Epoxy novolac-based chemically

Post-Exposure Bake Kinetics in Epoxy Novolac-Based Chemically Amplified Resists... 

On the other hand an insight into the specific resist chemistry should give further information on the expected behavior, rationalize the results obtained from microlithograpic experiments and help the resist process and formulation optimization. These goals motivated the study on the chemistry of epoxy novolac based chemically amplified resists presented here. 

In the present study, the chemical mechanism and the kinetics of acid initiated crosslinking reactions of epoxy novolac based chemically amplified resists are examined. FTIR and thermal analysis have been used as the basic methods for elucidating chemical mechanism. Lithographic results obtained in a number of different processing conditions are interpreted in the context of the proposed mechanism. 

Resist Chemistry. The basic chemistry of epoxy novolac based chemically amplified resists has been proposed in the past by Stewart et al. (9J. According to this the Bronsted acid generated either photochemically or through electron beam exposure from the onium salt induces acid catalysed polymerization of the epoxy functionality. This mechanism implies that the proton generated by the exposure is actually bound to the polymer. Since the lithography consequences of this mechanism are obvious we decided to seek possible experimental evidence for the proton binding in the resist film under conditions of lithographic interest. 

An unaccelerated epoxy novolac based vinylester base resin, offering excellent chemical resistance as well as high heat resistance. Suitable for pultrusion and filament winding. 

Although DGEBA resins provide the backbone of most epoxy formulations, they may be blended with other types to achieve modifications. Epoxy novolacs, having higher functionality, increase the cross-linking density, which improves heat resistance but decreases impact resistance. Incorporation of epoxidized oils increases flexibility at the expense of heat and chemical resistance. Low-viscosity polyfunctional epoxies based on polyols or polyhydric phenols reduce viscosity and can increase functionality without impairing cured properties. Monofunctional reactive diluents will also decrease viscosity and form part of the polymer backbone, to impart a measure of flexibility without the possibility of migration. Properties of commercially available epoxy resins and diluents from various suppliers are listed in Table 1. 

While epoxy resins are known for excellent chemical resistance properties, the development and commercialization of epoxy vinyl ester resins in the 1970s by Shell and Dow offered enhanced resistance properties for hard-to-hold, corrosive chemicals such as acids, bases, and organic solvents. In conjimction with the development of the structural composites industry, epoxy vinyl ester resin composites found applications in demanding environments snch as tanks, pipes and ancillary equipment for petrochemical plants and oil refineries, automotive valve covers, and oil pans. More recently, epoxy and vinyl esters are used in the construction of windmill blades for wind energy farms. Increasing requirements in the composite industries for aerospace and defense applications in the 1980s led to the development of new, high performance multifunctional epoxy resins based on complex amine and phenolic structures. Examples of those products are the trisphenol epoxy novolacs developed by Dow Chemical and now marketed by Huntsman (formerly Ciba). 

Trisphenol Epoxy Novolacs. In the 1980s, new trifunctional epoxy resins based on tris[4-(2,3-epoxypropoxy)phenyl]methane isomers were introduced by Dow Chemical to help close the performance gap between phenol and cresol epoxy novolacs and high performance engineering thermoplastics (57). These products were later sold to Ciba-Geigy and continued to be marketed under the TACTIX 740 and XD 9053 trade names by Huntsman. 

The increased functionality yields cured adhesives with higher crosslink densities resulting in higher temperature performance and increased chemical resistance. The functionality of commercially available, phenol based epoxy novolac resins varies from 2.3 to 6.0. Epoxy novolac resins can also be produced from substituted phenols like creosol and po-lyhydioxy phenols such as resorcinol. In the United States, Dow Chemical and Ciba-Geigy supply both phenol and cresol based epoxy novolac resins. 

Pure meto-cresol has been used for manufacture of synthetic musk— musk ambrette, used as a fixative to perfumes, for manufacture of synthetic Thymol and Menthol amd also leather preservative p-chloro-meto-cresol, synthetic pyre-throids, and lastly for manufacture of 2,3,6-trimethylphe-nol—an intermediate for vitamin E. o-Cresol has been used for manufacture of Coumarin and some derivatives which are employed in perfumery as fixative. o-Cresol has also been used for making Novolac and epoxy resins and also for the herbicides based on di-nitro-oAt/io-cresol, etc. In sum, individual cresols have been very successfully converted to important intermediates in the organic chemical synthesis. It is expected that further development work will lead to synthesis of many more organic chemicals of vital importance. While new chemicals using individual cresols are in the pipeline... 

Vinyl esters are thermosetting resins that consist of a polymer backbone with an acrylate or methacrylate termination. The backbone component of vinyl ester resins can be derived from epoxide, polyester or urethane but those based on epoxide resins have most commercial significance. Bisphenol A epoxy formed vinyl esters were designed for chemical resistance and commonly formulated for viscosity for use in filament winding of chemical containers. Typically styrene is used as a reactive dilutent to modify viscosity. Phenolic novolac epoxies are used to produce vinyl esters with higher temperature capability and good solvent resistance, particularly in corrosive environments, and their FRP composites have demonstrated initial economy and better life cycle costs compared with metals. 

The effects of solvent exposure on the viscoelastic properties of several vinyl ester resins (phenohc-novolac epoxy, propoxylated bisphenol-A fu-marate, urethane and bisphenol-A epoxy based) and various unsaturated polyester resins (terephthalic or isophthaUc acid with a standard glycol based) containing 10wt% glass fiber were studied [130]. The results of dynamic mechanical analysis showed that the influence of exposure time to the solvent as well as the influence of temperature depended on the styrene content and chemical composition of the studied resins, while the amount of cobalt octoate used for the synthesis as the accelerator had no influence on the viscoelastic properties of the prepared materials after solvent exposure. It was also found that not fully cured urethane vinyl ester and the terephthahc acid-based unsaturated polyester resins showed excellent resistance to sulfuric acid exposure. However, interactions between the tested resins and petroleum could possibly occur through intermolecular bonding between the non-polar chains of the cured resins and the solvent. 

Figure 8 Chemical formulae of phenolic resins used to prepare epoxy-phenolic adhesives based on Gunei Kagaku phenol novolac 61, Dow Chemical OCN ortho-cxQsoX novolac 62, Mitsui Toatsu para-xylene-modified phenol novolac 63, phenolic ortho-ortho-vtsoX of para-cresol 64, and poly (para-hydroxy styrene) 65.

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