Page 52 - Chip Scale Review_May June_2021-digital

Optimized UV-curing epoxy materials for lens imprint
When developing or selecting a UV-curable imprint material,
it is critical to consider and optimize a large number of material
properties [6]. These properties can be grouped into four groups as
shown in Figure 2.

Figure 2: Many different (and partially competing) properties need to be carefully
optimized and balanced.
Rather than trying to optimize a single property (or a group of
properties), it is key to balance all these properties. Even the most
process-friendly material would need to withstand reliability testing
and vice versa. For the work described in this paper, we have chosen
to use DELO KATIOBOND OM614, which is a UV-curable epoxy
material that has been developed for optical imprint applications.
Besides having a refractive index of ~1.5 (close to standard glass), it
shows excellent clarity (low haze) as needed for optical elements.
In terms of process capability, we want to focus on two properties:
1) compatibility with the stamp, and 2) shrinkage. During the imprint
process, the liquid imprint material is brought into contact with the
stamp (PDMS in the work described in this paper). While still in liquid
phase, the imprint material might migrate into the stamp material
(Figure 3), giving rise to a change of the stamp geometry by swelling.
This might affect the shape of the replicated lens and result in poor
optical quality. When cured, it is important that the adhesion of the
replicated optics to the stamp material is as low as possible. High
adhesion would not only make the demolding difficult, but would also
lead to higher wear resulting in reduced stamp lifetime, which would
increase overall process costs. In the case of DELO KATIOBOND
OM614, the formulation has been optimized so as to reduce both effects.

Figure 3: Typical acrylic or epoxy materials show high migration into PDMS. The
formulation of DELO KATIOBOND OM614 has been carefully optimized to reduce
such migration. The graph shows the weight gain of PDMS after 20 minutes of
contact with the imprint material.

50 Chip Scale Review May • June • 2021 [ChipScaleReview.com]
50