Online proceedings - EDA Publishing Association

al. used a modified LIGA (German acronym for LIthografie,
Galvanoformung, and Abformung) process to fabricate
microlenses by melting the deep x-ray irradiated pattern onto
a PMMA (poly-methyl methacrylate) substrate. Using this
technique, microoptical components of any desired shape can
be fabricated [6, 7]. The resulting components have smooth
and vertical sidewalls, lateral dimensions in the micrometer
range, and sag heights of several hundred micrometers. A
molding process (either injection molding or hot embossing)
is required before mass production can be achieved. The
microlens array mold or mold inserts play an important role in
the mass molding production process. This replication
process promises the desired profile as final products.
A new method for producing microlens array with large
sag heights was investigated for integrated fluorescence
microfluidic detection systems [8]. Three steps in that
production technique were included for concave microlens
array formations to be integrated into microfluidic systems.
The micro concave lens molds were then finished and ready
to produce convex microlens in PDMS material. Using a
LIGA-like process to fabricate microlens arrays is
considerably less expensive using a UV exposure tool instead
of deep x-ray lithography. A new microlens array fabrication
method using a UV proximity printing method has been
invented [9]. It uses a slice to control the gap size, resulting in
microlens array formation in the resist. However, this method
was limited to round microlens arrays with low sag heights.
They produced microstructures with smooth surfaces, high
yield rates, and good reliability.
The LIGA-like process provides microlens array
fabricators with high optical quality at low cost. By using the
vacuum pressure to form a microlens array was investigated
[10]. This vacuum suction technique is feasible for certain
microlens array fabrication sizes. Based on the LIGA-like
technology development, this paper will present the
promising technique using the LIGA-like process to pressing
microlens array and investigate the processing parameters for
making microlens array.
11-13
May 2011, Aix-en-Provence, France
Fig. 1. Illustration of the screen-printing process for microlens array
fabrication.
2.1 Contact angle measurement
The liquid contacts on a solid surface, there is a contact
angle between the solid and liquid drop surfaces. The surface
hydrophobicity of the substrate can determine the microlens
profile. It is necessary to find the contact angles between the
photoresist and different substrates. A surface tension
examiner (FTA200) was used to measure the contact angle.
An example to measure the contact angle between water and
copper coating substrate, the contact angle is 78.35°. The
contact angle between water and silver coating substrate is
60.98°. The contact angle between water and stainless steel
304 is 56.67°. A low contact angle between water and glass
substrate is 22.24° as shown in Fig. 2. The further
experiments to measure the contact angle between photoresist
AZ4620 and stainless steel 304, sopper coating substrate,
glass substrate, the resulted contact angle are 26.43°, 39.33°
and 42.42°. It means that the larger contact angle can result in
a high sag mirolens. From the above experiments, the glass
substrate is chosen for screen printing mirolens array in
photoresist.
II EXPERIEMNTS
The fabrication process mainly applies the LIGA-like
technology. In the conventional microlens array fabrication,
photoresist patterns are formed by lithography process, it
includes mask pattern design, photoresist coating, UV
exposure and development, and thermal reflow. Microlens
array in photoresist is formed by the above steps. The further
mass production will apply electroforming to replicate the
microlens array mold. A different approach is to pattern an
electroforming mold, then directly screen printing photoresist
patterns and thermal reflow formicrolens array fabrication. It
will be suitable for mass production of Microlens array by
using the same mold. The fabrication process is illustrated in
Fig. 1.
Fig. 2 Contact angle measurement of water and glass substrate.
2.2 Lithography process
The lithography process used a PET mask with pattern
layout design is illustrated in Fig. 3. Eight patterns with four
diameters 30, 45, 60, and 80μm as well as two different
spacings 20 and 40 μm are included. Since negative
photoresist JSR THB-126N was used, the resulted patterns
were micro-post array. The opening area is exposed to UV
lithography, monomers in negative photoresist are
cross-linking by photons. Micro-post array in photoresist
remains after development. Micro-post array height is
controlled by spin coating thickness. The relationship is
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