Diazonaphthoquinone dissolution

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 basic resist systems have remained essentially the same the positive photoresist composed of a novolac resin and a photoactive substituted diazonaphthoquinone dissolution inhibitor is the resist of choice. The current tools and resists will be able to print features as small as 0.5-0.7 (xm in a production environment. These systems are almost certainly the last generation of conventional-wavelength photolithographic systems. 

In our study of diazonaphthoquinone dissolution, the quartz crystal microbalance was determined to be the most suitable measurement tedinique because of its high predsion. We are interested in isolating the role of photoactive compound (PAC) in inhibiting dissolution of the matrix resin. There have been numerous theories proposed whidi address the issue of the role of PAC during development (12-16). Our goal is to understand the physical range of influence exerted on the polymer matrix by individual PAC moieties as well as the effect of PAC spatial distribution on overall development behavior. 

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 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). 

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. 

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. 

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.

One final example of the application of onium salt photochemistry in positive resist materials should be mentioned, because it does not include any postexposure acid-catalyzed processes and therefore does not encompass the principle of chemical amplification (79). Interestingly, Newman (79) has determined that onium salts themselves can inhibit the dissolution of novolac in aqueous base and that irradiation of such an onium salt-novolac resist restores the solubility of the resin in developer and leads to a positive-tone image. In this application, the onium salt behaves like diazonaphthoquinone in a typical positive resist. Recently, Ito (80) has reported also the use of onium salts as novolac dissolution inhibitors. 

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... 

Most of the ICs were obtained makinguseofaresistformulation discovered by Suss, namely, diazonaphthoquinone as PAC and novolak resin (Chart 12.1) as polymer matrix [12]. The resolution achieved with this resist formulation was smaller than 500 nm [13]. Novolak is obtained through a polycondensation reaction between formaldehyde and cresols [4,14]. The novolak resin is photochemically inert at 436 and 365 nm, and is easily soluble in basic developers due to its phenolic OH groups, but upon addition of naphthoquinone the dissolution rate decreases dramatically [15,16]. 

The resist system which supported the i-line technology for many years in an exclusive manner was so-called diazonaphthoquinone (DNQ)/novolac resists (Fig. 5). This type of resists originally invented for printing by Suss [2] is a two-component system consisting of a novolac resin and a photoactive compound (PAC), diazonaphthoquinone. The novolac resin is soluble in aqueous base in virtue of the acidic phenolic OH functionality. However, the lipophilic diazonaphthoquinone dispersed in the phenolic matrix inhibits the dissolution of the resin film in an aqueous base developer. UV irradiation of the photoactive compound results in formation of a highly reactive carbene, accompanied... 

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]. 

In addition to the oligomeric and polymeric dissolution inhibitors discussed earlier, small molecules bearing acid labile groups have been employed in 157 nm resist formulations [295, 312]. Representative examples are shown in Fig. 98. Some are better than others in dissolution inhibition of a copolymer of NBHFA and NBTBE (92 8). What is interesting is that a diazonaphthoquinone PAC developed for mid UV application (Fig. 99) [313] is surprisingly transparent and can inhibit the dissolution of PNBHFA even better than the small acid-labile dissolution inhibitors in Fig. 98 [312]. In contrast, the dissolution of PSTHFA cannot be efficiently inhibited either with diazonaphthoquinone, the small acid-labile lipophilic compounds in Fig. 98, or the carbon monoxide copolymer (Fig. 94) [312]. 

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. ... 

Dammel, Diazonaphthoquinone based Resists, p. 58, SPIE Press, Bellingham, WA (1993). 206t E Yeh, H.Y. Shi, and A. Reiser, A percolation view of novolak dissolution and inhibition, Proc. SPIE 1672, 204 (1992) Percolation view of novolak dissolution and dissolution inhibition, Macromolecules 25, 5345 5352 (1992). 

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