Chemical amplifiers

L First manufacturing use of chemically amplified resists Plasma-developed resist first described X-ray proximity lithography demonstrated Bis-azide rubber resists introduced DNO-novolac resist for microelectronics introduced Photoresist technology first applied to transistor fabrication DNO-novolac resist patented by Kalle... 

Fig. 18. Chemistry of PTBOCST chemically amplified resist. Pattemwise exposure creates small quantities of acid. In a subsequent heating step, pendant TBOC groups are cleaved under acid catalysis. The exposed and unexposed areas can then be differentiated on the basis of solubiUty.

Acid-C t lyzed Chemistry. Acid-catalyzed reactions form the basis for essentially all chemically amplified resist systems for microlithography appHcations (61). These reactions can be generally classified as either cross-linking (photopolymerization) or deprotection reactions. The latter are used to unmask acidic functionality such as phenohc or pendent carboxyhc acid groups, and thus lend themselves to positive tone resist apphcations. Acid-catalyzed polymer cross-linking and photopolymerization reactions, on the other hand, find appHcation in negative tone resist systems. Representative examples of each type of chemistry are Hsted below. 

Fig. 31. An acrylic terpolymer designed for chemically amplified resist applications. The properties each monomer contributes to the final polymeric stmcture are for MMA, PAG solubility, low shrinkage, adhesion and mechanical, strength for TBMA acid-cataly2ed deprotection and for MMA, aqueous...

Polymer Properties and Lithographic Performance in Chemically Amplified Resins. 

Acid—base reactions, anhydrous hydrazine, 13 567-568 Acid Black 63, 6 559 Acid blue, herbicide/algicide for aquaculture in U.S., 3 214t Acid catalysis, deep-ultraviolet chemically amplified resists based on, 15 163-181 Acid catalysts, 10 493. See also Acidic catalysts... 

Deep-ultraviolet chemically amplified resists, 15 163-181 Deepwater barges, 25 327 Deep-well turbine pumps, 21 68 Deesterification, of aspartame, 24 227 DEET, 2 549t Defaunation, 10 871 D,E,F color scale, 7 310 Defect Action Levels (DALs), 23 160 Defects, in silicon-based semiconductors, 22 232... 

Recently, nonionic acid precursors based on nitrobenzyl ester photochemistry have been developed for chemically amplified resist processes (78-80). These ester based materials (Figure 8) exhibit a number of advantages over the onium salt systems. Specifically, the esters are easily synthesized, are soluble in a variety organic solvents, are nonionic in character, and contain no potential device contaminants such as arsenic or antimony. In addition, their absorption characteristics are well suited for deep-UV exposure. 

A nonionic, non-volatile photoactive acid generator, 2,6-dinitrobenzyl tosylate has been recently reported and shown to be effective in chemically amplified resist systems (10). This ester is a nonionic compound that has a much wider range of solubility in matrix polymers and does not contain undesirable inorganic elements. While it is known to exhibit a lower sensitivity to irradiation than the onium salt materials, many structural variations can be produced to precisely vary the acid properties of the molecule and to control the diffusion of the AG in the polymer matrix (11). 

Three matrix polymers were chosen to evaluate the effect of polymer structure on the performance of chemically amplified resists. TBS was used as the reference material, while TBMS and TBSS were selected because of their ability to undergo... 

The chemically amplified resists reported here for deep-UV applications require a post-exposure thermal treatment process step to effect the deprotection reaction. This step has proven to be critical, and in order to understand the processing considerations it is instructive to discuss, qualitatively, the various primary and secondary reactions that occur with these systems during both exposure and PEB, ie ... 

From these highly idealized reactions, one can gain an understanding of some potential diffculties and process related concerns. For this system to work satisfactorily, it would be necessary for the radiation generated acid concentration, [H+], to remain constant. However, in most chemically amplified systems, undesired side reactions occur that prematurely destroy the acid, i.e., reactions with contaminants such as water, oxygen, ions or reactive sites on the polymer (reactions 2 and 3). 

The rates of these reactions depend upon the contaminant concentration and the inherent rate constants of the reactions. While the exact nature of these reactions differ for each type of chemically amplified system and are not fully understood, this generalized discussion is sufficient to understand many of the process issues. 

The process control of the post-exposure bake that is required for chemically amplified resist systems deserves special attention. Several considerations are apparent from the previous fundamental discussion. In addition for the need to understand the chemical reactions and kinetics of each step, it is important to account for the diffusion of the acid. Not only is the reaction rate of the acid-induced deprotection controlled by temperature but so is the diffusion distance and rate of diffusion of acid. An understanding of the chemistry and chemical kinetics leads one to predict that several process parameters associated with the PEB will need to be optimized if these materials are to be used in a submicron lithographic process. Specific important process parameters include ... 

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