Diazonaphthoquinone structure

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... 

In his pioneering work, Sus (1944) assumed that the final product of photodediazoniation of 2,1-diazonaphthoquinone (10.75) is indene-l-carboxylic acid (10.79, not the 3-isomer 10.78). He came to this conclusion on the basis of some analogies (in addition to an elemental analysis). Cope et al. (1956) as well as Yates and Robb (1957) found that the infrared spectrum of the product was consistent with an a,P-unsaturated acid. Later, Melera et al. (1974) verified the structure 10.78 by H NMR spectroscopy. Friedrich and Taggart (1975) showed that the equilibrium between 10.78 and 10.79 at 233 K lies on the side of the latter, but 10.78 clearly predominates at or above 0°C. Ponomareva et al. (1980) showed that not only 2,1-, but also 1,2-diazo-naphthoquinone yields indene-3- and not -1-carboxylic acid. 

The incorporation of PDMSX into conventional novolac resins has produced potential bilevel resist materials. Adequate silicon contents necessary for O2 RIE resistance can be achieved without sacrificing aqueous TMAH solubility. Positive resist formulations using an o-cresol novolac-PDMSX (510 g/mole) copolymer with a diazonaphthoquinone dissolution inhibitor have demonstrated a resolution of coded 0.5 pm L/S patterns at a dose of 156 mJ/cm2 upon deep-UV irradiation. A 1 18 O2 etching selectivity versus hard-baked photoresist allows dry pattern transfer into the bilevel structure. 

The familiar positive photoresists. Hunt s HPR, Shipley s Microposit, Azoplate s AZ etc., are all two-component, resist systems, consisting of a phenolic resin matrix material and a diazonaphthoquinone sensitizer. The matrix material is essentially inert to photochemistry and was chosen for its film-forming, adhesion, chemical and thermal resistance characteristics. The chemistry of the resist action only occurs in the sensitizer molecule, the diazonaphthoquinone. A detailed description of these materials, their chemical structures and radiation chemistry will be discussed in Section 3.5.b. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber 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 phenolic structure 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 carboxylic 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 solubility is controlled by chemical and polarity differences rather than molecular size. 

The first attempt to design a conventional photoresist resistant to RIE made use of trimethylsilylphenol (36), However, efforts to prepare an aqueous-base-soluble novolac from this monomer were frustrated by the hydrolytic instability of the bond between the aromatic carbon and the silicon atom. These problems were overcome by insertion of a methylene spacer between the aromatic ring and the silyl substituent. Thus, trimethylsilyl-methylphenol may be terpolymerized with ere sol and formaldehyde to afford stable, etching-resistant, aqueous-base-soluble resins see structure) (37). Formulation with a diazonaphthoquinone inhibitor affords a UV-sensitive resist (120 mj/cm at 405 nm) that acts as an etching mask for subsequent... 

Figure 2.9. The N-alkyldiazopiperidinedione structure is typical of the 1,3-diacyl-2-diazo compounds studied at IBM. Photolysis produces a carboxylic acid analogous to the chemistry of diazonaphthoquinones. These materials absorb strongly in the DUV region, but bleach completely as indicated in the spectra of a methanolic solution bottom). Resists formulated from these materials in novolac show residual unbleachable absorbance due to the resin as shown in the spectra of thin films (top).

AZ2400 is different from most other commercial positive photoresists in both formulation and response to mid-UV radiation. This resist is formulated with a resin that is relatively transparent in the mid-UV and l-oxo-2-diazonaphthoquinone-4-arylsulfonate (structure 3.3) rather than the 5-arylsulfonate (structure 3.2) that is commonly used in most commercial photoresists (24). 

Willson et al. (30) and Miller et al. (31) described a new mid-UV resist based on diazonaphthoquinone and a novolac resin speciScally designed for use in the mid-UV region. The novolac resin was chosen to be transparent above 300 nm. The structure of the naphthoquinone was designed with the aid of semiempirical molecular orbital calculations to provide increased optical absorbance at the 313-nm emission line. They found that 5-alkylsulfo-nates of diazonaphthoquinone exhibit a greatly improved extinction at both 313 and 334 nm over their aryl counterparts. Furthermore, these compounds photolyze to give substituted indenecarboxylic acids that are transparent above 300 nm, whereas the photoproducts of all of the corresponding aryl derivatives studied retain residual absorbance at 313 nm. They chose a mixed 4,5-disulfonate of an aliphatic diol (structure 3.4) as a spectrally matched sensitizer for the mid-UV resist. 

The role of the novolac resin is not as minor as it may seem as a base-soluble binder. While the aromatic nature of the resin provides high dry etch resistance, the novolac structures and properties such as the ratio of o-cresol to m-cresol, the ratio of ortho to para backbone linkages, the molecular weight, and molecular weight distribution all affect the dissolution behavior, thermal flow resistance, and lithographic performance. The optimization of the novolac and diazonaphthoquinone properties in conjunction with the improvement of the i-line step-and-repeat exposure tools has pushed the resolution limit of photolithography to a sub-0.5-pm regime [4]. 

For resists designed for applications in the near-UV region, good absorption at 365 nm (i-line) and at 405 nm and 436 nra (g-line) is required, and aromatic substituents present the best option. The most commonly used ballast compound is polyhydroxybenzophenone, where one to three (or even four) hydroxyl groups can be esterified with diazoquinone sulfonyl-chloride, as discussed above. A popularly used dissolution inhibitor is a substituted 2,3,4-trihydroxy benzophenone of stmcture (in), " where DQ stands here for diazonaphthoquinone. One other popular diazoquinone derivative found in commercial resists is the sulfonyl ester of cumylphenol. Similar materials such as shown in structure (IV) have also been employed in resist applications. ... 

A.D. Erlikh, N.P. Protsenko, L.N. Kurovkaja, and G.N. Rodionova, Zh. Vses. Khim. Obua., Pho tolysis of onaphthoquinonediazides Structure of substituted indenecarboxylic acids, 20, 593 (1975) [cited in A. Reiser, Photoreactive Polymers The Science and Technology of Resists, p. 187, John Wiley Sons, Hoboken, NJ (1989) R. Dammel, Diazonaphthoquinone based Resists, pp. 13 15, SPIE Press, Bellingham, WA (1993)]. 

Figure 4.42. Molecular structures and photoinduced reactions of common photoresists. Shown are (a) the diazonaphthoquinone (DNQ) positive tone photoresist, and (b) the SU-8 epoxy-based negative tone photoresist.

Resists are comprised of several components. The classic composition of negative photoresist is an azide sensitized rubber such as a polyisoprene polymer base with a bis-aryldiazide sensitizer, and a carrier solvent that makes the resist a liquid, allowing it to be spun on the wafer as a thin layer (sometimes additives are included such as dyes to control the light rays). A typical positive photoresist would be aphenol-formaldahyde Novolak resin structure and a diazonaphthoquinone sensitizer in a carrier solvent. 

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