CHAPTER 2 Fundamentals of CMOS Image Sensors Mixed-Signal IC Design for Image Sensor 2-1
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-2
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-3
Architecture of a CIS Mixed-Signal IC Design for Image Sensor 2-4
Photo-generated Carriers The amount of photo-generated carriers - Depends on the semiconductor material - Described by the absorption coefficient α Mixed-Signal IC Design for Image Sensor 2-5
Absorption Coefficient Absorption coefficient α Absorption length L abs Visible light (0.4 ~ 0.6um) L abs 0.1~10um Mixed-Signal IC Design for Image Sensor 2-6
α : Depends on Material Mixed-Signal IC Design for Image Sensor 2-7
Behavior of Minority Carriers Large absorption length of IR Photo-generated minority carrier in substrate Long life time & diffusion Crosstalk Need IR cut filter to suppress blur effect Mixed-Signal IC Design for Image Sensor 2-8
Diffusion Length vs. Impurity Conc. Diffusion lengths > 10um with concentration < 1E18cm -3 Mixed-Signal IC Design for Image Sensor 2-9
Quantum Efficiency and Sensitivity Sensitivity I L : Photocurrent, P 0 : unit light power Quantum efficiency # of generated carrier / # of input photons Maximum sensitivity (at η Q = 1) Mixed-Signal IC Design for Image Sensor 2-10
Sensitivity of Silicon Mixed-Signal IC Design for Image Sensor 2-11
Quantum Efficiency and Sensitivity Light intensity PN-junction structure Mixed-Signal IC Design for Image Sensor 2-12
Quantum efficiency Sensitivity Mixed-Signal IC Design for Image Sensor 2-13
QE: Dotted Sensitivity : Solid Mixed-Signal IC Design for Image Sensor 2-14
QE in P-N Junction Mixed-Signal IC Design for Image Sensor 2-
QE in P-N Junction Mixed-Signal IC Design for Image Sensor 2-
Structures of PDs, PGs, and PTrs. photogate photodiode V-phototransistor L-phototransistor Mixed-Signal IC Design for Image Sensor 2-17
Forward current PN Junction Diode Photocurrent of the pn-junction PD Mixed-Signal IC Design for Image Sensor 2-18
PD I V Curves Mixed-Signal IC Design for Image Sensor 2-19
Solar cell mode (get V oc from I L =0) PD mode (reverse biased ) Avalanche mode - Used in an avalanche photodiode (APD) : single photon detection Mixed-Signal IC Design for Image Sensor 2-20
Dark Current G R current (generation-recombination) Thermal generation Diffusion current Minority diffusion Surface leak current Si-SiO 2 interface Mixed-Signal IC Design for Image Sensor 2-
Dark Current Diffusion current T I diff Tunnel current - Band-to-band tunneling (BTBT) - Trap-assisted tunneling (TAT) Mixed-Signal IC Design for Image Sensor 2-22
Dark Current G R current (generation-recombination) Thermal generation Surface leak current Mixed-Signal IC Design for Image Sensor 2-23
Surface Recombination SiO 2 interfaces the surface of the silicon Produce surface states or interface states Carriers near the surface are trapped Degrade the quantum efficiency or sensitivity (critical for short wavelength light) Mixed-Signal IC Design for Image Sensor 2-
Dependence of I dark (T, V) Mixed-Signal IC Design for Image Sensor 2-25
Temporal Noise Shot noise Signal-to-noise ratio (SNR) for shot noise Mixed-Signal IC Design for Image Sensor 2-26
Temporal Noise Thermal noise - Johnson noise or Nyquist noise - Appears as ktc noise Mixed-Signal IC Design for Image Sensor 2-27
Spatial Noise Fixed Pattern Noise (FPN) Pixel uniformity Readout circuit uniformity Array FPN Column FPN Mixed-Signal IC Design for Image Sensor 2-28
Photon Transfer Curve Mixed-Signal IC Design for Image Sensor 2-29
Speed Some kinds of smart image sensors need a PD with a fast response - for optical wireless LANs - measure time-of-flight (TOF) Mixed-Signal IC Design for Image Sensor 2-30
The response of a PD is limited by (1) CR time constant (2) Transit time (3) Diffusion time of minority carriers Mixed-Signal IC Design for Image Sensor 2-31
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-32
Photogate Mixed-Signal IC Design for Image Sensor 2-33
Phototransistor Amplifies a photocurrent by a factor of the base current gain β - Not high, typically about 10 20 - Large variation ( produces FPN ) When base width increases, the quantum efficiency increases but the gain decreases Mixed-Signal IC Design for Image Sensor 2-34
Avalanche Photodiode (APD) High-speed response Optical fiber communication Ultra low light detection (single photon) Spike-current output (not for imaging) Require a high voltage over 100 V Low voltage operation possibility at sub-micron technology Mixed-Signal IC Design for Image Sensor 2-35
Avalanche Photodiode Structure Mixed-Signal IC Design for Image Sensor 2-36
Photoconductive Detector (PCD) Typically has a structure of n + n n + Gain originates from two parts - Long lifetime of holes τ p - Short transit time of electrons t tr Mixed-Signal IC Design for Image Sensor 2-37
MSM Photodetector Metal-Semiconductor-Metal (MSM) Photodetector A kind of PCD Easy to fabricate Ultra-fast photodetectors (GaAs MSM) Image sensors with a sensitivity in the UV region (GaN MSM) Mixed-Signal IC Design for Image Sensor 2-38
Structure of MSM Photodetector Mixed-Signal IC Design for Image Sensor 2-39
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-40
Architecture of a CIS Mixed-Signal IC Design for Image Sensor 2-41
Active & Passive Pixel Mixed-Signal IC Design for Image Sensor 2-42
Active & Passive Pixel Mixed-Signal IC Design for Image Sensor 2-43
Accumulation Mode Electrical floating PD as capacitor Accumulation of photo-generated carrier is interpreted as discharge process. R ph = 0.3A/W, PD area A = 100um 2, illumination L o = 100lux, 1lux~= 1.6x10-7 W/cm 2 Accumulation is necessary at video rate Mixed-Signal IC Design for Image Sensor 2-44
Accumulation Mode Junction capacitance (depends on applied bias V) For C PD (V) : linearly proportional to I ph + I d Mixed-Signal IC Design for Image Sensor 2-45
Voltage Drop of a PD Mixed-Signal IC Design for Image Sensor 2-46
Potential Description Mixed-Signal IC Design for Image Sensor 2-47
Mixed-Signal IC Design for Image Sensor 2-48
Behavior of Carriers in PD Mixed-Signal IC Design for Image Sensor 2-49
Pinned Photodiode (PPD) Also buried photodiode (BPD) Has less dark current than a conventional PD Has an accumulation region with complete depletion Need precise fabrication process control Mixed-Signal IC Design for Image Sensor 2-50
Photo-Generated Carriers in PPD Mixed-Signal IC Design for Image Sensor 2-51
Passive Pixel Sensor Advantages - very simple - large fill factor (FF) Disadvantages - large smear (?) - large k B TC noise - large column FPN Mixed-Signal IC Design for Image Sensor 2-52
On-Chip Column Amplifier Gain loss due to large C C Mixed-Signal IC Design for Image Sensor 2-53
Transversal Signal Line (TSL) Drastically reduces smear (?) k B TC noise is reduced Little switching noise Mixed-Signal IC Design for Image Sensor 2-54
Active Pixel Sensor, 3T-APS Basic pixel circuits of a 3T-APS Mixed-Signal IC Design for Image Sensor 2-55
Issues with 3T-APS It is difficult to suppress k B TC noise PD simultaneously acts as a photo conversion region. This constrains the PD design Full-well capacity and the conversion gain have a trade-off relationship Mixed-Signal IC Design for Image Sensor 2-56
Active Pixel Sensor, 4T-APS Basic pixel circuits of the 4T-APS Mixed-Signal IC Design for Image Sensor 2-57
Issues with 4T-APS The additional transistor reduces the FF Image lag may occur when the accumulated signal charge is completely transferred into the FD It is difficult to establish fabrication process parameters Mixed-Signal IC Design for Image Sensor 2-58
Incomplete Charge Transfer Mixed-Signal IC Design for Image Sensor 2-59
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-60
Addressing scanner decoder Mixed-Signal IC Design for Image Sensor 2-61
Typical method Random Access Mixed-Signal IC Design for Image Sensor 2-62
Random Access A different types of random access Mixed-Signal IC Design for Image Sensor 2-63
Source follower Readout Circuits Mixed-Signal IC Design for Image Sensor 2-64
Correlated Double Sampling (CDS) Eliminate the threshold mismatch (FPN) of readout path Eliminate thermal (ktc) noise from reset transistor Mixed-Signal IC Design for Image Sensor 2-65
3T CDS (with ktc noise) Mixed-Signal IC Design for Image Sensor 2-66
4T CDS (no ktc noise) Mixed-Signal IC Design for Image Sensor 2-67
Alternative CDS Mixed-Signal IC Design for Image Sensor 2-68
CDS with Differential Output Typical circuitry for CDS with a 4T-APS pixel circuit Mixed-Signal IC Design for Image Sensor 2-69
Timing chart of CDS Mixed-Signal IC Design for Image Sensor 2-70
4T Differential Delta Sampling (DDS) Mixed-Signal IC Design for Image Sensor 2-71
Rolling Shutter Mixed-Signal IC Design for Image Sensor 2-72
Shape Distortion Mixed-Signal IC Design for Image Sensor 2-73
Mechanical Shutter (Rolling Rst) Mixed-Signal IC Design for Image Sensor 2-74
Mechanical Shutter (Global Rst) Mixed-Signal IC Design for Image Sensor 2-75
Electronic Global Shutter Mixed-Signal IC Design for Image Sensor 2-76
Mode Change & Dead Frame Mixed-Signal IC Design for Image Sensor 2-77
Sub-Resolution Mixed-Signal IC Design for Image Sensor 2-78
Pixel Binning Mixed-Signal IC Design for Image Sensor 2-79
Analog Front End Mixed-Signal IC Design for Image Sensor 2-80
Analog-to-Digital Converters Chip-shared structure Pipelined ADC Flash ADC Column parallel structure Single slope ADC Successive approximation ADC Cyclic ADC Delta-Sigma ADC Mixed-Signal IC Design for Image Sensor 2-81
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-82
Reset Noise k B TC noise Mixed-Signal IC Design for Image Sensor 2-83
Reset Method Soft reset - V PD slowly reaches V dd V th - Image lag Hard reset - gate voltage is larger than V dd - reset action finishes quickly - k B TC noise Mixed-Signal IC Design for Image Sensor 2-84
Flushed Reset Hard reset first Then soft reset to reduce k B TC noise Mixed-Signal IC Design for Image Sensor 2-85
Dynamic Range The ratio of the maximum signal range to the minimum signal range Determined by two factors Noise floor (min.) Well charge capacity (max.) DR Full Well Charge 20log( Noise floor ) Mixed-Signal IC Design for Image Sensor 2-86
Color Reproduction There are three ways to realize color in a conventional CMOS image sensor - On-chip color filter type - Three imagers type - Three light sources type Mixed-Signal IC Design for Image Sensor 2-87
On-chip Color Filter Type Three colored filters are directly placed on the pixels - RGB or CMY complementary color filters Mixed-Signal IC Design for Image Sensor 2-88
Three Imagers Type High color fidelity Complicated optics Expensive Mixed-Signal IC Design for Image Sensor 2-89
Three Light Sources Type Mainly used in medical endoscopes Color fidelity is excellent Time to acquire a whole image is long Mixed-Signal IC Design for Image Sensor 2-90
Multiple Junction Pixel Mixed-Signal IC Design for Image Sensor 2-91
Pixel Sharing Mixed-Signal IC Design for Image Sensor 2-92
Zigzag Placement of RGB Pixels Mixed-Signal IC Design for Image Sensor 2-93
Comparison of Pixels Mixed-Signal IC Design for Image Sensor 2-94
Outline Photoelectric Effect Photodetectors CMOS Image Sensor(CIS) Array Architecture CIS Peripherals Design Considerations Evaluations of Image Sensors Mixed-Signal IC Design for Image Sensor 2-95
Evaluation Items Mixed-Signal IC Design for Image Sensor 2-96
Evaluation Environment Mixed-Signal IC Design for Image Sensor 2-97
Factors of Environment Light Source Imaging Lens Optical Filter Alignment Temperature Evaluation board Mixed-Signal IC Design for Image Sensor 2-98
Evaluation Methods Dark Characteristic Average dark current Temporal noise FPN (DSNU) Illuminated Characteristic Response Linearity Sensitivity Saturation SNR (Dynamic range) FPN (PRNU) Mixed-Signal IC Design for Image Sensor 2-99
Spectral Response Angular Response Smear Resolution Image Lag Defects Evaluation Methods Mixed-Signal IC Design for Image Sensor 2-100
Photoconversion Mixed-Signal IC Design for Image Sensor 2-101
Linear Saturation Mixed-Signal IC Design for Image Sensor 2-102
Dark FPN Measurement Mixed-Signal IC Design for Image Sensor 2-103
Spectral Response Mixed-Signal IC Design for Image Sensor 2-104
Incident Light Angle Control Mixed-Signal IC Design for Image Sensor 2-105
Angular Response Mixed-Signal IC Design for Image Sensor 2-106
Image Lag Measurement Mixed-Signal IC Design for Image Sensor 2-107