Download High-Speed Optical interconnections on Electrical Boards Using

High-Speed Optical Interconnections on
Electrical Boards Using Fully Embedded
Thin-Film Active Optoelectronic Devices
Sang-Yeon Cho
Department of Electrical and Computer Engineering
Duke University, Durham, North Carolina 27708, USA
Outlines
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Motivations
Backgrounds
Device fabrication and integration processes
Experimental and theoretical results of coupling
efficiencies for different integration structures.
Speed measurement results (impulse response and
eye-diagram).
Optical signal distributions using MMI couplers.
Thin film laser with integrated optical waveguides.
Optical interconnections on PWB.
Conclusions
Motivations
 Electrical interconnections:
- Impedance Matching Issues
- Electromagnetic Interference
- High Power Consumption
- Frequency Dependent Characteristics (Skin effects)
 Optical interconnections:
- No EMI, Easy Design, Frequency Independent
Characteristics
- Optical Alignment Issues
- Fabrication Compatibility and Integration Density Issues
Fully Embedded Optical Interconnections
using Thin Film Active OE Devices
Proposed Fully Embedded Optical
Interconnections Using Thin Film
Active OE Devices
Waveguide
Transmitter Circuit
Receiver
Rec’r/uP Circuit
Circuit
Thin film edge
emitting laser
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Receiver Circuit
Embedded Thin Film PD
Receiver Circuit
Substrate
Interconnection substrates and boards face latency and skew
problems for clock and critical data signals
Optical interconnections can address these problems if optical
signal distribution can be added at low cost to the system
One approach is to embed the optical active devices in the
waveguide on the board or substrate
Backgrounds: Inverted MetalSemiconductor-Metal Photodetector

MSM Advantage: Low
capacitance per unit area
compared to typical PIN PDs

MSM Disadvantage: Shadowing
by interdigitated electrodes
reduces responsivity

Inverted (I-) MSM: By inverting,
low capacitance per unit area is
maintained and responsivity is
dramatically improved because the
fingers are on the bottom
Contact pad
Metal electrodes
Semiconductor
Backgrounds: How the I-MSM
Works
Incident photons
Electric field line
Lines of equipotential
EHP #3
EHP #2
0V
+5 V
EHP #1
Heterogeneous Integration Process
for Embedded Thin Film Devices
Substrate
Initial device fab on
wafer on epilayer surface
Substrate
Mesa etching to stop
selective etch layer
defines separate devices
Substrate removal
using selective
wet etching
Thin film device bonding onto a host
substrate or IC using a transfer
diaphragm: single device or array of
devices
Embedded I-MSM in Optical
Waveguide Process
I-MSM
SiO2
Si
Chemical Mechanical Polishing
BCB
Inverted MSM on test probing
pad
Spin coating of waveguide cladding
material and CMP planarization step
Ultem
Spin coating of waveguide
core material
Reactive Ion Etching
SiO2
Optical waveguide channel
patterning with dry etch process
Design and Fabrication Issues
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Design of Integration Structure
- Optical Signal Coupling
- High Speed Operation
Device Fabrication and Integration Process
- Thermal Stability Issues
- Planarization Issues
Optical Signal Couplings for
Embedded Structures
Waveguide to Thin Film Photodetector
Waveguide Cladding
Waveguide Cladding
Waveguide Core
Waveguide Core
Waveguide Cladding
Waveguide Cladding
Evanescent Field Coupling
Direct Coupling
Thin Film Laser to Waveguide
Waveguide Cladding
Waveguide Cladding
Waveguide Core
Waveguide Core
Waveguide Cladding
Waveguide Cladding
Non-Embedded Coupling
Embedded Coupling
Calculation Results for Direct
Coupling Structures
Optical Waveguide
Photodetector
Core
Cladding
Electrical Interconnection
Board/Module/Chip
Optical Waveguide
Photodetector
Core
Cladding
Electrical Interconnection
Board/Module/Chip
Calculation Results for Evanescent
Field Coupling Structures
Optical Waveguide
Core
Photodetector
Electrical Interconnection
Board/Module/Chip
Cladding
Temperature Characterization of
Embedded I-MSM PDs in Polymer
Waveguides
Measured for 15 hours at 250 oC

I-MSM dark current was
severely degraded by
thermal cure process for
waveguide polymers

New Schottky metallization
(Pt/Ti/Pt/Au) and thermal
cure process utilized to
preserve dark current of IMSM; measured dark current
results using new
metallization shown at left as
a function of thermal
processing time at 250 oC
Characterization of Polymer Optical
Waveguide (Fiber Scanning Method)
Measured Propagation Loss
using Fiber Scanning Method
Propagation Loss at l= 1.3 mm: 0.36 [dB/cm]
(3.9 mm BCB/3 mm SiO2/Si)
Embedded I-MSM PDs in Polymer
Waveguides: Top Views
Embedded Thin-Film I-MSM PD
Electrical Metal Contact Pads
Polymer Waveguide
Embedded I-MSM PD in a BCB
Polymer Waveguide
Direct coupling structure(3.9 mm BCB/PD/3 mm SiO2)
Coupling Efficiency: Theoretical Result: 59.6%, Experimental Estimate: 52.2%
Embedded I-MSM PD in a BCB
Polymer Waveguide
Direct coupling structure(5.3 mm BCB/PD/1 mm BCB/3 mm SiO2)
Coupling Efficiency: Theoretical Result: 56.4%, Experimental Estimate: 52.4%
Embedded I-MSM PD in an
Ultem/BCB Polymer Waveguide
Evanescent Field coupling structure (1.8 mm Ultem/0.2 mm BCB/PD/3 mm SiO2)
Coupling Efficiency: Theoretical Result: 19.80%, Experimental Estimate: 9.39%
Electrical Impulse Response Setup
for an Embedded I-MSM PD in a
Polymer Waveguide
Electrical Impulse Response for an
I-MSM PD Embedded in a Polymer
Waveguide
Integration Structure (5.3 mm BCB/PD/ 1 mm BCB/3 mm SiO2/Si)
Electrical Pulse Response: 16.73 ps (FHWM) at 5V.
Size of detection area = 100 mm by 150 mm
Fully Embedded Optical
Interconnections integrated with
Electrical Interface Circuits
A Si CMOS TIA
Embedded MSM PDs in Polymer
Waveguide with Transimpendace
Amplifiers: Demonstration of
Chip-to-Chip Embedded Optical
Interconnections
Eye Diagram at
100 Mbps
A Commercial
Si-Ge TIA from
Maxim designed
for 2.5 Gbps
operation
2.5 Gbps Optical Interconnect
Chip picture with an
embedded I-MSM PD in a
Polymer Waveguide
Eye diagram at 1 Gbps and 2.5 Gbps
Balanced Optical Signal
Distributions Using Polymer MMI
Couplers
Applications:
 Global Clock Distributions
 Point-to-multipoint Signal Distributions
Infrared Imaging Setup
Fabricated MMI Couplers: 1by 8
and 1 by 16 Signal Splitters
Calculated Outputs Using FD BPM at 1.55 mm
Measured Infrared Outputs 1 by 8 and 1 by 16 MMI
1x4 MMI Coupler With an
Embedded 1x4 MSM Photodetector
Array: Photomicrographs
Embedded
MSM PD
array
Dimension for the MMI region:
160 mm wide, and 7610 mm long
Photoimageable Toray polymer used
to create MMI coupler.
Calculated and Measured Results
for the Fabricated 1x4 MMI coupler
The calculated maximum
power imbalance: 0.17%.
Output waveguides: 20 mm wide
and 20 mm separation (no PDs in
this sample).
DC Characteristics of the Embedded
1x4 PD Array
Device #4
Device #3
Device #1
Device #2
Measured Electrical Response
From Each Embedded PD
 Excellent balance from the MMI coupler
 Very uniform embedded MSM PD response
Thin Film InP/InGaAsP MQW
Gain-Guided Lasers
Thin film InP/InGaAsP
(5 mm thick) MQW laser
300 µm
width
Cleaved laser facet
20 µm stripe
p-contact
400 µm cavity
length
Characterization of Thin Film
InP/InGaAsP MQW Gain-Guided
Lasers
•L-I curve and near field pattern at 40 mA drive current – it lases!
Embedded Point to Point Optical
Interconnections
InGaAsP thin film laser, polymer waveguide, embedded thin film
InGaAs photodetector; test results
Embedded Optical Interconnections
on PWB
Thin film InGaAs-based photodetector (PD)
embedded in an optical waveguide core
High temperature PWB substrate
BCB planarization layer
PSB-K1 waveguide cladding layer
BCB waveguide core layer
Core Layer: BCB
Cladding layer: PSB-K1
Planarization layer: BCB
PWB substrate
PD
Challenges for PWB Compatible
Optical Interconnect Integration

Higher heat resistance PWB substrates
– High temp. waveguide process (~ 250 oC)

Smooth surface to minimize scattering of the
guided lightwave
– Surface roughness [mm order] of PWB may scatter the
guided lightwave, causing propagation loss

Adhesion and delamination of
PWB/cladding/core layers
PWB Materials & Surface
Planarization
Commercially available PWBs
Thickness: 1.6 mm
Woven glass cloth + Impregnated resin
Sufficient heat resistance for waveguide process
High Tg FR-4 with inorganic filler
Cyanate Ester with inorganic filler
Cyanate Ester
Polyphenylene Oxide
2mm
High Tg FR-4
Planarized with BCB layer
2mm
16 mm BCB planarization layer
Surface Profile and Planarization
for PWB
1. Surface Roughness of PWB
Rough surface
Long period(~few mm)
Middle period(~500mm)
Short period(~10mm)
2. Planarization using spun-on BCB (0.2 mm over 500 mm)
Waveguide Materials
Low temp. processing waveguide materials
Cladding: Polysiloxane (PSB-K1, Toray industry)
Refractive index 1.44 at l = 1.3 mm
Low optical loss (less than 0.1 dB/cm @1.3 mm)
Curable at 250 oC in air
Core: Benzocyclobutene (Cyclotene, Dow chemical)
Refractive index 1.54 l = 1.3 mm
Low optical loss (-0.3 dB/cm l = 1.3 mm)
Curable at 250 oC in nitrogen
Effects of Planarization
Typical surface profile before & after planarization
Waveguide loss comparison measured by fiber scanning method
High Tg FR4(1)
Non-planarized BCB
Surface roughness
Waveguide loss
Cladding:PSB(12 mm)/Core:
BCB(4 mm)
High Tg FR4(1)
Planarized BCB
Si wafer
+/-0.25[mm]
+/- 0.15
[mm]
+/-0.05
[mm]
1.4 [dB/cm]
0.56
[dB/cm]
0.49
[dB/cm]
Embedded Thin Film I-MSM PD
Embedded thin film MSM PD in waveguide on PWB substrate
Top view
Top view (infra red image)
Waveguide
(100mm)
PD
(150X300mm)
Responsivity of the embedded thin film MSM PD
Measured responsivity :0.64A/W at l = 1.3 mm
DC Characteristics and Impulse
Response
Measured FWHM of the impulse response was 22 psec.
Thus, the demonstrated optical interconnection
with the large (150 X 300 mm2) PD can be used in
high speed interconnection at around 10 Gbps.
Conclusions
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Evanescent field and direct coupling to embedded I-MSM PDs in
optical waveguides have been demonstrated and fully
characterized.
The embedded MSM impulse response was measured. ( FWHM:
16.73 ps for 100 mm by 150 mm detection area)
The eye-diagram of the embedded PD has been measured at 2.5
Gbps.
Balanced optical signal distributions have been demonstrated
using MMI couplers with embedded PD array.
Embedded Thin film edge emitting lasers in polymer waveguide
for optical interconnections have been demonstrated.
Optical interconnections using thin film embedded PDs and
polymer waveguides on PWB substrates have been demonstrated.
(Measured FWHM of electrical response: 22 ps for 150 mm by 300
mm detection area).