Dissolution inhibitors

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). 

Fig. 23. Representative protecting groups for phenolic and carboxylic acid-based systems, (a) The polymer-based protecting groups are fisted in order of increasing activation energy for acid-catalyzed deprotection, (b) Acid-labile monomeric dissolution inhibitors, a bifunctional system based on protected bisphenol A. (c) Another system that combines the function of dissolution inhibitor and PAG in a single unit.

Positive resists have as the photoreactive component a dissolution inhibitor that is destroyed in the regions exposed to the light. The resist is developed in an aqueous solution, where the exposed region dissolves away. The resists do not swell as much in the aqueous developer, allowing higher resolution. 

Aliphatic dissolution inhibitors, low molecular weight, 15 177 Aliphatic endoperoxides, 18 442, 443 Aliphatic fluorocarbon production Europe, 11 870-871 Pacific Rim/India, 11 871 United States, 11 869-870 Aliphatic fluorocarbons, surface energy of, 21 604... 

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). 

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)... 

Contact exposure at 248 nm using 20 wt % dissolution inhibitor. Behaves as a negative resist. 

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. 

SPP was prepared by dissolving a novolac resin dlazonaph-thoqulnone sulfonyl ester (DNQ) and APSQ In methyl Isobutyl ketone. A concentration of 20 wt% DNQ relative to APSQ Is sufficient for It to act as a dissolution Inhibitor. 

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. 

Novolac resins, as the oldest synthetic polymers, have played an important role 1n microelectronic Industry as positive photoresists. Studies of novolac dissolution have populated the literature a recent survey shows that the rate of dissolution 1s influenced by the concentration of the alkali, size of the cation, addition of salt, and the presence of dissolution Inhibitors (1-6). The voluminous experimental results, however, have not led to a clear understanding of the dissolution phenomena. Arcus (3) proposed an 1on-permeab1e membrane" model while Szmanda (1) and Hanabata (6) emphasized the Importance of secondary structures of novolac molecules, for Instance, Inter- or Intramolecular hydrogen bonding and the various isomeric configurations of the resins. These important contributions nevertheless point to a need for additional studies of the mechanism of dissolution. 

The role of the dissolution Inhibitor 1n our model rests on its ability to alter the path of water and ion transport 1n the solid. 

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). 

At the present time, most of the positive photoresists used in the manufacture of microcircuits consist of a low molecular weight phenolic resin and a photoactive dissolution inhibitor. This composite system is not readily soluble in aqueous base but becomes so upon irradiation with ultraviolet light. When this resist is exposed, the dissolution inhibitor, a diazoketone, undergoes a Wolff rearrangement followed by reaction with ambient water to produce a substituted indene carboxylic acid. This photoinduced transformation of the photoactive compound from a hydrophobic molecule to a hydrophillic carboxylic acid allows the resin to be rapidly dissolved by the developer. (L2,3)... 

Positive Photoresists. All positive photoresists used for conventional photolithography are two-component systems and operate on a mechanism that involves destruction of a dissolution inhibitor. These resists are formulated... 

Figure 7. Sequence of photochemical transformations of the quinonediazide sensitizer (dissolution inhibitor).

Several attempts have been made to redesign the traditional two-component near-UV positive resist systems to make them compatible with the deep-UV. Recall that the major problems associated with deep-UV exposure of conventional resists are related to non-bleaching of the o-quinonediazide sensitizer on exposure because of photoproduct absorbance, and strong absorption of the novolac resin. Willson and coworkers34 attempted to solve this problem using dissolution inhibitors based on 5-diazo Meldrums acid, which undergoes photochemical decomposition as follows ... 

An alternative system was recently described by Chandross et al. (35,36) based on optically transparent aqueous-alkali-soluble methyl methacrylate-methacrylic acid copolymer as the inert resin. The dissolution inhibitor was an o-nitrobenzyl carboxylate which undergoes photochemical decomposition to o-nitrosobenzaldehyde plus a carboxylic acid as follows ... 

Apart from multi-level layer resist systems, conventional positive-tone resists can be classified into two categories one-component and two-component systems. Classical examples of the former systems are polyfmethyl methacrylate), and poly (butene-1-sulfone) (2,3). Typical examples of the latter system are AZ-type photoresists, which are mixtures of cresol-formaldehyde-Novolak resins and a photoactive compound acting as a dissolution inhibitor... 

The structural variations of Novolak resins also influence how well they mix or form solid solutions with a dissolution inhibitor when resist films are cast onto substrates. This is a crucial problem for resist formulation. Usually, cresol-formaldehyde Novolak resins mix well with photoactive compounds like a... 

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