E-beam resists

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...

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.

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. 

The etch rate measurements for positive and negative-behaving e-beam resists are found in Table V. It is apparent that the etch resistance is lower the more sensitive the positive resist. The exception would be PMCN, which exhibits better dry-etch resistance than that which would be predicted based on e-beam sensitivity alone. Where e-beam sensitivity and etch resistance are needed, copolymerization becomes very important. This has been demonstrated for the MCN/MMA and MCA/MCN model copolymer systems in references 9 and 10, respectively. 

Dry-etch selectlvlties for several negative e-beam resists are also listed in Table V. They are more resistant than the positive e-beam resists of the Table except PMCN and the positive photoresists, AZ2400 and PC 129. The positive-behaving vinyl polymer resists tested are generally less resistant than the negative-behaving systems. This generality, however, does not hold for the photoresists tested, as the data of Table VII verifies. 

As was mentioned previously, resists based on the acid-catalyzed deblocking of poly(f-BOC-styrene) have been used also as e-beam resists (68). In fact, these materials are capable of <40-nm resolution in both the positive and negative modes. The sensitivity of these resists is six times that of PMMA. 

In addition to good sensitivity, issues for X-ray resist materials are analogous to those of optical and e-beam resists resolution, contrast, etch resistance, thermal stability, and adhesion. To stay competitive with e-beam and even optical lithography, X-ray lithography must have a resolution performance better than 0.5 p,m. An extensive list of X-ray resist properties has been collected in the literature (83, 116, 121). 

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