Many different types of non-traditional resist designs have shown promise for future generations of patterning, but there is a greater need for understanding and developing additives and ancillary materials for these novel resists compared to traditional polymeric positive tone systems which are quite mature. With the goal of meeting some of these needs, we carried out multiple different studies of negative tone molecular resists based on epoxide cross-linking. We have developed methods for controlling cross-linking in these materials using photo-decomposable nucleophiles (PDNs) which have shown resolution improvements in one resist from 26 nm down to 18 nm so far. Aqueous base developed systems have been made by introducing phenol groups to these resists. Although the first molecule designed successfully patterns in aqueous base, its performance is limited compared to organic solvent development. A series of di-functional epoxide molecular resists have been made and generally show much worse sensitivity than a fourfunctional epoxide resist. Underlayers (ULs) have been designed specifically to leverage the high reactivity of epoxides to create ULs that can cross-link to the resist. These ULs improve the adhesion of some of these molecular resists, but also show significant sensitivity improvements compared to imaging on bare silicon. Sensitivity in one resist was improved by 5-12 mJ/cm2 simply by using the UL.
A series of five negative tone epoxide functionalized molecular resists have been synthesized and have had their glass transition temperature (Tg) and lithographic contrast behavior characterized. Introducing rigid structural features in the form of aromatic rings to a resist was found to increase its glass transition temperature. All resists but one, BHPF-2Ep, were found to have poor film stability which required the use of an underlayer. A trend was observed where PEB conditions performed at temperatures much higher than the Tg of the molecular resist was found to induce propagation of polymerization outside of exposed regions. Di-functionalized resists were observed to have poor sensitivity due to their low degree of functionalization. A resist was synthesized (BHPF-2Ep) which was capable of resolving features down to 20 nm with an imaging dose of 70.5 mJ/cm2.
A negative tone, aqueous base developable molecular glass resist, 3Ep, is presented that is developable in both standard organic solvents and aqueous base developers. The resist shows slightly better imaging performance in organic solvent versus aqueous base and shows a shift of E0 away from zero dose. Compared to a previously reported 4Ep resist, 3Ep appears to have a more controlled polymerization rate at equivalent conditions, which results in higher-quality patterned features. 3Ep also requires use of an underlayer to avoid de-wetting during aqueous base development.