Recent Progresses of Visible Light Image Sensors

The Detector Seminar at CERN

February 23, 2018

Nobukazu Teranishi University of Hyogo / Shizuoka University

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

- Semiconductor device, which converts light image to electric signal. - Used in cameras.

Examples of applications

Smart- Movie Automobile Endoscope Iris verification Security phone Light Image Sensor Microlens Color filter

- IS sales amount has grown by camera phone. Camcoder PMP - IS spreads into various applications, DSLR Medical Smart glasses - “Others” includes scientific, Compact DSC Broadcast industrial, … Game Automobile Others Surveillance Bpcs PC/WEB camera 4 CMOS image sensor CCD image sensor Tablet 3

2 Camera phone

microlens (ML)

Color filter (CF)

Amplifier (SF)

Sotrage Pinning layer Transfer gate (TG) + P+ N+ + P e- P e- Silicon Ne- P Detective capacitance (Floating diffusion (FD)) Cross-section of frontside illumination type CMOS image sensor pixel

Inner Shell Excitation

Large depletion is needed.

Small surface dead layer is X-ray Near UV needed.

Visible light

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

©2018 N. Teranishi Classification of Dark Current and Blemish 8 Dark current FPN Average: Dark current (Dark current pixel-by-pixel fluctuation) ・Dark current shot noise ・Black level shift by temperature

Micro white blemish (Mid-range) White blemishes (Large-range)

Pixel Counts Pixel 3σ Very large level blemish by radiation damage

・Metals, especially transition metals, form mid-gap levels, which become GR centers. ・Generate dark current based on Shockley-Read-Hall (SRH) process 2 pn  ni U: Recombination Rate U  vthNt  Et  Ei  (Sze: “Semiconductor Devices” n  p  2ni cosh  Chap. 1 Eq.(59))  kT  Assuming the cross sections,  n   p   Conduction band Nt: GR center (trap) density ・Assuming depletion condition, then, n, p = 0, Et and Et=Ei because U becomes the largest; v n N U   th i t (Unit : 1/cm3s) 2 Valence band v n ・One GR center generates U   th i , (Unit : 1/s) Dark current by 2 mid-gap level ©2018 N. Teranishi Metal Contamination (2) 10 400 275 K a a a 20 Frames Two series of specific and periodic 0 1 60 s integration 300 b b b peaks, labeled as “a” and “b”. Poisson distribution 200 Assuming metals are distributed 2 as Poisson distribution, metal 100 Number of counts of Number 3 density per pixel can be derived.

0 0 1000 2000 3000 4000 5000 Signal (e-) σ s are derived from the pitches σ(cｍ2) Density(1/pixel) Density(cm-3) using the previous slide formula. a 1.8E-15 1.25 1.3E9 b 5.4E-16 0.15 1.5E8 ・ “Dark Current Spectroscopy” identifies metals and estimates concentrations. ・ It can measure very small amounts of metal contamination. (D. McGrath et al. (TI); “Counting of Deep-Level Traps Using a Charge-Coupled Devices”) ©2018 N. Teranishi 11 Contents

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

TG FD Pinning 1. Grounded P+ pinning layer Layer x x x+ xx prevents interface to be + P N + P N P depleted, and stabilizes PD electrically. P-Well ・Low dark current Storage N-Substrate ・Large saturation ・High blue sensitivity x: GR (generation-recombination) center 2. Complete electron transfer 0V OFF ・No image lag, ・No transfer noise

ON Signal Shallow P+ pinning layer

- Theoretical dark current reduction ratio using Shockley-Read-Hall Process: U Not depleted v N n 2 p  th t i  ~ 10 7 2 U Depleted vth Nt ni p 2ni Assuming hole density (p) at the P+ pinning layer is 1017 h+ cm-3

- Actual dark current reduction ratio:

Non-PPD (1982) PPD (2012) Unit Scheme CCD FSI CMOS Pixel size 23 x13.5 1.12 x 1.12 μm Dark current 1,300 5.6 e-/s/μm2 at 0.4 % 60℃ Still big reduction! ©2018 N. Teranishi Example of Dark Current Reduction by PPD 14

Conventional PD PPD If Dark current is reduced, dark current FPN, and dark current shot noise are also reduced.

Dark current FPN is suppressed, then, picture quality is much improved.

Saturation becoms12 times larger by PPD. ・ PN junction area is increased.

・ Acceptor density at pinning layer >> Donor density at storage

) 2

Pinning layer 10 /cm

11 8 10

x xx - x x P + 6 + N+ + (e 12.5 times PPD P e- P 4 - e N - e 2 Non-PPD Storage P region 0 Saturation 0 1 フォトダイオード電位2 3 4 PPD pixel cross-section Storage potential （V) Saturation by simulation 1984 B. C. Burkey et al.（Kodak) ©2018 N. Teranishi Pixel Shrinkage Trend 16

・ Shrinkage has been carried out steadily. ・ Lens volume and weight become 1/1000 when pixel size becomes 1/100, assuming that F-number, view angle and pixel number are constant, (um2) CCD 24 years CMOS

100

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

SPAD 4-Tr CMOS + High conversion gain + (Single photon avalanche diode) CMS (Correlated multiple sampling) n=1 n=2 n=3 n=0 n=4 128 samplings

n=5 n=6 n=7 (N. Dutton et al., VLSI Symposium 2014) ・ QVGA SPAD, 20 fps, room temperature (MW. Seo, S. Kawahito et al., IEEE EDL 2015) ・ In 2015, several organization reported ・ High avalanche gain makes following low noise < 0.3 e- rms. circuit noise negligible. ・ DEPFET (Max Plank) ・ Large dark count. Small fill factor ©2018 N. Teranishi Photon Counting (2) 19

(Seo, Kawahito et al, IEEE EDL, Dec. 2015)

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

Motivation of stack image sensors Sensor - Non-silicon material for sensor part Bump - Finer process technology for logic part - Smaller footprint - More flexibility of wiring Readout circuit - Isolation between analog and digital circuits Bump bonding

Approach - Bump bonding (In, solder Au, …) - SOI - Wafer-to-wafer oxide bonding + TSV - Wafer-to-wafer hybrid bonding SOI pixel detector (KEK Arai et al.)

Wafer-to-wafer oxide bonding + TSV(Through silicon via) at peripheral ・Smaller chip size → Smaller camera, lower cost ・Optimal processes for both sensor and logic ・More functions can be integrated. Suitable for large volume applications

・ Oxide bonding ・ Hybrid bonding + TSV(Through silicon via) Interconnection can be located Interconnections are at pixel array also. limited at only peripheral.

Cu2Cu optimized CMP < 150 ℃

400 ℃ BEOL CMP K. Shiraishi et al. (Toshiba), ISSCC 2016 CMP optimization Post-bond annealing ・ 2 um pitch interconnection Y. Kagawa et al. (Sony), IEDM 2016 E. Beyne (IMEC), 2018

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

１．To cover shortage of visible light and to increase sensitivity eg. Most illuminations have NIR. Night glow ・ Security use ２．Invisible for mankind and animal. ・ Gesture interface, ToF, ・ Animal observation at night ３．Eye safe laser (1.4-2.6um) ・ Longer than Si cutoff wavelength ４．Specific wavelength for each application ・ Window to living organisms ・ Fluorescent protein ・ Hemoglobin (Hb) ・ Bill validator (NIR and UV inks are used.) 5. Smaller sunlight background

・ Emission in the upper atmosphere - Photoionization by the sun - Luminescence by cosmic ray - Chemiluminescence ・ Light intensity at near IR is larger than that of visible light

・ 1.4 - 2.6 μm wavelength light hardly reaches retina. ・ Application： Night vision, range finder, obstacle detection ・ Note: Silicon cannot detect 1.4 - 2.6 μm wavelength light.

Cormea Aqueous Lens Vitreous

Total Transmittance

・ NIR, 700～900 (1400) nm, has a good transmittance of human body. ・ It can pass through even ３０ cm thick human body. Specific wavelengths ・Hemoglobin (Hb)： Window to living organisms Oxygen density in vein ・Myoglobin： Oxygen density in cardiac muscle Hemoglobin (Hb) and skeleton muscle Water ・ Cytochrome c oxidase (COX) ・ Melanin

Application Wavelength (nm) ・ Vein authentication ・ Optical topography Absorptance of hemoglobin and water https://www.an.shimadzu.co.jp/bio/nirs/nirs2.htm ・ Fluorescent protein ©2018 N. Teranishi NIR Fluorescence Imager (ICG Fluorescence)

・ Portable intraoperative diagnosis ・ Both excitation light and fluorescence light are inside the widow of living organisms.

NIR fluorescence camera Excitation light (760 nm) Fluorescence light (830 nm)

・ ToF, for example, uses active illumination or modulated illumination. ・ Longer wavelength at NIR, the sunlight background becomes smaller,

and SN ratio will be improved. Energy

・ Large depletion layer is needed to detect NIR or X-ray. For example, 900 nm wavelength NIR (equivalent with 5.4 keV X-ray) is absorbed half by 15 um silicon. ref: 3 um silicon for 700 nm 100 100 19.7keV 17.4keV 80 80 1050nm13keV 19.7keV 60 60 1000nmv 17.4keV 8.7keV 950nm 40 1050nm 40 900nm 1000nmv 13keV 850nm5.4keV 20 8.7keV 20

Intensity (%) Intensity 950nm 800nm Intensity (%) Intensity 0 0 0 100 200 300 400 500 600 0 10 20 30 40 50 Depth (um) Depth (um) Enlarge

・ Pyramid Surface, which enhances diffraction. ・ DTI（deep trench isolation) between pixels to suppress crosstalk

Increase effective sensor thickness and increase NIR QE.

33 %

18 %

・ Black silicon is formed on illuminated surface by femtosecond laser ・ As well as NIR QE is much improved, the cutoff wavelength is

enlarged beyond 1.2 μm. RelativeQE

Wavelength [nm]

Ge epitaxial layer Dislocation

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

Modulated light source

ToF image sensor ToF setup Intensity image

Emitted light Received light

ΔT Intensity Time ퟏ Distance image by ToF Distance = 풄∆푻 (풄: 퐥퐢퐠퐡퐭 퐯퐞퐥퐨퐬퐢퐭퐲) ퟐ (Kawahito Lab., Shizuoka University) ©2018 N. Teranishi http://www.idl.rie.shizuoka.ac.jp/study/project/tof/index_e.html 37 ToF Applications

・ Gesture input: game, medical, digital signage, automobile ・ Range finder, 3D measurement: automobile, dental, robot

Game Digital signage (EETimes Japan, 2011）

・To resolve the multi-pass errors is challenge at ToF.

(a) Normal (b) Reflector

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

Metal grid polarizer Stress image （Plastic lens） (Tokuda 2011 IISW) https://www.4dtechnology. com/

Antireflection R. Muller ISEurope2017 103

102 43.7 R R 1

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

Monocular 3D is required instead of binocular 3D ・Compact, ・No precise assembling is needed. Parallax

Camera diagram Pixel cross section Angular response

(S. Koyama et al.@Panasonic, ISSCC 2013)

Human sees photos and videos. Computer vision (Computers see photos)

+ Barcode + （Panasonic） New Functions Driving assistance Gesture input （Subaru）

1. Basics of Visible Light Image Sensors 2. Dark Current and Blemish 3. Pinned Photodiode (PPD)

Recent Progresses 4. Photon Counting Image Sensor 5. Stack 6. Near InfraRed (NIR) 7. ToF (Time of Flight) 8. Polarization Image Sensor 9. Monocular 3D Image Sensor

10. Queen Elizabeth Prize for Engineering

Purpose： The world’s most prestigious engineering prize, to celebrates world-changing innovations in engineering Citation： The creation of digital imaging sensors Winners： - Eric Fossum: CMOS image sensor - George Smith: CCD (Charge-Coupled Device) - Nobukazu Teranishi: Pined photodiode (PPD) - Michael Tompsett: CCD image sensor

Invitation letter

Reception at Guildhall hosted by the Lord Mayer of London

・ Proud and happy The best award for engineers ・ Lucky There are so many important and beneficial technologies. ・ Acknowledgement To family, teachers, seniors, colleagues, and everybody ・ Step-up I would like to grow up as a person of the Prize. ・ Education for engineers Good engineers are needed to evolve industries. So, I would like to introduce respectable engineers to young generation, and help them to learn technologies.

Anyone can take a photo anywhere, anytime!

Thank you !

Crosstalk effect：・Color mixture, degradation of chromatic S/N. ・Degradation of resolution ①Angled incident light

②Multi-reflection, Diffraction N N N

e ~3 um ③Charge diffusion Crosstalk Mechanism 1um P

・BSI is used for small pixel to reduce crosstalk and increase sensitivity. ・DTI suppress both optical cross talk and charge diffusion cross talk.

B-DTI (Back-side DTI) ・DTI is formed from backside. F-DTI (Front-side DTI) ・Metal is buried in DTI. ・DTI is formed from front side. ・Sufficient cross talk reduction ・Poly-silicon is buried in DTI. (Y. Kitamura et al.(Toshiba), IEDM,2012) ・Better DTI interface ©2018 N. Teranishi(JC Ahn et al.(Samsung）, ISSCC,2014) Dark Current Possible Causes 52 Large electric field at TG edge TG interface GR center Large electric field at P+ /N junction FD dark current TG RG RG off-leak PD interface GR center FD RD + STI P N+ N+ N P+ Charge flow P-Epitaxial from the outside

P+ Substrate

STI interface GR center Diffusion current Depleted region GR center

TG Off PPD x x x FD - P+ + On P+ N P+ - N - P (C) Pocket at the TG edge -Off- - - 0V

On On Potential (A) Small electric field (D) Pump back when the signal is large - - On CBarrier (E) Traps at the TG interface (B) Barrier at the PD edge On the next slide.

(E) Traps at the TG interface If the electron transfer path touches the interface, some electrons are captured by traps. - Some of them are detrapped in the following frames, causing lag. - Some of them are annihilated, causing non-linearity. TG Ideal case PPD x x x FD P+ N P+ P+ N With traps

P Outputelectrons x : Traps at the TG interface Signal electrons at PPD - The signal electron annihilation exhibits this kind of non-linearity. - A buried transfer path is needed to suppress these phenomena. ©2018 N. Teranishi Pixel Shrinkage Trend 55

・ Shrinkage speed becomes slower recently. ・ In 2017, 0.9 um pixel began to be mass produced. (um2) Microlens CCD Inner Microlens CMOS Shifted Microlens 100 Lightpipe Pinned PD BSI 10 Stack 50％ shrinkage DTI MinimumPixel Area in 3.5 years 1 80 85 90 95 00 05 10 15 Mass Production Year ©2018 N. Teranishi 56 Stack (3). Pixel-DRAM-Logic Oxide Bonding

1 Gbit 19.3 Mpixel

(WikiChip Fuse, 2018/02/3) ©2018 N. Teranishi On-chip Optics Functions 57 Light (1) Light collecting Microlens Lightpipe BSI (Back Side Illumination) Photo shield (2) Color filtering Color filter PD Vertical color separation Si (3) New specific functions Polarization image sensor Phase detection autofocus Computational photography

・RGB-IR image sensor is proposed to obtain IR image and RGB image simultaneously. ・Double bandpass filter in stead of conventional IR-cut filter is used to detect NIR. ・Color reproductivity is not sufficient. Conventional IR-cut filter Double Bandpass Filter

G B IR R IR

RGB-IR Image Sensor QuantumEfficiency 400 500 600 700 800 900 1000 1100