Positive photoresist photosensitization

Positive imaging techniques, 19 201 Positive ion spectroscopy, 24 107 Positive photochromism, 6 588 Positive photoresists, 20 280-281 Positive photosensitive polyimides,... 

Microlithography, Xerography. Because of their photosensitivity, polysilanes are under intense investigation for use as positive photoresist materials (94) (see Lithographic resists). They are particularly attractive because both wet and dry development techniques can be used for imaging (131,132). The use of polysilanes for xeroprinting has been reported (133). Thermal and optical sensors based on the photodegradation of polysilanes have been developed (134). 

Positive Photoresists. Positive resists are entirely different from negative resists. For the purposes of this discussion we restrict ourselves to visible-light-sensitive materials. Typically, these materials are mixtures of low-molecular-weight phenol-formaldehyde polymers and derivatives of naphtho-1,2-quinone diazide, the photosensitive component. The former is soluble in aqueous alkali, but the presence of the latter, a hydrophobic species, inhibits attack of this developer on the film. On irradiation the "sensitizer" is converted to a ketene, which, after reaction with water, forms a base-soluble carboxylic acid. Thus the irradiated part of the film is rendered soluble in the developer and it can be removed selectively. The important feature of this system is that the unirradiated areas are not swollen by the developer and the resolution of this material is quite high. It is possible to prepare gratings having several... 

Positive photoresist—A type of photosensitive polymer that leaves a barrier only where not exposed to light. 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

Photoresists are divided into two classes. Negative resists are materials whose solubilities are decreased by exposure to actinic radiation. The image is then developed by selectively dissolving the more-soluble, unexposed portion of the film. This basic process was invented 150 years ago and it is used in the manufacture of printing plates (63). Positive resists, on the other hand, are materials whose solubilities are increased by exposure, so that in development the exposed portion of the film is removed. The history of photoresists is described in Kosar s book (63), which is also an excellent review of the history of many photosensitive materials. 

Photosensitive onium salts provide the means for the development of novel positive and negative photoresists based on the concepts advanced in this paper. Work is currently proceeding both at this laboratory and in others to explore the unique opportunities for photoresists based on these materials as latent photochemical sources of strong protonic acids. 

Photosensitized degradation of poly(olefin sulfones) similar to the Hg(3P) photosensitized reactions of olefin sulfones make them subject to photodegradation in easily accessible wavelength regions. Almost all poly(olefin sulfones) have been reported only as positive tone electron beam resists (4). As the only exception, poly(5-hexene-2-one sulfone) has been reported as a positive tone photoresist with or without a photosensitizer, benzophenone (5). Because this polymer has a carbonyl chromophore, its photosensitivity is clearly derived from the polymer structure itself. 

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