Chemically amplified resist mechanisms

Exploration of Chemically Amplified Resist Mechanisms and Performance at Small Linewidths... 

The chemically amplified resist mechanism was the breakthrough that ultimately led to the next generation of 248-nm lithography. The initial work by Ito and Willson " employed the acid labile ferf-butyloxycarbonyl (t-BOC) group. The acid catalyzes the decomposition of the f-BOC groups, and thereby modifies the chemical composition. Several different trajectories can be followed from here, leading to both positive and negative tone resists. 

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...

It has been reported that the incorporation of nanoparticles such as fullerene and oligosilsesquioxane (POSS) to resist matrix improves the resist performance such as etch resistance and mechanical properties [107]. These resists are called nanocomposite resists. A single-component chemically amplified resist that incorporates not only POSS but also an acid generator into its main chain has been reported [108]. 

In contrast, the exposure mechanism of chemically amplified resists designed for EUV 13.5-nm exposure occurs by an entirely different mechanism from the mechanisms described above for DUV 248-nm and 193-nm resists. For one thing, the energy of EUV photons (92.4 eV) is 18 and 14 times higher than 248-nm and 193-nm photons, respectively. 

T. Iwamoto, M. Akita, T. Kozawa, Y. Yamamoto, D. Werst, D.A. Trifunac, and D. Alexander, Radi ation and photochemistry of onium salts acid generators in chemically amplified resists, Proc. SPIE 3999, 204 213 (2000) A. Nakano, K. Okamoto, Y. Yamamoto, T. Kozawa, S. Tagawa, T. Kai, H. Nemoto, and T. Shimokawa, Deprotonation mechanism of poly(4 hydroxystyrene) and its deriva tives, Proc. SPIE 5753, 1034 1039 (2005) T. Kozawa, A. Saeki, and S. Tagawa, Modeling and simulation of chemically amplified electron beam, x ray, and EUV resist processes, J. Vac. Sci. Technol. B 22(6), 3522 3524 (2004) T. Kozawa, A. Saeki, A. Nakano, Y. Yoshida, and S. Tagawa, Relation between spatial resolution and reaction mechanism of chemically amplified resists for electron beam hthography, J. Vac. Sci. Technol. B 21(6), 3149 3152 (2003). 

In the present study, the chemical mechanism and the kinetics of acid initiated crosslinking reactions of epoxy novolac based chemically amplified resists are examined. FTIR and thermal analysis have been used as the basic methods for elucidating chemical mechanism. Lithographic results obtained in a number of different processing conditions are interpreted in the context of the proposed mechanism. 

Resist Chemistry. The basic chemistry of epoxy novolac based chemically amplified resists has been proposed in the past by Stewart et al. (9J. According to this the Bronsted acid generated either photochemically or through electron beam exposure from the onium salt induces acid catalysed polymerization of the epoxy functionality. This mechanism implies that the proton generated by the exposure is actually bound to the polymer. Since the lithography consequences of this mechanism are obvious we decided to seek possible experimental evidence for the proton binding in the resist film under conditions of lithographic interest. 

While recent research regarding base catalyzed systems is now known (13, 14), the predominant diemistiy assodated with chemicalty amplified resists involves addolytic reactions. The add spedes is required for either crosslinking or deprotection reactions and is also often needed for depotymerization mechanisms. Add generator chemistry will be discussed separately since any of the available materials might find application in a chemically amplified resist composition. 

As with all chemically amplified resists, a major concern is, however, the latent image stabflity and the susceptibility to environmental conditions. With t-BOC deprotecdon systems, the influence of airborne nudeophilic contaminants has been recently demonstrated (23) the observadon of surface residues in a number of such materials (23, 24) may be traced back to the presence of ppb amounts of volatile bases. In the case of the acetal systems (19-21), the influence of trace bases is less pronounced, as even amine hydrochlorides are sdll sufficiendy addic to have some catatydc activity. Linewidth dianges with the interval between exposure and post exposure bake have been observed for both the t-BOC and the acetal systems. In the case of the t-BOC tystems, long intervals (several hoius) between exposure and post-e]q)osure bake will lead to a decrease of apparent sensidvity, which manifests itself as a linewidth inCTease, or, in extreme cases, as faUure to open the imaged areas. These effects are normally due to contaminadon by base traces, or, in cases where the presence of even ppb amounts of bases can be excluded, may be assumed to be the result of the same, unspecified chain terminadon (add annihilation) mechanism which is responsible for the containment of the calalytic reacdon to the immediate vicinity of the imaged resist. 

A new interesting concept to further increase the sensitivity of chemical amplification resists has been proposed primarily for 193 nm lithography. An acid amplifier in resist film, inert to radiation, produces a large number of acid molecules by its catalytic reaction with a photochemically generated acid (Fig. 10) [93,94]. In many cases only weak sulfonic acids such asp-toluenesul-fonic acid (tosic acid, TsOH) are generated by this mechanism and an aromatic structure incorporated into acid amplifiers to promote acid cleavage increases 193 nm absorption. 

H. Ito, Chemical amplification resists for microlithography, Adv. Polym. Set, 172, 149 (2005). Ito, Y. Maekawa, R. Sooriyakumaran, and E.A. Mash, Acid catalyzed dehydration A new mechanism for chemically amplified lithographic imaging, Polymers for Microelectronics, ACS Symp. Series 537, L.F. Thompson, C.G. Willson, and S. Tagawa, Eds., p. 64, American Chemical Society, Washington, DC. (1994). 

Photoacid diffusion behavior in t-BOC-blocked chemically amplified positive DUV resists under various conditions was studied. Based on the experimental results, it was confirmed that only one mechanism dominated the acid diffusion in the resist film, and two diffusion paths, i.e., the remaining solvent in the resist film and hydrophilic OH sites of base phenolic resin, existed. Moreover, the effects of molecular weight dispersion, acid structure, and additional base component on both acid-diffusion behavior and lithographic performance were revealed. Finally, the acid diffusion behavior in the resist film was clarified and the acid diffusion length that affected the resist performance could be controlled. 

Condensation polymerization mechanisms are probably the most prevalent in the design of chemically amplified negative resists and are the basis for the... 

The second type of chemically amplified depolymerization resist mechanism depends upon the incorportation of C-O bonds into the polymer backbone which can be cleaved by either hydrolysis or addolysis. This concept was first advanced by Crivello, who proposed that polymers such as polycarbonates and polyesters could undergo photo-induced add catalyzed hydrolysis reaction in polymeric film (9, 76). Although polymers could be designed to undergo catalytic chain cleavage in the presence of add, such an approach depends upon the inclusion of stoichiometic amounts of water in the polymer film. Uttle further work was reported on this concept until recently, when a new system for dissolution inhibition was described based upon the hydrolysis of polysilyl ethers in a novolac resin (24). 

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