Detectors 2 — CCDs and other photoelectric devicesObservational Astronomy 2019 Part 7 Prof. S.C. Trager
1
The photoelectric effectMost (but not all) astronomical detectors work on the basis of the photoelectric effect
...for which Einstein won his Nobel Prize!
Photons of sufficient energy hitting a metal surface will eject electrons
2
The photoelectric effectThis depends on the energy of the photon (or frequency or wavelength) but not the photon flux
The kinetic energy of the photo-electrons is linearly proportional to the energy of the photons:
Here W is the “work function” of the metal, corresponding to a minimum frequency νmin required to eject an e–
Ee� = E� �W = h⌫� �W = h(⌫� � ⌫min)
3
Photoconduction
Photoconduction occurs when the photon ejects an electron that drives a load
like in a solar cell (photocell)
That is, the photoelectrons are collected in some way to produce an electric current
4
PhotoconductionIn astronomy, we want to know how many photons have been detected
We could use a photocell to do this and measure the current generated to measure the photon flux
With amplification, this kind of detector is called a photomultiplier (tube) and is a kind of photoemissive detector
Very important in optical astronomy until 1980s
5
Photoconduction
Photomultipliers have now been (almost completely) replaced by non-photoemissive detectors which retain the ejected photoelectrons in the material
These sort of detectors allow you to store charge that can be read out later
6
PhotoconductionThe time during which this accumulation of charge occurs is called an integration
The measurement of this accumulated charge is the readout
This readout can cause a chemical change in the detector
like in the eye or in a photographic plate
...or a build-up of electrical charge in a potential well which is read out as a voltage
like in a CCD
7
Useful detector parameters
Quantum efficiency (QE)
The fraction of incoming photons converted into signal
QE is a function of wavelength (for reasons we’ll soon see)
CCDs have QEs up to ~95%
photographic film has QEs of ~few%
8-1
Useful detector parameters
Quantum efficiency (QE)
The fraction of incoming photons converted into signal
QE is a function of wavelength (for reasons we’ll soon see)
CCDs have QEs up to ~95%
photographic film has QEs of ~few%0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
350 450 550 650 750 850 950
WEAVE CCD Quantum Efficiency
8-2
Useful detector parametersSpectral response
Wavelength (frequency) range over which photons can be reliably detected
given by QE(λ)
Noise
uncertainty in output signal
combination of photon counting noise and systematic noise (like read noise)
9
Useful detector parameters
Linearity
degree to which the output signal is linearly proportional to the incident photon flux (number per unit time)
photographic plates are linear over only a small range of fluxes
CCDs much better — but not perfect!
10
Useful detector parametersDynamic range
maximum variation in signal representable by detector
Pixels
“picture elements” — individual, independent detecting elements in detector
Time response
minimum time interval over which changes in photon flux are detectable
11
Semiconductors
Nearly every astronomical detector (except the eye and some X-ray and γ-ray detectors) is based on semiconductors
12
Semiconductors
Elemental semiconductors are those from column IVa in the Periodic Table of the elements
Si (silicon) and Ge (germanium) are the most commonly used
13-1
Semiconductors
Elemental semiconductors are those from column IVa in the Periodic Table of the elements
Si (silicon) and Ge (germanium) are the most commonly used
13-2
Semiconductors
In these materials, the outermost (valence) shell of electrons contains 4 of the 8 possible electrons
These elements want to form covalent bonds, sharing one e– with each of four other similar atoms in large lattices
14
SemiconductorsIn crystal form, column IVa elements form covalent-bonded, diamond-like structures
note that C, the element that makes diamonds, is a column IVa element
In these crystals, e–’s are strongly held in their bonds
15
Semiconductors
Elements in columns Ib, IIb, IIIa, Va, VIa, and VIIa can be used as or in compound semiconductors:
diatomic molecules spanning column IVa symmetrically
16
Semiconductors
Examples of compound semiconductors:
AgBr: photographic plates
GaAs: optical phototubes
InSb, HgCdTe, InGaAs: NIR detectors
17
Semiconductors
Compound semiconductors combine elements with 3 and 5 (or 2 and 6 or even 1 and 7) electrons in their valence shells to form covalent bonds
18
SemiconductorsTo understand how semiconductors work, we need to generalize the concept of the “work function” of the photoelectric effect
It is more correct to call this “work function” the bandgap energy Eg
This is the energy required to free bound electrons to make them (storable as) free electrons
Thus the kinetic energy of the free electron isEe� = E� � Eg
19
SemiconductorsThe ability of a detector (a semiconductor) to create photoelectrons depends on the bandgap energy and the frequency of the incoming photons
Detectors with different bandgap energies are sensitive to different frequencies
Bandgap energies for different materials
20
Semiconductor Eg (eV) Wavelengths
InSb 0.18 NIR
Ge 0.67 NIR
Si 1.11 NIR, optical
GaAs 1.43 optical
AgBr 2.81 optical
SiC 2.86 optical
insulator (NaCl) >4
21
Semiconductors
A remarkable property of lattice-structure materials is that they have a “ground state” and “excited states”
just like a single atom!
22
SemiconductorsElectrons in a crystal lattice can exchange between ground states in their covalent bonds...
energies in the valence band
...and excited states...
energies in a conduction band
...via photon emission and absorption
23
SemiconductorsHowever, in a crystal lattice, the allowed (permitted) states occupy bands of very-closely-packed energy levels
The valence bands are ground states that are normally completely filled
The conduction bands are excited states that are normally completely empty
24
SemiconductorsThese bands are separated by energy levels that are forbidden
Thus the energy difference between the top of the valence band and the bottom of the conduction bands is the bandgap energy
An electron must absorb at least Eg to become excited into the conduction band
25
SemiconductorsHow do these bands come about?
In order to satisfy the Pauli exclusion principle...
fermions (like electrons) cannot share the same quantum state
...energy level splitting occurs when atoms are brought close together
26
SemiconductorsThis splitting eventually forms psuedo-continuous energy bands when enough atoms are closely packed
To conduct electricity, electrons must move freely
But in a solid, bound electrons remain bound
27
SemiconductorsIf the valence band is full, no electrons may move in this band
like in a semiconductor or an insulator
If an electron receives enough energy to excite it into an unfilled level in a conduction band, it can move freely to conduct electricity
Bandgap energies for different materials
28
Semiconductors
If the valence band is not full...
like in Li, K, and Na solids
...then electrons can move in the valence bands Bandgap energies for different
materials
29
Semiconductors
If the conduction band(s) broaden enough to overlap the valence bands, then electrons can find new states in which to move
like in familiar metals: Fe, Sn, Pb
Bandgap energies for different materials
30
Semiconductors
The usefulness of semiconductors comes from the fact that their bandgap energies are equivalent to frequencies (wavelengths) in the optical and NIR regimes
Detectors can detect photons with energies higher than the Eg of their material
Thus Eg defines the red side of the detector’s spectral response
31
Semiconductor Eg (eV) Wavelengths
InSb 0.18 NIR
Ge 0.67 NIR
Si 1.11 NIR, optical
GaAs 1.43 optical
AgBr 2.81 optical
SiC 2.86 optical
insulator (NaCl) >4
32
Semiconductors
For example, AgBr and AgCl crystals in standard photographic plates have Eg’s that correspond to λ<4400 Å
33
Dark current
Unfortunately, electrons can be thermally excited as well as excited by photon absorption
The probability that an electron with an energy near the top of the valence band is thermally excited into the conduction band follows a Fermi-Dirac distribution
34
Dark currentAt high temperatures, this distribution can be approximated by a Maxwellian distribution, so the probability for thermal conduction is
where T is the temperature and
/ exp(�Eg/2kT )
k = 1.381⇥ 10�16 ergK�1
35
Dark currentAt room temperature, kT≈0.025 eV
so
for Si
Given the exponential distribution, Si is a much better conductor at room temperature than at lower temperatures, where it is an insulator
Eg/2kT = 22
36
Dark current
The electrical current generated by thermal excitation is known as dark current
Because you can’t discriminate between thermally-excited electrons and photoelectrons, you want to minimize dark current by cooling your detector
37
Dark current
This is done by placing the detector in a dewar cooled by liquid N (for CCDs) or liquid He (for mid-IR to submm detectors)
38
Electron and “hole” migration and electrical current
When an electron is ejected into a conduction band, it leaves behind an empty “position” or a hole
We can think of the energy levels in the solid being filled by electrons or holes
39
Electron and “hole” migration and electrical current
If a hole is created, it can be filled by an electron from a neighboring atom
but this leaves a new hole, and the hole is said to migrate through the valence band
40
Electron and “hole” migration and electrical current
Although these holes are not real particles, they can behave as if they were
it is often convenient to discuss them as “positive” counterparts of electrons with mass, charge, and velocity
The total electric current in a semiconductor has contribution from both conduction e–’s and hole “e+’s”
41
Doping
We can dramatically alter the conductivity of a semiconductor by “preloading” it with an excess of electrons or holes
This is done by doping, where valence 3 or 5 elements are added to valence 4 elements
done in the liquid phase or via high-speed injection
42
DopingThe net effect is to generate some allowed energy levels in the normally forbidden bandgap into which holes in the valence band increase conductivity (“p-type” doping) or in which additional electrons exist to jump into the conduction band (“n-type” doping)
In either case, the normal bandgap energy is “short-circuited”
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Doping
N-type doping
A column Va element with 5 valence electrons is added to a column IVa crystal
1 excess electron per atom easily ejected into the conduction band
conduction band
valence band
Eg
EcEi
Ev
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DopingN-type doping
The effect is to decrease the effective bandgap energy by some Ei
This allows (at normal temperatures) an excess of electrons available for conduction
conduction band
valence band
Eg
EcEi
Ev
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Doping
P-type doping
A column IIIa element with 3 valence electrons is added to a column IVa crystal
Results in an electron deficiency — a surplus of holes
conduction band
valence band
Eg
Ec
EiEv
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DopingP-type doping
Atoms want to capture electrons from valence 4 elements
holes migrate and conductivity increases
The effect is to decrease the effective bandgap energy by some Ei
conduction band
valence band
Eg
Ec
EiEv
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DopingP-type doping
Effectively, p-type doping creates an energy level above the valence band from which electrons can jump to the conduction band
“binds” a hole into the valence band
Allows for electrical conduction of holes
conduction band
valence band
Eg
Ec
EiEv
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Doping
Remember:
“n-type” for negative charge: more electrons in conduction band
“p-type” for positive charge: more holes in valence band
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Doping
Why do we dope semiconductors?
To change the wavelength sensitivity of a semiconductor (detector) by changing the (effective) bandgap energy
and to change the electrical conductivity
useful for making junctions
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Pixels
The basis of a pixel in a semiconductor detector is a junction called a metal oxide semiconductor (MOS) capacitor
Insulator (SiO2)
substrate (semiconductor)
+ Gate
10 μm
++++++
- - - - - -
MOS capacitor
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Pixels
MOS capacitors are made by covering a semiconductor with a thin (10 µm) layer of an insulator like SiO2, then evaporating Al onto this layer to make a small gate (electrode) on top
Insulator (SiO2)
substrate (semiconductor)
+ Gate
10 μm
++++++
- - - - - -
MOS capacitor
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Pixels
If we add a small positive charge to the gate, free electrons will move towards the gate (and the holes will move away), but they can’t cross the insulator
This is therefore a capacitor
Insulator (SiO2)
substrate (semiconductor)
+ Gate
10 μm
++++++
- - - - - -
MOS capacitor
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PixelsIf however we make a substrate with a p-type doped semiconductor and put the gate at, say, +10 V, the holes move away from the gate as before
but now there are virtually no free electrons to move closer to the SiO2 region
this zone near the insulator is called the depletion zone
Pixel
Insulator (SiO2)
+10 μm
++++++
p-doped semiconductor
V
x
depletion zone
–
Gate (+10V)
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Pixels
If there are no thermal electron-hole pairs — i.e., the device is cold — then only photoelectrons will collect in the depletion zone
Pixel
Insulator (SiO2)
+10 μm
++++++
p-doped semiconductor
V
x
depletion zone
–
Gate (+10V)
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Pixels
The depletion zone is called a well where photoelectrons are stored
The depth of the well is proportional to the applied voltage, which is called the bias voltage
Pixel
Insulator (SiO2)
+10 μm
++++++
p-doped semiconductor
V
x
depletion zone
–
Gate (+10V)
56
PixelsThe maximum number of electrons — the maximum charge — a pixel can hold is called the (full) well capacity
For the WFC on the INT, the full well capacity is 2×105 e–
Clearly, by creating and storing photoelectrons in the pixel wells, we can integrate
Pixel
Insulator (SiO2)
+10 μm
++++++
p-doped semiconductor
V
x
depletion zone
–
Gate (+10V)
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ReadoutHow do we monitor the total charge collected in the pixels?
Two possibilities:
1) switch the gate voltage and drive the electrons into the substrate where they can collected and read out as current
This is called a charge injection device and is typical for NIR detectors
Readout (CID)
Insulator (SiO2)
–
10 μm
p-doped semiconductor
–
Gate (–10V)
substrate current (to amplifier)
58
independent pixels
Insulator (SiO2)
p-doped semiconductor
+
++++++
-
V1 +
++++++
-
V2
Readout
2) Imagine that two gates are placed close together — 1µm — on the same insulator over the substrate
Then their depletion zones can communicate
59
Readout
If the voltages are changed appropriately — clocked — electrons will the deeper well
This is the basis of the charge transfer mechanism
CCD
Insulator (SiO2)
p-doped semiconductor
+ V1 +
++++++
V2
-
++++++
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CCDs
By placing multiple gates along the substrate and moving the voltages — and thus moving the charges — from gate to gate, we have a charge-coupled device: a CCD
61
CCDsIn reality, most astronomical CCDs use three gates per pixel to read out
A group of gates with a common electrical link is called a phase
For each pixel, we have one group of each (of three) phase(s)
Each phase alters its voltage with a distinct clock-like signal, alternating between high and low states
62
CCDsBy clocking these phases in a timing sequence, we can change phase states to move the packets of stored charges in a single direction
making sure one high state separates each charge packet to prevent mixing information from consecutive pixels
Because three-phase electronics are difficult to manufacture, they’re expensive!
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Charge transfer efficiency (CTE)
Unfortunately, when we transfer charge from one pixel to the next, charges can get left behind
This poor charge transfer efficiency (CTE) results in a blurring of the signal due to charge trailing behind and getting mixed with later packets 0
1.75
3.5
5.25
7
0 1 2 3 4 5 6 7 8
e–/p
ixel
pixel
64-1
Charge transfer efficiency (CTE)
Unfortunately, when we transfer charge from one pixel to the next, charges can get left behind
This poor charge transfer efficiency (CTE) results in a blurring of the signal due to charge trailing behind and getting mixed with later packets
e–/p
ixel
pixel0
1.75
3.5
5.25
7
0 1 2 3 4 5 6 7 8
64-2
Charge transfer efficiency (CTE)
CTE is the fraction of any charge packet passed from one depletion zone the next
Only a very small CT ineffeciency is acceptable!
Let N0 be the number of electrons originally under the gate and Nt be the number of electrons transferred to the next gate
Then CTE = 1� N0 �Nt
N0
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Charge transfer efficiency (CTE)
Consider a simple case in which N0=100 and Nt=99, so only one electron doesn’t get transferred
Doesn’t sound too bad, eh?
Consider that on a three-phase CCD with 2K pixels along the rows, the charge packet furthest away from the readout has to make 6144 transfers!
Even if our case only has to make 100 transfers, we’re left with only (100 e–)×(0.99)100=37 e–!
66
Charge transfer efficiency (CTE)
Applying this reasoning to the realistic case, we require CTEs of >0.99999 or better!
Two physical mechanisms make electrons “want” to transfer from one well to the next:
Self-induced drift from electrostatic repulsion: in (fairly) full wells, electrons repulse each other into the adjacent well. For a 15 µm gate with 3×105 e–, the exponential decay time of this process is τ=0.002 µs. As the electrons transfer, this repulsion decreases, and...
67
Charge transfer efficiency (CTE)
...simple thermal diffusion takes over. This depends on temperature (of course): at T=300 K, τth~0.026 µs, while at T=77 K, τth~0.1 µs
This relates to why it takes so long to read out a CCD:
The faster you clock the phases, the less time there is to move the electrons from one depletion zone to the next. For good CTE, we need to clock the gates much more slowly than the thermal diffusion time constant to ensure nearly 100% charge transfer
68
Charge transfer efficiency (CTE)
The clock speed determines the readout time of a CCD
The CTE can then be written as
where m is the number of transfer phases, T is the slower of the two mechanisms we just discussed (i.e., thermal diffusion), and t is the duration that gates are in each voltage state
CTE = (1� e�t/T )m
69
Charge transfer efficiency (CTE)
This suggests that we can run CCD clocks at 10s of kHz and still have good CTE...
but this still means a big CCD takes ~a minute to read out
70
Charge transfer efficiency (CTE)
There are at least two other problems that can affect CTE:
fringing fields are depletion zones affected by neighboring gate fields, if the gates were improperly shielded
this is a design flaw
71
Charge transfer efficiency (CTE)
traps are poorly-shaped depletion zones that “trap” electrons
these are caused by poorly-shaped electrodes (design flaw), diffusion of implanted dopants, lattice defects in the Si substrate, impurities, or radiation damage
in space-borne CCDs, traps grow over time, badly affecting CTE
trap trap
72
CCD architectures
Linear-readout CCD cameras are 1D detectors capable of making 2D images
This can be done by either moving the detector relative to the object
like in a photocopier or a scanner
or moving the object relative to the detector
like in a fax machine
73
CCD architectures
2D readout architectures:
The simplest 2D CCD readout is the line-address readout
we arrange rows of CCD-linked pixels parallel to one another
74
CCD architectures
at the end of the rows, we arrange a column of CCD-linked pixels, called a serial register or a multiplexer (MUX)
75
CCD architecturesWe then readout the CCD with the following algorithm:
1) shift all rows by one pixel into the MUX
2) read out all MUX pixels in order by shifting charges along the MUX to the amplifier — a field effect transistor (FET)
3) when the MUX is completely empty, repeat from 1)
76
CCD architecturesThe image is then assembled row-by-row
Note the correspondence of physical MOS pixels and picture element pixels:
the physical rows of the CCD MOS pixels correspond to the image rows of the “picture element” pixels
but the physical MOS pixels are not coupled by column — this takes place in the MUX
77
CCD architecturesThere’s a problem with these detectors:
they still collect photons while being “clocked out”
This can result in smearing of the image
Either readout very quickly — with poor CTE
or cover the array during readout with a shutter
78
CCD architecturesAs the CCD clocks out at the MUX, the amplifier puts out an analog signal (a voltage) which is then converted into a digital signal using an analog-to-digital converter (ADC):
here G is the gain, the number of e–’s combined to make one “count” in the picture
G is actually the inverse gain, but most people call it the gain
signal (ADU) =1
G(Ne� ± �RN )
79
CCD architectures
The dynamic range of the image is limited by the ADC:
15 bit=215=32768 distinct values
16 bit=216=65536 distinct values
80
CCD architecturesThe amplifier introduces a noise into the signal called the read noise σRN (or just RN)
This noise is independent of the signal and can limit in the accuracy of measurements in the “photon-starved” regime (when Ne–<σRN2)
We call this the “read-noise-limited” regime
81
BiasWe’d like to use (nearly) all of the dynamic range of the ADC, i.e., as many of the 216=65536 possible values as we can
..but we need to guard against “negative” values due to readnoise and possible variations in the ground level of the CCD electronics
82
Bias
We (normally) add a bias level to shift all pixel values into the positive range
Typical bias levels are between a few hundred and a few thousand counts (ADU)
83
BiasIn a well-behaved CCD, the ADU distribution of a “zero” or “bias” frame...
a frame wiped of all charge, “exposed” for 0 seconds and then readout
...should be a Poisson distribution with a mean of the bias level and a width equivalent to the readnoise (in ADU): σRN=RN/G
84
BiasUnfortunately, during readout, the reference voltage can drift, changing the bias level
If this happens, we should use overscanning
some number of extra reads of the amplifier — beyond the number of physical pixels — are made
85
BiasThese “overscan pixels” represent pixels with zero charge
they tell you what the ADU signal for zero voltage is
This is done for every CCD row and this overscan region is subtracted off the image at each row
Modern CCDs don’t really need this, but old ones (like SITE3 from LCO) did!
86
BinningIf readnoise will be a problem with an observation, pixel binning is a solution
Signal from adjacent pixels can be combined before reaching the readout amplifier
in effect, creates one image pixel from multiple physical pixels
87
Binning
Because the dimensions of the final image are smaller, but the angular size of the image is the same as in the unbinned case, we have sacrificed resolution to lower the noise
88
BinningHow have we reduced the noise?
In two ways:
1) fewer amplifier readouts for the same (angular) picture area
2) binned pixels have more counts than the original individual pixels
89
BinningThus, the final signal is increased with respect to the readnoise
Consider the total variance of four unbinned pixels:
=
4X
n=1
Ni
!+ 4�2
RN
�2
TOT=
4X
n=1
�Ni + �2
RN
�
90
BinningNow consider the variance of one binned pixel containing those four physical pixels:
So in the readnoise-limited case, 2×2 binning decreases the (read)noise by a factor of 2
�2
TOT,binned =
4X
n=1
Ni
!+ �2
RN
91
Binning
Thus binning effectively increases sensitivity at faint signal levels
we require less integration time to detect a given object
...at the expense of loss of resolution
92
Binning
There is another gain:
chip readout is faster with binning because fewer amplified reads are required
for example, 2×2 binning is ~4× faster than unbinned readout
93
BinningHowever, one should always try to Nyquist sample the image
that is, make sure that the PSF (e.g., a star) is sampled by at least 2 pixels across its FWHM
Undersampled images are very difficult to deal with!
undersampling
fully sampled
undersampled, pixel centred
undersampled, corner centred
94
CCD characteristics
Let’s try to understand the wavelength dependence of QE:
the dominant source comes from the ability of photons of different energies to penetrate Si
95
CCD characteristicsIf Fλ(0) is the flux of photons of wavelength λ incident on the front surface of a Si CCD, then the flux at depth z is
where αλ is the coefficient of intrinsic absorption and is a function of both λ and temperature T
F�(z) = F�(0)e�↵�z
96
Coefficient of intrinsic absorption in Si
α (μm-1)
λ (Å) T=300 K T=77 K
4000 5 4
6000 0.5 0.25
8000 0.1 0.005
10000 0.01 0.002
97
CCD characteristicsPhotons are pretty much stopped by four scale lengths (~4/αλ)
so basically all blue (4000 Å) photons are stopped after ~1 µm of Si
roughly half of the photons make it past one scale height
98
CCD characteristicsso NIR photons (1 µm) can make it past >200 µm in cold Si
but remember that NIR photons with λ>1.1 µm don’t have enough energy to make electron-hole pairs (i.e., Eγ<Eg) in Si and will pass right through
99
CCD characteristics
So the blue sensitivity is limited by the weak penetration of photons
many blue photons absorbed before even reaching the depletion zone
the SiO2 insulator layer makes this even worse!
Therefore we need thin CCDs for blue sensitivity
backside-illuminated CCDs
100
CCD characteristics
Sensitivity in the red requires a thick substrate to absorb the weakly-interacting photons
Therefore we need thick CCDs for good red sensitivity
frontside-illuminated CCDs
101
Frontside-illuminated CCDsThick substrate and surface layers are ok for red photons up to λ≤1.1 µm
this is because αλ is small and these photons can travel >500 µm
the thicker the CCD, the more sensitive to red photons: more electron-hole pairs, so higher QE
Frontside-illuminated CCD
Insulator (SiO2)
p-doped semiconductor
~500 μm
10 μm
++++++
-depletion zone~5 μm
redphoton
+
102
Frontside-illuminated CCDs
But electrons can get “lost” on their way to the depletion zone in a thick substrate
and dark current increases with substrate depth (more electrons to thermally excite)
and crossing the gate and SiO2 layer limits to QE to ≈50%
Frontside-illuminated CCD
Insulator (SiO2)
p-doped semiconductor
~500 μm
10 μm
++++++
-depletion zone~5 μm
redphoton
+
103
Frontside-illuminated CCDsThere are ways to make thick CCDs blue sensitive
add a thin layer of florescent material that converts blue photons to red photons
typically PAH molecules are used, or lumigen phosphorous (highlighter ink!)
Thin “laser dyes” in layers can be used
Note that laser dyes and PAHs are carcinogenic, so not nice to apply!
Frontside-illuminated CCD, blue-enhanced
SiO2
p-doped semiconductor
~500 μm
10 μm
++++++
-
depletion zone~5 μm
bluephoton
+
redphoton
florescentlayer
104
Backside-illuminated CCDs
Because blue photons are absorbed within a few µm of Si, it is desirable to avoid the gate and SiO2 layers
Therefore, illuminate the CCD from behind: backside-illuminated CCD
Backside-illuminated CCD
SiO2
p-doped semiconductor ~10 μm
10 μm
depletion zone~5 μm
bluephoton
SiO2(~0.02 μm)
glass
105
Backside-illuminated CCDsBut to collect photoelectrons efficiently and without significant losses, we want to form them near or in the depletion zone
we need a thin CCD
With an ≈10 µm substrate, we can achieve ≥80% QE — at a loss of red sensitivity
Requires ≈1 µm accuracy in thickness to avoid large QE variations — difficult to acheive
Backside-illuminated CCD
SiO2
p-doped semiconductor ~10 μm
10 μm
depletion zone~5 μm
bluephoton
SiO2(~0.02 μm)
glass
106
Backside-illuminated CCDsThe thinning is done using strong acids to dissolve the substrate
dangerous and tricky!
The thinned CCD is very fragile and can bend (like a potato chip!) as the support structure of the CCD cools and shrinks
glass is glued to backside
Backside-illuminated CCD
SiO2
p-doped semiconductor ~10 μm
10 μm
depletion zone~5 μm
bluephoton
SiO2(~0.02 μm)
glass
107
Backside-illuminated CCDsThinned CCDs are thus expensive
especially those with few defects or traps
After thinning, there is unavoidable oxidation of Si on the backside
leaves ≈20 Å of SiO2 that can trap UV photons
can be avoided using UV flooding or “flash gates” (negatively-biased gates)
Backside-illuminated CCD
SiO2
p-doped semiconductor ~10 μm
10 μm
depletion zone~5 μm
bluephoton
SiO2(~0.02 μm)
glass
108
Linearity
CCDs are very linear until very near the full-well depth, although some are better than others
Always good to check by taking dome flats of various exposure times on a cloudy night!
SITE#3 chip at LCO October 1998
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Blooming
If a source is so bright that it produces more photoelectrons than the full-well depth of a pixel, the excess electrons will (on a normal CCD) “spill over” into adjacent, charge-coupled pixels
this is called blooming
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BloomingBlooming tends to cause degraded CTE
so blooming affects all pixel that must subsequently pass through that gate
Some CCDs have “anti-blooming”
a barrier and/or a drain (a negatively charged gate) placed between pixels
For this reason, it’s not a good idea to saturate the whole detector...
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Cosmetic defectsDead pixel: a pixel unresponsive to light due to defective gate, depletion zone, substrate, etc.
Hot pixel: a pixel with much larger dark current than its neighbors; looks like a cosmic ray but always in the same place
if a hot pixel has a stable dark current rate, can subtract the effect (except for the increased noise) by subtracting a dark frame of equal exposure time but taken with a closed shutter
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Cosmetic defects
Bad column: A defective pixel with a defect (like a deep trap) that affects CCD and “destroys” all charge packets that pass through it
results in a “bad column” in the final image
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Cosmetic defectsA significant limitation to making large CCDs is the frequency of defects
If a bad pixel occurs in the MUX, the chip is useless
If the chance of a bad pixel in 10–4, one out of every five 2k×2k CCDs will have a bad pixel in the MUX!
It is extremely difficult to get a perfect chip, and the cost of a chip is ~inversely proportional to the number of defects
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FringingIn thinned CCDs, the backside is (usually) bonded to glass to reduce “potato chip ripple” using ~1µm of glue
If illuminated with monochromatic light – like a bright sky line in a spectrograph – an interference pattern will be generated by in- and out-of-phase reflections in the glass
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FringingThis can also happen off the front and back of the detector material
Small thickness variations in the CCD (~0.1 µm) horizontally change the interference from constructive to destructive
yields large-scale banding or fringing pattern on the image
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Fringing
Fringing is a serious problem when doing
imaging in narrow-band filters, as the sky and source appear as effectively “monochromatic” to the CCD
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Fringingbroad-band imaging in filters whose wavelength coverage includes bright sky lines
like [OI]5577 in V
or the bright, variable OH lines in the R and I filters
note that the SDSS i filter is tuned to avoid the strongest lines
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Fringingspectroscopy in the red with a thinned CCD
bright, narrow sky lines are monochromatic
limiting factor for some spectrographs on big telescopes
better now: thick chips now used more frequently
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Fringingspectroscopy of objects with strong emission lines, like planetary nebulae
Fringing is very difficult to remove
Best to use thick chips in the red and use anti-reflection (AR) coatings otherwise
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Cosmic raysA serious problem in CCD images external to the CCDs themselves is cosmic rays
CRs are high-energy particles that interact with the semiconductor and produce many electron-hole pairs
If a CR comes into the CCD at an angle, it leaves a streak of affected pixels
CR incidence is higher at higher altitudes because there is less atmospheric attenuation
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Cosmic raysCRs are very severe for HST and other spacecraft with CCD detectors
To deal with CRs, we take multiple images of the same “scene” and “median stack” the images, discarding aberrant (CR) points
Note that thicker CCDs have a higher probability of intercepting CRs
CR-like events can also come from the detector’s surroundings, like radioactive glasses and coatings in the telescope or instruments
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