CMOS Image sensors - Introduction | dautreppe.photonique ...dautreppe.photonique.grenoble.cnrs.fr/sites/all/images... · Dautreppe 2015 | Vaillant Jérôme | 8 décembre 2015 | 7

CMOS Image sensors

Jérôme Vaillant - CEA-LETI

8 décembre 2015

| 2Dautreppe 2015 | Vaillant Jérôme | 8 décembre 2015



CMOS image sensor in a nutshell

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• CMOS image sensor today?

• How does it works?

• What’s next?

What’s CMOS image sensor?


• CMOS image sensor today? Brief history Market share / main players

• How does it works?

• What’s next?




• How does it works? Basics 15 years of strong development Role of optics? Today’s performances

• What’s next?





• What’s next? Applications New ways of sensing








Brief history of digital image sensor

Boyle & Smith (Bell lab)

Optical mouse sensor1980

1988

1995

2003 340Mpix

1979

1995

1993

2013 1.5Gpix


Brief history of digital image sensor


Pixel size race

2000 2015

320x240 (0.08Mpix) 5344x4016 (21Mpix)







CMOS / CCD market share in 2014

Data from Yole Développement report 2014 « Status of the CMOS Image Sensors Industry »


Vendors


Manufacturers












Camera module teardown

What we are taking about today


• Integration of additional features is one of the main differences between CMOS and CCD: Addressing & readout, charge-voltage conversion, ADC, … Bias & timing generation and control, communication stream, …

Closer look

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Even closer : the pixel


• Sensing light: photodiode

• Adressing / readout / reset

Pixel structure

VREAD

VDD

VRST

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How does it works?

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How does it works?

1- Reset

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How does it works?

2- Reference level sampling

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How does it works?

3- Integration

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How does it works?

3- Integration

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How does it works?

4- Signal sampling

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How does it works?

1- Reset

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The pixel


• Tri-stimulis theory Any color can be expressed as the weighted sum of 3 independent colors Base for image: additive synthesis and RGB primary color

A world of color


• Tri-stimulis theory Any color can be expressed as the weighted sum of 3 independant color Base for image: additive synthesis and RGB primary color

• How to sense 3 color on a rectilinear array? Bayer pattern proposed (and patented) in 1976

A world of color

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• Due to Bayer pattern the color image is subsample on 3 different planes

From raw Bayer image to color image


• Due to Bayer pattern the color image is subsample on 3 different planes Missing data are interpolated



• Due to Bayer pattern the color image is subsample on 3 different planes Missing data are interpolated Artefacts may appear



Camera module teardown

What is that?

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CMOS image sensor basics: a summary







• Remember, in 15 years, minimum pixel size decrease form 5.6µm to 1µm pixel: Area of pixel (i.e. light collection) divided by 31! Number of photon per pixel is strongly decreasing:

Strong development driven by pixel size race

1000 ph/pix 510 ph/pix 287ph/pix 154 ph/pix 98 ph/pix 63 ph/pix 32 ph/pix

5.6µm

4.0µm3.0µm

2.2µm 1.75µm 1.4µm 1.0µm


• Relationship between scene and pixel illuminance: Illuminance 𝐸𝑠 of elementary surface 𝑑𝑆

• Uniform, lambertian, diffusion reflection coefficient 𝑅

Radiance of 𝑑𝑆 : 𝐿 = 𝑅𝐸𝑆/𝜋

Light flux collected by the lens: 𝑑𝜙 = 𝐿𝑑𝑆.cos 𝜃𝑠.𝜋 𝐷2

4 cos 𝜃𝑙

𝑝2

• dS normal to line of sight (cos𝜃𝑠 = cos 𝜃𝑙 = 1) 𝑑𝜙 =𝑑𝑆.𝐷2

4𝑝2𝑅𝐸𝑠

• 𝑑𝑆𝑝𝑖𝑥 & 𝑑𝑆 conjugated 𝑑𝑆𝑝𝑖𝑥 = 𝛾2𝑑𝑆 et 𝑝 =𝛾−1

𝛾𝑓

• Lens overall transmission: 𝑇

• Standard configuration: R=18%, T=85%, f/D=2.8, « 1 Epix ≈ 5‰ Escene !!!

How many photons per pixel in standard photography?

𝐸𝑝𝑖𝑥 =𝑇𝑑𝜙

𝑑𝑆𝑝𝑖𝑥=

𝑅𝑇

4 𝛾 − 1 2 𝑓𝐷

2 𝐸𝑠


• Sunny day: Es=100 000lux => Epix = 500lux ~ 60 000 ph/pix

• Cloudy day: Es=5 000lux => Epix = 25lux ~ 3 000 ph/pix

• Low light interior: Es=50lux => Epix = 0.25lux ~ 30 ph/pix

Some use cases 1µm pixel, f/2.8, tint=30ms

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• Remember, in 15 years, minimum pixel size decrease form 5.6µm to 1µm pixel: Area of pixel (i.e. light collection) divided by 31! Number of photon per pixel is strongly decreasing

• It is mandatory to drastically improve pixel performances to increase image quality when pixel size decrease Increase signal:

• Quantum Efficiency: photon to electron conversion

Decrease noise: • Readout• Dark current• Reset noise









• 3T pixels suffer from “reset noise”: Signal is sampled at the end of integration, BUT reference level is sample

before the integration

Some developments done:

1- from 3T to 4T

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Reset noise (Johnson noise) known as kTC: 𝜎𝑒− =1

𝑞2𝑘𝐵𝑇𝐶 𝑒−

~ 25 e- for a pixel with C= 2fF vs. ~ 30 ph/pix at 50lux (pixel 1µm, f/2.8, tint=30ms)

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1- from 3T to 4T

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Noise known as kTC: 𝜎𝑒− =1



• Suppressed by 4T architecture: True Correlated Double Sampling (CDS) Use specific “pinned” photodiode

=> reduce dark current by curingdefect at Si-oxide interface

But less space for the photodiode => lower fill-factor, lower QE



1- from 3T to 4T

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Noise known as kTC: 𝜎𝑒− =1



• Suppressed by 4T architecture: Reduce noise Use specific “pinned” photodiode

=> reduce dark current by curingdefect at Si-oxide interface

But less space for the photodiode => lower fill-factor, lower QE



1- from 3T to 4T

http://www.canon.com/v-square/movie.html?id=t010








• A microlens is formed on top of EACH pixel To compensate the fill-factor reduction => increase the QE


2- microlens optimization

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• A microlens is formed on top of EACH pixel To compensate the fill-factor reduction Where to place it? It depends on the field height



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• Microlens benefits: Increase QE Improve FTM / spatial resolution by managing light inside the optical stack




• Advanced high density CMOS have: Thick back-end stack optimized for electrical performances Low(SiO2) / high(Si3N4) refractive index sandwiches


3- CMOS back-end (optical stack) optimization

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• Advanced high density CMOS have: Thick back-end stack optimized for electrical performances Low/high refractive index sandwiches

• Dedicated process developed for image sensor Improve QE and FTM Strong constraints on process & design


3- CMOS back-end (optical stack) optimization

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• Other solutions for BE optimization


3- CMOS back-end (optical stack) optimizationS

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Light guide « air gap » High refractive index light guide

Double microlens Microcavity


• Ultimate BE optimization: Suppress it!


3- CMOS back-end (optical stack) optimizationS

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Backside illumination







• As seen before, huge efforts have been done to improve CMOS image sensor High cost of development Very competitive environment Optics at pixel scale (~1µm)

• Optical simulations: Visualize light propagation inside pixel to identify critical parts in design and

process Evaluate innovative solutions in advance because all these optimization need to

be secured

• Development of dedicated tools: Ray-tracing for large pixels (>3µm)

Role of optical simulations in CMOS image sensor development


• From pixel layout and process caracteristics, generation of 3D model

• Add microlens and color filters

• Count rays at different planes Inside optical stack Inside silicon

Ray tracing for pixel simulation

Vaillant, Hirigoyen, SPIE Electronic Imaging 2004 “Optical simulation for CMOS imager microlens optimization”





be secured

• Development of dedicated tools: Ray-tracing for large pixels (>3µm) Electromagnetic simulations using FDTD below 3µm



• Solve Maxwell equation inside the structure: Use FDTD software: Finite Difference Time Domaine Time domain propagation of short light impulse Differential equations solve by finite difference

• Same methodology for model generation

• Access to E, H fields everywhere

Electromagnetic simulations

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A. Crocherie, et al., Proc. SPIE 2008, vol. 7100, “Three-dimensional broadband FDTD optical simulations of CMOS image sensor”





be secured

• Development of dedicated tools: Ray-tracing for large pixels (>3µm) Electromagnetic simulations using FDTD below 3µm Opto-electrical simulation flow



• Using FDTD software for optical part

• Coupled with electrical software for a photon-to-electron flow

Opto-electrical simulation tool

Crocherie et al., From photons to electrons: a complete 3D simulation flow for CMOS image sensor, IISW2009







• Pixels pitch range from 0.9µm to 200µm

• Sensor size range from 2x1.5mm² to 20x20cm²

• Image definition from 0.3MPix to 120Mpix

Large range of sensor and pixel sizes

AWAIBA 250x250-pixel NanEye for medical

applications


• Impressive QE, even for 1.1µm:

Today’s performances

H. Tian, Architecture and Development of Next Generation Small BSI Pixels, IISW 2013

H. Rhodes., BSI CMOS Imager Sensors with RGBC Technology,Image Sensors Conference 2013


• Low noise:

Today’s performances

Performance Sony1 Samsung2 OmniVision3 TSMC4

Full well [e-] 5000 6200 4500 5200

CVF [µV/e-] 63 110

Read noise [e-] 2.2 2.0 1.3

Dark current @ 60°C [e-/s] 3 6 7 4

Green sensitivity [e-/(Lux.s)]Under 3200KUnder D65

32403700

32004020

33403930

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[1] E. Funatsu, Small pixel CIS Technology and Image Quality Evaluation Method,Image Sensors Conference 2012[2] J. Ahn, et al., 1/4-inch 8Mpixel CMOS Image Sensor with 3D Backside-Illuminated 1.12μm Pixel with Front-Side Deep-Trench Isolation and Vertical Transfer Gate, ISSCC 2014[3] H. Rhodes., BSI CMOS Imager Sensors with RGBC Technology, Image Sensors Conference 2013[4] S. Takahashi., High Full Well Capacity and Low Dark Current Pixel Design for 1.1µm and 0.9µm Pixel in a 45nm BSI CMOS Image Sensor Process Technology, VLSI Technology and Circuits, 2014



• How does it works? Basics 15 years of strong development Today’s performances



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Consumer

Professional


Broad range of applications…

Mobile devices

Photography & video

Security Medical

Automotive

Machine vision

3D interaction


…needing faster, higher, wider CMOS imagers

High dynamic

Ultrathin sensors

Hyperspectral

Extended spectral

sensitivityVery low

noise

High speed Time of Flight

Low power consumption



• How does it works? Basics 15 years of strong development Today’s performances




• Photodiode is replaced by an avalanche diode Very fast detection (~100ps) Used for distance ranging (few cm to few m) Used to see light travelling

New sensors:1- SPAD Single Photon Avalanche Diode

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• For 2D+ imaging

• Sample part of light field function Enable depth map estimation Enable post refocus

New sensors:2- Multipupil/plenoptics cameras

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• Back side illumination and wafer stacking: Wafer dedicated to image sensor bonded Wafer dedicated to logic and processing

New sensors:3- High density, high performance sensors

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Thank you for your attention

Any question?

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Documents

CMOS Image sensors - Introduction | dautreppe.photonique ...dautreppe.photonique.grenoble.cnrs.fr/sites/all/images... · Dautreppe 2015 | Vaillant Jérôme | 8 décembre 2015 | 7