The effect of different proton fluences on the performance of two E2V Technologies CCD47-20 devices was investigated with particular emphasis given to the analysis of 'random telegraph signal' (RTS) generation, bright pixel generation and induced changes in base dark current level. The results show that bright pixel frequency increases as the mean energy of the proton beam is increased, and that the base dark current level after irradiation scales with the level of ionization damage. For the RTS study, 500 pixels on one device were monitored over a twelve hour period. This data set revealed a number of distinct types of pixel change level fluctuation and a system of classification has been devised. Previously published RTS data is discussed and reviewed in light of the new data

Ambrosi, RM

Holland, AD

Hutchinson, IB

Robbins, MS

Smith, DR

English

Brunel University Research Archive

Proton induced leakage current in CCDsDavid R. Smith*a, Andrew D. Hollanda, Mark S. Robbinsb, Richard M. Ambrosia,Ian B. HutchinsonaaUniversity of Leicester, Space Research Centre, University Road, Leicester, LE1 7RH, UKbE2V Technologies, Waterhouse Lane, Chelmsford, CM1 2QU, UKABSTRACTThe effect of different proton fluences on the performance of two E2V Technologies (formerly Marconi AppliedTechnologies, formerly EEV) CCD47-20 devices was investigated with particular emphasis given to the analysis of‘random telegraph signal’ (RTS) generation, bright pixel generation and induced changes in base dark current level. Theresults show that bright pixel frequency increases as the mean energy of the proton beam is increased, and that the basedark current level after irradiation scales with the level of ionisation damage. For the RTS study, 500 pixels on onedevice were monitored over a twelve hour period. This data set revealed a number of distinct types of pixel charge levelfluctuation and a system of classification has been devised. Previously published RTS data is discussed and reviewed inlight of the new data.Keywords: CCD, proton, leakage current, random telegraph signal, bright pixel, damage1. INTRODUCTIONA better understanding of proton damage effects in Charge Coupled Devices (CCDs) is important for the continued useof CCDs in space based applications where the required resolution, sensitivity, operating speed, size and number ofdevices used in focal plane instruments is increasing. Examples of upcoming applications are the proposed XEUS1 andGAIA2 missions. For this study, two E2V Technologies (formerly Marconi Applied Technologies, formerly EEV)CCD47-20 devices were irradiated with protons, using the particle accelerator facilities at Birmingham University, UK,and AEA Technologies, Harwell, UK. The aim of the experiment was to see how 10 MeV equivalent proton fluences ofdifferent mean energies affected the operational characteristics of the CCD. The fluences given were representative ofmission fluences expected to be received by a typical orbiting spacecraft3.It was expected that pixels exhibiting RTS behaviour would be generated as a result of the proton irradiations. There wasan emphasis on the analysis of these pixels because the mechanism behind their behaviour and generation is not wellunderstood. The charge levels in RTS pixels can be very high and many are clearly visible at room temperature havingimplications for optical uses of CCDs.2. EXPERIMENTAL METHODThe CCD47-20 is a three phase device and has both an image and store section, each with 1024 pixels × 1024 pixels of13.0 µm × 13.0 µm, forming a sensitive region of 13.3 mm × 13.3 mm. The resistivity of the silicon is 20 Ωcm-1, with adepletion depth of ~5 µm.2.1 CCD 9211-5-3 irradiationProton beam uniformity over the target region was examined using a photodiode in pulse counting mode, with the resultdisplayed on a spectrum analyser, before the CCD was positioned in a vacuum chamber and attached to the end of theproton beam line. Across the CCD area, the beam uniformity was found to be ±15 %. The flux reaching the photodiodein 1 minute was measured several times to calibrate the beam flux, to measure the stability of the beam and to ensure therequired proton doses could be given over a suitable time scale. The error associated with the dosimetry was estimated to* Corresponding author: drs@star.le.ac.uk; phone +44 (0)116 252 3519; fax +44 (0)116 252 2464.be ~20 %. Once the proton beam characteristics were determined, the target CCD was mounted in the vacuum chamberwith all pins grounded to avoid potential static damage.Two irradiations were carried out with CCD 9211-5-3 at a temperature of 22 oC. For the first irradiation, a 6.5 MeVproton beam was used to give a flux of 8.8 × 107 protons cm-2, equivalent to 1.5 × 108 10 MeV protons cm-2, to one thirdof the CCD. The rest of the CCD was covered with an aluminium shield to prevent the protons damaging that part of thedevice. It should be noted that the store section of the CCD47-20 has its own aluminium shield, although this is not thickenough to stop the protons passing through it. The time taken to carry out the irradiation was 47 seconds.For the second irradiation, two thirds of the CCD were shielded with aluminium while the rest of the device wasirradiated through 100 µm of copper. The copper had the effect of reducing the mean energy of the protons to 2.0 MeV.The same flux of protons was given to the CCD as for the 6.5 MeV irradiation. The 10 MeV equivalent proton dosegiven in this case was 3.6 × 108 protons cm-2. This irradiation was carried out in 100 seconds.For each irradiation the photodiode was positioned ~2 cm in front of the shielded section of the CCD to accuratelymonitor the proton flux reaching the CCD in real time. For the second irradiation, the photodiode was also covered with100 µm of copper in order to measure the same proton flux and mean energy as that reaching the CCD (Fig. 1). Thecentral part of the CCD remained unirradiated as a control area.Fig. 1: This diagram shows the irradiated areas of the CCD47-20 (9211-5-3) and the dose each area received. Also shown is theposition of the photodiode for each irradiation.2.2 CCD 9211-4-4 irradiationThe dedicated proton damage beam line at the tandem accelerator facility, run by AEA Technologies in Harwell,Didcot4, was used to irradiate CCD 9211-4-4. The CCD was mounted on a sample plate and rotated into the path of theproton beam. An aluminium shield was used to prevent protons reaching one half of the CCD, the unshielded sectionreceiving a 10 MeV equivalent proton fluence of 3.0 × 108 protons cm-2 (Fig. 2). The irradiation was carried out with allCCD pins grounded, at a temperature of 22 oC, in a time of 9 seconds. The beam profiling and dosimetry were carriedout by the Harwell staff and the given dose was accurate to ~20 %.Mean energy of protons: 6.5 MeVProton fluence received: 8.8 × 107 cm-210 MeV equivalent proton dosereceived: 1.5 × 108 cm-2Mean energy of protons: 2.0 MeVProton fluence received: 7.8 × 107 cm-210 MeV equivalent proton dosereceived: 3.6 × 108 cm-2Fig. 2: This diagram shows the irradiated area of CCD47-20 (9211-4-4) and the dose received.3. EXPERIMENTAL RESULTSAfter successful completion of the proton irradiation experiments, changes in the leakage current and bright pixelpopulations of CCD 9211-5-3 were investigated, while the 500 brightest pixels of CCD 9211-4-4 were monitored over a12 hour period to characterise any fluctuating pixels.3.1 Analysis of CCD 9211-5-3The mean dark current level increased in the areas of each CCD that were irradiated (Fig. 3). At a temperature of 24 oC,the unirradiated region in the centre of the CCD had a dark current level of ~2500 electrons, while the dark current levelsfor the 6.5 MeV and 2.0 MeV mean proton energy irradiations were ~4500 electrons and ~13,500 electrons respectively.The ratio of the change in dark current level from the 2.0 MeV and 6.5 MeV beams, after normalising for delivered flux,is ~6.A large number of bright pixels were generated as a result of each irradiation. To deduce the percentage of the totalnumber of pixels that became bright pixels after irradiation, a region of interest containing 350,000 pixels was examinedin each irradiated area. The percentage of pixels with charge levels > 5σ, where σ is the rms noise in the image, abovethe mean background level in the 6.5 MeV and 2.0 MeV irradiated areas were 0.94 % and 0.58 % respectively. Thepercentage of pixels with a charge level above a particular threshold level was also investigated. The results are plottedin Figure 4, which shows the trend for both of the irradiated CCD areas.Fig. 3: Plots of the sum average of 1000 rows in the image section of each CCD showing the increase in dark current andgeneration of bright pixels as a result of each irradiation.Mean energy of protons: 10 MeV10 MeV proton dose received:3.0 × 108 cm-2Fig. 4: The percentage of CCD pixels in each irradiated area with a charge level greater than a given threshold levelabove the mean background dark current.3.2 RTS Analysis of CCD 9211-4-4The 500 brightest pixels of the CCD were selected and monitored every 12 seconds over a period of 12 hours. The CCDwas stabilised at a temperature of -10 oC while the data was collected. Using software developed for the purpose, thecharge level of each pixel over the 12 hour period was investigated. Of the 500 pixels studied, 242 were found to exhibita fluctuating charge level. A number of different types of fluctuation were observed that could be classified into one offive different categories. For each classification, the amplitude of the oscillation or transition was above 5σ of the meanpixel level of a ‘Flat’ reference pixel, where σ was 50 electrons, and the noise distribution was Poissonian in nature. Thecategories were called: Wave, Bi-Stable, Multi-Stable, Bi-Stable with Wave, and Stable. The characteristics of each aredescribed below:• Wave – The pixel amplitude varies with a sinusoidal oscillation• Bi-Stable – The pixel shows sharp amplitude transitions between 2 distinct levels• Multi-Stable – The pixel shows sharp amplitude transitions between more than 2 distinct levels• Bi-Stable with Wave – The pixel shows sharp amplitude transitions between 2 distinct levels, while the amplitudealso varies in a sinusoidal fashion• Flat – The pixel shows no oscillatory nature, or transitions, that are visible above the pixel noiseExamples of each type are shown in Figure 5, spaced out to avoid overlap. Gaps in the data were caused by loss of framesynchronisation for short periods during data collection. The number of pixels in each category is shown in Table 1.Classification Number of PixelsStable 258Wave 71Bi-Stable 150Multi-StableBi-Stable with Wave2299Table 1: The number of pixels exhibiting different types of fluctuation, from a total sample of 500 pixels.0.00.11.04 6 8 10 12 14 16Sigma Above Mean Darkcurrent LevelPixels Above Threshold (%)6.5 MeV irradiated area,8.8E7 fluence received2.0 MeV irradiated area,7.8E7 fluence receivedA = Stable; B = Wave; C, D, E = Bi-Stable; F = Multi-Stable; G, H = Bi-Stable and WaveFig. 5: Examples of recorded random telegraph signals from a CCD47-20 operating at a temperature of –10 oC.4. DISCUSSION4.1 Ionisation and displacement damageIonisation damage in a CCD results in holes drifting and becoming trapped close to the silicon-oxide interface producinga ‘flat-band’ voltage shift at the interface. In addition, passage of holes through the interface creates extra interface statesleading to an increase in the leakage current of the device. The change in dark current of a device after irradiation shouldtherefore be proportional to the amount of ionisation damage caused. Using the Stopping Range of Ions in Matter(SRIM) code5, the ratio of energy deposited per unit distance into silicon for the 6.5 MeV and 2.0 MeV proton beamsused to irradiate CCD 9211-5-3 is found to be ~4. This value is comparable to the ~6 ratio of the changes in 6.5 MeVand 2.0 MeV base dark current levels observed after irradiation of the device, in reasonable agreement with the theory.0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0Time (hours)Pixel Amplitude (displaced to avoid overlap)ABCDEFGHThe slight difference in the ratios is due to the SRIM ratio being calculated for single proton energy values and not aspectrum of proton energies.Previous work has shown that the number of bright pixels generated after proton irradiation scales with the amount ofdisplacement damage, and therefore scales with the Non-Ionising Energy Loss (NIEL) function6. Using SRIM again, theNIEL value for the 6.5 MeV and 2.0 MeV proton energies are 1.7 and 4.6. There should therefore be a factor ~3 morebright pixels in the 2.0 MeV irradiated CCD area, compared to the 6.5 MeV irradiated CCD area. The recordedexperimental data show this is not what was observed, with ~2 times more bright pixels generated in the 6.5 MeV regionthan the 2.0 MeV region. This ratio is for pixel amplitudes above 5σ from the mean base dark current level. Increasingthe bright pixel threshold from 5σ to 15σ (Fig. 4), the bright pixel populations in both irradiated areas behave in asimilar way, each obeying power law.4.2 Fluctuating pixelsAn RTS pixel is characterised by sharp transitions in amplitude with high time constants. The pixels classified as ‘Bi-Stable’ pixels in this study were RTS pixels exhibiting standard RTS behaviour7, the amplitude of the pixel switchingbetween two distinct levels. The high and low state time constants were predominantly of order several tens of minutesto hours at the –10 oC monitoring temperature. This is in agreement with previous work8 that investigated the affect oftemperature on RTS pixels, showing that the time constants increase as the temperature is lowered, the time betweenamplitude changes becoming many hours and even days when operating at –20 oC.Pixels showing more than two distinct amplitude levels were also observed and classified as ‘Multi-Stable’ pixels. Of theRTS pixels generated after the 10 MeV proton irradiation, ~5 % were ‘Multi-Stable’. This is comparable tomeasurements made in previous studies9 where the fraction of generated RTS pixels exhibiting multi-stable behaviourafter irradiation with different 10 MeV proton doses was found to be between ~1 % - 15 %.No correlation between the pixel dark current pedestal level and the RTS amplitude was observed, in agreement withprevious studies8. Pixels with high and low RTS amplitudes are observed at both high and low dark current pedestallevels.The smooth oscillation observed in a number of pixels had a period of ~70 minutes, and was due to thermal cycling ofthe temperature controller. The amplitude of the oscillation increased proportionally with the mean pixel dark currentlevel, becoming visible above the noise in all pixels with a mean dark current level above ~2200 electrons. Theoscillation is not a radiation induced effect, and removing the ‘Wave’ classification from the collected data reveals that~35 % of the 500 monitored pixels had generated RTS characteristics. This is in good agreement with a study involvingE2V Technologies CCD02 and Hamamatsu S5466 CCDs, irradiated with electrons (90Sr) and neutrons (252Cf)10, wherethe fraction of generated bright pixels exhibiting RTS properties was found to be 40 %.A suggested mechanism for explaining the RTS phenomena is a change in configuration of a bi-stable, or multi-stable,defect11, 12 located in the bulk of the CCD depletion region. The time constants measured vary from ~1 minute to > 8hours while the measured amplitudes vary from ~250 electrons to ~850 electrons. The variation in the high and low stateRTS time constants and pixel amplitudes within the same device indicate that another parameter such as the electric fieldstrength must also be influencing the bi-stable defect. A possible explanation is that the reconfiguration of the defectinvolves a change in orientation of its dipole moment in relation to the surrounding electric field7.The RTS amplitudes observed are larger than those expected from Shockley-Read-Hall generation processes usuallyassociated with dark current levels in CCDs. The large amplitudes could be caused by clusters of defects, where onedefect has an influence on the generation rate of the surrounding defects, or by Poole-Frenkel emission. Poole-Frenkelemission is due to field-enhanced thermal excitation of trapped electrons into the conduction band. If a defect within apixel is in a high field region, the generation rate of the defect will be higher13.5 CONCLUSIONSAfter irradiation with protons the mean dark current level of a CCD47-20 increases proportionally with the amount ofionisation damage received. From a sample of the 500 brightest pixels generated after an irradiation of 3.0 × 108 10 MeVprotons cm-2, ~35 % exhibited RTS behaviour. Of the RTS signals generated, ~5 % showed transitions between morethan 2 distinct amplitude levels. The transition amplitudes and time constants observed are comparable to previouslypublished work, supporting the theory that configuration changes of a bi-stable, field-enhanced bulk defect are the causeof the phenomena.Further work will involve the study of proton irradiation induced bright pixels in CCD47-20 (9211-5-3), comparing RTSpixels from both the 6.5 MeV and 2.0 MeV irradiated areas, and bright pixels present in the unirradiated part of theCCD. The time resolution of the RTS monitoring will be improved to ~10 samples per second in an attempt to revealmore structure in the data and improve on the current understanding of the phenomena.ACKNOWLEDGMENTSThe authors would like to thank Mike Smith at Birmingham University, UK, and Keith Jones at Harwell, UK, for theirassistance during the experimental phase of this study, and E2V Technologies for the CCDs used in this work.REFERENCES1. M. Bavdaz, J. A. M. Bleeker, G. Hasinger, G. G. Palumbo, A. J. Peacock, A. N. Parmar, M. J. Turner, J. Trümper, J.Schiemann, “X-ray evolving universe spectroscopy mission (XEUS)”, Proc. SPIE, vol. 3766, 1999.2. GAIA Science Advisory Group, “GAIA: Composition, Formation and Evolution of the Galaxy. Concept andTechnology Study Report”, ESA-SCI(2000)4, European Space Agency, 2000.3. A.  Holmes-Siedle, S. Watts, A. Holland, “Further radiation evaluation of X-ray sensitive charge coupled devices(CCDs) for the XMM telescope”, Final report on ESTEC Contract No. 8815/90/NL/LC(SC), Brunel University,UK, 1995.4. R. M. Ambrosi, A. D. T. Short, A. F. Abbey, A. A. Wells, D. R. Smith, “The effect of proton damage on the X-rayspectral response of MOS CCDs for the Swift X-ray telescope”, Nuc. Inst. and Meth. A, vol. 482, pp. 644-652,2002.5. J. F. Ziegler, The Stopping and Range of Ions in Matter, IBM – Research, Yorktown, NY 10598, USA.6. P. W. Marshall, C. J. Dale, E. A. Burke, G. P. Summers, G. E. Bender, “Displacement damage extremes in silicondepletion regions”, IEEE Trans. Nuc. Sci., vol. 36, no. 6, pp. 1831-1839, 1989.7. I. H. Hopkins, G. R. Hopkinson, “Random telegraph signals from proton-irradiated CCDs”, IEEE Trans. on Nuc.Sci., vol. 40, no. 6, pp. 1567-1574, 1993.8. I. H. Hopkins, G. R. Hopkinson, “Further measurements of random telegraph signals in proton irradiated CCDs”,IEEE Trans. Nuc. Sci., vol. 42, no. 6, pp. 2074-2081, 1995.9. I. H. Bond, “Radiation damage in optical charge coupled devices”, Ph.D. thesis, University College London, 1996.10. K. D. Stefanov, “Radiation Damage Effects in CCD Sensors for Tracking Applications in High Energy Physics”,Ph.D. thesis, Saga University, 2001.11. A. Chantre, “Introduction to defect bistability”, Appl. Phys., A48, pp. 3-9, 1989.12. G. D. Watkins, “Metastable defects in silicon: hints for DX and EL2”, Semicond. Sci. Technol., vol. 6, pp. B111-B120, 1990.13. J. R. Srour, R. A. Hartmann, “Enhanced displacement damage effectiveness in irradiated silicon devices”, IEEETrans. on Nuc. Sci., vol. 36, no. 6, pp. 1825-1830, 1989.

Displacement damage extremes in silicon depletion regions”,

Enhanced displacement damage effectiveness in irradiated silicon devices”,

Further measurements of random telegraph signals in proton irradiated CCDs”,

Further radiation evaluation of X-ray sensitive charge coupled devices (CCDs) for the XMM telescope”,

Introduction to defect bistability”,

Metastable defects in silicon: hints for DX and EL2”,

Radiation Damage Effects in CCD Sensors for Tracking Applications

Radiation damage in optical charge coupled devices”,

Random telegraph signals from proton-irradiated CCDs”,

The effect of proton damage on the X-ray spectral response of MOS CCDs for the Swift X-ray telescope”,

Proton induced leakage current in CCDs

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