Cresol-formaldehyde

The epoxy cresol—novolak resins (2) are prepared by glycidylation of o-cresol—formaldehyde condensates in the same manner as the phenol—novolak resins. The o-cresol—formaldehyde condensates are prepared under acidic conditions with formaldehyde—o-cresol ratios of less than unity. 

Reaction media play an important role in m-crcsol-paraformaldehyde reactions.22 Higher molecular weight resins, especially those formed from near-equi-molar m -cresol - formaldehyde ratios, can be obtained by introducing a water miscible solvent such as ethanol, methanol, or dioxane to the reaction. Small amounts of solvent (0.5 mol solvent/mol cresol) increased reaction rates by reducing the viscosity and improving homogeneity. Further increases in solvent, however, diluted the reagent concentrations to an extent that decreased the rates of reaction. 

Materials DNQ was synthesized by esterification of o-cresol formaldehyde novolac resin (Mw=900) and of l,2-diazonaphthoquinone-5-sulfonyl chloride. The esterification rate was ca. 0.4. The novolac-based resist used in EB lithography to compare with SPP was... 

Pale crepe, 20 cresol-formaldehyde resin, 2 hexamethylene tetramine, 3 dicumyl peroxide 50min at 140° C Methyl methacrylate 91 95 12 -—... 

Apart from multi-level layer resist systems, conventional positive-tone resists can be classified into two categories one-component and two-component systems. Classical examples of the former systems are polyfmethyl methacrylate), and poly (butene-1-sulfone) (2,3). Typical examples of the latter system are AZ-type photoresists, which are mixtures of cresol-formaldehyde-Novolak resins and a photoactive compound acting as a dissolution inhibitor... 

The most popular photoactive compound, 1, is a substituted diazonaphthoquinone shown below together with a cresol-formaldehyde Novolak resin, 2 (5). There are many varieties of photoactive compounds that generate... 

A mixture of three isomeric cresols is used in a commercially available cresol-formaldehyde Novolak resin. This mixed Novolak resin, Varcum resin (12), provides adequate properties as a host resin for near-UV- and mid-UV-photoresist applications. Gipstein and his co-workers prepared pure cresol-formaldehyde Novolak resin from each isomeric cresol and compared their spectroscopic and resist characteristics (13). Their data on the UV-absorption spectra of each cresol-formaldehyde Novolak resin together with the commercially available Varcum resin are as follows the absorbances of 0.2 jim thick Novolak films at 250 nm are 0.165(Varcum), 0.096(o-cresol), 0.092(m-cresol), and 0.055(p-cresol). The so-called "window" in the UV absorption at around 250 nm is a maximum with the p-cresol-formaldehyde Novolak resin, while the other isomeric cresol and formaldehyde Novolak resins yielded similar UV absorptions at this wavelength. The smallest UV absorption at 254 nm is an advantage for the p-cresol-formaldehyde Novolak when the resin is used for a deep UV photoresist with a suitable photoactive compound (14). 

Table 1. Comparison of Molecular Weights and Absorbances of Cresol-Formaldehyde Novolak Resinsa...

Bulky AlkyllAryl Substituted Phenol-Formaldehyde Novolak Resins. Low molecular weight cresol-formaldehyde Novolak resins tend to have high solubility rates in alkaline developers. To increase developer resistance, Novolak resins containing a hydrophobic chain incorporated on a portion of the phenol group were synthesized, as shown below (17). As the number of alkyl... 

The structural variations of Novolak resins also influence how well they mix or form solid solutions with a dissolution inhibitor when resist films are cast onto substrates. This is a crucial problem for resist formulation. Usually, cresol-formaldehyde Novolak resins mix well with photoactive compounds like a... 

Chlorinated Novolak Resins. Mixtures of a cresol formaldehyde Novolak resin and a photoactive compound cross-link at electron doses far smaller than the dose required for the Novolak resin alone (11). The reason for this accelerated cross-linking is the reactions between the ketene (an intermediate formed from the photoactive compound upon irradiation) and the Novolak resin. This reaction may be reduced by using a Novolak resin modified for this purpose, or by using certain additives. The rationale for developing a halogen-substituted Novolak resin is the control of the reaction between the intermediate ketene and the Novolak. 

Development of Resist Patterns. Development was done in AZ2401 developer diluted with 2 to 5 times its volume of water AZ2401 is an aqueous solution of KOH with a surfactant. When the resist films were exposed to electron beam doses of 5 iC/cm2 at 25 keV, it usually took 1.5 to 2.0 min for complete development of the images using a diazo-naphthoquinone sensitizer with o-chloro-cresol-formaldehyde Novolak resin in (1 3) AZ2401/water developer. With poly(2-methyl-l-pentene sulfone) the chlorinated Novolak resin exposed to I juC/cm2, it took 2.0 min in (1 4) AZ2401 developer for complete image development. 

Figure 1. Dissolution rates of a composite resist made of a diazonaphthoquinone sensitizer and o-chloro-m-cresol-formaldehyde Novolak resin after 5 /cm2 electron beam exposures. Note this kind of an induction period appeared only in the high-molecular-weight fraction resin.

The preparation method is similar to that for the cresol-formaldehyde Novolak resin with a molar ratio of cresol/benzaldehyde = 1.1 in acidic conditions. We have prepared varieties of substituted m-cresol-benzaldehyde Novolak resins, and 1-, and 2-naphthol-4-hydroxybenzaldehyde Novolak resins in the same manner. Almost all of these benzaldehyde Novolak resins give excellent resist films when spin-coated onto silicon or silicon dioxide substrates after being dissolved, together with a photoactive compound, in a solvent like 2-... 

The UV-absorption spectra of these Novolak resins vary widely depending upon substitution. However, the m-cresol-benzaldehyde Novolak resin is characteristic in its transparency within 300-320 nm in comparison with the cresol-formaldehyde resin. The chlorinated Novolak resin made of 2-chloro-5-methylphenol and formaldehyde has a slightly stronger UV absorption in this wavelength range, but weaker absorption in the range of 250 and 300 nm in comparison with a commercially available cresol-formaldehyde Novolak resin, as shown in Figure 4. 

Figure 4. UV-absorption spectra of Varcum resin (solid line), o-chloro-m-cresol-formaldehyde Novolak resin (long dotted line), and m-cresol-benzaldehyde Novolak resin (short dotted line) 0.32- thick films were...

Figure 7. Wide band ESCA spectra of o-chloro-m-cresol-formaldehyde Novolak resin (a) original films, (b) after 1.5 hr UV exposure.

The dispersion agents used are primarily lignin sulfonates (sulfite cellulose liquors) condensation products of naphthalene, sulfuric acid, and formaldehyde condensation products of m - and o-cresol, formaldehyde, and 2-hydroxynaphtha-lene-6-sulfonic acid [93-01-6] or mixtures of these products [46], Literature on dispersing agents is relatively scarce [47-49],... 

Chart 2.2. This DQN system employs a cresol-formaldehyde novolac resin as the matrix material. The resin is rendered photosensitive by addition of a diazonaphthoquinone that undergoes photolysis to produce a ketene intermediate that rapidly reacts with water present in the resin to yield an inde-necarboxylic acid. The lipophilic diazoquinone serves to reduce the solubility of novolac films in aqueous base. Photolysis leads to production of an acidic photoproduct that renders exposed areas of the film soluble in aqueous base. 

Conventional positive photoresists consist of a matrix resin and a photoactive compound. The matrix resin is a cresol-formaldehyde novolac resin (structure 3.1) that is soluble in aqueous base solution, and the photoactive compound is a substituted diazonaphthoquinone (structure 3.2) that functions as a dissolution inhibitor for the matrix resin. As outlined in Scheme 3.1 (20), the photoactive compound undergoes a structural transformation upon UV radiation, known as WolflFrearrangement, foUowed by reaction with water... 

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