Diazonaphthoquinones

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. 

Fig. 11. Synthesis of DNQ photosensitizers found in commercial resists, (a) Condensation of l,2-diazonaphthoquinone-5-sulfonyl chloride with 1,2,3-trihydroxybenzophenone. Often the reaction is not carried to completion so the product is a mixture of monodi- and trisubstituted products, (b)...

Condensation of l,2-diazonaphthoquinone-4-sulfonyl chloride withp-ciimylpheno1. 

The solubHity properties of the PAG itself can play an important role in the overaH resist performance as weU (50). SolubHity differences between the neutral onium salt and the acidic photoproducts can be quite high and wHl affect the resist contrast. In fact onium salts can serve as dissolution inhibitors in novolac polymers, analogous to diazonaphthoquinones, even in the absence of any acid-sensitive chemical function (51). 

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. 

Evidence for the existence of a ketene intermediate was first obtained by Nakamura et al. (1972) in a study of the photolysis of 2,1-diazonaphthoquinone-5-sulfonic acid by flash photolysis in aqueous solution. An intermediate with a strong absorption at 350 nm and a lifetime of approximately 2 ms was found. 

Photosensitive reactions, chromium application, 6 560-561 Photosensitization, 9 385 of singlet oxygen, 26 804 Photosensitizers, 14 300 23 374-375 diazonaphthoquinone, 15 161-163 Photostability of Al-halamines, 13 100-101 of organic semiconductors, 22 210 Photostimulated drug delivery systems, 9 61, 81... 

The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxymethyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-sty-rene) to afford a negative-tone resist material which does not swell upon development in aqueous base. 

Tetramethylammonium hydroxide, TMAH, (Fluka Chemicals) was diluted with distilled water from a 25 wt % aqueous solution. In all cases the diazonaphthoquinone dissolution inhibitor used was Fairmont Positive Sensitizer 1009 (Fairmont Chemical Company). The syntheses of the PDMSX oligomers and novolac-PDMSX block copolymers have already been reported (11). The dimethylamine terminated poly(dimethyl siloxane), =510 g/mole (Petrarch), was used as the PDMSX component or to prepare higher molecular weight analogs. 

Optimization of the deep-UV exposure and aqueous TMAH development steps for all three parent phenolic resins formulate with the diazonaphthoquinone dissolution inhibitor resulted in the resolution of positive tone 0.75 pm L/S patterns at a dose of 156, 195 and 118 mJ/cm2 for the o-cresol, 2-methyl resorcinol and PHS materials, respectively (Table V). The copolymers prepared with a 4400 g/mole PDMSX resulted in TMAH soluble films at >11 wt % silicon however, the feature quality was extremely poor in each case. Figure 6 shows an SEM photomicrograph of a 2-methyl resorcinol-PDMSX copolymer using (a) 20 and (b)... 

Figure 6 Scanning electron microscope photograph of coded 0.75 pm line-space images obtained with the 2-methyl resorcinol-PDMSX copolymer ( = 4400 g/mole) containing (a) 20 wt % and (b) 30 wt % diazonaphthoquinone dissolution inhibitor.

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 deep UV induced reactions appear to be slightly different from X-ray and EB induced reactions. Deep UV exposure in air can induce an increase in solubility of SPP, indicating that indenecar-boxylic acid is produced. IR spectra of SPP exposed to deep UV are shown in Figure 11. In this case, we used a mono-functional dissolution inhibitor, tert-amylphenol diazonaphthoquinone sulfonyl ester, instead of a multi-functional sensitizer, DNQ, because the IR spectrum of a mono-functional ester is easier to interpret than that of DNQ. The SPP containing this mono-functional ester also exhibits an image reversal reaction with almost the same characteristics as the SPP with DNQ. 

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... 

HPR-206 Positive Photoresist (Olin-Hunt) Mixed Isomer Novolac + Diazonaphthoquinone Photoactive Compounds 120-140... 

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 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. 

Experimental methods for determining 0 are well documented (2). These experiments are conveniently carried out and require only a method of producing reasonably narrow-bandwidth radiation, a method of measuring the flux of that radiation per unit area, and a UV-visible spectrophotometer. The quantum efficiency of typical diazonaphthoquinone sensitizers of the type that are used in the formulation of positive photoresists ranges from 0.2 to 0.3, whereas the quantum efficiency of the bis-arylazide sensitizers used in the formulation of two-component negative photoresists, ranges from 0.5 to 1.0. 

The photoactive compounds, or sensitizers, that are used in the formulation of positive photoresists, are substituted diazonaphthoquinones shown in Figure 17. The substituent, shown as R in Figure 17, is generally an aryl sulfonate. The nature of the substituent influences the solubility characteristics of the sensitizer molecule and also influences the absorption characteristics of the chromophor (79). The diazonaphthoquinone sulfonates are soluble in common organic solvents but are insoluble in aqueous base. Upon exposure to light, these substances undergo a series of reactions that culminate in the formation of an indene carboxylic acid as depicted in Figure 17. The photoproduct, unlike its precursor, is extremely soluble in aqueous base by virtue of the carboxylic acid functionality. 

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. 

Figure 17. A schematic representation of positive resist action in diazonaphthoquinone-novolac resists. Photolysis of the sensitizer inhibitor) produces acid which allows the exposed areas of the resist to be selectively dissolved (developed) in aqueous base.

Figure 19 shows the ultraviolet absorption spectrum of a typical diazonaphthoquinone and a common novolac resin. The naphthoquinone sensitizer has a strong absorbance at the 365 nm., 405 nm., and to a lesser extent the 436 nm. mercury emission lines. There are two diazonaphthoquinone isomers that are used in commercial photoresist formulations that are available at this time. The 5-arylsulfonates are by far the most commonly used. A spectrum of a representative of this class of materials is depicted in Figure 20. The 5-arylsulfonate materials are characterized by an absorbance maximum at approximately 400 nm. and a second, slightly stronger maximum at approximately 340 nm.

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.

Figure 19. Absorbance spectrum of a typical diazonaphthoquinone sensitizer (in solution) and a cresylic acid novolac (film). The wavelengths of principle mercury emission lines are labeled.

Figure 21. Absorbance spectrum of a l-oxo 2 diazonaphthoquinone-4-arylsulfonate. These materials differ spectrally from the 5 isomers shown in Figure 20 and are used in certain commercial resist formulations.

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