Effects Of Defocus On Photolithographic Images Obtained With Projection-Printing Systems

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Abstract
The images etched in a positive photoresist layer by means of polychromatic exposures and all-reflective projection printing methods can be described by a model presented elsewhere. For that model, the lateral intensity distributions of the optical image need be computed only at a single effective exposure wavelength that is compatible with a properly focused resist image profile. Although the illumination in the optical system we used was partially coherent, we successfully used the effective enhanced numerical aperture for the system, and calculated the lateral intensity distribution in the optical image on the assumption of incoherent illumination. In this study, we deposited a layer of Al over a Si substrate wafer which we then coated with a layer of quartz and a layer of positive photoresist. We calculated the dynamic exposure response of the photoresist film by using the modulation transfer function of a defocused perfect lens in order to simulate the resist images from zero to a few Rayleigh units of defocus. Simulated and experimentally determined resist image profiles and linewidths were compared and shown to be in good agreement. The results of this comparison lead us to believe that defocusing at the time of our study was caused by a tilt of the wafer along the scan axis.© (1978) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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