Resist materials chemical amplification

A number of new resist materials which provide very high sensitivities have been developed in recent years [1-3]. In general, these systems owe their high sensitivity to the achievement of chemical amplification, a process which ensures that each photoevent is used in a multiplicative fashion to generate a cascade of successive reactions. Examples of such systems include the electron-beam induced [4] ringopening polymerization of oxacyclobutanes, the acid-catalyzed thermolysis of polymer side-chains [5-6] or the acid-catalyzed thermolytic fragmentation of polymer main-chains [7], Other important examples of the chemical amplification process are found in resist systems based on the free-radical photocrosslinking of acrylated polyols [8]. 

The seminal work on deep-UV resist materials which incorporate chemical amplification was started at IBM San Jose s Research Laboratory in 1979 when FrSchet and Willson first prepared poly(4-t-butyloxycarbonyloxy styrene) and end-capped copolymers of o-phthalaldehyde and 3-nitro-l,2-phthalic dicarboxaldehyde. 

The latest addition to this list of dry developing resist materials is a contribution from IBM s San Jose Research Laboratory (66-67) that evolved from efforts to design positive-tone resist materials that incorporate chemical amplification. These efforts were stimulated by the fact that the quantum yield of typical diazoquinones of the sort used in the formulation of positive photoresists is 0.2 to 0.3 thus, three or four photons are required to transform a single molecule of sensitizer. This places a fundamental limit on the photo-sensitivity of such systems. 

In order to circumvent this sensitivity limitation, the San Jose researchers sought to design resist materials that incorporate chemical amplification of the sort that characterizes the silver halide photographic emulsion system. In these systems a single photo event initiates a cascade of subsequent chemical reactions that ultimately result in the intended function. 

Ito, H. Willson, G., "Chemical Amplification in the Design of Dry Developing Resist Materials," SPE Regional Technical Conference, Ellenville, New York, Nov. 1982. 

Several groups have investigated three-component systems encompassing both chemical amplification and dissolution inhibition. As stated earlier, Smith and Bonham (63) reported resist materials composed of a binder resin (novolac), a nonpolymeric compound containing acid-labile functional groups such as acetals, and a trihalomethyl-substituted 5-triazine acid photogenerator. The acid-labile compound acts as a novolac dissolution inhibitor in a manner analogous to the action of DNQ in conventional positive resists. However, in this case, the inhibitor is not photochemically active. Instead,... 

One final example of the application of onium salt photochemistry in positive resist materials should be mentioned, because it does not include any postexposure acid-catalyzed processes and therefore does not encompass the principle of chemical amplification (79). Interestingly, Newman (79) has determined that onium salts themselves can inhibit the dissolution of novolac in aqueous base and that irradiation of such an onium salt-novolac resist restores the solubility of the resin in developer and leads to a positive-tone image. In this application, the onium salt behaves like diazonaphthoquinone in a typical positive resist. Recently, Ito (80) has reported also the use of onium salts as novolac dissolution inhibitors. 

Over the past few years we have been interested in the design of new types of resist materials which generally possess high sensitivities due to structural features which allow for the occurrence of radiation initiated repetitive processes. The three main approaches we have investigated to-date all maximize the use of available protons through "chemical amplification" they are the following ... 

The chemical amplification idea appeared when it was necessary to develop photoresist material having high sensitivity, high resolution, and good plasma etch resistance. It was desirable primarily when the 248 nm exposure became the requirement of the industry. And the result of implementation of the idea was very good. Chemical amplification as a basic ideology of the photoresist creation partly worked at the 193 nm millstone. ... 

We are continuing to explore the limits of the PBOCST and polyphthalaldehyde resist systems, as well as other materials that incorporate chemical amplification. 

H. Ito and C.G. Willson, Chemical amplification in the design of dry developing resist materials, Technical Papers of SPE Regional Technical Conference on Photopolymers, p. 331, Society of Plastics Engineers, Brookfield, CT (1982). 

Faced with the shortcomings of the polyphthaldehyde resist (presented helow in chemical amplification resists based on depolymerization), the search for chemically amplified DUV resists resulted in a quick switch to more stable materials based on poly(p-hydroxystyrene), a phenolic polymer that Willson et al. were studying as a potential replacement for novolac. They observed that poly(p-tert-butoxycarbonyloxystyrene) (PBOCST), which is poly(vinyl phenol) protected with tert-butoxycarbonyl groups (t-BOC), is far more stable than the unprotected p-hydroxystyrene and could be purified and polymerized under controlled conditions. The resulting protected polymer could be easily deprotected thermally by heating it to 200°C or to a much lower temperature (100°C) by treatment with acid generated from the exposure of onium salts, just as in the poly(phthaldehyde)... 

How the balance between the above-named requirements are struck in each of the major advanced resist-processing schemes in use today is discussed below, along with the advantages and drawbacks of each technique. In particular, we discuss the material basis of the resolution limit issues of resists, especially as they concern those based on chemical amplification systems, since these constitute the majority of resists in use in advanced lithographic processing today. 

Each approach has its characteristic advantages and disadvantages due to the underlying technology and the materials issues involved (see the remainder of this section). In contrast, SLR schemes are relatively simple processes, can have moderate levels of resolution and etch resistance, and good hnearity, but they suffer from reflective swing problems and small depths of focus, and are limited to low aspect ratios. Irrespective of the resist process approach chosen, chemical amplification continues to be the dominant exposure mechanism of the imaging layer. 

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