Figure 39 Mechanism of reducing volume shrinkage based on the reversible disulfide-bond.[125]
For the photoresist system containing ADSDA, it does not contain any additional PI due to the arylthiyl radical generated by ADSDA possesses a good capability of initiating photopolymerization. The photoresists with ADSDA exhibited a capability of photopolymerization in the absence of PI, and their double bond conversion can reach up to 86 %, at the same time, the volume shrinkage of the polymerized photoresists with ADSDA could even drop to 0.56 % owing to the “breakage–recombination” reversible reaction of disulfide bonds.
Oxygen inhibition of acrylate-based resists limits the potential of UV-NIL for high throughput, however, the photopolymerization system of thiol–ene leads to extremely little oxygen inhibition, simultaneously, it also has the properties of high monomer conversions, low shrinkage, homogeneous mechanical properties. Besides, the etch resistance and thermal stability of cured resist are important because these properties allow good pattern transfer. The inorganic polyhedral oligomeric silsesquioxane (POSS) combines many desirable properties as an NIL photoresist such as optical clarity, high thermal stability, high density, high Young’s modulus, and a low dielectric constant, meanwhile, it does not affect the resolution of the NIL pattern.[133-135] Based on this, Yin et al.[136] developed a novel POSS containing mercaptopropyl (POSS-SH), the structures of each component of the hybrid UV-NIL resist are displayed in Figure 40. POSS-SH can enhance the thermal properties, dimensional stability and etching resistance of UV-NIL resist, and reduce the volume shrinkage. The hybrid resist can obtain the high aspect ratio patterns as showed in Figure 41, which is beneficial for graphics transfer onto Si.
Figure 40 Chemical structures of each component of the hybrid UV-NIL resist.[136]