Improving resolution in photolithography with a phase-shifting mask

TLDR

A phase‑shifting mask uses a transparent layer to create opposite optical phases between adjacent apertures, causing destructive interference that reduces diffraction and nearly doubles resolution under partial incoherence (σ < 0.3) and yields substantial improvements for σ < 0.7, with only a minimal loss of depth of focus. Experiments show a 40 % increase in usable resolution, achieving up to 1000 lines/mm, and demonstrate that phase‑shifting masks enable proximity printing with larger mask‑wafer gaps, making them a highly desirable solution for enhancing lithography resolution in the VLSI/VHSIC era.

Abstract

The phase-shifting mask consists of a normal transmission mask that has been coated with a transparent layer patterned to ensure that the optical phases of nearest apertures are opposite. Destructive interference between waves from adjacent apertures cancels some diffraction effects and increases the spatial resolution with which such patterns can be projected. A simple theory predicts a near doubling of resolution for illumination with partial incoherence σ < 0.3, and substantial improvements in resolution for σ < 0.7. Initial results obtained with a phase-shifting mask patterned with typical device structures by electron-beam lithography and exposed using a Mann 4800 10X tool reveals a 40-percent increase in usuable resolution with some structures printed at a resolution of 1000 lines/mm. Phase-shifting mask structures can be used to facilitate proximity printing with larger gaps between mask and wafer. Theory indicates that the increase in resolution is accompanied by a minimal decrease in depth of focus. Thus the phase-shifting mask may be the most desirable device for enhancing optical lithography resolution in the VLSI/VHSIC era.