Viewpoint architectures Methods scanning Methods light detection

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Viewpoint architectures Methods scanning Methods light detection

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CMOS image sensors: Concepts
Viewpoint architectures
Methods scanning Methods light detection Silicon CMOS Floor plan CMOS image sensor Basic CMOS pixels Special CMOS pixels Custom designed sensors
Viewpoint performance
Silicon versus Film CMOS versus Units sensitivity Color sensitivity
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CMOS image sensors Concepts
Methods scanning
Image sensors classified according mechanism scanning focal plane
Solid state CID, CSD, Photogate): transfer Diode array passive pixel: switches Active pixel CMOS: voltage Field emitter arrays
solid state
image tubes: scanning electron beam opto-mechanical scanning photographic emulsion: chemical memory biological systems: parallel processing
scanning e-beam rotating prisms mirrors
photo-resistive discharge
vidicon
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CMOS image sensors Concepts
Classification method light detection
main circuit equivalents
Photo resistive detector variable resistor
ePHOTORESISTIVE I[A]
Photo voltaic detector current source
PHOTOVOLTAIC
I[A]
more light
more light
V[V]
V[V]
f(light power)
V/f(light power)
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CMOS image sensors Concepts
practice there types light receptors available standard CMOS
Photo gate
Poly GATE
Nwell
Photodiode
Photo transistor (BJT)
junction p-i-n junction
Inversion layer substrate junction
Well-substrate junction internal amplification
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CMOS image sensors Concepts
must adhere standard CMOS many other light receptors become available:
Light detectors (not even CMOS)
Avalanche photodiode (APD) P-I-N diodes Field emitter arrays Position Sensitive Detectors (PSD) Buried Channel
Silicon
non-standard CMOS
Buried diodes, pinned diodes Photo resistors Bolometers (temperature sensitive impedance) Pyro-electric detectors (temperature sensitive polarisation)
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CMOS image sensors Concepts
Solid-state detectors EM-radiation
Silicon used
virtually short wavelengths
visible range (400nm 700nm)
(30nm 300nm)
X-ray source=accelleration charges) gamma rays (source nuclear transitions) cosmic rays (source stellar nuclear transitions) High-Energy particles (electrons, nuclear fission, sub-nuclear physics experiments)
Silicon photo diodes photo resistors Silicon band transition: cut-off: 1200nm photon energy 1.12 direct SI-detectors with surface treatment indirect phosphorescent (scintillator) screen Silicon diodes, high energy photons have probability absorption. Solid-state ionization chambers
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CMOS image sensors Concepts
Silicon competition from other semiconductors longer wavelengths
Near infrared (NIR 800-2000 nm):
Mid-infrared (MIR 3-5um) Thermal infrared (TIR 8-12um) infrared 1000 scientific)
Microwave (300um radio, (sub THz)
1100 Silicon photodiode detection band-to-band transitions lower bandgap semiconductors (IV, III-V, II-VI): InSb, PbTe, InGaAs, photo-electric effect silicides: Si:PtSi, Si:CoSi (Schottkybarrier diodes) direct transitions ternary very low-bandgap semiconductors: HgCdTe, InGaAs, PbTeSn, photo-electric effect heterojunctions silicides detection uncooled bolometers (heat detectors) material with bandgap <20meV conceptually impossible impurity transitions extrinsic semiconductors (e.g. Si::P, GaAs:Si, (cryogenic) cryogenically cooled bolometers (thermoresistive, superconductive, non-coherent: infrared (cryogenic) coherent detection: heterodyne receivers coherent detection only
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CMOS image sensors Concepts
Solid-state detectors EM-radiation
0.01 1e-3 1e-4 1e-5 1e-6 1e-7 1e-8 1e18 1e11 10cm 1e10 1GHz U.V. infrared visible infrared diode photoresistor phosphors bandgap semiconductors (PV, photoelectric effect heterojunctions impurity band transitions (extrinsic semic) bolometers microwave mixers heterodyning 10nm 100nm 10um 100um 1e17 1e16 1e15 1e14 1e13 1e12 1MHz
h[eV]
[Hz]
1e21
1e20
1e19
1e-13 1e-12 1e-11 1e-10
micro wave radio. radar
P-I-N diodes ionization chambers wire detectors bolometers
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CMOS image sensors Concepts
Floorplan typical CMOS image sensor
sequencer pixel
Yaddressing pixel matrix
interface postprocessing
column amplifiers analog X-addressing general output
imaging core featured sensor
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CMOS image sensors Concepts
Basic CMOS pixel concepts
Let's make simple image sensor
what improved here?
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CMOS image sensors Concepts
Basic CMOS pixel concepts
most simple pixel CMOS:
1-transistor passive pixel
selection column
node
photo diode
time
Pixel voltages versus time
Schematic
advantages drawbacks?
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CMOS image sensors Concepts
Basic CMOS pixel concepts
most straightforward active pixel:
active pixel
select
reset
reset
node output
photo diode column output
time
Pixel voltages versus time
Schematic
advantages drawbacks?
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CMOS image sensors Concepts
Special CMOS pixels
Pixels detectors with defined voltage bias
Many reasons exist deviate from basic integrating passive active pixels
These examples more complex "pixels" that suitable read photo receptor while maintaining constant bias voltage over receptor
photoresistive detectors: thus bandgap material, with breakdown voltage voltage dependent dark current: Isignal Vbias*Z avalanche photo diodes require very precise bias voltage detectors with V-dependent memory effects (cryogenic) cancellation large detector capacitance response speed correction non-linear diode capacitance
feedback charge amplifier
reset ZERO-BIAS
AC-coupling
"direct-injection"
Integration capacitor Output signal reset
reset
detector
reset
Vout
detector Bias
Vout
BIAS
detector
BIAS
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CMOS image sensors Concepts
Special CMOS pixels
Pixels detectors with high currents
low-level photo current from visible light large dark currents handled easily small storage large background signal, IRcapacitors normal pixels. detectors, very intense light sources These pixels fail following cases: modulated information, beam chopping
Charge amplifier with offset current subtraction
separation
difference with reference detector, bridges,
reset
offset current
offset current
pass filter
dark reference
Vout
detector
Vout
illuminated
detector
BIAS
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CMOS image sensors Concepts
custom designed image sensors
Only "standard" needs solved "standard" components
special geometry
speed
sensitivity
system price
Wafer scale sensor (10x10 cm2) Round shape matched image Downlink limited Filtering pattern recognition Limited Very large diodes Logarithmic response Single chip processor
system dimensions
Application Algorithm Data reduction on-chip preprocessing parallelism spectral range light conditions high dynamic range Sensor cost Manufacturing cost Number components Size Weight
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CMOS image sensors Concepts
Silicon versus Film
Silicon image sensors film completely different systems
Similarity exists only beginning process:
Photon detection happens excitation bound electron unbound state product same: image
Differences
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CMOS image sensors Concepts
silicon film
Silicon
Size: grain bi-stable: becomes black not.
Film
Random placement randomly sized overlapping grains.
Regular pattern abutted pixels
Size typical .10.
High dynamic range pixel. It's signal varies continuously. >1e4 pixel possible
dynamic range patches homogenous film limited (about 20). ratio (but dynamic range) patches grows with number grains patch.
Response fundamentally linear:
photon electron
photons must absorbed grain create latent image. This creates strong non-linear effect light levels. limited smallest grain size. aliasing. Unlimited integration time. advantage: homogeneity
MTF: regularity pixel matrix causes aliasing
Dark current limits integration time
advantage: electronic processing
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CMOS image sensors Concepts
Film concept non-linear
pixel 100% photon grains
silicon photodiode
photon electron QE=100%)
film grains
creation latent image requires least, ideally) photon conversions. Transmission probability that grain remains white cumulative Poisson probability:
photon grains
probability white
exp(-
1-transmittance saturation
average number photons that grain during exposure time
ideal film: grain contrast fill factor 100%.
exposure [photons/information carrier]
non-linearity "bad"?
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CMOS image sensors Concepts
image thin line thin line
Image thin line
Pixel array
Film grains
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CMOS image sensors Concepts
Silicon Film: quality criterion
->aliasing
Pixel size
1000
100000 saturation electrons pixel
grains equivalent pixel film very small grains large, overlapping grains
diffraction limited grains
pixel
grains
MTF<->S/N?
lp/mm
1000
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CMOS image sensors Concepts
technology
History: diode arrays: 1967 invented 1970 became quickly preferred image sensor technology active pixels 1980
Technology: Silicon MOS, but: Photodetection buried diode buried channel charge transfer closely spaced electrodes
Historically optimized optical detection
vertical cross section through register
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CMOS image sensors Concepts
technology survives because really better
Standard CMOS
Modified CMOS
less dark current (<100pA/cm2) limited modifications compared standard technology mainstream extra steps
high (<50 noise electrons) dark non-uniformity best PRNU (1.10% p/p) very dark current (10pA/cm2) technology optimized optical detection rare technology ADC, logic chip,
limited serial scanning complicated driving interfacing
lower (>20 noise electrons) chip correction Higher PRNU higher dark current (nA/cm2) standard technology developed VLSI logic mainstream technology co-integration logic smart sensors random addressing digital interfacing single supply operation
significantly different:
spectral response (400.1000 minimal pixel size (3.5 charge storage unit area chip size number pixels: limited lithography geometrical stability
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CMOS image sensors Concepts
CMOS versus sensor performance
origin differences performance photo charge voltage conversion
passive pixel diode array active pixel
temporal noise fixed pattern noise
local amplifier local intelligence
multplexing charge packets lossless transfer charge sense amplifier register
Multiplexing switches photodiode node charge sense amplifier output
Multiplexing local amplifier output (charge sense) amplifier every pixel
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CMOS image sensors Concepts
Charge sense amplifier critical part
Qnoise=Vn*Cin
Just Qsignal=Vsignal*Ctransimpedance
Charge amplifier Transimpedance amplifier
(from photo diode)
high (bus lines) reasonable optimized unfavorable high signal bandwidth correlated double sampling complicated intermediate fill-factor
Diode array passive pixel
CMOS active pixel (APS)
small amplifier pixel, optimized noise: lowest small signal bandwidth correlated double sampling complicated fill factor
lowest possible single amplifier, thus optimal design lowest possible high signal bandwidth correlated double sampling possible high fill-factor CCD)
best performance
noise electrons noise electrons
noise electron
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CMOS image sensors Concepts
Units sensitivity
Sensitivity: easy measure hard define?
What does sensitivity mean?
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CMOS image sensors Concepts
Sensitivity
unit V.m2/W.s V/lx.s Ratio between light sensitive pixel area total pixel area. 100% Obscuration Factor) Output [V/s] versus input [W/m2] Output [V/s] versus input [lx] Empirical procedure obtain equivalent film speed obtain ratio between photo charge pixels output voltage. "Effective capacitance". Ratio between photo current incoming light power given wavelength Ratio between number generated electrons number "impinging photons".
good conversion light power voltage
Quantity
Charge conversion factor (Ceff)
Spectral response (SR)
Quantum efficiency
Fill factor (FF)
Sensitivity
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CMOS image sensors Concepts
Sensitivity: faint signal fixed shutter time
unit
Quantity Temporal noise
Fixed pattern noise (FPN) spatial noise Noise charge (Qnoise) Number noise electrons (#e-) Number noise photons (#h) Noise equivalent power (NEP) "electrons" "photons" W/Hz cmHz/W
obtain consecutive samples output voltage pixel. otherwise indicated, silently assumes noise dark, i.e. most favorable condition Static spread (dark) voltages pixels array
Specific noise equivalent power (NEP*) Specific detectivity (D*)
Background limited integrated performance (BLIP). Sometimes called "shot noise limited" Sensitivity
Noise equivalent Saturation
Ceff Ceff Light power equivalent noise pixel frame N*Ceff/tint/SR normalized framerate NEP* framerate only case white noise NEP* normalized pixel area framerate) other performance units obtained using dark noise (N), shot noise lowest signal level ("background") that will occur reality. BLIP system, limiting noise shot noise. thus "ideal" Lowest (environment) average light level "good" sensor operation, nominal speed, above, light level corresponding noise above, light level corresponding saturation nominal operation conditions
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CMOS image sensors Concepts
Sensitivity
unit
ratio signal uncertainty thereof
Quantity Signal Noise ratio (S/N SNR) Differential small-signal (dS/dN dSNR) Noise equivalent contrast ratio (NECR)
Dynamic range (DR)
Generalized dynamic range
Linear dynamic range (LDR) Photo response non-uniformity (PRNU) Background limited integrated performance (BLIP). Sometimes called "shot noise limited"
bits
obtain Output signal voltage range output signal noise dark Output signal saturation voltage output signal noise same signal level Noise measured certain signal level that signal. ability discriminate between nearby gray levels. =1/dSNR Saturation intensity noise equivalent intensity linear system this same S/N. More general definition ratio between upper lower intensities which dSNR==1. Largest intensity which dVolt/dIntensity linear Static spread Ceff pixels array other performance units obtained using dark noise (N), shot noise lowest signal level ("background") that will occur reality. BLIP system, limiting noise shot noise. "perfect" Number (useful) bits digital output
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CMOS image sensors Concepts
Sensitivity
faint signal unlimited shutter time
unit A/cm2 W/m2
Quantity Dark current (Idark)
Dark signal (Vdark)
Autosaturation time tauto
Dark Dark flux Background limited integrated performance (BLIP). Sometimes called "shot noise limited"
obtain (apparent) photodiode current dark pixel normalized unit area (apparent) signal voltage [drop] dark, dark current Longest possible integration time, where dark signal consumes complete output voltage range Light intensity level equivalent dark current Empirical procedure obtain equivalent film speed other performance units obtained using dark noise (N), shot noise lowest signal level ("background") that will occur reality. BLIP system, limiting noise shot noise. thus "perfect"
W/m2 (visible light only) W/m2 ±180 (over full Silicon diode band) either given environment camera specification), focal plane sensor specification).
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CMOS image sensors Concepts
color sensitivity CMOS
There ways give 3-color sensitivity area image sensor
Slow Static objects only Only controlled environment optimal resolution optimal speed "normal" sensor with extra layers color information same pixel only color pixel available: reconstruction? artifacts? non-standard CMOS color separation time domain multiplexing
Photonics West 2000 Short Course CMOS image sensors Concepts
contra
sequential illumination with standard -sensor
light sources filters
triple sensor red-green-
Delicate construction
blue dichroic filters
red-green-blue polymer color
filter pattern
Vertically spectrally separate
photodiodes
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vertically stacked photodiodes pixel
Color filter array pixel array (every pixel only color)
light
absorbsion
photo diodes
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CMOS image sensors Concepts

Viewpoint architectures Methods scanning Methods light detection

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