Model Novolac resins

The "C-NMR assignments for carbon atoms in binuclear phenolic compounds based on the 3-methylphenol-methanal reaction are summarized in Scheme 38. The analysis is based on model novolac resins that were synthesized for just this analysis [235]. 

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. 

Noroxycodeine, synthesis of 1275, 1276 Novolac resins 367, 1457-1460 model compounds of 1463-1469 reactions with HMTA 1470-1475 structure of 1469 synthesis of 1460-1463 Nuclear magnetic resonance spectroscopy 335-367... 

The key empirical facts surrounding the dissolution of novolac that have been established are outlined below. It was on the basis of these facts that mechanistic models of novolac dissolution were proposed. The dissolution of novolac, with the exception of a brief initial phase, proceeds with a constant rate and follows case II mass transfer kinetics (also called case II Fickian diffusion). In low-molecular-weight novolacs, the width of the gel layer is on the order of 10 nm. Arcus has demonstrated the existence of this gel layer in a high-molecu-lar-weight phenolic novolac resin, as shown in the laser interferogram trace of Fig. 11.34. This interferogram shows three separate sets of reflections, corresponding to the interface between the polymer matrix and the silicon wafer, the interface between the polymer matrix and the swollen gel, and the interface between the gel layer and the developer solution. ... 

As discussed earlier in the secondary structure model, Templeton et al. have shown that the steric arrangement of the methylene links in the novolac resin can have a profound effect on its dissolution rate and on lithographic performance. Using molecular mechanics, these authors have calculated the equilibrium secondary structures of cresol-formaldehyde oligomers. They found that the secondary structure of these molecules determines the relative positions of the hydroxyl groups in the novolac matrix, and hence the possibility of intramolecular hydrogen bonding. ... 

Numata and Kinjo (52) have shown rubber-modified isocyanurate-oxazolidone resins may be effectively modified with carboxyl-reactive nitrile liquids. The viscoelastic behavior of models using a polyglycidyl ether of phenol-formaldehyde novolac resin and di-phenylmethane-4,4 -diisocyanate is discussed. Such resins have suggested utility in thin films as electrical varnishes. 

C-NMR spectroscopy was used extensively to investigate the chemical structures of phenolic resins (11,15-18) and related model compounds (17-19). These studies provided useful information on the complex chemical structure of PF resins. C-NMR spectroscopy was also used to study the chemical structure and to determine the copolymer composition of novolac resins prepared from 2-methylphenol, 3-methylphenol or 4-methylphenol (20,21) or from mixtures of... 

Diffusion Model. In 1986, Arcus presented the membrane model (5) for the dissolution of novolac resins. While this model does little to elucidate the molecular-level interaction of DNQ with novolac, it does provide a framework in which to... 

Lim ASC, Solomon DH, Zhang X. Chemistry of novolac resins. X. Polymerization studies of HMTA and strategically synthesized model compounds. J Polym Sci A 1999 37 1347. 

The formation of a phenolic resin is often formally separated into two steps, though it probably should be three. If we use a three-step model, the first step is activation of the phenol or aldehyde. The second step is methylolation, and the third is condensation or chain extension. In addition to the clarity provided by the formalism, these steps are also generally separated in practice to provide maximum control of exothermic behavior, with the strategy being to separate the exotherm from each step from that of the others as much as possible. As there are significant differences in the activation step and in the details of the methylolation and condensations steps of novolacs and resoles, we will treat the two types separately. 

The commercial importance of phenol-formaldehyde resins has resulted in extensive studies of these systems, with the aim of identifying the reaction mechanisms and intermediates that occur during subsequent polymerization reactions. However, the complexity of Novolac-type systems has made a detailed understanding of the subsequent chemical processes and their relationship to the physical properties of the final polymerized product difficult. Thus, it is necessary to simplify the system in order to more readily unravel this complexity. Model compounds are frequently used to understand complicated chemical systems and their application to phenol-formaldehyde systems has been well documented . ... 

Chiou and Letton (1992) examine the chemorheology of a complex industrial resin, Hercules 3501-6, containing a major epoxide (TGDDM), two minor epoxides (alicyclic diepoxy carboxylate and epoxy cresol novolac), a hardener (diaminodiphenyl sulfone, DDS) and a Lewis-base catalyst (boron trifluroride monoethylamine complex). They found that both the conventional WLF model (Equation (5.26)) (Ferry, 1980) and a modified WLF model (Equation (5.27) were appropriate to describe the chemorheology ... 

Ea, above and below Tg. Three case studies illustrate the range of applicability of the bending beam setup and factors contributing to the stress state. The first is a comparison of two polymers for interlayer dielectrics PMDA-ODA (pyromellitic acid dianhydride - oxydiamine) and a bis-benzocyclobutene. The second is of a neat epoxy resin commonly used for microelectronics encapsulation (epoxidized ortho-cresol novolac cured with a phenolic novolac). The third is a screen-printable polyimide coating used for protection of the integrated-circuit chip. An outline of our stress model is sketched, and example results are presented. 

While the membrane model implies that the overall rate of the dissolution process is controlled by the rate of diffusion of developer components into the matrix, another model proposed by Garza et al. " maintains that in some resins, the deprotonation of the phenol groups of the novolac can be the rate-determining step (see below). 

The secondary structure model, proposed by Templeton et al., relates the secondary structures of novolacs [see Figs. 11.36(a) and (b)] to their dissolution behavior. The authors distinguished between structures where intermolecular bonds between novolac molecules predominate and those with predominantly intramolecular hydrogen bonds and they correlated these to dissolution behavior. They found that, for example, for novolacs made from p-cresol, the secondary structure of the resin brings the OH groups of the phenols together to such an extent that... 

This study explores the feasibility of developing positive tone resists that can be cast and developed from water alone. Its purpose is to determine which approaches might be suitable for the design of a positive tone water-soluble resist. Each of the components of an eventual resist are explored separately with the help of model reactions to develop guidelines for the design of an eventual positive-tone water-soluble resist system. Since neither the matrix resin (Novolac) nor the photoactive compound (diazonaphthoquinone) components of classical i-line resists (7) are water soluble, resists incorporating chemical amplification (2,3) were targeted. 

Reiser expanded the diffusion model for dissolution of novolac 13-24) using percolation theory (25, 2d) as a theoretical framework. Percolation theory describes the macroscopic event, the dissolution of resist into the developer, without necessarily understanding the microscopic interactions that dictate the resist behavior. Reiser views the resist as an amphiphilic material a hydrophobic solid in which is embedded a finite number of hydrophilic active sites (the phenolic hydrogens). When applied to a thin film of resist, developer diffuses into the film by moving from active site to active site. When the hydroxide ion approaches an active site, it deprotonates the phenol generating an ionic form of the polymer. In Reiser s model, the rate of dissolution of the resin. .. is predicated on the deprotonation process [and] is controlled by the diffusion of developer into the polymer matrix (27). 

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