Tech Briefs Magazine - May 2022 - PIT-23

DELO KATIOBOND
DELO PHOTOBOND
70
65
60
55
50
45
40
Abbe number
Figure 3. Range of refractive index and Abbe number currently available using DELO's imprint materials
(Figure: DELO)
In a typical UV-NIL process, a liquid
UV-curing (adhesive) material is deposited
onto a substrate. The stamp is
brought into contact and the liquid material
flows into the stamp's cavities.
Next, UV exposure begins, curing the
liquid material which becomes solid. After
this solidification has finished, the
stamp can be removed, resulting in an
imprinted structure on the substrate.
While a lot of variation can be found in
the details, generally every UV imprint
process follows this scheme.
UV-Curable Polymer
Specifications
Imprint materials for optical applications
can have a multitude of different
properties, dictated by the function
they must fulfill. The refractive index is
one of the key properties when selecting
a UV-curable optical imprint material.
While some applications might work
with a broad range of refractive indices,
optical designers often prefer one refractive
index over another due to design
or manufacturing constraints, and
often applications require a specific refractive
index.
For example, grating couplers for augmented
reality or core materials for light
guide applications need a high refractive
index (>1.7); whereas cladding materials
for optical waveguides demand a low refractive
index (<1.45). These requirements
are met by chemically different
formulations, based on different polymer
families with individual overall
strengths. The refractive index of epoxy-based
materials typically ranges from
1.45 to 1.6. For acrylic materials, the
range can be from 1.35 to 1.9 and above.
35
30
25
20
15
1,84
1,82
1,8
1,78
1,76
1,74
1,72
1,7
1,68
1,66
1,64
1,62
1,6
1,58
1,56
1,54
1,52
1,5
1,48
1,46
1,44
1,42
1,4
1,38
1,36
1,34
Other optical properties are also important.
A diffusor material, for example,
needs to contain a precise amount of
shaped particles to realize the desired
scattering profile. A lens material with
low haze and high transmission ratio requires
a high level of purity. Completely
light blocking materials have to combine
high optical densities with small layer
thicknesses. For other applications, these
materials need to act as optical filters in a
specific wavelength range. The combination
of such optically tailored properties
offers almost complete design freedom
as well as enabling complex device architectures
that are easy to fabricate.
Another fundamental aspect, which is
frequently disregarded, is the thermo-mechanical
behavior of an imprint
material. Take, for example, a monolithic
lens design. It requires a very hard material
with a high glass transition temperature
because the hardness directly
translates to good mechanical stability
and scratch resistance. In contrast, a material
for large area imprint of thick optical
structures on a glass carrier needs to
be softer so that it can compensate for
thermal stress created by different coefficients
of thermal expansion of glass and
the imprint material.
For these different scenarios, users
can choose from a wide range of soft
and hard materials. Generally speaking,
their Young's modulus can be designed
to be as low as 400 MPa (soft) and as
high as 6,500 MPa (hard). Regarding
temperature stability, UV-curable epoxies
can be equipped with a glass transition
temperature that can reach almost
200°C if necessary.
Both acrylic and epoxy materials pass
industry-standard temperature and humidity
tests (THT test, typically at 85°C
and 85% r.h. with storage times varying
Figure 4. Both lenses (left) and complex nanostructures (right) can be made from UV-curable polymers (magenta only for illustration purposes, the material
itself is completely colorless and transparent in reality) (Images: DELO)
Photonics & Imaging Technology, May 2022
23
PIT Application Briefs 0522_1.indd 23
Cov
ToC
4/18/22 2:21 PM
Refractive Index @589 nm

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