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CMOS Detector Technology
Markus Loose
Alan Hoffman
Vyshnavi Suntharalingam
Rockwell Scientific
Raytheon Vision Systems
MIT Lincoln Laboratory
Scientific Detector Workshop, Sicily 2005
CMOS - 1
Outline
•
•
•
•
General Concept & Architecture
Common Features of CMOS Sensors
Stitching Technology Enables Large Arrays
Monolithic CMOS
• Hybrid CMOS
– History of Hybrid CMOS
– ROIC Input Cells
– Detector Materials & Properties
Markus Loose
Alan Hoffman
• Low Noise Through Multiple Sampling
• CMOS Processing and General Limitations
• Emerging Technologies
– Vertical Integration
– Geiger-Mode Avalanche Photodiode Arrays
• Comparison: CMOS vs. CCD for Astronomy
CMOS - 2
Vyshi Suntharalingam
Collection of High-Performance CMOS Detectors
InSb 2K x 2K,
25 µm pixels
3D stacked CMOS
wafer sandbox
HgCdTe 2K x 2K, 18 µm pixels
Monolithic CMOS
4K x 4K, 5 µm pixels
HgCdTe 4K x 4K mosaic,
18 µm pixels
CMOS - 3
HgCdTe 2K x 2K,
20 µm pixels
General CMOS Detector Concept
CCD Approach
Photodiode
CMOS Approach
Photodiode
Amplifier
+
Pixel
Charge generation &
charge integration
Array Readout
Charge transfer
from pixel to pixel
Sensor Output
Output amplifier performs
charge-to-voltage conversion
CMOS - 4
Charge generation,
charge integration &
charge-to-voltage conversion
Multiplexing of
pixel voltages:
Successively
connect amplifiers
to common bus
Various options possible:
- no further circuitry (analog out)
- add. amplifiers (analog output)
- A/D conversion (digital output)
Control
&
Timing
Logic
(opt.)
Vertical Scanner
for Row Selection
General Architecture of CMOS-Based Image Sensors
Pixel Array
Bias
Generation
& DACs
(optional)
A/D conversion
(optional)
Digital
Output
Horizontal Scanner
/ Column Buffers
CMOS - 5
Analog
Amplification
Analog
Output
Common CMOS Features
• CMOS sensors/multiplexers utilize the same process as modern
microchips
– Many foundries available worldwide
– Cost efficient
– Latest processes available down to 0.13 µm
• CMOS process enables integration of many additional features
–
–
–
–
–
–
Various pixel circuits from 3 transistors up to many 100 transistors per pixel
Random pixel access, windowing, subsampling and binning
Bias generation (DACs)
Analog signal processing (e.g. CDS, programmable gain, noise filter)
A/D conversion
Logic (timing control, digital signal processing, etc.)
• Electronic shutter (snapshot, rolling shutter, non-destructive reads)
– No mechanical shutter required
• Low power consumption
• Radiation tolerant (by process and by design)
CMOS - 6
Special Scanning Techniques Supported by CMOS
• Different scanning methods are available to reduce the number of
pixels being read:
– Allows for higher frame rate or lower pixel rate (reduction in noise)
– Can reduce power consumption due to reduced data
Windowing
• Reading of one or
multiple rectangular
subwindows
• Used to achieve
higher frame rates
(e.g. AO, guiding)
Subsampling
• Skipping of certain
pixels/rows when
reading the array
• Used to obtain
higher frame rates
on full-field images
Random Read
Binning*
• Random access
(read or reset) of
certain pixels
• Selective reset of
saturated pixels
• Fast reads of
selected pixels
• Combining several
pixels into larger
super pixels
• Used to achieve
lower noise and
higher frame rates
* Binning is typically less efficient
in CMOS than in CCDs.
CMOS - 7
Astronomy Application: Guiding
• Special windowing can be used to
perform full-field science integration in
parallel with fast window reads.
 Simultaneous guide operation and science
data capture within the same detector.
• Two methods possible:
– Interleaved reading of full-field and window
• No scanning restrictions or crosstalk issues
• Overhead reduces full-field frame rate
– Parallel reading of full-field and window
• Requires additional output channel
• Parallel read may cause crosstalk or conflict
• No overhead  maintains maximum full-field
frame rate
Full field row
Window
Full field row
Full field row
Window
CMOS - 8
Full field row
Full field row
Window
Electronic Shutter: Snapshot vs. Rolling Shutter
• Snapshot Shutter
• Rolling Shutter (Ripple Read)
– All rows are integrating at the same time.
– Typically more transistors per pixel and
higher noise.
– Each row starts and stops integrating
at a different time (progressively).
– Typically less transistors per pixel and
lower noise.
Row 1
integration time
Row 1
Row 2
integration time
Row 2
Row 3
integration time
Row 3
Row 4
integration time
Row 4
Row 5
integration time
Row 5
start
integrating
CMOS - 9
stop
integrating
Read pixels of
selected row
start 2nd integration if
pixel supports
“integrate while read”
integration time
integr
integration time
start
integrating
inte
integration time
int
integration time
i
integration time
stop
integrating
Read pixels of
selected row
Stitching Enables Large Sensor Arrays
• The small feature size of modern CMOS processes limits the maximum
area that can be exposed in one step (so-called reticle) to about 22 mm.
• However, larger chips can produced by breaking up the design into
smaller sub-blocks that fit into the reticle.
– Sub-blocks are exposed one after
another
– Some blocks are used multiple
times
– Ultimate limit is given by wafer size
Stitched CMOS Sensor
horiscan1
horiscan2
V
1
array
array
array
V
2
array
array
array
V
3
array
array
array
Reticle
horiscan2
horiscan1
22mm
V
1
CMOS - 10
V V
2 3
array
CMOS-Based Detector Systems
• Three possible CMOS Detector Electronics Configurations
Single Chip
– All electronics integrated
in sensor chip
– Small, low system power
– Not always desirable
(high design effort, glow)
Detector Array
Discrete Electronics
– Assembly of discrete
chips and boards
– Large, higher power
– Reusable, modular, only
PCB design required
Dual Chip
– All electronics integrated
in a single companion chip
– Small, low system power
– Can be placed next to
detector => low noise
Detector Array
Requires ext.
ADC, bias
and/or clock
generation
Includes
ADC, bias &
clock
generation
Analog
output
ADC
Detector Array
Requires ext.
ADC, bias
and/or clock
generation
Bias Clocks
Analog
output
DAC
Digital data
Logic
Bias
Clocks
ASIC
Memory
Digital data
Acquisition System
CMOS - 11
Acquisition System
Digital data
Acquisition System
Monolithic CMOS
• A monolithic CMOS image sensor combines the photodiode and the
readout circuitry in one piece of silicon
– Photodiode and transistors share the area => less than 100% fill factor
– Small pixels and large arrays can be produced at low cost => consumer
applications (digital cameras, cell phones, etc.)
3T Pixel
Reset
SF
PD
Select
Read Bus
photodiode transistors
4T Pixel
Reset
Pinned PD
p+
n+
SF
TG
n+
p-sub
Select
Read Bus
CMOS - 12
Complete Imaging Systems-on-a-Chip
• Monolithic CMOS technology has enabled highly integrated,
complete imaging systems-on-a-chip:
– Single chip cameras for video and digital still photography
– Performance has significantly improved over last decade and is
better or comparable to CCDs for many applications.
– Especially suited for high frame rate sensors (> Gigapixel/s) or
other special features (windowing, high dynamic range, etc.)
• However, monolithic CMOS is still limited with respect to
quantum efficiency:
– Photodiode is relatively shallow
=> low red response
– Metal and dielectric layers on
top of the diode absorb or
reflect light
=> low overall QE
– Backside illumination possible,
but requires modification of
CMOS process
• Microlenses increase fill factor:
photodiode
CMOS - 13
2 Mpixel HDTV CMOS Sensor
Quantum Efficiency of a CMOS sensor
Si PIN
NIR AR coating
Si PIN
UV AR coating
3T pixel
w/ microlenses
Outline
•
•
•
•
General Concept & Architecture
Common Features of CMOS Sensors
Stitching Technology Enables Large Arrays
Monolithic CMOS
• Hybrid CMOS
– History of Hybrid CMOS
– ROIC Input Cells
– Detector Materials & Properties
Markus Loose
Alan Hoffman
• Low Noise Through Multiple Sampling
• CMOS Processing and General Limitations
• Emerging Technologies
– Vertical Integration
– Geiger-Mode Avalanche Photodiode Arrays
• Comparison: CMOS vs. CCD for Astronomy
CMOS - 14
Vyshi Suntharalingam
CMOS Processing Evolution for Hybrid Focal Planes
Indium bump hybrid
invented, circa 1975
1975
1980
1985
1990
1995
2000
MOS w/surface channel CCD
PMOS or NMOS
CMOS
CMOS ultimately "won" due to ease
of design and availability of foundries
CMOS - 15
2005
Sensor Chip Assembly (SCA) Structure:
Hybrid of Detector Array and ROIC Connected by Indium Bumps
Detector Array
Indium bump
Detector Array
Silicon Readout Integrated Circuit (ROIC)
Mature interconnect technique:
– Over 4,000,000 16,000,000 indium bumps per SCA demonstrated
– 99.9% interconnect yield
• Also called a Focal Plane Array (FPA) or Hybrid Array
CMOS - 16
Number of Pixels per Array
CMOS SCA Revolution
1E+09
MWIR arrays
1E+08
Moore's law with 18 month doubling time
predicted
1E+07
1E+06
1E+05
1E+04
1E+03
1E+02
1980
1985
1990
1995
2000
2005
2010
Year First used in Astronomy
• Large CMOS hybrids revolutionized infrared astronomy
• Growth in size has followed "Moore's Law" for over 20 years
– 18 month doubling time
CMOS - 17
Input Circuit Schematics
SFD
DI
reset switch
Output S/F FET
CTIA
input
FET
detector
enable switch
load
Cfb
driver
CMOS - 18
Cint
Three Most Common Input Circuits
for CMOS ROICs
Circuit
SFD
(Source Follower per
Detector)
also called "Self
Integrator"
Advantages
•
•
•
•
simple
low noise
low FET glow
low power
CTIA
(Capacitance
Transimpedance
Amplifier)
• very linear
• gain determined by
ROIC design (Cfb)
• detector bias remains
constant
DI
(Direct Injection)
• large well capacity
• gain determined by
ROIC design (Cint)
• detector bias remains
constant
• low FET glow
• low power
CMOS - 19
Disadvantages
• gain fixed by detector
and ROIC input
capacitance
• detector bias changes
during integration
• some nonlinearity
Comments
Most common
circuit in IR
astronomy
• more complex circuit
• FET glow
• higher power
Very high gains
demonstrated
• poor performance at
low flux
Standard
circuit for high
flux
Temperature and Wavelengths of
High Performance Detector Materials
Si PIN
InGaAs
SWIR HgCdTe
MWIR HgCdTe
InSb
LWIR
HgCdTe
Si:As IBC
Approximate detector temperatures for dark currents << 1 e-/sec
CMOS - 20
Detector Material Choices for CMOS Hybrid Arrays
Detector
Material
Spectral
Range*, m
Operating
Temp***, K
Si PIN
0.4 – 1.0
~ 200
InGaAs
0.9** – 1.7
~ 130
HgCdTe:
1.7m
2.5 m
5.2 m
10 m
InSb
Si:As IBC
(BIB)
0.9** – 1.7
0.9** – 2.5
0.9** – 5.2
5 – 10
General Comments
• All detectors can have:
– 100% optical fill factor
– 100% internal QE (total QE
depends on AR coat)
• Exception: Si:As is 40-70%
between 5 and 10 m
~ 140
~ 90
~ 50
~ 25?
0.4 – 5.2
~ 35
5 – 28
~7
• ROICs are interchangeable
among detectors (except Si:As)
• HgCdTe and InGaAs require
special packaging due to CTE
mismatch between detector and
ROIC
* Long wave cutoff is defined as 50% QE point
** Spectral range can be extended into visible range by removing substrate
*** Approximate detector temperatures for dark currents << 1 e-/sec
CMOS - 21
Noise in CMOS SCA/Hybrids
• Temporal
– White (uncorrelated) noise
• Reduced by multiple sampling
– 1/f (drift) noise
• Not a limiting factor in most astronomy focal planes
• Fixed pattern noise
– Caused by residual non-uniformity after calibration
• Can be reduced (eliminated?) by calibrating at multiple points in the
dynamic range
• Random Telegraph Signal (RTS)
– Randomly occurring charge trapping/detrapping events
– Process, design and characterization dependent
• Personal experience: have not seen this
CMOS - 22
CMOS SCA Sampling Techniques
Reset begins integration
• Periodic sampling of detector signal possible during a long integration
• Two general methods of white noise reduction by multiple sampling
– Fowler sampling: average 1st N samples and last N samples; then subtract
– Sample up the ramp (SUTR): fit line (or polynomial) to all samples
CMOS - 23
Example of Noise vs Number of Fowler Samples
100 sec integrations
in all cases
Bare multiplexer
2 e-
Data courtesy of Dr. Craig McMurtry, University of Rochester
CMOS - 24
Example of Fowler and SUTR
Sampling in Uncorrelated (White) Noise Limit
Relative S/N
12
6% difference
10
Peak at Fowler N/3
8
Fowler
SUTR (100 samples)
6
10
20
30
40
Number of Fowler Pairs
CMOS - 25
50
Hybrid CMOS Summary
• CMOS ROIC
– Wide choice of processing foundries and analog circuits
– "System on a chip" is possible
• Clocks & biases
• A/D & DAC
• Any digital function
• Detectors
– Wide choice of detector materials
– Interchangeability among detectors and ROICs
• SCAs
– Up to 4K x 4K arrays successfully hybridized
CMOS - 26
Outline
•
•
•
•
General Concept & Architecture
Common Features of CMOS Sensors
Stitching Technology Enables Large Arrays
Monolithic CMOS
• Hybrid CMOS
– History of Hybrid CMOS
– ROIC Input Cells
– Detector Materials & Properties
Markus Loose
Alan Hoffman
• Low Noise Through Multiple Sampling
• CMOS Processing and General Limitations
• Emerging Technologies
– Vertical Integration
– Geiger-Mode Avalanche Photodiode Arrays
• Comparison: CMOS vs. CCD for Astronomy
CMOS - 27
Vyshi Suntharalingam
Process Comparison
CCD
CMOS
> 35 years of evolution
“Trailing edge” fabs
Economics of scale accelerate progress
Lower fabrication cost, Foundry access
High resistivity (deep depletion) substrates
Controlled temperature ramps & stress control
Epi doping optimized for digital CMOS
Scalable to 300mm
Buried channel
Multiple oxidation cycles
Complex implant engineering
Rapid Thermal Processing (RTP)
Single gate dielectric thickness
Multiple gate dielectric thicknesses
Doped polysilicon (single type)
Complementarily doped polysilicon
Silicided polysilicon and FET source/drain
Highly nonplanar surfaces
Conservative design rules
Fine-line patterning
Multiple metal layers (dense routing)
Vulnerable to space-radiation-induced traps
Highly suitable for long-term space-based
applications
2m
2m
Four-Poly OTCCD
CMOS - 28
180-nm SRAM cell
2m
Stacked via to poly
<0.25m CMOS Technology Features
Feature
0.35 - 0.60 m
0.18 - 0.25 m
Field Isolation
LOCOS
STI
Voltage
3.3 - 5V
1.8 - 2.5V
Gate Oxide
70 - 125A
32 - 50A
Device
Polycide/Poly
Salicide
Junction Profile
Graded Junction
Shallow Junction
Planarization
SOG and Reflow
CMP
Thermal Budget
Furnace Anneal
RTP
Spacer Etch
Oxide spacer
SiN spacer
Dielectric Material
SiO2
SiO2/SiN/SiON
CMOS - 29
Periphery
Pixel
ONO spacer
CMOS Pixel Process Flow
Poly
STI
Double S/D imp
Oxide
Deposit oxide
Spin coat organic material
Organic material
Etch-back and remove oxide
Photo resist
Remove organic material
Pattern oxide (photo/etch)
Silicide
Form silicide on peripheral devices
Adapted from S. Wuu, TSMC
CMOS - 30
Cross Sectional TEM Photograph of Pixel
Silicide gate
non-silicide S/D
0.3um
0.4um
Courtesy S. Wuu, TSMC
CMOS - 31
Limitations of Standard Bulk CMOS APS
Pixel Layout
• Fill factor tradeoff
– Photodetector and pixel transistors share
same area
– PD from Drain-Substrate or Well-Substrate
diode
photodiode
OUT
• Low photoresponsivity
– Shallow, heavily doped junctions
– Limited depletion depth
– Absorption and reflection in poly, metal, and
oxide layers
– Surface recombination at Si/SiO2 interface
– QE*FF > 60% is good, many < 20%
• High leakage
– LOCOS/STI, salicide
– Transistor short channel effects
RST
VDD
ROW
VDD
ROW
n+
p-well
• Substrate bounce and transient coupling
effects
p-epi
p+ Substrate
CMOS - 32
OUT
RST
Field Oxide
n-Well
p+
Advantages of Vertical Integration
Addressing
Conventional Monolithic APS
3-D Pixel
Light
PD
pixel
PD
3T
pixel
ROIC
Processor
Addressing
A/D, CDS, …
• Pixel electronics and detectors
share area
• Fill factor loss
• Co-optimized fabrication
• Control and support electronics
placed outside of imaging area
• 100% fill factor detector
• Fabrication optimized by
layer function
• Local image processing
– Power and noise
management
• Scalable to large-area focal
planes
CMOS - 33
Approaches to 3D Integration
(To Scale)
Tier-1
3D-Vias
3D-Vias
10 m
Tier-2
10 m
Photo Courtesy of RTI
Bump Bond used to
flip-chip interconnect
two circuit layers
CMOS - 34
10 m
Two-layer stack using
Two-layer stack with
insulated vias through Lincoln’s SOI-based vias
thinned bulk Si
Four-Side Abuttable Goal
• 3-D CMOS imagers tiled for large-area focal planes
• Foundry fabricated daughter chip bump bonded to non-imaging
side
pixel
Foundry
Chip
Tile with
Daughter Chip
mechanical mockup
CMOS - 35
Tiled Array
8 mm
Cross Sections Through 3-D Imager
SOI-CMOS
(Wafer 2)
SEM cross section
Photodiode
(Wafer 1)
8 m
decorated
Transistor
CMOS Vias
3D-Via
Bond
Interface
Diode
CMOS - 36
Pixel
5 m
Four-Side Abuttable
Vertically Integrated Imaging Tile
• Wafer-Scale 3D circuit stacking technology
– Silicon photodetector tier
– SOI-CMOS address and readout tier
• Per-pixel 3D interconnections
– 1024x1024 array of 8mx8m pixels
– 100% fill factor
– >1 million vertical interconnections per imager
Front Illuminated
CMOS - 37
Back Illuminated
Presented at 2005 ISSCC
Geiger-Mode Imager: Photon-to-Digital Conversion
Pixel circuit
Digital
timing
circuit
photon
APD
• Quantum-limited sensitivity
• Noiseless readout
• Photon counting or timing
CMOS - 38
Digitally
encoded
photon
flight time
APD/CMOS array
Lenslet
array
Focal-plane
concept
3-D Laser Radar Sensor Development
• Objective: single flash, non-scanned 3D area imager
– Pixel stores range, not intensity, information
• 3-D imaging provides
– Robust object recognition
• relatively independent of lighting, reflectivity
– Separates objects behind foliage, camouflage
Active intensity image
SUV behind camouflage
3-D Brassboard image
SUV
QuickTime™ and a decompressor are needed to see this picture.
Npe= 105
CMOS - 39
Technology Development Evolution
Discrete 4x4
arrays
4x4 arrays wire
bonded to
16-channel
CMOS readout
APD’s
1996
2001
CMOS - 40
32x32 arrays fully
integrated with
32x32 CMOS
readout
3D Laser Radar Focal Plane (3D)2
• Laser radar focal plane based on singlephoton-sensitive Geiger-mode
avalanche photodiodes
– 64 x 64 demonstration circuit (scalable)
– Pixel size reduction from 100 m to 30 m
– Timing resolution reduction from 1 ns to
0.1 ns
– 100x reduction in voxel volume
Tier-3: 1.5V FDSOI CMOS
Tier-2: 3.3V FDSOI CMOS
APD
APD
Tier-1: Avalanche Photodiode
VISA APD Pixel Circuit (~250 transistors/pixel)
Pseudorandom
counter circuit
3D-Integrated Tier1/Tier-2 wafer pair
electrical test
vehicle
APD drive/sense
circuit
150 mm
CMOS - 41
Avalanche PD
Outline
•
•
•
•
General Concept & Architecture
Common Features of CMOS Sensors
Stitching Technology Enables Large Arrays
Monolithic CMOS
• Hybrid CMOS
– History of Hybrid CMOS
– ROIC Input Cells
– Detector Materials & Properties
Markus Loose
Alan Hoffman
• Low Noise Through Multiple Sampling
• CMOS Processing and General Limitations
• Emerging Technologies
– Vertical Integration
– Geiger-Mode Avalanche Photodiode Arrays
• Comparison: CMOS vs. CCD for Astronomy
CMOS - 42
Vyshi Suntharalingam
Comparison CMOS vs. CCD for Astronomy
Property
CCD
Hybrid CMOS
Resolution
> 4k x 4k
2k x 2k in use, 4k x 4k demonstrated
Pixel pitch
10 – 20 µm
18 – 40 µm, < 10 µm demonstrated
Typ. wavelength
coverage
400 – 1000 nm
400 – 1000 nm with Si PIN
400 – 5000 nm with InSb or HgCdTe
Noise
Few electrons
Few electrons with multiple sampling
Shutter
Mechanical
Electronic, rolling shutter
Power Consumption
High
Typ. 10x lower than CCD
Radiation
Sensitive
Much less susceptible to radiation
Control Electronics
High voltage clocks, at Low voltage only,
least 2 chips needed
can be integrated into single chip
Special Modes
Orthogonal Transfer,
Binning,
Adaptive Optics
Windowing, Guide Mode,
Random Access, Reference Pixels,
Large dynamic range (up the ramp)
Silicon PIN hybrid detectors have become a serious alternative to CCDs providing a
number of significant advantages, specifically for large mosaic focal plane arrays.
CMOS - 43
Conclusion
CCD
It’s happening!
CMOS
CMOS - 44