Multilayer Resist Systems For Optical And E-Beam Lithography

Abstract

A three layer resist system using spin-on intermediate layer has been described for optical and e-beam lithography. A thick bottom layer of positive photoresist, 0FPR800 (Tokyo Ohka) was spun on to the substrate, flood-exposed by a UV light, and baked for planarization. A thin intermediate layer of spin-on glass (SOG) or spin-on indium tin oxide (ITO), and a thin top layer of OFPR800 or polymethyl methacrylate (PMMA) were succes-sively spun on to the bottom layer. Transfer of the resist image pattern into the inter-mediate layer was performed anisotropically by reactive ion etching (RIE) in a C₃F₈ plasma for SOG or a CC1₄ /N₂ plasma for ITO. The pattern in the intermediate layer was then replicated in the bottom OFPR800 layer by RIE in an 0₂ plasma. Experimental details are described. The planarizing characteristics of bottom layer, OFPR800 increase with UV exposure and increasing baking temperature. For optical lithography, the complexity of bottom layer processing is discussed with particular emphasis on the planarizing character-istics, the absorption of the exposing wavelength, and the alignment accuracy. For e-beam lithography, spin-on indium tin oxide (ITO) has been developed to prevent the charging up of e-beams in thick bottom layer. High resolution and good CD (critical dimension) control is achieved on the topographic substrate in optical and e-beam lithography.