Beam resists

Other investigations dealt with straight-chain molecules (oi-tricosenoic acid) in which the penultimate and final carbon atoms at the hydrophobic end are connected by a double bond [91, 92]. The material does not polymerize as rapidly as those described before when irradiated by UV light, however, but it is readily polymerized when bombarded with an electron beam. It was thus thought to be an optimal material for the fabrication of electron beam resists. 

Lercei M J, Tiberio R C, Chapman P F, Craighead H G, Sheen C W, Parikh A N and Aiiara D L 1993 Seif-assembied monoiayer eiectron-beam resists on GaAs and Si02 J. Vac. Sc/. Technoi. B 11 2823-8... 

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...

Although these polymers have iaadequate stabiUty and processibiUty for most plastics appHcations, the abiUty to undergo scission back to the gaseous monomers has afforded some utility ia fabrication of electron-beam resists for photoUthography. Polybutene sulfone (251) and polyhexene sulfone (252) have been developed for this small-volume but high value appHcation. 

The pursuit of further miniaturization of electronic circuits has made submicrometer resolution Hthography a cmcial element in future computer engineering. LB films have long been considered potential candidates for resist appHcations, because conventional spin-coated photoresist materials have large pinhole densities and variations of thickness. In contrast, LB films are two-dimensional, layered, crystalline soHds that provide high control of film thickness and are impermeable to plasma down to a thickness of 40 nm (46). The electron beam polymerization of CO-tricosenoic acid monolayers has been mentioned. Another monomeric amphiphile used in an attempt to develop electron-beam-resist materials is a-octadecylacryUc acid (8). 

We have also performed preliminary imaging experiments usin E-beam exposure, these experiments indicate that the 80 20 copolymer is a sensitive E-beam resist material which requires an exposure dose of < ljiC/cm. Further experiments involving both E-beam and X-ray exposure are in progress. 

COP, the familiar negative e-beam resist developed at Bell Laboratories, is an example of a one-component negative resist system. COP is a copolymer which has excellent film-forming characteristics, resistance to etchants, and intrinsic radiation sensitivity. 

Figure 6. Dissolution kinetics as a function of dose for an experimental e beam resist. Note that a 3 pClcm dose causes 1 pm of resist to dissolve in 848 sec at which time 0.62 pm of unexposed resist remains undeveloped. The data was generated on the FT AH Film Thickness Analyzer, Figure 7.

Figure 11. A sensitivity plot for a positive-tone experimental e-beam resist. The data is from Figure 8.

Attempts to improve the sensitivity of PMMA through synthesis of analogs while preserving its attractive processing characteristics occupied resist chemists for several years during the early 70 s and research in the area of acrylate radiation chemistry continues to produce new results. The first electron beam resist used in device manufacturing can be considered to... 

An early commercial interest in poly (olefin sulfones) was sparked by the low raw materials cost, but this interest waned when it became apparent that thermal instability is a general characteristic of this class of materials. In 1970 Brown and O Donnell reported that poly (butene-1-sulfone) is degraded by gamma radiation with a G(s) approaching 10, making it one of the most radiation-sensitive polymers known (38-39). The potential for use of this radiation sensitivity in the design of electron beam resists was quickly realized by several members of the electronics industry. Bell Laboratories, RCA, and IBM published studies demonstrating the potential of poly (alkene... 

C The Epoxy Resists. The first negative tone electron beam resist materials with useful sensitivity were based on utilizing the radiation chemistry of the oxirane or epoxy moiety. The most widely used of these materials, COP (Figure 32) is a copolymer of glycidyl methacrylate and ethyl acrylate and was developed at Bell Laboratories (43,44). COP has found wide applicability in the manufacturing of photomasks. The active element... 

Figure 32. COP, Bell Laboratories negative e-beam resist. The resist is a copolymer of glycidyl methacrylate and ethyl acrylate.

Figure 33. The change in developed line-width with vacuum storage time after exposure for three commercially available, negative e-beam resists.

Another interesting positive-tone polyacrylate DUV resist has been reported by Ohno and coworkers (82). This material is a copolymer of methyl methacrylate and glycidyl methacrylate. Such materials are negative e-beam resists, yet in the DUV they function as positive resists. Thermal crosslinking of the images after development provides relief structures with exceptional thermal stability. The reported sensitivity of these copolymers is surprising, since there are no obvious scission mechanisms available to the system other than those operative in PMMA homopolymer, and the glylcidy side-chain does not increase the optical density of the system. 

Liutkis, J. Parasczak, J. Shaw, J. Hatzaski, M. "Poly-4-Chlorostyrene, A New High Contrast Negative E-Beam Resist," SPE Regional Technical Conference, Ellenville, New York, Nov. 1982, p 223. 

Figure 3. Size of clear and opaque 2.0 fim features as a function of exposure dose for a negative electron beam resist. The dose Dp that results in the correct feature size is denoted as the "sensitivity .

Methods 1 and 3 have been utilized in dry developed resist systems. To our knowledge, there are no resist systems commercially available that depend on post-exposure treatment other than the post-curing effect in negative electron beam resists mentioned earlier. Since such systems are still largely in the research phase we will not discuss them here but rather refer the reader to the literature for more detailed descriptions (44-50). 

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