Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
photoresist for microlens array formation.
B. Microlens Photolithography
Figure 4 shows the proposed micro probe array fabrication
process. Desired patterns are transferred from the designed
microlens array mask in the proximity printing ultraviolet
(UV) lithographic process. In this experiment, a microlens
array mask was fabricated using the thermal reflow process
onto a glass. The each pohotoresist microlens with a
diameter of 60μm , 70μm and the pitch distance for two
adjacent microlens was 90μm. The upper and lower rows
were arranged in equidistance. The Silicon wave substrate
was then spun with a layer of positive photoresist (AZ4620)
18μm thick. The spin condition was 500rpm for 40 seconds.
Prebaking in a convection oven at 90℃
for 3 minutes is a
required procedure. This removes the excess solvent from
the photoresist and produces a slightly hardened photoresist
surface. The mask was not stuck onto the substrate. The
sample was exposed through the microlens array mask using
a UV mask aligner (EVG620). This aligner had soft, hard
contact or proximity exposure modes with NUV (near
ultra-violet) wavelength 350-450nm and lamp power range
from 200-500 W. A slice of glass was inserted between the
photoresist and microlens array mask to create a gap shown
in Fig. 4a. The gap was adjusted to 100μm. Exposure was
then conducted for about 40 seconds. The threedimensional
array was completed after exposure and dip
into the developer for 2 minutes and cleaning with deionized
water. The micro-cone probe tips mold was produced as
shown in Fig. 4(b).
Fig. 4. Flow chart for micro probe array fabrication, (a) proximity UV
exposure, (b) photoresist molding, (c) Ni electroforming, (d) micro probe
array peel-off.
C. Fabrication of micro metal probe tip
Electroforming was carried out in a 10 L electroforming
tank. The electroplating process requires a conductive layer
to be deposited if the substrate itself is non-conductive.
Therefore, a seed layer of copper (250 nm) was deposited
usinganE-beam evaporator. The substrate is connected to a
cathode, with nickel pellets acting as the anode. An in-tank
circular filtration system including a filter tube and carbon
treatment was used. The filter used in this work has 5 lm
pores, which is the finest commercially available density
tube. High purity is required in the electroforming process to
avoid impurity deposits onto the microstructures. The
template was placed into a Ni electroplating bath to form the
metallic micro probes shown in Fig. 4c. The detailed
ingredients of the Ni electroplating bath are listed in Table I.
The deposition of Ni was uniformly controlled using an air
pump for agitation to mix the electrolyte bath. Because of
the micrometer range feature size at the end, a very slow
deposition rate at 1 ASD was applied for 2 h to maintain the
completed step coverage and duplication quality. The
sample microstructure was observed as shown in Fig. 4d.
Optical microscopy (OM) and a 3D surface profiler were
used to measure the characteristics of the resulting
microcone probe array structures.
TABLE I Ni electrolyte composition
Ni (NH2SO3)2_4H2O 500 (g L -1 )
Boric acid 45(gL -1 )
Current density 1 ASD (A dm -2 )
pH 4
Temperature 50 °C
Agitation
Air pump
Wetting agent 3 (mL L -1 )
Ⅳ. RESULTS AND DISCUSSION
The lithography using microlen array mask can produce
the micro-cone probe tips mold. As shown in Fig. 5, under
the conditions of 150 °C high temperature and 10 minutes,
patterns of microlens were successfully formed in the
aperture stops over the whole glass substrate by using OM
observation. From measurement, the microlens array was
found to have a diameter of 60μm and height of 5μm.
According to the experimental results, the micro cone probe
arrays mold were classified after development using
different focal length and diameter of lens. Fig. 6 shows
micro probe arrays mold by using OM observation. As the
exposure gap is 100μm and pohotoresist microlens with a
diameter of 60μm, micro probe arrays mold with concave
cone were formed. Then, using the exposure gap is larger;
the photoresist structure is flat. The UV light and photoresist
of gap is less; a flat down micro cone mold was formed. A
small exposure gap is not suitable for micro cone mold
fabrication because the thick photoresist will not have
enough thickness to produce concave structures. The
concave micro cone structure surface is quite smooth. The
micro metal probes after the electroforming process. The Ni
micro probes was fabricated. The 3D surface of Ni micro
cone probe profiler was measured using 3-D surface profiler
in Fig. 7. The fabricated structure was fine and had clear
surface. Using the proximity ultraviolet (UV) lithography
methodtofabricate micro cone probe mold and furthermore
replication of Ni micro cone probe array is practicable.
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