Ulrich Neukirch

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.

Time-resolved performance analysis of a second-order PMD compensator

Design, test, and performance requirement and analysis for a polarization-mode-dispersion compensator (PMDC) with four degrees of freedom is presented. The performance is analyzed on the basis of time-integrated and time-resolved bit-error ratio (BER) measurements. Signal impairments are generated by both, first- and higher-order emulators. The probability distributions of bit errors measured over many one second intervals exhibit very long tails. Therefore even a PMDC with a good average BER performance may result in a significant total outage time for a given system.

High-accuracy measurement of the optical transmittance of optical bulk materials at deep-ultraviolet wavelengths

A measurement setup that is capable of measuring the internal transmittance of fused-silica prisms at 193 nm with a precision better than 0.01%/cm (3sigma) is presented. Its application to materials and wavelengths other than those that were chosen here for demonstration is straightforward. A lack of any standards makes it impossible to determine the absolute accuracy (also called measurement uncertainty) experimentally; however, calculations indicate that it is almost within the same margin as the precision.

Exposure and compositional factors that influence polarization induced birefringence in silica glass

Silica glass exhibits a permanent anisotropic response, referred to as polarization induced birefringence (PIB), when exposed to short wavelength, polarized light. The magnitude of the PIB has been empirically correlated with the OH content of the glass. Our recent studies pertaining to PIB have focused on careful characterization of PIB, with particular emphasis on understanding all of the contributions to the measured birefringence signal and finally extracting only that signal associated with birefringence arising from exposure to a polarized light beam. We will demonstrate that a critical contributor to the total birefringence signal is birefringence that comes from exposure beam inhomogeneities. After subtracting beam profile effects we are able to show that PIB is proportional to the OH content of the glass. Polarized infrared (IR) measurements were performed on glasses that developed PIB as a consequence of exposure to polarized 157-nm light. These studies reveal that there is preferential bleaching of a specific hydroxyl (OH) species in the glass with OH aligned parallel to the incident polarization undergoing more bleaching than those perpendicular. Further, we observe a very strong correlation between the measured PIB of these samples and the anisotropic bleaching. From these studies we propose a mechanism that can explain the role of hydroxyl in PIB.

Advances in the use of birefringence to measure laser-induced density changes in fused silica

Birefringence mapping of fused silica samples is used to measure density change in the material after exposure to excimer laser radiation. The proper techniques and methods that should be used to perform the exposure of the samples and the analysis of the birefringence results will be discussed. The quantitative analysis of birefringence measurements includes the correct subtraction of the initial birefringence of the sample and the comparison with a theoretical birefringence map calculated for a 1 ppm unconstrained density change under consideration of material and exposure parameters. Proper experimental conditions include the use of samples with low initial birefringence and a round circularly polarized laser beam with top-hat intensity profile.

New method for high-accuracy measurements of the internal transmittance in the deep-ultraviolet spectral region using prism-shaped samples

A method capable of measuring the internal transmittance Ti of fused silica @193 nm with a precision better than 0.01 %/cm (3σ) is presented. The basic idea is to vary the optical pathlength during the measurement within one and the same prism-shaped sample by moving the latter through the optical test beam. In comparison to the standard multiple-sample experiment this greatly relaxes the requirements for the repeatability of surface preparation. Lack of any standards makes it currently impossible to determine the absolute accuracy experimentally. However, calculations indicate that it is very likely within 0.02 %/cm (3σ). The application to materials and wavelengths other than what were chosen here for demonstration is straightforward.