D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

September 19th 2012 – Oxford, UKTopical Workshop on Electronics for Particle Physics – TWEPP 2012

David Stoppastoppa@fbk.eu

Fondazione Bruno Kessler (FBK)Center for Scientific and Technological Research

Trento, Italy

Emerging Research Topics in Advanced Solid-State Image Sensors

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Outline

• Image Sensor Evolution

• CMOS Image Sensor Technology and SoA

• Conventional Imagers: solution to any problem?

• Emerging Research Topics:

• Time-Resolved Imaging: Single-Photon

• Time-Resolved Imaging: 3D Imaging

• Multispectral Imaging: Terahertz

• Conclusions and future perspective

Part 1

Part 2

Part 3

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Image Sensors History

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Evolution of “Image Sensors”

500BCE - 1816 1816 - 1900

No Storing Era!

Niepce Camera

Eastman “Kodak”

Film Storing

Camera Mass Production

Leica (1925)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

First (Electronic) Image Sensors• 1966 – Phototransitor array (Westinghouse)

• 1969 – CCD, Smith and Boyle (BellLabs)

• 1974 – 512x320 CCD imager (Sony)

• 1983 – 1Mpixel CCD camera (TI)

• 1985 – Color array (Hitachi)

• 1987 – CCD Broadcast camera (NEC)

• 1990 – Passive pixel array (Univ. of Edinburgh)

• 1996 – 1Mpixel array (AT&T, JPL)

• 1997 – CMOS Active Pixel

• 1996 – 66Mpixel CCD (Philips)

• 2002 – 14Mpixel CMOS (FillFactory)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CMOS Imaging Revolution

In 2008 More MPC than Films: Digital Imaging Era

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CIS Technology and State-of-the-Art

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CMOS Image Sensor Technology

Pinned photodiode:• 1/10 dark current• Integration capacitance is small (floating diffusion)• Correlated-Double-Sampling -> no more kT/C• Sharing of in-pixel electronics

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CMOS Image Sensor Technology

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CMOS Image Sensors SoA

Year

Fea

ture

Siz

e [u

m] Pixel Pitch

CIS Technology Node

Logic Gate Length

S.-H. Hwang – Samsung

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

• 65nm CMOS-CIS, Pinned photodiode:

Pixel pitch <1.1um

Global shutter, DR>80dB

• Extra pixel-level circuitry (8um pitch):

Rolling shutter, DR>140dB

• In-pixel Buried SF, High-Gain Column Amplifier and CMS:

PN<0.7e

• Special Column-level ADCs:

UHDTV, 33Mpixel@120fps

This is what we can rely on…

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

“Conventional” Imagers:Solution to any problem?

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Lifetime Imaging

Source: Becker&Hickl Website

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

PET/MRI Scanners•Detector

A scintillator crystal converts the incoming gamma-rays into visiblephotons;

Photon “shower” hits the sensor spread in space, close in time;

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

3D Imaging

Capture for each point of the scene not (only) the intensity but the distance from the sensor

• We live in a three-dimensional world• We have 3D perception

2D

f(x;y)=Intensity f(x;y)=(Intensity, Distance)

3D

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

3D Imaging: ToF

TargetD

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

We need image sensors with sub-nanosecond time resolution (all), and single-photon

sensitivity (not for 3D)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CMOS Single-Photon Detectors(Part 1)

How to achieve Single-photon sensitivity and sub-nanosecond resolution?

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Photodiode, APD, SPAD

• A SPAD is a photodiode biased beyond its breakdown voltage (Geiger mode)

Photo-multiplication effect allows for

SINGLE PHOTON DETECTION

20

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

SPAD Operation

• Operation Loop:

1. Entering the Geiger region at VB+VE (meta-stable point) 2. Avalanche 3. Quenching 4. Recharging to 1

VE: Excess bias voltage

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

SPAD with a simple pn Junction?

• At the edges (shallow junctions, microplasmas) high electric fields

• Premature breakdown at the sensor periphery

Active area!

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Desirable active area

Key point: guard-ring structure is needed!

SPAD with a simple pn Junction?

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

SPADs in CMOS Technologies

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

GR#1: Low-doping Diffusion

• In Deep-submicron high doping concentration, shallow implants -> High DCR

• Quasi-neutral field region at edges -> Long diffusion tail• Limited scalability • Require HV processA. Rochas at Al., Rev. Sci. Instrum., 2003

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

GR#2: STI and Retrograde NWell

• Suitable for deep-submicron technologies• Compatible with any triple-well process• Non optimal scalability• Excellent DCR performance

J. A. Richardson et al., Photonics Tech. Lett., 2009

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

SPAD-based Imagers: 1. Megaframe Sensor (Digital)2. SPAD with analog readout

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

The MEGAFRAME Projectwww.megaframe.eu

EPFL (E. Charbon), Univ. of Edinburgh (R. Henderson), STMICRO (L. Grant, J. Richardson), Univ. of Pavia (S. Donati), FBK (D. Stoppa)

• Goal: Create a high-speed, CMOS-based FLIM image sensor

• Use a single-photon avalanche diode (SPAD) and a TDC in every pixel– Eliminate scanning, gating/shuttering– Increase frame rate– Decrease exposure time, fit time– Move towards video-rate FLIM

• Recover fill factor losses with microlenses

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pixel Architecture

• 1.2V transistors• Two rings implemented: 50ps and 170ps delay

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

MEGAFRAME Sensors

• 130nm imaging process• 160x128 array of pixels• 50x50um2 pixel• Pixel includes SPAD; 10b,

55ps TDC; 10b memory• Transistors: 45M

MF128 SensorMF32 Sensor

C. Veerappan et al., ISSCC’11D. Stoppa et al., ESSCIRC’09

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

TDC ArchitectureRing fine state

• 1.2V transistors• Two rings implemented: 50ps and 170ps delay

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

On-chip Calibration

• The TDCs are locked to that of an integrated PLL that contains a replica of the TDC ring oscillator

• This provides global process, voltage, & temperature stabilization, when locked to a stable external clock

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Blue laser Red laserTDCs Uniformity

Jitter and Uniformity

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Ref. Array size

TechnologyPixel pitch

Fill Factor

In-pixel circuit

[1] 128x128 0.35um HV 25um 6%Inverter + active quenching

transistors (TDC at column level)

[2] 60x48 0.35um HV 85um 0.5% 2 gated counters

[3] 128x96 130nm CIS 44.6um 3.2%Time-Of Flight extraction

circuit

[4] 160x128 130nm CIS 50um 1% Time-to-Digital Converter

[5] 3x3 90nm CIS 5um 12.5%Inverter + passive

quenching (3T)

[1] C. Niclass et al., ISSCC 2008[2] C. Niclass et al., ESSCIRC 2008[3] R. Walker et al., ISSCC 2011[4] C. Veerappan et al., ISSCC 2011[5] R.K. Henderson et al., IEDM 2010

Fill Factor Issue with SPAD Sensors

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

SPAD Quenchingcircuit

Gate Counter

Output: number of counts inside the observationtime window

Inputphotons

All n-MOS 12-transistor pixel

(digital pixel requireshundreds of transistors)

Analog pixel schematic diagram

Observation window

Analog Approach

L. Pancheri et al. 2011

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Array size: 0.8 x 0.8 mmPixel pitch: 25umFill factor: 20.8%

SPAD Sensor with Analog Readout

L. Pancheri et al. 2011

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pixel output histogram

0

1 23

4

5

6

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0 2 4 6 8 10 12 14 16

Sig

nal [

V]

Time delay [ns]

1.1ns

2.7ns4.4nsGate width FWHM:

Time-gating Performance

SPAD Sensor with Analog Readout

L. Pancheri et al. 2011

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Time-Resolved Compact Pixels for 3D ToF Imaging

(Part 2)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Phase-sensitive light detection

Received Light Echo

Electrical Demodulation Signal

DC Component

LP Filter

G(t) = sin(ωmt)

R(t) = K sin(ωmt – Δφ)

Iph(t)= K/2 [cos(Δφ) – cos(2ωmt – Δφ)]

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Demodulationsignal

Received light

Iph

Cint

ΔVout µ cos(Δφ)

Δφ µ TOF

1x

Demodulation pixel concept

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Demodulating detectors:

• Photogate-based devices

• Pinned photodiode devices

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Basic Photogate Demodulator

VG2 > VPG > VG1

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

VG2 < VPG < VG1

Electron transport speedlimited by diffusion: From Si substrate From PG to D1 and D2

Basic Photogate Demodulator

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Electron diffusion time

n

2DIFF

DIFF D

Xt

Few microns for high frequency modulation

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pixel electronics: basic readout

• 1-tap pixel: 3T readout• Compact - high fill factor• Readout of 4 sequential frames is needed

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pinned photodiode demodulator

• Available in CIS processes

• 100% contrast in DC• Small bandwidth due

to:– Lateral diffusion– Residual potential

barrier between PD and FD

V. Berezin, et al., US Patent 2003/0213984A1, 2003D. Stoppa, et al., Proc. ISSCC, 2010

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pixel scaling

• Small pixel size increases device BW• Larger pixel size recovered by binning

Ref. Pixel pitch [μm]

Binning Contrast [%]

Mod. Frequency [MHz]

[1] 12 1 35 5

[2] 6 2x2 n.a. 10

[3] 3.65 4x4 52.8 20

[1] S.-J. Kim, et al., IEEE Electron Dev. Lett., 2010 [2] S.-J. Kim, et al., Proc. VLSI Symp., 2011[3] S.-J. Kim, et al., Proc. ISSCC, 2012

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pixel size and resolution

QVGA

VGA

[1] R. Lange, IEEE J. Quantum Electron., 2001[2] T. Oggier et al., Proc. SPIE, 2004[3] T. Möller et al., Proc.1st Range Imaging

Research Day at ETH, 2005[4] S. Kawahito et al., IEEE Sensors J., 2007[5] L. Pancheri et al., Proc. SPIE 2010

[6] S.J. Kim et al., Proc. VLSI Symp., 2011[7] L. Pancheri et al., Proc. ISSCC 2012[8] S.J. Kim et al., Proc. ISSCC 2012[9] W. Kim et al., Proc. ISSCC 2012

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

“Imaging Waves”

Terahertz Radiation Detectors(Part 3)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

The THz GapX Ray UV VIS IR uW, RF Radio

100um 1mm

ElectronicsOptics

H. Sherry et al., ISSCC’12

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

How to Detect THz?

• CMOS QE<0.001% THz Radiation

Micrometer Antenna

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Antenna…and then?

fT<300GHz for CMOS

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Solution 1: Antenna+uBolometer

THz Radiation

Antenna

Load

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

MUTIVIS project

Multispectral Terahertz, Infrared and Visible Imaging and Spectroscopy

Multispectral

optics

Cam

eracontroller

FP

Asensor

Scene

VisibleInfrared

Terahertz

Tunable narrow band Terahertz Source: 0.5-5 THz, =100 GHz

Imaging + spectroscopydemonstrator

THz radiation

Multisp. image

Camera

Source

Airport, train station, etc.

Field test in pilot application airport security

R & D Validation and Field Test

Imaging sensor

THz source

Demonstrator

proof of principle proof of principle development prototype prototype

technologicalmaturity

• BOSCH• CEA-LETI• FBK• Rainbow Photonics• Flughafen Zurich• ETH Zurich

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Pixel Architecture

• Visible: photodiodes in CMOS (400-900nm)• Infrared: bolometers above-IC (8-14m)• THz: antenna + bolometers above-IC (1-3THz)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Sensor Architecture

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Hybrid Sensor Fabrication Steps

CMOS

productio

n

• Wafer production with special finishing• Wafer level testing & die level characterization

BOLO

process

• Additional wafer level steps• Wafer level testing, selection and dicing

Vacuum

packagin

g

• Selection of proper multispectral window• Packaging in standard IR vacuum package

System

integratio

n

• Low-noise supplies & reference• FPGA waveforms generation

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

• Propagation of waves through electron plasma [Dyakonov, Shur]

• Oscillation of electron density– Faster than current due to charge

transport– Resonant detection possible

• Solution of differential equations expressing charge in space and time

s

L

s

Lsh

Tk

qV

TkC

mqLjTk

qV

TkC

mqLjms

qVV

B

eff

BoxB

eff

Box

rfDS

2cos

2exp

21

1

exp2

1

1

422

2

222

20

222

20

2

2

Solution 2: All-CMOS THz Detector

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Fully CMOS THz Pixel

H. Sherry et al., ISSCC’12

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

THz Imager Readout

H. Sherry et al., ISSCC’12

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Fully CMOS THz Camera

H. Sherry et al., ISSCC’12

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Solution 3: Schottky diode

R. Han et al., ISSCC’12

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Conclusive Remarks on SPADs

• There are many applications requiring:

Single-Photon and Time-Resolved Imaging

• CMOS SPADs available in different technologies

• Performance improvements in the last 5 years

• More research groups on the subject

• Pixel pitch is shrinking, fill factor increasing

• New application domains on the horizon (PET, Lab-on-chip)

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Conclusive Remarks on 3D-ToF

• 3D Imagers are becoming more and more popular

• ToF techniques take advantage of:

- Improvements in the lighting industry (LEDs, SS-Lasers)

- CMOS Image Sensors technologies

- Increased interest from big players • Challenges for Portable Devices Market:

- Dramatic reduction of the power consumption

- Color 2D co-integration with 3D

- Outdoor operation

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Conclusive Remarks on THz

• THz Imaging is increasingly popular

• New detectors are under development:

- cost reduction

- reasonable pixel pitch to obtain scannerless imaging

• Still a lot of work to be done:

- lack of THz sources (at reasonable cost)

- difficult to find calibrated testing labs

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Thank you!

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

CIS technology: 1.4um pixels

Front-side CIS (Aptina)R. Fontaine - Chipworks

Back-side CIS (Fujifilm/Toshiba)R. Fontaine - Chipworks

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

3D-TOF with in-pixel PD-SD

R. Walker at Al., ISSCC’11

• Phase Shift is directly measured at pixel-level by event-driven PDSD;

• 128x96-pixel array implemented in 130nm CIS;

• s=160mm @ 2.4m

• Pixel Pitch 44.6um, FF=3%

• First implementation of “real” D-TOF: output data volume greatly reduced

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

ΔVout

Δφ

AMP

OFF

V0

V1

V2

V3

4 samples per pixel are needed

Phase shift measurement

02

13

VV

VVatanΔφ

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Mutivis Architecture

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

Lifetime Imaging Challenges

• Each pixel must be capable of:

- Detect light with Single-Photon Sensitivity - Measure the distribution of photons arrival time:

Range: 1n-100ns; Resolution: <100ps

• Main Issues:

- Pixel dimension and fill factor- Uniformity along the pixel-array- Power consumption- Very high time precision

D. Stoppa – TWEPP’12 – Oxford, UK – 19th September 2012

TOF Image Sensors Challenges

• Light is fast! c=3x108m/s 1cm=>66ps High Time Accuracy

• High Dynamic Range

• High Sensitivity (QE, FF)

• Scannerless 3D Imagers: pixel dimensions, uniformity, power consumption etc.

• Immunity to background illumination