7 March 2008 A novel photo-thermal setup for determination of absorptance losses and wavefront deformations in DUV optics
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Abstract
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); doi: 10.1117/12.775453; https://doi.org/10.1117/12.775453
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