Measurement of wavefront distortions in DUV optics due to lens heating

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 Lab 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 on-line 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 new sensor, which is capable to record relative changes in the range of λ/10000 (corresponding to deformations of < 100 pm), allows registration and precise characterization of induced wavefront distortions by real-time Zernike analysis. On the other hand, the photo-thermal technique can be employed for a rapid assessment of the material quality, since the extent of transient wavefront deformation is directly proportional to the absorption losses. When used in orthogonal test geometry on cuboid samples, quantitative determination of both surface and bulk contributions to the overall absorption can be obtained by comparison with thermal theory. Along with a description of the new technique we present photo-thermal measurements on various fused silica samples under 193 nm irradiation. The data are compared with theoretical results obtained from a semi-analytical solution of the heat diffusion equation. Excellent agreement is achieved regarding both shape and extent of the lens heating effect.