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7 March 2008A novel photo-thermal setup for determination of absorptance losses and wavefront deformations in DUV optics
Lens heating due to absorbed UV laser radiation can diminish the achievable spatial resolution of
the lithographic process in semiconductor wafer steppers. At the Laser- Laboratorium Göttingen a
measurement system for quantitative registration of this thermal lens effect was developed. It is
based upon a strongly improved Hartmann-Shack wavefront sensor with extreme sensitivity,
accomplishing precise online monitoring of wavefront deformations of a collimated test laser beam
transmitted through the laser-irradiated site of a sample. Caused by the temperature-dependent
refractive index as well as thermal expansion, the formerly plane wavefront of the test laser is
distorted to form a rotationally symmetric valley, being equivalent to a convex lens. The observed
wavefront distortion is a quantitative measure of the absorption losses in the sample. Thermal
theory affords absolute calibration of absorption coefficients.
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K. Mann, A. Bayer, U. Leinhos, T. Miege, B. Schäfer, "A novel photo-thermal setup for determination of absorptance losses and wavefront deformations in DUV optics," Proc. SPIE 6924, Optical Microlithography XXI, 69242P (7 March 2008); https://doi.org/10.1117/12.775453