Positive working electron-beam resists

Positive-Working Electron-Beam Resists Based on Maleic Anhydride Copolymers... 

POHL ET AL. Positive- Working Electron-Beam Resists... 

Poly(methyl methacrylate), PMMA, has remained the standard by which to judge positive-working electron-beam resists for over a dozen years (1>.2 .3 4) Hundreds of rival polymers have been disclosed. Most of them exceed PMMA in sensitivity. However, the combination of properties which include stability, sensitivity, contrast, adhesion, and solubility have kept PMMA in the limelight. 

Photochemical and Radiation Sensitive Resists Table 1 Some Substituted Acrylate Positive-working Electron Beam Resist Polymers... 

A variety of techniques have been used in the present work to establish the relative sensitivity of positive electron-beam resists made from copolymers of maleic anhydride (Table I). The term sensitivity is used rather loosely at times. In the most practical sense, sensitivity is a comparative measure of the speed with which an exposure can be made. Thus, the exposure conditions, film thickness, developing solvent and temperature may be involved. Most often, the contrast curve is invoked as a more-or-less objective measure of sensitivity. The dose needed to allow removal of exposed film without removing more than about 70% of the unexposed film can be a measure of sensitivity. The initial film thickness and the developing conditions still must be specified so that this measure is not, strictly speaking, an intrinsic property of the polymeric material. 

Bowden and his coworkers(j).) proposed a new type of positive electron beam resist which consists of an alkali-soluble novolac and polymeric dissolution inhibitor. The positive working mechanism of this new type positive resist( NPR ) is similar to that for the conventional positive photoresist 10). It was also found that poly(2-methylpentene-l sulfone)( PMPS ) is good as a polymeric dissolution inhibitor for NPR(lil). In addition, it was clarified that one of the difficulties with NPR is phase separation in the resist films(10)(n). 

Resists used to define circuit patterns are radiation-sensitive and may be either positive- or negative-working. As a result of the fine lines, there has been movement away from optical lithography and into the mid- or deep-uv regions. Developmental work has also been focused on electron beam, x-ray, and ion-beam exposure devices and resists (9,10). 

Another interesting, DUV-sensitive, planarizing layer for the exposure-PCM scheme is poly(dimethyl glutarimide) (PMGI) (structure 3.7). Exposure of PMGI to DUV light or electron beam radiation results in main-chain scission therefore, this resist is positive working. 

In the PCM systems just described, both top and bottom resists are positive working. Residual exposure of a bottom positive resist during UV or electron beam imaging of a positive top resist is acceptable. However, when a negative resist is used as the top layer, the residual exposure may reduce the contrast of a bottom positive resist. The several PCM systems involving the use of a negative resist as a top layer listed in Table 3.8 (imaging layers 3-6) indicate that if there is a sufiScient sensitivity difference between the top and the bottom resists, the residual exposure can be tolerated. 

An electron beam is of much shorter wavelength than the radiation used in standard microlithography and, therefore, provides greater resolution possibilities. For resist exposure a beam of electrons can be used whose position is controlled by a computer-driven beam deflector, thus obviating the need for a mask. Both positive- and negative-working resists are used for electron beam lithography. 

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