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Optics Communications 245 (2005) 137–144
www.elsevier.com/locate/optcom
Fabrication and analysis of 2 · 2 thermo-optic SOIwaveguide switch with low power consumption and
fast response by anisotropy chemical etching
Jingwei Liu *, Jinzhong Yu, Shaowu Chen, Jinsong Xia
State Key Laboratory on Integrated Optoelectronics, Chinese Academy of Sciences, Institute of Semiconductors, Hai-Dian,
P.O. Box 912, Beijing 100083, PR China
Received 2 May 2004; received in revised form 21 September 2004; accepted 4 October 2004
Abstract
A low power consumption 2 · 2 thermo-optic switch with fast response was fabricated on silicon-on-insulator by
anisotropy chemical etching. Blocking trenches were etched on both sides of the phase-shifting arms to shorten device
length and reduce power consumption. Thin top cladding layer was grown to reduce power consumption and switching
time. The device showed good characteristics, including a low switching power of 145 mW and a fast switching speed of
8 ± 1 ls, respectively. Two-dimensional finite element method was applied to simulate temperature field in the phase-
shifting arm instead of conventional one-dimensional method. According to the simulated result, a new two-
dimensional index distribution of phase-shifting arm was determined. Consequently finite-difference beam propagation
method was employed to simulate the light propagation in the switch, and calculate the power consumption as well as
the switching speed. The experimental results were in good agreement with the theoretical estimations.
� 2004 Elsevier B.V. All rights reserved.
PACS: 42.82.Ds; 4282.Et; 42.82.�mKeywords: Thermo-optic switch; Silicon-on-insulator; Anisotropy chemical etching; Finite-element method; Finite-difference beam
propagation; Switching time; Power consumption
0030-4018/$ - see front matter � 2004 Elsevier B.V. All rights reserv
doi:10.1016/j.optcom.2004.10.004
* Corresponding author. Tel.: +861082304511; fax:
+861082305052.
E-mail address: [email protected] (J. Liu).
1. Introduction
Optical switch is one of the key devices for opti-
cal communication systems. It plays an important
role in the applications including optical cross con-
nection (OXC) and optical add-drop multiplexing
ed.
Fig. 1. Schematic structure of the 2 · 2 TO MZ-MMI switch.
138 J. Liu et al. / Optics Communications 245 (2005) 137–144
(OADM) systems [1]. Planar waveguides technolo-
gies, based on silica-on-silicon [2–4] and silicon-
on-insulator, are very popularly employed for
optical switching application, and some switches
with small port count have been fabricated [5–8].A 2 · 2 MZ optical waveguide switch often
consists of two paired couplers (i.e. multi-mode-
interference (MMI) couplers or directional inter-
ference couplers) joined by two phase-shifting
arms. The couplers act as 3 dB splitter and comb-
iner. With silicon, there are two possible choices
for phase-shifting: thermo-optical (TO) effect and
free carriers (FC) dispersion effect. The FC effectrequires high free carrier injection to reduce index
significantly [9]. Unfortunately, such high current
density contributes to rising of temperature that
will increase the refractive index. Therefore, the
effect of carrier injection will be weakened seri-
ously. Additionally, the free carriers inevitably
induce considerable absorption loss. Therefore, a
good choice for phase-shifting is TO effect sincesilicon has high thermo-optical coefficient and
thermal conductivity.
Fischer et al. [6] have fabricated 2 · 2 thermo-
optical switch, which showed an 85 mW switching
power, but no results about the switching speed
were reported. In 2003, House et al. [5] realized
high speed 2 · 2 thermo-optical switch with a
10-ls switching time, but the device showed a highswitching power of 440 mW. In order to get
smoother interface, we fabricated 2 · 2 thermo-
optic switch recently by anisotropy chemical
etching in stead of conventional dry-etching [8].
However, in the reported device, switching speed
is rather low (about 60 ls) [8].Furthermore, it is important to picture temper-
ature field in the phase-shifting arm accurately. Aone-dimensional (1D) method can be used to esti-
mate the temperature simply [10]. But the SOI
waveguide often has a large area rib-shape
cross-section in order to achieve a high coupling
efficiency. On the other hand, the waveguide will
have a trapezoidal cross-section instead of a regu-
lar rectangular one if the wet etching method is
adopted. Then, the 1D method is not suitablefor describing the temperature field and determin-
ing switching speed as well as power consumption
accurately.
In this paper, a 2 · 2 thermo-optic switch with
MMI couplers was fabricated on SOI wafer. A
bonding and back-etching SOI (BE-SOI) wafer
with a 5-lm top silicon layer and a 1-lm buried sil-
icon oxide was used here. Anisotropy chemicaletching was used similar with [8]. Blocking
trenches were etched near the phase-shifting arms
to shorten the device length and reduce the power
consumption. A top cladding layer is thin enough
to reduce power consumption and switching time.
The device achieved good characteristics including
the low switching power of 145 mW and the fast
switching speed measured to be about 8 ± 1 ls.These values are much better than our former
results [8].
Two-dimensional finite element method (2D-
FEM) has been chosen for temperature field simu-
lation to overcome the drawbacks of the previous
1D method. According to this temperature field, a
new 2D index distribution of the phase-shifting
arm was obtained, and then, finite-difference beampropagation method (FD-BPM) was used to simu-
late its light propagation. Meanwhile, the power
consumption and the switching speed were
determined.
2. Design
Fig. 1 shows a structure of the proposed 2 · 2
TO-MZ switch with MMI splitter and combiner.
It consists of four input/output ports with S-bend
connectors, two MMIs, two straight phase-shifting
arms, two heaters, and some blocking trenches.
Considering single-mode condition for SOI rib
waveguide with trapezoidal cross-section [11], the
J. Liu et al. / Optics Communications 245 (2005) 137–144 139
width of single-mode waveguides is 5 lm, and the
etching depth is designed to be 1.5 lm. The MZ
arms and the heaters are 4500 and 3380 lm long,
respectively. S-bends are used to enlarge the dis-
tance of neighbor ports to 127 lm. Radius of theS-bend is set at 50,000 lm to ensure low bend radi-
ation loss. In general, self-images are bigger than
the original fields in MMIs. In order to reduce en-
ergy loss and obtain large fabrication tolerance,
tapers are used at the end of the MMIs to connect
the following single-mode waveguides.
The MMIs are connected by two straight phase-
shifting arms instead of conventional S-bends toshorten the device length. To avoid mode cou-
pling, 5 lm wide blocking trenches are introduced
on both sides of each arm, as shown in Fig. 1. An-
other important advantage of using trenches is to
isolate heat transfer between the two arms. Such
transfer will reduce switching power greatly, which
was proved by our results of simulations and
experiments.MMI coupler is based on the self-imaging effect
of multi-mode waveguide [12]. There are three
kinds of interference in MMI, which are general,
paired, and symmetric interference. In this paper,
the general interference is utilized.
3. Analysis and simulation
A cross-section of the phase-shifting arm is
shown in Fig. 2. The arm is structured as follows:
A buried silicon oxide layer was grown by thermal
Fig. 2. Cross-section of the phase-shifting arm.
oxidation on a silicon substrate. Subsequently, a
silicon layer was bonded on the buried oxide layer.
The waveguides and the MMIs were formed on
this bonded silicon layer. Dry thermal oxidation
was applied to form a 0.12-lm top cladding layer.As heaters, aluminum film with a thickness of 0.5
lm were evaporated and patterned. For getting
smoother interface, the waveguides aligned to
Æ110æ-crystal direction were etched by KOH. Side-
wall angle theta of the trapezoidal cross-section is
54.74�, which arises from chemical etching [13].
Both steady and transient temperature fields in
the heated arm are analyzed by 2D-FEM basedon the cross-section shown in Fig. 2. The 2D mod-
el is sufficient for simulation because the arm�slength is much greater than its width. What we
cared is not absolute temperature, but temperature
difference. Such difference is almost independent
from environment temperature. Therefore, it is
reasonable to assume the boundaries of the section
are Direchlet ones and the environment tempera-ture is 0 K. With such assumptions, the calculated
temperature is just equal to the temperature differ-
ence that is requisite for the simulation. Table 1
summarizes data of material used for calculation.
When an electrical current is applied to one of
the heaters, the optical phase change in the heated
arm can be expressed as
DU ¼ onoT
LDT2pk; ð1Þ
where (on/oT) (= 1.86·10�4 K�1) is the thermo-op-
tic coefficient of silicon. DT is the temperature
change over the heater length L. The temperature
difference which is needed to induce a p shift in thearm can be given by
DT ¼ k2Lon=oT
: ð2Þ
Table 1
Material properties
Material Density
(kg/m3)
Specific heat
(J/gK)
Thermal conductivity
(W/mK)
Si 2330 752 140
SiO2 2210 89 1.38
Al 2700 864 240
Air 1.29 1007 0.026
Fig. 4. The cross-output versus time.
Fig. 5. Distribution of index difference in the heating arm.
140 J. Liu et al. / Optics Communications 245 (2005) 137–144
The cross-output intensity I of the switch as a
function of temperature difference can be ex-
pressed simply as [4]
I ¼ 1� sin2 p � on=oT � DT � Lh
k
� �: ð3Þ
According to the formulas and structure
parameters, the temperature difference of 1.2 K isrequired to realize a switching operation from a
cross state to a bar state. The steady temperature
distribution at a heating power of 140 mW is illus-
trated in Fig. 3. It demonstrates that the tempera-
ture difference of cross-section center area is
around 1.2 K and the trenches isolate the thermal
diffusion effectively. A transient temperature field
was also analyzed to determine the center temper-ature as a function of time. Then according to (3),
we could curve the response of the optical intensity
against time in a heating-and-cooling period as
shown in Fig. 4. Considering that the upper and
lower traces correspond to the heating and cooling
steps respectively. Therefore, the ‘‘fall’’ time and
‘‘rise’’ time of optical intensity denote the rise time
and fall time of the switch. In Fig. 4, the two val-ues are estimated to be about 8 and 7 ls,respectively.
Then FD-BPM is employed to simulate the
light propagation in the arms and the MMIs. Un-
like conventional process, index distribution of
phase-shift arm is no longer uniform. A 2D distri-
bution of the index change shown in Fig. 5 is pic-
tured according to the temperature distribution
Fig. 3. Temperature distribution in the phase-shifting arm
when steady-state thermal analysis is applied.
shown as Fig. 3. Simulation results of BPM are
shown in Fig. 6(a)–(d). From these results, we
can determine the optimal length of the MMIs.
The top width of the MMIs is 30 lm. The freespace wavelength is 1550 nm. Fig. 6(a) and (b)
show that the optimal length of the first MMI is
5220 lm for two images. Fig. 6(c) shows one good
image at the end of the second MMI with the same
length of 5220 lm when the switch works at cross
state. When one of the arms is heated at heat
power of 140 mW, the switch will work at bar
state, and as shown in Fig. 6(d), the image appearsat the opposite side, which proves the validity of
the thermal analysis. Small peaks lie in the middle
area of the waveguide (see Fig. 6(b)–(d)), which
will induce additional energy loss and degrade
the extinction ratio. This problem can be partly
Fig. 6. Simulated self-imaging in the MMI: (a) input field at the entrance of the first MMI; (b) two images at the end of the first MMI;
(c) one image at the end of the second MMI while the switch works at the cross state and (d) one image at the end of the second MMI
while the switch works at the bar state and one of arms is heated. The length and the width of MMI are 5220 and 30 lm, respectively.
J. Liu et al. / Optics Communications 245 (2005) 137–144 141
solved by widening the waveguide and choosing
paired interference.
Fig. 7. Output near-field profile of the cross state (a) and the
bar state (b).
4. Experiment and results
The fabrication process is shown as: first, a 0.15
lm thick SiO2 mask film was grown thermally,then the waveguides patterns whose direction were
paralleled to the Æ110æ orientation of the wafer
were defined with photolithography technology.
The rib waveguides were etched to a depth of 1.5
lm by saturated KOH solution. Then another
0.15 lm thick SiO2 film, acted as the mask of the
second chemical etching for the trenches, was
grown by thermal oxidation. After patterning themask by photolithography, the wafer was etched
to the buried SiO2layer by KOH solution. Then,
a 0.12-lm thick SiO2 film that acted as the top
cladding layer was grown thermally. Metal heater
was evaporated and patterned on the two phase-
shifting arms. Finally, the end facets of the wave-
guides were polished carefully.
The characteristics of the device were measuredby coupling laser from a single-mode fiber directly
to the device itself. Output near-fields were moni-
tored by a CCD system. Fig. 7(a) and (b) show the
output near-fields when the switch worked at cross
and bar states, respectively. In Fig. 8, solid and
dashed curves show the basic switching character-
istics measured for two orthogonal polarization
states at 1550 nm. As denoted, the switching
1530 1535 1540 1545 1550 1555 1560 1565 1570-18.0
-17.5
-17.0
-16.5
-16.0
-15.5
-15.0
-14.5
-14.0
-13.5
-13.0
-12.5
-12.0
Inse
rtio
n Lo
ss (
dB)
Wavelength (nm)
Ref 0 DegreeRef 45 DegreesRef -45 DegreesRef 90 Degrees
Fig. 9. Wavelength dependence of insertion loss on transmis-
sion maximum of switch.
0 20 40 60 80 100 120 140 160 180 200 220 240-20
-18
-16
-14
-12
-10
-8
-6
-4
-2
0
2
14.5 dB
145 mw
143 mw
Out
put P
ower
(dB
,a.u
.)
Electrical Power (mw)
Fig. 8. The output power of the two output ports versus
heating power.
142 J. Liu et al. / Optics Communications 245 (2005) 137–144
powers are 143 and 145 mW, respectively, lessthan the value of 235 mW reported in [8]. We have
fabricated a similar switch without trench and the
thickness of top cladding was about 0.8 lm. In
this device, the switching power consumption
was 420 mW [14]. These prove that the blocking
trenches and the thin top cladding layer are effi-
cient to reduce switching power. Due to limit of
technique, the two arms could not be made thesame, so initial powers about 20 and 46 mW,
respectively, are needed for the best extinction
ratio.
The extinction ratios are 18.5 and 15.1 dB,
respectively. However, the polarization-dependent
offset between the two switch transmission mini-
mas limits the extinction ratio to 14.5 dB. Rea-
sons for such low extinction ratios may bederived from the thickness unevenness of the top
layer and fabrication error. The thickness fluctua-
tion is about ±0.5 lm which is severe comparing
to the total thickness of 5 lm. For the same
SOI wafer, wider MMI will produce the self-im-
ages with higher quality because of more modes
being excited, but the size of device will be
enlarged at the same time.The insertion loss was measured by a fiber–
chip–fiber system. End facets of fibers did not
touch the chip directly, so there were air gaps be-
tween the chip and the fibers. Therefore the system
was a fiber–air–chip–air–fiber one in fact. The sys-
tem reflections included two fiber–air reflections
and two air–chip reflections.
The measured insertion loss of a straight wave-
guide, which is 3 cm long and 5 m wide, is �10 dB
at 1550 nm. The total reflection loss and coupling
loss, for both input and output, were calculated of
�3.5 and �3.4 dB, respectively. The propagationloss of the straight waveguide could be estimated
to be �1 dB/cm. The calculated results are ideal
compared with the experiment ones. So the propa-
gation loss can only be less than �1 dB/cm. The
insertion loss of the 2 · 2 switch is �14 dB. Com-
pared with the value of the straight waveguide, a
surplus loss of �4 dB should derive from energy
radiation in the S-bend connectors and the imper-fect interference because of design and fabrication
errors in MMIs.
Interferometric devices are susceptible to
changes in insertion loss and extinction ratios
as functions of wavelength. Fig. 9 shows the
wavelength dependence of insertion loss when
different polarization incident lights were used.
It is seen that a ripple of ±0.8 dB about an aver-age value of �14 dB loss at the C band (1530–
1570 nm). Across this band, the extinction ratio
limited by polarization-dependent offset changes
within a range of 13.9–14.5 dB according meas-
ured results. However, when the wavelength
increases to 1580 nm or more, the insertion loss
and the extinction ratio will deteriorate
significantly.
J. Liu et al. / Optics Communications 245 (2005) 137–144 143
The polarization dependent losses (PDL) were
also measured at different wavelengths. In the
worst case across the whole C band, the PDL
was 1.56 dB, and the best value of 0.56 dB was
attributed at 1550 nm.There are two reasons for which switching char-
acteristics are susceptible to the polarization of
incident light: (1) asymmetry in the layout of the
metal tracking to the device. This will cause a
non-uniform stress field across the arms [5]; (2) dif-
ference of thermal expansion coefficient between
silicon and silica. This difference will cause a resid-
ual stress during heating process, such as thermaloxidation and metal evaporating.
Because of the high heat conductivity of silicon,
SOI thermo-optic switch has a much faster re-
sponse speed, in comparison with silica or polymer
waveguide switches whose switch time was 180 lsand 5 ms [4,15], respectively. Fig. 10 shows the re-
sponse of cross-output against time when a square
wave pulse was applied to one of the heaters. Therise time and fall time were measured to be about
8 ± 1 and 5 ± 1 ls, respectively. These speeds are
much faster than the former results [8]. The meas-
ured results are in good agreement with theoretical
estimations. As shown in Fig. 2, the generated heat
by heater will be conducted to cladding layer first.
The heat conductivity of silicon is about 100 times
greater than SiO2s. Therefore, reducing the thick-ness of this SiO2 layer, the speed of heat conduct-
ing will be increased regardless the arm is heated
or cooling. The lower switching power and the
Fig. 10. Switching characteristic for the switch.
shorter switching time prove that a thinner top
cladding layer is advantageous to both power con-
sumption and speed.
The improved researches by FEM implied that
if the thickness of buried SiO2 increased, the powerconsumption would reduce obviously, but the
speed would also slower remarkably. Simulation
results show that, for the same structure, doubling
the buried layer to 2 lm would lower the power to
about 87 mW, but meanwhile, it would increase
the switching time to about 50 ls.
5. Conclusion
We have fabricated low power 2 · 2 thermo-op-
tic switch with fast response on SOI by anisotropy
chemical etching. Blocking trenches were etched
on both sides of the phase-shifting arms. A thin
top cladding SiO2 film was thermally grown. The
device obtained superior characteristics includinga low switching power of 145 mW and a fast
switching speed measured to be about 8 ± 1 ls.2D-FEM was applied to simulate the temperature
field of the phase-shifting arm. Then, FD-BPM
was used to simulate the light propagation in the
switch. Good agreement between the results of
measurement and simulation was achieved.
Acknowledgement
This work was supported in part by the Na-
tional Science Foundation of China (Grant No.
69896260) and The Ministry of Science and Tech-
nology ‘‘973’’ plan (No. G2000-03-66) and ‘‘863’’
plan (No. 2002AA312060).
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