Diazonaphthoquinone sensitizer

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. 

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 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.

Alkali-Developable Positive-Photosensitiye Polyimide Based on Diazonaphthoquinone Sensitizer... 

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. 

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). 

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. 

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 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.

Scheme I. Photolysis of a diazonaphthoquinone (DNQ) positive-resist sensitizer. The reaction leads to a carbene (1), which undergoes a Wolff rearrangement to give a ketene (2). Finally, this ketene can react with water present in the resin to give an indenecarboxylic acid (ICA) (3).

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... 

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 avoidance of swelling during processing is a necessary but not sufiScient characteristic of a viable resist system for use in micrometer and submicrometer lithography. For such systems to have practical utility, they must also function with extremely high sensitivity to maximize productivity. The efiSciency of crucial photochemical transformations is characterized by the quantum yield for the process expressed as molecules transformed per photons absorbed. The quantum yield of typical diazonaphthoquinones is from 0.2 to 0.3. Thus, three or four photons are required to transform a single molecule of sensitizer. This places a fundamental limit on the photosensitivity of such systems. 

Typical resists include cyclized polyisoprene with a photosensitive crosslinking agent (ex bisazide) used in many negative photoresists, novolac resins with diazoquinone sensitizers and imidazole catalysts for positive photoresists, poly(oxystyrenes) with photosensitizers for UV resists, polysilanes for UV and X-ray resists, and polymethacrylates and methacrylate-styrenes for electron-beam resists (Clegg and Collyer, 1991). Also note the more recent use of novolac/diazonaphthoquinone photoresists for mid-UV resists for DRAM memory chips and chemically amplified photoacid-catalysed hydroxystyrene and acrylic resists for deep-UV lithography (Choudhury, 1997). 

The enhancement of spectral absorptivity at 313nm was accomplished by the addition of singlet photosensitizers to ester derivatives of diazonaphthoquinone (DQ). These photosensitizers absorb incident photons and transfer the energy to the DQ derivative which is then converted to the alkaline soluble product without the interference of the sensitizer. Theoretically, the mechanism of energy transfer in the solid phase involves dipole-dipole interaction because of the insignificant extent of... 

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