HAL Id: hal-01061469
https://hal.archives-ouvertes.fr/hal-01061469
Submitted on 6 Sep 2014
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entic research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diusion de documents
scientiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
Fully etched apodized grating coupler on the SOI
platform with –0.58 dB coupling eciency
Yunhong Ding, Christophe Peucheret, Haiyan Ou, Kresten Yvind
To cite this version:
Yunhong Ding, Christophe Peucheret, Haiyan Ou, Kresten Yvind. Fully etched apodized grating
coupler on the SOI platform with –0.58 dB coupling eciency. Optics Letters, Optical Society of
America - OSA Publishing, 2014, 39 (18), pp.5348-5350. �10.1364/OL.39.005348�. �hal-01061469�
Fully etched apodized grati ng coupler on the SOI
platform with −0.58 dB coupling efficiency
Yunhong Ding,
1,
* Christophe Peucheret,
2
Haiyan Ou,
1
and Kresten Yvind
1
1
Technical University of Denmark, Department of Photonics Engineering, Kongens Lyngby, Denmark
2
University of Rennes, FOTON Laboratory, Lannion 22300, France
*Corresponding author: yudin@fotonik.dtu.dk
Received July 3, 2014; revised August 1, 2014; accepted August 6, 2014;
posted August 7, 2014 (Doc. ID 215256); published September 9, 2014
We design and fabricate an ultrahigh coupling efficiency (CE) fully etched apodized grating coupler on the silicon-
on-insulator (SOI) platform using subwavelength photonic crystals and bonded aluminum mirror. Fabrication error
sensitivity and coupling angle dependence are experimentally investigated. A record ultrahigh CE of −0.58 dB with a
3 dB bandwidth of 71 nm and low back reflection are demonstrated. © 2014 Optical Society of America
OCIS codes: (130.0130) Integrated optics; (230.3120) Integrated optics devices; (230.1950) Diffraction gratings;
(050.2770) Gratings.
http://dx.doi.org/10.1364/OL.39.005348
Grating couplers are attractive thanks to their ability to
directly couple light from nanowire waveguides to stan-
dard single-mode fibers (SSMFs). The biggest advantage
of grating couplers is the large alignment tolerance and
absence of need for chip cleaving, making wafer-scale
testing possible. Traditional grating couplers are uniform
and are shallowly etched in order to introduce a proper
scattering strength [
1–8]. However, their coupling effi-
ciency (CE) is sensitive to both the etching depth and slot
width [
1]. Moreover, the CE of such uniform shallowly
etched grating couplers is limited not only by power leak-
age to the substrate, but also by the intrinsic mode mis-
match between the gratings and SSMFs. For applications
where only fully etched silicon waveguides are present
on a photonic-integrated circuit, such as multimod e mul-
tiplexers [
9] or multicore fiber couplers [10], fully etched
grating couplers are preferred [
11–16] in order to simplify
the fabrication process.
Table
1 summarizes the performances of state-of-the-
art fully etched and shallowly etched grating couplers
that have been demonstrated over the past few years.
The highest CE of −0.62 dB was demonstrated for a
shallowly etched grating coupler with an aluminum (Al)
mirror, which was realized by etching through the silicon
substrate followed by Al deposition [
8]. However, the op-
timum thickness of the lower cladding (i.e. buried oxide
(BOX) layer), which is critical for maximizing the CE,
might not correspond to that of commercial silicon-on-
insulator (SOI) wafers.
In this Letter, we demonstrate an ultrahigh CE fully
etched apodized grating coupler on the silicon-on-
insulator (SOI) platform using subwavelength photonic
crystals (PhCs), an Al mirror, and adhesive bonding to
a silicon wafer. With optimum upper and lower SiO
2
clad-
ding thicknesses, −0.58 dB CE with a wide 3 dB band-
width of 71 nm are achieved. Such CE is, to the best
of our knowledge, the highest ever reported for grating
couplers.
The proposed grating coupler is based on flip-chip
bonding of a silica-clad fully etched silicon PhC grating
coupler on a silicon carrier wafer, as schematically de-
picted in Fig.
1. The thickness of the top silicon-device
layer is 250 nm. Artificial materials are introduced for
the scattering units, with refractive indices n
i
and lengths
of scattering units l
i
changed along the grating [11]. SiO
2
is used as upper and lower cladding material with thick-
nesses of h
u
and h
d
, respectively. A 100 nm Al mirror is
introduced below the lower cladding. Another layer of
SiO
2
is introduced beneath the Al mirror and is bonded
to the silicon carrier wafer using a benzocyclobutene
(BCB) layer. The coupling angle θ is designed to be 15°.
In the design, the width d
i
of the artificial material slots is
fixed to be d
0
345 nm.
The coupling wavelength λ of the grating unit is
given by
λ l
i
n
eff;i
l
i
;n
i
− n
up
sin θ; (1)
where n
up
is the refractive index of the uppermost clad-
ding (air). n
eff;i
is the effective refractive index of the
Table 1. Summary of Published Experimental Results
for Grating Couplers
Fully Etched Shallowly Etched
CE (dB)
3dB
BW (nm) Ref.
CE
(dB)
3dB
BW (nm) Ref.
−3.76 68 [
12] −1.6 65 [2]
−3.7 60 [
13] −1.9 70 [3]
−4.6 83 [
14] −1.2 48 [4]
−2.3 60 [
15] −1.6 80 [5]
−1.65 60 [
11] −1.5 54 [6]
−4.7 >140 [
16] −2.7 60 [7]
−0.58 71 This work −0.62 67 [
8]
Fig. 1. Schematic structure of the proposed grating coupler.
5348 OPTICS LETTERS / Vol. 39, No. 18 / September 15, 2014
0146-9592/14/185348-03$15.00/0 © 2014 Optical Society of America
scattering unit, which is determined by l
i
and n
i
. The light
scatters with a power leakage factor 2α
i
when it propa-
gates through each scattering unit [
11]. By jointly opti-
mizing n
i
and l
i
, one can tune 2α
i
while maintaining
the scattering angle of 15° at 1550 nm. In order to obtain
a Gaussian output profile Gz with beam diameter of
10.4 μm, corresponding to that of an SSMF, the power-
leakage factor distribution 2αz should satisfy [
6,11]:
2αzG
2
z∕
1 −
Z
z
0
G
2
zdz
: (2)
Figure
2(a) shows the designed distributions of n
i
and l
i
of the grating coupler. PhCs with a triangular lattice with
lattice constant of 2d
0
∕3 can be used for the artificial
material slots [
11,12], and the hole size D
hole
can be deter-
mined by the effective index approximation [
11]. Accord-
ing to the effective index approximation, the optimized
hole size of the designed grating coupler has a feature size
D
hole;min
of 70 nm. Considering that such a dimension is
beyond the capability of some fabricatio n methods such
as conventional deep ultraviolet (DUV) lithography, alter-
native optimum designs restricting D
hole;min
to 100 nm and
150 nm are also shown in Fig.
2(a). The CE of the trans-
verse electric (TE) mode is then investigated by a two-
dimensional (2D) eigenmode-expansion method (EME)
as a function of h
d
with h
u
set to 1000 nm, as shown in
Fig. 2(b). The CE depends periodically on h
d
, and reaches
a local maximum at h
d
1600 nm. With h
d
1600 nm,
the CE is further calculated by changing h
u
. One can find
that the CE is moderately influenced by the value of h
u
,
and can reach its maximum when h
u
1000 nm. With
h
d
1600 nm and h
u
1000 nm, the CE is then calcu-
lated as a function of wavelength for the original design
with D
hole;min
70 nm, as well as for the designs with re-
stricted D
hole;min
of 100 nm and 150 nm, as shown in Fig. 3.
A highest CE of −0.43 dB (corresponding to 91%) is
predicted for the original design at 1560 nm, with a
3 dB bandwidth of 76 nm. In addition, the 100 nm feature
size design shows negligible CE degradation and the
150 nm design exhibits only 0.4 dB CE degradation, indi-
cating that our design is compatible with most fabrication
methods. Considering that the scattering cell length l
i
changes along the grating coupler, which is different from
the case of uniform grating couplers, it is important to in-
vestigate the coupling-angle dependence. As shown in
Fig.
3, a 2° coupling-angle change results in an 18 nm peak
coupling wavelength shift, which is slightly smaller than
previous demonstrations [
8]. In addition, the peak CE
does not degrade as the coupling angle is changed.
In order to validate our design, the device was fabri-
cated on a commercial SOI sample with top silicon thick-
ness of 250 nm and BOX layer thickness of 3 μm. A single
step of standard SOI processing, including e-beam lithog-
raphy and inductively coupled plasma (ICP) etching, was
first used to fabricate the grating coupler and silicon
nanowire waveguide simultaneously. An 800 nm thick
layer of SiO
2
was then deposited on top of the grating
coupler. Considering the surface is not flat after SiO
2
deposition, another 800 nm borophosphosilicate glass
(BPSG) was deposited and annealed at 950°C for 30
minutes in nitrogen, giving a planarity across the grating
Fig. 2. (a) Designed l
i
and n
i
distributions of the grating cou-
plers, with D
hole;min
of 70, 100, and 150 nm. Simulated CE as a
function of (b) lower cladding thickness with h
u
1000 nm
and (c) upper cladding thickness with h
d
1600 nm.
Fig. 3. Simulated CE as a function of wavelength for designed
grating couplers with different feature sizes.
Fig. 4. (a) Scanning electron microscopy (SEM) and (b) opti-
cal microscopy images of the fabricated grating coupler.
(c) Measured CE for the fabricated coupler with Al mirror with
and without 8 nm hole size change as well as for the same gra-
ting coupler fabricated on the same type of SOI wafer without
Al mirror.
September 15, 2014 / Vol. 39, No. 18 / OPTICS LETTERS 5349
region better than 100 nm. Afterward, 100 nm Al was de-
posited on top of the BPSG, followed by another 1 μm
SiO
2
deposition. Then, about 2 μm BCB was spun on both
the sample and silicon-carrier wafer. The sample was
then flip-bonded on the silicon carrier wafer and ther-
mally cured in an oven. The substrate of the chip was
then removed by ICP fast etching stopping on the BOX
layer. Finally, the BOX layer was thinned to 1 μm by buf-
fered hydrofluoric acid (BHF) etching.
Figures
4(a) and 4(b) show details of the fabricated de-
vice. In order to test the CE, two identical grating couplers
were fabricated, with a 700 μm long single-mode straight
waveguide introduced in between. The waveguide width
was tapered from 12 μm for the grating couplers to 450 nm
for the single-mode waveguide with 500 μm tapering
length. The CE was obtained by η
0
− η
s
∕2, where η
0
is
the grating-to-grating transmission and η
s
is the loss of
the single-mode silicon waveguide, with propagation loss
of 2 dB∕cm measured by the cut-back method. Figure
4(c)
shows the measured CE as a function of wavelength for
the designed grating coupler with the bonded Al mirror.
The CE for the same grating coupler fabricated on the
same type of SOI wafer but without Al mirror is also
shown. A significant improvement provided by the
bonded mirror is confirmed. The tolerance to fabrication
error was investigated by changing the size of all the holes.
A diameter change of the holes dD
hole
of 8 nm resulted in a
peak coupling wavelength shift of only 23 nm without
peak CE degradation. Such coupling wavelength shift
could be compensated by adjusting the coupling angle.
The slight peak coupling wavelength deviation compared
to the simulations is believed to be due to fabrication er-
ror, e.g., the hole size error and top silicon thickness error
of the SOI sample.
The CE of the fabricated grating coupler was further
characterized by changing the coupling angle, as
shown in Fig.
5. One can find that a 2° coupling angle
change results in an 18 nm peak coupling wavelen gth
shift, which agrees well with the simulation. In addition,
the highest CE of only −0.58 dB with 3 dB bandwidth
of 71 nm was obtained at a coupling angle of 13°. The
back reflection of the grating coupler was extracted
from the contrast of the Fabry–Perot (FP) fringes of the
fiber-to-fiber transmission spectrum, and is lower than
1.4% at the peak coupling wavelength, which is com pa-
rable to previous demonstrations [
12–16].
In summary, we have designed and demonstrated a
fully etched fiber-to-chip grating coupler using subwave-
length PhCs and bonded Al mirror. A record-high CE of
−0.58 dB with 3 dB bandwidth of 71 nm and low back
reflection were demonstrated. In the present work, the
use of BCB is not compatible with conventional CMOS
processes, and it also may influence the thermal perfor-
mance. However, thermocompression bonding using alu-
minum [
17], which is a CMOS-compatible process, can be
used to solve the heat-sinking problem as well as to sim-
plify the fabrication process. In addition, the Al mirror
could be introduced during the fabrication of the SOI wa-
fers, which would also simplify the fabrication process.
This work is supported by the Danish Council for
Independent Research (DFF-1337-00152 and DFF-1335-
00771).
References
1. D. Taillaert, W. Bogaerts, P. Bienstman, T. F. Krauss, P.
Van Daele, I. Moerman, S. Verstuyft, K. De Mesel, and R.
Baets, IEEE J. Quantum Electron. 38, 949 (2002).
2. F. Van Laere, G. Roelkens, M. Ayre, J. Schrauwen, D.
Taillaert, D. Van Thourhout, T. F. Krauss, and R. Baets,
J. Lightwave Technol. 25, 151 (2007).
3. Y. Tang, Z. Wang, L. Wosinski, U. Westergren, and S. He,
Opt. Lett. 35, 1290 (2010).
4. X. Chen, C. Li, C. K. Y. Fung, S. M. G. Lo, and H. K. Tsang,
IEEE Photon. Technol. Lett. 22, 1156 (2010).
5. D. Vermeulen, S. Selvaraja, P. Verheyen, G. Lepage, W.
Bogaerts, P. Absil, D. Van Thourhout, and G. Roelkens,
Opt. Express 18, 18278 (2010).
6. C. Li, H. Zhang, M. Yu, and G. Q. Lo, Opt. Express 21, 7868
(2013).
7. L. He, Y. Liu, C. Galland, A. E.-J. Lim, G.-Q. Lo, T. Baehr-
Jones, and M. Hochberg, IEEE Photon. Technol. Lett. 25,
1358 (2013).
8. W. S. Zaoui, A. Kunze, W. Vogel, M. Berroth, J. Butschke, F.
Letzkus, and J. Burghartz, Opt. Express 22, 1277 (2014).
9. Y. Ding, H. Ou, J. Xu, and C. Peucheret, IEEE Photon. Tech-
nol. Lett. 25, 648 (2013).
10. Y. Ding, F. Ye, C. Peucheret, H. Ou, Y. Miyamoto, and
T. Morioka, in European Conference and Exhibition on
Optical Communication (ECOC, 2014), paper We.1.1.3.
11. Y. Ding, H. Ou, and C. Peucheret, Opt. Lett. 38, 2732 (2013).
12. L. Liu, M. Pu, K. Yvind, and J. M. Hvam, Appl. Phys. Lett. 96,
051126 (2010).
13. R. Halir, P. Cheben, J. Schmid, R. Ma, D. Bedard, S. Janz, D.
Xu, A. Densmore, J. Lapointe, and Í. Molina-Fernández, Opt.
Lett. 35, 3243 (2010).
14. M. Antelius, K. B. Gylfason, and H. Sohlström, Opt. Express
19, 3592 (2011).
15. X. Xu, H. Subbaraman, J. Covey, D. Kwong, A. Hosseini, and
R. T. Chen, Appl. Phys. Lett. 101, 031109 (2012).
16. Q. Zhong, V. Veerasubramanian, Y. Wang, W. Shi, D. Patel,
S. Ghosh, A. Samani, L. Chrostowski, R. Bojko, and D. V.
Plant, Opt. Express 22, 18224 (2014).
17. H. Lin, J. T. M. Stevenson, A. M. Gundlach, C. C. Dunare,
and A. J. Walton, Microelectron. Eng. 85, 1059 (2008).
Fig. 5. Measured CE as a function of wavelength for different
coupling angles.
5350 OPTICS LETTERS / Vol. 39, No. 18 / September 15, 2014
