Wavelength invariant Bi/In thermal resist as a Si anisotropic etch masking layer and direct-write photomask material

Abstract

Bilayer Bi/In thin film thermal resists are Bi and In films which form an etch resistant material at ~7 mJ/cm 2 laser exposures with near wavelength invariance from visible to EUV. New simulations predicted that Bi/In film of 15/15nm absorbs substantially at 1 nm, which projects single pulse exposure sensitivity of ~16 mJ/cm 2 , hence suggesting good sensitivity to X-ray range. Thermal modeling has confirmed the exposure time/optical energy requirements for Bi/In. Exposed and developed Bi/In resist etches slower than silicon dioxide in alkaline-based silicon etchants TMAH, KOH, and EDP, making it a better masking layer for anisotropic Si etching. Also Bi/In has been used to create a direct-write photomask as its optical transmission changes from OD>2.9 before laser exposure to OD<0.26 after exposure. Both Bi/In anisotropic etching and direct write masks have been combined to successfully build test photocells with V-groove surface textures by using Bi/In masked silicon anisotropic etching and the other layers created using regular lithography but with Bi/In masks. These devices showed no operational differences from those created with regular resist processes. Investigation of resist interactions with Silicon after laser exposure and strip were done with Auger surface analysis which showed no detectable Bi or In contamination on substrates and no substrate sheet resistance change. X-ray diffraction and Rutherford back scattering tests suggest that the converted Bi/In may involve oxides.

Materials modeling and development for use in double-exposure lithography applications

“…Chapman et al have investigated inorganic thermal resist systems 12 that use Bi/ In bilayers as an etch masking layer for silicon. However, the use of Bi and In metals are not compatible with the photolithography process because the target semiconductor devices are very susceptible to metal contamination.…”