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Download fileFinite Element Simulations of Filling and Demolding in Roll-to-Roll UV Nanoimprinting of Micro- and Nanopatterns
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posted on 2022-02-25, 20:03 authored by Johannes Götz, Asier Alvarez Rueda, Stephan Ruttloff, Ladislav Kuna, Maria Belegratis, Ursula Palfinger, Dieter Nees, Paul Hartmann, Barbara StadloberRoll-to-roll UV nanoimprinting
is a powerful method for the mass
fabrication of nano- and microstructured surfaces, which are highly
interesting for many technological applications (e.g., in the fields
of optics, electronics, biomimetic, and microfluidics). When setting
up a production process based on this technique, one of the main challenges
is the prevention of defects (mainly entrapped air during filling
and fractures during demolding). This can be cost- and time-intensive
as it is mainly done by trial and error. An improved theoretical understanding
of defect generation and its prediction for certain material and process
parameters is therefore desirable. To accomplish this, we developed
COMSOL-based two-dimensional (2D) and three-dimensional (3D) computer
simulations for the two key stages of UV nanoimprinting (filling
and demolding) and validated them by corresponding roll-to-roll as
well as step-and-repeat experiments. Regarding filling, the investigated
parameters are template and substrate contact angles; resin viscosity,
velocity, and thickness during filling; as well as feature geometry.
In summary, it is beneficial for filling to have low template contact
angles; high substrate contact angles; low resin viscosity and velocity;
as well as inclined sidewalls, low-aspect-ratio features, and a sufficient
resin thickness (whereby lack of one of these factors can be compensated
by others). Interestingly, nanoscale features are much easier to fill
than microscale features in practice (which is not due to reduced
bubble trapping but due to enhanced gas dissolution). Regarding demolding,
we studied the sidewall angle, fillet radius, size, and elastic modulus
of the features. In addition, we compared demolding by roll-to-roll
and by step-and-repeat considering the radius of rotation and we decoupled
bending, adhesion, and friction to investigate their relative contributions.
We could demonstrate quantitatively that for demolding, it is advantageous
to have small features, inclined sidewalls, rounded corners, and a
large radius of rotation. The dominant effect for nanostructures is
adhesion, whereas for microstructures, it is friction. Moreover, demolding
by tilting (step-and-repeat) exerts less stress on the imprint than
demolding using a roll-to-roll approach. Finally, we present a 3D
demolding simulation that identifies the most vulnerable positions
of a geometry. From the lessons learned from our filling and demolding
simulations, we could demonstrate the defect-free roll-to-roll UV
nanoimprinting of a challenging pattern (cuboids with vertical sidewalls).
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substrate contact anglesreduced bubble trappingmany technological applicationsimproved theoretical understandingexerts less stresstwo key stagesmainly entrapped airfinite element simulationsproduction process basedcould demonstrate quantitativelyvertical sidewalls ).sufficient resin thicknesslow resin viscosityroll uv nanoimprinting3d demolding simulationresin viscosityuv nanoimprintingcould demonstratebased twoprocess parametersmainly doneinclined sidewallsdemolding simulationscomputer simulationsothers ).microfluidics ).demolding ).whereby lackvulnerable positionstherefore desirablesidewall anglerounded cornersroll approachrelative contributionsregarding demoldingpowerful methodnanopatterns rollmuch easiermicrostructured surfacesmass fabricationmain challengeslessons learnedinvestigated parametershighly interestingg .,free rollelastic modulusdominant effectdeveloped comsoldemolding usingdecoupled bendingcorresponding rollcompared demoldingchallenging patterncertain material