724 CHINESE OPTICS LETTERS / Vol. 7, No. 8 / August 10, 2009
Research on the resolution of micro stereo lithography
Xiang Wang (
)
∗
, Jijun He (
ÛÛÛ
FFF
), Xiangtao Pei (
777
), Peng Shao (
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),
Jiaru Chu (
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), and Wenhao Huang (
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)
Department of Precision Machinery and Precision Instrumentation, University of Science and Technology of China,
Hefei 230026, China
∗
E-mail: wxyf@ustc.edu.cn
Received November 4, 2008
Micro stereo lithography is a kind of technology utilizing the solidified effect that photo curable polymer
will appear under ultraviolet (UV) laser exposure. It is widely used in three-dimensional (3D) micro
fabrication. We get the experimental values of a pair of UV laser curing coefficients, absorption coefficient
and critical curing energy, of curable resin by fitting the calculation results of the Gaussian beam theory
and experimental curing results. The theoretical relation between the curing unit’s shape and the exposure
features of time and intensity of convergent Gaussian beam is presented. The calculation and experimental
results of curing unit under d ifferent conditions agree well with each other. This research offers a steady
base for further research about th e improvement of resolution.
OCIS codes: 220.3740, 220.4000, 220.4610.
doi: 10.3788/COL20090708.0724.
Micro stereo lithography (MSL) is a method derived from
the technology of rapid prototyping (RP) in the field
of laser-based micr o fabrication. The basic principle is
that, under an ultraviolet (UV) exposure, some kinds of
polymers can absorb photons and cause a photochemi-
cal reaction, leading to pho to p olymerization
[1]
. MSL is
attracting global concern for its three-dimensional (3D)
micro fabrication ability. Kobayashi et al. resear ched the
polymerization of freely movable 3D microstructure us-
ing conductive photo curable resin
[2]
. Zha ng et al. used
a solution consisting of curable resin and c e ramic sus-
pens ion to lead a layer-by-layer polymerization on silicon
substrate
[3]
. Products of micros tructure using the tech-
nology of UV p olymerization of liquid resin and lithog-
raphy with mask are being provided by Micro-TEC-D
company
[4]
. It is now one of the most importa nt research-
ing realms in 3D micro fabrication
[5,6]
.
MSL uses scanning UV beam to focus on the UV cur-
able resin, leads to the polymeriz ation, and then form-
ing points, lines, planes, and finally the layer-by-layer
3D microstructure. In this letter, we analyze the reso-
lution of MSL through researching the relation between
the characteristics of single curing unit and fo c al spot by
the principle of micro photo polymerization. A mathe-
matical model is constructed based on the exp erimental
data. This will offer a steady base for the future 3D mi-
cro fabrication study.
According to the photon absorption theory, after ex-
posure, the molecules will turn into excited sta te and
energy will be tra ns mitted or reflected in co mpound sys-
tem. With the interaction of chromophore (chemical
bonds or groups that can absorb photons), polymeriza-
tion and cross linking reaction will happen, resulting in
phase transformation and photo curing
[7]
. Photo cur ing
is a kind o f photochemical reaction resulting from the ab-
sorption of photon by photo-curable compound. When
the absorption happ e ns, if the absorbed photons’ energy
lets the molecules reach the critical value of excitation,
the liquid resin’s photo curing can appear. Generally,
the main properties to evaluate the liquid photo curable
resin include physical properties, optical properties in a
sp e cific wavelength, and mechanical prop e rties. Analy-
sis of influences on photo polymerization is mainly about
its optical performance parameter. Photo sensitive resin
used in MSL research is mainly UV curable resin. And
the principal coefficients reflecting the curing property
contains critical curing energy and absorption coefficient.
The feature value of a particular kind of resin is different
when the wavelength of UV exposure is different, so only
through the curing experiments and theoretical calcula-
tion can the feature para meters be fixed.
The experimental system is schematically shown in
Fig. 1. The process of exper iment is as follows. The UV
laser exits from quartz fiber, passes through the cover
slip, reaches the container full of liquid resin, and reacts
with resin, while single point curing unit is produced. In
the experiment, the fiber has a numerical aperture (NA)
of 0.37 and a radius of 0.6 mm. Its working distance
(from the tip face of the fiber to the liquid level of resin
including the depth of cover slip) WL = 0.38 mm and
the exposure spot radius on liquid level is 0.9 mm. The
intensity of exposure spot can be measured by UV in-
tensity meter (UV-A type Developed by Beijing Normal
University, whose measurable peak value of wavelength
is 365 nm, minimum resolution is 0.1 µW/cm
2
, a nd accu-
racy is ±5%). Usually, the photo poly merization of UV
curable resin obeys the Beer-Lambert law of absorption
which can be expressed as
[8,9]
I = I
0
exp(−αz), (1)
Fig. 1. Principle of experimental system.
1671-7694/2009/080724-04
c
2009 Chinese Optics Letters