Novolac copolymer, matrix resin

Figure 17 provides an overview of the function of the diazoquinone/novolac materials. The matrix resin is a copolymer of a phenol and formaldehyde. The generic term for this class of polymers is novolac (18) meaning "new lacquer" and describes the purpose for which they were first developed. The chemical industry produces millions of tons of novolac each year where its end use is that of a thermoset resin and adhesive. Novolac is commonly used, for example, as the principle adhesive in the manufacture of plywood. 

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). 

Research has also been aimed at the development of more-transparent base-soluble matrix resins. For example, novolacs prepared from pure p-cresol absorb less strongly at 250 nm than do typical photoresist novolacs containing a mixture of cresol isomers. Unfortunately, p-cresol novolac is only sparingly soluble in aqueous base and has limited usefulness (28, 57). Other examples of more-transparent matrix resins include poly(dimethyl glutarimide) (PMGI) (58) and copolymers of methyl methacrylate (MMA) and methacrylic acid (MAA) [P(MMA-MAA)]. 

Reichmanis et al. (38, 40), Wilkins et al. (39), and Chandross et al. (41) redesigned both the dissolution inhibitor and the matrix resin. They evaluated a variety of o-nitrobenzylcholates (structure 3.6) that are initially insoluble in alkaline developer but are cleaved upon UV radiation to form base-soluble species. As shown in Scheme 3.3, irradiation of the o-nitro-benzyl ester results in rearrangement and degradation to generate a carboxylic acid and o-nitrosobenzaldehyde (R = H in Scheme 3.3) (42). The matrix resin chosen is a copolymer of methyl acrylate and methacrylic acid that is far more transparent in the DUV than novolac resins and is soluble... 

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