Chemically amplified resist systems

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. 

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). 

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 ... 

A number of PAGs have been synthesized for use in chemically amplified resist systems. The choice of PAG to use for each specific application is dependent on a number of factors, including the nature of the radiation, quantum efficiency of acid generation, solubility, miscibility with resin, thermal and hydrolytic stability, plasticization effect, toxicity, strength and size of generated acid, impact on dissolution rates, cost, etc. Figure 7.10 shows classes, while Table 7.5 is a list of typical photochemical acid generators currently in use in chemical amplification resists. 

Scheme 7.27 The chemically amplified resist system invented by G.H. Smith and J.A. Bonham of 3M. The photogenereated acid cleaves the lipophilic tetrahydropyranal ether to generate the base soluble phenol.

Since the introduction of chemically amplified resist systems to DUV technology, the environmental stability and bake latitudes have been the major concern of this type of chemistry. Ketal resist systems have been very robust towards fiiese issues. The methoxypropene protected polyhydroxystyrene resist is our first initial work on ketal system. 

Chemically amplified resist system is a promising technology to attain hi resolution and high sensitivity for sub-quarter micron device fabrication. However, air-borne contamination (1-3), such as ammonia mainly generated from conventional adhesion promoter, hexamethyldisilazane (HMDS), severely affects this kind of resist. It causes surface insoluble layer of resist patterns, which results in failure of the pattern fabrication. 

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. 

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 ... 

It should be pointed out that the solubility change in all chemically amplified resists (CARs) occurs only in the dark reaction during the PEB. Some of the CARs such as acetal and ketal systems have low activation energy, so deprotection can occur with low-temperature PEBs or even at room temperature. In the absence of thermally driven diffusion such as in acetal and ketal resist systems, BARCs must be used for resists with such low PEB temperatures, which are significantly lower than the soft bake temperature, in order to control reflectivity issues associated with standing waves. 

Resist systems based on PBOCSt turned out to be very sensitive towards airborne impurities. These difficulties were overcome by employing another chemically amplified resist, a random copolymer consisting of p-hydroxystyrene and t-butyl acrylate (see Chart 9.3). 

Semiconductor manufacturing applications, using 1.0 nm X-rays as the exposure source, require resist sensitivities on the order of 50-100 nJ/cm in order to meet the desired throughput. Conventional resists, where an interaction between the exposing radiation and resist directly defines sensitivity, have not been able to meet this need. Attention has turned to chemically-amplified resists where the exposure creates a species which catalyzes multiple chemical events during the post-exposure bake(PEB). There are a variety of resist systems that can demonstrate chemical amplification, but Shipley SAL 605 - a negative-tone resist showing sensitivities on the order of 100 nJ/cm2 - is the system chosen for this quantitative study. 

Recently, this polythiophene derivative was evaluated as a topcoat discharge layer that was applied to a typical novolac resist [26]. The polymer was found to be very effective at eliminating resist charging. In another study [71], a water-soluble polythiophene derivative referred to as ESPACER was applied to a chemically amplified resist, and charging was eliminated during e-beam writing. In this system, the ESPACER was removed by a water wash prior to the development of the resist. 

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. 

Figure 14 Basic schematic of the function of a chemically amplified resist based on photoacid-catalyzed deprotection reactions. In the simplest two-component system, exposure of a PAG produces an acid that subsequently causes the catalytic deprotection of the protected polymer resin.

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