FABRICATION OF PHASE-SHIFTING MASKS WITH SHIFTER OVERCOAT

Phase-shifting masks have demonstrated the potential to extend the application of existing optical lithography tools by simultaneously enhancing their resolution, depth of focus, and exposure latitude. This paper identifies a processing sequence for the fabrication of phase-shifting masks using spin-on-glass (SOG) as a shifter overcoat. The SOG is applied uniformly, range of 11 nm, across conventional 5-in. mask substrates. A selective wet etch for patterning the SOG is also demonstrated. Using this processing sequence, alternate aperture, phase-shifting masks are fabricated, and both focus and exposure latitude are evaluated. Additionally, the role of shifter wall angle and level-to-level alignment on the sized rim shifter are explored. It was found that a 45-degrees shifter wall angle resulted in approximately 10% degradation in the aerial image of a 0.35-mu-m feature as compared with a vertical wall angle, and an alignment error of 0.25-mu-m on the reticle resulted in pattern placement errors of 0.06-mu-m. In order to circumvent second level lithography constraints several self-aligned schemes have been proposed. Two previously published techniques are reviewed and their implications on the shifter overcoat mask are discussed. Finally, having identified several of the limitations related to the overcoat process, issues related to the fabrication of phase-shifting masks with the shifter material beneath the chromium are defined.