Charge diffusion effects in CCD X-ray detectors

Nuclear Instruments and Methods 216 (1983) 431-438 North-Holland Publishing Company

CHARGE

DIFFUSION

EFFECTS

431

IN CCD X-RAY DETECTORS

il. Experimental results D.H. LUMB and G.R. HOPKINSON * X-rc(v Astronon~v Group, Lewester Un~vermtv, Leicester, I.EI 7Rtl. England Received 21 February 1983 and in revised form 24 May 1983

The quantum efficiencies of two GEC MA328 epitaxial CCDs (thicknesses 12 and 30 ttm) have been measured at X-ray energies in the range 2.3-22 keV. The data are consistent with the assumption that only absorption events in the depletion layer contribute to the single photon X-ray peak. This is confirmed by pulse height spectra which show that charge diffusion effects cause events produced deep in the silicon to give signals of lower energy. In addition, the comparison of spectra where diffused events are either clustered or rejected gives an indication of the amount of radial spreading in those events.

I. Introduction It was predicted in paper I ** that pulse height spectra, obtained by single X-ray photon counting with a C C D imager, should show an X-ray peak with a low energy tail: the position of the X-ray peak being proportional to X-ray energy and the tail being associated with charge generated below the depletion region (where there is no drift field). The existence of partially recombined events from the field-free region has been reported before [1,2]. In this paper experimental data are presented for two epitaxial C C D s of nominal thickness 12 and 30 ttm (corresponding to field-free region thicknesses of 5 and 23 ttm). The results are in agreement with the predictions, thus indicating that the distinction between completely collected events from the depletion region and partial events from the field-free region is correct.

2. Instrumentation 2.1. Device details

The C C D s used were MA328 devices, manufactured by G E C U K pie. These are 120 x 150 element, front-side illuminated, frame transfer devices with three-level, three-phase overlapping polysilicon electrodes [3]; they are no longer commercially available, having been superceded by the larger (385 x 576) P8600 imagers. The devices were operated with a constant integra-

* Present address: Sira Limited, Chislehurst, Kent, England. ** See Nucl. Instr. and Meth. 216 (1983) 423-429. 0167-5087/83/0000-0000/$03.00

:,) 1983 North-Holland

tion time of 0.31 s (corresponding to a pixel rate of -- 60 kHz). During this time the image clock levels were kept steady (no interlace - charge storage under phases 2 and 3) whilst charge was sequentially transferred from the storage area (120 x 75 elements) to a 128 element output register and floating diffusion amplifier. The C C D s were constructed of a p-type epitaxial layer ( N a = 1015/cm3) on a low lifetime n substrate ( N d = 10JS/cm3). The electrodes in the image area define a pixel with dimensions 30 x 36 ttm 2. (This includes 6 ttm wide channel stop diffusions which separate the columns.) The epitaxial layer thicknesses were nominally 12 and 30 p,m + 20%. A large number of devices ( - 20) were made available to us by the manufacturers from 'engineering grade" stock, i.e. the devices had insufficient 'cosmetic' quality at room temperature to allow retail as TV imagers. Complete laboratory testing at low temperatures ( - 170 K) was possible with only three epitaxial devices. The remainder (including some bulk devices) either had initial faults (e.g. interelectrode shorts) which prevented successful imaging or failed during testing (e.g. through faults in the output gate structure). Those devices with a working output section were used for measurement of transistor characteristic curves and readout noise. The CCDs showed a considerable variation in output node capacitance (Cr)). The best noise performance (25 electrons rms, total; 16 electrons rms for the CCD) was obtained from a device with Ct~ = 0.3 pF [4,5], however a more typical value was 0.6 pF. High values of (-'D and low on-chip amplifier gains resulted in a total readout noise in the range 60--90 electrons rms for the majority of devices. Of the three devices used for X-ray imaging, one showed poor charge transfer along the column direction

D.H. l.umh, (ZR. Hopktnson / Charge diffusion effects 11

,:1.32

and could only be used with a light bias equivalent to several thousand electrons, and a temperature above - 5 0 ° C . Data from this device are not discussed here. One device ( 4 6 6 / 2 / 2 5 : 3 0 p.m expitaxial) had good charge transfer efficiency under all conditions and the remaining chip ( 4 6 6 / 5 / 1 0 : 1 2 p.m, expitaxial) required a small light bias of a few hundred electrons otherwise some charge smearing appeared in the row (output register) direction. In the later two devices fixed pattern variations in the imagers were found to be small (much less than the readout noise) provided that small areas ( - 20 × 20 pixels) were used and obvious defects (e.g. bad columns) avoided.

2.2. Electroni~w Fig. 1 is a block diagram of the system used for C C D operation and data collection. Clock pulses for the C C D were generated by a hard-wired TTL logic board and the MOS levels were obtained by strobing analogue gates between the high and low vohages, set manually by multi-turn potentiometers. High speed current buffers were used to drive the gate signals to the C C D clock pins. The dc bias pins were fed from low noise operational amplifiers, the voltage again being set manually 16]. The C C D was bootstrapped with a FET [5] to minimise the output capacitance and operated with an 8.2 k~'2 resistive load to ground. The signal was ac coupled to an Analog Devices 50J F E T operational amplifier (gain 30) with a diode in the feedback loop to ensure that positive going reset feedthrough spikes did not saturate the amplifier. The 50J output was de restored at the start of each reset phase, prior to correlated

double sampling with a differential averager (or gated integrator) with a gain of - 10 for an integration time of - 5 p.s [4]. A second logic board was employed to generate timing signals for selecting a region of interest (e.g. single pixels or a subarray) from the serial data stream. The resulting data was monitored with a Canberra Series 30 pulsc height analyser and recorded with an Interface Standards ADS 120 analog digitiser. The latter is a C A M A C module with a capacity for 256K 12 bit words. Input and output were controlled by an Interface standards CC85 microcomputer programmed in F O R T H (at the present time the ADS 120 module has not been fully commissioned: the ADC, although nominally 12 bit, exhibits considerable differental nonlinearity and binning of 10 or more channels is necessary to achieve acceptable pulse height spectra). Images of individual frames could be displayed (under the control of a BBC microcomputer) or software used to cluster or discriminate (i.e. reject) spread evcnts before producing pulse height spectra.

2.3. Crvostat assembly The C C D was mounted in an evacuated, liquid nitrogen cooled cryostat, similar to the one described by Jorden and Van Breda [7]. The device temperature was monitored with a platinum resistance sensor, which was used with a servo system designed to control the temperature to within = 0.1 K. A typical operating temperature was - 9 0 ° C . The X-rays were produced either from radioactive iron-55 or cadmium-109 sources, or fluorescent lines from a Henke source. These sources were calibrated using a Harwell X e / C O 2 proportional

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D.H. Lumb. G.R. Hopktnson / Charge diffuston effects 11 counter with a thin beryllium window. The radiation flux at the C C D was kept low so that the event rate was much less than 1 / p i x e l / 0 . 3 s integration time and event overlapping was negligible. A light bias could be provided by supplying a stabilised voltage to a LED mounted in the cryostat.

3. Cosmic ray effects The interaction of cosmic ray particles with silicon devices results in the generation of bursts of charge (both points and tracks). The total amount of charge created depends on the type and energy of the particles and on the device geometry. Calculations, based on the known sea-level particle flux, have recently been made by Ziegler and Landlord [8]. For a collection depth of 10--30 ~tm they predict a total event rate of 0 . 0 2 / c m : - s, with most of the events producing a charge - 1 0 3 electrons (charge bursts large enough to cause soft errors in silicon memories occur much less often and the rate is more likely to be determined by the alpha particle flux from the chip package itself). A detailed comparison with observations is difficult because the

low energy electron and proton fluxes are mostly produced by nuclear interactions with nearby materials, however event rates - 0 . 0 6 / c m 2 • s have been reported in C C D images taken at altitude (where the cosmic ray flux is higher) [9,10]. The pulse height spectra presented in this paper were each obtained from a chip area of 5 x 1 0 3cm2 and total exposure time - 180 s. so that the probability of contamination by cosmic ray events is small (the predicted event rate is = 0.02/spectrum). Marcus et al. [10] have noted that cosmic ray events often have the appearance of tracks with a point like head (charge production in the depletion layer) and a diffuse tail (generation in the field-free region), however full frame exposures of ~ 1/2 hour duration are needed to observe a significant number and we have not yet attempted this experiment.

4. Results

4.1. Pulse height spectra Fig. 2 is an image obtained from the 30 p.m expitaxial device ( 4 6 6 / 2 / 2 5 ) and shows several events,

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produced by X-rays from an SSFe source. A is a partial event, the signal being approximately I / 2 that of the completely collected B events. Notice that the charge from either does not spread much beyond one pixel. Figs. 3 and 4 show pulse height spectra from 4 6 6 / 5 / 1 0 for 5.9 and 22 keV radiation and figs. 5 and 6 are the corresponding spectra for the thicker device. To obtain clustering of events, blocks of 3 × 3 pixels ,,,,ere taken and those signals which were more than 30 away from the zero peak were added together; alternatively, a spread event could be discriminated out by setting the signals to zero before histogramming. It is seen that the spectra have the general appearance of an X-ray peak with a tail extending towards the zero (or no X-ray) peak; this is to be expected since events can be produced either in the depletion region (and contribute to the X-ray peak) or below it in the field-free region (and partially recombine, forming pulses of lower energy) as explained in Paper I. Note that no attempt was made to keep the voltage gain of the electronics system constant for each spectrum (the gain depends on the device operating conditions). Also the number of A D C channels which were binned was not fixed (values were 10 for the 55Fe, 20 for the mgCd and 16 for the Ge source. Hence the horizontal scales of figs. 3-6 are not the same. The predicted spectra in the figures were calculated using the known energies and strength ratios of the K,, and K/~ lines, the I / e absorption depths for silicon ( -30 p.m at 6 keV, -- 1000 p.m at 22 keV) and taking the ratio of field-free region to depletio,l region depth in the thin and the thick devices as 1.1 and 4.0 respectively'. These ratios correspond well with predictions from the manufacturers data for the device dimensions: (5.0_+ 2.4)/(7 + 1) = 0.7 _+ 0.4 and (23 _+ 6)/(7 + l ) = 3.3 _+ 1. Table 1 shows the values of count rate and noise needed to obtain good fits to the spectra. For a given source, approximately the same count rate for the X-ray peak was found for the two devices, as would be expected from a dependence solely on the depletion region depth. From figs. 3 and 4 it is seen that the clustered and

discriminated spectra are similar for the thin expitaxial device which indicates that the majority of events occur within one pixel. This is to be expected since section 3 of Paper I predicts that, for most events, the radius within which 80% of the charge is collected is approximately the field-free region depth or --5 p,m in this case. For the thick epitaxial device (figs. 5 and 6) the radial spreading is sufficient for event discrimination to be partially successful, but the data suggest that - 20~ of events are still contained within one pixel: thus the radial spreading must be ~ 20 /*m .. a value again comparable with the field-free region depth. Since the Fano factor for silicon is =0.12 [11] the photon shot noise is small, hov,.ever from table 1 it can be seen that the noise in the X-ray peak is aot~roximately twice that in the zero peak. A factc, r of - ¢~2 will be due to the clustering process itself (we have seen that most of the charge is contained within one or two pixels so that normally only two pixels are co-added). The excess noise is probably due to a combination of causes such as splitting of completely collected events between two pixels such that the overflow is lost in the noise, non-uniformities in any charge transfer inefficiency, statistical fluctuations in the amount of charge collected from the field-free region due to absorption at the bulk-epi boundary and variations in diffusion length and resistivity' within the silicon (note that this increase in noise will not necessarily be present in a better quality chip). Also included in table 1 are data obtained using a Henke fluorescent source with a germanium target (giving characteristic X-rays at 9.9 and 1 I. 1 keV). Fig. 7 shows a pulse height spectrum from the thin expitaxial device with no light bias. It is seen that the presence of charge transfer inefficiency produces a displacement of the X-ray peak (in the output register 10% of the charge is smeared from one pixel to the following one if the light bias amounts to less than a few hundred electrons). Since all the events are spread, discriminalion removes the X-ray peak entirely. Finally. fig. 8 shows the linear relationship obtainable between X-ray energy and number of electron-hole

Table I. Estimates of noise and count rate from the pulse height spectra. Nominal epi-layer thickness (p~m)

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Rms e's (fwhm, keV) Noise in zero peak

Noise in X-ray peak

110 (0.95) 96 (0.83) 85 (0.74) 46 (0.40) 74 (0.64) 1 IO (0.95)

165 144 170 140 220 132

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pairs produced, when charge smearing is absent. The slope corresponds to one electron- hole pair per 3.6 + 0.2 eV deposited, as expected [12]. The data were obtained directly from PHA spectra with adjustments for changes in gain: i.e. for each pulse spectrum the conversion factor between output vohage and number of j

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D.IL Lumh. G.R. Hopkinson / Charge dtffusion effects 11

count rates in table 1 were used. The predictions were based on the assumption of a 7 # m depetion layer depth and an electrode structure composed of four layers: 0.12 p.m of gate oxide, 0.05 p.m of silicon nitride, 0.5 ~ m polysilicon (for the electrodes) and 0.5 # m silicon dioxide (for the interphase isolation). A 3 # m overlap between adjacent phases was also assumed. The good agreement reinforces the interpretation that the quantum efficiency (above 4 keV) depends primarily on depletion depth (above 4 keV, absorption in the electrode structure produces less than 9% loss). A measurement was also made for Mo X-rays at 2.3 keV. At this energy the quantum efficiency is largely influenced by the nature of the electrode structure. The figure shows that good agreement between experiment and prediction is possible without precise modelling of the (rather complicated) 3-phase structure.

5. Conclusion The effects of charge difffusion and recombination on X-ray spectra obtained with C C D imagers have been demonstrated. The experimental results agree well with the predictions of Paper I. Although the readout noise of the MA328 C C D s studied is poor compared with more modern devices (for the larger G E C P8600 chips the noise is typically 10-20 electrons rms) the achieved energy resolution ( - 1 keV fwhm for discriminated events) is a considerable improvement on that obtainable with most other types of detector, e.g. imaging proportional counters (lithium drifted silicon detectors with LED switched F E T amplifiers, as used in nuclear physics, continue to have the lowest readout noise and hence the best energy resolution). If the relative effects of charge diffusion can be reduced by using deep depletion devices [13] (which also improves the quantum efficiency) whilst keeping the readout noise below 20 electrons ( - 170 eV fwhm) then C C D s will be excellent detectors for X-ray imaging and spectroscopy.

The authors are grateful to the staff of G E C PIc and in particular to D.J. Burt (of the Hirst Research Centre) for the supply of C C D s and for technical assistance. At Leicester, A. Wells is thanked for his continuing interest. The work was supported by funds from the Science and Engineering Research Council in the form of research assistantships ( G R H and DHL) and a CASE studentship (DHL). D H L also acknowledges support from the ICI Educational Trust.

References [1] R.C. Catura and R.C. Smithson. Rev. Sci. Instr. 50 (2979) 219. [2] D.M. McCann, M.C. Peckerar, W. Mend. D.A. Schwartz. R.E. Griffiths, G. Polucci and M.V. Zombeck. Proc. SPIE 217(1980) 118. [3] II.T. Brown. GEC J. Sci. Technol. 43 (1977) 125. [4] GR. Hopkinson and D.It. Lumh, J. Phvs E 15 (1982) 1214. [5] D.H. i,urnb and G.R. Hopkinson, Proc. SPIE 331 (1982) 82. [6] D.II. Lumb, Ph.D. Thesis, Leicester University (1983). [7] P.R. Jorden and I.G. Van Breda, Proc. SPIE 290 (1981) 113. [8] J.F. Ziegler and W.A. Lanford, J. Appl. Phys. 52 (1981J 4305. [9] A. Fowler, P. Waddell and L. Mortara, Pr~. SPll£ 290 11981) 34. [10] S.L Marcus, R.E. Nelson and C.R. gynds, Prtx:. SPIE 172 (1979) 207. [11] F.S. Goulding and D.A. Landis, IEEE Trans. Nucl. Sci. NS-29 (1982) 1125. [12] C.Canali, M. Martini, G. Ottavini and A. Allerigi Quaranta. IEEETrans. Nucl. Sci. NS-19(197219. [13] M.C. Peckerar, D.H. Mc('ann and L. Yu. Appl. Phys. Lett. 39 (1981) 55.