Possibility of Limiting the Radiation
Damage Effects in CCDs
Mark Robbins
Space and Communications Group
Marconi Applied Technologies
mark.robbins@eev.com
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HST CTE Workshop 2000
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Overview
l Introduction
l Primary damage
n
n
Voltage shift
Bulk damage
l Results of primary damage (Bulk only)
n
n
Dark signal, RTS, DSNU, Spikes
CTE Degradation
l Structures for CTE improvement
n
n
Charge injection & dump drain
Supplementary buried channel (SBC)
l Not covered are surface dark signal issues
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Introduction
l Effects of radiation damage can be reduced
by:
n
reducing the damage to the material/structure
(primary damage) e.g.
reducing the charging of the oxide
u modifying defects created in the bulk Si
u
n
reducing the effects of the damage e.g.
setting biases to accommodate voltage shift
u changing the temperature
u choosing appropriate CCD structure
u choosing appropriate operating mode
u
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Primary Damage: voltage shift
l Charging of the oxide increases the effective
bias applied to gates
l For ‘Standard’Marconi CCD process
(for Co60 or p>10 MeV)
n
n
Unbiased during irradiation ~14 mV/krad(Si)
Biased during irradiation ~100 mV/krad(Si)
l Can reduce shift by modifying the process
n
n
Produced TV imagers for decommissioning
type applications
Survive in excess of 1 Mrad(Si) whilst
operating
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Marconi Radiation Tolerant Process
6
"Standard":
100
mV/krad(Si)
5
Voltage Shift (Volts)
X: 17 mV/krad(Si)
4
3
Y: 3.3 mV/krad(Si)
2
1
Z: 1.5 mV/krad(Si)
0
0
200
400
60
Co
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600
800
1000
Ionising Dose (krad(Si))
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1200
Ionisation Damage: voltage shift
l The Voltage shift that can be survived is
dependent on the required device
performance. Very approximately, for most
applications:
n
n
~1.5 - 2 Volt shift can be accommodated by
choosing optimum biases prior to irradiation
~3 - 4 Volt shift can be accommodated by
tracking the CCD biases during irradiation.
l The device will continue to image after
greater shifts but with significantly reduced
performance.
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Primary Damage: bulk damage
l 3 phases to defect creation
n
n
Generation of vacancies, interstitials and
multivacancies due to atomic displacement
Diffusion and reaction e.g.
I+Cs>Ci, Ci+Cs>CC, V+O>VO, V+P>VP, V+V>VV
n
Annealing of the damage
l Possible to affect defects created
n
Defect engineering (See CERN ROSE
collaboration)
Bulk dark signal dependent on divacancies,
independent of material type and scales well
with NIEL
u Reaction of interstitials and vacancies is
dependent on material (e.g. p-channel CCD)
u
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Bulk Damage: dark signal
l Mean level proportional to NIEL and volume
of depleted silicon
− 6616 
∆s(e / p / s ) ≈9.85 ×10 − 6 ×V ×Φ ×NIEL ×T 2 exp

T


l For 1011 10 MeVp/cm2 bulk dark signal will be:
n
n
~9 nA/cm2 for CCD47 (non IMO at 293K)
~7 nA/cm2 for CCD55 (IMO at 293K)
l Independent of silicon type (high/low res.)
l Anneals
n
Factor 2 reduction after ~ 3 months
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Bulk Damage: DSNU
l Dark Signal Non Uniformity (DSNU)
n
Caused by the stochastic variation in the
energy deposited by the protons
Dark signal distribution at 298K from a CCD47-20 Non
IMO after 3.36 1010 60 MeV p/cm2
0.4
Probability Density
0.35
Measurement
Theory
0.3
0.25
0.2
0.15
0.1
0.05
0
-3
-1
1
3
5
Dark Signal - Mean (nA/cm2)
n
DSNU dependent on pixel volume
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Bulk Damage: dark signal spikes
l Dark signal outside main distribution
n
Caused by field enhanced emission from
defects in high field regions
Field enhanced emission from Coulombic defects
CCD47, 1 Phase high, 3d Pool Frenkel
Enhancement Factor
14
12
10
8
6
4
2
0
0
1
2
3
Distance Into Silicon (µm)
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6
4
5 12
10
4
2
0
8
Distance Across Pixel (µm)
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Bulk Damage: CTE degradation
l N-channel CCD
n
n
Dominated by trapping at Si-E and V-V
After 1010 10 MeV pcm-2 there will be:
~2 1011 cm-3 Si-E centres
u ~3 1010 cm-3 V-V centres
u
l Interaction between the charge packet and
traps is complicated
n
n
n
n
CTE dependent on signal level (density)
Temperature
Clock timing
Nature of the image being observed
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CTE Degradation: trapping theory
l An enlightening analytical relationship can be
obtained for uniformly spaced/sized charge
packets with no background
(
)

 t0 
 te 
⌠ 1 − exp − t g (1/ τe + 1/ τ c )



Slost = Nt 
dV ×
−
− exp
−
exp






1 + τc / τe
⌡
 τe 
 τe 

te = time allowed for emitted charge to rejoin signal
t0 = time between charge packets
t g = time under gate
τc = capture time constant = 1/(σnv th n )
τe = emission time constant = exp(E / kT ) /(σn X nv th n )
Nt = trap density
n = signal density (varies across pixel, signal size dependant)
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CTE Degradation: trapping theory
l Low CTI (=1-CTE) if τe >> t0
n
traps remain filled by preceding charge packet
l Low CTI if τe << te
n
trapped signal rejoins the charge packet
l Low CTI if τc >> tg
n
n
signal not trapped in time spent under a gate
note tc ∝ 1/n and will vary across the charge
packet
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Bulk Damage: CTE degradation
Effect of temperature and clock timing (τc << tg)
1
t0 = 30 ms, te = 66 µs
t0 = 30 ms, te = 33 µs
t0 = 1 ms, te = 33 µs
0.9
0.8
Relative CTI
0.7
0.6
0.5
0.4
V-V
Ec-E = 0.21eV
0.3
σnXn = 5 10
-16
cm2
0.2
Si-E
Ec-E = 0.44eV
0.1
σnXn = 2 10
0
100
150
200
250
Temperature (K)
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-15
cm2
300
Bulk Damage: CTE degradation
l Effect of reducing the time spent under a gate
Readout register of CCD01, Rφ1+Rφ2+Rφ3 = 12 µs, T=250K
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Bulk Damage: CTE degradation
l Number of traps the signal interacts with is
dependent on the signal volume
l Volume occupied by the signal is dependent
on the signal size
l The smaller the signal the greater the number
of traps ‘seen’per signal electron
n
smaller signal ⇒ greater CTI
l Prediction is complicated by the fact that
charge distribution in the pixel is not uniform
l Require 2d/3d device simulation
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Bulk Damage: CTE degradation
CCD02: ~270k Electrons
signal density (cm-3)
2.4E+16
2.0E+16
1.6E+16
1.2E+16
8.0E+15
4.0E+15
9.5
0.0E+00
0.0
6.6
0.1
0.2
0.3
0.4
0.5
3.6 distance across
pixel (µm)
distance into silicon (µm)
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Bulk Damage: CTE degradation
CCD02: ~820 Electrons
signal density (cm-3)
2.0E+15
1.6E+15
1.2E+15
8.0E+14
4.0E+14
9.5
0.0E+00
0.0
6.6
0.1
0.2
0.3
0.4
0.5
3.6 distance across
pixel (µm)
distance into silicon (µm)
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CTE Improvement: background signal
Modelled CCD02 (2d/3d)
11
-3
Trap Density = 1.4 10 cm Dwell time per phase = 0.66 µs,
-15
2
σn = 2 10 cm , τe = 100 µs
0.0006
T = 273K
0.0005
CTI
0.0004
0.0003
150 electrons background
500 electrons background
2000 electrons background
18000 electrons background
Hopkins et al 1994, 150 e background
Hopkins et al 1994, 16000 e background
0.0002
0.0001
Equivalent 10 MeV proton fluence ~7.2 109 cm2
0
0
50000
100000
150000
Signal Size (electrons)
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Structures for CTE Improvement
Charge injection structure
Supplementary buried channel (SBC)
Dump drain
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CTE Improvement: charge injection
l Inject a row of charge at the ‘top’of the
Emission Time Constant (µs)
device
l Reduces t0 and keeps traps filled
l Effectiveness depends on the emission time
constant of the traps
1000000
Si-E
-15
2
σnXn = 2 10 cm
E = 0.44 eV
100000
10000
1000
100
10
1
0.1
0.01
100
V-V
-16
2
σnXn = 5 10 cm
E = 0.21 eV
150
200
250
300
Temperature (K)
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CTE Improvement: charge injection
Injected rows of charge at the start of integration
Object being observed
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CTE Improvement: charge injection
XMM-EPIC: Marconi CCD22 (baselined for SWIFT)
50% Mission Proton Fluence, T=180K
No charge
injection
Charge injection
recovery
Zero proton fluence spectral response is 118 eV FWHM @ 4510 eV
Data supplied courtesy of Paul Bennie, Space Research Centre,
Leicester University, UK.
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CTE Improvement: dumping charge
l Use Dump drain to rapidly dump unwanted
rows
l Effectively window around the required object
l Increases line rate for most of the transfers
n
n
reduces t0 with a possible improvement in CTI,
dependent on temperature
if tg << τc for most of the transfers CTI will be
improved
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CTE Improvement: SBC
l CTI dependant on number of traps ‘seen’by
the signal
n
n
restricting signal to smaller volumes generally
improves CTE (provided tg>τc)
Use smaller pixels or confine signal to a
smaller volume within a pixel:
u
Use supplementary buried channel (SBC)
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CTE Improvement: SBC
CCD43 Simulation 4 µm SBC
Distribution of 17000 electrons signal
Signal Density
Depth into Si
(µm)
0.6
0.4
0.2
5
6
7
8
9
10
Distance Across Pixel (µm)
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CTE Improvement: SBC
CCD43 Simulation 2 µm SBC
Distribution of 15000 electrons signal
Signal Density
Depth into Si
(µm)
0.6
0.4
0.2
4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 10.5
Distance Across Pixel (µm)
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CTE Improvement: SBC
SBC in CCD43 (2d/3d Simulation)
Number Trapped per 3 Phase
Transfer
10
1
0.1
100
No SBC
5µm SBC
4µm SBC
3µm SBC
2µm SBC
1µm SBC
Trap Density = 1.4 1011 cm-3,
Dwell time = 1 s, σn = 10-15 cm2, τe = 1
ms
1000
10000
100000
Signal (electrons)
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CTE Improvement: SBC
SBC in CCD43 (2d/3d Simulation)
Ratio CTI/ CTI with no SBC
1.4
Trap Density = 1.4 1011 cm-3,
Dwell time = 1 s, σn = 10-15 cm2, τe = 1
ms
1.2
1
0.8
0.6
5µm SBC
4µm SBC
3µm SBC
2µm SBC
1µm SBC
0.4
0.2
0
100
1000
10000
100000
Signal (electrons)
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CTE Improvement: SBC
SBC in CCD43 (2d/3d Simulation)
Number Trapped per 3 Phase
Transfer
10
Trap Density = 1.4 1011 cm-3,
Dwell time = 5 µs, σn = 10-15 cm2, τe = 1
ms
1
0.1
100
No SBC
5µm SBC
4µm SBC
3µm SBC
2µm SBC
1µm SBC
1000
10000
100000
Signal (electrons)
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CTE Improvement: SBC
SBC in CCD43 (2d/3d Simulation)
Ratio CTI/ CTI with no SBC
1.4
Trap Density = 1.4 1011 cm-3,
Dwell time = 5 µs, σn = 10-15 cm2, τe = 1
ms
1.2
1
0.8
0.6
5µm SBC
4µm SBC
3µm SBC
2µm SBC
1µm SBC
0.4
0.2
0
100
1000
10000
100000
Signal (electrons)
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CTE Improvement: SBC
SBC in CCD43 (2d/3d Simulation)
Ratio CTI/ CTI with no SBC
1.4
1.2
Trap Density = 1.4 1011 cm-3,
-15
2
σn = 10 cm , τe = 1 ms
1
0.8
0.6
0.4
5µm SBC 1s Dwell
5µm SBC 5µs Dwell
2µm SBC 1s Dwell
2µm SBC 5µs Dwell
0.2
0
100
1000
10000
Signal (electrons)
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100000
CTE Improvement: SBC
l Reducing SBC width reduces CTI for small
signals
l However, variation in width ⇒ variation in
channel potential, worse for narrow SBC.
n
Creation of potential pockets and traps
Max Potential (Volts)
11
1 µm SBC
2 µm SBC
3 µm SBC
4 µm SBC
5 µm SBC
10
9
8
Marconi CCD43
7
2.5
3.5
4.5
5.5
6.5
7.5
8.5
9.5
10.5
11.5
12.5
Distance Across Pixel (µm)
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Marconi device summary
l Example of CTE control features on Marconi
devices
Device Example
SBC DD IS
Programme/Type
CCD01 SLAC VERTEX
CCD12 JET-X
CCD15 XMM-RGS
CCD22 XMM-EPIC
CCD25 MERIS
CCD30 Spectroscopy
CCD42 Large area Astro
CCD43 Large area Astro
CCD44 Large area Astro
CCD47 Gen purpose Sci
CCD57 Gen purpose Sci
CCD64 SXI
etc…
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Date
1985
1991
1994
1992
1993
1993
1995
1996
1997
1996
1997
1998
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Conclusion
l Various operating schemes and structures
have been devised to reduce the observed
degradation
l Must consider what trade-offs are involved
n
not all solutions are appropriate
e.g. using 2 µm SBC for images >10,000 e is
not the best solution
l Modelling can yield useful insights
n
desirable to know actual device structure
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