Off-axis illumination is a promising optical microlithography technique which can be used to improve the image quality of line-space patterns. With this method the image is produced by the zero and first order diffracted beams. Due to the intensity difference between these two order diffracted beams the contrast of the image cannot be unity. This paper demonstrates the optical enhancement that can be achieved by a combination of interferometric phase shifting and off-axis illumination. In such an arrangement the mask is illuminated symmetrically from both the front and back sides, and not two but in fact four–(two zero and two first)–order beams produce the image. We show experimentally that the contrast of the image can be improved if the phase difference between the reflected and transmitted beams is π, and the intensity of the transmitted beam is about 13% of the reflected beam. This improved quality image with feature sizes of 0.4 µm was recorded in a photoresist using an Ar+ ion laser operating at 457.9 nm.
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