Optical Techniques for Fabrication of Microball Lens Arrays
C. T. Pan1, S.C. Shen2, J. Z. Zha1
1
Department of Mechanical and Electro-Mechanical Engineering, and Center for Nanoscience
& Nanotechnology, National Sun Yat-Sen University, Taiwan
2
Mechanical Industry Research Laboratories, Industrial Technology Research Institute,
Hsinchu 310, Taiwan
Abstract: -In the study, a novel concept of batch-fabrication microball lens array has been successfully
demonstrated on the silicon wafer. Not only can this batch-fabrication microball lens provide accurate
coupling distance to improve the optical coupling efficiency in free space, but also can replace
traditional way such as aspheric lens or expensive GRIN without sacrificing performance and can
reduce micro-assemblage cost. The result reveals that the aspect ratio of H/D plays a significant effect
on the formation of microball lens array, where H is the thickness of photoresist, and D is the diameter
of the photoresist. Measurement result shows that using 635 nm wavelength laser, the optimum
coupling distance is about 8m with an insertion loss below -1.3 dB (coupling efficiency about 73%).
Key-Words:- coupling efficiency, microball lens, critical aspect ratio, photoresist, insertion loss
1
Introduction
robotics
vision,
optical
scanner,
and
The major objective for using microlens is to
high-definition projection displays [2]. Both
enhance the brightness of illumination and
higher accuracy and lower cost of microlens
simplify the light guide module construction.
fabrication methods are needed to meet the
In a laptop display, a 25% increase in light
rapid growth of commercial devices.
output was reported for using microlens
Refractive lenses in microscale offer
technology [1]. Other micro-optical functions
several
and devices, such as focal plane optical
reduced wavelength sensitivity compared to
concentration,
diffractive optics (necessary for broadband
enhancements,
optical
color
efficiency
features:
significantly
beam
application), the possibility of really large
shaping, and miniature optical scanning,
numerical apertures, and a high light
have shown the potential needs in the
efficiency [3].Several fabrication techniques
industry.
have
Miniaturizing
separation,
important
devices
using
been
applied
to
the
fabrication
micro-optics promise to revolutionize many
refractive microlenses. One way to fabricate
electro-optical systems – from video cameras,
refractive microlens is by melting cylindrical
video phones, compact disk data storage to
posts
1
of
photoresist
[4,5].
Upon
the
development of very large scale integration
these devices. It offers an all-optical network
(VLSI)
techniques,
to increase the data exchange speed and light
coherent arrays of refractive microlenses are
propagation performance. [15].Microscale
made in the surface of silicon using a
Fresnel lenses, refractive lenses, beam
combination of lithography and reactive ion
splitters, gratings and precise optical mounts
etching (RIE) technique [6, 7]. A laser
have been tested and characterized for 1x2
writing system for
of
and 2x2 optical switch [16].Furthermore,
continuous-relief micro-optical elements in
Toshiyashi et al [17]presented 2x2 matrix
photoresist was described by Gale et al
optical switch by surface micromachining
[8].Besides, analogous microlens array was
technique. They employed a commercially
fabricated
ablation
available collimated beam fiber (CBF) that
[9].Micro-optics printing technology offers
was equipped with a spherical lens (1mm in
by printing a number of droplets on substrate
diameter). However, most of the approaches
and
array
require a lot of time to align micro-optical
[10].Microlenses ranging in diameter from
components. These procedures are time
20μm to 5 mm have been fabricated. Using
consuming
deep
Furthermore, size of commercial microball
based
processing
by
forming
the
fabrication
excimer
circular
laser
microlens
x-ray
lithography
to
fabricate
micro-optical
components
shows
and
lack
of
precision.
great
lens components is from several hundred m
potential for mass production [11].Lee et al.
to several mm. It requires more space to
used the modified LIGA process to fabricate
accommodate the components.
microlens
by melting the
deep
x-ray
There are a lot of existing papers talking
irradiated pattern in the PMMA substrate
about how to batch-fabricate microlens array
[12].Micro-optical
[18-20],
components
of
any
but
little
about
batch-fabricate
sidewalls,
the
conventional method to fabricate microball
micrometer range, and heights up to several
lens is forming a high refractive index ball
hundred micrometers can be achieved.
lens by melting glass at the fiber’s terminal
Following a molding process (either injection
to enhance fiber optical coupling efficiency
molding or hot embossing), the fabrication of
[21, 22].On the other hand, Cox et al
optical components in mass production can
[23]applied micro-inkjet printer to form
be achieved [13, 14]. However, most of the
microlens at the fiber’s terminal to enhance
mentioned
fiber performance. But, it is hard to control
complicated
dimensions
above
process
approaches
to
in
require
fabricate
the
lens.
to
desired shape with smooth and vertical
lateral
microball
how
The
the size and position of microlens.
microlens.
In this paper, the authors present an
A commercial microball lens can be
effective process utilizing a dual polymeric
applied in MOEMS. The micro-optical
layers
switch (an indispensable element for optical
batch-fabrication of microball lens array. The
communication systems) is representative of
3-D schematic drawing of optical coupling
2
to
achieve
fast
and
neat
platform integrated with microball lens to
The schematic flow chart of how to
enhance coupling efficiency is shown in
fabricate micro-ball lens is illustrated in
Fig.1(a).The cross-sectional view of two
Fig.3.First, the PI was smoothly spin-coated
microball lenses between optical fibers at an
on the wafer followed by AZ4620 spread on.
accurate
in
The exposure and development were carried
Fig.1(b).Due to the benefit of providing
out to build a cylinder of dual layer on wafer.
accurate alignment distance between fibers
The wafer was then put in oven to reflow to
and microball lens, the light emitted from the
produce microball lens array.
distance
as
illustrated
input fiber can be precisely transmitted to the
opposite fiber or redirected to the other
orthogonal one.
3
2
3.1 Effects of Primary Material
Diameter
Experimental Method
Results and Discussions
The material system of various cover size has
2.1 Experimental Principle
the
As shown in Fig.2, the photoresist (labeled as
sin 1  sin  2  sin  3
M2) and under-layer polymer (labeled as M1)
are spin-coated on substrate followed by
same
contact
angle,
namely
, thus
d1 d 2 d 3


 constant
r1
r2
r3
exposure and development. When the system
is afterwards heated over Tg temperature of
(2)
photoresist (M2) in oven, it begins to show
Therefore small diameter of developed
the liquid behavior and to reduce the surface
material produces small curvature radius of
energy. The Young’s Model describes a liquid
microlens.
drop on a solid surface as:
TSA  TLS  TAL cos 
(1)
where  is the equilibrium contact
3.2 Effects of PI Thickness
angle, TAL is the surface tension of the
liquid, TSA is the surface energy of the
The contact angle of a single layer of
photoresist can not reach up to 90 oC after
solid and TLS is the solid-liquid interface
reflow to form a ball-shape lens, as
energy.
illustrated in Fig. 4(a). However, with the PI
underneath,
the
upper
photoresist
can
successfully form a ball lens, as shown in Fig.
2.2 Experimental Set-up
4(b). As a further step to the early reference
In the study, AZ4620 and SP431 (PI) were
[24], it shows the thicker the PI (slower
selected as M2 and M1 to be photoresist and
spinning speed) in the tested range, the more
under-layer polymer.
effective is the assisting PI, thus the smaller
3
radius of microlens is produced at all levels
lower than the Tg temperature of PI material.
of diameter as a consequence.
Its Tg is between 175 oC and 180 oC. As to PI,
its Tg is about 300 oC. Once the thermal
energy input goes beyond the threshold, the
3.3 Effects of Protoresist Aspect Ratio
formation process is started and is then
The aspect ratio (H/D) plays a significant
controlled literally by the aspect ratio of
role on the curvature radius of the formed
photoresist layer.
microball lens (R), where H is the thickness
of photoresist, and D is the diameter of the
photoresist or PI before reflow process.
3.5 Coupling Efficiency
Either the smaller diameter (D) will produce
Visible 653 nm laser diode was collimated
smaller radius, or the thicker (H) photoresist
into the microball lens with 40m in
will produce smaller microball lens. It is
diameter. The coupling efficiency was
attributed to the higher surface energy
measured by light intensity with variation of
difference driving the formation of ball shape.
the distance between microball lens and
One notices further that beyond the aspect
power meter. Each coupling position from
ratio of around 0.3, the minimum of
microball lens to power meter was measured.
curvature radius subject to the applied
Fig. 6, reveals the experimental measurement
diameter of the dual cylinder will be reached.
of coupling efficiency is a function of
Fig. 5, illustrates that when the applied
distance from microball lens and power
reflow temperature is higher than the Tg of
meter. In this optical coupling measurement,
photoresist, and at a ratio of H and D less
optimum coupling distance is about 8m and
than 0.3, thus the cylinder of dual layer
the insertion loss is about -1.3 dB. The
becomes mushroom-like profile, as shown in
maximum coupling efficiency is approximate
Fig. 5(a). Otherwise, at a ratio of H and D
73 %.
around 0.3, the cylinder changes to a
ball-like structure, as shown in Fig. 5(b). The
4
final reflowed shape changes to micro-ball
Conclusions
In this study, an effective method using the
array from micro-mushroom by changing the
dual layer of upper photoresist and PI
ratio of diameter D and height H. when the
beneath is investigated. The results show that
aspect ratio of the patterned photoresist is
reflowed microball is major controlled by the
larger than 0.3, the reflowed shape will
aspect ratio of photoresist and the applied
always be a microball shape.
diameter of the primary material when
heat-treated adequately. Thicker polymer is
found more advantage for forming smaller
3.4 Effects of Reflow Temperature
balls, while the reflow temperature is of little
The oven temperature was set at 190 oC, 220
effect in the tested range. The experimental
o
C and 250 oC to reflow AZ4620, which is
4
the monolithic fabrication of microlens
arrays, Vol.27, 1988, pp. 1281-1284.
[5] M. C.Hutley, Journal of Modern Optics,
Optical techniques for the generation of
microlens arrays, Vol.37, 1990, pp.
253-265,.
[6] M. Stern and T. R. Jay, Optical
Engineering, Dry etching for coherent
refractive microlens arrays, Vol.33,
No.11, 1994, pp. 3547-3550.
[7] M. E. Matamedi, M. P. Griswold, and R.
E. Knowlden, Proc. SPIE, Silicon
microlenses for enhanced optical
coupling to silicon focal planes, Vol.1544,
1991, pp. 22-32.
[8] M. T. Gale, M. Rossi, J. Pedersen, and H.
Schutz, Optical Engineering, Fabrication
of
continuous-relief
micro-optical
elements by direct laser writing in
photoresists, Vol.33, No.11, 1994, pp.
3556-3566.
[9] K. Zimmer, D. Hirsch, and F. Bigl,
Applied Surface Science, Excimer laser
machining for the fabrication of
analogous microstructures, Vol.96-98,
1996, pp. 425-429.
[10] W. R. Cox, T. Chen, and D. Hayes,
Optics & Photonics News, Micro-optics
fabrication by ink-jet printing, Vol.12,
Np.6, 2001, pp. 32-35.
[11] J. Gottert and J. Mohr J, SPIE:
Micro-Optics,
Characterization
of
micro-optical components fabricated by
deep-etch x-ray lithography, Vol.2,
No.1506, 1991, pp. 170-178.
[12] S.-K. Lee, K.-C. Lee, and S. S. Lee,
Journal
of
Micromechanics
and
Microengineeing, A simple method for
microlens fabrication by the modified
LIGA process, Vol.12, 2002, pp. 334-340.
[13] H. Yang, M.-C. Chou, A. Yang, C.-K.
Mu, and R. F. Shyu, Proc. of SPIE,
Realization of fabricating microlens
array in mass production, Vol.3739, 1999,
pp. 178-185.
[14] H. Yang, C.-T. Pan, and M.-C. Chou,
Journal
of
Micromechanics
and
Microengineering, Ultra-fine machining
tool/molds by LIGA technology, Vol.11,
2001, pp. 94-99.
[15] Huang, L.-S., Lee, S.-S., Motamedi, E.,
Wu, M.-C., and Kim, C.-J., 48th IEEE in
Electronic Components and technology
Conference, MEMS packaging for micro
result reveals that the critical aspect ratio
(H/D) to produce a ball is found around 0.3
for the current process. Namely, when the
aspect ratio is larger than 0.3, the final
reflowed shape changes to microball profile
from
micro-mushroom
profile.
The
experimental result of optimum coupling
distance between fiber and microball lens is
about 8 m and coupling efficiency is
approximate 73 %. The new method is based
on the thermal reflow of two polymeric
layers to batch-fabricate microball lens array.
Therefore, not only can the fabrication
process provide accurate coupling distance
through lithography process to enhance
coupling efficiency, but also can reduce
micro-assemblage cost.
Acknowledgement
The author would like to thank Dr.
Tung-Chuan Wu and Dr. Min-Chan Chou at
MIRL of ITRI in Taiwan for their guidance,
and National Science Council (NSC) for their
financial supports to the project (granted
number: NSC92-2622-E110-009-CC3 and
NSC92-2212-E110-029).
References:
[1] B. Ezell, Information Display, Making
microlens backlights grow up, Vol.5,
2001, pp. 42-45.
[2] M. E. Motamedi, Optical Engineering,
Micro-opto-electro-mechanical system,
Vol.33,
No.11,
1994,
pp.
3505-3517.
[3] S. Sinzinger and J. Jahns, Microoptics,
WILEY-VCH Verlag GmbH, 1999, pp.
85-103,.
[4] Z. D. Popovic, R. A. Sprague, and G. A.
N.Connell, Applied Optics, Technique for
5
mirror switches, Vol.25-28, 1998, pp.
592-597.
[16] M.C. Wu, L.Y. Lin, S.S. Lee, and C.R.
King, Int. J. High Speed Electronics and
Systems, Free space integrated optics
realized by surface micromachining,
Vol.8, No.2, 1997, pp.283-297.
[17] H. Toshiyoshi, and H. Fujita, IEEE J.
Microelectromech. Systems, Electrostatic
micro torsion mirrors for an optical
switch matrix, Vol.5, NO.4, 1996, pp.
231-237.
[18] D. Daly, R. F. Stevens, M. C. Hutley,
and N. Davies, Meas. Sci. Technol, The
manufacture of microlenses by melting
photoresist, Vol.1, 1990, pp.759-766.
[19] Zoran D. Popovic, Robert A. Sprague,
and G. A. Neville Connell, Applied
Optics, Technique for Monolithic
Fabrication of Microlens Arrays, Vol.27,
No.7, 1988, pp.1281-1284,.
[20] M. C. Hutley, R. F. Stevens, and D. Daly,
Optical Connection and Switching
Networks for Communication and
Computing,
The
manufacture
of
microlens arrays and fan-out gratings in
photoresist, 1990, pp. 111-113.
[21] Vera Russo, Giancarlo C. Righini,
Stefano Sottini, and Silvana Trigari,
Applied Optics, Lens-ended fibers for
medical applications: A New Fabrication
Technique, Vol.23, No.19, 1984, pp.
3277-3283.
[22] G. D. Khoe, J. Poulissen, and H. M. de
Vrieze, Electronics Letters, 17th, Efficient
coupling of laser diodes to tapered
monomode fibers with high-index end,
Vol.19, No.6, 1983, pp.205-207.
[23] W. Royall Cox, Chi Guan, Donald J.
Hayes and David B. Wallace, Int. J.
Microcircuits
Microjet
&
Elect.
printing
interconnects,
of
Vol.23,
Packaging,
micro-optical
No.3,
2000,
pp.346-351.
[24] S.C. Shen, C.T. Pan, M.C. Chou, and
H.P. Chou, Journal of Magnetism and
Magnetic
optical
Materials,
switch
communication,
for
Electromagnetic
optical
Vol.239,
network
2001,
pp.610-613.
(a) 3-D schematic drawing
Fig.2 Theoretical surface tensions in a
dual-layer system for microball lens
fabrication.
(b) Cross-sectional view
Fig.1
Three dimensional schematic
drawing and cross-sectional view
of coupling platform integrated
with microball lens.
6
Figure 5 Shape profile of microball lens
formation
Coupling efficiency of microball lens
(40m in diameter of microball lens).
Fig.3 Schematic illustration of
micro-ball lens fabrication
Figure 6 Measurement result of microball
lens.
(a) single layer after reflow
(b) dual layer after reflow
Figure 4 Reflow Characteristics
(a)mushroom shape
(b) ball shape
7