Dissolution inhibition resists

While "conventional positive photoresists" are sensitive, high-resolution materials, they are essentially opaque to radiation below 300 nm. This has led researchers to examine alternate chemistry for deep-UV applications. Examples of deep-UV sensitive dissolution inhibitors include aliphatic diazoketones (61-64) and nitrobenzyl esters (65). Certain onium salts have also recently been shown to be effective inhibitors for phenolic resins (66). A novel e-beam sensitive dissolution inhibition resist was designed by Bowden, et al a (67) based on the use of a novolac resin with a poly(olefin sulfone) dissolution inhibitor. The aqueous, base-soluble novolac is rendered less soluble via addition of -10 wt % poly(2-methyl pentene-1 sulfone)(PMPS). Irradiation causes main chain scission of PMPS followed by depolymerization to volatile monomers (68). The dissolution inhibitor is thus effectively "vaporized", restoring solubility in aqueous base to the irradiated portions of the resist. Alternate resist systems based on this chemistry have also been reported (69,70). 

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 most widely used positive resists are those that operate on the basis of a dissolution inhibition mechanism. Such resists are generally two-component materials consisting of an alkali soluble matrix resin that is rendered insoluble in aqueous alkaline solutions through addition of a hydrophobic, radiation-sensitive material. Upon irradiation, the hydrophobic moiety may be either removed or converted to an alkali soluble species, allowing selective removal of the irradiated portions of the resist by an alkaline developer. 

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. 

Interest in solution inhibition resist systems is not limited to photoresist technology. Systems that are sensitive to electron-beam irradiation have also been of active interest. While conventional positive photoresists may be used for e-beam applications (31,32), they exhibit poor sensitivity and alternatives are desirable. Bowden, et al, at AT T Bell Laboratories, developed a novel, novolac-poly(2-methyl-l-pentene sulfone) (PMPS) composite resist, NPR (Figure 9) (33,34). PMPS, which acts as a dissolution inhibitor for the novolac resin, undergoes spontaneous depolymerization upon irradiation (35). Subsequent vaporization facilitates aqueous base removal of the exposed regions. Resist systems based on this chemistry have also been reported by other workers (36,37). 

Several groups have investigated three-component systems encompassing both chemical amplification and dissolution inhibition. As stated earlier, Smith and Bonham (63) reported resist materials composed of a binder resin (novolac), a nonpolymeric compound containing acid-labile functional groups such as acetals, and a trihalomethyl-substituted 5-triazine acid photogenerator. The acid-labile compound acts as a novolac dissolution inhibitor in a manner analogous to the action of DNQ in conventional positive resists. However, in this case, the inhibitor is not photochemically active. Instead,... 

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). 

A cresol novolac resin has been synthesized which exhibits a much greater dissolution-inhibiting effect than in various commercially available novolac or phenolic resins. Using this resin, a positive electron beam resist was prepared and its exposure characteristics were examined. A tetramethylammonium hydroxide aqueous solution was used as the developer. The sensitivity reaches 3xl0 6 C/cm2 without post-exposure baking. It was found that the sensitivity to double exposure was much higher than that to single exposure with the same total dose. A similar phenomenon was also... 

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]. 

The soft bake process can affect the solubility properties of some resists in the developing solvent. For instance, in DNQ/novolac resists, the solubility of the exposed resist as a function of prebake temperatures shows a maximum at around 120°C (Fig. 11.16). Four zones can be distinguished in this plot (i) a no-bake zone where residual solvent and dissolution rates are high, (ii) a low-temperature zone (up to 80°C) where the dissolution rate shows an appreciable decrease due to solvent removal, (iii) a mid-temperature zone (80-110°C) where the DNQ is thermally and preferentially converted to indene carboxylic acid, leading to an increase in dissolution rate, and (iv) a high-temperature zone (>120°C) where the film densification takes place, DNQ is thermally decomposed, the film is depleted of water, and the novolac resin is cross-linked, resulting in dramatic dissolution inhibition. ... 

The reaction does not occur in the unexposed areas because of the dissolution inhibition of the DNQs that remains unchanged there, or because of the lack of chemical amplification reaction there. As a result, the unexposed resist film... 

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). 

For a positive resist based on dissolution inhibition chemistry such as the DNQ/novolac resist system, the rate of the reaction tr between the developer with the resist (comprised of resin R, a photoactive compound that acts as a dissolution inhibitor M, but which is converted to product P on exposure to UV light, which in turn enhances the dissolution rate of the resin) is given by... 

For 193 nm applications, evaluation of a series la) of cholate based dissolution inhibitors suggested that the dissolution inhibition of methacrylate-based resins by these derivatives is largely a function of the hydrophobicity of the cholates employed. The observed relative order of hydrophobicity and dissolution inhibition was lithocholate (1 pendant hydroxyl) > deoxycholate == ursocholate (2 pendant hydroxyls) > cholate (3 pendant hydroxyls). Experiments using monomeric dissolution inhibitors such as t-butyl cholate (la), t-butyl deoxycholate (lb), t-butyl lithocholate (Ic), t-butyl lithocholate acetate (2) with the P(NB/MA) acrylate resins afforded resist systems that exhibited low contrast, poor adhesion, dark erosion (unexposed resist film loss) and were incompatible with industry standard 0.26 N TMAH developers. 

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