Lecture Notes on Photonics and Optoelectronics Vol. 1, No. 1, June 2013
Benzocyclobutene (BCB 4022-35) Single Mode
Rib Waveguides
Fikret. G. Aras and E. Öz Orhan
Gazi University, Institute of Science and Technology, Department of Advanced Technologies, Teknikokullar, Ankara,
Turkey
E-mail: goncaras@gmail.com, eliforhan@gazi.edu.tr
O. Salihoglu
Bilkent University, Department of Physics,Advanced Research Laboratory, Bilkent, Ankara, Turkey
Abstract—Design, fabrication and characterization of a
photodefinable benzocyclobutene (BCB 4022-35 is a product
of DowTM) single mode rib optical waveguide is presented
in this work. At first, the refractive index of polymer film is
measured by the method of prism coupling. We design the
single mode rib waveguide based on the geometrical
adjustment of rib width, total waveguide height using
conventional R-soft BPM and effective index methods.
Based on the design results, BCB waveguides were
fabricated by using photolithographic process with wet
etching, which is quite complicated and time consuming. In
this work, the insertion and propagation losses of waveguide
are measured by using the conventional cut back method.
The insertion losses of waveguide are 3.95 dB and 4.08 dB
for TE and TM modes respectively. The propagation losses
of waveguide are 1.09dB/cm and 1.20dB/cm for TE and TM
modes respectively. 
Index Terms—Benzocyclobutene
waveguide, single mode
I.
(BCB
4022-35),
cyclobutene). Benzocylobutene (BCB), a product of
DowTM [2] is a photodefinable polymer. Due to excellent
planarization, low dielectric constant, low moisture
uptake, low optical loss, high chemical resistance and
high glass-transition temperature, BCB is then often used
for optical waveguides in recent years [3].
It is well known that benzocyclobutene (BCB) has
been widely used for flat panel display, interlayer
dielectrics and microelectronics packing applications.
Recently, it is also used for making waveguides [4]-[7]
because BCB has a high glass-transition temperature of
over 350 C and high optical transparency over the wide
wavelength range from ultraviolet to infrared [8].
Moreover, BCB on silicon substrate is compatible with
the standard processing techniques of integrated circuits
[9]. Ibrahim et al. [10] discussed the characterization
process of benzocylobutene (BCB 4022-40) and its
realization into optical devices.
In this paper, we will report on fabrication and
characterization process of optical waveguides using
organic photodefinablebenzocyclobutene (BCB 4022-35)
polymer from DowTM [1].
rib
INTRODUCTION
Integrated optics are having an increasing impact on
the development of lightwave communication systems
with applications such as high speed broadband switching
and high speed interconnects for local area network.
Undoubtedly, the major performances of integrated optics
rely on the waveguiding component. Extensive interest
has been shown in the development and application of
polymers for various optically active and passive
waveguide devices because of potential of these materials
for low cost and high performance commercial products
[1]. Polymers are expected to play an important role in
the realization of integrated optical devices for
applications in the fields of optical communications,
optical data processing, electro-optic and thermo optic
switching devices, directional couplers, nonlinear optics
etc. Classes of polymers used in integrated optics include
acrylates, polyimides, polycarbonates, and olefin (e.g.,
II.
We have designed the single mode rib waveguide
based on the geometrical adjustment rib of width, total
waveguide height to the single mode condition for rib
waveguide with large cross section obtained by effective
index method proposed by Soref et al. [11]. Based on the
previous theory Petermann [12], Soref et al. [11]
proposed condition for the single-mode propagation in rib
waveguides with large cross section taking into account
cutoff values from numerical solutions. This formula (1)
has been frequently used later.
(1)
Here,
Manuscript received November 29, 2012; revised January 6, 2013;
accepted January 17, 2013.
©2013 Engineering and Technology Publishing
doi: 10.12720/lnpo.1.1.14-17
DESIGN OF WAVEGUIDE
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Lecture Notes on Photonics and Optoelectronics Vol. 1, No. 1, June 2013
= 1 for TE modes
Figure 2. Mode profile of fundamental mode and computed mode
spectrum.
for TM modes
nf, ns and nc are refractive indices of the guiding
region, the substrate and air respectively. k=2π/ ,  is
the wavelength. The constant c is given as 0.3 which was
found from an approximation to a BPM numerical
solution. However, it has been recently stated that c
values of 0 or -0.05 give better single mode condition for
rib waveguide design purposes. Plot of theoretical curve
for single mode limit and schematic of a rib waveguide
are given in Fig. 1.
III.
WAVEGUIDE FABRICATION
We have designed fabricated and characterized BCB
single mode rib waveguide in this work. A schematic
diagram of waveguide structure is shown Fig. 3. For
convenience, a silicon wafer (100) is used as the substrate.
BCB was chosen specifically for the core. After a
conventional semiconductor cleaning, a 4 m SiO2 was
grown on substrate to form a lower cladding layer by
using PECVD. This was followed by the photolitography
step where a mask aligner having I-line UV exposure at
365 nm wavelenght was used to pattern.The photoresist
mask was patterned by the buffered oxide etching. After
the wet etching step, a BCB was coated at 5000 rpm and
cured at 250 C for 1 h in nitrogen environmental.The
refractive index of cured BCB film was then measured by
prism coupling method.
Figure 1. Single mode condition for rib waveguide ( for H = 3.5 µm ).
As can be seen in Fig.1, the zone under the curve
shows the experimentally verified single mode (SM)
propagation region. In the other zone multimode (MM)
waveguiding condition is satisfied. This is a stronger
criterion for the single mode propagation since it allows
waveguide designers to be certain that they would have
single mode waveguides. According to single mode
condition, waveguide simulation was carried out by
usingR-soft BPM which based on Finite Difference
method in this work. Fig. 2 shows computed mode
spectrum and the mode profile of fundamental mode.
©2013 Engineering and Technology Publishing
Figure 3. Schematic diagram of waveguide structure (left) and optical
micrograph and imaging of SEM (right).
Table 1 shows the refractive indices of BCB films for
TE polarization. Finally waveguide was cleaved at the
facets for optical coupling.
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Lecture Notes on Photonics and Optoelectronics Vol. 1, No. 1, June 2013
TABLE I.
REFRACTIVE INDICES OF BCB FILMS FOR TEPOLARIZATION
The surface roughness of a waveguide device is
important as it can become the determinant for optical
applications, particularly because scattering loss is more
sensitive to the surface roughness in case of evanescent
modes [13-15]. To survey the surface morphology, the
root mean square (RMS) surface roughness was measured
by atomic force microscopy (AFM). The surface
morphologies of coated polymer (spin coated and cured)
are shown Fig. 6.
BCB (4022-35)
Wavelenght
(nm)
Cure
temperature
(C)
Spin
speed
(rpm)
Refractive
index
632,8
250
5000
1,5845
1553,0
250
5000
1,5606
632,8
300
2500
1,5680
1553,0
300
2500
1,5471
IV.
CHARACTERIZATION
In order to characterize for a waveguide loss a
conventional cutback method has been adopted in the loss
measurement. A single mode fiber is used to couple
1550,0 nm laser source in to polished and facet of the
polymer waveguide. The output is measured using the
Germanium (Ge) photodetector and the near field profile
is imaged onto an infrared camera integrated with beam
analyzer software. Fig. 4 represents a schematic diagram
regarding the optical injection set-up and the
measurement of optical losses.
Figure 6. A surface morphology of a coated of BCB polymer which
was spun at 5000 rpm an cured 250C.
The surface roughness of coated polymer was RMS =
0.322 nm. The result is given in Fig. 6 which gives a
surface roughness in root mean square (RMS) of 0.322
nm, which indicate good coating quality.
V.
RESULT AND DISCUSSION
Optical waveguides based on a photodefinable BCB
(4022-35) polymer have been demonstrated utilizing
silicon as a substrate and thin lower cladding of SiO2 with
4 m thickness. Polymer characterization using prism
coupling technique provides useful ideas on its abilities in
optoelectronic applications. The photolithography and
wet etching technique were utilizing throughout the
fabrication process. For single mode application, a core
dimension of 3.5 m was fabricated, which is further
characterized for loss using the cutback method. The
propagation losses of the polymer single mode waveguide
for TE and TM modes are 1.09 dB/cm and 1.20 dB/cm
respectively. The insertion losses of waveguide for TE
and TM modes are 3.95 dB and 4.08 dB respectively.
Figure 4. Optical waveguide devices measurement setup.
The results of loss measurement of BCB optical
waveguides for TE and TM modes are shown in Fig. 5.
The slope of the line indicates that the propagation loss in
the waveguides is 1.09 dB/cm and 1.20 dB/cm
respectively. The insertion losses for TE and TM modes
are 3.95 dB and 4.08 dB.
ACKNOWLEDGMENT
This work was supported by Gazi University Research
Fund under Project No 18/2010-02. The authors would
like to thank Professor A. Aydınlı who is director of
Advanced Research Laboratory in Bilkent University for
help in waveguide fabrication and for numerous useful
discussions. The authors are grateful to John H. Jackson
from Metricon Corp. Pennington NJ, USA for prismcoupling measurements.
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Figure 5. Measurement output power for several waveguide lengths
and image of waveguideoutput for 1550,0 nm laser coupling and single
mode profile.
©2013 Engineering and Technology Publishing
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Dr. Elif Orhan is an Associate Professor of
department of Advanced Technologies from
Graduate School of Natural and Applied
Sciences at Gazi University in Turkey. She has
served as a Chairman of the department from
2008 to 2013. In 1993, she received her B.S.
degree in Physics Education from the Atatürk
University and then received her M.Sc. degree
in department of Physics in 1996. In 2000, she
received her Ph.D. degree in the field of X-Ray Fluorescence (XRF)
Spectroscopy from the Atatürk University of Erzurum, Turkey. She was
elected as a reviewer of the Turkish Scientific and Technical Research
Council and in 2012 and she was a reviewer of journal of X-Ray
Spectroscopy in 2001-2003 and journal of Radiation Physics and
Chemistry in 2002. She was the recipient of International Scientific
Publication Incentive Awards, from Turkish Scientific and Technical
Research Council, Atatürk University and Gazi University. She was a
member of Group of Specialist from 2008 to 2012 at Gazi University
and she is a member of Committee of Specialist at Gazi University. She
is a member of the Turkish Physics Academy. Dr. Orhan has focused
her research efforts on Coster-Kronig yields and chemical effects in the
field of X-Ray Fluorescence (XRF) Spectroscopy and on optical
waveguides and micro disk/ring add- drop multiplexers in the field of
Integrated Optics. She has 26 research papers and cited 155 according to
Web of Science. Dr. Orhan is the technical editor in 2013 the 2nd
International Conference on Electronics and Opto-electronics Science.
She is now Associate Professor at Gazi University of Advanced
Technologies, Ankara.
Dr. Omer Salihoglu received his B.S. in
Physics from middle east technical university
(METU), Ankara, Turkey in 2001 and Ph.D. in
Physics from Temple University, Philadelphia,
PA in 2009. His areas of research interest
include superlattice infrared photodetector
technologies, ultrafast crystallization of
semiconductors and graphene based plasmonics.
He is currently postdoc at Bilkent University in
Ankara Turkey.
©2013 Engineering and Technology Publishing
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