The photoelectric effect Detectors 2 — CCDs and other ...sctrager/teaching/OA/Detectors2.pdf ·...

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


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


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


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


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


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


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”

43

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

44

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

46

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

47

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

48

Doping

Remember:

“n-type” for negative charge: more electrons in conduction band

“p-type” for positive charge: more holes in valence band

49

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

50

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

51

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)


+ Gate

10 μm

++++++

- - - - - -

MOS capacitor

52

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)


+ 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)

54

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

++++++


V

x

depletion zone

–

Gate (+10V)

55

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

++++++


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

++++++


V

x

depletion zone

–

Gate (+10V)

57

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


–

Gate (–10V)

substrate current (to amplifier)

58

independent pixels

Insulator (SiO2)


+

++++++

-

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)


+ V1 +

++++++

V2

-

++++++

60

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!

63

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


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


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

65


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


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


...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


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


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


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


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)


~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)


~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


~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


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


SiO2


10 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


SiO2


10 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)


SiO2


10 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

109

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

110

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...

111

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

112

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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