We examine theoretically and experimentally the characteristics of in-focus and out-of-focus images of simple, well-defined phase objects. Theoretical calculations are based on the theory of partial coherence, and a simple calculation for imaging with coherent light demonstrates distinctive aspects of bright-field images. Experiments are performed with a well-corrected microscope, equipped for the precise control of illumination conditions and focus position. Theoretical and experimental results agree, although the contrast in the experimental images is often lower than expected. Also verified by experiment is a (to our knowledge) previously uninvestigated linear response in the intensity modulation of defocused, coherent images of thin, phase objects. The near-focus behavior of phase object images differs in symmetry from the more-familiar behavior of opaque object images.

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While Hopkins1 showed that pure phase objects can be seen in bright field microscopy with coherent illumination, very little quantitative study of such system performance has been carried out to date. We evaluate intensity distributions of images, in and out of focus, from planar phase and amplitude objects in bright field microscopy. This includes numerical calculations, based on the theory of partial coherence, and their quantitative comparison to experiments with test objects. Objects that we consider are slits or bar charts with known transmission properties. They are fabricated by photolithographic methods and their structure is verified by electron microscopy. In particular, we observe the effect of defocus under partially coherent illumination for objects near in size to the resolution limit of the light microscope. We observe that images of phase objects in partially coherent illumination are asymmetric about the focal plane, in agreement with theory, although the contrast is often lower than predicted. Also, we calculate spatial frequency content of the image intensity patterns and analyze the effects of scalar theory breakdown at high aperture angles.

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Process improvements attributed to the use of bottom anti-reflective coatings (B.A.R.C.s) are well documented. As our experience with these materials improves, so does our understanding of additional optimization. Recent supplier experiments suggest an increase in the thickness of AZ® BARLi (bottom anti-reflective layer i-line) solution to reduce photoresist swing curve ratios. Also, changes in thin film stack on common substrates can adversely affect the degree of photoresist reflective notching. It is therefore of extreme importance to determine optimum thickness(es) of a B.A.R.C. material to ensure maximum process potential. We will document several process effects in the conversion of a SRAM test device (0.38 -O.45jtm) from a 650A to a 2000A BARLiTM fi thickness using conventional i-line photolithography.Critical dimension (CD) uniformity and depth of focus (DOF) are evaluated. Defect density between the two processes are compared before and after etch employing optical metrology and electrical test structures. Sensitivity of overlay as a function of BARLiTM fip thickness is investigated as well.