Astronomical Telescopes and Instruments, Lecture 12:...

Lecture 13: Detectors 1

Outline

1 Overview2 Photoconductive Detection3 Charge Coupled Devices4 CMOS and CMOS Hybrid Detectors5 Array Detector Properties6 Array Detector Data Reduction7 Array Detector Problems

Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Telescopes and Instruments, Lecture 12: Detectors 1

Photon Detection at Different Wavelengths

from Lena et al., Observational Astrophysics, Second Edition

Photoconductive Detection

Operation Principleillumination changes conductance/resistance of photoconductorconductance σ0 increases due to excess charge carriers insemiconductorintrinsic semiconductors: charge carriers = electron-hole pairsextrinsic semiconductors: charge carriers = electrons (n-type) orholes (p-type)spectral responsivity determined by energy/band gaponly photons with energies > gap are absorbedcharge carriers create excess current flow

Responsivity of a Photoconductor

material with conductivity σ0 produces current density~j given by

~j = σ0~E

~E : electric field from bias voltage VB accross photoconductor| ~E | in Volt·m−1

σ0 in Ohm−1·m−1

current density~j at microscopic scales

~j = Nq~v

N: volume density of free charge carriersq: elementary charge~v : drift velocity of charges in applied electric field

drift velocity ~v = µc~E , µc the mobility of charge carrier

Conductivityintrinsic semiconductor: distinction between electron conductionand hole conductionmobility µn for electronsmobility µp for holes, µn ≈ 3µp

current density~j = −nq ~vn + pq ~vp

with n, p electron/hole densities, ~vn/ ~vp electron/hole driftvelocities (opposite directions)q: elementary charge, positive signtherefore

σ0 = q(nµn + pµp)

reduces to σ0 = qnµn and σ0 = qpµp in case of heavily dopedn-type, p-type extrinsic semiconductors

Light on Semiconductor

monochromatic photon flux F (λ0) on n-type semiconductorequilibrium between generation rate of excess conductionelectrons and recombination rate:

d∆ndt

= g − ∆nτl

∆n: equilibrium number of excess electrons per unit volume (=excess carrier concentration)τl lifetime of electrons against recombination

generation rate g:

g =ηλ0F (λ0)

ηλ0 : photon detection efficiencyd thickness of photoconductor material

Light on Semiconductor (continued)conductivity:

σ0 = qnµn

equilibrium:d∆n

dt= g − ∆n

τl= 0

generation rate:

g =ηλ0F (λ0)

dincrease in conductivity ∆σ = σ − σ0 from:

∆σ = qµn∆n =qµnηλ0F (λ0)τl

qµnηλ0τl

hcΦ(λ0)

Φ(λ0): monochromatic radiation flux in WattA: illuminated area of the photoconductorλ0: wavelength of monochromatic photon flux

Charge Coupled Devices (CCD)

Operation PrincipleCCD: array of capacitorstypically metal-oxide-semiconductor (MOS) capacitor made fromsilicon (Si) and silicon dioxide (SiO2, insulator)charge position in capacitor electrostatically controlled by voltagelevelsdynamical application of voltages and relative phases:

injected charges due to electron-hole pairs generated by photonsare stored in capacitorbuilt-up charge (charge packet) can be transferred acrosssemiconductor substrate

CCD arrays for imaging in near-infrared up to 1.1 µm, visible,and X-ray range

Charge Storage in a CCD

2 types of charge coupled structurescharge packets stored very close to interface betweensemi-conductor (Si) and overlaying insulator (SiO2) (surfacechannel CCDs, SCCDs)charge packets stored some distance away from surface ofsemiconductor (bulk or buried channel CCDs, BCCDs)

both devices are very similardiscuss SCCDs since their concept is easier to understand

Single CCD Electrode

single CCD-electrode. VG positive, QInv

and QD negative

metal gate, separated by thin oxidelayer (few 0.1 µm) from p-typesemiconductor (hole-conduction)without voltage bias to gate, uniformdistribution of holes (majority freecharge carriers) in p-typesemiconductorgate electrode positive, holes arerepelled beneath gatedepletion layer (devoid of freecharge) is createdincreased gate voltage extendsdepletion region into semiconductorpotential at semiconductor/insulatorinterface (φS) becomes positive

Single CCD Electrode (continued)

single CCD-electrode. VG positive, QInv

and QD negative

voltage high enough⇒ surfacepotential φS attracts electrons (i.e.minority charge carriers in the p-typematerial) to surfaceelectrons form extremely thin (≈ 0.01µm thick), but very (charge) denselayer, the inversion layerelectrons reside in deep potentialwell at semiconductor surface, do notrecombine with holes, since holesare repelled from depletion layerlight on single CCD electrodecreates electron-hole pair: electronsstored in inversion layer, holesrepelled from depletion region

Charge Transport in a CCD

charge packet transport and clocking waveforms (from Beynon & Lamb, 1980)

all CCD electrodes at minimum bias (≈ 2 V) to ensure that eachMOS capacitor operates in inversion-modepotential well under 10 V electrode is much deeper than thoseunder the 2V electrodes

Charge Transfer Efficiency

charge transfer mechanisms are not perfectcharge transfer efficiency (CTE) is ratio of charge transferred toinitial chargetypical values of CTE are of the order 0.99999 for a good device

CCD Structure

must limit extent of potential well in orthogonal directionlateral confinement with channel-stop diffusion, heavily dopedregion of semiconductor relative to neighboring regionsregion has large conductivity σ relative to surrounding material,quenches surfaces potential φS; no depletion region can beformed1-D columns or rows are implemented in CCD structure alongwhich charge transfer occurs, isolated from neighboring columns(rows)at channel stop diffusion (p+ or n+ material), field oxideseparating it from gate structure is much thicker to preventbreak-down of p+ (n+) contact to conducting gate electrodes

Charge capacity and transfer speed in CCD structures

maximum amount of charge in CCD pixel depends mainly onclock voltages and electrode areaconsidering an SCCD, full-well storage capacity is in goodapproximation given by oxide capacity under gate electrode(AelecCox ) and gate voltage VG:

AelecCox =QInv

VG=⇒ QInv = AelecCoxVG

Cox is oxide capacitance per unit area, Aelec is electrodegeometric areawith Aelec = 10 × 20µm2, tox = 0.1 µm (Cox = εox/tox ) and VG =10 V: in that case QInv = 0.6 pC ≈ 3.6·106 electrons.

Charge Transport

intrinsic speed of charge transport in CCD governed by transportequation, depending on the time constants for self-induced drift,thermal diffusion and fringe field drift.in SCCD: time constant for self-induced drift is a function ofcharge density, Cox and the interelectrode spacing.For Cox = 1 pF, QInv = 1012 cm2 and spacing 25 µm, the timeconstant τSi = 0.14 µstime constant for thermal diffusion of an electron packet (Dn ≈ 10cm2·s−1) amounts to 0.25 µshigh frequency limit would appear to be a few MHz, however thefringing field of the neighboring gate electrodes aid the transferconsiderably, especially when thermal diffusion is dominant andclocking frequencies up to 15 MHz can be used for SCCDs

Focal Plane Architectures

Architecture of frame-transferCCd array (from Theuwissen,1995)

astronomical CCD imagingarrays can be subdivided intofull-frame and frame-transferarraysinterline-transfer arrays areoften used in commercialCCD camerasCCD has photosensitive arrayand a memory array coupledto a linear output registerfull-frame device lacksstorage sectionshutter interrupts illuminationduring readout

Frame Transfer Operation

Working principle of frame-transfer CCD (from Theuwissen, 1995)

transfer needs to be done quickly to prevent disturbance by lightfalling on the image section during read-outduring readout, all CCD cells in image array are again biased inintegration mode

Wavelength response of CCDs

from Beynon & Lamb (1980)

at optical wavelengths: illumination through front surface orback-surface (back-illumination)front illumination:

poly-silicon gate electrodes that transmit lightstrongly wavelength dependent absorption and interferenceeffects occurring in thin poly-silicon gate layer (≈ 0.5 µm) and thinoxide layer (≈ 0.1 – 0.2 µm)blue-responsivity strongly suppressed by absorption inpoly-silicon gate

Top left: back-side illuminated FT imager with 1260× 1152 pixels. Top right: a full frame CCD with 3k× 2k pixels, each of 9

µm× 9 µm. Middle: a low-cost FT sensor with 270,000 pixels, total diameter of the image section is 3.2 mm. Bottom left:

HDTV IT image sensor with 2M pixels, 11 mm diagonal. Bottom right: FT CCD for broadcast applications with 500,000 pixels.

CCD Responseback-illumination requires thinning of silicon substrate forphoton-generated charges to reach potential wellscharge transport can be aided by building an electric fieldgradient into the semi-conductor by increasing substrate dopingconcentration in regions close to silicon surfacethis accelerates photon-generated carriers towards front surfaceand potential wellsparticularly useful for increasing blue-responsivity where chargecarriers are generated close to rear silicon surfaceresponse can be further improved by minimizing the reflection oflight from back surface employing a λ/4 thick layer of siliconmonoxide at wavelength of interestquantum efficiency of about 50 % with back-illumination can beraised to a peak efficiency of about 90% by using the properantireflection coating

CCDs from X-rays to the Infrared

Penetration depth of silicon as a function of wavelength (from Theuwissen, 1995)

wavelengths > 1 µm: photo-electric absorption coefficient insilicon is too low (hνIR < bandgap)⇒ all photons pass throughsiliconwavelengths < 0.4 µm, > 10 nm: absorption very high in siliconand silicon oxide layer

Infrared CCDswavelengths longer than 1 µm, for which the photo-electricabsorption coefficient in silicon is too low (hνIR < bandgap) andall photons pass through silicon without being absorbedinfrared photons need to be converted first into electrons, e.g. bymeans of so-called Schottky-barrier structures in which pixelsare used made out of platinum-silicide (PtSi)array of these detectors is then coupled to a CCD read-outsystemresponsivity in the thermal IR can theoretically be extended inthis way to approximately 5.6 µmhowever, quantum efficincy of PtSi CCD detectors is only about1%

CCDs at Ultraviolet Wavelengths

UV-sensitive phosphor on top of active area: down-convertsenergy of UV-photons to longer wavelengthsback-illumination: due to high absorption, substrate of CCD mustbe thinned to about 10 µm (expensive, difficult)deep depletion of lightly-doped, high-resistivity substrate:depletion region under CCD gates extends to back ofsiliconwafercharge carriers generated by UV illumination swept to front sideinto potential wells of deep depletion layerdoes not require extreme thinning: 50 µm adequate

CCDs for X-ray AstronomyCCDs useful for X-ray astronomy when X-ray photon fluxsufficiently low to register (small) charge packet associated withsingle X-ray photonexposures with no more than one X-ray photon per 100 pixels toobtain both spectral and spatial information simultaneouslymagnitude of charge packet represents energy of absorbedX-ray photondeep depletion CCDs (30 – 50 µm) provide high quantumefficiency (> 90%) over wide X-ray range (0.2 – 10 nm)ideal imaging spectrometer behind grazing incidence X-raytelescopesback-illumination avoids problem of penetrating the gatestructure and oxide layer, superior response to low-energyX-rays (2 – 10 nm)deep depletion layer minimizes effect of charge diffusion ofX-ray-generated charge cloud, since electric field causes cloudto quickly drift into potential wellcharge loss when X-ray photons are absorbed at boundary oftwo or more pixels: charge splitting between these pixels willoccur; fix in softwareX-ray CCDs require very high CTE value (CTI = 10−5 – 10−6) toretain the intrinsic, quantum noise dominated, spectral resolutionCCD cooled to about 200 K to reduce influence of thermal noise,read-out noise

hubblesite.org

Charge Coupled Device (CCD)

imagingu.com/articles/threephase.html imagingu.com/articles/microscopyimaging.html

CCD Readout Architectures

imagingu.com/articles/microscopyimaging.html

Binning

imagingu.com/articles/binning.html

Digital Imaging

imagingu.com/articles/digitalimagebasics.html

Complementary Metal Oxide Semiconductor (CMOS)Detectors

CMOS and CCD Pixels

www.dalsa.com/shared/content/OE_Magazine_Dueling_Detectors_Janesick.pdf

CMOS and CCD Cameras

www.dalsa.com/shared/content/Photonics_Spectra_CCDvsCMOS_Litwiller.pdf

CMOS vs. CCDCMOS advantages over CCD:

standard semiconductor processinglow power consumption (≈ 1% of CCD)random access to regions of interestblooming and streaking much reduced compared to CCDsadditional electronics can be integrated on chip and in pixel (smartsensor)non-destructive readout

CMOS disadvantages:small geometric fill factor (microlenses can help)typically larger read noise

CMOS Hybrid Detectors

combine CMOS readout multiplexer bonded to a photosensitivematerial layer pixel by pixelcombines sensitivity of CCD with CMOS readout flexibilitycan also use HgCdTe or InSb for infrared sensitivitydisadvantages:

expensive due to pixel-by-pixel bondingdifferential thermal expansion of materialsimage lag

Array Detector Properties

Overviewquantum efficiencybiasdark currentflat field

Quantum Efficiency

imagingu.com/articles/microscopyimaging.html

quantum efficiency: conversion of photons⇒ electronsabsorption: avoid opically dead structures above or within pixelreflection: 70% loss at 250 nm without anti-reflective coatingtransmission: no photons generated in photosensitive volume(problem in near-infrared and soft X-rays)

Dark Current

imagingu.com/articles/ccdsnr.html

Flatfield

http://www.not.iac.es/instruments/notcam/sci-grade-arr.html

Array Detector Data Reduction

Raw Frame

Reduced Frame

Bias + Dark

Flat Field

www.sc.eso.org/˜ ohainaut/ccd/Christoph U. Keller, Utrecht University, C.U.Keller@uu.nl Astronomical Telescopes and Instruments, Lecture 12: Detectors 40

Typical Array Detector Data Reductionscience frame S, exposure time tSdark frame D, exposure time tDbias frame B, zero exposure timeflat field frame F , exposure time tFcorrected (calibrated) image

S′ =S − tS

tD(D − B)− B

F − tFtD

(D − B)− B

F − tFtD

(D − B)− B often normalized such that mean of S′ =mean of S

Gain, Read Noise, Saturation Determination

gain (G) between arbitrary digital units (ADU, A) and number ofphoto-electrons (e): A = G · enoise in e is given by σ2

e = eand therefore σ2

A = G2σ2e = G2e

gain G determined from G =σ2

Array Detector Problems: Read Noise

Array Detector Problems: Bias Shift

Array Detector Problems: Cross Talk

Array Detector Problems: Blooming

Cosmic Rays

www.sc.eso.org/õhainaut/ccd/CCD_artifacts.html

Array Detector Problems: Fringes

Other Array Detector Problemsphoton (shot) noisedark current noise; dark current is reduced by factor of 2 forevery 7 K of coolingnon-linearity of electronic amplificationimage lag

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