Novolak photoresists

Novolak photoresist Novolak process Novolak resin... 

The pH-rate profiles of the enol of l-indene-3-carboxylic acid and of its ketene precursor, formed from either l-diazo-2( 1 //jnaphthalenone or 2-diazo-l(2//)naphthalenone by photochemical deazotization and Wolff rearrangement, are shown in Fig. 5.36 The first and second acidity constants of the diol, p = 1.9 and p aE = 8.3, are evident from the downward curvature of log (k /s-1) at these pH values. The photo-Wolff rearrangement of diazonaphthoquinones is the active principle of Novolak photoresists. 

The DNQ/novolak photoresists used for 436 and 365 nm optical lithography did not meet the requirements for photolithography at 248 nm, and the CA photoresists were employed at this wavelength. Besides the low quantum yield, an important aspect was the high absorbance of novolak at 248 nm. The photoresists for 248 nm were based on polyhydroxystyrene (PHOST) and modified PHOST, which met the requirements for this exposure wavelength. 

Po]y(vinyl alcohol) (PVA) is found to undergo a dramatic change in morphology when prepared as a thin film ( 1000 A) between a siloxane polymer and an add-generating novolak photoresist, and heated to 115 or more for a few minutes. When the siloxane is stripped off in chlorobenzene, the ei sed PVA film is wrinkled and insoluble in water (although soluble in acetone). 

The majority of 2-methylphenol is used in the production of novolak phenoHc resins. High purity novolaks based on 2-methylphenol are used in photoresist appHcations (37). Novolaks based on 2-methylphenol are also epoxidized with epichlorohydrin, yielding epoxy resins after dehydrohalogenation, which are used as encapsulating resins in the electronics industry. Other uses of 2-methylphenol include its conversion to a dinitro compound, 4,6-dinitro-2-methylphenol [534-52-1] (DNOC), which is used as a herbicide (38). DNOC is also used to a limited extent as a polymerization inhibitor in the production of styrene, but this use is expected to decline because of concerns about the toxicity of the dinitro derivative. 

Positive photoresists, by contrast, are based on water-soluble novolak resins with naphthalene diazoquinone sulfonate (NDS) as the photosensi-tiser. On photolysis the NDS causes a rearrangement in the polymer to yield nitrogen gas plus an indene carboxylic acid. This latter functional group considerably increases the solubility of the polymer, hence solubilising those areas of the polymer that had been exposed to light. 

As insulation between the coil and the magnetic core, a hard-cured (to 200°C) photoresist insulator is patterned. It is a novolak polymer or polyimide about 5 fim thick, which is popular for its high insulator and photolithographic properties. This provides electrical insulation as well as a planar surface for subsequent deposition of copper cods. 

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... 

A mixture of three isomeric cresols is used in a commercially available cresol-formaldehyde Novolak resin. This mixed Novolak resin, Varcum resin (12), provides adequate properties as a host resin for near-UV- and mid-UV-photoresist applications. Gipstein and his co-workers prepared pure cresol-formaldehyde Novolak resin from each isomeric cresol and compared their spectroscopic and resist characteristics (13). Their data on the UV-absorption spectra of each cresol-formaldehyde Novolak resin together with the commercially available Varcum resin are as follows the absorbances of 0.2 jim thick Novolak films at 250 nm are 0.165(Varcum), 0.096(o-cresol), 0.092(m-cresol), and 0.055(p-cresol). The so-called "window" in the UV absorption at around 250 nm is a maximum with the p-cresol-formaldehyde Novolak resin, while the other isomeric cresol and formaldehyde Novolak resins yielded similar UV absorptions at this wavelength. The smallest UV absorption at 254 nm is an advantage for the p-cresol-formaldehyde Novolak when the resin is used for a deep UV photoresist with a suitable photoactive compound (14). 

Poly(methylmethacrylate) is a well-known single component positive photoresist, whereas a diazoqui-none ester and a phenolic novolak resin is a two component positive photoresist. As anticipated, radiation will render the scission or rupture the main and side chain of the polymer making it soluble in an alkaline developing solution. Hydroxide solutions such as KOH and tetramethylammonium hydroxide are commonly used as the developer for positive photoresist. 

Conventional Photoresists. PE rate ratio values for several positive photoresists are also included in this study (see Table II), because several of these novolac resin containing formulations also function as positive e-beam and x-ray resists. Generally speaking, these formulations are more dry-process compatible than most of the vinyl systems (see also ref.2). This is due primarily to the aromatic nature of the novolak resins in the photoresists. Thus, the photoresist PE rate ratio data is close in value to those of the aromatic vinyl and negative behaving polymers. 

New positive-type photoresist systems based on enzymatically synthesized phenolic polymers were developed [55]. The polymers from the bisphenol monomers exhibited high photosensitivity, comparable with a conventional cresol novolak. Furthermore, this photoresist showed excellent etching resistance. The oxidative polymerization of bisphenol-A proceeded by fungal peroxidase from Coprinus cinereus (CiP) in aqueous isopropanol [56]. CiP also catalyzed the oxidative... 

M. Hanabata, Y. Uetani, and A. Furuta, Novolak design for high resolution positive photoresists. II. Stone wall model for positive photoresist development, Proc. SPIE 920, 43 (1988). 

Hanabata, Y. Uetani, and A. Furuta, Novolak design for high resolution positive photoresists. II. stone wall model for positive photoresist development, Proc. SPIE 920, 349 482 (1990). C.G. Willson, R. Miller, D. McKean, N. Clecak, T. Tompkins, D. Hofer, J. Michl, and J. Downing, Design of a positive resist for projection lithography in the mid UV, Polym. Sci. Eng. 23, 1004 (1983) M.K. Templeton, C.R. Szamanda, and A. Zampini, Dissolution kinetics of positive photo resists the secondary structure model, Proc. SPIE 771, 136 (1987). 

In order to understand the changes, it is necessary first to know the components of the resist and the chemistry of the exposure step. For positive photoresists, the mechanism was elucidated by Pacansky and Lyerla (14). Typical resists are a solution of a naphthoquinone diazide photoactive compound (PAC) and a cresol formaldehyde novolak resin in one or more high boiling point industrial solvents. The number average molecular weight of the resin is quite low, on the order of 1000, with a polydispersity of about 10 (13). During the normal exposure step, the PAC, in the presence of water, absorbs light and is transformed into a... 

The importance of m,p-cresol novolak resins to photoresist formulation makes the supply of a range of copolymer compositions and molecular weights useful. In novolak resin synthesis, the growing polymer chains compete with cresol monomer for formaldehyde. As conversion increases, the polymer competes better than the monomer, so that the supply of phenolic monomer is never exhausted, and the amount of unreacted monomer changes with extent of reaction. Since different monomers react at different rates, this ensures not only that copolymer composition will not be the same as the charge ratio of the monomers, but also that it will change over the course of the reaction. The model we describe uses relative monomer reactivities to predict copolymer composition. 

The earhest photoresists used in integrated circuit manufacture consisted of polymers that were rendered insoluble by photo-cross-linking and thus operated in the negative tone mode. For instance, partially cycUzed poly(cis-isoprene) containing a bisazide as additive served for a long time as the workhorse resist material in photohthography applications [15]. This system has already been described in Section 7.2.3. Subsequently, Novolak-based positively functioning sys-... 

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