CMOS Image Sensor Pixel Design and · PDF file1 No Security: Public Information Defining the Future of Digital Imaging™ February 1st 2017 Boyd Fowler CMOS Image Sensor Pixel Design

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No Security: Public Information
Defining the Future of Digital Imaging
February 1st 2017
Boyd Fowler
CMOS Image Sensor Pixel Design
and Optimization

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Defining the Future of Digital Imaging *
Outline
Introduction
Photodetectors
Pixel circuitry
Active pixels
Global shutter pixels
Performance optimization

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CMOS Image Sensor (CIS) Architecture
Charge is not transferred outside the pixel area
Multiple functions integrated with the sensor array such as amplification, CDS, ADC, readout sequencing and digital processing

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CIS Performance Parameters
Quantum efficiency (QE)
Modulation transfer function (MTF) /
spatial resolution
Read noise
Conversion gain
Full well capacity
Dark current
Lag
Shutter efficiency

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Photodiodes
Advantages
High FWC
High QE
Disadvantages
High dark current
Low conversion gain
No in pixel charge transfer

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Photogates
Advantages In pixel charge transfer
Low dark current for buried channel
Disadvantages
Low blue QE for FSI operation
Charge transfer lag (read out
time)

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Pinned Photodiodes
Advantages In pixel charge transfer
Low dark current High QE
Disadvantages Lower FWC
Charge transfer lag (read out time)

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TCAD Device Design
Process and device simulation to optimize the sensor performance (FWC, lag/charger transfer speed)
Typical design parameters that are optimized include implant doses and energies

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Light Gathering
Micro-lenses Focus light on photodetector
Increase effective fill factor
Reduce optical crosstalk
Anti-reflective coatings
=21
2+1
2
1 = 20

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Frontside and Backside Illumination
FSI structure works well for larger pixels > 2-3um, but suffers
from low QE and high pixel crosstalk as pixel size shrinks
BSI always has better QE and less optical crosstalk than FSI

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Deep Trench Isolation and Buried Color Filters
DTI is used to reduce substrate carrier diffusion and
therefore increase MTF
BCFA is used to reduce optical crosstalk and improve
MTF
PD PD
DTI
BCFA
CMG

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FDTD Optical Simulation
Finite difference time domain electro-magnetic equation
solver
Critical for pixels as their size becomes similar to the
wavelength of the illumination Optical confinement methods for continued scaling of CMOS image sensor pixels
C. C. Fesenmaier, Y. Huo, and P. B. Catrysse, Opt. Express 16, 20457 (2008)

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Outline
Introduction
Photodetectors
Pixel circuitry
Active pixels
Performance Optimization

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3T Active Pixel (3T APS)
First demonstrated by P.
Noble in 1968
High full well capacity
High dark current
KTC readout noise
High speed readout

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3T APS Readout Circuitry

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4T Active Pixel (4T APS)
First described with a photo-gate by E. Fossum in 1994
First described with a pinned photodiode by P. Lee in 1997
In pixel charge transfer enables CDS and removes kTC reset noise
Low dark current due to buried channel photodetectors
Lower read noise due to separation between photodetector and floating diffusion capacitance
Lower fill factor than 3T APS

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4T APS Readout

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1.75T Active Pixel
First demonstrated by M.
Mori in 2004
Reduced pixel size
Increased fill factor
Slower readout speed
(1/2)
Higher read noise due to
shared floating diffusion

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Rolling/Global Shutter Operation
Global Shutter Rolling Shutter

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Rolling Shutter Artifacts

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Voltage Mode Global Shutter Pixel
8T / 2C 5.5um pixel
[Meynants 15]
High shutter
efficiency typically ~
80dB+
KT/C noise limits
low light
performance due to
size of C1 and C2
Large pixel size /
low fill factor

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Stacked Voltage Mode Global Shutter Pixel
5.5T/1.25C 3.75um pixel [Kondo 15]
Very high shutter efficiency > 120dB

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Charge Mode Global Shutter (Gate Storage)
Smaller pixel size than voltage mode GS [Meynants 15]
In pixel charge transfer allows for CDS and complete kTC noise suppression
Buried channel storage gate is needed for low dark current operation
Light Shield
Storage
Gate

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Outline
Introduction
Photodetectors
Pixel circuitry
Active pixels
Performance optimization

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Quantum Efficiency Optimization
Frontside / backside illumination
EPI thickness
Anti reflective coatings
Color filters materials
Pixel size
Micro-lenses

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Quantum Efficiency FSI/BSI and EPI
Thickness
Thicker EPI improves NIR QE but reduces MTF
BSI has better QE than FSI

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MTF Optimization
Frontside / backside illumination
EPI thickness
Photodetector depletion depth
Illumination wavelength
Pixel size
Buried color filters
Deep trench isolation
System optics

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MTF FSI/BSI and EPI Thickness
Thicker EPI reduces MTF

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MTF FSI/BSI and Illumination Wavelength
MTF is better for short wavelengths for FSI
MTF is better for longer wavelengths for BSI
MTF of FSI is typically better than BSI

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Read Noise Optimization
In a well designed CIS the read
noise is limited by the transistor
connected to the FD
When read noise is limited by the
transistor connected to the FD
then it is proportional to the total
input capacitance [Centen 91]
2 =++
2
() ()2
Read noise is also limited by the
read out bandwidth and the
excess or 1/f noise of the input
transistor

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Read Noise Optimization II
Therefore higher conversion
gain typically leads to lower
read noise, but this limits
dynamic range and full well
capacity
Read noise is not a single
parameter, it is a distribution

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Full Well Capacity Optimization
Surface mode photodetectors
have higher capacitance and
therefore higher FWC (q=cv)
Photodiodes
Surface mode photo-gates
Larger pixels have higher
potential FWC
Lower conversion gain enables
hi

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CMOS Image Sensor Pixel Design and · PDF file1 No Security: Public Information Defining the Future of Digital Imaging™ February 1st 2017 Boyd Fowler CMOS Image Sensor Pixel Design