Fabrication of x-ray masks using anisotropic etching of (110) Si and shadowing techniques

Abstract

We describe a process for producing x-ray masks of grating patterns with extremely smooth line edges. The technique developed by Flanders, in which a square-wave profile relief grating in polyimide is obliquely shadowed with an x-ray absorber, is followed, except that the original square-wave structure is produced in (110) silicon by anisotropic chemical etching rather than in SiO2 by reactive ion etching. In this way, significant improvements in edge acuity are achieved because relief grating sidewalls are defined by atomic (111) planes. Holographic lithography is used to expose grating patterns in AZ 1350 over a thin Si3N4 layer on the (110) Si. The Si3N4 is patterned by reactive ion etching and serves as the mask for anisotropically etching the square-wave-profile grooves. At the proper crystallographic orientation the groove sidewalls are defined by (111) planes, and groove bottoms are approximately flat. The structure in Si is then transferred to polyimide which is obliquely shadowed and forms the x-ray mask. Grating patterns replicated in PMMA using the CK x-ray (4.5 nm) show straight lines with uniform widths and edges smooth to ∠±4 nm.