Illumination Optimization for Optical Semiconductor Metrology
Bryan M. Barnes, L Howard, Richard M. Silver
Uniform sample illumination via K hler illumination, is achieved by establishing a pair of conjugate focal planes; a light source is focused onto the condenser lens aperture while the image of the field aperture is focused at the plane of the specimen. Placement accuracy of these elements along the optical axis will determine the quality of the spatial homogeneity of the illumination in the sample plane. Imaging is therefore are inherently sensitive to the tolerance of this alignment. Measuring three-dimensional features on semiconductor wafers has demonstrated an additional requirement for illumination angular homogeneity, as this is critical for robust matching between experimental results and numerically modeled results. We outline our techniques for aligning a custom-built microscope that features a 12 mm diameter conjugate back focal plane, beginning with the establishment of an optomechanical datum. We then illustrate the techniques used to establish the quantitative degree of spatial and angular homogeneity through quantifying the resultant illumination while apertures are scanned across this conjugate back focal plane. Examples of the correlation between homogeneity and optical metrology are provided.