An introduction to CCD operation.docx

An introduction to CCD operation.docx

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AnintroductiontoCCDoperation

AnintroductiontoCCDoperation

Thebasics

AChargeCoupledDevice（CCD）isahighlysensitivephotondetector.TheCCDisdividedupintoalargenumberoflight-sensitivesmallareas（knownaspixels）whichcanbeusedtobuildupanimageofthesceneofinterest.Aphotonoflightwhichfallswithintheareadefinedbyoneofthepixelswillbeconvertedintoone（ormore）electronsandthenumberofelectronscollectedwillbedirectlyproportionaltotheintensityofthesceneateachpixel.WhentheCCDisclockedout,thenumberofelectronsineachpixelaremeasuredandthescenecanbereconstructed.

Thepicturehereshowsa"typical"CCD.TheCCDitselfisprimarilymadeofsiliconandthestructurehasbeenalteredsothatsomeofthesiliconatomshavebeenreplacedwithimpurityatoms.

ThefigurebelowshowsaverysimplifiedcrosssectionthroughaCCD.ItcanbeseenthattheSiliconitselfisnotarrangedtoformindividualpixels. Infact,thepixelsaredefinedbythepositionofelectrodesabovetheCCDitself.Ifapositivevoltageisappliedtotheelectrode,thenthispositivepotentialwillattractallofthenegativelychargedelectronsclosetotheareaundertheelectrode.Inaddition,anypositivelychargedholeswillberepulsedfromtheareaaroundtheelectrode.Consequentlya"potentialwell"willforminwhichalltheelectronsproducedbyincomingphotonswillbestored.

AsmoreandmorelightfallsontotheCCD,thenthepotentialwellsurroundingthiselectrodewillattractmoreandmoreelectronsuntilthepotentialwellisfull（theamountofelectronsthatcanbestoredunderapixelisknownasthefullwellcapacity）.TopreventthishappeningthelightmustbepreventedfromfallingontotheCCDforexample,byusingashutterasinacamera.Thus,animagecanbemadeofanobjectbyopeningtheshutter,"integrating"foralengthoftimetofillupmostoftheelectronsinthepotentialwell,andthenclosingtheshuttertoensurethatthefullwellcapacityisnotexceeded.

AnactualCCDwillconsistofalargenumberofpixels（i.e,potentialwells）,arrangedhorizontallyinrowsandverticallyincolumns.ThenumberofrowsandcolumnsdefinestheCCDsize,typicalsizesare1024pixelshighby1024pixelswide.TheresolutionoftheCCDisdefinedbythesizeofthepixels,alsobytheirseparation（thepixelpitch）.InmostastronomicalCCDsthepixelsaretouchingeachotherandsotheCCDresolutionwillbedefinedbythepixelsize,typically10-20µm.Thus,a1024x1024sizedCCDwouldhaveaphysicalareaimagesizeofabout10mmx10mm.

HowisaCCDclockedout?

ThefigurebelowshowsacrosssectionthrougharowofaCCD.EachpixelactuallyconsistsofthreeelectrodesIØ1,IØ2,andIØ3.Onlyoneoftheseelectrodesisrequiredtocreatethepotentialwell,butotherelectrodesarerequiredtotransferthechargeoutoftheCCD.Theuppersectionofthefigure（section1）showschargebeingcollectedunderoneoftheelectrodes.TotransferthechargeoutoftheCCD,anewpotentialwellcanbecreatedbyholdingIØ3high,thechargeisnowsharedbetweenIØ2andIØ3（section2）.IfIØ2isnowtakenlow,thechargewillbefullytransferredunderelectrodeIØ3（section3）.TocontinueclockingouttheCCD,takingIØ1highandthentakingIØ3lowwillensurethatthechargecloudnowdriftsacrossundertheIØ1electrodes.Asthisprocessiscontinued,thechargecloudwillprogresseitherdownthecolumn,oracrosstherow,dependingupontheorientationoftheelectrodes.

Thefigurebelow（calledaclockingdiagram）showstheprogressionunderwhicheachelectrodeisheldhighandlowtoensurethatchargeistransferredthroughtheCCD.

Initially,IØ2ishigh-usuallytoaround12V,andthechargeisheldunderthatelectrodeasin

（1）previously.WhenIØ3isheldhigh,andIØ2istakenlow（usually0V）,thechargemigratesundertheIØ3electrode（asin

（2））.Finally,takingIØ1highandIØ3lowtransfersthechargeunderIØ1（asin（3））.Thisprocessisrepeatedintransfer2andtransfer3,thechargehasnowbeenmovedthreepixelsalong.Thisprocessisknownaschargecoupling（henceCCD）.

FormostoftheCCD,theelectrodesineachpixelarearrangedsothatthechargeistransferreddownwardsalongthecolumns.Hence,duringtheCCDclockingoperation,rowsaretransferreddownwardstothefinalrow（thereadoutregister）whichisusedtotransferthechargeineachpixeloutoftheCCDsoitcanbemeasured.

Inthereadoutregister,theelectrodesarearrangedsothatthechargeistransferredinthehorizontaldirection,alongthereadoutregister.

Howthechargeismeasured

ThefinalprocessontheCCDisthereadingofeachpixelsothatthesizeoftheassociatedchargecloudcanbemeasured.Attheendofthereadoutregisterisanamplifierwhichmeasurethevalueofeachchargecloudandconvertsitintoavoltage,atypicalconversionfactorbeingaround5-10µVperelectronwith"typical"fullwellvaluesbeingabout100,000electronsorso.

ACCDcamerawillconsistoftheCCDchip,andassociatedelectronics,whichisusedatthispointtoamplifythesmallvoltageontheCCD,removenoisecomponents,digitisethepixelvaluesandoutputthevaluesofeachpixelforexample,toaPC,wheretheimagecanbeprocessedinsoftwareandtheimagedisplayed.TheCCDisananaloguedevice,andtheanaloguevoltagevaluesareconvertedintoadigitalformbythecameraelectronics.

SomeaspectsofCCDbehaviour

QuantumEfficiency

Noteveryphotonfallingontoadetectorwillactuallybedetectedandconvertedintoanelectricalimpulse.ThepercentageofphotonsthatareactuallydetectedisknownastheQuantumEfficiency（QE）.Forexample,thehumaneyeonlyhasaQEofabout20%,photographicfilmhasaQEofaround10%,andthebestCCDscanachieveaQEofover80%.Quantumefficiencywillvarywithwavelength.

Wavelengthrange

CCDscanhaveawidewavelengthrangerangingfromabout400nm（blue）toabout1050nm（Infra-red）withapeaksensitivityataround700nm.However,usingaprocessknownasbackthinning,itispossibletoextendthewavelengthrangeofaCCDdownintoshorterwavelengthssuchastheExtremeUltravioletandX-ray.

DynamicRange

Theabilitytoviewbrightandfaintsourcescorrectlyinthesameimageisaveryusefulpropertyofadetector.Thedifferencebetweenabrightestpossiblesourceandthefaintestpossiblesourcethatthedetectorcanaccuratelyseeinthesameimageisknownasthedynamicrange.WhenlightfallsontoaCCDthephotonsareconvertedintoelectrons.Consequently,thedynamicrangeofaCCDisusuallydiscussedintermsoftheminimumandmaximumnumberofelectronsthatcanbeimaged.AsmorelightfallsontotheCCD,moreandmoreelectronsarecollectedinapotentialwell,andeventuallynomoreelectronscanbeaccommodatedwithinthepotentialwellandthepixelissaidtobesaturated.ForatypicalscientificCCDthismayoccurataround150,000electronsorso.Theminimumsignalthatcanbedetectedisnotnecessarilyoneelectron（correspondingtoonephotonatvisiblewavelengths）.Infact,thereisaminimumamountofelectronicnoisewhichisassociatedwiththephysicalstructureoftheCCDandisusuallyaround2-4electronsforeachpixel.Thus,theminimumsignalthatcanbedetectedisdeterminedbythisreadoutnoise.

Intheexampleabove,theCCDwouldhaveadynamicrangeof150,000:

4（takingtheuppernoiselevel）.But-thisdynamicrangeisalsodependentontheabilityoftheelectronicstobeablefullydigitiseallofthisdynamicrange（seethemoredetailedCCDinformationfordiscussionsonelectronicsresolution）.

Linearity

Onthewhole,theeyeisnotalineardetector（exceptoververysmallvariationsinintensity）andhasalogarithmicresponse.Animportantconsiderationinadetectorisitsabilitytorespondlinearlytoanyimageitviews.Bythiswemeanthatifitdetects100photonsitwillconverttheseto100electrons（ifwehad100%QE）andifitdetects10000photons,itwillconverttheseto10000electrons.Insuchasituation,wesaythatthedetectorhasalinearresponse.Sucharesponseisobviouslyveryusefulasthereisnoneedforanyadditionalprocessingontheimagetodeterminethe'true'intensityofdifferentobjectsinanimage.

Noise

OneofthemostimportantaspectsofCCDperformanceisitsnoiseresponse.ThereareanumberofcontributionstothenoiseperformanceofaCCD,thesearebrieflylistedhere:

∙Darkcurrent-i.ethermallygeneratednoise.Atroomtemperature,thenoiseperformanceofaCCDcanbeasmuchasthousandsofelectronsperpixelpersecond.Consequently,thefullwellcapacityofeachpixelwillbereachedinafewsecondsandtheCCDwillbesaturated.Darkcurrentcanbemassivelyreducedbycooling.Forexample,thenoiseperformanceoftheCCDcouldbereducedfromthousandsofelectronsatroomtemperaturetoonlytensofelectronsperpixelpersecondat-40degreesC.Bycoolingdowntotemperaturesbelowabout-70degreesCdarkcurrentcanbevirtuallyeliminated（substantiallybelowoneelectronperpixelpersecond）.AsecondwayofreducingnoiseistoslightlyaltertheCCDprocessingtechniquetoproduceaMulti-Pinned-Phase（MPP）CCD.Thistechniquecanreducethedarkcurrenttoverylowlevels（afewhundredelectronsperpixelpersecondatroomtemperature）.

∙Readoutnoise -theultimatenoiselimitoftheCCDisthereadoutnoise.ThereadoutnoiseoriginatesfromtheconversionoftheelectronsineachpixeltoavoltageontheCCDoutputnode（atypicalvaluewouldbearound4µVperelectron