Pass - a chalcogenide-based lithography scheme for I.C. fabrication

This paper contains details of a multi-layer resist scheme which provides excellent resolution capability as well as reducing many of the problems associated with the use of conventional microlithographic resists. In the "PASS" (planarized arsenic/sulfur/silver) scheme, surface planarization is first achieved by spinning on a layer which is self levelling by spin-casting. A thin film of A3S67 (at. %) is then vacuum deposited on this layer and topped with silver. The planar nature of the resist scheme reduces the problems of focus variations at steps in the circuit topology for optical lithography. During exposure, the silver diffuses rapidly into the As-S with little lateral spread. The As-S compound is soluble in a CF₄ plasma whereas the As-S-Ag ternary compound is extremely insoluble under the same conditions. We can therefore dry develop the active layer. The unremoved ternary is then used to selectively protect the planarizing layer during the subsequent dry etch of this material. In experimental studies, the resist system exhibited extremely high resolution; contrast is typically in excess of 13 for optical illumination and electron-beam direct writing has produced 35 nm lines spaced by 35 nm in the active layer.