<title>Conjugate twin-shifter for the new phase-shift method to high-resolution lithography</title>

Abstract

This paper describes a new phase shift method for 0.3 micrometers optical lithography. Phase shift lithography provides very high resolution, but current techniques suffer from high contrast levels at the shifter edges and from asymmetric optical intensity profiles. A new method uses conjugate twin-shifters to overcome these problems. This new method provides high resolution for both positive and negative resists and assures symmetric intensity profiles in bright field areas. This method uses two different phase shifts, of (pi) /2 and 3(pi) /2, respectively, placed alternately in the adjacent line pairs. While maintaining the desired (pi) phase shift between adjacent lines, the phase difference between the shifter elements and the mask substrate is reduced to (pi) /2, thus providing optimum resolution while avoiding undesired printing of the shifter edges. Symmetric intensity profiles are obtained by requiring that both shifters provide the same degree of phase shift relative to the substrate; i.e., the phase of the substrate is midway between the phases of the two shifters. The conjugate condition is defined by this phase relationship. The functional characteristics of this new method have been examined both theoretically and experimentally. Experimental results were obtained using a commercially available i-line stepper with 0.50 Numerical Aperture (N.A.) and partial coherence (sigma) -factor 0.50. Mask phase shifters were fabricated of sputtered SiO2 film. Tests were made using novolac-based positive resist.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.