Novolac polymers

Though novolacs have been used in formulating both positive and negative-working photoresists, the vast majority are used in the former. A few examples are shown in Table 2.4. In the case of positive-working resists the desired amount of a photo-active group can be added to the novolac resin as a preformed compound (polymers 1 and 2 in Table 2.4) or 

Polymer 3 in Table 2.4 contains no ionizable group, and can be added to polymer 2 as the PAC-containing component. 

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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. 14. Molecular weight characteristics of novolac resins. Shown is the size-exclusion chromatogram for a typical commercial novolac polymer. The unsymmetrical peak shape reflects the multimodal molecular weight distribution of the polymer.

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). 

Ti incorporation was also found to be a function of the background (residual) pressure in the GFC. Table I lists Ti incorporations for 3 different m-cresol novolac polymers that were treated with TiCLt f°r 1 min. after evacuation of the reaction cell to 110 and 220 mtorr. The polymers that were treated at 220 mtorr had 1.5 times more Ti than those treated at 110 mtorr. Similar experiments conducted with HB-HPR 206 films show a linear relationship between the residual pressure in the GFC and thickness of the resulting Ti02 layer. 

Liu YL. Flame-retardant epoxy resins from novel phosphorus-containing novolac. Polymer 2001 42 3445-3454. 

SEC study of epoxy novolac polymers, such a low molecular weight peak was assigned to a phenol- or cresol-like residue (5). Considering the molecular... 

Bilayer resists are formulated mostly from organosilicon polymers as the imaging layer and novolacs polymers as the underlayer. Many organosilicon resists for bilayer resist systems have been reported for use in near-UV, DUV, mid-UV, electron-beam, and x-ray applications, a good review of which has been provided... 

Novolac polymers are soluble in a variety of common organic solvents. Resist films are typically cast from organic solutions (propylene glycol monomethyl ether acetate, or PGMEA, is a common solvent) to form isotropic, glassy, thin films. These polymers are also soluble in aqueous alkaline solutions due to the presence of the acidic phenolic functionalities on the polymer backbone. It is this base solubility that is exploited in the development process. In order to print a relief image in a photoresist, it must be possible to modify the dissolution rate of the resist upon exposure to radiation. It is not the... 

Thick Resist Lithography, Fig. 3 Schematic picture of the mechanism by which DNQ-novolac resists function. Addition of DNQ inhibitor (/) to novolac polymer (AO results in a dramatic reduction in the dissolution rate of the polymer in aqueous alkaline developer solutions. Exposure of the DNQ-novolac resist mixture causes a chemical reaction converts the diazonaphthoquinone to... 

All near-UV positive photoresists are two-component systems the polymeric material is a low molecular weight novolac polymer (Figure 6.5) and the sensitiser is a derivative of a 1,2-diazonaphthoquinone (DNQ) (20%-50% by weight). DNQ forms a complex with the phenol groups of the novolac resin and prevents the dissolution of the latter in an aqueous base. 

Exposure of the resist to UV light results in photodecomposition of the sensitiser to an unstable ketocarbene. This reacts with water to produce the base-soluble indene carboxylic acid, which no longer inhibits dissolution of the novolac polymer in aqueous base [4] (Figure 6.6). 

Several attempts have been made to design two-component deep-UV positive systems with increased sensitivity. An example is a system based on a novolac polymer and 5-diazo Meldrum s acid as a dissolution inhibitor [20]. The Meldrum s acid derivative provides a bleachable chromophore at 250 nm, which is converted to volatile compounds on irradiation according to the scheme shown in Figure 6.12,... 

Materials. The epoxy novolac polymers used in this study were purchased fi-om Dow (Quatrex 3710) and from Shell (Epikote 164, Epon). They were used either as received or after fractionation. Details on the fractionation procedure and material characterization have been published elsewhere (8). Triphenylsulfonium... 

Table 1. Tg values for epoxy novolac polymer and resist samples.

NEC has investigated a benzoguanidine-modified phenolic novolac polymer as a multirole hardener and flame retardant in epoxy polymers for electronic applications. 

The addition of diazonapthoquinones to novolac films leads to a reduction in the dissolution rate of the polymer film in aqueous base relative to the dissolution rate of the pure polymer. This difference in rate can be as large as several orders of magnitude. Diazonaphthoquinone is therefore termed a dissolution inhibitor due to its ability to retard the dissolution rate of the novolac film. As shown above, DNQ undergoes a series of reactions upon exposure to radiation to form a carboxylic acid product. This carboxylic acid, unlike the hydrophobic DNQ, is soluble in aqueous base and its presence in the polymer film does not retard the dissolution rate in aqueous base. In fact, the presence of the carboxylic acid often serves to increase the dissolution rate of the novolac several orders of magnitude relative to the pure novolac polymer. It is this change in the dissolution rate of the novolac film due to the reaction of... 

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