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Modeling, Design, and Demonstration of Ultra-miniaturized and High Efficiency 3D
Glass Photonic Modules
Bruce C. Chou^, Sandeep Razdan*, Haipeng Zhang*, Jibin Sun*, Terry Bowen*, Vanessa Smet, Gee-Kung
Chang, Venky Sundaram, and Rao Tummala
3D Systems Packaging Research Center,
813 Ferst Drive NW, Georgia Institute of Technology, Atlanta, GA, USA
*TE Connectivity, Menlo Park, CA, USA
^[email protected], (352)-359-3182
Abstract This paper presents the modeling, design, and
demonstration of an ultra-miniaturized 2.5D optical
transceiver module using ultra-thin glass interposers with
electrical and optical through vias. The 3D Glass Photonics
(3DGP) technology with double sided attach of electrical and
photonics ICs can achieve ultra-high bandwidth with
improved power efficiency at lower cost than other photonic
integration such as silicon photonics and organic boards. Thin
glass substrates with 60um diameter through vias were
fabricated with copper plated electrical vias and polymer-
filled optical vias, formed simultaneously. Re-distribution
layers were fabricated on top of these integrated vias for
electrical interconnections. The 2.5D optical module produced
this way features flip-chip bonded VCSEL and driver chips.
Initial measurements of the optical vias showed 1.2 dB of loss.
Keywords: 2.5 D glass interposer, optical vias, optical
transceiver.
I. Introduction
Due to intrinsic lower propagation loss and higher channel
capacity of photons versus electrons, optical interconnects
have gradually replaced electrical interconnects on bandwidth-
critical nodes at shorter distances. Since the early 2000s,
intense efforts in optical interconnects, beyond board-level,
achieved impressive research results, including IBM’s
Terabus, Fraunhofer IZM’s EOCB (Electro-Optical Circuit
Board), and Intel’s silicon photonics research that spearheaded
many groundbreaking accomplishments [1 – 3]. Silicon
photonics, which leverages high density CMOS technology
and high index contrast between silicon and silicon dioxide, is
no doubt the most suitable for intra-chip optical
communications. However, silicon photonics cannot
realistically extend beyond a single die due to fabrication cost.
Furthermore, the huge refractive index mismatch between
silicon-based optical waveguides and glass-based optical
fibers introduces high loss and requires costly sub-micron
alignment. The fiber-to-waveguide transition happens at board
and chip level, as shown in Figure 1. In fact, packaging cost
and fiber-to-waveguide loss are two of the biggest factors why
optical communications have yet to replace electrical
communications at board and chip levels in spite of decades of
research efforts [4]. To go beyond board-level optical
communications, the three metrics of energy efficiency (Joules
per bit), density, and cost of packaging optoelectronics
systems must be optimized simultaneously [5].
Figure 1. Evolution of optical communications
The 3D Glass Photonics (3DGP) concept aims to provide a
simple and low cost photonics packaging solution utilizing
ultra-thin glass interposer technology [6,7]. Glass offers
several advantages over silicon and organic substrates for
photonics packaging: optical transparency, reflective index
matching to glass fiber, low-loss electrical signaling
capability, good thermal isolation, excellent dimensional
stability, and large panel processing. The 3DGP research aims
to demonstrate the advantages of glass substrate in the
following four research areas, as illustrated in Figure 2:
1. Fine-pitch and low-loss optical vias in glass.
2. Ultra-thin glass interposer with both optical and electrical
vias.
3. 3D assembly of dies such as Photonic Integrated Circuit
(PIC) on glass interposers.
4. Fiber-to-die transition.
Figure 2. The four focus areas of 3D Glass Photonics (3DGP)
978-1-4799-2407-3/14/$31.00 ©2014 IEEE 1054 2014 Electronic Components & Technology Conference
Initial modeling and fabrication of passive photonic
devices in a glass interposer, namely polymer-based optical
vias and waveguides, were presented earlier [8]. The current
paper goes beyond this prior work in higher level of module
integration. A VCSEL-based direct modulation optical
transceiver was chosen to showcase the capability of 3DGP
technology. Prior work has demonstrated the feasibility of a
2D optical transceiver module on glass substrate, but the
thickness and dimension of such a module can be further
miniaturized [9]. An ultra-thin 2.5D optical transceiver
module is fabricated, featuring high density optical and
electrical vias using a simplified process that can be modified
for panel level processing. The 2.5D optical transceiver
module is targeted at size reduction, electrical/optical loss
reduction and lower cost enabled by large panel fabrication
processes, precisely as shown on the top side of Figure 2.
The rest of the paper is organized as follows. Section II
covers the design of optical transceiver module and Beam
Propagation Model (BPM) of optical wave propagation
through glass interposer with through package vias (TPV).
Section III describes the simplified fabrication and assembly
steps used to build the 2.5 D transceiver module. Section IV
shows the optical characterization results, with a summary and
conclusion in Section V.
II. Modeling & Design
A typical 40 G short-reach optical transceiver module
consists of a 4-channel laser source made of either VCSELs or
EELs driven by a CMOS driver on the transmit side, and a 4-
channel Photo-Detector (PD) array feeding into an amplifier
such as a Trans-Impedance Amplifier (TIA) on the receiver
side [10]. The optical modulation scheme is the simple On-Off
Keying (OOK) scheme and the lasing wavelength used is
typically 850 nm rather than 1550 nm, which uses multi-mode
fibers (MMF) with 62.5/125 um core/clad diameter rather than
single-mode fibers. The connection from the optoelectronic
devices to MMF is through a 45 degree mirror and then to a
standard MT-connector mounted on the PCB.
The electrical design of the 2.5 D transceiver module is
completed in collaboration with TE Connectivity, and features
impedance matched differential signal lines at minimized
distances between the dies. The optoelectronic dies and
CMOS dies used are provided by TE specifically for 40 G
application. The optimized dimension of the interposer is 5 x 5
mm2, which achieves 4x area reduction comparing to 10x10
mm2 reported in [9]. Shown in Figure 3 is a simplified (to
protect proprietary design details) side-by-side comparison of
the 3DGP transceiver module versus the previously reported
module. The reduction in area is achieved by 1) shorter
electrical signal traces from the CMOS dies to the bumps
through the use of electrical vias and 2) possibility of dies on
top of bumps by utilizing both layers. In addition, thermal vias
are deployed to help heat dissipation on a smaller area. Further
reduction in area is achievable through 3D assembly.
Figure 3. Simplified diagram showing area reduction of the
3DGP module (right) versus previously published module
(left) [9].
One of 3DGP’s research areas is the fabrication of fine-
pitch optical vias. Depending on the thickness of the
interposer, required laser/photo detector pitch, and fiber core
diameter, an optical via might not be necessary.
Beam Propagation Model is used to determine the need for
optical via. BPM is used in favor of the more accurate but
more computationally intensive Finite-Difference Time-
Domain (FDTD) method. BPM is sufficient because the
critical dimensions (>20 um) are much larger than the
wavelength of interest (0.85 um), and the lack of sharp angles
along the path of light ensures paraxial model is reliable. BPM
is implemented using MATLAB. Light propagation is
simulated for three cases: 1) bare glass, 2) bare glass with
polymer lens, 3) bare glass with fully filled polymer vias. In
our model, the light wave travels through VCSEL-underfill-
interposer-gel-fiber interface. The VCSEL is assumed to be 30
um away from the surface of glass, the glass thickness, t,
varies from 100 um to 200 um, and the fiber is assumed to be
30 um from the exit side. The via has an entrance diameter of
60 um and a taper angle of 2.8°. The three cases are illustrated
in Figure 4.
Figure 4. BPM simulation setup for the three cases.
The modeling results for t = 100 um is shown in Figure 5.
The spreading of the light wave within the distance between
VCSEL and fiber is short enough that bare glass is the
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simplest and best approach. In fact, a comparison of the
optical power measured at the fiber core showed the fiber core
will be able to capture close to 100% of the optical power in
the bare glass case. While the addition of via did reduce the
loss in the simulation, the added fabrication steps and optical
loss due to surface and sidewall roughness could rendered the
use of optical via excessive.
On the other hand, the modeling results for t = 200 um
showed that the optical loss in the bare glass case is no longer
negligible due to the dispersion of light, as shown in Figure 6.
As shown in simulation, an optical via can help guide the
beam to the fiber core through a 200 um thick interposer.
Similar simulation had been completed for t = 300 um and
the loss at the fiber end was measured. The optical loss,
calculated in dB as 10*log(Pout/Pin), for the three cases from
100 to 300 um has been plotted in Figure 7. The optical loss is
effectively controlled to less than -0.5 dB at 300 um while the
optical loss for the bare glass and polymer lens case exceeded
-2 dB due to dispersion. The interposer built by GT-PRC
features thin glass between 100 ~ 130 um thick. In this range
via is not needed to guide light into a MMF as the dispersion
has not spread beyond the core region.
However, if a thicker interposer is used or SMF is needed
for higher wavelength, an optical via will be the best choice
with the most consistent dispersion control among the three.
Figure 5. BPM simulation for t = 100 um.
Figure 6. BPM simulation for t = 200 um.
Figure 7. Calculated optical loss from BPM simulation.
III. Fabrication
Two fabrication processes were explored: electroless
plated seed layer on dielectric laminated glass and sputtered
seed layer on bare glass. The detailed process steps are
illustrated in Figure 8. Both processes are implemented to
compare the advantages and disadvantages of each. In the
processed below, steps 1 ~ 5 are panel level processes while
step 6 is done at the coupon level currently. Panel level
processing on a 150 mm x 150 mm glass panel with 5 mm
square coupons can yield 576 coupons per panel; therefore,
potentially reducing the cost per coupon drastically.
Figure 8. The two 3DGP substrate process sequences
explored.
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III.1 Laminated Glass Process
The laminated glass approach has been developed and
optimized by GT-PRC [7]; however, the optically opaque
dielectric laminate restricts optical waveguide formation on
glass. The laminated glass approach nevertheless presents a
viable option when only optical vias are needed. For the
demonstrator, a 6” x 6” 100 um thick glass panel from Asahi
Glass Company (AGC) is laminated on both sides by RXP-4
dielectric from Rogers. RXP-4 is chosen for its higher
temperature tolerance (>350 C), which is important for
assembly. 60 um vias are drilled by AGC at 250 um pitch, to
match typical VCSEL pitch. Electroless process by Atotech is
used to plate the copper seed layer. Electrical vias are exposed
after photoresist patterning and plated to desired thickness,
while optical vias are left bare.
The optical material used is the photo-definable Cyclotene
4024 from Dow Chemical, which is based on
bisbenzocyclobutene (BCB) chemistry [11]. BCB exhibits
higher refractive index comparing to glass and a high Tg;
therefore making it suitable for the assembly process
involving AuSn solder. Processing of optical waveguides and
fully filled optical vias on glass has been reported in our
previous work and will not be discussed in detail [8]. In
current work, an attempt was undertaken to use BCB as
passivation in addition to optical structures. If successful, the
number of process steps can be reduced. Unfortunately, the
process had not been optimized at writing and severe warpage
made assembly unfeasible. On the other hand, the optical vias
were filled completely, while the electrical vias were filled
conformally. Figure 9 shows the panel-view of finished
demonstrator containing 289 interposers, and Figure 10 shows
the cross-sectional view of the electrical and optical vias from
one of the coupons.
Figure 9. 3DGP demonstrator panel using laminated glass
process on 100 um glass provided by AGC.
Figure 10. Cross-section of optical and electrical vias using
laminated glass process.
III.2 Bare Glass Process
The bare glass process utilizes Corning’s bare glass via
formation technology to drill 60 um vias on 130 um thick 6”x
6” glass panel, also provided by Corning [12]. Titanium and
copper seed layers were sputtered on bare glass by Tango
Systems Inc. Based on the results from laminated glass, BCB
was only used for optical via filling to reduce warpage, while
solder resist from Hitachi Chemical was used for the
passivation layer. ENEPIG (Electroless Nickel, Electroless
Palladium, Immersion Gold) process by Atotech was applied
to create gold finish on the surface. The completed panel
featuring 289 interposers is shown in Figure 11. The panel was
diced after surface finish to obtain the singulated interposers
for assembly. VCSEL die with AuSn solder joints was flip-
chip bonded first, followed by flip-chip bonding of driver die
with tin-silver solder joints. The receiver dies were not
assembled in the current demonstrator. Underfill was
dispensed after bonding and cured in oven in air. The cross-
section of the glass interposer with bonded VCSEL and driver
dies is shown in Figure 12.
Figure 11. 3DGP demonstrator panel using bare glass process
on 130 um glass provided by Corning.
Optical via Electrical vias
250 um
250 um
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Figure 12. SEM of cross-sectioned interposer with VCSEL
and driver.
IV. Characterization:
Optical loss characterization had been performed on via
level, where the loss through the optical via was measured
using direct abutment method. The measurement setup is
shown in Figure 13. A standalone 850 nm VCSEL was used to
drive a single mode fiber (SMF), which was fastened to a xyz
micro-positioner. The DUT, which in this case is the glass
interposer, was mounted on a second xyz micro-positioner. A
photodetector, mounted on top of an FR4 board, was attached
to the third xyz micro-positioner. The schematic of the sectup
is shown in Figure 13a), while a picture of the actual setup is
shown in Figure 13b).
Figure 13. a) Schematic of optical loss measurement setup. b)
Picture of setup showing DUT.
Figure 14. BPM of dimpled via showing light dispersion at the
exit side of the via.
Relative optical loss can be measured by normalizing the
receiving power calculated using PD voltage and current
reading with respect to VCSEL power with no DUT (through
air). Measurements performed on the 100 um laminated glass
samples were recorded in Table I. The loss is higher in the
optical vias than BPM simulation predicted, while the loss is
comparable for the bare glass case. The higher loss through
the optical via could be caused by either the surface or
sidewall roughness of the optical via. In the BPM simulation
of the filled via with surface dimples, as shown in Figure 14,
visible optical loss due to dispersion could be observed, while
simulation showed 0.48 dB loss. Since BPM could not capture
the loss due to micron level surface roughness, the lower loss
in simulation was expected.
Table I. Optical loss measurement of optical vias
Interface Measured Loss Simulated loss
Air Normalized to 0 dB 0 dB
Bare Glass 0.25 dB 0.23 dB
Optical via 1.2 dB 0.14 dB / 0.48 dB
V. Conclusion:
A 2.5D short-reach optical transceiver module was
demonstrated using GT-PRC’s 3D glass photonics technology
using both the laminated glass and the bare glass approach.
The bare-glass approach was used for assembly trials due to
lower warpage. The optical transceiver interposer featured
reduction in dimensions in the x, y, and z direction, thus
achieving more than 10x improvements in density compared
to previously- published results. Both optical and electrical
vias were integrated in the interposers. Optical measurements
and simulations confirmed 0.48 to 1.2 dB loss in the optical
via at 100 um thickness which could be reduced with process
improvements.
SMF Tip
PD
DUT
SMD fiber (Φ 125 um)
PD
Glass Interposer
200 um VCSEL
Driver
130 um thick glass interposer
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Acknowledgement:
The authors would like to thank William Vis, Jialing Tong,
Timothy Huang, and Vijay Sukumaran for their generous help
in fabrication. Furthermore, the authors would like to thank
Chris White and Jason Bishop for lab support. The authors
would also like to thank Meg Gerstner and Jim Toth from TE
Connectivity for their leadership and guidance.
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