Optical Interconnects: A Viable Solution for Interconnection Beyond - PowerPoint PPT Presentation

Optical Interconnects: A Viable Solution for Interconnection Beyond 10 Gbit/sec Ray T. Chen, Univ. of Texas at Austin On IEEE ISPD Conference 3-20-2007

Introduction: Projection of Bandwidth

Potential Markets for UT’s IP Portfolio  Optical backplane  Optical PC Board  Passive Waveguide Components  Active Optical Components  Optical Biosensor Source IBM

Fully Embedded Board Level Fully Embedded Board Level Optical Interconnection Optical Interconnection  Unique Architecture for Optical PWB (Printed Writing Board) ; All the optical components are interposed inside the PCB Solve the package problem / Reduce Cost Effects Micro-via Cu Trace 45  micro-mirror 1x12 PIN 1x12 VCSEL Photodiode VCSEL array 12-channel Polymer Waveguide [109 cm ] Optical PCB

Lamination of Optical Waveguide Film Lamination of Optical Waveguide Film & Integration of Thin Film VCSEL & Integration of Thin Film VCSEL  12-Channel Polymer Waveguide  Cross Section View & 45 o Micro-Mirror of Laminated Optical Layer Optical Layer (~170  m) 250  m PSA (Pressure Sensitive Adhesive) Film (100 / 200  m) PCB Substrate  Cu Transmission Lines for VCSEL (or PD) Integration VCSEL PCB Sub Optical Layer PSA film Optical layer Top Emitting VCSEL via PCB Sub VCSEL 2 mm - PSA (Pressure Sensitive Adhesive) Film : 100 / 200  m Optical Layer - Optical Waveguide Film Layer = ~ 170  m Cu Trans. Lines (thickness = ~ 10  m) Bottom Emitting VCSEL

Fully Embedded Board Level Optical Interconnection 45  micro-mirror Micro-via Optical PCB Cu Trace Photodiode Waveguide array Cross section view of optical VCSEL PCB array R. T. Chen, et al, Proc. IEEE, 88 , 780-793 (2000).

Fabrication of Flexible Optical Waveguide Film(1)  Physical Dimensions of Waveguide Structures SEM image of No. of channels : 12 45 o Micro-mirror Cross-Section : 50 X 50  m 2 Channel to channel separation : 250  m Total Length : ~ 109 cm Curvatures : 3.68 cm / 1.72 cm

Master Waveguide Structures for Mold Channel waveguides Mirror surface PR Waveguide St. Output Input

Optical Bandwidth Measurement of the 51 cm Long Waveguide The 3-dB optical bandwidth is determined to be 150GHz for the 51cm long waveguide

Polyimide Based 1-to-48 Fanout H-tree Optical Waveguide on Si-Substrate (c) (d)

Optical Signal Distribution in a Network Card Optical Signal Distribution in a Network Card

Substrate Removed 1 X 12 GaAs VCSEL Array  Flatten Optical Layer to Facilitate Embedded Structure  ~ 10  m Thickness VCSEL Formation - Mechanical Lapping : ~ 50  m - Chemical Wet-etching (Citric Acid : H 2 O 2 ) : ~ 10  m X 50 X 50 ~ 10  m 200  m GaAs Sub Original VCSEL Substrate Removed VCSEL (~ 10  m) on GaAs Substrate

Fabrication of Cu Transmission Lines & Gold Stud Bump for Flip Chip Integration  Design of Cu Transmission Lines for Flip Chip Integration Separated 12 N-contact Transmission Lines 250  m 2 mm Common P-contact Transmission Lines Cu Plated Transmission Lines

Integration of VCSEL and PIN Photodiode with Optical Waveguide Film  Photolithography UV-Aligner  UV-Curable Adhesive 1 x 12 PIN 1x12 VCSEL Photodiode 12-channel Polymer Waveguide [109 cm ]

Speed Measurement of Substrate Removed 850 nm Wavelength VCSEL  BER/Q-factor/Jitter RMS  Eye-diagram [ V bias = 2.0 V / I bias = 5.0 mA ] 30 -log[BER] / Q-factor / Jitter RMS 25 Q-factor -log[BER] Jitter RMS 20 15 10 5 V bias = 2.0 V NRZ mode I bias = 5.0 mA PRBS = 2 31 -1 0 2 4 6 8 10 12 Ampl = 0.5 V Jitter RMS = 4.6 ps Frequency [GHz] Offs = 0 V Q-factor = 5.18 Freq.= 10 Gb Eye width = 71.7 ps

Speed Measurement of Substrate Removed VCSEL and PIN Photodiode  Frequency Response  Frequency Response of 850 nm PIN Photodiode of 850 nm VCSEL -30 -15 -18 -33 -21 f -3dB = 10 GHz -36 -24 Response [ dB ] -27 -39 Response [ dB ] -30 -42 -33 -36 -45 -39 -48 -42 1.0 mA VCSEL Output Power 2.0 mA -45 -51 0.1 mW 2.5 mA -48 0.5 mW 3.0 mA -54 -51 1.0 mW 4.0 mA -54 -57 1.5 mW 5.0 mA -57 2.0 mW 5.5 mA -60 -60 0.0 5.0G 10.0G 15.0G 20.0G 0.0 2.0G 4.0G 6.0G 8.0G 10.0G 12.0G 14.0G Frequency [ Hz ] Frequency [ Hz ]

Coupled Field Thermal-Electric Analysis for Heat Generation of Thin Film VCSEL  Material Properties (thermal/electrical)  2-D Modeling of VCSEL & Physical Dimensions for Coupled Field Analysis Electrical Resistivity Thermal Thickness Light Output Conductivity [ohm-um  [um] [W/um-K] Top contact Active Region P-DBR  r  k r = 1.2 x 10 -5 3.30 Bottom  z  k z = 1.0 x 10 -5 Current contact Top Mirror Aperture N-DBR  r  k r = 1.2 x 10 -5 3.497  z  k z = 1.0 x 10 -5 Bottom Mirror GaAs sub.  r  z  k r = k z = 4.5 x 10 -5 10 ~ 200 Au  r  z  x   k r = k z = 3.15 x 10 - 0.2 4 GaAs Substrate Copper  r  z  x   k r = k z = 3.98 x 10 - 0 ~ 200 4 Polymer  r  z   k r = k z = 2.0 x 10 -7 30 ~ 100 Cu Heat Sink (Thermal-Via)  Coupled Field Analysis    2   V 1  V  Thermal Analysis Electric Field Analysis        r  0 - Electrical Potential :   - Joule Heating in DBR z  r   2 - Electrical Potential z r r  r    - Heat Generation - Current Density 2 2 in Active Region    V   V          - Joule Heating In DBR : q z r    z   r 

Experimental & Simulation Results Thermal Resistances(R th ) of Thin Film VCSEL  Temperature Distribution  Thermal Resistances of Thin Film VCSEL inside Thin Film VCSEL as a Function of Substrate Thickness Thermal Resistance [  T/  P diss ] 3000 Simulation 2800 Measured 2600 2400 2200 2000 1800 1600 0 20 40 60 80 100 120 140 160 180 200 220 Substrate Thickness [  m] Simulation & Experimental Parameters Substrate Thinning (200  m 10  m) - Bias Conditions : 5 mA / 2 V - Thickness of VCSEL = 10  m ~ 200  m 40 % Thermal Resistance Reduction - Substrate Cooling Condition by TEC (25 o C)

Effects of Thermal-Via Structures for the Fully Embedded Thin Film VCSEL  Temperature Distribution  Thermal Resistances of Thin Film VCSEL of Thin Film VCSEL with Thermal Via as a Function of Substrate Thickness 5000 o C/W] 4500 VCSEL Polymer layer 4000 Thermal Resistanc [ Closed Thermal-via 3500 3000 Open Blind Via Structure D = 200  m / L = 200  m 2500 D = 200  m / L = 100  m Closed Blind Via Structure D = 200  m / L = 200  m 2000 D = 200  m / L = 100  m 0 20 40 60 80 100 120 140 160 180 200 220 VCSEL Polymer layer Substrate Thickness [  m ] Optimized VCSEL Thickness Open Thermal-via = 44  m ~ 72  m Fabricated via-hole on the optical film layer (D = 200 mm, Aspect ratio = 0.5)

Silicon Thin Film Photonic Crystal Waveguide Modulator

Photonic crystal structure in nature Opal, the best known periodical structure in nature.

Schematic of Fully Embedded External Modulator for Analog Signal Transmission EO Waveguide Modulator Vias CW Laser Diode Driving Electrode

Fully-Embedded Silicon Thin Film Nano- Photonic Crystal Waveguide Modulator • In-plane structure: Photonic crystal waveguide • High dispersion enhances modulation efficiency, up to 100 times

Key Performance Improvement Conventional Proposed Si PCW Improvement Mach-Zehnder Modulator Factor Modulator ~ 4mm ~ 40 um 100 X reduction Size 10X to100X Power ~ 0.3 W ~ 0.01 W consumption reduction No integration Potential for high Integration N/A potential density integration * Conventional Mach-Zehnder modulator performance represents typical specifications.

SEM Micrographs & Key Facilities 3-D Image 2-D Image High smoothness, exact round shape JEOL JBX-6000FS/E E-Beam Nano-Lithography Rough Plama-Therm 790 Si sidewall FEI Strata DB235 without and SiO 2 Reactive Dual Beam SEM/FIB post- Ion Etching (RIE) Focus Ion Beam (FIB) Nano-characterization System etching nano-polished endface oxidation

Photonic Crystal MZI Modulator air-trenches 100  m 100  m PCW PCW electrodes electrodes Y-junction of the MZ modulator, the rib Micrographs of Mach-Zehnder(MZ) waveguide splits as it extends up. Two 4  m modulator: electrodes, pads, and wide air-trenches (etched through the entire photonic crystal waveguides (in upper silicon layer) separate the rib lighter color) waveguides from the surrounding silicon.

Photonic Crystal MZI Modulator - more SEM micrographs Electrodes Electro-pad Electrode Rib waveguide PCW Electrode Electro-pad 6 4 2 Current (mA) 0 -2 -4 -6 -80 -60 -40 -20 0 20 40 60 80 Voltage (V)

High-speed measurement set-up

Switching characteristics : Modulation traces Operating wavelength: λ =1541 nm λ = 1541 nm and I π = 7.1 mA Applied voltage: V on = 2 V, V off = -1 V Modulation depth = 92 % 1 Gbit/sec 29