Diazoquinone sensitizer

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). 

The positive resist materials evolved from discoveries made by the Kalle Corporation in Germany who developed the first positive-acting photoresist based on the use of a novolac matrix resin and a diazoquinone photoactive compound or sensitizer. The original materials were designed to produce photoplates used in the printing industry. These same materials have been adopted by semi-conductor fabrication engineers and continue to function effectively in that more demanding application. 

The latest addition to this list of dry developing resist materials is a contribution from IBM s San Jose Research Laboratory (66-67) that evolved from efforts to design positive-tone resist materials that incorporate chemical amplification. These efforts were stimulated by the fact that the quantum yield of typical diazoquinones of the sort used in the formulation of positive photoresists is 0.2 to 0.3 thus, three or four photons are required to transform a single molecule of sensitizer. This places a fundamental limit on the photo-sensitivity of such systems. 

Wilkins and coworkers have redesigned both the sensitizer and the matrix resin (78-79). They have tested a variety of o-nitrobenzyl esters of cholic acid as sensitizers. These substances, like the diazoquinones, are insoluble in aqueous base but undergo a photo-reaction that yields base soluble products. The matrix resin chosen for the new sensitizer materials is a copolymer of methyl methacrylate and methacrylic acid that is far more transparent than novolac resins in the DUV. The new resist materials are reported to have useful sensitivity (ca. 00mJ/cm ) and extremely high contrast. The resist formulation is essentially aliphatic in nature and would be expected to be less stable to dry etching environments than the aromatic-based novolac resin materials (24). 

Although many types of compounds have been tested as sensitizers in phenolic host resins (Novolacs, Resols, etc.) (S), all commercial positive resists employ aromatic diazoquinones of some type which photochemically generate base soluble products via Wolff rearrangement initiated by the loss of nitrogen (6). A staggering variety of diazoketones have been synthesized and evaluated for lithographic purposes, but derivatives of J[ and 2 are most commonly employed (5). 

H. Meier and K. Zeller, The Wolff rearrangement of diazo carbonyl compounds, Angew. Chem. Int. Ed. (English) 14, 32 (1975) M. Kaplan, D. Meyerhofer, Response of diazoquinone resists to optical and electron beam exposure, RCA Rev. 40, 169 (1979) Sensitivity of diazoquinone resists to optical and electron beam exposure, Polym. Eng. Sci. 20, 1073 (1980), D. Ilten, R. Sutton, Spectrophotometric determination of photoresist photosensitivity, J. Electrochem. Soc. 119, 539 (1972) B. Broyde, Exposure of photoresists, J. Electrochem. Soc. 117, 1555 (1970). R. Dammel, Diazonaphthoquinone based Resists, p. 15, SPIE Press, Bellingham, WA (1993). " ibid., p. 17. 

The TBOC based resists are examples of resists which can be aqueous alkaline developed. The PHOST resin used in acid sensitive resists are more transparent in the deep UV region than their novolak counterparts and thus have been the focus of more investigations. Previous attempts to use di-azquinone novolak resist in the deep UV region are hampered by the higher absorbance of the novolak resin and the diazoquinone photoproducts (4). Only thin films less than 500 run thick can be used for the deep UV region (4). 

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