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viscosity of the imprint materials induces the forces resulting from the flow of the working stamp 2 (Figure 1b). Dispensing
resistance against the Z-axis movement. viscous epoxy between the two stamps with epoxy with precise volume and position is
However, the active WEC technique can arbitrary geometries. These forces can lead extremely important in attaining the desired
sustain a force up to 3kN, therefore, it plays to a resulting shear force that generates a final RLT while avoiding an overflow of
a crucial role in accurately reaching the shift in lateral directions. Furthermore, the the epoxy. In the next steps, the fiducials
targeted Z-position thereby spreading the poor contrast of the fiducials on the PDMS on stamp 2 are aligned to the grabbed
epoxy to the desired thickness. Through stamp (filled with epoxy) during the imprint position of the fiducials on stamp 1. The
its three-axis movement, it also allows for process makes it challenging for the pattern alignment with respect to grabbed positions
a controlled re-leveling of the layers in the recognition and automatic alignment is needed because of the large gap of up
machine if forces should induce changes in systems. To mitigate this issue, back side to 3mm between the two stamps, which
the parallelism [4]. alignment (BSA; higher depth of focus) makes it impossible to keep the targets of
The use of silicone polymer stamps is applied. BSA gives the opportunity to both layers in focus at the same time. The
on both sides of the imprint ensures capture and train fiducials of the top stamp combination of using the grabbed target
the required high transfer fidelity. The before the alignment features are concealed position from stamp 1 and the live image
combination of polydimethylsiloxane by the epoxy, and allows for a direct view from stamp 2 allows for reliable pattern
(PDMS) and a rigid glass back plate of the targets on the bottom stamp without recognition and high accuracy alignment,
provide, both the large area rigidity, and interference of structures on the top stamp. independent of the process gap between the
the local flexibility, that is needed to A general process flow of fabricating two working stamps. In this paper, the RLT
imprint monolithic arrays. The PDMS monolithic lens arrays is represented of the monolithic lens array is predefined
part of the stamp assists the conformal in Figures 1a-c. To begin with, the top to be 200µm. Hence, the realignment and
imprinting and thereby minimizes the working stamp 1 is secured by vacuum on Z-axis movement are performed until
trapped air. Simultaneously, the rigid the stamp holder and the bottom working the target gap of 200µm is reached. After
carrier provides the required in-plane stamp 2 is held by vacuum on the bottom reaching the targeted alignment and RLT,
rigidity while imprinting to achieve chuck. Once, the top and bottom stamps are the epoxy is exposed for 6min by using the
good TTVs. In order to achieve the very fixed on their corresponding tooling, the high intensity i-line band (365nm) from a
good overlay accuracies needed for best WEC process levels the two stamps with broadband SUSS UV-LED unit (Figure 1c)
optical performance, performing an X-Y respect to each other. The alignment targets [5]. In the final step, the cured free-standing
alignment after each Z-axis movement step on the working stamp 1 are captured using monolithic lens array is separated from
is obligatory until the final gap (and hence, the BSA microscopes (Figure 1a). Then, the working stamps, using a SUSS imprint
the target RLT) is reached. This is due to the epoxy is dispensed on the center of the separation tool (Figure 1d).

Figure 1: a) Performing WEC between the two working stamps and the process of capturing and training fiducials from stamp 1; b) Training fiducials on stamp 2 and
alignment between the two working stamps with epoxy dispensed on stamp 2; c) Reaching final imprint gap and UV-exposure of the epoxy; d) Separation of the monolithic
stack from the working stamps.

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