Novolac matrix resin

This study explores the feasibility of developing positive tone resists that can be cast and developed from water alone. Its purpose is to determine which approaches might be suitable for the design of a positive tone water-soluble resist. Each of the components of an eventual resist are explored separately with the help of model reactions to develop guidelines for the design of an eventual positive-tone water-soluble resist system. Since neither the matrix resin (Novolac) nor the photoactive compound (diazonaphthoquinone) components of classical i-line resists (7) are water soluble, resists incorporating chemical amplification (2,3) were targeted. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber 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 phenoHc stmcture 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 carboxyHc 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 solubiHty is controUed by chemical and polarity differences rather than molecular size. 

The workhorse of the VLSI industry today is a composite novolac-diazonaphthoquinone photoresist that evolved from similar materials developed for the manufacture of photoplates used in the printing industry in the early 1900 s (23). The novolac matrix resin is a condensation polymer of a substituted phenol and formaldehyde that is rendered insoluble in aqueous base through addition of 10-20 wt% of a diazonaphthoquinone photoactive dissolution inhibitor (PAC). Upon irradiation, the PAC undergoes a Wolff rearrangement followed by hydrolysis to afford a base-soluble indene carboxylic acid. This reaction renders the exposed regions of the composite films soluble in aqueous base, and allows image formation. A schematic representation of the chemistry of this solution inhibition resist is shown in Figure 6. 

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. 

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. 

Figure 18. Diazonaphthoquinone-novolac resist. The novolac (Novolak) matrix resin is prepared by acid catalyzed copolymerization of cresol and formaldehyde. The base insoluble sensitizer, a diazohaphthoquinone, undergoes photolysis to produce a carbene which then undergoes Wolff rearrangement to form a ketene. The ketene adds water which is present in, the film, to form a base soluble, indenecarboxylic acid photoproduct.

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)]. 

Knowledge that silyl substituents may be incorporated into standard resist chemistry to effect etching resistance has prompted several workers to evaluate silylated novolacs as matrix resins for conventional positive-photoresist formulations. Typically, these resists operate via a dissolution inhibition mechanism whereby the matrix material is rendered insoluble in aqueous base through addition of a diazonaphthoquinone. Irradiation of the composite induces a Wolff rearrangement to yield an indenecarboxylic acid (Figure 4), which allows dissolution of the exposed areas in an aqueous-base developer (35). 

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... 

Two interesting attempts to redesign the dissolution-inhibitor-matrix-resin systems have been reported. Grant et al. (37) proposed an alternative inhibitor for novolac resins. They found that 5-diazo-Meldrum s acid (structure 3.5) exhibits an intense, bleachable absorbance at 254 nm with a sharp... 

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... 

In chemical development, the matrix resin of the resist system dissolves in the developer through a chemical reaction. Examples of resists that use chemical development include positive resists composed of novolac resins and DNQs, as well as positive chemical amplification resists based on phenolic, acrylate, and ali-cyclic polymers. These resists are developed with a 0.26-N aqueous solution of tetramethylammonium hydroxide. The exposed resins with phenolic and acidic functional groups dissolve in the developer via the chemical reactions... 

The most widely used epoxy resins are reaction products of either bisphenol A or a novolac phenolic resin with epichlorhydrin. When used to manufacture corrosion-resistant structures for use in the chemical process industry, epoxy resins are generally hardened with either aromatic or cycloaliphatic amines. The hardeners for epoxy resins are, with few exceptions, added at levels varying from 20phr (parts per hundred resin) to lOOphr. This means that the hardener is actually quite a high proportion of the matrix resin and has quite a profound effect on the mechanical and corrosion properties of the cured resin. Thus the selection of the most suitable hardener is critical to the eventual success of the application. Epoxy resins have viscosities of several thousand mPas at room temperature, which makes it much more difficult to wet out glass fibre efficiently with them than with polyesters. Wet-out therefore involves heating the resin formulation to between 40°C and 60°C to reduce the viscosity to less than 1000 mPas. 

The Choice of a Matrix Resin. As was mentioned earlier, the common matrix resins for today s lithography are phenolic resins such as Novolac and poly(4-hydroxystyrene). Though some of our early work had involved simple water soluble alcohols such as poly(vinyl alcohol), schemes for their reversible in situ insolubilization were sometimes complicated by irreversible processes or side-reactions. As a result we chose to test the water-soluble linear polymer that is obtained by free-radical polymerization of 2-isopropenyl-2-oxazoline, 1. Monomer 1 can be polymerized through a variety of techniques, as shown in Scheme 1 (6,7). Both radical or anionic polymerization conditions lead to a polymer containing pendant oxazoline rings, while a more complex structure is obtained under cationic conditions as both the vinyl and the oxazoline moieties are reactive. 

An interesting system consisting of a novolac matrix resin, an onium salt photoadd generator and silanol compounds that act as dissolution promoters for novolac resins in aqueous base was described recently (37, 38). Compounds such as diphenylsilanediol (DPS) are readily soluble in aqueous base and may in fact ino ase novolac solubQity in aqueous media by as much as a factor of S. Upon exposme to light followed by post-exposure bake, acid catalyzed condensation of the silanol additive results in formation of a polysiloxane. While silanols are dissolution promoters, polysiloxanes are hydrophobic, aqueous-base insoluble resins that may act as dissolution inhibitors. Suffident differential solubility is achieved between the exposed and unexposed areas of a resist film resulting in negative tone images. Polysiloxanes are dtemate dissolution inhibitors that may be used in these processes (39). 

Specific strength Epoxy-novolac Resin matrix for filament wound motor case... 

Positive photoresist formulations consist of a novolac resin and an appropriate diazonaphthoquinone dissolved in organic solvent. Common solvents include ethyl cellosolve acetate, diglyme, etc. These formulations are spin-coated and then baked to remove the coating solvent. They provide films in which the sensitizer is randomly distributed through the novolac matrix. 

From this result on MRS, we expected that a combination of phenolic-resin-based resist and aqueous alkaline developer would lead to etching-type dissolution and non-swelling resist patterns. In this paper, we report on a new non-swelling negative electron beam resist consisting of an epoxy novolac, azide compound and phenolic resin matrix (EAP) and discuss the radiation chemistry of this resist. 

Materials. Epoxy novolac, DEN-431, obtained from Dow Chemical Co. was selected as the epoxy component. A 3,3 -diazidodiphenyl sulfone synthesized in our laboratory (5) was used as the azide compound. Poly(/7-vinyl phenol) obtained from Maruzen Oil Co. was used as the phenolic resin matrix. The coating solvent was cyclohexanone. The developer used in this study was 0.1 N tetramethylammonium hydroxide aqueous solution. 

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