Detector I: PMTs, MCPs, CCDs, CMOSdmawet/teaching/ay122a_detector_1.… · PMTs, MCPs, CCDs, CMOS...

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Detector I: PMTs, MCPs, CCDs, CMOS Ay122a: Astronomical Measurements and Instrumentation, fall term 2015-2016 D. Mawet, Week 6, November 11, 2015

Transcript of Detector I: PMTs, MCPs, CCDs, CMOSdmawet/teaching/ay122a_detector_1.… · PMTs, MCPs, CCDs, CMOS...

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Detector I: PMTs, MCPs, CCDs, CMOS

Ay122a: Astronomical Measurements and Instrumentation, fall term 2015-2016D. Mawet, Week 6, November 11, 2015

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General properties

General expression for signal coming from 1 detection element of the detector (pixel):

x(t) = x0 (t)+ f Φ(ν )dνΔν

∫ I(θ,ν, t)P(θ )dθΔΩ

∫%

&'

(

)*

specific intensity of radiation

arriving at detector

angular response of the detector

spectral response of the detector

dark signal (in the absence of an incident

signal)

function f characterizes input-output relation of the

detector

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Amplitude detectors

• Measures the instantaneous amplitude of the electric or magnetic field of a wave of frequency ν:

• Instantaneous linearity between the signal and the amplitude

=> linear, or coherent detector

x(t) = Re E0 exp(2πiνt +φ)[ ]

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Quadratic detectors

• Delivers a signal proportional to the mean power of the wave, i.e. integrated over the “integration time” ΔT

• N(t) describes the arrival of photons, a Poisson process

=> non-linear (linear in intensity), power, or intensity, incoherent detector

x(t) = 1ΔT

E( "t )t

t+ΔT

∫ E*( "t )d "t = 1ΔT

N( "t )t

t+ΔT

∫ d "t

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Historical evolution of quadratic detectors (source, from eyeballs to electrons: http://collections.ucolick.org/exhibits_on_line/E2E.1/)

19th century eyepieces

Drawing of Jupiter by James E. Keeler, 1890. This beautifully executed drawing of the planet Jupiter was

made at the eyepiece of Lick's Great 36-inch Refractor, by the gifted young astronomer James Keeler.

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Historical evolution of quadratic detectors cont’d (source, from eyeballs to electrons: http://collections.ucolick.org/exhibits_on_line/E2E.1/)

Star clouds in the Southern Milky Way, 1895. The image is from a three-hour exposure made

by E. E. Barnard.

Photographic plates, late nineteenth century

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Historical evolution of quadratic detectors cont’d

Photomultiplier tube (1930s)

Memoire original J. Optics (Paris), 1987, vol. 18, n” 4, pp. 167-176

OBSERVATOIRE DE PARIS S, place Janssen, 92195 Meudon principal Cedex

LE CINQUANTENAIRE DE LA CAMERA ELECTRONIQUE DE LALLEMAND

G. WLERICK

RESUME : On decrit comment I’idee est venue a A. Lallemand de combiner, dans un msme recepteur. les proprietes remarqua- bles de la plaque photographique (grande surface. possibilite d’integration pendant un temps tres long) et les qualites de la photoclectricite (reponse lineaire, bon rapport signal sur bruit). Ainsi est nee la (G Camera electronique n. O n indique ensuite la seric de developpements qui ont et& realises en France et a I‘Etranger. pendant les 50 dernieres annees, et on note les nombreuscs applications des recepteurs electronographiques a I‘Astronomie.

MOTS CLES : KEY WORDS : Camera elcctronique Elcctronographic Electronographie Electronographic camera

Fifty years of the Lallemand electronic camera

SUMMARY : Around 1934. A. Lallemand had the idea to combine. in a single instrument. the remarquable properties of the photographic plate (large surface. large integrating time) and the efficiency of the photocmission (linear response to flux, good signal to noise ratio). The first experiments are recalled. WC then describe the various developments that took place in France and other Countries during the last 50 years. We mention also the original applications of the electronographic receivers to As- tronomy.

I. - LA PERIODE HISTORIQUE

Andre Lallemand m’a raconte qu’il a eu l’idee du N telescope electronique D, un jour de 1934, alors qu’il se rendait, en train. de Paris a Strasbourg ; le train se trouvait alors a la hauteur de Chiiteau- Thierry. I1 faut expliquer ce qu’est un telescope electronique et essayer de deviner par que1 chemine- ment Lallemand est arrive a cette idee.

Le principe du telescope electronique est simple ( f ig . l ) . On place, dans une enceinte videe, une photocathode et une emulsion photographique. On forme une image optique sur la photocathode ; de chaque point eclair6 partent des photoelectrons ; un systeme de lentilles Clectroniques ies accelkre et les focalise ; ces electrons arrivent donc en un point de I’emulsion photographique et I’energie de chacun est suffisante pour rendre plusieurs grains d’argent developpables. Le processus global est plus efficace que celui qui intervient dans la photographie classi- que.

Photocathode

6lectrodes d’acc616ration et d e

*(‘ tocatisation

Amulsion pour i lectrons

FIG. 1. - Principe du telescope Plectronique : l’instrument astro- nomique, lunette ou telescope, forme une image optique sur une phorocathode ; les photoelectrons h i s sont acc&ltrt!s et focalists ; i ls reforment une image << Clectronique Y qui est enregistree sur une Pmultion photographique.

170 G. WLI’<IWK

optique Clectronique dCrivCe de celles des convertis- seurs de I’armee allemande, une photocathode semi- transparente de type SbCs,, introduite par (( trans- fert )) : la cathode est stockCe dam une ampoule ; au moment de I’utilisation, I’ampoule est brisCe et la cathode est placCe dans sa position de travail.

Beaucoup d’efforts sont deploy& pour rendre le tube operatiomel : i l faut conserver la sensibilite de la cathode, ameliorer les images, supprimer les emissions parasites, sClectionner les emulsions. Les resultats sont retraces dans une serie de publications de Lallemand et Duchesne.

I1 n’est pas possible de relater I’ensemble des trnvaux entrepris avec la Camera Clectronique depuis 19.50. qu’il s‘agisse de developpements techniques, d’expCriences de physique ou d’observations astro- nomiques. DejS, en 1977, G. Lelievre recensait plus de 200 publications ou communications sur I’electro- nographie, en se restreignant aux travaux effectuCs avec les cameras developpees en France.

J’indiquerai seulement quelques jalons :

- 1951. Experiences en laboratoire, gain de 100 par rapport a la photographie, note intitulte cc Sur un rtcepteur ideal de photons et sa realisation )) [S]. - 1952. Premiere observation sur le ciel, a Paris

[g]. La camera utilisCe existe encore au Laboratoire de Physique astronomique, rebaptise Laboratoire Andre Lallemand, et i l serait souhaitable qu’elle entre au MusCe de I’Observatoire ou au MusCe de la

J . Optics (Pnris). 1987, vol . 18, no 4

Villette, au mCme titre d’ailleurs que certains photo- multiplicateurs. - 1956-1957. Installation, en coopCration avec

Ch. Fehrenbach, d’une camera <i I’Observatoire de Haute-Provence. Obtention, pour la premiere fois en France. de spectres de galaxies peu brillantes, avec le plus grand telescope disponible S I’Cpoque, de dimension pourtant modeste ( D = 1,2 m ) [IO]. Dans les annees 60, une skrie de galaxies actives seront CtudiCes ainsi par M. Chopinet et R. Augarde. - 1956. Lallemand et Duchesne reqoivent le

grand Prix de l a Recherche Scientifique et Techni- que.

III. - INSTALLATION D’UNE CAMERA h L’OBSERVATOIRE LICK

- 1959. Sur l a suggestion d’un astronome ameri- cain M. Walker, Lallemand et Duchesne sont invites h I’Observatoire de Lick, en Californie (fig. 4) ; on venait d’y mettre en service, un telescope de 3 m qui etait alors le plus grand apres celui du Mont Palomar. Le 25 octobre, un spectre du noyau de la galaxie Messier 31 (NCbuleuse d’Andromede) montre que le noyau forme une entite physique distincte du reste de la galaxie (fig. 5 ) ; sa masse est de 13 millions de Solei1 et i l tourne 100 fois plus vite que le reste de la Galaxie [ 1 I , 121 ; c’est, a ma

FK;. 4. - Une Camera Plectronique est ins!allC.e 2 I‘Ohservoroire Lick en 195Y. E l k foncrionne a11 foyer coudc‘ du rilescope de 3 m er ser! de rc‘ceppreur pour l’ohjecrif de chamhre d’un grand specrrographe. D e gauche a droire, on voir : Maurice Duchesne. Gerald Kron. Andre Lullemand e! Merle Walker.

J. Optics (Paris), 1987, vol. 18, no 4 G. WLERICK 169

deuxieme, il signale la poursuite des experiences avec diverses emulsions photographiques et Ccrit (fig. 2) : Cet appareil constitue un veritable compteur de photons, la plaque photographique jouant le r61e de totaliseur d’impulsions en m&me temps qu’elle reproduit fidelement le point d’arrivee de chaque photon c’est-a-dire l’image lumineuse projetee sur la photocathode. D’apres les resultats obtenus avec les compteurs de photons, on peut se faire une idee de la sensibilite que l’on peut atteindre avec ce dispositif.. . >>

Esclangon attire l’attention de 1’Academie sur les perspectives nouvelles ouvertes, par ce travail, dans les domaines de la photometrie et de la spectroscopie des objets celestes.

De 1937 a 1939, Lallemand poursuit son axvre avec des moyens accrus ; des savants prestigieux, tels que Louis de Broglie et Jean Perrin, lui appor- tent leur soutien. I1 reussit a enregistrer des etoiles artificielles et a obtenir un gain de temps de pose par rapport a la photographie ; ce gain est encore modeste mais tout de mCme superieur a 4. La guerre Cclate, le materiel experimental est detruit mais Lallemand sauve les cliches. La description du tube (fig. 3) et les resultats ne seront publies qu’en 1945 [6, 71.

FIG. 3. - Schtma du rkcepteur realise‘ par Lallemand dans la periode 1936-1939. En C se trouve la photocathode, E est un kcran cathodoluminescent qui sert pour la mise au point de l’image electronique; lorsque la mise au point est effectuke, l‘icran est remplack, grcice a une commande magnktique externe, par une emulsion photographique (paru dans ( 6 ) , p. 67, 1945).

En 1939, Lallemand est affecte aux Laboratoires du C.N.R.S. a Bellevue et developpe des recepteurs pour 1’Armee. De 1940 a 1943, il rCside a Clermont- Ferrand ou 1’Universite de Strasbourg a et6 replike. En 1943, il prend, sur le conseil de A. Danjon, un poste d’astronome adjoint a I’Observatoire de Paris ; il Cree un service au titre significatif : Laboratoire de Physique astronomique >>. Comme a Clermont, il a tres peu de moyens, mais il entreprend, clandesti- nement, l’etude de detecteurs infrarouges avec un officier de marine rendu a la vie civile. Cela lui vaut d’Ctre nomme, en 1945, Chevalier de la Legion d’Honneur.

11. - L’APRES-GUERRE ; LES CAMERAS

ELECTROSTATIQUE ELECTRONIQUES A FOCALISATION

A la liberation, Lallemand se rend compte qu’il ne lui est pas possible de reprendre d’emblee le develop- pement du <( telescope electronique >>. I1 connait l’ampleur de la t k h e et sait qu’il doit auparavant rassembler une equipe et des moyens : il faut des mecaniciens, des souffleurs-verriers, des specialistes du vide pour realiser les photocathodes ; il faut aussi etudier 1’Optique electronique. I1 reussit a creer l’equipe et 6 obtenir les moyens en combinant trois activites : l’etude, pour la Defense Nationale, de recepteurs infrarouges (un laboratoire de la DCCAN a existe pendant plus de 20 ans a 1’Observa- toire) , la cooperation avec l’industrie, la realisation de (( photomultiplicateurs d’electrons B .

Lallemand realise ces photomultiplicateurs suivant une conception originale, en les adaptant aux besoins de la photometrie astronomique ; il Cree des versions speciales pour les domaines visibles, UV et IR ; il cherche le meilleur rapport signal sur bruit (rende- ment quantique eleve de la cathode, emission para- site faible). Ces photomultiplicateurs d’electrons ont equipe, pendant 20 ans, les observatoires fraqais et etrangers ; l’un d’eux a permis a Oort et Walraven de decouvrir que le rayonnement de la Nebuleuse du Crabe (reste de la Supernova de l’an 1054) est de type synchrotron. Un prolongement fecond a &e, dans les annees 50, la realisation, aux Etats-Unis, sous la direction de Jean-Pierre Gausse (Schlumber- ger, E.M.R.) de photomultiplicateurs qui ont vole, avec succes, dans les Satellites artificiels.

Dans les annees 1945-1949 Cgalement, Maurice Duchesne entreprend sous la direction de Lalle- mand, une these dont une large partie est consacree a 1’Optique Clectronique ; il met au point des metho- des de calcul de la position du plan image, du grandissement et des aberrations. On se rend compte de la difficulte de realiser de bonnes images electroniques quand on voit les aberrations de certains tubes utilises avant la guerre.

En 1950, Lallemand juge que le potentiel du laboratoire est suffisant pour reprendre les travaux sur le telescope Clectronique, bient6t rebaptise Camera electronique, et il propose a M. Duchesne de s’associer a lui. Ce fut le debut d’une collabora- tion tres fructueuse et, pendant plus de 30 ans, Duchesne a consacre son activite a l’electronogra- phie et a ses applications en Astronomie et en Physique.

Depuis 1939, des progres ont Cte accomplis dans les domaines qui inttressent les tubes-images : pho- tocathodes, ecrans fluorescents, emulsions sensibles aux electrons, technique du vide, optique Clectroni- que. Lallemand s’oriente vers un recepteur different du prototype : la nouvelle camera comporte une

Lallemand electronic camera (1930s)

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Historical evolution of quadratic detectors cont’d

Television scanning detectors (1950s)

Microchannel plates (MCP) & image intensifiers (1950-60s)

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Historical evolution of quadratic detectors cont’d

Charge Coupled Device (CCD, 1970s)

Complementary metal-oxide semiconductor (CMOS, 1970s)

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Fundamental detector characteristics

• Quantum Efficiency f(λ): N(detected ph)/N(input ph)

• Size, Number of pixels

• Noise characteristics: dark current, readout noise

• Cosmetics (bad pixels)

• Linearity (to intensity): threshold and saturation

• Dynamic range

• Uniformity (flat field), Stability

• Cost

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Quantum efficiency evolutionThe Evolving Quantum Efficiency!

Rods and cones in the human retina!

Photographic emulsion grains!

The Evolving Quantum Efficiency!

Rods and cones in the human retina!

Photographic emulsion grains!

Rods and cones in the human retina

Photographic emulsion grains

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Photographic platesPhotosensitive silver halide particles (10-30 microns)• Typical QE ~ 2-3%, but large

formats available; can be digitized

• Non-uniform

• Messy development process

• Non-linear response

Classical Photography!Typical QE ~ 2-3%, but large formats available; can be digitized!

A problem: non-linear response! (H-D curve)!

Also: non-uniform!

And messy …!

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Photoelectric effect: photocathode

Wikipedia: A photocathode is a negatively charged electrode in a light detection device such as a photomultiplier or phototube that is coated with a photosensitive compound. When this is struck by a quantum of light (photon), the absorbed energy causes electron emission due to the photoelectric effect.

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Photomultiplier tubes (PMT)

A dynode, usually made from BeO or MgO, is held at a positive potential, such that when it is struck by a single energetic electron, the dynode will emit several electrons. The next dynode in the chain is held at a slightly larger potential, such that an electric field accelerates electrons liberated from the first dynode towards the second, where the electrons again liberate additional electrons. The process continues in a cascading fashion until the final anode is reached; if 6-8 dynodes are chained together, then a single photoelectron incident on the first can generate 106-108 electrons at the anode.

Typical QE ~ 5-10%UV/B sensitive,

poor in R/IR

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Application of PMTs: image intensifiers

• An image intensifier amplifies light signals by: 1. converting photons to electrons via the photoelectric effect 2. accelerating the electrons them via electrostatic forces 3. focusing the electron beam, electrostatically or magnetically 4. having them impact on an output phosphor releasing a shower of

photons 5. recording the output photons using a photographic emulsion or

some more modern detector (or indeed the human eye).

• The gain = N(output photons) / N(input photons); multi- stage image intensifiers can reach total gains up to ~ 106-8

• Image intensifiers are now used very little in the optical, where CCDs dominate, but are still used in the UV

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MCP is a modern image intensifier

• A thin disk of Pb oxide glass with many microscopic channels/pores running parallel to each other from one face to the other

• Pores are either slanted or curved, to allow the electrons to hit the walls to provide the gain, and to absorb positive ions produced from residual gas before they generate a cascade

• A potential of a small number of kiloVolts is applied between one face and the other

• Each channel acts like a tiny image intensifier: electrons hitting the walls eject additional electrons resulting in a cascade of electrons

• It still needs a photocathode and an output phosphor • Advantages over conventional image intensifiers:

• Channels confine the electron shower => better resolution • Voltages are lower (~2 kV instead of ~30 kV for gain of 106)

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Microchannel plates (MCPs)

• Effectively arrays of PMTs • Still used in X-ray, UV (e.g., in GALEX) • Also for some night vision applications

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Photon counting detector

• Run an image intensifier at high gain (~106), and image the output phosphor onto a CCD or similar detector • For each photon incident at the photocathode there is a large splash of

photons at the detector. • Read this out and centroid, record {x,y,t} • Build up time-resolved image photon by photon • If more than one photon arrives in a particular location within the frame time

of the detector then one or both will be lost • There is a limit to the count rate (per pixel and per frame) • You cannot remove saturation by taking short exposures • Useful in the UV/Xray, where photon rates are low

• Photon counting detectors have no readout noise and thus a potential advantage for all ultra-low light level app’s

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CCD 101

• CCD = charge coupled devices

• Invented in 1970 (bell labs)

• Developed in the 1960s as memory storage devices!

• In the 1980s, their use became widespread

• By the 1990s, they had taken over almost all imaging applications

• QE of CCD boosted telescope gathering power by 2 orders of magnitude

Ay 122a - Fall 2012!Detectors (UV/Opt/IR)!

S. G. Djorgovski!Some of the earliest astronomical CCD images, obtained in the early 1970’s at P200 (and Mt. Lemon?), by Westphall, Gunn, et al.!

Uranus! A distant cluster; R lim ≈ 24.5 mag!890 nm Uranus CCD image (1975, JPL & UoA), 61”, Mt Bigelow

The first 8-bit CCD, this chip consists of twenty-four closely packed MOS capacitors (the narrow rectangles in the football-field-like grid in the center). The thick rectangles at either end

of the grid are input/output terminals.

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Charge generation via photoelectric effectAn incoming photon excites an electron from the the valence band to the conduction band: hν > Eg

Eg = energy gap of material Critical wavelength: λc (μm) = 1.238 / Eg (eV)

conduction band

valence band

E

Ege-

Material name Symbol Eg (eV) λc (μm) Op. Temp. (*)

Silicon Si 1.12 1.1 163-300

Mer-Cad-Tel HgCdTe 1.00-0.09 1.24-14 20-80

Indium Antimonide InSb 0.23 5.5 30

Arsenic dope Silicon Si:As 0.05 25 4

(*) to keep dark current low (thermal electrons)

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An electron-volt (eV) is extremely small

• The energy of a photon is VERY small • The energy of a 2.5 μm photon is 0.5 eV

• Drop a peanut M&M® candy from a height of 2 inches

• Energy is equal to 6 x 1015

eV (a peanut M&M® is~2g)

• This is equal to 1.2 x 1016

SWIR photons • The number of photons that will be detected in ~1 million images from the

James Webb Space Telescope (JWST) • A 2-inch peanut M&M

® drop is more energy than will be detected during the

entire 5-10 year lifetime of the JWST!

1eV = 1.6 10-19 J (J=joule)1J = N m = kg m sec-2 m

1 kg raised 1 meter = 9.8J = 6.1 1019 eV

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CCD as a conveyor belt

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD as a conveyor belt cont’d

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CCD conveyor belt Animation

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Top view of a CCD

But Nowadays, Charge Coupled Devices (CCDs) Are The Detectors of Choice"

(in visible, UV, and X-ray)!

Silicon chip

Metal,ceramic or plastic package Image area

Serial register On-chip amplifier A whole bunch of CCDs on a wafer!

Nearly ideal detectors in many ways!Counting photons in a pixel array!

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Manufacturing based on Silicon micro-electronics fab process

CCD on a Si wafer

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Structure of a CCD

The diagram shows a small section (a few pixels) of the image area of a CCD. This pattern is repeated. Every third electrode is connected together. Bus wires running down the edge of the chip make the connection. The channel stops are formed from high concentrations of Boron in the silicon.

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Structure of a CCD cont’dStructure of a CCD

On-chip amplifier at end of the serial register

Cross section of serial register

Image Area

Serial Register

Once again every third electrode is in the serial register connected together.

Below the image area (the area containing the horizontal electrodes) is the ‘Serial register’ . This also consists of a group of small surface electrodes. There are three electrodes for every column of the image area

Below the image area (the area containing the horizontal electrodes) is the ‘Serial register’ . This also consists of a group of small surface electrodes. There are three electrodes for every column of the image area. Once again every third electrode is in the serial register connected together.

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CCD up close!(note scale: 100 µm !

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Internal Photoelectric Effect in Doped Silicon

• Incoming photons generate electron-hole pairs • That charge is collected in potential wells applied on the surface • Thermally generated electrons are indistinguishable from photo- generated

electrons => Dark Current => keep the CCD cold! • Silicon is transparent to photons with E < 1.26eV (λ ≈ 1.1 μm) -=> Red Cutoff!

Need a different type of detector for IR ...

Incr

easi

ng e

nerg

y

Valence Band

Conduction Band

1.26eV

•  Thermally generated electrons are indistinguishable from photo-generated electrons Dark Current keep the CCD cold! •  Silicon is transparent to photons with E < 1.26eV (λ ≈ 1.05 µm) Red Cutoff! Need a different type of detector for IR …

Hole Electron

How Does A CCD Work?!Internal Photoelectric Effect in Doped Silicon!

•  Incoming photons generate electron-hole pairs •  That charge is collected in potential wells applied on the surface

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p-n junction

space-charge region = depletion layer

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p-n junction

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p-n junction inside a CCDElectric Field in a CCD

n p

Potential along this line shown in graph above.

Elec

tric

pote

ntia

l

Cross section through the thickness of the CCD

Region of maximum potential, where the electron packet accumulates

+V

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A grid of electrodes establishes a pixel grid pattern of electric potential wells, where photoelectrons are collected in “charge packets”

pixe

l bo

unda

ry

Charge packet p-type silicon n-type silicon

SiO2 Insulating layer Electrode Structure

pixe

l bo

unda

ry

inco

min

g ph

oton

s

How Does A CCD Work?!A grid of electrodes establishes a pixel grid pattern of electric potential wells, where photoelectrons are

collected in “charge packets”!

Typical well (pixel) capacity: a few 105 e- . Beyond that, the charge “bleeds” along the electrodes.!

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Clocking the CCD = charge transfer = implementing an electronic conveyor belt

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Clocking animation

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Slow scan CCD

The most basic geometry of a Slow-Scan CCD is shown below. Three clock lines control the three phases of electrodes in the image area, another three control those in the serial register. A single amplifier is located at the end of the serial register. The full image area is available for imaging. Because all the pixels are read through a single output, the readout speed is relatively low. The red line shows the flow of charge out of the CCD.

Slow Scan CCDs 1.

The most basic geometry of a Slow-Scan CCD is shown below. Three clock lines control the three phases of electrodes in the image area, another three control those in the serial register. A single amplifier is located at the end of the serial register. The full image area is available for imaging. Because all the pixels are read through a single output, the readout speed is relatively low. The red line shows the flow of charge out of the CCD.

Image area clocks

Serial Register clocks Serial Register

Output Amplifier

Image Area

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Slow scan CCD cont’d

A slightly more complex design uses 2 serial registers and 4 output amplifiers. Extra clock lines are required to divide the image area into an upper and lower section. Further clock lines allow independent operation of each half of each serial register. It is thus possible to read out the image in four quadrants simultaneously, reducing the readout speed by a factor of four.

Slow Scan CCDs 2.

A slightly more complex design uses 2 serial registers and 4 output amplifiers. Extra clock lines are required to divide the image area into an upper and lower section. Further clock lines allow independent operation of each half of each serial register. It is thus possible to read out the image in four quadrants simultaneously, reducing the readout speed by a factor of four.

Upper Image area clocks

Lower Image area clocks

Amplifier C

Amplifier A Amplifier B

Amplifier D

Serial clocks C Serial clocks D

Serial clocks A Serial clocks B

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Video CCD

In the split frame CCD geometry, the charge in each half of the image area could be shifted independently. Now imagine that the lower image area is covered with an opaque mask. This mask could be a layer of aluminum deposited on the CCD surface or it could be an external mask. This geometry is the basis of the ‘Frame transfer’ CCD that is used for high frame rate video applications. The area available for imaging is reduced by a half. The lower part of the image becomes the ‘Store area’.

Video CCDs 1.

Image area clocks

Store area clocks

Amplifier

Serial clocks

Image area

Store area

In the split frame CCD geometry, the charge in each half of the image area could be shifted independently. Now imagine that the lower image area is covered with an opaque mask. This mask could be a layer of aluminium deposited on the CCD surface or it could be an external mask. This geometry is the basis of the ‘Frame transfer’ CCD that is used for high frame rate video applications. The area available for imaging is reduced by a half. The lower part of the image becomes the ‘Store area’.

Opaque mask

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Video CCD cont’d

Once the image is safely stored under the mask, it can then be read out at leisure. Since we can independently control the clock phases in the image and store areas, the next image can be integrated in the image area during the readout. The image area can be kept continuously integrating and the detector has only a tiny ‘dead time’ during the image shift. No external shutter is required but the effective size of the CCD is cut by a half.

Video CCDs 3.

Once the image is safely stored under the mask, it can then be read out at leisure. Since we can independently control the clock phases in the image and store areas, the next image can be integrated in the image area during the readout. The image area can be kept continuously integrating and the detector has only a tiny ‘dead time’ during the image shift. No external shutter is required but the effective size of the CCD is cut by a half.

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These are cheap to produce using conventional wafer fabrication techniques. They are used in consumer imaging applications. Even though not all the photons are detected, these devices are still more sensitive than photographic film.

They have a low Quantum Efficiency due to the reflection and absorption of light in the surface electrodes. Very poor blue response. The electrode structure prevents the use of an Anti-reflective coating that would otherwise boost performance.

The amateur astronomer on a limited budget might consider using thick CCDs. For professional observatories, the economies of running a large facility demand that the detectors be as sensitive as possible; thick front-side illuminated chips are seldom if ever used.

Thick front-side illuminated CCDThick Front-side Illuminated CCD

These are cheap to produce using conventional wafer fabrication techniques. They are used in consumer imaging applications. Even though not all the photons are detected, these devices are still more sensitive than photographic film.

They have a low Quantum Efficiency due to the reflection and absorption of light in the surface electrodes. Very poor blue response. The electrode structure prevents the use of an Anti-reflective coating that would otherwise boost performance.

The amateur astronomer on a limited budget might consider using thick CCDs. For professional observatories, the economies of running a large facility demand that the detectors be as sensitive as possible; thick front-side illuminated chips are seldom if ever used.

n-type silicon

p-type silicon

Silicon dioxide insulating layer Polysilicon electrodes

Inco

min

g ph

oton

s

625µm

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The silicon is chemically etched and polished down to a thickness of about 15microns. Light enters from the rear and so the electrodes do not obstruct the photons. The QE can approach 100% .

These are very expensive to produce since the thinning is a non-standard process that reduces the chip yield. These thinned CCDs become transparent to near infra-red light and the red response is poor. Response can be boosted by the application of an anti-reflective coating on the thinned rear-side. These coatings do not work so well for thick CCDs due to the surface bumps created by the surface electrodes.

Almost all Astronomical CCDs are Thinned and Backside Illuminated.

Thinned back-side illuminated CCDThinned Back-side Illuminated CCD

The silicon is chemically etched and polished down to a thickness of about 15microns. Light enters from the rear and so the electrodes do not obstruct the photons. The QE can approach 100% .

These are very expensive to produce since the thinning is a non-standard process that reduces the chip yield. These thinned CCDs become transparent to near infra-red light and the red response is poor. Response can be boosted by the application of an anti-reflective coating on the thinned rear-side. These coatings do not work so well for thick CCDs due to the surface bumps created by the surface electrodes.

Almost all Astronomical CCDs are Thinned and Backside Illuminated.

n-type silicon

p-type silicon

Silicon dioxide insulating layer Polysilicon electrodes

Inco

min

g ph

oton

s

Anti-reflective (AR) coating

15µm

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Front-side vs back-side illuminated CCDs

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CCDs are not perfect

Cosmic rays

Hot Spots (high dark current, but sometimes LEDs!)

Bright Column (charge traps)

Dark Columns (charge traps)

QE variations

CCDs Are Not Perfect …!

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Bias

• The CCD amplifier also introduces a “bias level” to the output voltage, typically a few hundred electrons

• The bias level is measured from the “overscan” region and subtracted off

• “Bias structure” may also be present in a 2D image

• The electronics as well as the physical make-up of a CCD can also imprint a faint background structure on the images.

low-level structure in the bias

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Charge Transfer Efficiency (CTE)

• How efficiently can charge be moved across the pixels and the readout register? Will every electron be moved or will some be lost?

• The earliest CCDs had a CTE of only ~98%

• Today CTE is typically better than 99·995% in commercial devices (“4 nines”)

• Much higher in scientific devices - 99·9999% (5-6 “nines”)

• Poor CTE means that not all of the photons which arrived on the CCD will be counted, and the further from the readout register the worse the effect

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Linearity, gain and readout noise

1/g

CCD linear response

Film non-linear response

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Flat field (inter-pixel gain non-uniformity)

(a) (b) (c)

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Saturation and blooming in a CCD

pixe

l bo

unda

ry

Phot

ons

Blooming in a CCD 1.

The charge capacity of a CCD pixel is limited, when a pixel is full the charge starts to leak into adjacent pixels. This process is known as ‘Blooming’.

Phot

ons Overflowing

charge packet

Spillage Spillage pi

xel

boun

dary

The charge capacity of a CCD pixel is limited, when a pixel is full the charge starts to leak into adjacent pixels. This process is known as ‘Blooming’. The response of the pixel becomes non-linear, but the charge is conserved!!!

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Sources of noise in a CCD

• Readout Noise: Caused by electronic in the CCD output transistor and in the external circuitry; typically σ RON ~ 2-3 e-

• Dark Current: Caused by thermally generated electrons in the CCD. Eliminated by cooling the CCD.

• Photon Noise: Also called “Shot Noise”. Photons arrive in an unpredictable fashion described by Poissonian statistics.

• Pixel Response Nonuniformity: Also called “Pattern Noise”. QE variations due to defects in the silicon and manufacturing. Removed by “Flatfielding”

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State-of-the-art: HSC at Subaru telescope

Hyper Suprime-Cam (HSC): a 900-megapixel ultra-wide-field camera

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State-of-the-art: ZTF at Palomar

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Reducing a CCD image

Flat Field

Bias Image

Flat -Bias

Sci. -Dark Output Image

Flt-Bias Sci-Dk

Dark or Bias

Science Frame

Reducing A CCD Image!

Raw data!

Calibration exposures !

… which you measure, analyse, and flux-calibrate with images of standard stars!

Flat = image of a uniformly illuminated surface (a dome, sky, etc.)!

Bias = a zero integration image!

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CMOS detectors

• CMOS = Complementary Metal Oxide Semiconductor; it’s a process, not a particular device

• Each pixel has its own readout transistor. Could build special electronics on the same chip. Can be read out in a random access fashion.

• Noisier, less sensitive, and with a lower dynamical range than CCDs, but much cheaper; and have some other advantages (e.g. speed)

• Not yet widely used in astronomy, but might be (LSST?)

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CCD vs CMOS

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Sources• http://spiff.rit.edu/classes/phys445/lectures/ccd1/ccd1.html

• Observational astrophysics, 2nd edition, P. Lena

• S. G. Djorgovski (Caltech, Ay122a, 2012)

• J.W. Beletic notes (optics in astrophysics, R. & F.C. Foy editor, NATO Science Series)

• C. Pikachowski, Indiana University Bloomington, (http://www.astro.indiana.edu/classweb/a540/)