Silicon-on-Insulator Microring Add-Drop Filters With Free Spectral Ranges Over 30 nm

Abstract

We demonstrate highly compact optical add-drop filters based on silicon-on-insulator microring resonators. The microring resonators have a small radius of 2.5 mu m and a very large free spectral range ~ 32 nm at the 1.55 mu m communication band. The propagation loss in such small micoring resonators was experimentally determined and shown to be extremely important in designing microring add-drop filters with low add-drop crosstalk, low drop loss, and maximally flat drop passband. For box-like channel dropping responses, second-order optical add-drop filters with two coupled microring resonators are designed and demonstrated, and the simulation matches well with the experiment. Devices were patterned with electron-beam lithography. Two fabrication procedures utilizing different polarity of resists were introduced and compared, and the process with negative resist resulted in much smaller sidewall roughness of waveguides, thus reducing the propagation loss in microring resonators.

Full text

Page 1

Purdue University
Purdue e-Pubs
Birck and NCN Publications Birck Nanotechnology Center
1-15-2008
Silicon-on-insulator microring add-drop filters with
free spectral ranges over 30 nm
Shijun Xiao
Purdue University
Maroof H. Khan
Birck Nanotechnology Center, Purdue University, mhkhan@purdue.edu
Hao Shen
Birck Nanotechnology Center, Purdue University, shen17@purdue.edu
Minghao Qi
Birck Nanotechnology Center, Purdue University, mqi@purdue.edu
This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for
additional information.
Xiao, Shijun; Khan, Maroof H.; Shen, Hao; and Qi, Minghao, "Silicon-on-insulator microring add-drop filters with free spectral ranges
over 30 nm" (2008).Birck and NCN Publications.Paper 180.
http://docs.lib.purdue.edu/nanopub/180

Page 2

228JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 2, JANUARY 15, 2008
Silicon-on-Insulator Microring Add-Drop Filters
With Free Spectral Ranges Over 30 nm
Shijun Xiao, Member, IEEE, Member, OSA, Maroof H. Khan, Student Member, IEEE,
Hao Shen, Student Member, IEEE, and Minghao Qi, Member, IEEE
Abstract—We demonstrate highly compact optical add–drop
filters based on silicon-on-insulator microring resonators. The mi-
croring resonators have a small radius of 2.5
free spectral range
32 nm at the 1.55
The propagation loss in such small micoring resonators was ex-
perimentally determined and shown to be extremely important in
designing microring add–drop filters with low add–drop crosstalk,
low drop loss, and maximally flat drop passband. For box-like
channel dropping responses, second-order optical add–drop filters
with two coupled microring resonators are designed and demon-
strated, and the simulation matches well with the experiment.
Devices were patterned with electron-beam lithography. Two
fabrication procedures utilizing different polarity of resists were
introduced and compared, and the process with negative resist
resulted in much smaller sidewall roughness of waveguides, thus
reducing the propagation loss in microring resonators.
m and a very large
m communication band.
Index
tical devices, microring add–drop filters, microring resonators,
microstructure fabrication, photonic integrated circuits, sil-
icon-on-insulator.
Terms—Electron-beamlithography, integratedop-
I. INTRODUCTION
S
gratedcircuitsdowntomicrometerlengthscale,withsubmicron
cross-sections for individual waveguides. In SOI configuration,
high-index-contrast between the core (silicon) and the bottom
claddingmaterial(silicondioxide)enablessub-wavelengthlight
confinement in the core with very little optical leakage into
the silicon substrate, thus making SOI suitable for highly inte-
grated photonic devices with very sharp bends, e.g., microring
add-drop filters [1]–[3], where the bending radius can be on
the order of several micrometers. SOI microring add-drop fil-
ters are promising for WDM signal processing in a chip. The
miniaturization of filtering devices may reduce the cost and the
complexity of WDM systems, and SOI devices can benefit from
low-cost fabrication using well-developed CMOS technologies
ILICON-ON-INSULATOR (SOI) has become an attractive
technology to enable the miniaturization of photonic inte-
Manuscript received January 31, 2007; revised August 4, 2007. This work
was supported in part by a grant from the Defense Threat Reduction Agency
(DTRA) under Contract HDTRA1-07-C-0042 and in part by the National Sci-
ence Foundation under Contract ECCS-0701448.
S. Xiao was with the School of Electrical and Computer Engineering and the
Birck Nanotechnology Center, Purdue University, West Lafayette, IN 47907
USA. He is now with the National Institute of Standards and Technology,
Boulder, CO 80305 USA (e-mail: sxiao@boulder.nist.gov).
M. H. Khan, H. Shen, and M. Qi are with the Birck Nanotechnology Center,
Purdue University, West Lafayette, IN 47907 USA (e-mail: mhkhan@purdue.
edu; shen17@purdue.edu; mqi@purdue.edu).
Digital Object Identifier 10.1109/JLT.2007.911098
[4]. Unfortunately, large free spectral range (FSR), box-like re-
sponse with maximally flat passband and fast filtering roll-off,
and high out-of-band signal rejection (filtering contrast) are still
major challengesfor SOI microringoptical add-drop filters. Re-
cently, a group reported microring add-drop filters both in sil-
icon nitride (SiN) [5], [6] and in silicon [7]. However, their re-
ported micoring resonators have a FSR of 20 nm (microring’s
radius
m) and 16 nm (microring’s radius
SiN devices and Si devices, respectively. In order to achieve a
large FSR over 30 nm for a sufficiently large spectral window
covering the entire C band
matured erbium-doped fiber amplification technology, the mi-
croring’s radius should be less than 3 m [1].
In this paper, we demonstrate a very large
1.55 m in SOI microring add-drop filters, and the microring’s
radius is 2.5
m. Nevertheless, the propagation loss in small
microring resonators can be large, which is attributed to the
bending loss (the major one) as well as sidewall roughness scat-
tering [8]. This nontrivial propagation loss has increased signif-
icantly the complexity to design microring add-drop filters with
maximally flat channel dropping passband, low drop loss and
lowadd-dropcrosstalk,anditalsosetsalowerlimitontheband-
width achievable in these filters. Low-loss microring resonator
is therefore the key to implement high-performance add-drop
filters.Inthiswork,thepropagationlossinmicroringresonators
isminimizedsignificantlybyincreasingthewaveguidewidthby
only 100–150 nm. The propagation loss in microring resonators
isanalyzedwitharelativelynewmethodreportedbyusrecently
[9]. For low add-drop crosstalk and low channel dropping loss,
a complete design of SOI microring add-drop filters is verified
with detailed experimental results. For box-like responses with
flat passband, a second-order add-drop filter with two mutu-
ally coupled microring resonators was designed, fabricated and
characterized. Although the loss in microring resonators is op-
timized, it is still nontrivial, and the condition for flat passband
in add-drop filters is then modified compared to that with the
loss neglected [10]. With simulation, we illustrate the signifi-
cant effect of the nontrivial propagation loss as well as micror-
ings’ mutual coupling in the design of second-order add-drop
filters. Thesecond-order filter’sdesign wasverifiedand demon-
strated with our detailed experimental results. All simulations
agree well with experiments. Compared to the majority of pre-
viously reported microring add-drop filters, our demonstrated
microring add-drop filters have the smallest footprint for inte-
gration and the largest FSR covering almost the whole spectral
windowofCband.Byincorporatingthisworkwithourrecently
reported work on multiple-channel filters [11], we believe that
m) for
–1565 nm) supported by
nm at
0733-8724/$25.00 © 2008 IEEE
Authorized licensed use limited to: Purdue University. Downloaded on November 24, 2008 at 13:06 from IEEE Xplore. Restrictions apply.

Page 3

XIAO et al.: SILICON-ON-INSULATOR MICRORING ADD-DROP FILTERS WITH FREE SPECTRAL RANGES OVER 30 NM 229
these demonstrated results here are very useful to implement
high-performancemultiple-channelfiltersthataretrulycompat-
ible with WDM systems for multiple users.
All microring add-drop filters were patterned with the Vistec
(formerly Leica) VB6 electron-beam lithography (EBL) system
installed in the Birck Nanotechnology Center at Purdue Univer-
sity. A significant advantage of this state-of-the-art EBL tool
is the high acceleration voltage (100 kV) that mitigates elec-
tron-beam proximity effects [5]. With a beam spot diameter of
around 6 nm according to specifications, we used a beam de-
flection step of 2 nm to reduce digitization errors discussed in
previous reports [5]. Two different fabrication strategies were
investigated and compared. In contrast to the conventional pat-
terning method, which uses a positive-tone electron-beam re-
sist [5], a relatively new procedure, based on a negative-tone
resist, is superior in providing much smoother device sidewalls,
which reduces the process complexity and provides much faster
device turn-around time. The smoother waveguide sidewall re-
duces significantly the sidewall roughness scattering loss.
The remainder of this paper is structured as the following.
Section II discusses and compares two different fabrication pro-
cesses for SOI devices. Section III presents basic theories to
design microring add-drop filters and extract the propagation
loss in microring resonators. Section IV presents our experi-
mental work on the optimization of the propagation loss in mi-
croring resonators. We present both simulation and experiment
for the synthesis of second-order microring add-drop filters in
Section V. Section VI is the conclusion.
II. DEVICE FABRICATION
ThebasicstructureofSOIwaveguidesconsistsofthreelayers
from bottom to top: the silicon substrate, the buried oxide layer
and the silicon core (Fig. 1(a)). The oxide layer is 3 m thick in
our case, which is enough to suppress the optical power leakage
from the silicon core to the substrate. The height of the wave-
guidecoreisfixedat250nminourcase,whichis alsothethick-
ness of the top silicon layer of our SOI wafers. The waveguide
widthcanbeadjustedvialithography.Thewaveguideisapprox-
imatelyofsinglemode(thelowestTE)at1.55 mforawidthup
to 600 nm in microring resonators. The lowest TM and higher
order modes have a much larger loss and resonate at different
wavelengths in resonators.
Two strategies were applied for the device fabrication.
One was with the positive resist poly-methyl-methacrylate
(PMMA) and the other one was with the negative resist hy-
drogen silsesquioxane (HSQ). Fig. 1(a)–(f) shows the flow
for the PMMA process. Our devices were fabricated on sil-
icon-on-insulator (SOI) wafers (from SOITEC). The device
patterns were exposed in a 200 nm-thick PMMA with the
100 kV EBL system. The high accelerating voltage of 100 kV
significantly reduces waveguide width dependence on whether
another feature is present at close proximity. Such width varia-
tions are especially prominent at the coupling regions between
resonators and bus waveguides, leading to smaller gaps and
wider waveguides. In EBL, the primary electron beam excites
the substrate and generates secondary electrons. Unfortunately,
such electrons are generated in an area that is much larger than
Fig. 1. Schematic of the PMMA-based fabrication procedure. (a) structural
configuration of the sample, (b) electron-beam exposure and development, (c)
chromium deposition via electron beam evaporation, (d) lift-off by immersing
the wafer in a solvent that dissolves undeveloped PMMA and a complementary
patternistransferredintothechromiumlayer,(e)reactive-ionetchingofsilicon,
(f) removal of chromium mask layer via wet etch.
Fig. 2. Schematic of the HSQ-based fabrication procedure. (a) structural con-
figuration of the sample; (b) electron-beam exposure followed by resist devel-
opment, (c) reactive-ion etching of Si in a Chlorine based plasma. (d) Remove
HSQ mask layer in HF solution (Optional).
the spot of the primary beam, and contribute to the exposure
of resist. Thus the total dose the resist receives at a specific
location is also dependent on the exposed pattern in close
proximity to itself. This effect is called “proximity effect”
[5]. Thus, a negative bias is typically applied to the pattern
layout in order to pre-compensate the proximity effect. At high
accelerating voltages, the range of the secondary electrons are
large, up to 50 m, therefore providing a uniform background
dose that are less sensitive to the local patterns. This advantage
helped to eliminate the requirement of pre-compensation of
waveguide width in our exposure. After the exposed resist was
developed, we deposited a 40 nm-thick chromium layer with
electron-beam evaporation. This was followed by a lift-off
procedure to remove the unexposed PMMA and to leave the
chromium pattern as a mask for the etching of silicon. We used
gas at a pressure of 35 mtorr in a PlasmaLab reactive-ion
Authorized licensed use limited to: Purdue University. Downloaded on November 24, 2008 at 13:06 from IEEE Xplore. Restrictions apply.

Page 4

230JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 26, NO. 2, JANUARY 15, 2008
Fig. 3. Scanning electron micrograph of ring waveguides fabricated with the
PMMA(top)andtheHSQ(bottom)respectively.FortheHSQsample(bottom),
the HSQ mask layer was removed with wet etch for micrograph purpose only.
Clearly, the HSQ process yields much smoother waveguides.
etch (RIE) toolto etch through the250 nmsiliconlayer. Finally,
the chromium mask was removed via wet chemical etch.
Fig. 2(a)–(d) shows the flow of fabrication with the HSQ on
the same type of SOI wafers. The same device patterns were
exposed in a 200 nm-thick HSQ with the 100 kV EBL system.
Still, no pre-compensation of waveguide width was applied.
In contrast to the PMMA-based process, the developed HSQ
pattern is a good etch mask without further need of pattern
transfer through the lift-off procedure. This simplified process
reduces the waveguide roughness caused by the lift-off. Then,
we used inductively-coupled-plasma (ICP) reactive-ion-etch
(RIE) to etch through the 250 nm silicon layer. The chamber
pressure was 5 mTorr and the gases were
rates of 15 sccm and 2.5 sccm, respectively. The exposed HSQ
pattern can be removed in a diluted HF solution for a very short
time period. However, the HF solution also etches slightly the
bottom cladding oxide layer. The HSQ pattern can also be
annealed at high temperature (1100–1200
oxide layer. As the HSQ has a refractive index
low absorption loss at 1.55 m bands [12], it doesn’t affect the
optical performance appreciably in high-index-contrast silicon
waveguides. Therefore, it was kept intact as a top cladding layer
in our case for device characterization. Fig. 3 shows two scan-
ning-electron micrographs of fabricated microring waveguides
with the PMMA (the top picture) and HSQ (the bottom picture)
respectively. The sidewall roughness is noticeably reduced on
the waveguide fabricated with the HSQ process. Thus, the HSQ
process was adopted to fabricate the microring add-drop filters
for optical characterization.
and Ar with flow
) to convert to an
and a very
III. DESIGN OF MICRORING ADD-DROP FILTERS
Fig.4showsthebasicconfiguration(top)ofanadd-dropfilter
based on a microring resonator with its schematic responses
(bottom). In a symmetrically coupled microring resonator, two
bus waveguides have the same width
pling distance
to the microring resonator.
onator waveguide’s width, which may be different from
is the average radius of the microring.
pling coefficient between the bus waveguide and the resonator,
which depends exponentially on the coupling gap
is the propagation power loss coefficient per round-trip in the
and the same cou-
is the res-
.
is the power cou-
, and
Fig. 4. Theoretical model (top) and schematic power responses (bottom) of a
symmetrically coupled microring add-drop filter (first order).
microring resonator. According to the theory developed in [9],
[10], close to each center resonance wavelength, power trans-
mission responses of the through port and the drop port are
written respectively as
(1.a)
(1.b)
where FSRis the freespectral range of theresonator’s response,
and
is the center resonance wavelength. Equation (1.b) indi-
cates a first-order Butterworth filter. The add-drop crosstalk is
defined as
expressed by
definedas
at
in dB. The through extinction is de-
finedas
by
in dB. For low add-drop crosstalk
and low drop loss, the waveguide power coupling coefficient
mustbemuchlargerthanthepropagationpowerlosscoefficient,
i.e.,
. As there is always a nonzero propagation loss
in practical devices, for sufficiently large waveguide
coupling to achieve low add-drop crosstalk and low drop loss,
the gap between the bus waveguide and the microring resonator
can’t be too large due to the fact of
a decayingcoefficient.Accordingto(1.b), thechanneldropping
dB bandwidth is
at, which is
in dB. The drop loss is
, whichis expressedby
at,whichis expressed
[11], whereis
dB
(2)
With constraints
drop loss, the channel dropping
limited by the waveguide coupling coefficient
the drop
dB bandwidth for high spectral resolution, it is
veryimportanttominimizethepropagationlossintheresonator
so that weaker waveguide coupling can be used. With FSR,
for low add-drop crosstalk and low
dB bandwidth is mainly
. To minimize
Authorized licensed use limited to: Purdue University. Downloaded on November 24, 2008 at 13:06 from IEEE Xplore. Restrictions apply.

Page 5

XIAO et al.: SILICON-ON-INSULATOR MICRORING ADD-DROP FILTERS WITH FREE SPECTRAL RANGES OVER 30 NM231
Fig. 5. Scanning-electron micrographs of one fabricated microring add-drop
filter.
, and
dB,
andcan be calculated by
and
[9].
For microring resonators, we have
or
the group index in SOI waveguides with core cross section of
nmnm and a top HSQ mask layer of 200 nm.
For a large FSR over 30 nm or larger to cover a sufficiently
largeopticalspectrumwindowsupportedbyerbium-dopedfiber
amplifiers, the radius of SOI microring should be
smaller. To our best knowledge, there was few reported work
on microring add-drop filters with a large FSR over 30 nm as
the relatively large propagation loss (we will show in the next
section) in such small microring resonators might have been the
bottleneck. In this paper, we optimize the filtering performance
of SOI microrings with a radius of 2.5 m. The propagatin loss
in such small microring resonators can be significantly reduced
by increasing the waveguide width over a range of 100–150 nm
while the height remains constant.
, where at 1.55 m is
m or
IV. PROPAGATION LOSSES IN MICRORING RESONATORS
Fig. 5 is a scanning-electron micrograph of a fabricated mi-
croring resonator in add-drop configuration. The microring ra-
dius is 2.5 m, and the waveguide’s width is
bus waveguide’s width is
thebus waveguideand themicroringwaveguideis
We used a tunable laser source to characterize responses of mi-
croring resonators. The laser was coupled into the bus wave-
guidethroughalensedsinglemodefibertipmountedonanXYZ
nanopositioningstage.Afiber-basedpolarizationcontrollerwas
used at the input to adjust the polarization to excite the funda-
mental TE mode with the lowest propagation loss. The output
was collected by another lensed single mode fiber tip mounted
on a second XYZ nanopositioning stage and then fed into an
optical power meter. To obtain a power transmission spectrum,
the laser wavelength was swept at a step continuously while the
output power of each wavelength was recorded sequentially.
Fig. 6 plots the measured responses of the microring add-
drop filter shown in Fig. 5, and Fig. 6(b) is a zoom-in view of
Fig. 6(a) at one resonance wavelength.For comparison, theoret-
ical curves (solid lines) are also plotted in Fig. 6(b) according
to the discussion below. The maximum filtering contrast (out of
band signal rejection) is
dB. The measured channel drop-
ping loss is
dB, and the measured add-drop crosstalk
is
dB. For the resonance
imum transmission of the through-port is
nm. The
betweennm, and the gap
nm.
m, the min-
dB
Fig. 6. (a) Measured add-drop responses of the fabricated first-order microring
resonator. The waveguide coupling gap is 200 nm. (b) A zoom-in view with
simulated responses plotted in solid line. The simulation is based on (1.a) and
(1.b). The drop-port response is normalized to the through-port response by set-
ting the maximum through-port transmission at 0 dB when not at the resonance.
(through-port extinction
ping
dB bandwidth is
nm. The extracted
is
the theoretical add-drop cross-talk is
discrepancy between the theoretical result and the experimental
result could be due to slightly different fiber-to-waveguide cou-
plings or waveguide propagation losses between through-port
and drop-port during measurement, and this explains why there
is a shift of 1–2 dB in the vertical axis between theoretical
curve and experimental curve for the drop-port’s response in
Fig. 6(b). The propagation loss is
or
the corresponding intrinsic quality-factor is
maximum quality-factor that can be achieved). We also fabri-
cated and tested microring add-drop filters with different ring
width
of 550, 500 and 450 nm, and we found that the
propagation loss increases dramatically as the ring width is re-
duced. For resonance wavelengths
propagation losses are
and
of 550, 500 and 450 nm respectively. The waveguide
power coupling coefficient doesn’t vary much compared to that
for
nm, e.g.,
nm.Fig.7plotsthepropagationlossinmicroringresonators
m for different
nm. The four data points are connected with lines to guide the
dB). The channel drop-
nm, and the FSR is
, and the extracted is
. The theoretical drop loss isdB, and
dB. The small
dB/round-trip, and
(the
m, the extracted
,
for
for
of 450, 500, 550 and 600
Authorized licensed use limited to: Purdue University. Downloaded on November 24, 2008 at 13:06 from IEEE Xplore. Restrictions apply.