Transcript DeepImproving Depletion CCDs the red response of CCDs. An Introduction Simon Tulloch www.qucam.com Charge Collection in a CCD. Charge packet pixel boundary pixel boundary incoming photons Photons entering the CCD create electron-hole pairs.

DeepImproving
Depletion
CCDs
the red response of
CCDs.
An Introduction
Simon Tulloch
www.qucam.com
Charge Collection in a CCD.
Charge packet
pixel
boundary
pixel
boundary
incoming
photons
Photons entering the CCD create electron-hole pairs. The electrons are then attracted towards
the most positive potential in the device where they create ‘charge packets’. Each packet
corresponds to one pixel.
n-type silicon
Electrode Structure
p-type silicon
SiO2 Insulating layer
Deep Depletion CCDs 1.
Electric potential
The electric field structure in a CCD defines to a large degree its Quantum Efficiency (QE). Consider
first a thick frontside illuminated CCD, which has a poor QE.
Cross section through a thick frontside illuminated CCD
In this region the electric potential gradient
is fairly low i.e. the electric field is low.
Potential along this line
shown in graph above.
Any photo-electrons created in the region of low electric field stand a much higher chance of
recombination and loss. There is only a weak external field to sweep apart the photo-electron
and the hole it leaves behind.
Deep Depletion CCDs 2.
Electric potential
In a thinned CCD , the field free region is simply etched away.
Cross section through a thinned CCD
There is now a high electric field throughout the
full depth of the CCD.
This volume is
etched away
during manufacture
Problem :
Thinned CCDs may have good blue
response but they become transparent
at longer wavelengths; the red response
suffers.
Red photons can now pass
right through the CCD.
Photo-electrons created anywhere throughout the depth of the device will now be detected.
Photons no longer have to pass through the electrode structure to reach active silicon.
Deep Depletion CCDs 3.
Electric potential
Ideally we require all the benefits of a thinned CCD plus an improved red response. The solution is to use a
CCD with an intermediate thickness of about 40mm constructed from Hi-Resistivity silicon. The increased
thickness makes the device opaque to red photons. The use of Hi-Resistivity silicon means that there are no
field free regions despite the greater thickness.
Cross section through a Deep Depletion CCD
Problem :
Hi resistivity silicon contains much lower
impurity levels than normal. Very few wafer
fabrication factories commonly use this
material and deep depletion CCDs have to
be designed and made to order.
Red photons are now absorbed in
the thicker bulk of the device.
There is now a high electric field throughout the full depth of the CCD. CCDs manufactured in this way
are known as Deep depletion CCDs. The name implies that the region of high electric field, also known as
the ‘depletion zone’ extends deeply into the device.
QE Improvement
e2v technologies
varying silicon thickness models -100C astro, NIR AR-coat QE
100%
100-300 um
90%
80%
40 um
70%
QE
60%
50%
"high-rho" silicon (100-300 um)
40%
e2v dd silicon (40um)
30%
20%
10%
0%
450
-
Si thickness AR coat
16 um
300um, NIR
250 um, NIR
200um, NIR
150um, NIR
100um, NIR
40um, NIR
16um, NIR
550
Standard silicon (16um)
650
750
nm
850
Spectral response for differing silicon thickness
16um = standard thinned CCD
40um = E2V Deep Depletion CCD
300um = LBNL SNAP/DEC project CCDs
950
1050
QE Improvement
QE improvement in detectors at ING
100
90
Quantum Efficiency %
80
70
60
50
40
EEV12
30
TEK4
MARCONI2
20
MITLL3
10
0
350
450
550
650
750
850
Wavelength nm
EEV12 and TEK4 are standard thinned CCDs.
MARCONI2 and MITLL3 use deep depletion silicon
950
LBNL ‘Hi-Rho’ CCDs for SNAP and DEC projects
Electric potential
Under thinned standard Si.
n p
Extra thick Deep Depletion Si.
Photoelectron lost to
recombination
Photoelectron
detected
With Deep Depletion Si.
Extra thick Deep Depletion Si + Bias Gate
Transparent bias gate
40mm
300-600mm
-200V
LBNL ‘Hi-Rho’ CCDs for SNAP and DEC projects
Requires an applied backside potential to ensure good PSF
VSUB = 5V
20V
115V
Deep Depletion CCDs 4.
Fringing will also be reduced since less light
reflects from backside of CCD causing interference
Images illuminated by 900nm filter with 2nm bandpass
Thinned Marconi CCD
>50% fringe amplitude
CCID20 Deep Depletion CCD
Test data courtesy of ESO
A Cryogenic Deep Depletion Camera
CC1D20 2k x 4k x 15mm pixels