CHARACTERIZATION OF OPTICAL PROPERTIES OF SU-8 AND
FABRICATION OF OPTICAL COMPONENETS
Om Prakash Parida
1 and Navakant Bhat
2
DOI, DRDL, Kanchanbagh, Hyderabad-581, Dept. of ECE, IISc. Bangalore-560012
2
[email protected], [email protected]
2
Abstract: SU-8 is a negative tone, near UV resist. Due to its unique properties, it is widely used as a structural
material in diverse fields of MEMS. Its excellent optical transparency beyond 400nm makes it a preferred
material for the fabrication of large variety of optical components and systems. UV-Visible spectroscopy and
Spectroscopic ellipsometry were carried out for characterizing the optical properties of SU-8. Process parameters
were optimized for the fabrication of different types of optical components like photonic bandgap structures,
optical waveguides, splitters, directional couplers and gratings using Laser and electron beam lithography.
1. INTRODUCTION SU-8 is a relatively new polymer material in the
field of MEMS introduced by IBM in 1989. It is an
epoxy based chemically amplified, negative tone,
near UV photo-resist as well as a low cost X-ray and
e-beam resist [1]-[3]. It is a highly functionalized
molecule with 8-epoxy groups, polymerized by
cationic photo-polymerization which is induced by
Lewis acid during UV illumination. Subsequent
heating during post-exposure baking (PEB) activates
cross-linking and regenerates the acid catalyst which
causes significant enhancement of the resist
sensitivity and results in high mechanical strength
and thermal stability of the structure. Chemical
structure of SU-8 monomer and standard SU-8 film
processing sequence are presented in figure-1.
Figure-1: (a) Structure of SU-8 monomer and
(b) basic SU-8 processing sequence
Cross-linked SU-8 films are highly resistant to a
large number of acids, bases and solvents. Due to its
unique physical, chemical, mechanical,
electromagnetic and bio-compatible properties, it has
been explored as an important material in diverse
fields of MEMS manufacturing not only as a resist
material but also as the core structural material [4].
Because of its excellent optical transparency and
favorable optical parameters, SU-8 has been
preferred as a suitable material for the fabrication of
optical waveguides, gratings, Bragg filters and
reflectors, photonic bandgap structures, on-chip light
sources, optical pressure sensors, opto-fluidic
systems and MOEMS [5]-[8]. Optical properties and
parameters specified in the datasheets are limited and
obtained under specific processing conditions, hence
cannot be straightway acceptable for a different
processing environment. So extensive characteri-
zations for optical properties of SU-8 under different
processing conditions were carried out. ‘Ultraviolet-
Visible spectroscopy’ and ‘Spectroscopic
ellipsometry’ [9] - [10] of SU-8 films were carried
out to evaluate the absorption characteristics and
refractive index of SU-8 respectively. Process flow
was optimized for the fabrication of optical
components like 2-D photonic bandgap structures,
optical waveguides and splitters, 4-port directional
couplers and optical gratings.
2. OPTICAL CHARACTERIZATION
2.1 Absorption characteristics of SU-8
Absorption characteristics of SU-8 were studied
by carrying out ‘Ultraviolet-Visible (UV-Vis.)
Spectroscopy’ of SU-8 films using ‘Cintra 40 UV-
Visible Spectrometer’. A new formulation was
prepared by thoroughly mixing SU-8 2150 and SU-8
thinner (Microchem, USA) in an optimized ratio and
spin coated on a 2.5cm X 2.5cm clean glass substrate
to get 10µm SU-8 film. The sample was then soft
baked (SB) on a flat leveled hot plate at 650C for 3
minutes followed by 8 minutes at 950C and then
exposed to UV radiation for 45 seconds using
‘MJB-3 single-sided mask aligner’. After exposure,
the sample was post-exposure-baked (PEB) on the
hot plate at 950C for 9 minutes. During UV-Vis.
spectroscopy, the PEB SU-8 sample was exposed to
radiations varying from λ=190nm to 900nm.
The sample was then hard-baked (HB) on hot plate at
2000C for 30 minutes and again UV-Vis.
spectroscopy was carried out. Absorption spectra of
the PEB and HB samples are shown in figure-2. It
can be observed from the absorption spectra that
absorption starts increasing sharply below 350-
360nm. Absorbance peaks around 290nm for the
PEB film whereas for the HB film it peaks around
315nm. The HB film also shows a little higher
absorption than the PEB film. This red shift in the
absorption peak and enhanced absorption in case of
the HB film may be attributed to a structural
rearrangement most probably by the removal of
dangling bonds and/or formation of conjugation in
the structure because of relatively longer heating of
the film at a higher temperature. From both the
spectra, it can be observed that beyond 400nm,
absorption is very low and go on decreasing with
higher wavelengths. This indicates that PEB and HB
SU-8 films exhibit excellent optical transparency
above 400nm. This makes SU-8 an excellent polymer
material for a variety of optical applications. Optical
bandgap of 3.4eV for SU-8 was evaluated from the
absorption spectrum of the HB film.
200 300 400 500 600 700 800 9000
0.2
0.4
0.6
0.8
1
Wavelength in nm
Normalized Absorbance
Absorption spectrum of SU-8 films
PEB film
HB film
Figure-2: Absorption spectra of post-exposure baked
(PEB) and hard-baked (HB) SU-8 films.
2.2 Refractive index of SU-8
Refractive index (n) of SU-8 was evaluated by
carrying out spectroscopic ellipsometry of SU-8
films. 2cm X 2cm Silicon wafer pieces were piranha
cleaned and dehydration baked on hot plate at 2000C
for 10 minutes. 60nm and 1.2µm films were spin
coated on the Si substrate and soft baked (SB) on hot
plate at 650C for 1 minute followed by for 2 minutes
at 950C and exposed to UV radiation for 15 seconds
using MJB-3. After exposure, the samples were
baked (PEB) on hot plate at 650C for 1 minute and at
950C for 1 more minute. The samples were then
hard-baked (HB) on hot plate at 2000C for 30
minutes. After each stage of processing i.e. SB, PEB
and HB, the samples were put for spectroscopic
ellipsometry and refractive index ‘n’ as function of
wavelength was evaluated. It can be observed from
figure-3 that for the PEB SU-8 film, refractive index
monotonically decreases with increase in wavelength
and the thicker (1.2µm) SU-8 film has marginally
higher refractive index than the thinner (60nm) film.
Similar trend was observed for the SB and HB SU-8
films. Variation in the refractive index among the
SB, PEB and HB cases is much smaller for the thin
film than for the thick film (figure-4 & 5).
300 400 500 600 700 8001.55
1.6
1.65
1.7
1.75
1.8
1.85
1.9
Wavelength in nm
Refractive index
Plot of refractive index of thin and thick PEB SU-8 films
Thin (60 nm) film
Thick (1.25um) film
Figure-3: Refractive index (n) of PEB SU-8 film as
function of wavelength.
300 400 500 600 700 8001.55
1.6
1.65
1.7
1.75
1.8
1.85
1.9
Wavelength in nm
Refractive index
Plot of refractive for thick (1.2um) SU-8 film
SB film
PEB film
HB film
Figure-4: Refractive index (n) of 1.2µm thick SU-8
film as function of wavelength. Large variation in ‘n’
can be observed for different processing conditions.
300 400 500 600 700 8001.55
1.6
1.65
1.7
1.75
Wavelength in nm
Refractive index
Plot of refractive index for thin (60nm) SU-8 film
SB film
PEB film
HB film
Figure-5: Refractive index (n) of 60nm thin SU-8
film as function of wavelength. Variation in ‘n’ is
relatively small for different processing conditions.
Table-1: Values of refractive index ‘n’ at 365nm and
620nm for SB, PEB and HB SU-8 films
‘n’ /
Sample
365nm
(60nm
film)
365nm
(1.2µm
film)
620nm
(60nm
film)
620nm
(1.2µm
film)
SB 1.6410 1.6649 1.5776 1.5976
PEB 1.6396 1.7720 1.5755 1.6950
HB 1.6339 1.6387 1.5696 1.5726
Values of ‘n’ at 365nm and 620nm are presented in
table-1. It can be observed that at 365nm, ‘n’ varies
from 1.63 to 1.77 and at 620nm; it varies from 1.56
to 1.69 due to variations in film thickness and film
processing conditions. The extinction coefficient (k),
which is the imaginary part of the complex refractive
index and gives a measure of the fraction of light lost
due to scattering and absorption per unit distance in
the film, was found to be decreasing with wavelength
and very close to zero beyond 400nm (figure-6).
300 400 500 600 700 800-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
Wavelength in nm
Extinction coff.
Plot of extinction coff. of SU-8 films
PEB film
HB film
Figure-6: Plot of extinction coff. (k) of 1.2µm PEB
and HB SU-8 films as function of wavelength. ‘k’ is
negligibly small beyond 400nm.
3.FABRICATION OF OPTICALCOMPONENTS
3.1 Photonic band-gap structures
Photonic band-gap structures or photonic crystals
(PC) are periodic structures (1D, 2D or 3D) in which
periodic spatial modulation in the refractive index
leads to coherent scattering of light and alters the
modes of propagation of light of wavelengths
commensurate with length scale of periodicity. A
PBS is characterized by its band gap where light with
certain wavelength cannot propagate through it.
Figure-7: Photonic bandgap structures made of
circular SU-8 micropillars
2D-PCs of different shapes and dimensions were
fabricated with SU-8. 2µm of SU-8 film was spin
coated on a 2cm X 2cm silicon substrate. The sample
was soft baked on hot plate at 650C for 1 minute
followed by for 2 minutes at 950C. The photonic
bandgap structure pattern was directly written on the
sample by ‘Microtech LW 405 Laser Writer’. As this
Laser writer operates at 405nm where sensitivity of
SU-8 is low, 6 times writing of the pattern (at an
optimized setting of the Laser writer parameters) was
carried out to ensure proper cross-linking. The
sample was then post-exposure baked on hot plate at
650C for 1 minute followed by at 95
0C for 1 minute.
For thicker films longer baking times and more
number of Laser writings were required. Figure-7
shows the microscopic image of a 2D PC consisting
of an array of circular micropillars of height 2µm,
diameter 2µm and inter-pillar spacing of 5µm.
3.2 Optical waveguides, splitters and 4-port
directional coupler
Excellent optical transparency of SU-8 makes it
suitable for fabricating optical components like
optical waveguides, splitters and directional couplers.
When surrounded by suitable cladding medium,
SU-8 core can allow light to propagate through it by
total internal reflection.
Figure-8: Single mode SU-8 optical waveguides of
width 5µm, height 1µm and spacing 90µm.
(a) (b)
(c) (d)
Figure-9: (a) Symmetric Y-splitter, (b) asymmetric Y-
splitter, (c) T-splitter, (d) 4-port directional coupler.
1µm of SU-8 (SU-8 2002) was spin coated on an
oxidized Si substrate (0.9µm of SiO2 was thermally
grown by wet oxidation). The sample was soft baked
on hot plate and the patterns were directly written
(6 times) on the sample by the ‘LW-405 Laser
writer’. Figure-8 shows the microscopic images of
single mode optical waveguides of width 5µm and
thickness 1µm. As refractive index of SiO2 is around
1.45 whereas that of SU-8 is around 1.56 (at 620nm),
light in the SU-8 core is guided by the SiO2 layer
below and air (‘n’=1) at the other 3 sides acting as
the cladding layers. Figure-9 shows symmetrical and
asymmetrical Y-splitters, T-splitter and a 4-port
directional coupler fabricated with SU-8 by direct
laser writing following the above process flow.
3.3: Optical gratings
Grating is an optical component with a regular
pattern which splits light into several beams
travelling in different directions depending upon the
spacing of grating and the incident light so that it acts
as a dispersive element. Optical gratings of varying
periodicity were fabricated with SU-8 by electron
beam lithography (EBL). 1.5cm X 1.5cm glass
substrate was properly cleaned in chromic acid and
10nm of Au was sputter deposited on it to avoid
drifting during EBL. A new SU-8 formulation was
spin coated on the glass sample to get 150nm of
SU-8 film. The sample was then soft baked on hot
plate at 650C for 1 minute followed by at 95
0C for
1 minute. The grating patterns were directly written
on the SU-8 film by ‘Raith-e-Line’ electron beam
lithography machine with an optimized setting of the
EBL parameters. Figure-10 shows the scanning
electron microscopic image of the 150nm SU-8
grating with a spacing of 50nm.
Figure-10: SU-8 gratings of width, height 150nm and
spacing 50nm, fabricated on glass substrate by EBL.
4. CONCLUSION
Process parameters were optimized for the
fabrication of SU-8 films. From UV-Visible
spectroscopy of SU-8 films (10µm thick), it was
observed that optical absorption increases drastically
below 350-360nm and peaks around 290nm and
310nm for the PEB and the HB films respectively.
Beyond 400nm very low absorption was observed.
Thus SU-8 exhibits excellent optical transparency
above 400nm and hence becomes suitable for
fabrication of optical components. Spectroscopic
ellipsometry of SU-8 films indicated that irrespective
of the film processing conditions, thicker film
(1.2µm) exhibits higher refractive index than the
thinner film (60nm) and at any wavelength, the
variation in the refractive index under different
processing conditions (SB, PEB and HB) is less for
the thin film than for the thicker film. Further
research can be taken up to study the exact nature of
thickness dependency of refractive index of SU-8
films in the nano-scale. In the wavelength range of
365nm to 620nm, refractive index of SU-8 was found
to be varying from a minimum of 1.56 (for a 60nm
HB film at 620nm) to 1.77(for 1.25µm PEB film at
365nm). Extinction coefficient was found to be very
small (maximum 0.007 at 300nm) and decreases very
close to zero beyond 400nm. Processing sequences
were established and relevant process parameters
were optimized for the fabrication of a variety of
SU-8 based optical components like 2D photonic
bandgap structures, waveguides, symmetric and
asymmetric Y-splitters, T-splitters, 4-port directional
couplers and gratings by Laser writing and electron
beam lithography. Optical characterization of these
components is in the process. Further research can be
taken up to utilize these components in optical
sensors, MOEMS, OICs and nano-photonics.
ACKNOWLEDGEMENT
We thank MCIT for their support through Centre
of Excellence in Nanoelectronics. We also thank
Prof. S. A. Shivsankar, Prof. G. Hegde, Prof. V.
Venkatraman, Prof. M. Ashokan, and Tamilarson of
IISc. for their support and valuable suggestions at
crucial points of this work.
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