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.

Date Published: 7 March 2008
PDF: 10 pages
Proc. SPIE 6924, Optical Microlithography XXI, 69242P (7 March 2008); doi: 10.1117/12.775453

Published in SPIE Proceedings Vol. 6924:
Optical Microlithography XXI
Harry J. Levinson; Mircea V. Dusa, Editor(s)