Fabrication of Sub-Micro Thin Layer Structures using Cyclopentanone Mix Solution as a Modified SU-8 3050

Journal of Science & Technology 118 (2017) 060-064
60
Fabrication of Sub-Micro Thin Layer Structures using Cyclopentanone
Mix Solution as a Modified SU-8 3050
Nguyen Quang Long1, Nguyen Thi Thai1, Chu Thi Xuan1*, Nguyen Minh Hang2, and
Pham Duc Thanh1, Mai Anh Tuan1
1Hanoi University of Science and Technology - No. 1, Dai Co Viet Str., Hai Ba Trung, Ha Noi, Viet Nam
2Institute of Applied Science and Technology, Ministry of Science and Technology
Received: April 27, 2016; accepted: June 9, 2017
Abstract
The SU-8 3000 series have been formulated for improved adhesion to the substrate and reduced coating
stress than SU-8 2000 series. It is being used to fabricate microstructures in the range of thickness from
some up to several hundred micrometers. However, it is hard to fabricate the layer of the SU-8 3000 series
at sub-micrometer thinness. In this paper, we propose a procedure to reduce the thickness of the SU-8 3050
thin film. By mixing the SU-8 3050 with Cyclopentanone (CP) with different volume ratios, we can control
viscosity of the mixture. Therefore, the thickness of the fabricated photoresist layer at 2000 rpm spincoating
can be from 540 nm to 5330 nm. The mixing procedures of SU-8 fabrication have been optimized in order to
enhance the unity of the thin layer photoresist. Some other fabrication factors are also investigated.
Keywords: Thin SU-8 photoresist, Cyclopentanone, Sub-micro thin layer.
1. Introduction*
Recently, there are many materials with
improved properties such as non-toxicity,
biocompatibility, stability under various
environmental conditions and the fabrication with
simplicity and low-cost in the fields of biology and
biotechnology. Wide range of both organic and
inorganic materials such as silicon, porous silica,
porous alumina, and various nanoparticles has been
proved to be biocompatible and widely used for
various biological and biotechnological applications.
Similarly, polymers have been gaining much interest
as valuable structural materials in biotechnology due
to their biocompatibility and ease of structuring with
sizes down to few nanometers. Among various
polymers, SU-8 is an epoxy based negative
photoresist which has found a broad range of
biomedical and biotechnological applications due to
their biocompatibility, high thermal, mechanical, and
chemical resistance [1,2]. Since its invention in the
nineties, SU-8 has been extensively used in micro-
and nanotechnologies and in MEMS for fabricating
scaled-down components and devices such as micro-
and nanochannels, membranes, microfluidic devices,
cantilevers, biosensors and more.
SU-8 (glycidyl ether of bisphenol A) is an acid-
catalyzed, near-UV photoresist based on EPON SU-8
epoxy resin which was developed and patented by
IBM (US patent No. 4882245) in 1989 [3]. SU-8
* Corresponding author: Tel.: (+84) 163.362.0899
Email: xuan@itims.edu.vn
dissolved in cyclopentanone and gamma-
butyrolactone solvents are classified as SU-8 2000
and SU-8 3000 series respectively. SU-8 has eight
epoxy groups per monomer and the number eight in
SU-8 refers to the eight epoxy sites present in each
monomer.
Referring to the physical texture of SU-8, it is a
highly viscous liquid and the viscosity is determined
by the ratio of solvent to EPON resin mixed. The
viscosity influences the thickness of the resist film
when spin-coated on a substrate. The thickness of
SU-8 film can vary from 2 µm to 300 µm by single
spin coating of SU-8 and reach mm range by multiple
spin coating [4]. Previous research has shown that the
micro and nanostructures with very high aspect ratio,
larger than 20, can be achieved by using SU-8
photoresist [5]. SU-8 photoresist has good
biocompatibility that is suitable for bioapplications.
In addition, this photoresist is thermally and
chemically stable due to its high cross-linked matrix.
Upon the radiation exposure (UV, e-beam or X-ray),
SU-8 gets cross-linked and becomes insoluble to the
developer solution. The physical properties of SU-8
are listed in references [6,7,8].
SU-8 are widely used in many domains. It was
first applied to the MEMS devices in IBM in 1995
[10]. The use of SU-8 in MEMS can be distinguished
into two applications: temporary and permanent
applications. In temporary applications, SU-8 may be
removed or not used in the final MEMS devices.
Popular examples can be listed as using SU-8 as the
mold for electroplating [10], or soft lithography [11].
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In permanent applications, SU-8 is widely used in
microfluidics [12], optical communication devices
[13], and optical waveguide [14]. From the
microfluidic point of view, SU-8 is widely used as a
mold thanks to strong adhesion to many types of
substrates and highly sensitive to the UV expose [3].
In microlense fabrication, SU-8 is a favorite material
because of many reasons such as: transparence, ease
of microfabrication and mass production; reduction of
lens weight; enable the microscale various focus
length and array of liquid microlens fabrication [15].
Depending on the viscosity of SU-8, different
structures with different size and high can be
obtained. Normally, the viscosity range of SU-8
allows for film thicknesses of several µm to more
than 100 µm by single spin coating. In our previous
study, we have fabricated SU-8 3050 film with
thickness from 25 to 125 µm in a single coat [11].
However, in some applications such as micro and
nanolens fabrication, SU-8 thin layer in the order of
sub micrometer is needed. This can be reached by
diluting the resist solution with the solvent. Various
works have been reported using gamma-butyrolacton
to dilute SU-8 [5]. Another solvent has been used to
dilute SU-8 2000 series is cyclopentanone (CP) [16].
However, there is no systematical study on the
diluting SU-8 3000 series with cyclopentanone.
This paper aims at fabricating SU-8 thin layer in
the order of micrometer and sub micrometer
thickness with SU-8 3050 using cyclopentanone as a
diluting solution.
2. Experiment
2.1 Materials and methods
SU-8 3050 photoresist was purchased from
MicroChem. Cyclopentanone was purchased from
Sigma-Aldrich.
The thickness of this layer was also measured by
Scanning Electron Microscope (SEM) technique and
profilometer. By SEM technique, the sample was
tilted at 35 degrees to calculate the thickness of the
SU-8 layer. The thickness was calculated based on
the equations: ℎ = (1)
Where:
h is the real thickness of the SU-8 layer;
d is the high of projection of the SU-8 layer on the
substrate measured by SEM;
φ is the inclined angle of the sample.
The Veeco Detaak 150 Profilometer is used to
measure the roughness of the surface. It uses a stylus
to track the surface variations. From the function of
position, we can measure the thickness of the sample.
2.2 Mask design
This mask was designed in Corel Draw
including hinge and lens with different sizes from
0.05 mm to 2.842 mm (Fig. 1)
Fig. 1. Mask design for SU-8 mold fabrication.
2.3 Mixing procedure
In order to mix SU-8 photoresist with
Cyclopentanone, two different process were applied
(Fig. 2).
Fig. 2. Mixing process of SU-8 3050 and
Cyclopentanone.
In mixing procedure 1, the first step was to put
SU-8 3050 and Cyclopentanone at fixed ratios into a
bottle, then the solution was stirred in 30 minutes.
Finally, bubbles in the mixture solution were
removed by centrifugation. In mixing procedure 2,
after stirred, the solution was boiled using hotplate at
90 oC. In this work, we focus on the effect of heating
procedure and mix ratios. The SU-8:Cyclopentanone
mix ratios 1:1, 1:1.5, 1:2, 1:3, 1:4, and 1:5 were
investigated.
Mixing procedure 1 Mixing procedure 2
SU-8 3050
+Cyclopentanone
SU-8 3050
+Cyclopentanone
Stirring magneticStirring magnetic
Heating at 90 0C
Centrifugation Centrifugation
Mixing solution Mixing solution
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2.4 Lens fabrication procedure
The mixture solution at different volume ratios
was then used as the photoresist for the basic
photolithography process. Before lithography
process, the Si substrate was oxidized at 1050 oC in
30 min to obtain 200 nm SiO2 layer. The lens
fabrication is illustrated in Fig. 3.
Fig. 3. Photolithography process.
Step 1: Clean the wafer with piranha solution and
then bake at 200 oC in 30 min.
Step 2: Spin-coat the mixed solution at 500 rpm for
10 s (ramp: 100 rpm/s), slope, and then 2000 rpm for
30 s (ramp: 100 rpm/s).
Step 3: Pre-bake the sample at 95 oC for 2 min.
Step 4: Expose the sample to UV light (250 W, 12
mW/cm) for 50 s.
Step 5: Post-bake the sample at 65 oC for 1 minute
then 95 oC for 2 min with ratio of SU-8:
Cyclopentanone equal 1:1, for 1 min with ratio 1:1.5,
for 30 s with ratios 1:2, 1:3 and 1:4 respectively.
Step 6: Develop the sample with SU-8 developer for
5 min with ratio of SU-8:Cyclopentanone equal to
1:1, for 3 min with ratio 1:1.5, for 2 min with ratios
1:2, 1:3, and 1:4.
Step 7: Rinse the sample with IPA and then dry with
nitrogen.
Step 8: Hard-bake the sample at 180 oC for 20 min.
3. Results and discussion
3.1 Influence of heating
Fig. 4 (a) shows the surface of the 4-inch Silicon
wafer which is spin-coated with the solution of
mixing procedure 1 (SU-8 3050/CP equal to 1:3,
2000 rpm, 30 s, non-heated). The obtained solution is
uniform and no gas bubble was observed (Fig. 5).
The mixed solution was then used as photoresist layer
for the micro-prism fabrication. However, we can
also observe that gas bubbles appeared in photoresist
layer after coating on the SiO2 surface. It means that
SU-8 and Cyclopentanone were not completely
mixed or the mixed solution was not well adhered on
the SiO2/Si surface.
Therefore, we have modified the mixing process
in order to promote the mixture quality of the SU-
8/CP. After mixing with magnetic stirrer, the mixture
solution was heated at 90 oC in 30 min (mixing
procedure 2). Fig. 4 (b) shows that there is no bubble
formed after spin coating. It means that the heating
process favored the unity of the mixture. The result
proved that the prepared solution using mixing
procedure 2 can be used to create perfect photoresist
layer for photolithography process.
Fig. 4. Images of silicon wafer after being spin-
coated with the mixed solution, a) no heating mix
solution (mixing procedure 1); b) heating mix
solution (mixing procedure 2).
Fig. 5. Image of the mixture of SU-8 and
Cyclopentanone.
3.2 Dependence of SU-8 layer thickness on volume
of the Cyclopentanone in mixture solution.
The microscope image of the SU-8 mold after
dipping the wafer in developer solution is shown in
Fig 6. Shape of molds for lens fabrication was formed
as design after photolithograph process.
The SEM image (Fig. 7) of the device’s
structure shows the well-defined shape. SU-8 is the
upper section, and the lower part is the silicon
substrate. The thickness of this layer was approximate
(a) (b)
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5.14 µm in all the surface of the wafer (ratio of SU-8
3050:Cyclopentanone = 1:1). To verify the thickness
of this layer, profilometer was used on different
points of the device. The results showed that when
the ratio of SU-8 3050 : Cyclopentanone equal 1:1,
the thickness of the layer was about 5.23 µm (Fig. 8).
These results were similar with the calculated one
based on SEM images.
Fig. 6. The SU-8 hinge and lens image fabricated
using photolithography process.
Fig. 7. SEM image of mixture at volume ratio 1:1.
Fig. 8. Profilometer images of micro-device at
volume ratio 1:1
a) 1:1.5 b) 1:2 c) 1:3 d) 1:4
Fig. 9. Profilometer images of micro-device at
different volume ratios.
Fig. 10. Dependence of layer thickness on SU-8:
Cyclopentanone volume ratio.
The same procedure was done with other
samples of different mixed volume ratios of SU-8
3050: Cyclopentanone equal to 1:1.5, 1:2, 1:3, 1:4,
and 1:5 respectively. The profilometer images of lens
structure for different mixed ratios SU-8/CP are
showed in Fig. 9.
As shown in Fig. 10, the results showed that
when the ratio of SU-8 3050: Cyclopentanone
decreases, the thickness of photoresist decreases. To
specific, the layer’s thickness were 5330 nm, 2500
nm, 1380 nm, 880 nm and 540 nm when the mixing
ratios of SU-8:Cyclopentanone were 1:1, 1:1.5, 1:2,
1:3 and 1:4 respectively. The relation between the
thickness and SU-8: Cyclopentanone volume ratio
exhibited to be logarithmic. The volume ratio 1:5 is
too diluted for use as photoresist. There is no color
change of the photoresist after UV insolation and
neither lens nor hinge were observed after
development.
5.14 m
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4. Conclusion
The mixing process for SU-8 3050 and
Cyclopentanone was presented in this paper. The
heating was proved to play an important role in
reducing bubble and favoring the unity of the
mixture. After the process, the mixed solution can be
used as photoresist to fabricate the microstructures.
Some different volume ratios of SU-
8:Cyclopentanone was described. The thickness of
thin layer formulated by SU-8 3000 series following
this process is similar to that of SU-8 2000, which
allows to extend the applications of SU-8 3000.
Acknowledgment
This work was financially supported by Hanoi
University of Science and Technology, Grant No.
T2016-PC-125.
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