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]