Acid photogenerators

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

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 ... 

The resist films were irradiated with the same incident doses used for the determination of acid content. A postbake was performed at 100°C for one minute. The carbonyl absorption was measured before and after irradiation and postbake. The difference was used to determine the number of t-BOC groups removed for a given weight of resist film. By dividing the number of t-BOC groups by the amount of photogenerated acid, the catalytic chain length was determined (Table II). 

Defined as mmol of carbonyl loss per mmol of photogenerated acid. 

Figure 1. Comparison of crosslinking catalyzed by added acid and photogenerated acid for crosslinker 1.

Xeotron also uses a DMD processor to produce arrays in microchannels (Figure 2.6). Standard oligonucleohde phosphoramidite-based S5mthesis is performed. The virtual masking is directed instead toward deprotechon of a pho-tolabile acid. The process is called PGA or photogenerated acid. Once the acid is liberated, the trityl group on the attached base can be removed to allow base extension to occur. Stepwise pelds of 98.5% are reported for the XeoGhip. 

Note 3 An example of chemical amplification is the transformation of [(fert-butoxy-carbonyl)oxy]phenyl groups in polymer chains to hydroxyphenyl groups catalyzed by a photogenerated acid. 

Recently, many similar systems have been reported in the literature. Examples include acid-photogenerating compounds in combination with t-BOC-protected maleimide or hydroxyphenyl maleimide copolymers (69, 70), tert-butyl ethers of phenolic resins (71), silylated phenolic resins (72, 73), and polycarbonates that contain acid-labile linkages in their backbone (74, 75). Onium-salt-photogenerated acid has also been used in another two-component system to catalytically depolymerize polyphthalaldehyde (76). 

Gao, X., LeProust, E., Zhang, H., et al. (2001) A flexible light-directed DNA chip synthesis gated by deprotection using solution photogenerated acids. Nucleic Acids Res. 29, 4744-4750. 

Photoresist compositions have been prepared that consist of silsesquioxanes containing sulfonamide substituents. These materials are useful in multilayer resist systems that provide contrast upon exposure to photogenerated acid. 

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