Acid-catalyzed deprotection, chemically

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

Figure 4.28. Molecular structures and photoinduced reactions of common photoresists. Shown (top) is the positive tone resist containing the active diazonapthoquinone (DNQ) chromophore group. Chemical amplification (CAM) reactions are illustrated in (i)-(iii). Reaction (i) represents photoinduced acid generation step (ii) is an acid-catalyzed deprotection mechanism (positive tone resist) and step (iii) is an acid-catalyzed crosslinking mechanism (negative tone resist).

The conversion of PBOCST to PHOST in the solid state can be conveniently monitored by IR spectroscopy because the fBOC carbonyl absorption at 1755 cm-1 shrinks in intensity, accompanied by appearance of a phenolic hydroxyl absorption at 3500 cm 1 as the acid-catalyzed deprotection proceeds (Fig. 15). Because this is the most classical and simplest form of the chemical... 

A new type of copolymer resist named ESCAP (environmentally stable chemical amplification photoresist) has recently been reported from IBM [163], which is based on a random copolymer of 4-hydroxystyrene with tert-butyl acrylate (TBA) (Fig. 37), which is converted to a copolymer of the hydroxystyrene with acrylic acid through photochemically-induced acid-catalyzed deprotection. The copolymer can be readily synthesized by direct radical copolymerization of 4-hydroxystyrene with tert-butyl acrylate or alternatively by radical copolymerization of 4-acetoxystyrene with the acrylate followed by selective hydrolysis of the acetate group with ammonium hydroxide. The copolymerization behavior as a function of conversion has been simulated for the both systems based on experimentally determined monomer reactivity ratios (Table 1) [164]. In comparison with the above-mentioned partially protected PHOST systems, this copolymer does not undergo thermal deprotection up to 180 °C. Furthermore, as mentioned earlier, the conversion of the terf-butyl ester to carboxylic acid provides an extremely fast dissolution rate in the exposed regions and a large... 

S Chemically amplified methacrylate negative resists based on acid-catalyzed deprotection and development in supercritical CO2... 

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. 

The first chemically amplified resist was designed by G.H. Smith and J. A. Bonham of the 3M Company for which they were granted a U.S. patent in 1973. It was used in the acid-catalyzed deprotection of tetrahydropyranal ether from a polymer of tetrahydropyranal ether of novolac to generate a base-soluble phenol (novolac), as shown in Scheme 7.27. Incidentally, this work was never published in the open literature and was never exploited by 3M, and remained lost in the patent literature for a considerable length of time. 

Other reported chemical amplification resist polymers based on the acid-catalyzed deprotection of ether-protected groups include the hydrolysis of tri-methylsilyl (TMS) ether polymer and the alcoholysis of tetrapyranal (THP) -protected group polymer all of these lead to conversion to PHOST (see Scheme 1.2% ) ... 

Scheme 7.29 Chemical amplification mechanism of acid-catalyzed deprotection of PBOCST.

One of the most important ester-protected polyhydroxystyrene-based resist copolymers, ESCAP (environmentally stable chemically amplified photoresist), developed at IBM, is based on the random copolymerization of 4-hydroxystyrene with tert-butyl acrylate (XXX).On exposure, this resist copolymer is converted to a copolymer of 4-hydroxystyrene with acrylic acid through photoinduced acid-catalyzed deprotection of the tert-butyl group (see Scheme 7.34). Because this resist system can be annealed at temperatures near its Tg in a process that hlls up the free volumes (voids in the resist matrix), thus preventing the out-diffusion of photoacids from the matrix and in-diffusion of airborne bases into the resist, neutralization reactions between the photoacids and bases in the resist matrix (otherwise known as poisoning) are reduced, thus allowing... 

In contrast to chain scission, chemical amplification (CA) employs an acid catalyzed deprotection or crossfinking reaction, which changes the solubility of the resist in the exposed areas. Therefore, a typical CA resist consists of a matrix polymer and a photoacid generator (PAG). The PAG is converted into a strong acid on absorption of a photon. The acid catalyzes the deprotection or crosslinking reaction and is regenerated (Scheme 2), which explains the amplification effect in the system. A variety of acid generator chemistries are available, both ionic and nonionic, but are not considered here. 

This methodology, applied to the MEM-protected thiohydroxamic ester of mycophenolic acid 1871. gave the requisite bromide 88 in 43% chemical yield. Treatment with K " CN, followed by hydrolysis of the resulting [ C]nitrile 89 and acid-catalyzed deprotection converted 88 into l C187. precursor to the immunosuppressive drug mofetil". ... 

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. 

Table 5.1 shows the results of the chemical assembly of compounds 2 and 3 using the two different deprotection techniques. As shown in the Table, under both basic and acid catalyzed conditions, mononitro compound 2 formed SAMs that are consistent with its theoretical calculated thickness. On the other hand, SAMs of compound 3 formed thinner layers under basis conditions and thicker layers under acid conditions when compared to the theoretical values. The thinner layer might be due to the formation of bond angles smaller than 180° or to incomplete SAM formation the thicker layer is likely a multi-layer. 

In 1979, Frechet and Willson put forward a very productive idea of a chemical amplification that was used in the development of a new generation of photoresists.They decided to use a photoresist comprising of a photochemical acid generator (PAG) and a polymer that was able to switch from hydrophobic to hydrophilic in the course of acid catalyzed hydrolysis. The PAG reacts with light to produce an acid catalyst. During a subsequent postexposure bake, the catalyst diffuses and reacts with the polymer component, causing many reaction events in the polymer and recovers the acid catalyst. The acid molecules catalyze the deprotection reaction and provide a prerequisite for chemical amplification. The number of the reaction events initiated by single quantum absorption has been estimated to be of order of 100. ... 

Acetals and ketals have attracted a great deal of attention recently as protecting groups of PHOST due to their lower activation energies of deprotection than fBOC and terf-butyl esters. While the majority of chemical amplification resists require PEB to accelerate acid-catalyzed reactions, deprotection of ac-... 

The solubility of the PAG in the solid polymeric matrix can influence CA resist performance (63). This is a particular issue for the ionic oniiun salts if the polarities of polymer and PAG are sufficiently different, phase segregation will occur, and the poor mixing of photoacid and polymer effectively reduces the efficiency of catalyzed deprotection. Additionally, resist contrast can be affected when the solubility of the photoproducts differs from that of the original onium salt. In fact onium salts can serve as dissolution inhibitors in novolac polymers, analogous to diazonaphthoquinones, even in the absence of any acid-sensitive chemical function on the poljuner (27). 

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