Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks

The application of the phase-shift method allows a significant resolution enhancement for proximity lithography in mask aligners. Typically a resolution of 3 mu m (half-pitch) at a proximity distance of 30 mu m is achieved utilizing binary photomasks. By using an alternating aperture phase shift photomask (AAPSM), a resolution of 1.5 mu m (half-pitch) for non-periodic lines and spaces pattern was demonstrated at 30 mu m proximity gap. In a second attempt a diffractive photomask design for an elbow pattern having a half-pitch of 2 mu m was developed with an iterative design algorithm. The photomask was fabricated by electron-beam lithography and consists of binary amplitude and phase levels.