Resists based on chemical amplification

As discussed previously, an optional postexposure, predevelopment bake can reduce problems with the standing-wave effect in DNQ-novolac positive resists. However, such a postexposure bake step is indispensable in the image reversal of positive resists (37-41) and certain resists based on chemical amplification of a photogenerated catalyst (64-67, 77, 78). For both types of resists, the chemistry that differentiates between exposed and unexposed areas does not occur solely during irradiation. Instead, differentiation occurs predominantly during a subsequent bake. Therefore, to obtain acceptable CD control in these systems, the bake conditions must be carefully optimized and monitored. 

Kaimoto, K. Nozaki, S. Takechi, andN. Abe, Alicyclic polymer for ArF and KrF excimer resist based on chemical amplification, Proc. SPIE 1672, 66 (1992). 

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. 

Major advances in radiation sensitivity have been achieved by applying the concept of chemical amplification to photolithography [252-254]. Chemical amplification is related to a cascade of chemical reactions that are initiated by a single photon. In fact, strategies based on chemical amplification have played a key role in the development of novel resist systems, as will be outlined in the following subsection. It is to be noted, that the chemical amplification concept has served the semiconductor industry as a workhorse for the past few decades. 

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

A shown in Fig. 7.1, there are two hroad categories of chemical amplification resists based on their imaging mechanisms, namely, (i) those based on acid-catalyzed main chain scission and (ii) those based on functional group polarity switch brought about by acid-catalyzed deprotection of lipophilic pendant groups, depolymerization, and Claisen rearrangement. 

Chemical amplification positive resists based on deprotection... 

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)... 

S Chemical amplification positive resists based on depolymerization 7.2.2.5.1 Poly(phthaldehyde) resist... 

In chemical development, the matrix resin of the resist system dissolves in the developer through a chemical reaction. Examples of resists that use chemical development include positive resists composed of novolac resins and DNQs, as well as positive chemical amplification resists based on phenolic, acrylate, and ali-cyclic polymers. These resists are developed with a 0.26-N aqueous solution of tetramethylammonium hydroxide. The exposed resins with phenolic and acidic functional groups dissolve in the developer via the chemical reactions... 

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