OFC 2018 – Short Course SC454
INTRODUCTION TO SILICON PHOTONICS CIRCUIT DESIGN
Wim Bogaerts Short Course 454 - OFC 2018
1 OFC 2018 – Short Course SC454 WHAT IS SILICON PHOTONICS?
The implementation of high density photonic integrated circuits by means of CMOS process technology in a CMOS fab
Enabling complex optical functionality on a compact chip at low cost
8 OFC 2018 – Short Course SC454 PHOTONIC INTEGRATED CIRCUITS (PIC)
Integration of (many) optical functions on a chip
Source: EECS Berkeley 9 OFC 2018 – Short Course SC454 INDUSTRIAL TAKE-UP EXAMPLES IN TELECOM/DATACOM/DATA CENTERS
• active optical cables (eg PSM4: 4x28 Gb/s on parallel fibers) • WDM transceivers (eg 4 WDM channels x 25 Gb/s on single fiber) • coherent receiver (eg 100 Gb/s PM-QPSK) • fiber-to-the-home bidirectional transceiver (eg 12 x 2.5 Gb/s) • monolithic receiver (eg 16x20Gb/s) • 40Gb/s, 50Gb/s and 100 Gb/s Ethernet (future: 400Gb/s) • …
10 OFC 2018 – Short Course SC454 WHY SILICON PHOTONICS?
Large scale manufacturing Scale
Submicron-scale waveguides
11 OFC 2018 – Short Course SC454 SCALING OPTICAL WAVEGUIDES: INDEX CONTRAST
III-V semiconductors index contrast ~ 10%
1mm
Glass waveguide 1mm index contrast ~0.1%
SOI wire
silicon wire: index contrast ~ 200%
12 OFC 2018 – Short Course SC454 WAVEGUIDES: SILICON PHOTONIC WIRES
High contrast waveguides • submicrometer dimensions 200 nm • small bend radius Si
500 nm SiO2
silicon-oxide
Si substrate
optical mode 13 OFC 2018 – Short Course SC454 HIGH INDEX CONTRAST: A BLESSING AND A CURSE Si Very tight confinement of light
SiO2 Very small bend radii : down to 1 mm [2um box]
Very dense integration of components on a chip
Sub-wavelength design freedom
Photonic crystals with extremely high quality cavities …
Cheben, OE, 2015 14 OFC 2018 – Short Course SC454
HIGHER CONTRAST, SMALLER CORES, TIGHTER BENDS
50mm 4 mm 4
Silica on silicon 0.3mm Contrast ~ 0.01 – 0.1 Indium Phosphide Mode diameter ~ 8µm Bend radius ~ 5mm Contrast ~ 0.2 – 0.5 Size ~ 10 cm2 Mode diameter ~ 2µm Silicon on insulator Bend radius ~ 0.5mm Contrast ~ 1.0 – 2.5 Size ~ 10mm2 Mode diameter ~ 0.4µm 10000 × Bend radius ~ 5µm Size ~ 0.1mm2
15 OFC 2018 – Short Course SC454 HIGH INDEX CONTRAST: A BLESSING AND A CURSE
Every nm3 matters
CMOS technology is the only manufacturing technology with sufficient nm-process control to take advantage of the blessing without suffering from the curse
Si
SiO2
[2um box]
16 OFC 2018 – Short Course SC454 WAVEGUIDES
Waveguide losses dominated by scattering. Use better litho + etch
1µm
sidewall roughness
17 OFC 2018 – Short Course SC454 COMPACT BENDS, TRANSITIONS, CROSSINGS
2µm rib waveguide
3 µm Waveguide Crossing: 97% transmission
wire waveguide 18 OFC 2018 – Short Course SC454 FIBER-TO-CHIP COUPLING
Vertical fiber interface: allows easy on-chip testing
10°
TE
19 OFC 2018 – Short Course SC454 WAVELENGTH FILTERING FUNCTIONS
Light is a wave: interference at the 100nm scale • interferometers • resonators 1 e.g. ring resonator 0.9 0.8 fit: Q = 15600±500 0.7
0.6 Transmission λ 0.5
0.4
0.3
Normalized Normalized transmission 0.2
0.1 Transmission Transmission λ fit: Q = 15400±180 λ 0 1564.5 1565 1565.5 1566 Wavelength [nm] 20 OFC 2018 – Short Course SC454 WAVELENGTH FILTERING FUNCTIONS
Arrayed waveguide grating
0 output star coupler: different phase delays create -5 a phase front focussing into different output waveguides -10
-15 dispersive delay lines:
-20 each wavelength feels a different phase delay -25
-30 input star coupler: light is distributed -35 over many delay lines
-40 1545 1550 1555 1560 1565 1570 1575 1580
21 OFC 2018 – Short Course SC454 SENSITIVITY OF SILICON PHOTONICS WAVELENGTH FILTERS Silicon photonic waveguides are sensitive to • geometry • stress • temperature • … 휕λ ≈ 1 푛푚Τ 휕푤 푛푚 wavelength wire width 휕λ w ≈ 2 푛푚Τ 1nm ~20K 휕ℎ 푛푚 h Si wire height 1nm ~10K 휕λ SiO2 ≈ 0.08 푛푚ൗ 휕푇 퐾
temperature 22 OFC 2018 – Short Course SC454 INTEGRATED HEATERS FOR CONTROL
Different types of electrical resistors: Copper Tungsten metal, silicide, doped silicon traces pillars
oxide oxide Optional undercut to lower reduce thermal leakage. silicon resistor oxide
0.000006 silicon waveguide Measured Optical Power 0.000005 Optical Power Fit
0.000004 undercut region 1.5mW/휋 0.000003
0.000002 Optical Power (mW) Power Optical
0.000001
0 0 0.5 1 1.5 2 2.5 3 3.5 Electric Power (mW) 23 OFC 2018 – Short Course SC454 ELECTRO-OPTIC EFFECT IN SILICON: INJECTION VS. DEPLETION
Carrier injection p n — p-i-n diode in forward bias — Inject carriers into waveguides p n — Strong effect (many carriers) — Slow effect (~1GHz) Carrier depletion — p-n diode in reverse bias p n — Extract carriers from waveguide — Weaker effect p n — Fast effect (>40GHz) Carrier accumulation — Accumulation at oxide n p — Similar to capacitor — Fast
24 OFC 2018 – Short Course SC454 ELECTRICAL SIGNAL MODULATION 20µm
Add doped junction to silicon waveguide: modulate refractive index 1mm • travelling wave modulator • ring resonator modulator
metal p n
metal
waveguide
25 OFC 2018 – Short Course SC454 GERMANIUM ELECTRO-ABSORPTION MODULATOR
Advantages
▪Uses existing Ge-detector technology ▪Compact, optical BW > 35nm ▪3dB BW > 40GHz Next steps
▪Ge → SiGe to reach C-band
Gupta, S., et al. 50GHz Ge Waveguide Electro-Absorption Modulator Integrated in a 220nm SOI ▪Ge-laser Photonics Platform. In Optical Fiber Communication Conference (Vol. 1, pp. 5–7) 2015.
26 OFC 2018 – Short Course SC454 FAST AND EFFICIENT PHOTODETECTORS
Integrated Germanium Photodetectors
— 1 A/W responsivity — > 70 GHz bandwidth — 3nA dark current — 1 V operation
27 OFC 2018 – Short Course SC454 INTEGRATION ON WAFER SCALE
wavelength filtering signal modulation light transport
detection
Compatible with CMOS processing
28 OFC 2018 – Short Course SC454 ALL PHOTONIC FUNCTIONS ARE THERE…
light source
…except for the laser.
30 OFC 2018 – Short Course SC454 LASER INTEGRATION ON SILICON PHOTONICS
Transfer III-V laser material to the silicon Photonic chip
III-V wafer
DVS-BCB
SiO2 SiO2 Silicon waveguide
Buried oxide
31 OFC 2018 – Short Course SC454 LASER INTEGRATION ON SILICON PHOTONICS Different laser types DFB, DBR, microdisk, external cavity, multiwavelength, modelocked, …
32 OFC 2018 – Short Course SC454 SILICON PHOTONICS ENABLES LARGE SCALE PHOTONICS
>10000 optical functions on a chip optical guiding, filtering, detection and modulation efficient fiber-chip coupling external or integrated light sources
33 OFC 2018 – Short Course SC454 MORE THAN JUST PHOTONS
Silicon photonics goes beyond the optical chip software configuration
1000s electronic feedback loops
1000s electrical IOs
100s optical IOs
10s RF signals 10000s optical elements
34 OFC 2018 – Short Course SC454 THE PHOTONIC CHIP IS JUST A PART OF THE SYSTEM
integrated package
analog digital software user electronics photonics electronics
35 OFC 2018 –fiberShort Course arrays SC454 PACKAGING TECHNOLOGY
• Combining photonics and electronics • Fiber interfaces • RF connections • Thermal and mechanical multi-core fibers
shielded packages
36 OFC 2018 – Short Course SC454 FABLESS SILICON PHOTONICS
Many fabless Silicon Photonics companies have emerged • from direct collaboration with fabs (Luxtera, ...) • starting from MPW (Caliopa, Genalyte, Acacia)
Established players are also partnering • e.g. Finisar with ST • Many keep their fab a secret
How to enter as a new (fabless) startup?
37 OFC 2018 – Short Course SC454 SMALL BUILDING BLOCKS → LARGE CIRCUITS
μm-scale building blocks thousands – millions components cm-scale chips
Photonics Very Large Scale Integration (VLSI)
38 OFC 2018 – Short Course SC454 COMPLEXITY AS AN ENABLER Integrated Electronics • billions of digital gates: unprecedented logic performance • millions of analog transistors: unprecedented control — (even with imperfect components: enabled by design!)
More More More elements complexity functionality
Integrated Photonics (Silicon Photonics) • technological potential of 10000+ photonic elements on a chip • not even scratched the surface of what this could do
39 OFC 2018 – Short Course SC454
PHOTONIC CIRCUIT DESIGN
40 OFC 2018 – Short Course SC454 ENABLING COMPLEXITY IN PHOTONICS
Industrial PIC technology platforms (Si, InP, …) • demonstrations of sensors, spectrometers, … • commercial products
But: fairly simple circuits ~ 1970s ICs
More complexity is enabled by design methods • Design capture: translating ideas to circuits • Circuit simulation (electrical+photonic) • Variability analysis on circuits • Yield prediction and improvement
41 OFC 2018 – Short Course SC454 COMPLEX CIRCUITS ≠ COMPLICATED BUILDING BLOCKS You can do a lot with a few building blocks
Electronics: Transistors, Resistors, Diodes, … Photonics: Waveguides, Directional couplers, …
Complexity emerges from connectivity
But you need to support complexity - Accurate models - Variability - Parasitics
42 OFC 2018 – Short Course SC454 DESIGNING PHOTONIC INTEGRATED CIRCUITS
Can we learn from electronic ICs?
• Millions of analog transistors • Billions of digital transistors • Power, timing and yield • First time right designs
• Very mature Electronic Design Automation (EDA) tools! • A well established design flow
Can we repurpose this for photonics?
43 OFC 2018 – Short Course SC454 DESIGN ENVIRONMENTS ARE EMERGING
Combinations of Photonics Design and EDA Physical simulation combined with circuit design Physical and functional verification First PDKs with basic models
44 OFC 2018 – Short Course SC454 WHAT IS A DESIGN FLOW?
Design is the creation of a plan or “ convention for the construction of an object or a system ” Design Flow
a repeatable pattern of activity, “usually involving multiple tasks with a specific set of outcomes”
45 OFC 2018 – Short Course SC454 WHAT IS THE PURPOSE OF A DESIGN FLOW?
idea/ a working chip concept
to translate an idea into a WORKING chip.
46 OFC 2018 – Short Course SC454 A TYPICAL DESIGN CYCLE
idea/ design simulate design check verify fabricate test concept function function layout design design device rules function
Front-End Back-End
design flow time
49 OFC 2018 – Short Course SC454 A GREAT IDEA?
idea/ design Questionssimulate todesign be askedcheck verify fabricate test concept function function layout design design device • What should my devicerules do?function • What are its operational principles? • How well should it perform? • Where/How will it be used?
To go from an idea to a design, you need
SPECIFICATIONS
design flow time
52 OFC 2018 – Short Course SC454 DESIGN CAPTURE AND SIMULATION Capture design intent idea/ design simulate design check inverify a functionalfabricate descriptiontest concept function function layout design design device rules •functionunderlying equations • behavioral models • flow of information
This typically results in a schematic circuit
design flow time
53 OFC 2018 – Short Course SC454 DESIGN CAPTURE
Select/construct functional blocks Connect them together
• Netlist: list of connections (“Nets”) and which components the nets are attached to. • Schematic: arm1 graphical representation of
a netlist, with placements dc dc
Example: arm2 Mach Zehnder Interferometer 54 OFC 2018 – Short Course SC454 SCHEMATIC EDITOR drag and dropping components and drawing connections make waveguides explicit if needed
parametrization component libraries
different connections (waveguides, direct scriptability optical, electrical)
interface to specify circuit simulation I/O ports 55 OFC 2018 – Short Course SC454 HIERARCHY dc
Netlists are hierarchical wg
• Hierarchical cells: ring1
contain another netlist wg wg • Atomic cells: contain a circuit model arm1
dc dc
arm2 Example: Ring-Loaded Mach Zehnder Interferometer 56 OFC 2018 – Short Course SC454 WAVEGUIDES IN PHOTONIC SCHEMATICS
What are waveguides?
• Simple connections between building blocks
• the length and shape does not really matter phase sensitive • it should just provide a good connection (delay in MZI) separate building • similar as an electrical wire block • Functional building blocks with a certain phase/time delay • length and shape are very important just a direct (logical) waveguide • should be treated as a building block connection arm2 link to the output The distinction is important
grating splitter combiner grating coupler coupler
arm1 57 OFC 2018 – Short Course SC454 DESIGN CAPTURE AND SIMULATION Capture design intent idea/ design simulate design check in verifya functionalfabricate descriptiontest concept function function layout design design device rules • functionunderlying equations • behavioral models • flow of information
This typically results in a schematic circuit
Simulated at an abstract level
Optimization: an iterative process
design flow time
58 OFC 2018 – Short Course SC454 COMPONENT SIMULATION ≠ CIRCUIT SIMULATION
Physical models Circuit simulations Accurate, slow Approximate, fast Based on actual geometries Based on functional description Best model = reality
behavioural models
59 OFC 2018 – Short Course SC454 MODELS
Should allow simulation in a larger circuit - based on equations - based on measurement data - based on EM simulations
Photonics: Nothing really standardized • No standardized simulation method • No standard model description • No standard signals
60 OFC 2018 – Short Course SC454 OPTICAL VS. ELECTRICAL CIRCUIT SIMULATION zoom optics = electric… at very high frequency • ultra-small time steps (fs) • ultra-long simulations (1012 time steps) • high-bandwidth signals (200THz)
period ~5fs impractical.
complex envelope Solution: analytic signal • amplitude • phase = complex amplitude on carrier of carrier
61 OFC 2018 – Short Course SC454 PHOTONIC CIRCUIT SIMULATIONS
Same as electronics? No. Photonics does not fit in Spice
Effort-flow systems
Electrical Voltage Current Fluidic Pressure Flow Thermal Temperature Heat Flow* Mechanical Force Motion Photonic? E-field H-field
Not the best formalism for photonics (too high frequency, much more than an RF wave)
62 OFC 2018 – Short Course SC454 WAVE SCATTERING FORMALISM ≠ EFFORT FLOW FORMALISM
electrical and optical is not the same electrical: - nets with a voltage potential - current flowing through terms Vin solve with SPICE
gc_in wg V gnd vertical_in out in 흓 out in ps
ring splitter gc_out out2 out vertical_in optical: in out in - ports out1 out - links with a travelling wave pd - bidirectional in solve with signal propagator out gnd or wave scattering formalism PDout 63 OFC 2018 – Short Course SC454 LINEAR PHOTONICS: SCATTER MATRICES Generalized reflections of a propagating wave
4 Linear coupling between all ‘ports’ • waveguides 3 • modes
푆21 휔
to from frequency (wavelength) 2 dependent 1 S Includes reflection!
64 OFC 2018 – Short Course SC454 WHAT IS A PORT OF A WAVEGUIDE COMPONENT? Orthogonal states • Physically separated waveguides 3 physical • Each mode in the waveguide waveguides
Example: 6 “ports” 6x6 S-matrix
In practice: Only use the relevant modes (rest is “loss”) 2 guided modes
65 OFC 2018 – Short Course SC454 FREQUENCY DOMAIN OPTICAL CIRCUIT SIMULATOR Frequency domain • Linear systems • Described by scatter matrices (S-parameters) Circuit is solved as a single matrix (similar as RF)
Pro: • Very fast • Large circuits
Con: • No nonlinear effects >1000 wavelengths
66 OFC 2018 – Short Course SC454 FREQUENCY DOMAIN SIMULATIONS
Frequency domain simulations are very useful for calculating • Insertion losses • Backreflections • Dispersion (wavelength dependent behavior) • Wavelength filter response and can also be extended to model • Slowly varying effects • Certain optical nonlinearities
67 OFC 2018 – Short Course SC454 TIME DOMAIN OPTICAL CIRCUIT SIMULATION Calculate time response of a circuit • to a stimulus (or combination of excitations) • at certain output monitors • using discrete time steps
Pro: • Fast • Large circuits
Con: • Slower than frequency domain • Only response to specific stimulus excitations circuit monitors
68 OFC 2018 – Short Course SC454 TIME-DOMAIN OPTICAL CIRCUIT SIMULATION
Nodes • connected by signal lines (bidirectional) • an internal state • an algorithm to calculate output from inputs and internal state (differential equations, coupled-mode theory, custom code) every time step, in each node: • Input signals of last time step are read • Internal state is updated • Output signals are generated
in out in out in out in out
표푢푡 푡 = 푓(𝑖푛 푡 − 1 ) N steps delay 69 OFC 2018 – Short Course SC454 OPTICAL SIGNALS two directions An optical link carries an an optical signal… complex number
power phase signal line
wavelength: 2 × 2 × N × M N channels single WDM spectrum
not all simulators TE0 TM0 TE1 support all combinations mode/polarization: M modes
70 OFC 2018 – Short Course SC454 OPTICAL SIGNALS: EXAMPLE two directions Example: Single-λ link • One direction complex number • One wavelength power phase • On-off-keying: power • One mode: TE wavelength: 1 × 1 × 1 × 1 N channels single WDM spectrum
not all simulators TE0 TM0 TE1 support all combinations mode/polarization: M modes
71 OFC 2018 – Short Course SC454 OPTICAL SIGNALS: EXAMPLE two directions Example: WDM bidirectional link • two directions complex number • QPSK modulation: phase power phase • 32 wavelength channels • one mode wavelength: 2 × 2 × 32 × 1 N channels single WDM spectrum
TE0 TM0 TE1 mode/polarization: M modes
72 OFC 2018 – Short Course SC454 OPTICAL SIGNALS: EXAMPLE two directions Example: DWDM multimode link • two directions complex number • QAM64 modulation: phase power phase • 512 wavelength channels • 4 modes wavelength: 2 × 2 × 512 × 4 N channels single WDM spectrum
TE0 TM0 TE1 mode/polarization: M modes
73 OFC 2018 – Short Course SC454 SIMULATING PHOTONICS + ELECTRONICS
Real system: photonics + electronics Example: optical link
transmitter receiver Tx signal in Rx signal out
laser wg pd
74 OFC 2018 – Short Course SC454 SIMULATING PHOTONICS + ELECTRONICS
Circuit has optical and electrical parts: Some components overlap
electrical
Tx
optical Rx
laser wg pd
75 OFC 2018 – Short Course SC454 SIMULATING PHOTONICS + ELECTRONICS
Simulating everything in electrical simulator (SPICE – MNA) • Use native, verified models for electronics • Build Verilog-A models for photonics
Tx
Rx
Verilog-A models
laser wg pd
76 OFC 2018 – Short Course SC454 PHOTONICS IN VERILOGA
Encode time signals as ‘analytical’ signals (complex numbers)
Bus of two lines for bidirectionality
Modulation on an optical wavelength
C. Sorace-Agaskar, OpEx 23(21), 2015 77 OFC 2018 – Short Course SC454 SIMULATING PHOTONICS + ELECTRONICS
Simulate everything in a photonics simulator (Interconnect, Caphe, OptSim) • Optimized models and formalisms for photonics • Electronics models need to be mapped. No verified fab models
Tx
Rx custom models for photonic circuit simulator
laser wg pd
78 OFC 2018 – Short Course SC454 SIMULATING PHOTONICS + ELECTRONICS
Co-simulate with waveform exchange • Photonics and electronics in optimized model, executed sequentially • Output of one simulation = input of next simulation
SPICE simulator Tx
waveform exchange Rx (unidirectional)
Optical simulator
laser wg pd
79 OFC 2018 – Short Course SC454 SIMULATING PHOTONICS + ELECTRONICS
True cosimulation (photonics and electronics in lockstep) • Both photonic and electronic simulators run in parallel • Photonic and electronic model exchange data at each step
SPICE Mixed-signal simulator simulator Tx
full signal exchange Rx (bidirectional)
Optical simulator
laser wg pd
80 OFC 2018 – Short Course SC454 CO-SIMULATION
Optical and electrical co-design in Virtuoso Schematic Photonic simulation in Lumerical Interconnect
A. Farsaei, APC 2016, JTu4A.1 81 OFC 2018 – Short Course SC454 FROM FUNCTION TO LAYOUT
Layout: the patterns used for idea/ design simulate design check verify fabricate test concept function function layout design designfabricatingdevice a chip rules function • Geometric primitives • Placing of components • Connecting components
design flow time
82 OFC 2018 – Short Course SC454 LAYOUT Geometric patterns • Originally drawn by hand • Now drawn by computer • or programmed using scripts
Different layers • correspond to process steps: Mask layers • or to logical operations (e.g. Boolean operations)
Different purposes • Intent of the drawn shape: process, exclusion, annotation, …
83 OFC 2018 – Short Course SC454 LAYOUT: CIRCUITS Organized in (reusable) Cells - placement - transformations
Hierarchy: Cells contain other cells
Routing - Optical connectivity with waveguides - Electrical connectivity with metal wiring - Avoid crossings/shorts/disconnects
84 OFC 2018 – Short Course SC454 PARAMETERIZED CELLS (Or PCells)
Consists of • Parameters — that the user can supply • Evaluators — piece(s) of code that generate the content based on the parameters
Layout, model, symbol, netlist, …
Languages: • Open: Tcl, Python, Ruby • Proprietary: SKILL, Ample, SPT, … 85 OFC 2018 – Short Course SC454 alignment and snapping LAYOUT EDITORS drag and dropping at waveguide ports components
component parametrization libraries
optical and electrical pins
scriptability
interface to verification smart waveguide cells with automatic routing of waveguides (DRC and LVS) bend radius and flaring in long segments and electrical wires 86 OFC 2018 – Short Course SC454 SCHEMATIC DRIVEN LAYOUT (SDL)
Layout: the patterns used for idea/ design simulate design check verify fabricate test concept function function layout design design fabricatingdevice a chip rules function • Geometric primitives • Placing of components • Connecting component
Schematic Driven Layout: Derive information from circuit schematic • Component placement • Component connectivity
design flow time
87 OFC 2018 – Short Course SC454 SCHEMATIC DRIVEN LAYOUT (SDL) Derive the physical layout from the schematic — Generate the Layout (P)Cells — Place the Layout Cells — Connect the layout cells together
Not trivial to fully automate — What is the optimal placement? — Is the topology possible? — Constraints for length matching? — On which layer to route? — Waveguide bends and crossings?
Combination of manual + auto
88 OFC 2018 – Short Course SC454 PLACEMENT AND ROUTING
Photonic-specific constraints • ‘optical length’ and phase control • minimal bend radius • waveguide spacing • matching port direction • single routing layer!
89 OFC 2018 – Short Course SC454 PHOTONIC SDL TOOLS ARE EMERGING
Pure photonics or based on EDA tools
• define connections • place components • route waveguides
Luceda, Phoenix, Mentor Graphics, Cadence …
90 OFC 2018 – Short Course SC454 IS THE LAYOUT VALID?
Design Rule Checking idea/ design simulate design check verify fabricate test concept function function layout design design devicemeets the fabrication rules function rules of the fab?
• minimum features • layer combinations • overlaps • pattern density
design flow time
91 OFC 2018 – Short Course SC454 DESIGN RULE VIOLATIONS: EXAMPLES
92 OFC 2018 – Short Course SC454 PHOTONIC PROBLEMS WITH DRC?
DRC techniques were designed for electronics: 90-degree angles…
Silicon Photonics:
— All-angle waveguides – discretized… — Nanometer scale sensitivities — Arbitrary geometries (e.g. slot waveguides, PhC)
What is bad? What is intentional?
R. Cao, VSLI-SoC 2014 93 OFC 2018 – Short Course SC454 PATTERN DENSITIES Pattern density must be sufficiently uniform — Etch rate control — Avoid CMP dishing
Tiles are added
There must be sufficient room to add tiles — Slab areas (AWG) — Dense waveguide arrays — …
94 OFC 2018 – Short Course SC454 IS THE LAYOUT VALID?
Design Rule Checking idea/ design simulate design check verify fabricate test concept function function layout design design devicemeets the fabrication rules function rules of the fab?
• minimum features • layer combinations • overlaps • pattern density
An iterative process
design flow time
95 OFC 2018 – Short Course SC454 REAL-TIME DRC
Layout is checked on DRC errors as it is being generated Real-time feedback in editor Much faster to a DRC-clean design
96 OFC 2018 – Short Course SC454 FUNCTIONAL VERIFICATION
Does the layout idea/ design simulate design check verify correspondfabricate test to the circuit concept function function layout design design device rules function schematic?
Parasitic effects that were not in the schematic
design flow time
97 OFC 2018 – Short Course SC454 FUNCTIONAL VERIFICATION: LAYOUT VERSUS SCHEMATIC
Check Connectivity Check functionality Are the correct components placed? Did we use the right parameters? Are they properly connected? Does the layout perform the correct function? e.g. does the waveguide have the correct width (i.e. optical length) connected
not connected
engineered crossing not connected
98 OFC 2018 – Short Course SC454 FUNCTIONAL VERIFICATION
idea/ design simulate design check verify fabricate test concept function function layout design design device rules function
design flow time
99 OFC 2018 – Short Course SC454 POST-LAYOUT SIMULATION
Resimulate the circuit based on the actual layout Include lengths, crossings, reflections, …
100 OFC 2018 – Short Course SC454 FABRICATION “no plan survives contact with the enemy” H. von Moltke (misquoted)
idea/ design simulate design check verify fabricate test concept function function layout design design device rules function
design flow time
101 OFC 2018 – Short Course SC454 THE ACTUAL FABRICATION PROCESS
Layer depositions Al
Cu Cu Pattern definition (lithography) W W W W
oxide Pattern transfer (etch) W SiN [100nm] p a-Si [160nm] poly Ge + n n p n+ p Planarization Si + n +
BOX 2000nm Thermal treatment example: IMEC silicon Photonics Doping and implantation … and each step with imperfections and variability
102 OFC 2018 – Short Course SC454 LITHOGRAPHY: NOT PERFECT
Spatial low-pass filter — Minimum feature size — Minimum pitch — Pattern rounding
Example: Bragg grating
P ~ 290nm
103 OFC 2018 – Short Course SC454 OPTICAL PROXIMITY CORRECTIONS (OPC)
Overcome rounding: add OPC — serifs — cutouts
Makes mask hammerhead serif cutout serif more complex (and costly)
Not always possible without violating DR
104 OFC 2018 – Short Course SC454 FABRICATION: IN-LINE DATA
idea/ design simulate design check verify fabricate test concept function function layout design design device rules function
design flow time
105 OFC 2018 – Short Course SC454 IN-LINE PROCESS DATA Collect data from wafers as wg gap -> 160nm they are being processed — Line width — Etch depth
— Layer thickness red points: intentional excursions — …
Feed in design process — FRONT-END: Predict behavioural change — BACK-END: Adjust layout wg width -> 480nm
STATISTICS!
106 OFC 2018 – Short Course SC454 THERE ARE MANY SOURCES OF NON-UNIFORMITY
107 OFC 2018 – Short Course SC454 VARIABILITY: PREDICTING CIRCUIT YIELD
108 OFC 2018 – Short Course SC454 DESCRIBING VARIABILITY AT DIFFERENT LEVELS
w0 h Lring process conditions w1 exposure dose L Pπ resist age device geometry plasma density line width optical device properties slurry composition layer thickness effective index circuit properties … sidewall angle group index optical delay system performance doping profile coupling coefficients path imbalance insertion loss … center wavelength tuning curve crosstalk … … noise figures power consumption …
109 OFC 2018 – Short Course SC454 DIMENSIONAL DEPENDENCE OF A WAVEGUIDE
110 OFC 2018 – Short Course SC454 LEVELS OF VARIABILITY: CAREFUL WITH MAPPING
Geometry: width and thickness Model: neff and ng
w1,t1 => neff1
t2 w ,t => n t1 w1 2 2 eff2 w1,t2 => ng2 w2 w2,t1 => ng1
t ng t2 ng2
t1 ng1
w1 w2 w neff1 neff2 neff2
111 OFC 2018 – Short Course SC454 INTRA-DIE VARIABILITY
Variability has causes with different properties… test locations Optical extraction of linewidth and thickness
Line width Thickness
Weak correlation Strong correlation between neighbours between neighbours
112 OFC 2018 – Short Course SC454 VARIABILITY EFFECTS WORK ON DIFFERENT SCALES intra-die local pattern density layer thickness lithography nonuniformity
distance
time die-to-die exposure dose layer thickness plasma density CMP pattern
lot-to-lot wafer-to-wafer tool drift tool priming resist aging layer thickness wafer supplier 113 OFC 2018 – Short Course SC454 VARIABILITY ≠ VARIABILITY
Wafer – to – wafer variability
Die – to – die variability Thickness map Intra-die variability - mask-related linewidth map - distance related - stochastic
114 OFC 2018 – Short Course SC454 TESTING
Put the device on a measurement idea/ design simulate design check verify fabricate test setup and characterizeconcept functionits performancefunction layout design design device rules function
design flow time
115 OFC 2018 – Short Course SC454 HOW TO TEST?
Electrical, optical, or both? Wafer-scale testing -> grating couplers Testing after packaging? Need statistics? depends on application
116 OFC 2018 – Short Course SC454 CHALLENGE: DEFINING GOOD TESTS
You need to think about tests during the design stage — Which structures are representative? — How can I isolate them? — What parameters do I want to measure? — How will I analyse/fit the data?
Example: waveguide model • 푛푒푓푓 휆 -> polynomial? • loss(휆) -> polynomial? Parameters for your component models! • nonlinearities?
— What makes a good model? How to measure 푛푒푓푓?
117 OFC 2018 – Short Course SC454 OUR SIMPLE DESIGN FLOW
idea/ design simulate design check verify fabricate test concept function function layout design design device rules function
design flow time
118 OFC 2018 – Short Course SC454 OUR SIMPLE DESIGN FLOW
idea/ design simulate design check verify fabricate test concept function function layout design design device rules function
Exchange of Information?
design flow time
120 OFC 2018 – Short Course SC454 EXCHANGE OF INFORMATION Files - Layout: GDSII and OASIS - Netlist/Schematic: Spice, EDIF - Models: Spice, VerilogA, C++, Python - PCell code: Skill, Python , Tcl - Data: Touchstone, XML
Databases - proprietary - EDA standard: OpenAccess
121 OFC 2018 – Short Course SC454 DESIGNING IN CODE VERSUS GUI
Designing in Code Designing in GUI
123 OFC 2018 – Short Course SC454 DESIGNING IN CODE VERSUS GUI
Designing in Code Designing in GUI Pro: Pro: • Easy to reuse • Intuitive quick start • Easy to upgrade design • Visual feedback • Easy to share and version • WYSIWYG • Easy to parametrize • Quick point and click • Easy to document and make examples • Everything is numerically correct • Automate repetitive work Con: • Difficult to make complex things Con: • No calculations • Harder to learn • A lot of manual work • No immediate visual feedback • Easy make small (invisible) mistakes
124 OFC 2018 – Short Course SC454 DESIGNING IN CODE VERSUS GUI
Designing in Code Designing in GUI - parameter sweeps - schematic connectivity - calculated geometries - layout positioning (floorplanning) - circuit models - fixing the last DRC errors - automatic placement and routing - quick manual routing
125 OFC 2018 – Short Course SC454 DESIGN ABSTRACTIONS
System idea/ design simulate design check verify fabricate test design concept function function layout design design device rules function Behavioral simulations Higher Circuit level of design abstraction
Physical simulations Component design design flow time
126 OFC 2018 – Short Course SC454 ABSTRACTIONS IN A CIRCUIT DESIGN FLOW
System idea/ test design concept Behavioral design simulate check function function function simulations
Circuit design check layout rules design
Physical simulations Component design design flow time
127 OFC 2018 – Short Course SC454 ABSTRACTIONS IN A CIRCUIT DESIGN FLOW
System design System Behavioral simulations
Circuit design Circuit
Physical simulations Component Components design design flow time
128 OFC 2018 – Short Course SC454 PDK: INTERFACE FROM FAB TO DESIGNER
component libraries documentation support scripts circuit design and simulation
component design simulation, measurement FAB DESIGNER defining technology and verification rules PDK layout generation and verification
PDK for photonics
129 OFC 2018 – Short Course SC454 SUMMARY (Silicon) Photonics is growing towards a circuit platform • Technology supports larger circuits • A circuit-oriented design flow is emerging (similar to electronics) • Fabs are building PDKs
Challenges • Schematic-driven Layout for photonics • Variability: fabrication, performance, models • Verification: DRC and LVS • Design for manufacturability • Photonic-electronic-software stacks
130 OFC 2018 – Short Course SC454
DESIGN FLOW: FROM IDEA TO WORKING CHIP schematic device capture modeling circuit simulation idea circuit layout
a working chip design team 131 OFC 2018 – Short Course SC454
PRACTICAL SETUP
143 OFC 2018 – Short Course SC454 JUPYTER NOTEBOOKS interactive notebook • text, figures • formulas • python code simulation and design • built-in IPKISS
144 OFC 2018 – Short Course SC454 THE IPKISS DESIGN FRAMEWORK
Design framework for Photonic Integrated Circuits • Parametric design • Focus on reuse and automation
History • Developed at Ghent University – imec in 2000-2014 • Spin-off into Luceda Photonics in 2014 • Currently hundreds of users worldwide
145 146 OFC 2018 – Short Course SC454 THE IPKISS DESIGN FRAMEWORK measurement
circuit layout component layout One component definition
circuit for 3D geometry simulation Circuit design 1 Layout
Simulation circuit model physical model
146 147 OFC 2018 – Short Course SC454 THE IPKISS DESIGN FLOW measurement
Python script based circuit layout component layout
one single circuit component 3D geometry simulation definition
circuit model physical model
147 148 OFC 2018 – Short Course SC454 THE IPKISS DESIGN FLOW
Python script based
extremely flexible easy-to-read powerful engineering libraries industry standard
148 OFC 2018 – Short Course SC454 THE PICAZZO LIBRARY
A large library of photonic components
— waveguides and routing — crossings, splitters and couplers — wavelength filters — grating couplers and mode converters — generic modulator blocks Parametric and technology aware Validated on the IMEC technology platform
149 149 OFC 2018 – Short Course SC454 ADVANCED SPECTRAL FILTER DESIGN
Arrayed Waveguide Gratings Echelle Gratings • Fully parametric • Design from specifications • Integrated layout and simulation • Validated on fabricated devices
Measurement Result Simulation Result
150150 151 OFC 2018 – Short Course SC454 IPKISS NOTEBOOKS Explore your designs in a browser Very rapid experimentation Interactive code and plots Widely supported community
Powered by 151 OFC 2018 – Short Course SC454 FIRST NOTEBOOKS
Unfamiliar with Python?
/0_1_python_getting_started: basic Python tutorial /0_2_ numpy_and_plotting: Numpy and Matplotlib
Check if everything works and if you find your way around the notebook interface.
152 OFC 2018 – Short Course SC454 PRACTICAL 1. Connect WIFI / Ethernet 2. Open web browser (Chrome, Firefox, Opera) 3. Connect to Jupyter server (address will be provided on-site) 4. Log in with your personal ID/password
157 OFC 2018 – Short Course SC454 NOTEBOOK: INTERACTIVE ENVIRONMENT
Text and explanations
Executable python code
SHIFT+ENTER
to execute 158 OFC 2018 – Short Course SC454 NAVIGATING
Click here to go back to start
Create blank Folders notebook here with notebooks
159 OFC 2018 – Short Course SC454 NAVIGATING
Notebook: click to start
Running Notebook
160 OFC 2018 – Short Course SC454 PRESS H FOR ‘HELP’
Useful menu and toolbar
Keyboard shortcuts are extremely powerful
161 OFC 2018 – Short Course SC454 TAKE CARE OF MEMORY
Interactive plots consume resources. Close them when ready.
162 OFC 2018 – Short Course SC454 GETTING STARTED… - connect to the internet - open browser (Chrome, Firefox) - connect to notebook server: https://wsjupyler.intec.ugent.be - notebook login / password
Launch a notebook
Step 1: Copy the notebook
163 OFC 2018 – Short Course SC454 BUILDING CIRCUITS IN A NOTEBOOK Define schematics in python code • List building blocks (or subcircuits) • gc, splitter, wg • List internal connections • gc:out↔splitter:in, splitter:out2↔wg:in • List external ports • in ↔gc:vertical_in, out1 ↔ splitter:out1, out2 ↔wg:out
in out
wg out2 out2 vertical_in out in
out1 in gc splitter
out1 164 OFC 2018 – Short Course SC454 BUILDING CIRCUITS: AUTOPLACEANDCONNECT
Circuits with direct connections: no waveguide generation
out2 in2 dc2
4 components wg2 wg1
4 internal connections in1 dc1 out1
4 input/output ports
automatic placement auto-generate layout 165 OFC 2018 – Short Course SC454 BUILDING CIRCUITS: PLACEANDAUTOROUTE out_cross reflection Generate waveguides for connections vertical_in out out vertical_in dc in2 out2
in1 out1 vertical_in out out vertical_in 5 components in out_bar
4 internal connections
4 input/output ports
manual placement
auto-generate layout 166 OFC 2018 – Short Course SC454 USE HIERARCHY: YOU CAN USE A CIRCUIT AS A BUILDING BLOCK
Circuits can be nested out add Break up circuits into reusable parts
in sink
out add dc3
wg3 wg4 dc1
in
dc2 sink
167 OFC 2018 – Short Course SC454 THE SMALL PRINT ON COPYRIGHT The material on the server is copyrighted • The IPKISS toolset • The addon libraries • The notebooks
Please do not download the material to your own PC. It will probably not work as the server has a specific set of pre-configured utilities.
If you interested in using IPKISS, contact [email protected] If you are interested in using the course material, contact [email protected]
You can continue to use the server until 30 June 2018.
168 Further reading OFC 2018 – Short Course SC454
(invited)
Lasers and Photonics Reviews
170 OFC 2018 – Short Course SC454
Wim Bogaerts
Professor in Silicon Photonics
E [email protected] @PhotonicsUGent T +32 9 264 3324
www.photonics.intec.ugent.be
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