Experimental verification of high-numerical-aperture effects in photoresist

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Abstract
This work describes an experimental setup approximating the output of a 0.85 NA reduction stepper which is used at the limits of its resolution. The experimental method concentrates on verifying the numerical predictions of vector imaging theory. Since this theory is based on a plane-wave decomposition of the vector image field, two-beam and three-beam interference are the simplest forms. Alternating phase masks, attenuated phase masks, and standard masks can be represented by this arrangement. The setup uses a periodic grating mask to obtain diffraction orders, and then substitutes mirrors for the imaging lens which results in the desired beam interference at the image plane. A unique experimental process for obtaining the record of the image distribution is presented which results in decorating the image cross- section for analysis. SEM photographs reveal that beams of high obliquity have drastically different behavior within a photoresist film for S and P polarization for the two-beam case. The addition of a third central beam, with three-beam interference, results in a reduction in the difference between S and P polarized illumination.

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