Improving the red wavelength sensitivity of CCDs
Paul R Jorden
27 June 2010
Paul Jorden, Andrew Harris, Andrew Kelt, Pritesh Mistry, Pash Patel
e2v technologies, Chelmsford, UK www.e2v.com
Mark Downing
ESO, Garching, Germany
SPIE San Diego AS10 June 2010
Conf 7742
www.eso.org
High Energy, Optical, and Infrared detectors for Astronomy IV
Introduction
CCD imagers offer very high performance for astronomical imaging and
spectroscopy.
ÆInfrared sensors also offer very high performance but are not available in
the variety (and prices!) available for CCD sensors.
ÆThe cross-over point between the two sensor types is in the region of
1 µm wavelength.
ÆTraditional CCDs have finite silicon thickness and therefore limited
absorption (and efficiency) at wavelengths close to 1 µm.
The range of application for CCDs can be extended by increasing the silicon
thicknessThick silicon devices are described- for use up to the silicon cut-off of 1.1 µm
A new “high-rho” scientific sensor is introduced- the CCD261
Use of an extended wavelength range requires higher performance antireflection coatingsDevelopments of wide range multi-layer AR coatings are described
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Contents
•Introduction
to thicker silicon CCD development
•“Bulk”
CCDs
Intermediate thickness devices with enhanced red response
•“High-rho”
CCDs
Fully depleted sensors with silicon thickness > 100 µm
•Anti-reflection
coatings
Multi-layer coating developments for highest QE over a wide range
•Summary
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Thick CCD sensors- 1
A progression of thickness and red wavelength response•Traditional
CCDs- low resistivity silicon; 10- 16 µm thick; limited red QE
Epitaxial silicon; well established; excellent performance
•e2v
“deep depletion” CCDS- higher resistivity; 40 µm thick; better red QE
Epitaxial silicon; excellent performance; risk of minor blue PSF degradation
•e2v
“bulk” CCDs- highest resistivity; 70 µm thick improved red QE
Bulk silicon; good performance; any device type can be made in this material
•e2v
“high-rho” CCDs- highest resistivity; > 100 µm thick; maximum red QE
Bulk silicon; good performance; requires custom design and HV BSS operation
At a wavelength of 1000 nm QE is almost proportional to device thickness
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Thick CCD sensors- 2
A progression of thickness and red wavelength responseQE versus thickness; at -100°C; e2v astro multi-1 coating
100%
90%
80%
QE(%)
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
standard Si: 16µm
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Thick CCD sensors- 3
A progression of thickness and red wavelength responseQE versus thickness; at -100°C; e2v astro multi-1 coating
100%
90%
80%
QE(%)
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
standard Si: 16µm
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deep depletion Si: 40µm
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Thick CCD sensors- 4
A progression of thickness and red wavelength responseQE versus thickness; at -100°C; e2v astro multi-1 coating
100%
90%
80%
QE(%)
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
standard Si: 16µm
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deep depletion Si: 40µm
bulk Si: 70µm
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Thick CCD sensors- 5
A progression of thickness and red wavelength responseQE versus thickness; at -100°C; e2v astro multi-1 coating
100%
90%
80%
QE(%)
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
standard Si: 16µm
deep depletion Si: 40µm
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bulk Si: 70µm
high-rho Si: 150µm
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“Bulk” CCD devices- 1
•Bulk
silicon (non epitaxial) offers higher resistivity and therefore larger depletion depth
and thicker silicon thickness.
•Any
existing device could be made using this material- offering better QE than standard
epitaxial devices.
•Operation
at normal voltage levels (eg 10V clocks and 0V substrate) limits depletion
depth and therefore limits device thickness- to about 70 µm for typical bulk silicon
•Bulk
silicon does not benefit from intrinsic gettering of epitaxial silicon and can have
poorer cosmetic quality.
•Bulk
silicon CCD44-82 devices (2k X 4k) have been previously evaluated.
See Downing et al, “Bulk silicon CCDs…”, http://www.eso.org/sci/meetings/dfa2009/
Devices worked wll, but required -120°C operation for best white defect performance.
•Here,
we report on latest refinements to the manufacturing quality of these deviceswhich now demonstrate improved cosmetic performance- this allows lower white defect
levels and an elevation of operating temperature Æ -100°C increases red QE
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“Bulk” CCD devices- 2
Performance summary of bulk CCD44-82 tests
Property
Performance
Notes
Format
2048 X 4096; 15 µm pixels; full frame
Build standard
Bulk silicon, 70 µm thick; Backthinned
Spectral response
astro multi-2 AR coating (multi-layer)
See figure
Cosmetic quality
Grade-0 quality
See below
Responsivity
6.0 µV/e-
Readout noise
3.1 e- rms (at 20 kHz)
Dark current
0.2 e-/pixel/ hour (at -120°C)
Other formats possible
Nominal
Scaled from -100°C measurement
Fe55 measurement
(whole clock triplet)
CTE
Parallel
Serial
99.9998%
99.9996%
Non-linearity
<0.3% up to 100 ke-; < 1% up to 175 ke-
PSF
~ 1 pixel (400- 700 nm wavelength)
Operating voltages
Nominal; 0 to +10V clocks; Vss= 0V; OD= 31V
Operating mode
Non-inverted mode operation (NIMO or non-MPP)
220 ke- pixel full well
Not measured on this sample
[See Downing et al; 2009]
Full scientific quality demonstrated
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“Bulk” CCD devices- 3
Sample manufactured with e2v astro multi-2 AR coating (see later also)
Measured QE at -100°C, Bulk Silicon (70µm), Astro Multi-2 AR coating
100%
90%
80%
QE (%)
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Predicted QE
Measured QE
QE matches expected value
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“Bulk” CCD devices- 4
2 minute dark frame
Cosmetic
defect
type
Number
measured
Specification level
White pixel
defects:
0
Threshold level >100 e/pixel/hour at -120°C
White
column
defects:
0
At white pixel threshold; >
100 pixels long
Dark pixel
defects:
209
Threshold level > 20%
below local mean
Dark
column
defects:
1
At black pixel threshold ; >
100 pixels long
Traps:
3
Above 200 e-
650 nm flat field
At -100°C
Excellent cosmetics: Standard scientific defect specs achieved
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“High-rho” fully depleted CCD development- 1
High rho technology
Depletion region needs to extend from front to back to ensure no PSF degredation
High voltage back bias (BSS) needed to create full depletion
Front side (output circuits) need to operate at normal low voltage (FSS)
No leakage current must flow from BSS to FSS; guard diode is important
Front substrate p+
Front substrate p+
Guard diode
VGD
Guard diode
VFS
VGD
VFS
CCD electrodes
CCD electrodes
p-type substrate material
VBS
Large
leakage
current
Buried
channel
Back-surface p+
Optical input
Depleted
silicon
(a) Leakage current
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Buried
channel
Depletion edges meet
VBS
Back-surface p+
Depleted
silicon
Optical input
(b) Guard diode depletion isolates front and back
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“High-rho” CCD development- 2
The CCD261-84 high-rho device
An evolution of previous e2v high rho sensorssee Jorden et al, Proc SPIE 6276 (2006)
First Sample Results (June 2010)
B1
B1
Image Section B
B2
B2
2048 x 2048 pixels
Each 15 µm square
B3
B3
A1
Item
Key Parameters
Format
2048 X 4104; 15 X 15 µm ; 30.7 X 61.6 mm image
Package
Buttable; 40 pin PGA connector; 20 µm flatness
Outputs
2; split register- read from one or both outputs
Responsivity
12 µV/ e-
Read-noise
<2 e- noise floor
Pixel capacity
200,000 e- (design; measured to 100ke- so far)
Dark signal
Same as standard silicon devices
(Non-inverted operation mode)
CTE
99.9995% expected (for 3-phase triplet)
QE
40% QE at 1000nm wavelength
Cosmetics
Grade-1 quality achieved at -100°C
Operating temp.
-100°C typical
A1
Image Section A
A2
A2
2048 x 2056 pixels
Each 15 µm square
A3
A3
TGA
TGA
OSE
Register
section E
E1 E2 EF3 F2 F1
Register
section F
OSF
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“High-rho” CCD development- 3
CCD261 BI
Readout noise
RØ1
RØ2
SW
OG
ØR
RD
TGA
6
OD
Clamp
Reset
Noise ( e- rms)
JD
4
OS
Output
OS
CN
OP
External
load
First stage
load
Signal charge
Node
0V
2
Output
Internal
load
10k Ohms
RL
External
load
FS
0V
0V
BS
0
10
100
1000
10000
2-stage output circuit
optional JFET buffer
Pixel frequency (kHz)
predicted noise
measured noise
Responsivity: 12 µV/ eRead-noise: 3.5 e- rms at 500 kHz
System noise (3.5 e-) subtracted in quadrature
< 2 e- noise floor design
Measured by photon transfer- with care to use low
signals
see Downing et al, Proc SPIE 6276 (2006)
Reset drain current measurement gives same result
high responsivity, very low noise amplifier
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“High-rho” CCD development- 4
Dark current
Dark current
10000000
Measured from -20°C to -80°C
1000000
0V BSS & -70V BSS- no significant change in dark current
100000
Trend line drawn; exp (-6600/T) : 5 e-/hr expected at -100°C
e/pix/hr
10000
I.e. very similar dark current to standard silicon devices
1000
100
Lower temperature tests in progress
10
1
0.1
0.01
-120
-100
-80
-60
-40
-20
Temp (°C)
Meas 0 BSS
Meas -70 BSS
trend -70 BSS
Dark current follows typical scale law; no change with HV back bias; 5 e-/hr expected at -100°C
Quantum Efficiency
Not measured yet. QE expected to correspond to device design (thickness & coating)
See later
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“High-rho” CCD development- 5
Cosmetics
White defects analysed (below)
Dark defects not yet analysed
Defect
type
Number of
defects
Specification
level
-120°C
frames
BSS
= 0V
BSS =
-70V
White
pixels
8
9
>100 e-/pix/hr
@ -120°C
White
columns
0
3
> 100 pixels
long
White defects
Small change with full BSS voltage
(a) -70V, -80°C (b) -70V, -100°C (c) -70V, -120°C (d) 0V, -100°C
300 s dark images- showing effect of temperature and BSS voltage
(d Æ b): Minor change of white defects as BSS increases
(cÆbÆa): Modest progression of white defects with temperature
(b): High scientific quality even at high BSS and -100°C
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Cosmic rays dominate white
defects in long exposures
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“High-rho” CCD development- 6
Summary
Full operation of new CCD fully depleted sensor
Demonstrated with• 2048 X4096 format device
CCD261-84
• -100°C operation typical
• Good cosmetic performance
• Low read-noise and low dark current
• Performance to design parameters
Future plans
Complete characterisation of CCD261-84
Commercial supply of devices
Intended 4096 X 4096 format variant
Alternate thickness and AR coating variants
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“High-rho” QE
Designed for very high
red wavelength
response and
maintaining high UV QE
Quantum Efficiency
A multi-layer AR coating optimised for 330 to 1000 nm
Predicted high-rho (Si: 150µm) QE at -100°C
Multi-layer AR (optimised for 330-1000 nm)
100%
“astro multi”:
90%
Using existing
process
80%
QE (%)
70%
60%
“optimum multi”:
50%
Optimised
process
40%
30%
20%
10%
0%
300
QE at -100°C
400
500
600
700
800
900
1000
>40% for 330 - 1000 nm
>70% for 400 - 950 nm
Wavelength (nm)
astro multi
1100
optimum multi
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Anti-reflection coating refinement- 1
Extended red response and desire for blue response (U-Z bands)
requires wider-range anti-reflection coatings
QE at -100°C, Silicon thickness: 150µm, alternate AR coating types
100%
90%
80%
QE (%)
70%
60%
50%
40%
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
astro BB
astro multi-1
astro multi-2
astro multi-3
Spectral response of single layer (BB) compared to three e2v multi-layer coatings
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Anti-reflection coating- 2
Enhanced coatings designs tested with new materials and multi layers
Sample standard silicon multi-2 AR coating
QE at +20°C, Silicon thickness: 109µm
100
100%
90%
90
80%
80
70
QE at -100°C Silicon thickness: 150µm
multi-layer optimised
60
60%
QE (%)
QE (%)
70%
50%
100%
40%
30%
40
90%
30
20%
10%
80%
0%
70%
50
20
10
450
astro multi-2
550
QE (%)
350
650
60%
750
850
950
0
300
1050
400
500
600
Wavelength (nm)
Theory multi-2
50%
+20C measured
700
800
900
1000
wavelength (nm)
+20C
T= -100C predicted
-88C
Minimal change on cooling ( +20°C to -88°C)
40% performance (at +20°C)
Almost perfect
30%
20%
10%
0%
300
400
500
600
700
800
900
1000
1100
Wavelength (nm)
Predicted performance of optimised coating
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Anti-reflection coating- 3
AR Summary
•
Wide wavelength range (U-Z) benefits from multi-layer coatings
e2v has developed advanced multi-layer coatings:
High performance at red wavelengths together with maintained UV response
•
Multi layer AR coatings have advantages over single layer Hafnia (traditional) coatings:
Single layers give good peak response; multi-layers broaden range (with slight dip in middle)
•
•Latest
coatings show close to theoretical performance
•Coating
design is adjusted for application area
•Secondary
benefitsMinimised reflectivity (ghosts/ scattered light)
Minimised fringing
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Summary
Progression of development towards thicker CCDs for higher red QE
• Commercially available device types
Standard “Bulk” CCD44-82 (2K X 4K) shows excellent performance
• Operates at standard voltages; drop-in upgrade path
New “high-rho” CCD261-84 (2K X 4K) shows scientific performance
• Highest red response; good performance with one HV bias
Multi-layer AR coatings
• Enhanced performance for wide wavelength range
Acknowledgements
Ray Bell, Steve Bowring, David Burt, Paul Jerram, Andrew Pike, and Peter Pool
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