(628e) In-Situ Control of Cationic Polymerization Kinetics for Negative Tone Photoresists

Many negative tone photoresists (also referred to as resists) operate by the cationic polymerization of functional groups such as epoxides, vinyl ethers, and oxetanes to create a cross-linked, higher molecular weight film that is less soluble than the uncross-linked unexposed region. Negative tone photoresists may also operate by the acid catalyzed reaction of epoxide groups with pendant alcohols or carboxylic acids on the resist molecule or polymer. The widely used SU-8 photoresist operates by this cationic polymerization mechanism. Negative tone resists have many favorable properties compared to positive tone photoresists, but one major problem with these types of negative tone resists is that it is difficult to control the physical propagation of the polymerization. In conventional acid catalyzed positive tone resists, base quenchers can be added to neutralize the photoacid and prevent its diffusion outside of the exposed area and control its diffusion within the exposed area. This will not work as well or at all in these types of negative tone resists because the photoacid is not the primary ?active species?. It exists as an acid only long enough to protonate an epoxide which initiates polymerization. Furthermore, since the epoxides are often in large excess relative to both the photoacid generator and base, the probability of a photoacid reacting with a base before it reacts with an epoxide is small. Thus to control these systems, the cationic polymerization itself must be controlled. To that end we have studied multiple different methods of controlling cationic polymerization through film additives, direct modification of the cross-linker, and addition of photosensitive additives as well. The effect of these additives on the cationic polymerization has been studied using direct lithographic patterning and FT-IR study of the extent of cross-linking. We have developed and investigated chain transfer agents, chain termination agents, additives that generate additional initiators and photo-switchable chain termination agents. By reaction engineering of these materials, greatly improved performance has been achieved in these materials.