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Procedia Engineering 00 (2017) 000–000
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Procedia Engineering 216 (2017) 144–151
9th International
International Conference
Conference on
on Materials
Materials for
for Advanced
Advanced Technologies
Technologies(ICMAT
(ICMAT2017)
2017)
9th
100 Gbps (4 × 25 Gbps) Optical Receiver Module Packaged in
Chip-on-Board Based on Germanium Photodetector
Do-Won Kim*, Andy Eu Jin Lim, M. Kumarasamy Raja, Jason Liow Tsung Yang,
Vishal Vinayak Kulkarni, Surya Bhattacharya, and Guo Qiang Lo
Institute of Microelectronics (IME), A*STAR (Agency of Science, Technology, and
Research), 2 Fusionopolis Way #08-02, Innovis Tower, Singapore 138634
Abstract
100 Gbps (4 × 25 Gbps) optical receiver (Rx) module is demonstrated using Germanium (Ge) photodetector (PD) which is
fabricated through Silicon-photonics process using 750 ohm-cm of high-resistivity silicon oxide insulator (SOI) wafer. Transimpedance amplifier (TIA) and Ge PD are packaged with chip-on-board (COB) manner on a printed circuit board (PCB). High
speed PCB for the assembly of both electronic and photonic devices in COB package is precisely designed from the material
selection to the device footprint layout and transmission line design. The layout on PCB is optimized using high frequency
simulation tool of HFSS to minimize RF loss happening in the transmission line and electrical interconnection points of bond
wires. Electrical-optical (EO) S-parameter measurement for the Ge PD shows 22 GHz of transmittance (S21) 3dB bandwidth.
Photocurrents of the photodetector induced by the optical input power are analyzed for signal integrity both TIA ON and OFF
states. Photocurrent changes by the misalignment of the lensed optical fiber coupled to the edge coupler of the Ge PD shows that
3dB misalignment tolerances are 5.5 µm in the longitudinal and around +/-1 µm in the lateral directions. This COB packaging
technique of optical Rx module can be applied for the integration and assembly of the optical module of higher data rate of 100
Gbps and beyond.
© 2017 The Authors. Published by Elsevier Ltd.
© 2017 The Authors. Published by Elsevier Ltd.
Selection
ICMAT.
Selection and/or
and/or peer-review
peer-review under
under responsibility
responsibility of
of the
the scientific
scientific committee
committee of
of Symposium
Symposium 2017
2017 ICMAT.
Keywords: Receiver module, COB packaging, GE PD, silicon photonics.
1. Introduction
The explosive data use in both mobile and wired access is driving the emerging of optical data centers especially
by huge internet companies such as Google, Amazon, and Facebook, which are planning to invest for the upgrade of
data centers up to 2018 [1]. Ericsson estimated mobile and fixed data traffic will be 170.5 exabyte/month in 2020,
which is tremendous increase from 53.3 exabyte/month in 2014 [2]. This data usage has motivated high-speed
optical transceivers over 40 Gbps, and 100 Gbps to become dominant players for optical interconnections platforms
1877-7058 © 2017 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd.
Selection and/or peer-review under responsibility of the scientific committee of Symposium 2017 ICMAT.
10.1016/j.proeng.2017.10.019
2
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145
including silicon photonics integrated circuit (Si-PIC) based ones. Higher data rate of 400 Gbps and beyond is
expected to be commercially realized in near future.
Optical transceiver based on vertical cavity surface emitting laser (VCSEL) has been mainly applied for the
optical interconnections of both inter and intra data centers with lower cost especially for short reach links.
However, VCSEL based optical transceiver has critical drawbacks such as dispersion of the laser signal in the multimode fiber (MMF) for longer distance over 10 km, which is common for connections of inter/intra data centers, and
relatively higher cost of parallel link compared with coarse wavelength division multiplexing (CWDM) link for
longer reach [3]. As Intel announced commercialization of the Si-PIC based 100 Gbps Quadrature small form factor
(QSFP) optical transceivers in Aug. 2016, Si-PIC based CWDM optical transceiver with single mode fiber (SMF) is
a strong competitor to the VCSEL based one.
QSFP is an industry jointed standard of small form factor (SFF) committee specifications of SFF 8635 and SFF
8665 evolving from 4 × 10 Gbps to 4 × 28 Gbps to meet the demands on data-density [4-5]. IEEE 802.3ba specifies
the PHY layer of 40/100 Gigabit Ethernet standard in which 100 Gbps QSFP can be applied [6]. Recently IMEC in
Belgium demonstrated silicon PIC based 4 × 20 Gbps optical interconnection [7]. QSFP can be packaged and
assemble in COB manner for the data rate up to 100Gbps. For the higher data rates up to 400 Gbps, 800 Gbps, 1
Tbps, and 1.6 Tbps, new packaging technology should be developed. 2.5D/3D multi-chip module (MCM) using
interposer or through silicon via (TSV) has been intensively studied and reporting noticible RF performance [8-9].
In this study, we demonstrate chip-on-board (COB) packaged 4 channel × 25 Gbps (100 Gbps) optical receiver
(Rx) module using Ge photodetector (PD). The Ge PDs are fabricated at a commercial foundry with IME’s silicon
photonics technology platform. Typical bandwidth performance of Ge PD is over 20 GHz with ~10 nA of dark
current and larger than 1 A/W of responsivity for C-band operation [10]. COB packaging technique, even though its
data rate limitation caused by PCB board and bond wires, is still mainly applied for the packaging of high-speed
optical transceiver because of its specific features such as high reliability, low cost, well compatibility for mass
manufacturing, and good rework ability in laboratory level test [11]. When it is designed to have well optimized
compact footprints on board layout, smaller form factored optical modules is capable to be implemented.
2. 4 channel (4ch) Ge on Si photodetector
Fig. 1 (a) shows the die-attached and wire bond packaged vertical Ge pin PD array fabricated using IME’s silicon
photonics technology [10]. Each Ge PD has a 2.5 µm × 2.5 µm size of edge coupler of inversed Si nano-taper for
optical coupling of the external lensed optical fiber [See Fig. 1 (b)]. Lensed optical fiber which has 2.5 µm of spot
size is coupled to the edge coupler to supply optical modulated signal to Ge PD as shown in Fig. 1 (c).
(a)
(b)
(c)
Fig. 1. (a) Packaged 4ch Ge PD, (b) 2.5 µm × 2.5 µm of edge coupler array in Ge PD, and (c) coupled lensed
optical fiber of 2.5 µm of spot size to the edge coupler of the Ge PD.
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Fig. 2 (a) shows the measured frequency response of the Ge PD. This is measured using optical-electrical (OE)
measurement method of the network analyzer. The 3dB bandwidth is at around 22 GHz for both 1 V and 2 V of
biasing voltages. Fig. 2 (b) shows the measured photocurrent of the Ge PD itself. The dark current is around 10 nA
for -1 V of reverse biasing.
(a)
(b)
Fig. 2. (a) Measured frequency response, and (b) photocurrent of the Ge PD.
The performance of the packaged Ge PD on PCB is evaluated. Fig. 3 (a) shows the measured I-V curves of the
Ge PD packaged with TIA on PCB. It is measured with biasing power to the TIA both switched ON and OFF states.
The dark current range of the Ge PD shows almost similar around 2~30 nA for the both biasing power ON and OFF
states. The dark current region is shifted from -0.6~1.24 V at TIA OFF state to 2.24~4.37 V at TIA ON state due to
the bias provided by the self-biased from TIA, at its input to Ge PD. As shown in Fig. 3 (b) photocurrent changes by
input optical power changes. The 1550 nm laser from the tunable laser source is supplied to the Ge PD through the
edge coupler coupled with lensed optical fiber which has 2.5 µm of spot size. As optical power increases from
1.1dBm to 4.2dBm, PD photocurrent increases almost linearly within measured optical power range. The measured
eye diagram at 10 Gbps has increased vertical jitter when photocurrent goes over around 3 mA, which is shown in
Fig. 3 (b). This is due to the photo current saturation of TIA. The input overload current of the TIA is 4 mA.
(a)
(b)
Fig. 3. (a) Measured I-V curve of the Ge PD in a packaged state with TIA on PCB with TIA biasing on and
off respectively, and (b) Measured photocurrent by increasing input optical power. Eye diagrams at 10 Gbps
are shown for several selected photocurrent values.
Fig. 4 shows the photocurrent behavior by misalignment of the coupled lensed optical fiber to the edge coupler of
the Ge PD. Some photocurrent drops come from the fiber oscillation during movement. The optical input power to
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Ge PD is 9.58dBm. Measured photocurrent curve in Fig. 4 (a) is for the longitudinal misalignment, -x direction, and
Fig. 4 (b) is for the lateral misalignment, +/-y and +/-z directions. 3dB misalignments to the longitudinal and lateral
directions are 5.5 µm, and around +/-1 µm respectively. In photonics packaging, precise control of the alignment
state under submicron is required. Thus, when we assume the volume of the epoxy resin is 3 × 3 × 3 mm3, low
volumetric shrinkage less than 0.4% UV curable epoxy should be used for the fixture of the lensed fiber to the
silicon photonics ICs to make less misalignment than 1 µm.
(a)
(b)
Fig. 4. (a) Measured photocurrent of the Ge PD by misalignment to the longitudinal direction, -x, and (b) to
the lateral directions of +/-y, and +/-z.
3. Chip-on-board packaging and measurement results
3.1. RF loss management
In COB packaging for high-speed optical transceiver requires highly precise control of the RF and optical losses.
RF loss and optical loss should be controlled to be as minimum as possible for the high signal integrity. Optical loss
can be minimized by the optimized design of the coupler and accurate coupling of the optical fiber to the coupler.
Fiber coupling can be accomplished through both active and passive alignment methods [12-13]. In edge coupling of
the fiber to the PIC, active alignment is preferred since the edge coupler size relatively smaller than grating coupler.
The main RF loss comes from transmission lines on printed circuit board (PCB) and bond wire between ICs or IC and
PCB.
Dielectric loss, conductor loss, impedance mismatch, crosstalk, radiation loss, and conductor roughness on PCB
are RF loss sources to be controlled. These loss sources degrade signal integrity by reducing the amplitude of the
signal or making longer the rise/fall times with thicker jitter [14-15]. Dielectric loss is the signal energy attenuation
by the substrate dielectrics. It comes from the dissipation factor or loss tangent (δ) which is a function of the
dielectric material’s molecular structure [16]. Conductor loss is a loss coming from the metal transmission lines such
as surface roughness, DC and AC resistance, and skin effects [14-15]. Radiation loss and crosstalk loss are minor
when the transmission lines are optimally designed. Crosstalk loss is negligible by designing with 4W “rule of
thumb”, that is the distance between adjacent transmission lines should be larger than 4 times of the transmission
line width [17].
In COB packaging of this study, we applied wire bonding technique to electrically interconnect between ICs and
lines on PCB with Au bond wire of 1 mil of diameter. The RF signal transmission loss for bond wire is analysed
since the signal integrity is degraded even by short length of hundreds µm of bond wire in high-speed operation of
tens GHz. The effect of the number and length of bond wires is analysed using RF simulation tool of HFSS. Since
the bond wire connects between ICs or IC and PCB in free space, the dielectric loss can be ignored (loss tangent is
almost 0 for air). Thus the major RF loss source in wire bond is conductor loss, which is divided into DC loss and
AC loss. The DC loss, which degrades signal amplitude, is a loss from the DC resistance of conductor which
depends proportionally on the length and inversely proportionally on the cross-sectional area. Therefore shorter and
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multiple bond wire can have better signal integrity. In AC loss, which degrades amplitude as well as rise time and
fall time of the signal, signal tends to flow on the surface of the conductor with a certain depth that is called skin
depth decided by the conductor material. In a same way as for DC loss, thus shorter and multiple bond wires can
reduce the AC loss [15]. The HFSS simulation results are shown in Fig. 5 (a) and (b). The slope of the loss of 900
µm of bond wire length is 3 times larger than 300 µm of bond wire length. Multiple bond wires also reduce the RF
loss. In this Rx packaging, bond wire length is less than 300 µm with multiple bond wires to reduce the RF loss.
(a)
(b)
Fig. 5. (a) Simulated transmittance (S21) results of bond wire length, and (b) number of bond wires.
3.2. PCB design
To minimize the RF loss in transmission lines for the design of high-speed PCB, accurate control is required in
every design step from the material selection to the fabrication and assembly of PCB transmission lines and
footprints. Specialized dielectric core materials can be used for the high-speed PCB for optical transceiver
applications. Some of them which are generally used in manufacturing and R & D are summarized in Table 1. with
their major characteristics for high-speed PCB design application. For this study, Rogers RT5880 of 250 µm of
dielectric thickness and ½ oz. of copper layer thickness in both sides is used. Rogers RT5880 is used for only top
dielectric for the fabrication of 6-layered PCB. FR4 is used for other lower dielectric layers. This PCB layer structure
does not degrade signal integrity bur cost down around 6 times compared with whole 6-layered RT5880 structure.
The dielectric constant of the RT5880 is 2.2 @ 8~40 GHz, and its dissipation factor is 0.0009 @ 10 GHz, 23 ºC.
Table 1. Characteristic of the high-speed PCB dielectric materials.
Dielectric Material
Dielectric Constant, �r
Dissipation Factor, tan�
Data Rate
(Gbps)
FR4
4.34 @ 1 GHz
0.017
~10
Rogers RO3003
3.0 @ 8~40 GHz
º
~50a
º
0.0013 @ 10 GHz 23 C
Rogers RT5880
2.20 @ 8~40 GHz
0.0009 @ 10 GHz 23 C
~50a
Panasonic Megtron 6
3.38~4.07 @ 1~50 GHz
0.004 @ 12 GHz
~50a
a
The transmission line length is recommended shorter less than 3 cm for over 50 Gbps operation.
The microstrip high-speed data transmission lines of the PCB are designed to be impedance matched using RF tool
of HFSS. Fig. 6 (a), and (b) show the simulated transmittance (S21) results of the single-ended transmission line and
differential line respectively. For the single-ended line, the width of the transmission line is 0.75 mm. For the
differential line, the width is 0.55 mm, and gab distance between the lines is 0.35 mm.
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(a)
149
(b)
Fig.6. Simulated transmittance (S21) result of the microstrip transmission lines; (a) single-ended, and (b)
differential lines.
3.3. COB packaging of the electronic and photonic ICs
As shown in Fig. 7 (a), all the electronic and photonic ICs are packaged on the PCB. Four TIAs are die attached
using conductive thermally curable epoxy adhesive for GND connection to PCB common GND, and 4ch Ge PD is
die attached using nonconductive thermally curable polymeric adhesive. The Ge PD and TIA are electrically
connected by the single-ended transmission line on PCB and bond wires which connect pads of the TIA and PD.
TIA output pads are electrically connected through the bond wires to the differential transmission lines on PCB.
SMA connectors are used for the signal output ports linked to the sampling oscilloscope.
Fig. 7 (b) shows the package details of the four TIAs and Ge PD on PCB. The length of the bond wire is
controlled to be as short as possible around 300 µm with multiple bonding to reduce the parasitic effect to signal.
High-speed photocurrent signal of each channel is transmitted to the signal input of ach TIA through single-ended
transmission line. Current to voltage conversion as well as single-ended to differential conversion are carried out by
the TIA and the amplified high-speed voltage signal from the TIA is sent to the sampling oscilloscope connected to
the SMA connector through differential transmission lines on PCB.
(a)
(b)
Fig. 7. (a) COB packaged 4ch Rx module using 4ch Ge PD and commercial TIAs, and (b) wire bonded TIAs
and Ge PD.
3.4. Measured Eyediagrams
Fig. 8 shows the measurement set-up for 4 × 25 Gbps Rx module. Continuous wave (CW) with 1550 nm of
wavelength is provided from the tunable laser source. For the generation of the modulated optical signal at 25 Gbps,
commercial optical Tx module packaged with photonic modulator and driver IC is used. The modulated optical signal
at 25 Gbps is supplied to the each channel of the packaged 4ch Ge PD through the coupling of the lensed optical fiber
of which 2.5 um of spot size. Active alignment method is used to accurately couple the lensed optical fiber to the edge
coupler of the Ge PD. The peak alignment point of the lensed fiber is searched by monitoring the photocurrent.
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Fig. 8. Measurement set-up.
Fig. 9 shows the successfully measured 100 Gbps (4 × 25 Gbps) eye diagrams of the 4ch Rx module at the data
rate of 25 Gbps of each channel. The pseudo random bit sequence (PRBS) signal with a 10 -7-1 of duty cycle and ½
of mark ratio is supplied. The rise time and fall time of the signal is around 33 ps. And the signal amplitude is 204.8
mV which is a little smaller than the 225 mA of TIA typical output in the data sheet. Some portion of the thick jitter
comes from the long RF cable connected from SMA connector to Sampling Oscilloscope.
Fig. 9. Measured eye diagrams of the COB packaged 4ch Rx module at 25 Gbps.
4. Conclusions
100 Gbps of 4 × 25 Gbps Rx module is successfully demonstrated. 4ch Ge PD and 4 TIAs are packaged on PCB
with chip-on-board manner and interconnected by wire bonds and transmission lines on PCB. This can be applied
for 100 Gbps of QSFP based on silicon photonics ICs.
Acknowledgements
This work was supported by the Science and Engineering Research Council of Agency for Science, Technology,
and Research (A∗STAR), Singapore, and Exploit Technologies Pte Ltd (ETPL), Singapore under Silicon Photonics
Commercialization Flagship Grant ETPL13-R15GAP-0020.
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