A number of fused silica samples were evaluated for their resistance to densification by deep
ultraviolet (UV) radiation at 193nm wavelength. Density changes for all the samples equal the
product of a material dependent constant and the absorbed two-photon dose to a sub-linear
power of about 2/3. This dose dependence is consistent with earlier compaction studies using
UV, electron and gamma radiation.
We propose a fictive temperature model to describe fused silica structure; and the observed
stretched power dependence of compaction on deposited energy for ionization damage can be
explained by a simple network relaxation process. Experimental observations of isothermal-annealing
behavior of UV-induced compaction in fused silica agree very well with our
theoretical prediction; e.g. strong correlation between thermal recovery of compaction and the
compaction rates for different fused silica samples; preheat-treatment can manipulate the
compaction damage rates.
Fused silica is the optical material of choice for deep ultraviolet (DUV) lithographic systems. However, this material is subject to irradiation-induced compaction with ArF excimer radiation. Here we report direct, at-wavelength, wavefront measurements of DUV-laser-damaged fused silica samples performed using phase-shifting point diffraction interferometry (PS/PDI). Experimental results show that the damage anneals in the temperature range of 200 to approximately 600 degrees Celsius. Finally, the interferometric measurements are compared to birefringence studies performed on the same samples.
Different 1995 - 1996 grade experimental fused silica samples were evaluated for their resistance to UV-induced compaction at 193 nm under elevated sample temperature conditions. Stress induced birefringence was used as a sensitive compaction monitor. We found that compaction rate decreases with increasing sample temperature. Isochronal annealing experiments were performed on two different sets of pre-damaged fused silica samples. Annealing of compaction was observed at temperatures as low as 200 degree(s)C, and an activation energy of 0.1 eV was found.
Using birefringence monitoring, several experimental fused silicas are tested for 193-nm compaction durability. f N ( I J2 't 10.1 All samples exhibit densification that can be described by the equation: (Lip/p )u = 1C · l--7 · - · _.!!_J where 10 10 't 'tis the pulse length ('t0 = lns), Np is the pulse count, K is a constant, and I is the 193-nm energy density (I., = 1 mJ/cm2). The extracted value of K varies from 84ppB to 660ppB for experimental (1995-1996) fused silicas, as much as a factor of two improvement compared to (1990-1994) grades. The role of irradiation geometry in compaction is also investigated using finite element simulations. The net optical path difference formed for a given level of damage is found to increase by approximately 25% when the damage radii is increased from 30% to 70% of the total sample diameter in a relatively thin optical element. Keywords: damage, fused silica, two-photon, densification, compaction, lithography, 193-nm, stress-birefringence
Several experiments are reported to better predict the 193-nm radiation-induced compaction rates of fused silica at lithographic intensities. Birefringence monitoring is capable of measuring the compaction-induced birefringence distribution from modest total fluences. For example, 6 hours at 350Hz with a per pulse energy density of only 1.1mJ/cm2 produces a relative compaction of about 38 ppB and corresponding relative refractive index change of 11 ppB which is easily measured. Moreover, this value of compaction is more than would be predicted by extrapolation from past higher fluence experiments. Compaction follows a non- linear dependence on pulse count. Compaction rates drop with continuing irradiation but do not saturate in the 1-10 ppm compaction range. The dependence of compaction on intensity is found to accurately follow an intensity squared rule that is all data over a wide range of pulse energy densities can be plotted via a single curve. Using this result, one can now scale higher energy density tests to predict damage rates of materials at lower fluences. Although compaction rates can vary by a factor of three over all sample types, we find less than 25 percent variation in compaction rate among UV grade samples.
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