ENHANCED MICROLITHOGRAPHY USING COMBINED PHASE-SHIFTING AND OFF-AXIS ILLUMINATION

Off-axis illumination is a promising optical microlithography techniqu e which can be used to improve the image quality of line-space pattern s. With this method the image is produced by the zero and first order diffracted beams. Due to the intensity difference between these two or der diffracted beams the contrast of the image cannot be unity. This p aper demonstrates the optical enhancement that can be achieved by a co mbination of interferometric phase shifting and off-axis illumination. In such an arrangement the mask is illuminated symmetrically from bot h the front and back sides, and not two but in fact four - (two zero a nd 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 pi, and the intensity of the transmitted beam is about 13% of the reflected beam. This impro ved quality image with feature sizes of 0.4 mu m was recorded in a pho toresist using an Ar+ ion laser operating at 457.9 nm.