Resist images

Fig. 36. Representative bilayer resist systems. Both CA and non-CA approaches are illustrated (116—119). (a) Cross-linking E-beam resist, 193-nm thin-film imaging resist (b) acid-cataly2ed negative-tone cross-linking system (c) positive-tone CA resist designed for 193-nm appHcations and (d) positive-tone...

In each of these approaches, imaging is confined to the top of a single polymeric film by adjusting optical absorption. The penetration depth of the silylation agent and the attendant swelling of the polymer film must also be controlled to avoid distortion of the silylated image. Resists of this type are capable of very high resolution (Fig. 37). 

The topmost imaging resist in 2LR system needs not only high sensitivity and high resolution but excellent dry etching durability to 02 RIE. In addition to this, it must have the ability to coat a uniform, pin-hole free film on a substrate. It is preferable that the resist is solid at room temperature to be handled easily. In order to satisfy these characteristics, we designed a molecular structure of the topmost imaging resist as shown in Figure 1. 

The formulated resist composition is then spin coated on silicon wafers. The resist coating layer is exposed through a photomask at 193 nm, and then the exposed coating layers are post-exposure baked at 110°C. The coated wafers are then treated with a diluted aqueous tetramethylammonium hydroxide solution to develop the imaged resist layer and provide a relief image (61). 

Bilevel Processes. A bilevel system consists of a thick resist at the base and a thin imaging resist on top. Many variations have now been reported. Conventional image transfer into the bottom layer is accomplished... 

Figure 3.52. Patterns formed in a bilayer resist system consisting of a 1.2-p.m-thick planarizing layer of PMMA and a 0.4-[im-thick RD2000N imaging resist. Reproduced with permission from reference 19. Copyright 1984 Technical...

That polymerization is observed in this system even after irradiation has ceased is evidence of the occurrence of a chain reaction mechanism. This postirradia-tive polymerization leads to the gradual enlargement of features in the imaged resist, which degrades the resolution. The use of radical scavengers such as l,l-diphenyl-2-picrylhydrazyl (DPPH) has been demonstrated as an effective way of inhibiting this reaction. ... 

Also worthy of mention is the introduction of conductive electron-beam resists that eliminate the charging effect that is so troublesome in electron-beam lithography. Todokaro et al. accomplished this by using a partially chloromethylated poly(diphenylsiloxane) as the top imaging resist in a bilayer system, and using the ionically conductive ammonium salt of poly(p-styrene sulfonate) (XXI) as the bottom layer. 

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