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Absorption-induced lens heating within optical elements can diminish the achievable resolution of the lithographic process in semiconductor wafer steppers, as the distortion of the transmitted wavefront leads to deteriorations of the imaging performance. In reverse, sensitive monitoring of thermally induced wavefront distortions also establishes a procedure for fast and precise absorption measurements, since the extent of deformation is directly proportional to the absorption loss. In this paper a parallelized photo-thermal absorption measurement system based upon a Hartmann-Shack wavefront sensor with extreme sensitivity is presented, providing quantitative absorptance data of optical materials with sub-ppm resolution. Caused by the temperature dependence of the refractive index as well as thermal expansion, the initially plane wavefront of a probe beam is distorted into a convex or concave lens, depending on sign and magnitude of index change and expansion. Wavefront deformations as low as 50pm (rms) can be registered. Moreover, due to the spatial resolution of the employed wavefront sensor, the technique is insensitive to misalignments, allowing for a rapid and reliable assessment of material quality. Absolute calibration of the absorption data is achieved by comparison with a thermal calculation.
The method accomplishes not only to measure absorptances of plane optical elements, but also wavefront deformations and focal shifts in lenses as well as in complex optical systems. Thus, it is employed already at many places in the semiconductor industry for quality assurance of test optics and components. Along with a description of the technique we present results from absorption measurements on coated and uncoated optics as well as mirrors for 193nm and 248nm. Extensions to EUV optics are discussed.
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