of this presentation is to give the audience a general update of Cornings ULE Glass improvement effort for EUVL, with focus on EUV photomask requirements; it will include an overview of key ULE Glass properties, improvements that have been made, and a road map of work to be done.