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openEBL Fabrication run – Thermo-Optic Phase Shifters

(link to slides)

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Introduction

  • Need for tuning the optical phase
    • Tuning the optical phase is a critical requirement for photonic integrated circuits
    • Adjusting the phase in an interferometer allows for:
      • Compensating for fabrication variations and circuit temperature changes
      • Switches: provides a control signal to change from 0% to 100% transmission
      • Modulators: adjusting for the operating point bias
    • Adjusting the resonator round-trip phase of a resonator adjusts the central wavelength
      • Ring resonator filter, Bragg grating filter, etc.
  • Applications
    • Modulators, optical switches, reconfigurable circuits, tunable circuits, optical neural networks, quantum processors, sensors, etc.

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Overview

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Thermo-Optic Phase Shifters: Introduction

  • Different types of optical phase shifters
    • Thermo-optic heaters
    • Plasma dispersion
      • Carrier depletion, PN junction
      • Carrier injection, PIN junction
    • Emerging materials
  • Ideal optical phase shifter
    • Low loss
    • “Pure” phase modulation without a modulation of the amplitude
      • Low phase-dependant loss
    • High speed
  • Thermo-optic phase shifters are the only device in standard silicon photonics platforms that satisfy the low loss & “pure” phase modulation requirements.

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Overview

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Thermo-Optic Phase Shifters:�Operating Principle

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Operating Principles

  • Device that produces an optical phase shift, in response to an electrical input
  • Operating principle:
    • Resistor: Current input, power dissipated (heat) in the region in or near the waveguide
    • Heat: causes an increase in the temperature of the waveguide (∆T)
    • ∆T changes index of refraction of the materials, ∆n, via the thermo-optic coefficient
      • silicon:
    • ∆n changes waveguide effective index, ∆neff.
    • Light travelling down the waveguide experiences a phase shift
    • Phase shift in an interferometer causes the optical spectrum to shift, or the transmission amplitude to change for a given wavelength

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Overview

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Thermo-Optic Phase Shifters:�Performance metrics

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Performance metrics

  • Performance parameters
    • Efficiency (power for a π phase shift)
      • Example, 40 mW / π, for ANT reference design
    • Electrical: Voltage, current, resistance
      • Adjustable by designing the resistance
      • Example, 40 mW with 500 Ω heater: 8.9 mA and 4.5 V
    • Speed (rise and fall time)
      • Example, 10 µs, for ANT reference design

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Thermo-Optic Phase Shifters: Performance metrics

  • Physical parameters
    • Waveguide dimensions, e.g., 500 x 220 nm
    • Heater dimensions
      • Example, 500 µm long x 4 µm wide x 200 nm thick
      • Thickness of dielectric between the waveguide and heater, e.g., 2.2 µm (ANT)
  • Resistance
    • Resistance of the heater
      • With a sheet resistance of 3.04 Ω/sq (ANT)
      • Will have a resistance of 500 / 4 x 3.04 Ω = 380 Ω

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Overview

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Thermo-Optic Phase Shifters:�Implementations

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Implementations

  • All thermo-optic phase shifters are based on heating a waveguide
    • Silicon has a strong thermo-optic effect
    • Oxide and SiN have much weaker effects
  • Different approaches
    • Metal resistive heater on top
    • Metal resistive heater on the side
    • Doped silicon resistive heater on the side of the waveguide
    • Doped silicon resistive heater inside the waveguide
  • Example

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Overview

Oxide

Si Waveguide

TiW metal heater

Si handle wafer

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Thermo-Optic Phase Shifters: Implementations

  • Opportunities for optimization of the design
    • Multiple dissimilar waveguides running under a single heater
      • e.g., Ref. K. Murray (2015) https://doi.org/10.1364/OE.23.019575

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Overview

Oxide

Si

TiW metal heater

Si handle wafer

Si

Si

Si

Si

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Thermo-Optic Phase Shifters:�Device Simulation

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Device Simulation

  • Approach for simulating a thermo-optic phase shifter
    • Heat equation solver (Matlab, Lumerical, Python, etc)
      • Geometry
      • Material properties
      • Input electrical power
    • Map temperature onto index of refraction change in waveguide materials
    • Solve the waveguide mode (effective index)
    • Calculate the phase shift versus power
  • Solutions:
    • Steady-state response
    • Transient: heating up, cooling down

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Overview

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Thermo-Optic Phase Shifters: Device Simulation

  • In Matlab
    • See https://github.com/lukasc-ubc/SiliconPhotonicsDesign
    • Ch6 / Thermal_waveguide.m; “Silicon Photonics Design” textbook, Chapter 6
    • 2D solution – easiest way, very fast – assumes an effectively infinite structure

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Overview

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Thermo-Optic Phase Shifters: Device Simulation

  • Thermal heat transfer equation
  • 2D solution – easiest way, very fast
    • Approximation since it ignores thermal diffusion in 3rd dimension, e.g., heat being removed by the metal electrodes. Ok for long phase shifter (e.g., > 300 µm).
  • 2D Heat solver simulation in Python

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Overview

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Thermo-Optic Phase Shifters: Device Simulation

  • In Lumerical DEVICE and MODE:
  • Device
    • Materials
    • Geometry
    • Boundary conditions
    • Sources
    • Monitors
  • Mode
    • Material import
    • Effective index

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Overview

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Thermo-Optic Phase Shifters:�Compact model

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Compact model

  • Steady-state model
    • Could be extended to include transient response
  • Geometry parameters
    • Length
    • Assume all others are fixed
  • Optical response: Phase shift
    • ∆n vs. voltage or current
  • Electrical response
    • resistance
  • Example model (for ANT process)
    • Configured for TE 1550 with 500 nm Si waveguide
  • Suitable for implementation in different circuit solver tools

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Overview

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Thermo-Optic Phase Shifters:�Circuits

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Circuits

  • Mach-Zehnder switch 2x2
  • Analytic model
  • Lumerical Interconnect

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Overview

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Thermo-Optic Phase Shifters:�Fabrication process

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Fabrication process

  • Applied Nanotools (ANT)
    • Silicon waveguide layer
    • Oxide cladding
    • Tri-layer metalization
      • Heater metal
      • Routing metal
  • Details at https://www.appliednt.com/nanosoi/sys/resources/specs/#metal-tri
  • Example Mach Zehnder results from ANT:

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Overview

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Thermo-Optic Phase Shifters:�Layout Cells

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KLayout, SiEPIC-Tools, SiEPIC-EBeam-PDK

Slides that describe how to use KLayout and SiEPIC-Tools to create layouts:

https://docs.google.com/presentation/d/1MzOfMpBw1HopLr0Gsot7FymhEeohdGu5fkcXUJyoRsY

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Thermo-Optic Phase Shifters: Layout Cells

  • Thermo-optic phase shifter parameterized cell (PCell)
    • Waveguide
      • Length
      • Waveguide: choose from waveguides defined in Waveguides.XML
    • Heater
      • metal width, technology layer
    • Metal ports:
      • Locations, widths, technology layer

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Overview

Metal port

Metal port

Waveguide port

Waveguide port

SiEPIC-EBeam-PDK: EBeam library, wg_heater cell

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Thermo-Optic Phase Shifters: Layout Cells

  • Thermo-optic phase shifter parameterized cell (PCell)
    • Waveguide
      • Length
      • Width: Singlemode versus low-loss multimode waveguide
    • Heater
      • metal width, technology layer
    • Metal ports:
      • Locations, widths, technology layer

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Overview

Metal port

Metal port

Waveguide port

Waveguide port

SiEPIC-EBeam-PDK: EBeam library, wg_heater cell

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Thermo-Optic Phase Shifters:�Circuit Layout

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Circuit Layout

  • Example layouts
    • Please look in the EBeam PDK, folder Examples / thermo-optic.
    • EBeam_MZIwithHeater
    • EBeam_2StageMZIwithHeater
    • BraggGratingwithHeater

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Overview

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Thermo-Optic Phase Shifters: Circuit Layout

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MZI with one heater

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Thermo-Optic Phase Shifters: Circuit Layout

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Two-stage MZI with two heaters

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Thermo-Optic Phase Shifters: Circuit Layout

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Bragg grating with a heater

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Thermo-Optic Phase Shifters: Circuit Layout

  • Mach-Zehnder switch 2x2
    • With metal pads, grating couplers
  • Example measurement data
    • IV
    • Pout vs. voltage,
    • Pout vs. power
    • Optical spectrum, for two different electrical power values
  • Extraction of performance parameters from data
    • Resistance
    • Vπ
    • Pπ

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Overview

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Thermo-Optic Phase Shifters:�Design for Test rules

Dr. Lukas Chrostowski

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Thermo-Optic Phase Shifters: Design for Test rules

  • Probe station design rules
    • “opt_in” optical input labels
    • “elec” electrical input/output labels
    • Distances, GC pitch 127 µm, DC pitch 125 µm, etc.
  • Diagram of experimental setup, with example layout
    • XYZ chip stage, optical array, DC array, laser/detector, Source-meter, PC
  • Generating testing parameters / Creating Test Routines
    • Details on SiEPICLab software, install, usage, upload
    • Each user can create a custom test sequence for every device

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Overview

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Grating Couplers

  • Please use the Black Box grating couplers. These will be replaced prior to submission for manufacturing.
  • Library “EBeam”
  • O-Band 1310 nm:
    • TE polarization: GC_TE_1310_8degOxide_BB
    • TM polarization: GC_TM_1310_8degOxide_BB
  • C-Band 1550 nm:
    • TE polarization: GC_TE_1550_8degOxide_BB
    • TM polarization: GC_TM_1550_8degOxide_BB
  • Facing right, 127 µm pitch

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127 µm

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Labels for Grating Couplers

  • Optical labels should be placed at the tip of the grating coupler to be connected to the laser.
  • Labels should follow the format:�opt_in_<polarization>_<wavelength>_device_ <unique-device-id>
  • Each electro-optic device should have an “id” which is unique from any other on the same chip.
    • Illegal characters: underscore (_) and comma (,)

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Labels for Bond Pads

  • Electrical labels should be placed at the center of top bond pad
  • Labels should follow the format �elec_<unique-device-id>
  • <unique-device-id> must match the ID used on the optical IO to allow the software to associate the two (optical and electrical) ports together for the same device.
  • Illegal characters: underscore (_) and comma (,)

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Designing for Measurement

  • Bond pads should be 75µm by 75µm and be on a 125 µm pitch.
  • There should be at least two bond pads and the top-most pad should be ground.
  • Ensure that all bond pads for a device are to the right of the associated grating couplers (facing East) and that there is at least 300 µm between grating couplers and bond pads.
  • Pads should be less than 8000 µm from the associated grating couplers.
  • Grating couplers should be on a 127µm pitch.
  • No metal with 100 µm of the grating couplers (to avoid damaging the optical fibre array in case the metal burns)
  • Two independent electrical signals

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G1

125 µm

S1

G2

127 µm

>100 µm

>300 µm

>75 µm

S2

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Experimental Setup

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Camera

Fibre Array

Swept Tunable Laser

& Detector

XYZ Positioner

2x Source-Measure Unit (SMU)

Multi-Contact Wedge Probe

Wedge Probe Micropositioner Arm

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SiEPICLab

  • SiEPICLab: A python suite to enable automated electro-optic testing using motorized automated probe station.
  • https://github.com/SiEPIC/SiEPIClab

PC + SiEPICLab

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Instruments

Probe station

Instrument Drivers

Motor drivers

Instrument Control

Probe station movement

Optical Fine align

GDS-Motor coordinate mapping

Text measurement coordinates parsing

Measurement visualization and save

Automated measurement

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Installation

Steps:

  • You will need Python
    • E.g., Conda, VSCode, etc.
  • In the terminal:
    • pip install SiEPIC_TestCreator
    • SiEPIC_TestCreator –gui
  • A GUI should appear and you can define your test parameters
  • Step by step details at https://docs.google.com/presentation/d/1HXdqyqy5vN7JVm1cSUt2AW7CwgPD6p_se1ajgKRmObs

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Thermo-Optic Phase Shifters:�Layout Submission

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Space allocation

  • Chip allocation per design file
    • 470 µm height x 440 µm width
    • Merging performs cropping to maximum specified dimensions
    • Multiple design files per designer, unique filenames
  • Submission procedure
    • Keep the same file, and upload again when making changes

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Submission, merge aggregation

  • Filename requirements:
    • Layout, either
      • EBeam_heaters_<USERNAME>.oas, or .gds – layout binary file
      • EBeam_heaters_<USERNAME>.py – layout Python file
    • Test sequence:
      • EBeam_heaters_xxxx.yaml – test parameters file
  • Submit your design to the following repository:
  • Note:
    • the blackbox cells will be replaced before the layout is sent for manufacturing

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Overview

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Tested Example

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Example design

  • EBeam_amantejr_thermo_MZI.gds
    • Layout and measurement data
  • Single MZI test structures
    • ∆L = 356.474 µm
      • Features a ~97 µm long phase shifter
    • ∆L = 127.717 µm
      • Features a ~20 µm long phase shifter
  • Activity:
    • Analyze the experimental data
    • Plot MZI transmission spectrum for different voltages (0, 1, 2, 3 V)
    • Plot phase shift versus voltage

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Overview

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