A New High Performance CA Resist for E-beam Lithography
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ABSTRACT Three major lithographic applications have emerged for electron beam exposure tools: optical mask fabrication, direct writing for device fabrication, and more recently projection e-beam printing. The traditional mask making process uses poly(butenesulfone) resist. A wet etch process was adopted to generate patterns on chrome. Recently, shrinking dimensions, optical proximity correction features, and the complexity of phase shift masks have forced the industry to a chrome dry etch process. ZEP, a poly(methyl cc-chloroacrylate-co-a-methylstyrene) based resist, has been well accepted for most of the >180 nm device mask making. The acceptance of ZEP comes in spite of its low contrast, marginal etch resistance, organic solvent development, and concerns of resist heating associated with its high dose requirements. These issues have spawned interest in using chemically amplified resist (CAR) systems for direct write and mask making applications. We have developed a high contrast resist based on ketal protecting groups, KRS-XE, which is robust against airborne contamination and can be used for all forms of e-beam exposure in both chrome mask and silicon processing. This high contrast resist is processed with aqueous base developer and has a wide bake latitude. The development of KRS-XE has provided the capability of fabricating chrome masks for future generation (< 180 nm) devices and has potential for use with projection beam exposure systems.
INTRODUCTION Although the mass production of silicon devices is based on optical lithography, electron beam lithography (EBL) has been the mainstay of chrome photomask fabrication , and is now being considered for next generation lithography (sub-100 nm devices) using projection exposure.' The three major applications of electron beam exposure tools in semiconductor lithography are direct writing (DW) of electron beam resists (EBR) on chrome for a 4X photomask manufacture; DW on silicon for custom circuits; and potentially projection electron beam using a 4X silicon membrane scatter mask.i To enable performance of all of the EBL strategies, adequate EBR's are needed that satisfy the different requirements associated wvith each specific product (device or mask) and exposure system. Table I outlines some of the desired performance requirements for EBR's. The evolution of EBR's for Cr mask making has followed pathways that have focused on both the sensitivity and etch resistance. Among the first resists used was poly(butenesulfone), 2 at 10 kV and was used a chain scission (CS) positive-tone resist with a sensitivity of-3 ýtC/cm with wet etching of chrome. For chrome mask fabrication for < 200 nm devices, a dry etch with chlorine/0 2 gas' is necessary, and a CS resist based on poly(methyl- a -chloroacrylate 2 -co-cu-methylstyrene), ZEP, is now being used at doses!', of 8 tC/cm (10 kV). Despite the acceptance of ZEP, some enhancements are desirable such as higher contrast (> 2)', enhanced 2 at 10 kV) for better RIE resistance (< 2 to I resist/Cr etch ratio) , and lower dose (
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