Charge-coupled-device Charge -coupled- devicecharge-collection charge -collectionefficiency efficiency and and the the photon-transfer photontransfer technique JamesR. James R. Janesick Kenneth P. Klaasen Tom Elliott Propulsion Laboratory Jet Propulsion California Institute of Technology Oak Grove Drive 4800 Oak 4800 Pasadena, Pasadena,California California 91109 91109 performance unprecedented performance charge-coupled Thecharge Abstract. Abstract. The -coupled device has shown unprecedented and transfer, and charge transfer, response, charge spectral response, of spectral areas of the areas photon detector in the as aa photon as for potential for noise. Recent readout noise. Recent experience experience indicates, indicates, however, that the full potential realized in CCD's charge-collection the CCD's charge -collection efficiency efficiency (CCE) (CCE)lies lieswell well beyond beyond that that realized perforCCE perfordefinitionofofCCE presentaa definition we present paper we this paper In this devices. In available devices. currently currently available photon-transfer (the photonmance and and introduce a standard test tool (the transfer technique) for CCE compare CCE We compare parameter. We CCD parameter. importantCCD this important measuring and optimizing this in CCDs, discuss of CCDs, types of characteristics for different types discuss the the primary limitations in characteristics prospects for CCE performance, achieving achieving high high CCE performance,and andoutline outline the the prospects for future future improvement. charge-coupled terms: chargeSubject terms: Subject coupled devices; charge diffusion; diffusion; xx-ray -ray events; events; frontside frontside illuminaillumination; backside backside illumination. illumination. 1987). 972-980 Optical Engineering 26(10), 972 -980 (October 1987). CONTENTS 1. 1. Introduction Charge-collection 2. 2. Charge -collection efficiency efficiency (CCE) (CCE) Photon-transfer 3. 3. Photontransfer technique Ideal CCD camera 3.1. 3.1. Ideal 3.2. 3.2. Evaluation of constant K Evaluation of constant J 3.3. 3.3. Evaluation included events included split events and split Partial and 3.4. 3.4. Partial Photon-transfer 3.5. 3.5. Photon -transfer curve 3.6. Photon -transfer histogram Photon-transfer 3.6. Photon-transfer 4. 4. Photon -transfer use Frontside illumination (TI VPCCD) 4.1. 4.1. Frontside Backside illumination 4.2. 4.2. Backside illumination (TI (TI 3PCCD) 5. Future improvements in CCE Future improvements 5. Acknowledgments 6. Acknowledgments References 7. References 1. INTRODUCTION 1. CCDs CCDs in recent years years have have become become the premier detector for astronomical ground-based and ground spaceborne and many spaceborne use use in many -based astronomical instruments. They were were selected selected for for use use in in the Hubble Space (WF/PC), Telescope Telescope Wide Wide Field Field Planetary Planetary Camera (W F/ PC), the the GaliGalimany (SSI), and many Imager (SSI), State Imager Solid State Orbiter's Solid leo leo Jupiter Jupiter Orbiter's ground-based ground -basedimaging imagingand andspectroscopic spectroscopicapplications. applications. ProPro-ray imager on NASA's posed space applications include an xx-ray Space Advanced Advanced X-ray X -ray Astronomical Astronomical Facility Facility (AXAF), (AXAF), a Space Telescope Telescope Imaging Imaging Spectrometer Spectrometer (SIS), the Solar Optical TeleFlyby Rendezvous/Asteroid Comet Rendezvous/ the Comet scope scope (SOT), (SOT), and and the Asteroid Flyby Imaging Imaging Subsystem Subsystem (CRAF (CRAF ISS). received 1987; revised 13, 1987; received April 13, CH-102 Paper CH Invited Paper -102 received revised manuscript received 1987; received 19, 1987; 1987; accepted 15, 1987; May 15, accepted for for publication publication June 19, received by by Managing Managing the at the presented at 570-02, Paper570 of Paper revision of paper isis a revision 6,1987. July 6, Editor July Editor 1987. This paper -02, presented 1985, San Diego, 22-23, Aug.22 Arrays,Aug. Imaging Arrays, State Imaging SPIE conference Solid State -23, 1985, Calif. Calif. The The paper paper presented presented there there appears appears (unrefereed) (unrefereed) in in SPIE SPIE Proceedings Proceedings Vol. 570. Engineers. Photo-Optical 1987 Society of Photo © e 1987 -Optical Instrumentation Engineers. CCD limit CCD ultimately limit parameters that ultimately fundamental parameters The The fundamental efficiency charge-transfer (2) chargenoise,(2) read noise, (1) read are (1) performance performance are transfer efficiency charge-collection (4) charge and (4) (QE), and efficiency (QE), (CTE), (3) quantum quantum efficiency -collection efficiency efficiency (CCE). (CCE). At At their their present stage of development, it is (in the 4 noise (in read noise low read have low that have devices that possible to fabricate fabricate devices deferred excellent CTE range), excellent 15 e~ to 15 a range), CTE performance performance(<10 (<10e~ a deferred entire the entire over the performance over QE performance unsurpassed QE charge), charge), and and unsurpassed potenfull the However, -3 1 A. 11,000 to 1 from range spectral spectral range from 1 to 11,000 A.1 -3 However, potencharge-collection of charge tial of -collection efficiency efficiency lies lies well well beyond beyond that that of currently currently available available devices. devices. Optimization Optimization of this important manufacnew challenge for the CCD manufacrepresents aa new parameter parameter represents is required for many performance is CCE performance High CCE user. High turer and user. the which the to which spectrum to regions of the spectrum applications applications over over all all regions are CCDs are CCD is sensitive. In the visible range, for example, CCDs (charge sensitivity (charge highsensitivity demandhigh thatdemand trackers that used used in star trackers geometric high geometric with high loss) in conjunction with collection collection without loss) accuracy (collection (collection without significant charge diffusion). In applications spectrum, applications the spectrum, regions of the EUV regions the xx-ray -ray and and EUV the require confinement of signal charge to a single pixel pixel without without loss loss in in order order to accurately determine the energy of the incomincoming ing photon. perforCCE perforhigh CCE achieving high The means of measuring and achieving we present a useful Sec. 22 we InSec. paper. In mance is the subject of this paper. of parameters that definition for CCE performance in terms of definition is The definition CCD. The testing the CCD. when testing readily found are are readily found when responsible for are responsible that are divided divided into the two primary factors that and loss and chargeloss namely,charge performance,namely, CCE performance, of CCE the degradation degradation of Sec. 3 we In Sec. charge diffusion. In we introduce introduce the concept of photon measuring ofmeasuring way of standard way as aa standard used as technique used transfer, a technique CCE characteristics, characteristics, and develop develop the the theoretical theoretical foundations show We show based. We method isis based. photon-transfer which the photonupon which transfer method photon-transfer of the photonlimitations of the strengths and limitations transfer technique as it is used used in in measuring CCE characteristics of the CCD. In Sec. Sec. 44 we we apply apply the the photon-transfer photon -transfer technique in measuring measuring 10 No. 10 26 No. Vol. 26 ENGINEERING // October OPTICALENGINEERING 972 / /OPTICAL October 1987 1987 // Vol. Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx CHARGE-COUPLED-DEVICE PHOTON-TRANSFER TECHNIQUE CHARGE -COUPLED -DEVICE CHARGE-COLLECTION CHARGE -COLLECTION EFFICIENCY EFFICIENCY AND AND THE PHOTONTRANSFER TECHNIQUE CCE performance for frontsidefrontside- and andbackside backside-illuminated CCE performance -illuminated CCDs ultimately limit CCDs and and discuss discuss the the primary primary factors factors that ultimately CCE each device. device. Finally, Sec. 55 we we discuss discuss future future CCE for for each Finally, in Sec. considerations considerations for for further further improving improving CCE CCE for for the the CCD. CCD. 2. CHARGE-COLLECTION 2. CHARGE -COLLECTION EFFICIENCY CCE isis aarelatively relativelynew newCCD CCDperformance performanceparameter parameter that that has been Jet Propulsion Propulsion been defined, defined, measured, measured, and and optimized optimized at at Jet Laboratory (JPL) (JPL)and andelsewhere. elsewhere. CCE CCE measures measures the the ability of Laboratory the CCD to collect all signal charge generated from aa single single photon event event into a single single pixel. pixel. High CCE CCE performance performance is is especially especially critical critical for for EUV EUV and soft xx-ray -ray applications (e.g., soft xx-ray soft -ray imaging imagingspectrometers), spectrometers),where wherethe the ability ability of the CCD to accurately accurately determine determine the energy of photon CCD the energy of the photon depends upon collecting collectingthe thephotogenerated photogenerated charge properly. collection requires Experience has shown that complete charge collection requires that two criteria be met: met: (1) (1) There There must must be be no no trapping.centers trapping centers within the CCD to cause signal charge to be lost by recombination, (2) the individual photon must nation, and (2) the charge charge of an individual must be be collected collected within withinaa single singlepixel pixeland and must must not be allowed allowed to divide divide among among several several pixels. pixels. Charge Charge loss loss causes causes the the photon energy degrades energy to to be be underestimated, underestimated, while charge splitting degrades the precision of charge charge measurement measurement by requiring the summation of several noisy pixels. The loss and charge charge splitting splitting depends depends The degree degree of of charge charge loss upon where where in inthe thepixel pixelthe thephoton photonisisabsorbed. absorbed. Photons Photons that are absorbed within the frontside frontside depletion depletionregion region(see (see Figs. Figs. 55 given pixel pixelare are typically typically seen as as the the ideal ideal event event and and and 8) of aa given are called called "single-pixel "single -pixelevents." events."Photons Photons absorbed absorbed below the depletion region, where the electric field is weaker, create aa charge cloud until itit reaches reaches cloud that thermally diffuses outward until the rapidly changing wells at changing potential wells at the lower lower boundary boundary of the pixel pixel array. array. At At that point, the charge cloud may split into two pixels. two or more packets, which are collected in adjacent pixels. Events of this Events in in which which Events this type type are called "split events." Events charge conserved have have been been named named simply simply "partial "partial charge isis not conserved events" regions deep events" and and are are usually usually generated generated in regions deep within within the the CCD, where where loss occurs. loss of of carriers carriers through through recombination occurs. From this discussion, discussion, aa definition for CCE for an individual ual photon event event II can can be be presented presented through the formula CCE, CCEI = = (1) (1) tion and the the 800 800 ae remaining remaining(rspe_1) Upe-j) split between and collected by two pixels pixels (1 (Pse _,). For se_1). For this event, event, aa CCE1 CCEj of of 0.4 0.4 is is calculated are split calculated no no matter matter in in what what proportion proportion the the 800 800e~ a are split between between the two affected pixels. To determine the average CCE performance of a CCD for a large large number of of interacting interacting photons photons of ofthe thesame same energy, energy, many splitting and many events events are are measured measured for charge loss and splitting then averaged using using the equation N CCE _ i= Spe - I (3) Pse -I where N isis the the number of photon events where N events sampled. Equation (3) (3) is is used used regularly regularly in in the the laboratory laboratory in characcharacterizing mechanisms (the partial terizing the the two mechanisms partial and and split split events) events) responsible for performance of the CCD. CCD. responsible for degrading degrading CCE CCE performance of the However, manner described described by by Eq. Eq. (3) (3) However, measuring CCE in the manner amount of ofdata datareduction. reduction-since requires a considerable amount since many events must be integrated. Also, Also, Eq. Eq. (3) (3) is is usable over only a limited region (typically, (typically, A X< 30 A) A) because because for for limited spectral spectral region < 30 longer wavelengths wavelengths the signal generated generated by an an individual individual longer the signal photon becomes too small compared to the the CCD CCD read read noise noise floor to reliably resolve the individual event and determine the amount of of charge lost and the the number number of ofpixels pixels affected. affected. In this paper we describe describe another approach approach to to evaluating evaluating CCE performance for the CCD CCD that thatisis applicable applicable to toall all wavewaveThe new new technique technique(discussed (discussedininSec. Sec. 3) 3) isis lengths of interest. The based on the formula E CCE=-' (4) r/E isiscalled where CIE calledthe theeffective effectivequantum quantumyield, yield,aaquantity quantity that that measures the average number of of electrons electrons collected collected by by an an measures affected interacting photon photon of ofenergy energy Ex. Ex . The The affected pixel pixel for for an interacting effective quantum yield effective yield r%E rjE is isrelated relatedtotothe thepartial partial and split events events through E pe (5) Pse where Spe/ £pe /Pse the term termape £ -i!/Pse PSeisisthe theaverage average value value of the /Pse _j. 77; Pse-1 where where CCEj CCEI represents represents the the fraction of signal electrons, generated by by aa particular particular interacting interacting photon I, that ated that isis collected collected in any _j refers any single single affected affected pixel; pixel; ^e bpe_I referstotothe thepartial partial event event and and represents the number of signal signal carriers carriers generated by a photon collected by and collected by all all pixels pixels (the (the rest rest being being lost lost to recombination); PSe_1 Pse _, refers referstotothe thesplit splitevent eventand andrepresents represents the the number of pixels pixels that that collect collect signal signal electrons electrons generated generated by by a photon; Tjj is and m is defined defined as as the the ideal ideal quantum quantum yield, a quantity equal to the total number of electrons generated for an interacting interacting photon photon of energy (eV).The The ideal ideal quantum quantum yield energy EA Ex (eV). yield ir/j is directly photon energy energy and is is found found directly proportional proportionaltoto the the photon according according to to the the relationship E,, I)i 3.65 (A<1000A) (À<1000 A) .. (2) (2) As As an an example example of of using using Eq. Eq. (1), (1), assume assume that that an an interacting photon generates (17}), with recombinagenerates 1000 1000e~e (m), with 200 200e~e lost to recombina- PHOTON-TRANSFER 3. PHOTON -TRANSFER TECHNIQUE The ideal CCD, which does not generate generate split split or or partial partial events events perfect CCE CCE performance, performance,will will deliver deliver an an effeceffecbut exhibits perfect tive quantum yield equal the ideal ideal quantum quantum yield yield (i.e., tive quantum yield equal to the r;E rjj). Today's CCDs are rapidly progressing toward this this rlE = rl;). however, very placed on ultimate goal; however, very strict strict conditions conditions are placed in obtaining obtaining such such performance, performance, as as we we shall shall see see in in the CCD in Sec. technologies and Sec. 4. 4. Because Because of of the the various various CCD technologies and manumanufacturers involved in fabricating fabricating CCDs, a standard "test tool" performance over over aa very very large large spectral spectral for evaluating CCE performance range is required. In this section section we wediscuss discussthe thetechnique technique of ofphoton photon transfer, transfer, was used in the past to evaluate a test tool that was evaluate CCD CCD perforperforunits. 1 ItIt was was realized mance characteristics characteristics in in absolute units.' realized only recently that the photon photon-transfer recently -transfer technique also can be applied as method for for evaluating evaluating the the CCE CCE perforperforplied as a standard method mance of CCD. In In the thediscussion discussion that that follows, follows, we we first first mance of a CCD. develop necessary to to describe describe the the technique, technique, develop the equations necessary OPTICAL / October 1987 / Vol. OPTICALENGINEERING ENGINEERING / October 1987 / Vol.2626No. No.1010/ / 973 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx ELLIOTT JANESICK, KLAASEN, KLAASEN, ELLIOTT QE, QE1 INCIDENTI PHOTONSI PI INTERACTING INTERACTING PHOTONS PHOTONS ELECTRONS ELECTRONS INCIDENT INCIDENT PHOTONS PHOTONS INTERACTING INTERACTING PHOTONS PHOTONS L COLLECTED COLLECTED QE QE Al SV NE '?' *R2 R 7 DIGITAL DIGITAL NUMBER VOLTS VOLTS VOLTS VOLTS NUMBER NUMBER ELECTRONS ELECTRONS TRANSFERRED TRANSFERRED VOLT VOLT VOLT SIGNAL SIGNAL CHAIN CHAIN GAIN A/DC A/DC GAIN CCD CCD SENSITIVITY I _JJ Tft by through77; through A2 S(DN) SIDNI CC D ideal an ideal of an Schematic showing individual transfer functions of Fig. 1. Fig. 1. Schematic CCD camera. assuming assuming that that we we have have an an ideal ideal CCD CCD camera camera with with no no partial split event or split event generation. generation. We We show show that that the ideal quantum photon-transfer thephotonthrough the determined through be determined can be yield TJ transfer i can which camera, which CCD camera, typical CCD examine a typical We next examine approach. We approach. includes includes partial and split event generation, and show that the photon-transfer photontransfer technique technique gives gives aa reasonable reasonable approximation approximation rjE , defined yield77E, quantumyield for the effective quantum defined in Eq. (5), which in performance of the calculate the used to calculate turn is used the CCE performance the CCD sense. (4)], at least in a relative sense. [Eq. (4)], CCD camera Ideal CCD 3.1. Ideal 3.1. overall transfer Figure Figure 11 isis aa schematic schematic representation representation of the overall debe decamera. The camera can be ideal CCD camera. function of function of an ideal are functions, three five transfer scribed in in terms terms of of five transfer functions, three that are scribed related related to to the the CCD CCD and and two two that are related to the external signal processing processing circuitry. circuitry. The The input input to the camera is CCD signal given in given in units units of of incident incident photons, photons, and the final output of the achieved by is achieved camera by encoding encoding each each pixel's pixel's signal signal into into a camera is 12 to 16 bits. using 12 digital number bits. The output output typically using (DN), typically number (DN), signal signal SS (DN) (DN) resulting resulting from from aa given given exposure exposure of of the the CCD camera camera shown in Fig. 11 is given by _ _K 1 (10) J measuring by measuring possible to to determine determine the factors K and JJ by is possible It is each transfer function in Fig. 11 separately and then combining the of the because of However, because (9). However, and (9). (8) and Eqs. (8) these results as in Eqs. (which CCD (which the CCD of the parameters of of parameters number of uncertainty in a number Sv independently), we QEj, rj{rl, and Sv prevents us from knowing QED, great any great to any directly determine cannot determine KK or or J to practice directly cannot in practice we have developed a simple accuracy. Instead, we simple technique technique that that requires no knowledge knowledge of the the individual individual transfer functions to K and J. determine factors K determine the factors K constant K of constant Evaluation of 3.2. Evaluation 3.2. generate only one For the CCD stimulated with photons that generate one rl = (i.e., rj{ electron-hole electron -hole(e-h) (e -h)pair pairfor for each each interaction (i.e., (6) reduces A), Eq. (6) 3000 A), 1; 1; AX > > 3000 reduces to to the form PIK~-I S(DN) S (DN) = PIK (11) (I l) , interacting of interacting number of represents the PQE, represents the number PI = PQE, where PI photons per pixel. The constant K can be determined by relating the output (DN) a§ (DN) The variance as a| (DN). The signal SS (DN) (DN) to its variance, as which errors, which of errors, of Eq. (11) is found using the propagation of (11) is perfect (i.e., perfect CCD (i.e., ideal CCD the ideal yields the following following equation for the yields the charge collection collection and charge transfer): -( a S (DN)]2 as (DN) L ô PI aPI + [r 3S (DN)12 aK (12) + aR (DN) , PQE,r7i SvA,A2 ,, S(DN) S (DN) == PQE0);SvAlA2 (6) over all signal (DN) average signal where SS (DN) (DN) represents represents the the average (DN) over all where affected affected pixels, pixels, PP isisthe themean meannumber number of of incident incident photons photons per pixel on the CCD, QED definedasasthe theinteracting interactingquantum quantum QEj isisdefined pixel is the efficiency (interacting efficiency (interacting photons/incident photons/ incident photons), photons), rlrj{ is ideal ideal quantum yield defined by Eq. (2), Sv is is the sensitivity sensitivity of of is the electronic gain of A, is on-chip the CCD on -chipcircuitry circuitry(V/e~), (V/ el, A, the of the A2 isis the the camera V), and and A2 the transfer function of (V/V), camera (V/ analog -to- digital converter converter (DN/ (DN/ V). analog-to-digital QE, and The quantities QED and 77}mare are related related through QE = QE = (7) (7) ThQEI , where QE isis the the average average quantum quantum efficiency (electrons collected/incident ted/ incident photon). fundamental into fundamental (DN) into signal SS (DN) To convert the output signal convert the physical units, it isis necessary necessarytotofind findthe theappropriate appropriate factors factors to signal or signal photons or interacting photons either interacting DN units into either convert DN convert are defined by conversion are do this conversion that do electrons. The constants that the equations ,A2r! , J = (r);SvAIA2)-1 ^A2r1 » K = (SvAIA2)-I , , (8) (9) w (8) interacting phowhere where the the units units of of KK and and JJ are are e~/DN e / DN and interacting respectively. Note Note that that Eqs. Eqs. (8) (8) and and (9) (9) are are related related tons/ DN, respectively. 2 J a1 floor noise floor read noise the read where we where we have have added added in in quadrature the (e~)KIf2]. a* (e-) variance aR (DN) [see [see Fig. Fig. 1;1; a£ aÁ(DN) (DN) = aR -2]. a£ (DN) variance and (12) and Eq. (12) on Eq. differentiation on Performing Performing the required differentiation (i.e., variance (i.e., negligible variance has negligible constant KK has assuming assuming that that the constant variance in the variance for the expression for following expression aK thefollowing findthe wefind 0),we cjjt == 0), S (DN): 2 as (DN) = (-K I (DN) . (13) following the following statistics, the photonstatistics, ofphoton because of PI because Since ajsj aPl = PI Since (DN) a§ (DN) of SS (DN) and as terms of in terms K in constant K equation for the constant results: K = K S (DN) S(DN) (DN) as (DN) (DN)--4a 2(DN) (A>3000 (À>3000 A) . (14) used, with no be used, can be Equation (14) is a useful expression and can Equation (14) measurements in output measurements calibration, to convert output in DN further calibration, electrons. of electrons. directly into units of Evaluation of constant J 3.3. Evaluation 3.3. wavelengths longer For wavelengths longer than than 3000 3000 A, A, the the constants constants KK and and JJ we move into as we However, as 1]. However, = 1]. are equivalent equivalent [Eq. [Eq. (10), (10), rj{rl = multipleee-h spectrum, multiple -ray regions of the spectrum, -h the UV, EUV, and xx-ray resulting in pairs are generated generated by each interacting photon, resulting pairs in the value J. For these conditions, the decrease in and aa decrease > 11 and rji > 10 No. 10 26 No. Vol. 26 October 1987 // Vol. ENGINEERING // October / OPTICALENGINEERING 974 / OPTICAL Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx CHARGE-COUPLED-DEVICE PHOTON-TRANSFER TECHNIQUE CHARGE -COUPLED -DEVICE CHARGE-COLLECTION CHARGE -COLLECTION EFFICIENCY EFFICIENCY AND AND THE PHOTON -TRANSFER TECHNIQUE constant also can be found by by relating relating the the output output signal constant JJ also signal S (DN), (DN), given S given by by Eq. Eq. (6), (6), to to its its variance variance o$ as (DN). Through propagation of errors, the variance variance in in the the signal signal for for the the ideal ideal CCD can be expressed by as (DN) [âS(DN)2 â PI + °l\ avi + z 'as(DN)T . J 1e - K J aK + aR (DN) an; . (15) Differentiating Eq. quantum yield yield Eq. (15) (15) and and assuming assuming that that the quantum r/j has negligible variance partial or or split split event; event; rli has negligible variance (i.e., (i.e., no partial 61. = — 0), 0), we we find find a2. S(DN) S (DN) (XOOOOA) (X<3000 A) a£ (DN) - aR = aS| (DN) - J = (16) Equations (14) (14) and (16) Equations (16) form form the basis basis for for the the photon-transfer photon- transfer technique. By By simply simply measuring measuring the mean signal and its variance other spespeance for for both both visible visible photons photons and and photons at any other cific we can determine the values values cific wavelength wavelength of of illumination, illumination, we K Once the known, the ideal K and and J. J. Once the constants constants K K and and J are known, quantum yield yield for for photons at the wavelength under consideration can be calculated Eq. (10). (10). calculated through Eq. 3.4. Partial and and split split events events included 3.4. Partial Up to this point, we have assumed assumed no partial or or split split event event Up to this we have generation within the CCD (i.e., irjEE = rj{). We now show that rl;). We now show that = K/J with with partial partialand andsplit splitevents eventsincluded included gives the ratio K/J gives an an effective quantum yield yield rjE, rjE , which upper limit for the effective which in in turn gives performance for gives an an upper upper limit limit for for CCE CCE performance for the CCD as as defined of partial partial defined by by Eq. Eq. (4). (4). We We also also show show that that as as the number of and split events events within CCD decreases, decreases, the and split within the the CCD the ratio ratio K/J approaches the real real value value ofofrlE rjE and andininthe thelimit limit77E rjE equals rj^ rl;, when perfect CCE is achieved. To analytically these condianalytically solve solvefor for the the constant constant J under these tions, we we must must give give Eq. Eq. (6) (6)aanew newform form so so that that the variances variances of the partial and split split events are included when the overall overall varivariance of the the signal signal isiscalculated. calculated. Such Such an an equation equation for for the average signal in signal in any any given given pixel pixel can can be be written written in the form S(DN) S (DN) ==P171;M (CPe K -I 2(4 Pseat \-1-1 / S(DN) S (DN) ' P., + [as (DN) (DN) a£ (DN) (DN) (\P5e + P5e++ epe - aR (18) (18) a£ is the event event-to-event of pixels where ap -to -event variance variance in the number number of [ âS(DN)j2 ax = K [âS(DN)2 a , 3*. It can be be shown, shown, again again using using propagation propagation of errors, that the signal SS (DN), given by Eq. a| (DN) are Eq. (17), (17), and and its variance as effective quantum yield by related to the effective (17) Psell' where where M M isis the the average average number number of interacting interacting photons per pixel. informative to compare the behavior of the the signal signal It is informative to compare given ideal given by by Eq. Eq. (17) (17) to to the the signal given given in in Eq. (6) for the ideal CCD camera camera without or split split events. events. The The signal signal dedeCCD without partial or scribed in Eq. rj{ and Eq. (6) (6) isisproportional proportional to il; and is is not not influenced by CCE characteristics since CCE is is assumed assumed to be perfect. perfect. In the the case signal isis proportional case of of Eq. Eq. (17), (17), we wefind find that that the the signal proportional to to (£pe/ pixel is ape/ PSC) PSe)when whenthe thenumber numberof ofinteracting interacting photons photons per pixel small 1) and interactions are not adjacent adjacent to to each each small (i.e., (i.e., M M «~ 1) other this case, case, the amount of of signal signal other in in the the CCD CCD array. array. In this measured split event event behavmeasured isis dependent dependent on on both partial and split ior. However, when ior. However, when the the number number of of interacting interacting photons photons per per pixel Pse ), the pixel isis large large (i.e., (i.e., M M« ~ PSe), the signal signal is is dependent only on the partial event [i.e., PI^K"-l] 1 ] and the effects of [i.e., SS (DN) (DN) ==PIC,peK the split split event event are are averaged averaged out. a^ is is that collect signal electrons electrons per interacting photon and al event-to-event the event -to -eventvariance variancefor forthe the total total number of electrons collected. we have average of `ape collected. Here, we have assumed assumed that the average £pe -II _j/ Pse -i isis equal £pe _j divided by the average of Pse_I equal to to the average of Cpe1 Pse -i» "se _1,which whichbecomes becomesnearly nearlycorrect correct for for large large numbers numbers of of photon events. events. Equation (18) also can be written in the form K_ Je (19) where where Ee isis PSe Pse + 2(a£/Pse) Pse (a£/a£e) 2(4/ Pse) ++PSe(GV ape)and and K K and and J are as defined Eqs. (14) (14) and and (16), (16), which which we we normally normally measure defined in Eqs. measure using photon-transfer using the photontransfer technique. The true value value of of the the effective effective quantum quantumyield yield i rjE given by E given Eq. (19) As the the number number of Eq. (19) isis less lessthan thanK/J K/J by by the factor ce"I.1 . As partial and split split events events decreases, decreases, the accuracy of partial and the accuracy of K/J K/J improves improves and and in in the the limit limit isisexact exactwhen wheneE== 11 (i.e., (i.e., rjE rlE=_ rjj). Therefore, when measuring measuring the the effective effective quantum quantumyield yield using using the photon-transfer presence of partial and photon- transfer technique technique in the presence split K/J gives gives an an upper upper limit limit for for flE. rjE . For split events, events, the the ratio ratio K/J example, for the Texas Instruments (TI) 3PCCD 3PCCD (a (a CCD CCD type type Instruments (TI) discussed in Sec. Sec. 4), 4), we wefind findexperimentally experimentally that for individual 5.9 keV (Fe55) (Fe55 ) photon 5.9 keV photon events, events, one one out of of 11 11 events events splits splits between 22 pixels, pixels, with with only only aa few fewpartial partial events events observed. observed. For For this we calculate calculate an an average average P5e Pse of of 1.09 1.09 pixels pixels with with this CCD we variancesap = 0.166 0.166 and and a2 a^ -«0.0.Assuming Assumingthese thesevalues, values, we variances aP = find 0.72.Therefore, Therefore,the thetrue truevalue valueofofrlE rjE isis actuactufind that ce"I1 ==0.72. than the the value value of of,lE rjE measured measured using using the ally smaller by 0.72 than the photon-transfer photon- transfermethod method (i.e., (i.e., K/J). K/J). Even though the ratio ratio K/J K/ Jdoes doesnot notgive give an an exact exact value value for for the effective quantum quantity still the effective quantum yield, yield, this this quantity still is is useful useful in evaluating and optimizing CCE performance of the CCD, as we we shall shall see see in in Sec. Sec. 4. 4. Therefore, Therefore, unless otherwise indicated, we use K/J as as found found through throughthe we use the the ratio ratio K/J transfer thephotonphoton-transfer method as our standard standard measuring measuringtool toolinincomparing comparingrlE rjE and CCE performance for different CCDs under different operatoperating ing conditions, conditions, while keeping in in mind mind that that the absolute values of these 1 ). these quantities quantities are are lower lower (by (by e" c I). Photon-transfer curve 3.5. Photon -transfer curve The constants constants K K and JJ can can be be found found either either graphically graphically or or The throughEqs. Eqs. (14) (14) and and (16). (16). We We examine examine the the graphical graphical directly through approach first first because because the method method gives gives insight insight into into the approach mechanics of the photonphoton-transfer mechanics transfer technique. The constants K K and J can be found graphically by plotting "photon-transfer curve") a curve (called the "photontransfer curve ") of noise as (DN) as of signal signal SS (DN), (DN), typically typically for aa 20X20 20X20 pixel pixel as a function of array on on the the CCD. CCD. One One such such photonphoton-transfer curve isis shown transfer curve shown in Fig. 2. For For this this curve curve we we use use 7000 7000 AA illumination, illumination, which which guarantees that that TIE rjE = rl; rj{ = 1I and and therefore therefore can can be be used used in finding K. The The abscissa, abscissa, SS (DN), (DN), is is the average finding the constant K. signal level level of of the the 400 400 pixels pixels with with the the array array uniformly illuminated at some level. (Here, we assume that electrical offset and OPTICAL ENGINEERING / October 975 OPTICAL ENGINEERING / October1987 1987/ /Vol. Vol.2626No. No.1010/ / 975 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx ELLIOTT JANESICK, KLAASEN, ELLIOTT I I I I I I I I I I I TI3PCCD, _ TI3PCCD 4000 A X= X = 4000 30 - T= 7000 A 0 t 10 SIGNAL SHOT NOISE SLOPE = 1/2 ó z 20 READ NOISE, oR 101 SLOPE = 0 10 K = 2 e-/DN 100 100 100 J I I I i 101 102 102 103 103 10* 104 1.5 = K = 1.5 105 105 0.50 DN SIGNAL SIGNAL (S), (S), DN 1). (T?J = = 1). illumination (*1; Fig. 2. Photon -transfer curve curve using using 7000 7000 A illumination Photon-transfer Fig. 2. 0.75 1.0 1.25 1.50 1.75 2.00 2.25 2.50 PHOTONS/DN INTERACTING PHOTONS /DN Fig. Fig.4.4. Photon-transfer Photon -transferhistogram histogram using using 4000 A illumination 1). (T* = =1). (+F, 10000 10000 rq '' ,"'i Ea) , .,,.i "1 nE12.1Á1=1420 1000 1000 _ 71E11216Á1=3 77E(700011=1 ó .`a . 100 100 Z 2. 1A 1216 Á 10 7000 Á A 0.001 0.01 0.1 11 10 10 100 100 1000 1000 10000 100000 100000 10000 DN SIGNAL, SIGNAL, DN each energies, each photon energies, three photon at three taken at curves taken Photon-transfer Fig. Fig. 3. 3. Photon -transfer curves yielding aa different different effective quantum yield. signal were subtracted subtracted from the data before the signal dark current were level was wasdetermined.) determined.)The The ordinate, ordinate, as (DN), is the standard level exposure. deviation of the signal of those 400 pixels at each exposure. pixel-to-pixel CCDpixel theCCD afterthe found after -to -pixel deviation isis found The standard deviation accomplished nonuniformity has been removed. This can be accomplished same differencing (pixel (pixel by by pixel) pixel) two two frames frames taken at the same by differencing light level, level, calculating calculatingthe the standard standard deviation of the resultant as desired as yields the desired which yields difference,and and dividing dividing by by 2, 2, which difference, (DN). aR (DN), (DN), indicated indicated in Fig. Fig. 2, 2, represents the The read noise aR intrinsic noise associated associated with with the the readout circuitry, i.e., the intrinsic noise on-chip CCD on -chip amplifier amplifierand and any any other other noise noise sources sources that are level. As the signal is increased, the signal level. the signal of the independent independent of noise eventually becomes becomes dominated dominated by the shot noise of the 1/2. signal and and isis characterized characterized by by aa line line of slope slope 1/ 2. From Eq. signal the of the line of slope 1/1/2 the slope of the (14) we note note that the intersection of 2 line (14) we converdesired converthe desired represents the signal axis signal axis [i.e., [i.e., as as(DN) (DN) = 1]1] represents K. sion constant K. The same graphical approach can be used in determining same graphical Fig. 33 example,ininFig. Forexample, (16)].For [Eq.(16)]. the constant J when when rj{i >>1 1[Eq. same photon-transfer we transfer curves curves (taken (taken with the same show three photonwe show wavefields at waveflat fields CCD CCD camera camera and CCD) generated from flat corresponding The corresponding A. The 2.1 A. 1216 A, lengths lengths of of 7000 7000 A, A, 1216 A, and and 2.1 wavelengths these wavelengths of these each of intersections on the signal axis at each intersections 7000 A = =r]-m== 11 for 7000 . Since 1.62 X10~3 are 2.3, 0.77, 0.77, and 1.62 X 10 -3. Sincer]E77E photon-thisphoton forthis illumination,the thesignal signalatat as as (DN) (DN) = 11 for illumination, (i.e., K (i.e., constant K transfer curve represents represents the the value value of constant transfer curve wavethe wavefor the intersections, for two intersections, other two The other 2.3ee~/DN). K K ==2.3 -/ DN). The lengths lengths 1216 1216AAand and2.1. 2.1.A, A,represent represent values valuesfor for JJ that that can be be an yielding an J), yielding used with K to find (= KK//J), rjE (— find rlE conjunction with used in conjunction interacting per interacting pixel per affectedpixel peraffected 1420 e~ and 1420 average average of of 33e~ e and e per respectively. photon, respectively. the e~),),the 1610 el 5.9 keV; rj = 1610 (Ex = 5.9keV; 2.1 A the case In the case of of2.1 A(Ex An actual 1420 e~. of 1420 photontransfer technique yields yields aa K/J of e-. An photon-transfer individual measuring individual by measuring readily determined by 1215 e~ rjE of 1215 7k a isis readily (19) Eq. (19) e~ l ininEq. gives a value for CI photon which gives (5)], which [Eq. (5)], events [Eq. photon events of 0.88 for CCE perforlimit of of 0.85. 0.85. From Eq. (4), an upper limit mance is calculated calculated using K/ JJ found found from the photon-transfer photon- transfer while a true CCE of 0.75 is curve, while is calculated using individual good quite good level of CCE performance isis quite This level events. This photon events. by today's CCD standards. 3.6. -transfer histogram Photon-transfer 3.6. Photon by improved by be improved can be The of determining determining K K and and J can accuracy of The accuracy using Eqs. (14) (16) directly directly (as (as opposed to the graphical (14) and (16) 3.5). The signal S (DN) and the noise as Sec. 3.5). in Sec. used in approach used approach in the same manner as for the (DN) are found from the CCD in aR noise aR read noise photon-transfer photon- transfer curve curve discussed discussed above. above. The read is found found from a dark dark image. After applying these these formuformu(DN) is sensor, the sensor, across the subarrays across las to many different 20 X20 pixel subarrays call we call formwe intoaaform compiled into K (or J) are compiled of K the resulting values of resulting values using "photon- transfer histogram." An example histogram using a "photon-transfer very produces aa very 4000 A illumination illumination isis shown shown in in Fig. 4. ItIt produces 4000 A pixel 20X20 many20 Usingmany 1.5e e~/DN. accurate value of K ==1.5 -/ DN. Using X20 pixel device thedevice on the regions on those regions of those subarrays allows elimination of values for erroneous values give erroneous whichgive artifacts,which that contain blemish artifacts, contain blemish be easily recognized as can be behaved can well behaved not well K. Areas that are not as K. A shows. A Fig. 44 shows. as Fig. histogram, as main histogram, the main outside the points outside data points CCD entire CCD the entire over the be generated over similar histogram also can be 77E(= (= K/ J) at a specific wavelength of interest. This array for rjE type of histogram is quite valuable in characterizing the varivariability ability of CCE performance performance across across the array of the CCD. In rjE under different of rlE histograms of Sec. we depict depict the use of histograms Sec. 44 we operating conditions of the CCD. PHOTON-TRANSFER 4. PHOTON 4. -TRANSFER USE to technique to photon-transfer the photonapply the we apply section we In this section transfer technique In types of CCDs, different types two different fortwo performancefor measuring CCE performance CCD virtual-phase namely, the thick TI frontsideilluminated virtual -phase CCD frontside-illuminated three-backside-illuminated thinTITIbackside thethin (TI -illuminated three and the VPCCD) and (TI VPCCD) devices are discussed in conThese devices 3PCCD). These phase CCD (TI 3PCCD). discussed tests discussed in tests used in CCDs used siderable detail elsewhere. !1-~ 33 The CCDs charge-floor, charge noise floor, read noise same read the same here here have approximately the for performance for efficiency performance quantum efficiency transfer efficiency, efficiency, and quantum 10 No. 10 26 No. Vol. 26 976 / /OPTICAL October 1987 1987 // Vol. ENGINEERING // October OPTICALENGINEERING 976 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx PHOTON-TRANSFER CHARGE -COUPLED -DEVICE CHARGE-COLLECTION CHARGE -COLLECTION EFFICIENCY EFFICIENCYAND AND THE THE PHOTON -TRANSFER TECHNIQUE CHARGE-COUPLED-DEVICE SUBSTRATE 500 500 LAYER EPITAXIAL EPITAX IAL LAYER v, p 11012-cml p+ 10.011 -cm) FRONTS IDE THICK FRONTSIDE THICK ILLUMINATED SiO2 400 EPI INTERFACE STATES TII 1 Ec fl 2µm - - Ef ei GATE o o. 200 200 EVENTS SPLITEVENTS AND SPLIT 7 PARTIAL AND POTENTIAL z Mn Ka KQ, 100 100 - ee i 3.65eV/e~ 3.65 eVle- 300 r ? \ 300 POLYS I LICON 5µm -i V 1.4e"/DN 1.4e -IDN 1200 A p+ DIFFUSION - E µmr--- Fe55 5000 A ¡' fMn Kß QUAS I -FERMI LEVEL FOR ELECTRONS 200 200 FREE 800 600 10µm 240 µm FIELD 400 -H FIELD FREE REGIME REGIME SPLIT PARTIAL SPLIT PARTIAL EVENTS EVENTS 1200 DN SIGNAL, SIGNAL, DN 1000 1000 1600 1600 1400 1800 1800 2000 2000 CCD, frontside-illuminated Fe55 x-ray Fig. Fig. 6. 6. Fe55 x -rayhistogram histogram for for aa thick thick frontside -illuminated CCD, events. showing numerous partial and split events. h`- DEPLETION REGION 200 200 a FRONTSIDE THICK FRONTSIDE THICK ILLUMINATED 150 150 Fe55 100 8 100 ELECTRIC FIELD BACKSIDE BACKS IDE FRONTS IDE /IDEAL IDEAL 50 50 Cross section Fig. 5. Cross section of aa thick thick frontside-illuminated frontside -illuminated CCD, CCD, showing showing Fig. 5. and split event energy band bandstructure structure and and locations locations at at which which partial and energy generation occurs. I i 00 perforwavelengths used. the the wavelengths used. However, However, we we show show that that CCE performance for the the two two CCDs CCDs isis significantly significantly different different owing owing to to the the device. each device. in each generated in events generated split events number of partial and split illumination (TI 4.1. 4.1. Frontside illumination (TI VPCCD) the events for the split events we find find numerous numerous partial and split general, we In general, is device is frontside-illuminated thick thick frontside -illuminatedepitaxial epitaxialCCD. CCD. Such Such a device primary 5, which shows the primary schematically represented represented in in Fig. Fig. 5, schematically regions regions within within the the CCD CCD that that are responsible for generating split in the substrate region interact in events. Photons Photons that interact and partial events. charge cloud cloud that that has a high probability of recombiproduce aa charge holes in this region of holes owing to nation owing to the high concentration of existence of of bulk bulk interface interface states states at at the the epitaxial epitaxial interinterand the existence face. The physical size size of the the charge cloud also is likely to span the boundary boundary between between adjacent adjacent pixels. pixels. Such Such interactions interactions tend to to between interact between result result in in split splitand and partial partial events. events. Photons Photons that interact also region also depletion region frontside depletion interface and epitaxial interface the the epitaxial and frontside field-free thefield diffusionininthe charge diffusion to charge generate -free generate split events due to region and and partial events due to signal charge charge diffusing diffusing through through weak field field produced produced by by the p+ p+ diffusion diffusion into the bulk traps the weak 5). Fig. 5). (see Fig. interface (see at the epitaxial epitaxial interface Figure 66 shows shows the the response response of of the the TI TI VPCCD uniformly Fe55 x-ray illuminated by illuminated by an an Fe55 x -raysource sourcewith withaa mean mean flux flux of each pixel on of each DN of The DN pixels. The 500 pixels. per500 rayper approximately approximately i1 xxray appromeasured and approis measured charge is signal charge the containing signal the CCD containing response of Fig. Ideally, the response priately binned binned in histogram form. Ideally, 1150 DN 66 should should show show only only two two prominent prominent peaks, located at 1150 and Mn-Ktt theMn duetotothe (1780 e") 1270 DN (1780 and 1270 (1610 (1610e~) e) and e) due -Ka and events with F55 source. Mn-K0 Mn -Kßxx rays rays generated generated by by the F55 source. The The events signals below signals below these these two two lines linesare are the the result result of split split and and partial 200 200 400 400 J I i1 I i 1400 1200 1400 1000 1200 800 800 1000 t^), ee~ YIELD í'7E1, QUANTUM YIELD QUANTUM 600 600 2000 1800 2000 1600 16100 1800 for a thick r?E for yieldnE quantumyield effectivequantum of effective thick frontsideFig. Fig. 7. Histogram of CCD. illuminated CCD. observed. events observed. the events ofthe majority of events events and and constitute the majority yield r]E effective quantum yield Figure Figure 77 shows shows aa histogram of effective '7E TI same TI the same photon-transfer calculated calculated by by the photontransfer method method for the Fig. 6. shown in Fig. histogram shown used in generating the histogram VPCCD used The histogram is generated by uniformly stimulating the CCD and (Fe55 ) and pixel(Fe55) perpixel raysper about 55 xxrays of about mean flux of with aa mean with subarrays rjE for calculating calculating 17E forseveral severaldifferent different 40X40 40X40 pixel subarrays effective average effective anaverage Fig. 77 an from Fig. We find from sensor. We across across the sensor. which is pixel//interacting 900 ae~/ of 900 yield of quantum yield -/ pixel interacting photon, photon, which is 1610 ee~.. of1610 yieldrlirj{ of quantumyield ideal quantum significantly significantly less less than the ideal is readily calcudevice is thisdevice forthis performancefor The relative CCE performance (4). be 0.56 using Eq. (4). lated to be Backside illumination 4.2. 4.2. Backside illumination (TI (TI 3PCCD) substrate backside-illuminated the backside For the -illuminated CCD, CCD, in which the substrate epitaxial interface are removed (Fig. 8), the number of and epitaxial been has been It has reduced. It significantly reduced. is significantly events is split events partial and split achieved beachieved canbe 100%can QEofof100% internalQE an internal demonstrated that an (i.e., ^pe/Tjj and thinned and properly thinned that isis properly CCD that 1) for the CCD = 1) pe /77i= CCE determines CCE 6 The backside backside treated.4" treated.4 -6 Themain main factor factor that determines split is the split backside-illuminated performance performance for the backside -illuminated CCD CCD is event. backside-thebackside forthe eventsfor splitevents ofsplit numberof the number minimize the To minimize of field of electric field an electric that an illuminated illuminated CCD, CCD, itit is is important that entire the entire throughout the provided throughout V/cm 105 V/ greater than 105 greater cm be be provided field the field which the Regionsininwhich CCD. Regions the CCD. of the depth of photosensitive depth / Vol. 1987 / October OPTICAL ENGINEERING OPTICAL ENGINEERING / October 1987 / Vol.2626No. No.1010/ / 977 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx ELLIOTT KLAASEN, ELLIOTT JANESICK, KLAASEN, BACKSIDE BACKSIDE IDE FRONTS IDE FRONTS e" e" e nn p P*-—— 800 800 II DEPLETION , DEPLETION —^11 REGION P—— REG 1µm e^ ——I""'! E e e^ e 1 e" 11 600 600 e 500 500 Ev 400 400 - Si0 2 Si02 OVERLY-THIN OVERLY -THIN 300 300 - _^ 111 20 2QA THIN 17 volts Vnp VnP == 17 volts ^NW ~^ Ev\ 1 X. ACCUMULATION h*- ACCUMULATION ^ LAYER LAYER ^ BACKSIDE THIN THIN BACKSIDE ILLUMINATED ILLUMINATED F055 Fe55 700 700 - Ef BACKSIDE BACKSIDE CHARGE CHARGE 900 900 ^ - /IDEAL /IDEAL 1200 A 200 200 - 6-10µm 100 100 - backside-illuminated thin backsideFig.8.8. Cross Cross section section of of a thin illuminated CCD CCD that that is Fig. backside charged, charged,showing showing energy energy band band structure. structure. backside 00 i 0 200 200 400 400 It I I 1400 1200 1400 1200 1000 800 600 800 1000 600 e(%), e" YIELD 1'7E1' QUANTUM QUANTUM YIELD '. 1600 1600 2000 1800 1800 ' 2000 backside-IJE for yield TE for aa thin thin backside quantum yield effective quantum H istogram of effective Fig. Fig. 10. Histogram illuminated CCD. 3UU 500 2250 EVENTS == 2250 TOTAL EVENTS ': • TOTAL Ka Mn Ka ^ /- Mn ; «2.3e~/DN 2.3 e /DN <yo N (1620e") 11620e-1 375 r • 3. y 65 eV/e3.65 eV /e z 375 _U ; - .: : : 125 ~ i z 125 / (1780e"): (1780é I if 1250 ° 250 0 K : Mn Kß IrMn I Ka PEAK r ESCAPE ESCAPE PEAK ; CI 0 LrrdL***^*^^ 300 300 400 400 500 500 600 600 700 700 U, 1 _ 800 800 900 900 DN (S), DN SIGNAL SIGNALIS), backside-illuminated Fig. 9. 9. Fe55 x -rayhistogram histogramfor for aa thin backside -illuminated CCD, CCD, Fe x-ray Fig. events. split events. and split showing very few partial and large diffuse to aa large strength is lower permit the charge cloud to diffuse split. and split. pixel and one pixel size, making making itit likely likely to to overlap overlap more than one size, field high field thataahigh For the TI 3PCCD, it has been demonstrated demonstrated that using achieved using be achieved can be region can /zm region condition condition throughout throughout a 7 µm backside charging charging or or aa flash flash gate gate and and proper bias conditions backside n-channel across the nchanneland and substrate substrate (Fig. 8). across of a TI Fe55 x rays of toFe55 response to Figure 9 shows a histogram histogram response 3PCCD that is properly thinned (i.e., thinned to the frontside Figs. 6 and Comparing Figs. edge) and depletion edge) and backside treated. Comparing events isis split events and split partial and of partial 9, number of the number note that the we note 9, we significantly less less for for the the backside-illuminated backside -illuminated CCD CCD than than for significantly frontside-illuminated the frontside -illuminated CCD. effective the effective of the histogram of 10 shows Figure Figure 10 shows aa TI 3PCCD histogram photon-the photon using the generated using Fe55 x rays, generated for Fe55 yield for quantum yield on regions on two regions that two this figure that from this We note from method. We transfer method. transfer different produce different thin) produce overly thin) and overly the CCD (labeled thin and CCD (labeled yields. The overly thin region is quantum yields. quantum is slightly slightlythinner thinner than than depletion represents where the frontside depletion the thin region and represents exhibits region exhibits thicker region backside. The thicker edge has reached reached the backside. edge has field-free largerfield of aa larger because of -free region, degraded performance because and diffusion and which causes causes more more charge charge splitting splitting due due to diffusion which yield. The variability of results in a lower effective effective quantum quantum yield. more seen more be seen can be membrane can quantum yield across the CCD membrane rjE isisdisplayed which TIE 11, in which displayedininimage imageformat format as a clearly in Fig. 11, across the subarrays across 160,000 subarrays for 160,000 calculating K/J for result of calculating result right-hand upperright the upper of the view of expanded view 12 is an expanded -hand Figure 12 CCD. Figure 13 isis aa corresponding 11, and Fig. 11, and Fig. Fig. 13 corresponding printout of corner of Fig. rjE . It is seen fE. seen from from Figs. Figs. 12 12 and and 13 13 that that the quantum yield for this this device device varies varies considerably considerablyfrom from 1400 1400e~ e- in in the the corners corners regions). The middle (black regions). the middle in the (white (white regions) regions)toto 1200 1200e~a in The rjE are differences in in 77E arethe theresult result of of thinning thinning nonuniformities, differences middle the middle than the thinner than where the four corners are physically thinner employed. method employed. of the array because of the thinning method histogramsfor foraadifferent different TI TI 3PCCD rjE histograms 14 shows 77E Figure 14 (V ) across voltages(VnP) depletionvoltages frontside depletion biased with different frontside across n-channel channel and and p-substrate. p- substrate.55 The The distance distance that the edge of the nthe depletion region extends into the the substrate substrate isis proportional proportional field-free the field influences the therefore influences and therefore -free region region of the the V^2 and to V42 quantum effective quantum the effective that the see that we see figures we these figures From these CCD. From an lowered due is lowered V is as Vn significantly as yield decreases decreases significantly yield due to to an diffusion field-free increase increase in field -free material, material, whicn which causes more diffusion r]E image over the showsanan77E Figure1515shows splitting. Figure and charge splitting. two distinct areas that corV; note two Vnp — CCD CCD array for VnP = 71 V; Fig. in Fig. indicated in regions indicated thin regions overly thin and overly respond to the thin and respond 14(b). 1 6, isissimilar rjE histogram, histogram, shown shown in in Fig. Fig. 16, similar to to that that of The last r1E voltage of depletion voltage frontside depletion Fig. 10 elevated frontside an elevated has an 10 but has Fig. performance CCEperformance bestCCE thebest represents the Vn == 27 andrepresents 27 VV and V region, an thin region, overly thin the overly For the achieved for the the TI TI 3PCCD. For acnieved for which yields measured, which was measured, 1420 e~ yield of 1420 average quantum quantum yield a was CCE in CCE limit in this limit that this believe that We believe a relative CCD == 0.88. 0.88. We relative CCD factwo facprimarily to two due primarily is due 3PCCD is performance for the TI 3PCCD 5.9 keV for aa 5.9 size for diametersize cloud diameter initial cloud the initial First, the tors: First, keV photon photon therefore, )um); therefore, diameter « significant (cloud event is (cloud diameter ~ 0.6 µm); is significant event occurs pixels occurs pm pixels 3PCCD's1515Am TI 3PCCD's the TI between the splitting between charge splitting even if there there is is no no field-free field -freematerial. material. Second, Second, it isis known known that that regions, stop regions, channel stop field-free -freeregion region exists exists beneath beneath the channel a field occur. to occur. splitting to which which allows allows charge charge diffusion diffusion and charge splitting is where itit is Ref.7,7, where detailininRef. in detail discussed in are discussed Both of of these effects are CCE the CCE improves the pixel improves of the pixel size of the size increasing the shown that increasing performance further because of these two factors. CCE IN CCE IMPROVEMENTS IN 5. FUTURE IMPROVEMENTS 5. FUTURE backside-thebackside CCD,the theCCD, ofthe development of the development in the time in At this time frontside-illuminated thefrontside tothe superior to is superior device is illuminated illuminated device -illuminated because is because This is device in achieving achieving high high CCE performance. This device in backside -illuminated CCD allows field field control over most the backside-illuminated bulk of the photosensitve volume and elimination of neutral bulk can charge can photogenerated charge which photogenerated at which and trapping centers at trapping centers events. split events. and split partial and recombine to produce partial diffuse and recombine diffuse backside-illuminated the backside for the Improvements in CCE for -illuminated CCD area for first area principally in made principally will be made in two two areas. areas. The The first will be 10 No. 10 26 No. Vol. 26 ENGINEERING // October 1987 //Vol. / OPTICALENGINEERING 978 / OPTICAL Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx PHOTON-TRANSFER CHARGE-COUPLED-DEVICE CHARGE -COUPLED -DEVICE CHARGE-COLLECTION CHARGE -COLLECTION EFFICIENCY EFFICIENCY AND AND THE THE PHOTON -TRANSFER TECHNIQUE 1434 1415 1526 1434 1478 1526 1440 1420 1478 1323 1346 1382 1369 1348 1440 1382 1369 1248 125! 1323 1248 1251 1437 1475 1437 1471 1475 1441 1471 1420 1420 1441 1354 1314 1339 1420 1261 1344 1354 1273 1246 1261 1273 1431 1489 1431 1391 1489 1375 1391 1303 1413 1375 1255 1321 1344 1303 1230 1248 1255 1192 1192 1157 1230 1500 1428 1462 1500 1342 1462 1293 1337 1342 1416 1374 1293 1232 1416 1058 1058 1117 1149 1139 1232 1391 1358 1456 1391 1413 1456 1339 1402 1413 1280 1270 1328 1339 1131 1141 1129 1280 1131 1118 1141 1373 1411 1332 1373 1326 1332 1337 1399 1326 1183 1154 1241 1185 1270 1337 1099 1127 1183 1099 1461 1441 1324 1461 1280 1324 1186 1231 1280 1207 1142 1171 1116 1116 1186 1171 1116 1157 1157 1154 1207 1461 1369 1379 1461 1282 1379 1142 1180 1282 1151 1168 1171 1142 1217 1193 1151 1199 1221 1217 1199 1146 1236 1318 1319 1207 1172 1185 1191 1116 1152 1202 1146 1130 1130 1207 1157 1202 1325 1131 1157 1171 1117 1131 1198 1204 1220 1188 1121 1171 1166 1167 1198 1171 1159 1316 1216 1171 1121 1046 1216 1231 1203 1170 1170 1135 1207 1207 1134 1121 1175 1136 1187 1157 1204 1239 1175 1241 1264 1129 1202 1221 1279 1279 1241 1129 1094 1157 1146 1116 1157 1142 1175 1186 1146 1189 1203 1082 1082 1153 1175 1135 1063 1147 1189 CORNER QUANTUM T] E YIELDS,TIE QUANTUMYIELDS, Fig. 13. Printout region the region of the yield of quantumyield effectivequantum representing effective Printoutrepresenting shown in Fig. 12. Fig. 12. 600 Fig. Fig. 11. 11. Pictorial Pictorial display display of of the the effective effective quantum quantum yield yield for for aa thin thin backside-illuminated backside -illuminatedCCD, CCD,showing showing regions regionsof of different different thickness. thickness. ~~i——i——i— 1 I V np -17wlts • VnP=17volts 500 00 ,55 • Fe Fe55 g 400 - y 300|» €£. r OVERLY-TH IN OVERLY-THIN THIN z 100 (a) (a) 0 ———————————I________-^_________________iX^.L 00 200 200 . I 1800 2000 1600 1800 1400 1600 1200 1400 1000 1200 800 1600 600 800 400 600 2000 QUANTUM YIELD I77E1, e), e~ YIELD(*?E QUANTUM 600 VIP 7 volts np ==7voIts <V 500 - Fe 55 • Fe55 /OVERLY-THIN OVERLY-THIN, THIN THIN I \n h1 I 200 Fig. 11. right-hand upper right of upper view of Magnified view Fig. 12. Fig. 12. Magnified -hand corner corner ot of Fig. 11. i 100 o (b) <*>) improvement will willcome comefrom fromdepleting depleting the the entire entire photosensi­ photosensiimprovement depletion complete depletion Although complete CCD. Although tive volume volume of of the CCD. tive PC CD in certain already has has been been demonstrated demonstrated for for the the TI TI 33PCCD already of 77 maximum of only aa maximum regions on on the the array, array, depletion depletion extends extends only regions energy efficiency µm. Therefore, high energy efficiency and and cosmic cosmic ray ray back­ back/urn. sensor. characteristics are are still still poor for this sensor. rejection characteristics ground rejection fully depleting itit fully Increasing the the thickness thickness of of the the CCD CCD and depleting Increasing silicon) in resistivity silicon) (high resistivity deep depletion through through deep depletion technology technology (high in yield both high conjunction with with backside backside treatment treatment should yield conjunction CCE and and high high energy energysensitivity sensitivityfor forthe theCCD. CCD. Work Work at at several several CCE this accomplish this initiated to accomplish CCD manufacturers manufacturers has has been been initiated CCD goal. goal. CCE per­ increase CCE would increase jum) would Second, larger larger pixels pixels(—30 (-30 µm) perSecond, formance further further by by making making aa larger larger "target"for "target " for the the incoming incoming formance cloud event cloud initial event the initial effects of the reducing the effects photon and and reducing photon pixel is the pixel of the smaller fraction diameter. In In addition, addition, aa smaller fraction of is devoted devoted diameter. regions, stop regions, channel stop to "overhead" functions such such as the channel "overhead" functions to which tend tend to to increase increase the the number number of split events events by by diffusion. diffusion. of split which 0 200 200 400 400 600 600 800 1000 1200 1400 1400 1000 1200 QUANTUM YIELDP?E YIELD ITIEI, e), e~ QUANTUM 2010 2000 1600 1600 1800 1800 1600 1600 1800 2000 2000 1800 600 .V np =2volts 500 § I 400 5 o 300 fe a: I •^ 200 •z. 100 (c) o 200 400 600 600 800 1000 120Q 1200 1000 1400 1400 QUANTUM Y IELD I' El, e- Fig. 14. 14. Effective quantum backside-thinbackside foraathin histograms for yield histograms quantum yield Fig. illuminated CCD CCD using using frontside frontside depletion voltages of of (a) (a) 17 17 V, depletion voltages illuminated (b) 77 V, V, and and (c) (c) 2 V. V. (b) OPTICALENGINEERING ENGINEERING / October 1987 / Vol.2626No. No. 979 / / 979 1010 / Vol. 1 987 / October OPTICAL Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx JANESICK, KLAASEN, ELLIOTT ELLIOTT 7. REFERENCES REFERENCES 7. I.I. J. R. Janesick, Janesick, T. Elliott, Elliott, S. S. Collins, Collins, M. M. M. M. Blouke, Blouke, and and J. Freeman, Freeman, J. R. "Scientific charge charge-coupled devices;' Opt. Opt. Eng. Eng,26(8), 26(8),692 692-714 (1987). "Scientific -coupled devices," -714 (1987). 2. 2. M. M. Blouke, Blouke, J. R. Janesick, Janesick, T. T. Elliott, Elliott, J. J. E. E. Hall, Hall, M. M. W. W. Cowens, Cowens, and and M. M. J. R. P. J. May, "'Current status statusofof800X800 800X800charge charge-coupled-device imager P. J. May, "Current -coupled -device imager sensor," Eng. 26(9), 26(9), in in press press (1987). (1987), sensor," Opt. Opt. Eng. J, R. R. Janesick, Janesick, J. J. Hynecek, Hynecek, and and M. M. M. M. Blouke, Blouke,, "Virtual "Virtual phase phase imager imager for for 3. J. Galileo, "in C. Geary Geary and W. Galileo," in Solid Solid Stale State Imagersfor Imagers for Astronomy, Astronomy, J. J. C. and D. D. W. Latham, Proc. SPIE SPIE 290, 165-173 (1981)." Latham, ed., ed., Proc. 290, 165 -173 (1981). 4. J. J. Janesick, Janesick, T. T. Elliott, Elliott, T. T. Daud, Daud, J. J. McCarthy, McCarthy, and and M. M. Blouke, Blouke, "Backside "Backside charging of in Solid State Imaging Imaging Arrays, charging of the the CCD," CCD," in Solid State Arrays, K. K. N. N. Prettyjohns Prettyjohns and E. L. L. Dereniak, Dereniak, eds., eds., Proc. Proc. SPIE and E. SPIE 570, 570, 46-79 46 -79(1985). (1985). J. Janesick, Janesick, T. T. Elliott, Elliott, T. Daud, and 5. J. and D. D. Campbell, Campbell, "The CCD flash flash gate," T. Daud, "The CCD gate," in Astronomy Astronomy VI, VI, D. D. L. L. Crawford, in Instrumentation in Crawford, ed., ed., Proc. Proc. SPIE SPIE 627,543-582(1986). 627, 543 -582 (1986). Janesick, D. D. Campbell, Campbell, T. T. Elliott, Elliott, T. T. Daud, Daud, and and P. 6. J. Janesick, P. Ottley, Ottley, "Flash "Flash technology for CCD imaging imaging in in the theUV," UV,"ininUltraviolet UltravioletTechnology, Technology, technology for CCD R. Huffman, Huffman, ed., ed,, Proc. Proc. SPIE SPIE 687, 687, 36-55 R. 36 -55(1986). (1986). H. Marsh, Marsh, and and, S. S. Collins, Collins, "'Present 7. J... J. Janesick,, Janesick,T.T. Elliott,, Elliott, J. J. McCarthy, McCarthy, H. "Present and, future CCDs CCDs for forUV UVand andx x-ray scientificmeasurements," measurements," IEEE IEEE and future -ray scientific Trans. Nuc. Nuc. Sci. Sci.NS NS-32 (I),, 409 409-416 (.1985). Trans. -32 (1), -416 (1985). s Fig. effective quantum quantum y yield image for eld image for the histogram histogram of Fig. 15. 15. An An effective Fig. Fig. 14 (b). 14(b). 900 wu 1 800' 800 - • THIN BACKS I IDE THIN BACKSIDE ILLUMINATED' ILLUMINATED I I • 1 I 1 i,, a,A 700 700 z 600 1 600 0 I r • V »= 27 27 volts volts Vnp Fe 55 w 1 Z 200 1 200 00 ; THIN I CSL UJ 300 § 300 0o 1 |Lx/ THIN E 500 §500 o ó o 400 ó 400 100 100 1 R. Janesick: Janesick: Biography Biography and and photograph photograph appear appear with the Guest James R. Editorial issue. Editorial in this issue. OVERLY -THIN .OVERLY-THIN an/ A\ ^I i D"L IDEAL , 2DO , 400 , 600 600 , 800 800 , J. 1000 1000 1200 1200 Ai ., 1400 1600 1400 1600 1800 1800 2000 2000 QUANTUM YIELD I'k1, eQUANTUM YIELD(r?E ), e~ Fig. Fig. 16. 16. Histogram 3PCCD, H istog ra m of of best best effective effective quantum quantum yield yield for for the the TI Till 3 PC C D, using aa fronts frontside voltage of of 27 V. V. using id e voltage 6. ACKNOWLEDGMENTS 6. ACKNOWLEDGMENTS The authors acknowledge many rewarding conversations on The the subject of CCE (father of of the the TI the subject of CCE with with Morley Morley Blouke Blouke (father 3PCCD), Taher Daud, Andy Andy Collins, Dave Campbell, James DeWitt, Arsham Dingizian, DeWitt, Dingizian, and James McCarthy. McCarthy,., We We also also thank Deborah Durham thank Deborah this paper. Durham, for reviewing reviewing this paper. The The research described describedwas wascarried carried out out by by the Jet research, Jet Propulsion Propulsion, Laboratory, California California Institute of Technology, Technology, under concon­ tract with with the the National National Aeronautics Aeronautics and and Space Space Administration. Administration. tract Kenneth P. IP. Klaasen Klaasen is is the the supervisor supervisor of of the the P h otosci e nee Group G rou p at at JPL. J P L. He H e received recei ved his h i s BS BS Photoscience deg r e e iin n physics p hys i cs from fro m Calvin C a I v i n College, Co 11 eg e, Grand degree G r a nd Rapids, MS in in aerospace aerospace engiengi­ Rapids, Mien., Mich., and his MS neering University of Michigan Michigan in neering from from the University 19 6 9, He H e has h a s been bee n involved i n vo I ved in i n many m a n ysolar so I a r syssys 1969. tern exploration exploration missions, missions, including including Mariner Mariner tem 1 I 10, the and currently currently the the GaliGali10, the Viking Viking Orbiter, and | ^ leo Jupiter. He He has has served served as as the the leo mission to Jupiter. I experiment representative representative for experiment for the the imaging imaging experiments on on these these missions missions and and is is curcur­ experiments member of rently aa member of the the Galileo GalileoImaging ImagingScience Science Team. Team. His His primary primary responsibilities have have included included imaging imaging system system calibration, calibration, experiment experiment planning, and and mission mission operations. operations. He He has has published published papers papers on planning, on the rotation period period of of Mercury, Mercury, Venus Venus atmospheric atmosphericdynamics, dynamics,and andthe thephopho­ rotation of Mercury, Mercury, Mars, Mars, and tometry of and the the Martian Martianmoons, moons,Phobos P hobos and and Deimos, Deirnos, as well as as several several on on various various spacecraft as spacecraft imaging imaging instruments instruments and and experiments. 1 :/: Stythe T. T. Elliott Elliottwas wasborn bornininVan Stythe VanNuys, Nuys, CaliCali­ fornia. He He received received his his BA fornia. BA degree in geography from the California California State State University University at at NorthNorthPropulsion Laboratory Laboratory ridge. He joined the Jet Propulsion in 1979 1979 and and isis presently presently working workingon on developdevelop­ charge-coupled devices for NASA NASA space space ing charge -coupled devices imaging systems. imaging systems. He He received received aa NASA NASA in 1986. 1986. He He isis currently currently Achievement Award in involved with withthe thedevelopment developmentofofCCDs CCDs used in involved the wide /planetary Hubble wide field field/planetary HubbleSpace Space TeleTele­ scope camera. / OPTICALENGINEERING 980 / OPTICAL ENGINEERING // October October 1987 1 987 // Vol. Vol. 26 26 No. No. 10 10 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 08/21/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx