Rubber Cyclized

Schematic representation of the chemistry of the cyclized rubber - lns arylazide negative resists.

The first modern day negative photoresists were developed by the Eastman Kodak Company which utilized cyclized rubbers and cinnamic acid derivatives as photosensitive crosslinking agents (42). The first commercially important photoresist based on this chemistry was known as KPR, which was of a cinnamate ester of polyvinyl alcohol. It was introduced by Kodak in 1954. 

ODUR-110-WR, but no detailed lithographic evaluation has been published. Since the matrix resin is cyclized rubber, one would expect the same swelling limitation on resolution evident in conventional negative photoresists. 

Fig. 5. Chemistry of cyclized rubber—bis-azide negative acting resist, (a) Preparation of cyclized rubber resin from polyisoprene (b) photochemistry of aromatic bis-azide sensitizers. The primary photoproduct is a highly reactive nitrene which may combine with molecular oxygen to form oxygenated products, or may react with the resin matrix by addition or insertion to form polymer—polymer linkages.

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber 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 phenolic structure 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 carboxylic 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 solubility is controlled by chemical and polarity differences rather than molecular size. 

Synthetic polyisoprenes are superior to natural rubber in terms of consistency of properties. The are also freer of contaminants, and are preferred for applications that require lighter color, for personal care items and for derivatization to chlorinated and cyclized rubber products that are used in the adhesives and coatings industries. 

Chart 2.1. The negative-tone resists that were first used in semiconductor manufacturing were based on a matrix resin of synthetic rubber prepared by Ziegler-Natta polymerization of isoprene followed by acid-catalyzed cycliza-tion to improve the mechanical properties. This cyclized rubber was rendered photosensitive by addition of a bisarylazide that undergoes photolysis to produce a bisnitrene. The nitrene reacts with the cyclized rubber to create in-termolecular cross-links that render the exposed areas insoluble. 

The most widespread photoresists of negative tone consist of organic polymers and aromatic diazides dissolved in an organic solvent. In 1970-1980, the photoresists were based on cyclized rubber 10 and 2,6-di (4 -azidobenzylidene) -4-methylcyclohexanone... 

Photoresists, using cyclized rubber as a polymer and an aromatic diazide as a photoinitiator, swell strongly during development and have a very low contrast (7 = 0.5). Phenol- and cresolformaldehyde resins are frequently used as polymers and white azides as photosensitive components to avoid the last shortcoming. These azides 12 and 13 absorb at shorter wavelengths than the commonly used azide 11. 

Establishments primarily engaged in manufacturing synthetic rubber by polymerization or copolymerization. An elastomer for the purpose of this classification is a rubber-like material capable of vulcanization, such as copolymers of butadiene and styrene, or butadiene and acrylonitrile, polybutadienes, chloroprene rubbers, and isobutylene-isoprene copolymers. Butadiene copolymers containing less than 50 percent butadiene are classified in Industry 2821. Natural chlorinated rubbers and cyclized rubbers are considered as semifinished products and are classified in Industry 3069. 

Aromatic azides are stable photosensitive compounds and some of them mixed in cyclized rubber are practically used as photosensitive resines.(jJ Such photosensitive polymers as poly(vinyl p-azidobenzo-ate)(2) and poly(vinyl p-azidocinnamate)(3) are also investigated. These photosensitive resines are known to be spectrally sensitized by triplet sensitizersQ,2,3). But the energies E(T1) of the lowest triplet states T-) of aromatic azides have not been known, because these compounds emit no phosphorescence(4) and the trials on the measurement of their S0 >Ti absorptions are failed(4,5). 

Alpex. [Hoechst Celanese/Fine Chem.] Cyclized rubber resins for coatings and inks. 

Near contact printing Cyclized rubber 1 Kbit DRAM 8 12... 

The mechanism of the cross-linking reaction of cyclized rubber by photogenerated nitrenes, in other words, the photoprocess and the subsequent thermal reactions of azide photolysis in a polymeric substrate, are described by the following reactions (Scheme 6.6). ... 

Scheme 6.6 Reaction mechanism of the cross-linking reaction of cyclized rubber by photogenerated nitrenes.

Other characteristics of DNQ/novolac resists that have contributed to their lasting success in the semiconductor industry include their high etch resistance and the fact that they can be developed in environmentally benign aqueous base developers. In addition, the cyclized rubber/bis-azide negative tone resists did not image well at the Hg g-line, the exposure wavelength of the earliest commercially available wafer steppers, the introduction of which effectively brought about the complete and wholesale conversion of the IC industry to novolac resists from the cyclized rubber/bis-azide. ... 

Cross-linking of a linear polymer backbone by the light-induced decomposition of a photosensitizer to generate active species (e.g., cyclized rubber with azide type photosensitizer). 

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