Characterization of absorptance losses in optical materials using a high resolution Hartmann-Shack wavefront sensor

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 Gottingen 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.