InGaAsP thin-film microdisk resonators fabricated by polymer wafer bonding for wavelength add-drop filters

Abstract

10-/spl mu/m-diameter InGaAsP thin-film microdisk resonators have been fabricated using polymer-wafer bonding with benzocylobutene. This wafer bonding process is to provide strong two-dimensional mode confinement in the waveguide and reduce the optical propagation loss. The measured resonance linewidth at wavelength 1.55 /spl mu/m is about 0.22 nm with a free-spectral range of 20 nm. The narrow linewidth and large free-spectral range make these devices conducive to the applications in dense wavelength division multiplexed systems.

Full text

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, NOVEMBER 20001495
InGaAsP Thin-Film Microdisk Resonators Fabricated
by Polymer Wafer Bonding for Wavelength
Add–Drop Filters
Yong Ma, Gilbert Chang, Seoijin Park, Liwei Wang, and Seng Tiong Ho, Member, IEEE
Abstract—10- m-diameter InGaAsP thin-film microdisk res-
onators have been fabricated using polymer-wafer bonding with
benzocyclobutene. This wafer bonding process is to provide strong
two-dimensional mode confinement in the waveguide and reduce
the optical propagation loss. The measured resonance linewidth at
wavelength 1.55
m is about 0.22 nm with a free-spectral range of
20 nm. The narrow linewidth and large free-spectral range make
these devices conducive to the applications in dense wavelength di-
vision multiplexed systems.
Index Terms—Benzocyclobutene, InGaAsP, microdisk, polymer
wafer bonding, wavelength add–drop filters.
I. INTRODUCTION
W
tiplexed (DWDM) systems [1]. Microdisk or microring
resonators with narrow linewidth and large free-spectral
range (FSR) have the potentials of functioning as wavelength
add–drop filters [2]–[5]. Recently, waveguide-coupled mi-
crodisk and microring resonators with small sizes (diameter
m) fabricated by deep dry etching have been demon-
strated to have narrow resonance linewidths (
large FSRs ( 21.6 nm) in AlGaAs–GaAs material system [3],
[5]. However, to our best knowledge, there has not been much
work reported on InGaAsP–InP-based microdisk or microring
resonators.
In this letter, we report for the first time the fabrication and
experimental results of InGaAsP thin-film microdisk resonators
fabricated involving benzocyclobutene (BCB) polymer-wafer
bonding. The polymer-wafer bonding process enables us to
bond a submicron InGaAsP epitaxial layer onto a transfer sub-
strate with BCB polymer as the intermediate medium. There
are a few advantages associated with this bonding technique.
First, strong mode confinement can be achieved not only in
the lateral direction, but also in the vertical direction (direction
perpendicular to the wafer plane) as BCB has a much lower
refractive index (
) than InGaAsP. The strong mode
confinement in the vertical direction helps to reduce substrate
AVELENGTH add–drop filter is one of the essential
components in current dense wavelength-division-mul-
nm) and
Manuscript received June 8, 2000; revised July 24, 2000. This work was sup-
ported by DARPA/AFOSR Program under Award F49620-96-0262/P005 and
made use of MRSEC Control Facilities of Northwestern University supported
by the NSF under Award DMR-9632472.
The authors are with the Department of Electrical and Computer Engi-
neering, The Technological Institute, Northwestern University, Evanston, IL
60208 USA.
Publisher Item Identifier S 1041-1135(00)09588-4.
leakage loss. Second, the refractive index contrast in the lateral
direction is changed from about 3.4:1.0 to 3.4:1.53, which
helps to reduce the sidewall scattering loss. This is because
the amount of scattering due to sidewall roughness scales with
the contrast between the square of the refractive indices of the
waveguide and its surrounding.
II. DEVICE FABRICATION
The geometry of our microdisk resonators with coupling
waveguides is the same as that in [5]. The wafer structure with
a 0.4- m-thick InGaAsP (
on an InP substrate by molecular beam epitaxy. To prepare
the epitaxy sample for etching, a 400-nm-thick SiO
was deposited by plasma-enhanced chemical vapor deposition
(PECVD). This SiO layer acts as a hard mask for etching. The
wafer is then patterned through a soft mask of 180-nm-thick
2% polymethylmethacrylate (PMMA) using electron-beam
lithography. The pattern on the PMMA mask was subsequently
transferred to the underlying SiO
etching (RIE). Dry etching utilizing inductively coupled plasma
(ICP) with a gas mixture of Cl :Ar
transfer the pattern onto the epitaxy wafer through the SiO
hard mask at an elevated temperature of 250 C. The etching
depth was about 1.3 m. After the etching process, we removed
the SiO hard mask using buffered HF.
The above procedure defines a typical deeply etched ridge
waveguide structure. In such a structure, there is a strong lateral
modeconfinementbecauseofthelargeindexcontrast(3.4:1.0).
However, in the vertical direction, the refractive index contrast
between the guiding layer and the substrate is relatively small
(3.41:3.17). As the result, a large part of the guided mode will
enter into the substrate, which will cause a lot of scattering
lossesintothesubstrateduringthepropagationanddecreasethe
optical throughput.
To increase mode confinement, we need to increase the re-
fractive index contrast in the vertical direction. We can obtain a
large refractive index contrast by using polymer wafer bonding,
aspolymerusually hasamuchlowerrefractiveindexthansemi-
conductors. In our experiment, we use BCB as the polymer,
which has been utilized to fabricate low-loss waveguides and
modulators. The details of this bonding process can be found in
[6] and [7]. Basically, we first spin BCB on both the patterned
samplewafersandthetransfersubstrates(GaAsinourcase).We
thenflipthesampleoverandputitdirectlyontothetransfersub-
strate. We give some pressure or weight to push the two wafers
m) guiding layer is grown
layer
layer using reactive ion
(2:3) was employed to
1041–1135/00$10.00 © 2000 IEEE

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1496IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 11, NOVEMBER 2000
(a)(b)
Fig. 1.
microdisk resonator with coupling waveguide. (b) Cross section of a BCB
bonded waveguide.
(a) Scanning electron micrograph of a BCB bonded 10-?m-diameter
together and then put the whole wafer into a nitrogen-filled fur-
nace at 250 C for one hour. After that, BCB becomes fully
cured and the two wafers are very tightly glued together. Fi-
nally, theInP substrate was removedusing selectivewet etching
(HCl:H PO
2:3).
Fig. 1(a) shows the scanning electron microscope (SEM)
image of a 10- m-diameter BCB-bonded microdisk resonator.
The straight waveguide is for input coupling and the U-shape
waveguide is for output coupling. Light is input into port X and
partially coupled into the microdisk resonator and coupled out
by the U-shape waveguide through port Y when the wavelength
is on resonance. When off resonance, light will pass through
the straight waveguide and exit from port Z. We call port Y
the transmission port and port Z the reflection port. All the
coupling waveguides are tapered from 2
near the microdisk resonator. The length of tapered region is
500
m. The radius of U-shape waveguide is 25
between the coupling waveguide and disk resonator is about
0.18 m. This gap is filled with BCB polymer after the bonding
process, which reduces the lateral refractive index contrast
and increases the coupling efficiency. A cross section of the
bonded waveguide structure is shown in Fig. 2(b). From Fig.
2(b), we can see that the 0.4- m-thick InGaAsP guiding layer
is surrounded by BCB polymer and air. In this structure, the
BCB layer must be thick enough to prevent the guided mode
from leaking into the transfer substrate as GaAs has a higher
refractive index than BCB. But, it should not be too thick to
make the wafer cleaving difficult. In our experiment, the thick-
ness of BCB was chosen to be 3.0
the BCB layer is higher than the top of the waveguide. This is
because the etching depth (1.3 m) is larger than the waveguide
thickness (0.4
m). In the next section, the transmission and
reflection measurement results will be discussed.
m down to 0.4m
m. The gap
m. Note that in Fig. 2(b)
III. RESULTS AND DISCUSSION
The waveguide coupling was achieved by using end-firing
method.Thelightfromatunablediodelaserwithacenterwave-
lengthof1550nmwascoupledintothedevicefromportXusing
a high numeric aperture lens (N.A.
port Y or port Z was focused by another lens and imaged onto
). The output from
(a)
(b)
Fig. 2.
bonded microdisk resonator with coupling waveguide. —Experimental ???
Simulated (a) TE and (b) TM.
Reflectivity as the function of wavelenght of a 10-?m-diameter BCB
an IR camera and a photodetector. The insertion loss is about 15
dB. A movable pinhole was placed in front of the photodetector
to select either output.
The reflection spectra for the TE and TM modes of a 10-
m-diameter BCB-bonded microdisk resonator are shown in Fig.
2 (The TE polarization is parallel to the substrate). For the TE
case,therearethreesetsofresonancemodes.EachsethasaFSR
nm with a full width at half maximum
nm, which results in a finesse of 84. For the TM case, we
have 2 sets of resonance modes for which
nm, which gives a finesse of 91. Note that
the presence of the multiple sets of resonance modes is due to
the guiding layer not being single mode waveguide. Therefore,
higher order modes could be excited in the disk resonator. The
largeFSRisduetothesmalldiameterofthediskandthenarrow
linewidthisduetothelowcavityloss.Asweknow,inmicrodisk
resonators,whisperinggallerymodes(WGMs)canhavehigh-
values, where
TM resonance modes show
value as high as 8000.
We calculated the reflectivity
nm and
. In our case, both the TE and
by using the following:
(1)

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MA et al.: InGaAsP THIN-FILM MICRODISK RESONATORS1497
Fig. 3.
10-?m-diameter BCB bonded microdisk resonator for the TM case.
Measured transmissivity as the function of wavelength of a
where
maximum transmissivity and
;
coefficient of finesse given by
with
;
;
collective disk diameter;
loss coefficient in the disk resonator;
coupling coefficient between the coupling waveguide
and the disk resonator;
being the propagating effective refractive index.
To fit the experimental data, we first use the conformal transfor-
mation and WKB approximation as described in [8] to calcu-
late
. We substitute
at each wavelength by assuming
m for TE and
m for TM. The simulated results are shown in
Fig. 2 from which we see that the resonance wavelengths match
theexperimentaldata reasonably. Note thattheTEmodes suffer
more losses than the TM modes in our devices. This is probably
due to larger scattering loss in the TE mode case. From Fig.
2, the maximum transmissivity is about 40% for TE and 90%
for TM. Fig. 3 shows the measured transmissivity from port
Y for the TM case. Compared with Fig. 2(b), we can see that
;
into (1) to get the reflectivity
cm
cm
,
,
,
,
for most of the resonant wavelengths, which
means the total loss from disk resonator and coupling wave-
guide is small. The TE case has similar results. Note that the
location of the resonance dip is subjected to digitizing error and
the resolution limitation of wavelength scanning ( 0.05 nm).
We estimate the total loss to be about 0.7 dB for a 10- m-diam-
eter disk resonator and 1-mm-long tapered coupling waveguide
[7].Basedonourexperimentalresults,thedevicesreportedhere
mightfunctionaswavelengthadd–dropfilters.However,theuse
of single resonator cannot provide flat-topped filtering charac-
teristics which can be achieved with the use of multiple res-
onators [2].
IV. CONCLUSION
With polymer-wafer bonding process, we have success-
fully fabricated 0.4- m-thick thin-film InGaAsP microdisk
resonators with 10- m-diameter. These resonators display
a linewidth as narrow as 0.22 nm and a FSR as large as 21
nm. Further improvements, such as to achieve single mode
operation, higher order filtering characteristics and wavelength
tuning, will make these devices to be useful as wavelength
add–drop filters in DWDM applications.
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