1. 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

2. Outlines 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

3. Motivations Fully Embedded Optical Interconnections
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

4. Thin film edge emitting laser
Proposed Fully Embedded Optical Interconnections Using Thin Film Active OE Devices
Waveguide
Transmitter Circuit
Receiver Circuit
Receiver Circuit
Receiver Circuit
Rec’r / uP
Circuit
Thin film edge emitting laser
Embedded Thin Film PD
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

5. Backgrounds: Inverted Metal-Semiconductor-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

6. Backgrounds: How the I-MSM Works
+5 V
0 V
Incident photons
Electric field line
Lines of equipotential
EHP #1
EHP #2
EHP #3

7. Heterogeneous Integration Process for Embedded Thin Film Devices
Substrate removal using selective wet etching
Mesa etching to stop selective etch layer defines separate devices
Initial device fab on wafer on epilayer surface
Thin film device bonding onto a host substrate or IC using a transfer diaphragm: single device or array of devices

8. Embedded I-MSM in Optical Waveguide Process
Spin coating of waveguide cladding
material and CMP planarization step
Inverted MSM on test probing pad
Spin coating of waveguide
core material
Optical waveguide channel
patterning with dry etch process
Ultem
SiO2
I-MSM
BCB Si

9. Design and Fabrication Issues
Design of Integration Structure
- Optical Signal Coupling
- High Speed Operation
Device Fabrication and Integration Process
- Thermal Stability Issues
- Planarization Issues

10. Optical Signal Couplings for Embedded Structures
Waveguide to Thin Film Photodetector
Evanescent Field Coupling
Direct Coupling
Thin Film Laser to Waveguide
Non-Embedded Coupling
Embedded Coupling

11. Calculation Results for Direct Coupling Structures

12. Calculation Results for Evanescent Field Coupling Structures

13. Temperature Characterization of Embedded I-MSM PDs in Polymer Waveguides
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 I-MSM; measured dark current results using new metallization shown at left as a function of thermal processing time at 250 oC
Measured for 15 hours at 250 oC

14. Characterization of Polymer Optical Waveguide (Fiber Scanning Method)

15. 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)

16. Embedded I-MSM PDs in Polymer Waveguides: Top Views
Embedded Thin-Film I-MSM PD
Electrical Metal Contact Pads
Polymer Waveguide

17. 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%

18. 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%

19. 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%

20. Electrical Impulse Response Setup for an Embedded I-MSM PD in a Polymer Waveguide

21. 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: ps (FHWM) at 5V.
Size of detection area = 100 mm by 150 mm

22. Fully Embedded Optical Interconnections integrated with Electrical Interface Circuits
Eye Diagram at 100 Mbps
A Si CMOS TIA
A Commercial Si-Ge TIA from Maxim designed for 2.5 Gbps operation
Embedded MSM PDs in Polymer Waveguide with Transimpendace Amplifiers: Demonstration of Chip-to-Chip Embedded Optical Interconnections

23. 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

24. Balanced Optical Signal Distributions Using Polymer MMI Couplers
Applications:
Global Clock Distributions
Point-to-multipoint Signal Distributions

25. Infrared Imaging Setup

26. 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

27. 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.

28. Calculated and Measured Results for the Fabricated 1x4 MMI coupler
Output waveguides: 20 mm wide
and 20 mm separation (no PDs in this sample).
The calculated maximum
power imbalance: 0.17%.

29. DC Characteristics of the Embedded 1x4 PD Array
Device #4
Device #3
Device #1
Device #2

30. Measured Electrical Response From Each Embedded PD
Excellent balance from the MMI coupler
Very uniform embedded MSM PD response

31. Thin Film InP/InGaAsP MQW Gain-Guided Lasers
(5 mm thick) MQW laser
300 µm width
Cleaved laser facet
20 µm stripe p-contact
400 µm cavity length

32. Characterization of Thin Film InP/InGaAsP MQW Gain-Guided Lasers
L-I curve and near field pattern at 40 mA drive current – it lases!

33. Embedded Point to Point Optical Interconnections
InGaAsP thin film laser, polymer waveguide, embedded thin film InGaAs photodetector; test results

34. 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
PWB substrate
Planarization layer: BCB
Core Layer: BCB
Cladding layer: PSB-K1 PD

35. 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

36. 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
High Tg FR-4
2mm
Planarized with BCB layer
16 mm BCB planarization layer

37. 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)

38. 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 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

39. Effects of Planarization
Typical surface profile before & after planarization
Waveguide loss comparison measured by fiber scanning method
High Tg FR4(1)
Non-planarized BCB
Planarized BCB
Si wafer
Surface roughness
+/-0.25[mm]
+/ [mm]
+/-0.05 [mm]
Waveguide loss
Cladding:PSB(12 mm)/Core: BCB(4 mm)
1.4 [dB/cm]
0.56 [dB/cm]
0.49 [dB/cm]

40. 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

41. 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.

42. Conclusions
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: 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).