LCFI CHARGE TRANSFER INEFFICIENCY STUDIES FOR CCD VERTEX DETECTORS

8/6/2019 LCFI CHARGE TRANSFER INEFFICIENCY STUDIES FOR CCD VERTEX DETECTORS

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2.1. 0.44 eV Trap CTI Contribution

We consider partially lled traps to improve the simulation by representinga continuous readout process. The results from the initially empty andpartially lled traps are compared in Fig. 3. A negligible contribution tothe CTI from 0.44 eV trapping for partially lled traps (due to long emissiontime) are obtained. Thus, the 0.44 eV traps are ignored in further studies.

Temp (K)

C T I x 1 0 - 5

CTI using initially empty0.44eV traps, 50MHz.

Initially empty

150 200 250 300

Temp Κ

CTI using new code for 0.44eV

traps, 50MHz.

x 10-5

150 200 250 300

Figure 3. Left: CTI value for initially empty 0.44 eV traps. Right: Partially lled 0.44eV traps.

2.2. 0.17 eV Trap CTI Contribution

Figure 4 shows the CTI simulation for initially empty and partially lledtraps. A clear peak structure is observed. New experimental data will coverthe simulated temperature range.

Temp (K)

C T I x 1 0 - 5

SimulationSim. singletrap levels

ISE-TCAD Simulation50 MHz readout

0.17eV

0.44eV

150 200 250

Temp (K)

C T I x 1 0 - 5

Partial traps

Partial trap filling0.17eV traps, 50MHz

100 150 200 250

Figure 4. Left: CTI value for initially empty 0.17 eV traps, 0.44 eV traps and theirsum. Right: Partially lled 0.17 eV traps.

2.3. Frequency Dependence

The frequency dependence is shown in Fig. 5 for initially empty and par-

tially lled traps. For higher readout frequency there is less time to trapthe charge, and thus the CTI is reduced near the CTI peak region. At hightemperatures, the emission time is so short that trapped charges rejoin thepassing signal.

4. Initial Data Measurements

For the environment of a future Linear Collider a serial readout for CCDsis no longer an option. Instead column parallel technology, using readoutelectronics for each column, are in development to cope with the requiredreadout rate. CPC-1 is a prototype 2-phase CCD capable of 25 MHz read-out frequency. Initial measurements have been performed on an unirra-diated device in standalone mode, where four columns of the CCD wereconnected to external ADC ampliers. An Fe 55 source provides the sig-nal charge (Fig. 7). The determination of the CTI involves measuring thecharge reduction as a function of the pixel number from a known initialcharge. The results so far have shown small CTI values ( < 10− 5) for the

unirradiated CCD under normal operation conditions. It is possible to in-duce CTI-like effects by reducing the clock voltages used to transfer charge.For 1 MHz readout the CTI value was observed to increase sharply below1.9 V (peak-to-peak).

ADC-100 -50 0 50 100 150 200 250

F r e q

Channels 1,2&3

(5.9keV) Signal55Fe

Pedestal

noiseσ

/ADC-11 e≈Gain

Figure 7. Measured signal distributionfor an Fe 55 source.

Noise ≈ 60 e− .Frequency = 1 MHz.Integration time = 500 ms.T ≈ − 30 ◦ C.2000 frames.

4.1. CTI - Event Selection

The signal is induced by an Fe 55 source which provides isolated hits of about1620 electrons in order to determine the CTI value. Hits are located usinga 3x3 cluster method and selection criteria are applied: Pixel amplitude> 5σnoise , Σ8

i =1 |cluster i | < 8σnoise . Events are selected within ± 2σ of thesignal peak as shown in Fig. 8.

4.2. CTI - Determination

The CTI is determined from isolated pixel hits. The distribution of the

ADC amplitude Q against the pixel number gives CTI = −1

d Qd (Pixel) ,where Q0 is the intercept from a straight-line t. An example for an unir-

radiated device and low clock voltage is given in Fig. 9. The decrease of theclock voltage reduces the transfer efficiency which provides a possibility tomeasure CTI values as function of the clock voltage.

abs(Cluster) (ADC)Σ0 50 100 150 200 250 300 350

t r a l ( A

central:cluster {cluster<350&&central<200}

Isolated pixel hitsEntries 12896

/ ndf2χ 60.07 / 32

Constant 4.7±275.3Mean 0.1±129.8

Sigma 0.16±9.52

ADC40 60 80 100 120 140 160 180

C o u n

300Entries 12896

/ ndf2χ 60.07 / 32

Constant 4.7±275.3Mean 0.1±129.8

Sigma 0.16±9.52

Isolated pixel hits, ch 2.

Figure 8. Left: Isolated hit. Right: Extracted signal peak.

Pixel Number0 50 100 150 200 250 300 350 400

/ ndf2χ 254 / 276

p0 0.3252±136.7p1 0.001834±-0.02414

/ ndf2χ 254 / 276

p0 0.3252±136.7p1 0.001834±-0.02414

-510×1.334)±CTI = (17.558

CTI (file: c00567.dat)

Vpp (V)1.4 1.5 1.6 1.7 1.8 1.9 2

CTI vs Vpp (Measured)

20 MHz

Figure 9. Left: CTI determination. Right: CTI as function of clock voltage.

4.3. Dark Current

Some thermally generated electrons are captured in the potential wells.The collected charge is proportional to the integration time. 10 overclockssampled per frame are used as reference level. The gain (e − /ADC) iscalibrated from an Fe 55 source (at each temperature). From a t to J dc =T 3 exp( α − β/T ), a uniform dark current characteristics is observed acrossthe four channels (Fig. 10).

5. Summary

Radiation hardness effects have been simulated for a serial readout CCD,CCD58. A simple model has been developed and compares well with thesimulation which shows the strong dependence on the operating conditionsof the CCD. Measurements performed on a prototype column parallel CCD(CPC-1) display as expected no measurable CTI for the unirradiated sys-

tem. CTI-like effects were induced by reducing the clock voltage to explorethe CTI measurement method. Comparisons with experimental data arebeing carried out at a test-stand. The test-stand at Liverpool Univer-sity has been shown to operate in the required temperature range. The

Entries 299574

ADC Amplitude-30 -20 -10 0 10 20 30 40 50

C o u n

Entries 299574Dark Current

Overclocks

Active pixels

Dark current value

Integration time (s)0 0.5 1 1.5 2 2.5 3 3.5 4

/ p i x e l )

D a r k C u r r e n t ( e

/ ndf2χ 0.0009462 / -2p0 52.39±-1.54p1 31.69±230.7

/ ndf2χ 0.01764 / -2p0 53.83±6.732p1 34.58±495.6

/ ndf2χ 0.03987 / -2p0 46.86±3.858p1 83.8±1697

/ ndf2χ 0.00494 / -2p0 55.57±4.633p1 167.6±3041

/ ndf2χ 0.006973 / -2p0 59.74±3.882p1 288.9±5582

Dark Current

C°-20

C°-10

Figure 10. Upper left: Dark current

measurement method. Upper right:Dark current at different temperatures.Lower left: Dark current t to theoryexpectation.

performances of the CCD58 and CPC prototypes will be compared withsimulations. Experiences with the prototypes have already been gained atthe Rutherford Appleton Laboratory (RAL). The verication of the simu-lation results and the tuning of the simulation is important for the studies

to guide the future CCD development as vertex detector for a future LinearCollider.

Acknowledgments

This work is supported by the Particle Physics and Astronomy ResearchCouncil (PPARC) and Lancaster University. I would like to thank Kon-stantin Stefanov and James Walder for comments on the manuscript.

References

1. K. Stefanov, PhD thesis, Saga University (Japan), “Radiation damage effectsin CCD sensors for tracking applications in high energy physics”, 2001;

O. Ursache, Diploma thesis, University of Siegen (Germany), “Charge transferefficiency simulation in CCD for application as vertex detector in the LCFIcollaboration”, 2003;and references therein.

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