Polymers negative resist system

The value in units of incident dose per unit area for either a positive or negative resist system is of little value unless accompanied by a detailed description of the conditions under which it was measured. This description should include, at the minimum, the initial film thickness, the characteristics of the substrate, the temperature and time of the post- and pre-bake, the characteristics of the exposing radiation, and the developer composition, time and temperature. The structure, copolymer ratio, sequence distribution, molecular weight, and dispersity of polymers included in the formulation should also be provided. 

The concepts of positive or negative systems are by no means a strict classification since modifications of the original pre-polymer with additives, with suitable changes In the development processes can be utilized to obtain positive resists whilst the unmodified material Is a negative resist. For example, tetra-chloro dlazo cyclopentadienes in general are polymerisable by UV light to obtain negative resist systems. 

Acid-catalyzed silanol condensation to form insoluble networks has been also utilized in the design of aqueous base developable negative resist systems [419]. An aqueous base soluble silicone polymer was synthesized by a sol-gel reaction of a mixture of phenyltrimethoxysilane and 2-(3,4-epoxycyclohexyl)-ethyltrimethoxysilane. This polymer contained a high concentration of silanol OH groups and thus was soluble in aqueous base. 

Acid-C t lyzed Chemistry. Acid-catalyzed reactions form the basis for essentially all chemically amplified resist systems for microlithography appHcations (61). These reactions can be generally classified as either cross-linking (photopolymerization) or deprotection reactions. The latter are used to unmask acidic functionality such as phenohc or pendent carboxyhc acid groups, and thus lend themselves to positive tone resist apphcations. Acid-catalyzed polymer cross-linking and photopolymerization reactions, on the other hand, find appHcation in negative tone resist systems. Representative examples of each type of chemistry are Hsted below. 

The use of phenolic polymers in photocrosslinkable systems usually involves multicomponent systems which incorporate polyfunctional low molecular weight crosslinkers. For example, Feely et al. [9] have used hydroxymethyl melamine in combination with a photoactive diazonaphthoquinone which produces an indene carboxylic acid upon irradiation to crosslink a novolac resin. Similarly, Iwayanagi et al. [10] have used photoactive bisazides in combination with poly(p-hydroxy-sty-rene) to afford a negative-tone resist material which does not swell upon development in aqueous base. 

Dry-etch selectlvlties for several negative e-beam resists are also listed in Table V. They are more resistant than the positive e-beam resists of the Table except PMCN and the positive photoresists, AZ2400 and PC 129. The positive-behaving vinyl polymer resists tested are generally less resistant than the negative-behaving systems. This generality, however, does not hold for the photoresists tested, as the data of Table VII verifies. 

It was indicated earlier that swelling limits resolution in solvent-developed negative resists. It was also intimated that swelling effects could be minimized if there were a sufficient polarity change between the exposed and non-exposed areas of the type mentioned in the previous discussion of the PBOCST system. A similar principle was utilized by Hofer et al., (145-146) at IBM, based on ion pair formation. The resist consists of a polystyrene polymer to which tetrathiofulvalene (TTF) units have been attached. When spun down with an acceptor such as CBr4, a complex is formed which, on irradiation, undergoes an electron transfer reaction to form an ion pair ... 

Positive Photoresists. Positive resists are entirely different from negative resists. For the purposes of this discussion we restrict ourselves to visible-light-sensitive materials. Typically, these materials are mixtures of low-molecular-weight phenol-formaldehyde polymers and derivatives of naphtho-1,2-quinone diazide, the photosensitive component. The former is soluble in aqueous alkali, but the presence of the latter, a hydrophobic species, inhibits attack of this developer on the film. On irradiation the "sensitizer" is converted to a ketene, which, after reaction with water, forms a base-soluble carboxylic acid. Thus the irradiated part of the film is rendered soluble in the developer and it can be removed selectively. The important feature of this system is that the unirradiated areas are not swollen by the developer and the resolution of this material is quite high. It is possible to prepare gratings having several... 

We have successfully employed the trimethylsilylmethyl appendage to effect oxygen RIE resistance in both positive and negative acting electron-beam resist systems (10,11). The relatively compact nature of this substituent allows the preparation of glassy polymers useful for lithographic applications. The preparation and characterization of select trimethylsilylmethyl substituted resists will be presented in addition to a study of their radiation chemistry and lithographic properties. 

Next, the pattern is developed by dissolving the exposed parts of the pattern in the correct solvent. The quality of the pattern (contrast) depends on both the exposure and the development time, both of which must be experimentally determined for different resist systems and substrates. Once the pattern has been exposed and developed, the actual catalytically active metal(s) or oxide(s) is vapor deposited both within the holes in the pol3mier and on top of the polymer (step 5). This metal (or oxide) film should be discontinuous at the pattern boundaries in order to make possible the liftoff the remaining resist. This requires that the resist layer is thicker than the deposited film, and that the developed resist features have an undercut or negative profile... 

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