James F. Cameron

Photogeneration of polymeric amines: synthesis, photocrosslinking and photoimaging of copolymers containing photoactive carbamate pendant groups

Two families of copolymers containing 4-[(2-nitrobenzyloxy)carbamoyl]styrene as the photoactive component, with styrene or methyl methacrylate as the other component are described. The polymers are prepared by chemical modification of copolymers of 4-aminostyrene with styrene and methyl methacrylate, respectively, using 2-nitrobenzyl chloroformate. The photoactive carbamate groups of both copolymers can be removed by irradiation with UV light below 320 nm. Quantum-yield measurements and comparisons with small-molecule model compounds confirm that attachment to the polymer has no effect on the photolability of the 2-nitrobenzylcar-bamoyl protecting group. Quantum-yield values near 0.10 also compare favourably with those obtained for related small molecules. The photoactive polymers have been tested successfully in the UV-induced crosslinking of formulations containing epoxy groups.

Solid state quantum yield determination of a novel base photogenerator

Abstract The photoefficiency of a new class of base photogenerating materials, based on the photoremoval of the α,α-dimethyl-3,5-dimethoxybenzyloxycarbonyl (Ddz) group from masked amine-type bases, was investigated. The solid state quantum yield for the photogeneration of cyclohexylamine from its corresponding Ddz carbamate was evaluated at 280 nm using a number of quantitative IR methods. A value of 0.10 was found using the optimal method involving a comparison of the intensities of a strong IR reference band solely associated with the polymer matrix with the decreasing carbonyl absorption of the Ddz substrate. Comparable quantum yields were also found by using an intense band associated with the Ddz-masked amine, thus indicating the overall applicability and reliability of the method provided that careful choice of the reference band was made. The value of the quantum efficiency for the photodeprotection of Ddz-masked amines is sufficiently high for practical applications requiring the use of photogenerated base.

Photogenerated amines and their use in the design of a positive-tone resist material based on electrophilic aromatic substitution

The photogeneration of an active amine within a cationically curable polymer coating can be used to design a novel positive-tone resist material. The resist is based on a copolymer containing 4-hydroxystyrene as well as 4-acetoxymethylstyrene units; when heated in the presence of an acid, this copolymer crosslinks through an electrophilic aromatic substitution process. Therefore, a small amount of 2-nitrobenzyl toluene-p-sulphonate, that decomposes upon heating to produce toluene sulphonic acid, is added to the resist along with a thermally stable but photoactive carbamate that liberates an amine upon irradiation. Exposure of a film of the resist to 254 nm UV radiation results in the formation of a latent image consisting of amine molecules dispersed within the polymer film. The latent image is ‘fixed’ by heating; this liberates acid, which is neutralized where amine has been formed, but causes crosslinking of the polymer by a cationic process in those areas of the film where no amine has been produced. This resist, based on an image-reversal concept applicable to numerous cationically activated resists, can be developed in aqueous base and shows a good sensitivity of ca. 19 mJ cm–2.

New photolabile amino protecting groups: photogeneration of amines from [(3′,5′-dimethoxybenzoinyl)oxy]carbonyl carbamates

A novel class of photoiabile amino protecting groups, based on [(3′,5′-dimethoxybenzoinyl)oxy]carbonyl groups, is presented and their scope and versatility demonstrated by the photogeneration of free primary and secondary amines.

Photogenerated base as catalyst for imidization reactions

The photogeneration of free amines or diamines from soluble organic precursors such as photoactive 2-nitrobenzyl carbamates is useful for the imidization of polymers containing amic acid or amic ester moieties. The base-catalyzed reaction occurs at lower temperatures than the non-catalyzed thermal process and is applicable to the patterning of polymer coatings or thin films.

Cationic curing of polymer coatings: Evaluation ofo-nitrobenzyl tosylate as a thermally labile acid precursor

SummaryThe propensity ofo-nitrobenzyl tosylate to act as a thermal source ofp-toluenesulfonic acid useful in the cationic curing of polymers was established both in solution and the solid state using a combination of variable temperature1H NMR and quantitative infrared spectroscopy. Monitoring of the thermal generation ofp-toluenesulfonic acid within inert coatings of poly(styrene) or poly(methyl methacrylate) confirmed that the process was most efficient at temperatures between 100 and 110°C, though slow liberation of acid was observed at temperatures as low as 90°C.

Resist materials design: base-catalyzed chemical amplification

Many chemically amplified resists that function on the basis of acid catalysis of thermolytic reactions have been described as well as systems that function on the basis of free radical chain reactions. But there have been very few reports on the use of base catalysis of chemical transformations in resist materials. We describe here our initial results on base catalyzed chemically amplified deep-UV photoresists. Photogenerated amines were used as catalysts for the decarboxylation of carboxylic acids. Two approaches to building resists around this chemistry were investigated. (1) Decarboxylation of a low molecular weight carboxylic acid led to base induced dissolution inhibition of a phenolic polymer giving negative tone images. (2) A carboxylic acid polymer was synthesized which also is susceptible towards base catalyzed decarboxylation. Wet development of this resist material gives negative tone images. Site specific gas-phase silylation of the carboxylic acid allows the use of this material in a positive tone dry develop process. A 0.5 micrometers line-space pattern obtained by this dry develop process illustrates the potential of base-catalyzed chemical amplification.

Calibration and validation of projection lithography focusing by fluorescence detection of latent photoacid images in chemically amplified resist

We dope the commercial resist UVIII with the pH sensitive dye coumarin 6 (C6) to demonstrate a fluorescence microscopy technique for detecting latent photoacid images in exposed chemically amplified resist films. The spectroscopic properties of C6 in the resist matrix are investigated in order to select spectroscopic filters which optimize the fluorescence image contrast of exposed patterns. We apply this technique to focus calibration of a projection lithography system and show a significant correlation of the results to scanning electron microscope analysis of developed features. Hence, we demonstrate the utility of this technique as a fabrication line diagnostic for focus calibration and validation (i.e., proof of focus) prior to postexposure processing of the resist film.