This short, introductory review covers the still rapidly growing and industrially important field of ring opening polymerization (ROP). The review is organized according to mechanism (radical ROP (RROP), cationic ROP (CROP), anionic ROP (AROP) and ring-opening metathesis polymerization (ROMP)) rather than monomer classes. Nevertheless, the different groups of cyclic monomers are considered (olefins, ethers, thioethers, amines, lactones, thiolactones, lactams, disulfides, anhydrides, carbonates, silicones, phosphazenes and phosphonites) and the mechanisms by which they can be polymerized involving a ring-opening polymerization. Literature up to 2012 has been considered but the citations selected refer to detailed reviews and key papers, describing not only the latest developments but also the evolution of the current state of the art.

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Ring-Opening Polymerization—An Introductory Review

Historically, in the mass production of semiconductor devices, exposure tools have been repeatedly replaced with those with a shorter wavelength to meet the resolution requirements projected in the International Technology Roadmap for Semiconductors issued by the Semiconductor Industry Association. After ArF immersion lithography, extreme ultraviolet (EUV; 92.5 eV) radiation is expected to be used as an exposure tool for the mass production at or below the 22 nm technology node. If realized, 92.5 eV EUV will be the first ionizing radiation used for the mass production of semiconductor devices. In EUV lithography, chemically amplified resists, which have been the standard resists for mass production since the use of KrF lithography, will be used to meet the sensitivity requirement. Above the ionization energy of resist materials, the fundamental science of imaging, however, changes from photochemistry to radiation chemistry. In this paper, we review the radiation chemistry of materials related to chemically amplified resists. The imaging mechanisms from energy deposition to proton migration in resist materials are discussed.

https://iopscience.iop.org/article/10.1143/JJAP.49.030001/pdf

Radiation Chemistry in Chemically Amplified Resists

Externally stimulated initiator systems for cationic polymerization

The fabrication of multilayer organic light-emitting diodes through solution processing presents challenges, especially regarding dissolution of the first layer during deposition of a second layer. One possible approach to this problem is to insolubilize the first layer using cross-linking. Cross-linking has also been used to control the morphological stability and aggregation phenomena of the active organic materials. In this short review, we discuss the alternative chemically, thermally, and photochemically promoted cross-linking chemistries that have been examined in the context of organic light-emitting diodes including: the hydrolysis of silicon compounds to form siloxanes; the polymerization of styrene, acrylate, and oxetane groups; and the dimerization of trifluorovinyl ethers, benzocyclobutenes, and cinnamates.

Approaches to Solution-Processed Multilayer Organic Light-Emitting Diodes Based on Cross-Linking†

With the shrinkage of feature sizes, ever precise accuracy has been required for process simulators because of the importance of nanoscale resist topography such as line edge roughness. Formation processes of latent images in chemically amplified electron beam (EB), x-ray, and EUV resists are different from both chemically amplified photoresists used in optical lithography and conventional, nonchemically amplified EB resists. A new simulation scheme precisely based on reaction mechanisms is necessary to reproduce resist patterns for the postoptical lithographies. We proposed a method to simulate electron dynamics in chemically amplified resists and to calculate the acid distribution around an ionization point with a typical parameter set.

Modeling and simulation of chemically amplified electron beam, x-ray, and EUV resist processes

Irradiation of solutions of naphthalene and triphenylsulphonium trifluoromethanesulphonate initiates an electron transfer reaction which gives 1- and 2-phenylnaphthalenes, 2- and 4-phenylthiobiphenyls, and diphenyl sulphide. The phenylnaphthalenes are formed by an in-cage reaction between phenyl radical and naphthalene radical cation, whereas diphenyl sulphide is an escape product. A second cage process, oxidation of diphenylsulphide by naphthalene radical cation, generates diphenylsulphinyl radical cation; reaction of this intermediate with phenyl radical gives the phenylthiobiphenyl isomers. However, irradiation of anthracene and triphenylsulphonium triflate yields only phenylated anthracenes and diphenylsulphide, because anthracene radical cation cannot oxidize diphenylsulphide.

Novel photoinduced electron transfer reactions between naphthalene and triphenylsulphonium salts

Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, 3- and 4-phenylthiobiphenyl isomers by an in-cage reaction, whereas diphenylsulfide and other aromatic photoproducts are formed by cage-escape reactions with solvent. The cage and escape reactions both generate acid, which is the primary initiator for many chemically amplified resists. Photo-CIDNP has been used to characterize the cage and escape reactivity of radical intermediates formed from photolysis of triphenylsulfonium salts. In addition nanosecond laser flash photolysis studies have found key intermediates in the direct and triplet sensitized photolysis of these salts. The photophysics of a number of aromatic polymers was examined to understand how the polymer participates in the photoinitiation process. The polymer fluorescence was quenched by sulfonium salts in solution by a dynamic mechanism, whereas in polymer films the quenching was by a static mechanism. Fluorescence lifetimes for the polymers, estimated from the quenching plots in solution, were relatively short, 4-8 nsec and the values agreed well with those obtained by time-resolved spectroscopy.

Photochemical and photophysical studies on chemically amplified resists

Triphenylsulfonium bromide, triflate, tetrafluoroborate, hexafluoroarsenate, hexafluorophosphate and hexafluoroantimonate were irradiate in solution and in the solid state. The photoproducts, 2-, 3- and 4-phenylthiobiophenyl are formed by an in-cage fragmentation-recombination mechanism, whereas diphenylsulfide is formed by a cage-escape reaction. In dilute solution, the ratio of cage to escape products is approximately one for each salt. Also in concentrated solutions, an identical cage : escape ratio is observed for all salts except for the bromide, which favors the escape reaction. Photolysis of the salts in the solid state shows remarkable counter ion dependence and cage : escape ratios as high as 5 : 1 are observed. The mechanism for both in-cage and escape reactions is described.

Cationic Photoinitiators: Solid State Photochemistry of Triphenylsulfonium Salts

Photolysis of triphenylsulfonium salts in solution or polymer films gives 2-, 3-and 4-phenylthiobiphenyl isomers by an in-cage fragmentation-recombination mechanism, and the cage-escape sulfide product diphenylsulfide. Both the in-cage and cage-escape reactions result in acid production. Formation of the in-cage products is favored in viscous solutions, polymer films and in the solid state photolysis of triphenylsulfonium salts. However sensitization by photo-induced electron transfer, or triplet energy transfer, gives only the cage-escape sulfide product via the triphenylsulfur radical, or triplet phenyl radical - diphenylsulfinyl radical cation pair, respectively. In-cage and cage-escape products are also found from photolysis of diphenylhalonium salts. The importance of‘cage versus escape’ reactivity of onium salts in acid production and in the mechanistic interpretation of their photochemistry, in solution and polymer films, is discussed.

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The importance of cage versus escape reactivity in the photochemistry of onium salts

A scheme for imaging of polyimide films is described which is based on the amine-catalyzed imidization of poly(amic alkyl ester) precursor polymers. Films containing amine photogenerators along with poly(amic alkyl esters) are patterned by exposure followed by heating to partially imidize the exposed portion of the film. Negative images are developed by taking advantage of the greater solubility of the precursor polymer which is dissolved in an appropriate solvent mixture. A final cure is carried out to complete the imidization of the patterned film.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Nigel P. Hacker

Papers

Novel photoinduced electron transfer reactions between naphthalene and triphenylsulphonium salts

Photochemical and photophysical studies on chemically amplified resists

Cationic Photoinitiators: Solid State Photochemistry of Triphenylsulfonium Salts

The importance of cage versus escape reactivity in the photochemistry of onium salts

Base-catalyzed photosensitive polyimide