Photo‐induced cross‐linking mechanism in azide–novolac negative photoresists: molecular level investigation using NMR spectroscopy

Abstract

Negative photoresists are composed of a photoactive component (aromatic azides/bisazides) and cyclized rubber or novolac resin dissolved in an organic solvent. Hydrogen abstraction and/or insertion reaction of the reactive nitrene intermediate formed during photoirradiation of the azide result in a cross-linked network of the novolac resin. The molecular weight of novolac resin in the exposed part of the photoresist film thus increases compared with that of the unexposed part. This makes the exposed part insoluble in the alkaline developer. Exploiting this change in physical property, a pattern can be transferred to a substrate from a mask. A better understanding of the exact mechanism of cross-linking reactions is very important to the design of a high-performing negative photoresist. A quinone-imine-type complex has been proposed earlier involving the aromatic moiety of novolac resin as the reaction site. A more recent study focuses the attack of nitrene on the methylenic bridge and hydroxyl group of novolac resins, which were found to be responsible for the cross-linking reaction along with the aromatic moiety of novolac resin. However, in our study no evidence was found for the involvement of a methylenic hydrogen or aromatic moiety of novolac resin in the cross-linking reaction. The 1 H NMR, 13 C NMR and DEPT-135 spectra before and after photolysis indicate that the cross-linking site is predominantly the hydroxyl group of novolac resin. Multiple reaction sites of attack for the nitrene intermediate have been demonstrated in cashew nut shell liquid (CNSL)-based novolac resin by 1 H NMR spectroscopy, which in turn further increases the cross-linked network in the exposed part of a negative photoresist.

Cardanol‐based novolac resins as curing agents of epoxy resins

“…Although cardanol and its derivatives can be converted to polymers with better processability, hydrocarbon solubility and resistance to acids and alkalis than conventional phenol‐based systems, little has been published on the development of polymeric materials from CNSL and, in particular, from novolac‐type phenolic resins prepared from cardanol. This kind of resins have been studied and proposed only recently for applications in the fields of surface coatings,11 rubber reinforcement,12, 13 brake linings,14 composites,15, 16 photoresist for microlithography,17, 18 polyurethanes,19, 20 and few others 21–23. In the last years, cardanol‐based novolac resins have been also studied as modifiers of commercial epoxy resins to improve the mechanical properties and toughness of these 24, 25…”

Cross-Linking of Side Chain Unsaturated Aromatic Polyethers for High Temperature Polymer Electrolyte Membrane Fuel Cell Applications

“…During the thermal treatment the bisazide loses a nitrogen and a reactive nitrene is formed. This intermediate product was added to the propenyl groups to give aziridines and amines , as shown in Scheme . The used amounts of the neat cross-linkable polymers and the bisazide are listed in Table .…”

Reactive & Efficient: Organic Azides as Cross-Linkers in Material Sciences

“…In the final stripping process, depending on the type of photoresist, either the exposed (positive resist) or the unexposed (negative resist) area is soluble and is removed, leaving behind the desired pattern. Popular commercial negative photoresists consist of low molecular weight polymers derived from phenols and formaldehyde (so called novolacs) and aromatic bisazides as photosensitizers [40]. Scheme 3 shows the chemical mechanisms in bisazide photoresist systems [40].…”

“…Popular commercial negative photoresists consist of low molecular weight polymers derived from phenols and formaldehyde (so called novolacs) and aromatic bisazides as photosensitizers [40]. Scheme 3 shows the chemical mechanisms in bisazide photoresist systems [40]. Irradiation of the photoresist with UV light generates highly reactive singlet nitrenes which can react with the polymer matrix creating covalent bonds via (1) insertion into C-H bonds, (2) addition to double bonds forming aziridine rings which is the most common way, and (3) radical abstraction followed by recombination of radical polymer chains.…”