Quasi-Brewster angle technique for evaluating the quality of optical surfaces

the requirements for optical surfaces have drastically increased for the deep-ultraviolet (DUV) and vacuum-ultraviolet (VUV) spectral regions. Low optical loss and laser durable fluoride optics are required for microlithographic applications. A nondestructive quasi-Brewster angle technique (qBAT) has been developed for evaluating the quality of optical surfaces including both top surface and subsurface information. The negative quasi-Brewster angle shift at wavelengths longer than 200nm has been used to model the distribution of subsurface damage. By using effective medium approximation (EMA), the positive quasi-Brewster angle shift at wavelengths shorter than 200nm has been explained by subsurface contamination. The top surface roughness (TSR) depicted by the qBAT is consistent with atomic force microscopy (AFM) measurements. The depth and the micro-porous structure of the subsurface damage measured by the qBAT, has been confirmed by magnetorheological finishing (MRF). The technique has been used to evaluate optically polished CaF2 (111) and MgF2 (00 1) surfaces.