Photogenerated acid, reactions

Pos twe-Tone Photoresists. The ester, carbonate, and ketal acidolysis reactions which form the basis of most positive tone CA resists are thought to proceed under specific acid catalysis (62). In this mechanism, illustrated in Figure 22 for the hydrolysis of tert-huty acetate (type A l) (63), the first step involves a rapid equihbrium where the proton is transferred between the photogenerated acid and the acid-labile protecting group ... 

In each system, the primary photo-event is dissociation of the cationic photoinitiator to produce an acid. This reaction proceeds with a quantum efficiency that is characteristic of the particular initiator. The photogenerated acid then interacts with a carefully chosen polymer matrix to initiate a chain reaction, or acts as a catalyst, such that a single molecule of photogenerated acid serves to initiate a cascade of bond making or breaking reactions. The effective quantum efficiency of the overall process is the product of the photolysis reaction efficiency times the length of the chain reaction (or the catalytic chain length). This multiplicative response constitutes... 

Acid Intermediate Initiated Radiation Curable Coating Compositions. In this process, photogenerated acid intermediates can cause ring opening reactions of various oxirane monomers and prepolymers that can then further polymerize into three-dimensional network structures. 

The addition of base quenchers to resists has been shown to improve LER at the expense of photospeed (see Fig. 17.28). This stems from the neutralization reaction between the photoacid and the base quencher within the exposed area, resulting in the increase in the photogenerated acid concentration gradient, as well as chemical contrast at the feature edge. The enhanced chemical contrast at the feature edge translates into enhanced development contrast between the exposed and the unexposed areas of the film the result is a much sharper edge (lower LER) than would otherwise be the case. 

Photogenerated acids can also catalyze various other reactions, e.g. the cross-linking of polymers containing epoxide groups (see Chart 9.4), or Claisen and pinacol rearrangements in polymers as shown in Scheme 9.5. Resist systems operating on the basis of these reactions have been proposed [12, 13]. 

However, the exact nature of the catalytically active species formed in the photochemical reaction of W(CO)6 in tetrachlorocarbon remains a matter of speculation. Later, it was observed that the more active and tractable catalyst for the metathesis of alkenes and the polymerisation of alkynes can be photogenerated in reaction of W(CO)6 carried out in alkane solution containing catalytic amounts of Lewis acid such as the halides group four, thirteen and fourteen elements. In such a system even higher catalytic activity is achieved than in tetrachlorocarbon solution (Eq.2) [12, 15, 20]. 

Image Blur in CA Resists. The mobility of the acid catalyst in the film influences CA resist performance. At the molecular level, the proton must move from site to site to effect acid-catalyzed reactions in the resist film. If the mobility is too great, however, the initial spatial profile of photogenerated acid will be blurred and the final developed resist image will appear distorted. The problem becomes more acute at finer resolution. In those situations where diffusion of the... 

Figure 1. Schematic of the deprotection reaction showing the protected polymer, PBOCSt, and deprotected analog, PHOSt, Shown encircled is the protecting group cleaved by the photogenerated acid.

Sol-gel condensation polymerizations have also been carried out using either photogenerated acids or bases as catalysts. For example, Hanson and Jensen employed a (diphenylmethyl)trimethylammonium salt as a photolatent source of trimethylamine to conduct the sol-gel condensation of tetraethoxysilane (Scheme 7). Postirradiation heating at 65 °C was necessary to drive the reaction to completion. Crivello and Mao and Croutxe-Baghorn have reported similar sol-gel condensation polymerizations of alkoxysi-lanes conducted using diaryliodonium salts as photoacid generators. 

The concept of photogenerating a latent image that could be dry developed in a dry bake process has been described for certain copolycarbonates." In this process a polymer such as (8), containing a photoacid catalyst such as an iodonium" or sulfonium" salt, is exposed to UV irradiation to photogenerate acid. Upon heating, an acid-catalyzed thermolysis occurs in the exposed areas to degrade the initial polymer back to volatile monomeric fragments. This reaction is shown in equation (6) for (8). 

Zhou et al. [177] use a random copolymer of tetrahydropyranyl methacrylate (THPMA) and methyl methacrylate (MMA) polymer doped with BSB-S2 as the PAG for microfabrication. At the laser focal spot, the THPMA groups were converted to carboxylic acid groups due to photogenerated acid-induced ester cleavage reactions, and were therefore rendered soluble in aqueous base developer. Figure 45 shows the 3D microstructure produced by this method. By two-photon fluorescence imaging, it was found that the buried channels are open and a continuous connection was made between the two cavities. 

Photocatalytic oxidation is a novel approach for the selective synthesis of aldehyde and acid from alcohol because the synthesis reaction can take place at mild conditions. These reactions are characterized by the transfer of light-induced charge carriers (i.e., photogenerated electron and hole pairs) to the electron donors and acceptors adsorbed on the semiconductor catalyst surface (1-4). Infrared (IR) spectroscopy is a useful technique for determining the dynamic behavior of adsorbed species and photogenerated electrons (5-7). 

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