Sub-150 nm, high-aspect-ratio features using near-field phase-shifting contact lithography

Abstract

A simple technique, near-field phase shifting contact lithography, is described which can extend the resolution of contact lithography to less than 200 nm. Sub-150 nm, high-aspect-ratio features have been produced using chromeless phase-shifting masks, a Karl Suss MA-6 contact aligner and off-the-shelf SPR 510 photoresist. The technique is neither purely near field nor purely phase shifting considering subwavelength features have been patterned in resists whose thickness is twice the longest exposure wavelength. Conventional 5 in. chrome-on-quartz masks were reactive ion etched to generate precisely varied phase steps from 375 nm to 512 nm using standard CF4 etch chemistries and then stripped of chrome. These masks were then used to expose SPR 510 spun on bare silicon wafers to broadband and filtered UV irradiation. After development in an aqueous base developer, features were examined as a function of exposure wavelengths, shifter depth, resist thickness, resist type, exposure dose, and postexposure bake temperature and time. Features as small as 128 nm with aspect ratios of >5:1 have been printed with this technique with minimal sidewall roughness.