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SensorsCon 2013:
Residual Bulk Image in
CCD Image Sensors and
How to Tame it
Richard Crisp
Vice President and Chief
Technologist, Invensas Corp
March 6, 2013
Outline
Introduction to Invensas
Image Sensor Market and CCD-dominant applications
Residual Bulk Image (“RBI”) in long exposure CCD
images
Root causes of RBI
Characterization of RBI
Management of RBI
Impact on noise
Summary/Conclusion
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Who is Invensas?
Formation: Founded in 2011 as a wholly owned subsidiary of Tessera
Technologies, Inc. (Nasdaq:TSRA)
Goal: Develop and commercialize breakthrough semiconductor
interconnect solutions and IP in Mobile, Storage and Cons. Electronics.
Core Focus: “Interconnectology”: adv. interconnect, semiconductor
packaging, memory circuitry, modules, 3D TSV systems/architecture.
Company: 50+ Employees (1/3 PhD). Headquarters: San Jose, CA.
IP: ~1000 patents and applications.
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Invensas Product Capabilities
Courtesy ALLVIA
Courtesy ALLVIA
Invensas has advanced prototyping capabilities from wirebond and flip-chip to
3D-TSV Circuit Level to Board and System Level
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Invensas Technology Platforms
MEMORY (xFD):
• DIAP for Ultrabooks & Tablets.
• xFD for Servers and Datacenters.
WAFER LEVEL PACKAGING:
• Fan-Out Wafer Level Packaging
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• Stacked WLP for Flash Memory/SSD
MOBILE (BVA):
• Bond Via Array for Ultra Smartphones
• Cu-Pillar PoP for Smartphones/Tablets
3D-IC:
• Fine-Pitch TSV Interposers
• 3D Memories
• 3D Capacitors
3DIC Total Integration Solution & Team
System
Design
Component
Selection
Hardware
Design
Signal Integrity
Analysis
Algorithm
Design
SEMATECH’s 3D
Enablement Center
RTL Design
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Prototype
Build
Embedded
Firmware Design
Functional Test
Image
Characterization
Developing Robust and Cost Competitive 3D-SIC Packaging Solutions
o
o
o
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Know How – Extensive body of practical knowledge to enable the bleeding edge 3D-SIC technology
today.
Expertise – Experienced personnel (50+ engineers/scientists) to produce TSV based devices and
develop the next generation of 3D-SIC assembly processes.
Capabilities – Access to complete front end prototype development line and internal packaging
assembly technology to develop robust and cost competitive packaging assembly technology.
Invensas 3DIC Solutions
Ability to produce “Xilinx Vertex-7 like” designs.
Equivalent node to TSMC. Underpinned with IP.
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System Integration: Invensas Si-Interposer Assembly
Micro-bumped Die
Micro-bumped Die
Si-Interposer
Micro-bumped Interconnects
Si Interposer
3-4-3 Build Up Substrate
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C4 Interconnects
CCD Market and Key Applications
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CCD market share
Declining proportion of total sensors market
But dominant in scientific applications
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Scientific Applications Dominated by CCD
Long Exposure
(minutes not
milliseconds)
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Life on the other end of the curve
Most of the industry volume
CMOS smartphone cameras etc
Focus area of this talk
(long exposure CCD)
Worries:
Quantum Efficiency
Dark signal
Residual Image
Photons
Source: Catrysse
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RBI example in long exposure CCD scientific image
Image with RBI
(is that a nebula beside the bright star?)
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RBI example in long exposure CCD scientific image
Image with RBI
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Actual starfield
(the “nebula” was RBI)
Slow RBI decay revealed in Long Exposure Dark Images
Light Image
2hours later
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Immediately following
3hours later
1hour later
4hours later
5 minute dark exposures at -20C operating temperature
Dark Image Example Showing Accumulated RBI
Five minute dark exposure
following
four light exposures
-20C operating temperature
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Root cause of RBI
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Basic CCD Structure
KAF39000
39Mpixels @
6.8 microns
49 x 36.8 mm
Phase 1
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Phase 2
Phase 3
Phase 1
CCD Exposure and Readout
PHOTONS PARTICLES
PIXELS
HORIZONTAL
SHIFT
REGISTER
ELECTRONS
VERTICAL
SHIFT
REGISTERS
MEASUREMENT
Source: Janesick
SERIAL READOUT
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Charge Transfer for Readout by Phase Clocking
Negative charge (e-)
Follows clock high phase
Possible for Trapped Charge
Leakage Into Subsequent
Images
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RBI Mechanisms
Epi interface trapping sites
o Spectral dependence
Stress-induced trapping sites in lattice from crystal growth
process
o Swirling shapes in darks
Random bulk defects in crystal lattice
o No spectral dependence or swirling shapes
EPI
Bulk Wafer
Wafer cross section
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Trapping sites
Residual
stresses
from crystal
growth
(boule
rotates as
pulled from
melt)
Wafer Mapping example
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Die 1
Die 2
Die 3
Die 4
Die 5
Die 6
Die 7
Die 8
Die 9
Characterization
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RBI Trap Decay Measurement
Flood/Flush
Dark
yes
Detectable
RBI?
no
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Fills traps, flushes pixel
Traps leak during dark
exposure adding signal vs
reference dark
Comparison made to leakagefree reference dark
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27
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KAF09000 RBI trap exhaustion vs temperature
Trap Exhaustion Time vs Temperature
NIR Flooded RBI Trap Exhaustion Time (seconds)
1000000
Trap Exhaustion Time
100000
10000
1000
15
10
5
0
-5
-10
Temperature (C)
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-15
-20
-25
-30
-35
Management of RBI
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Management of RBI
A method to manage RBI is to begin all integrations with filled traps
The procedure is:
o Flood the CCD using LED at least 10x past saturation
o Flush the saturated sensor
o Begin integration
Flood Sensor
Flush sensor
Integrate
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Management of RBI: Flood-Flush-Integrate protocol
Image
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Subsequent dark
Amplifier luminescence reduced via flood/flush/integrate
30 minute dark with no flood.
Significant amplifier luminescence
Post-flood 30 minute dark frame
Negligible amplifier luminescence
The result hints at a power-up transient bias condition
initiating impact ionization in CCD’s output amplifier
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Amplifier luminescence
GATE (V G = 0 V)
0.0
NITRIDE
OXIDE
DRAIN
V DD = 20 V
0.2
0.4
ELECTRIC
FIELD
0.6
DEPLETION
EDGE
0.8
PINCHOFF
REGION
1.0
1.2
DEPLETION
EDGE
1.4
1.6
7.6
8
8.4
8.8
9.2
DISTANCE, MICRONS
Source: Janesick
Source: Janesick
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9.6
Power Up Transient
Sensor array is saturated at power-up
15V
-> output Source Follower transistor is in pinch-off
High drain field causes impact ionization -> luminescence
λ
7V
3V
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Luminescence loads nearby RBI traps and gradually decays
Dark FPN
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DARK FPN: without and with light flood
No Light Flood
(900 second dark)
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With Light Flood
(900 second dark)
DARK FPN: Before and after Calibration
With Light Flood
not calibrated
(900 second light)
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With Light Flood
After calibration
(dark subtraction and flat-fielding)
(900 second light)
Noise considerations
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Impact on noise: charge leaking from traps
camera _ noise = read _ noise + dark _ signal
2
Thermal_charge + trap_leakage_charge
[ignoring dark fixed pattern noise]
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Maximum operating temperature
camera _ noise = read _ noise + dark _ signal
2
(time-invariant)
(time-dependent)
The maximum operating temperature is reached when the two terms are equal
for the desired exposure time
Noise terms
under radical
Dark signal noise term: f(temp, time)
temp1
temp3
Read noise term: ~constant
t1
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temp2
time
t2
t3
10000
Trap Leakage During Exposure (e-)
Arrhenius Plot of
KAF09000 RBI Trap Leakout
vs
Temperature
vs
Time
5 minute exposure
10 minute exposure
15 minute exposure
1000
20 minute exposure
25 minute exposure
30 minute exposure
225 e-
-37.7C
-49.3C
15 e- read noise
equivalency limit
dynamic range = 6,000:1
or 75.56 dB
-54.7C
-19.2C
-30.3C -44.6C
100 e-
-57.9C -68.5C
-43.1C
10 e- read noise
equivalency limit
dynamic range = 9,000:1
or 79.08 dB
100
-31.5C
-50.8C
-63.8C
25 e-76.1C
-59.9C
-47.3C
-68.5C
5 e- read noise
equivalency limit
dynamic range = 18,000:1
or 85.106 dB
-87.8C
-81.9C
10
3
+60.3C
3.5
4
4.5
-23.0C
-73.0C
1000/T (1/K)
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5
5.5
6
-106.3C
Maximum Operating Temperature vs Time for KAF09000 Meeting Dark
Shot Noise-Limited Constraint
Commencing Exposure With Filled RBI Traps
0
5 e- noise target
-10
Operating Temperature (C)
-20
10 e- noise target
-30
15 e- noise target
-40
-50
-60
-70
-80
-90
-100
0
5
10
15
20
Exposure Time (minutes)
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25
30
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Summary
RBI can disrupt the integrity of scientific images unless
managed
RBI is severe in the evaluated sensor (Eng Grade KAF09000):
residual image detectable 30 hours after exposure with modest
cooling (-30C)
Flood-Flush-Integrate protocol is effective at removing RBI
o Also minimizes observed amplifier luminescence
Prefilled trap leaked charge is typically much larger than dark
signal charge for moderate cooling (down to –30C) in typical
duration scientific exposures (5 – 30 minutes)
Dark Fixed Pattern Noise is observed to be altered due to nonuniform distribution of trapping sites. This DFPN is completely
removed by dark subtraction provided the traps are placed in a
fully-filled state prior to integration
Deep cooling (down to approx -90 C) indicated to minimize shot
noise associated with prefilled trap leakage for half hour
exposures with 5 e- dark shot noise target
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THANK YOU
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