2794 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004

High-Energy Electron Irradiation of DifferentSilicon Materials

S. Dittongo, L. Bosisio, M. Ciacchi, D. Contarato, G. D’Auria, E. Fretwurst, and G. Lindström

Abstract—The effects of 900 MeV electron irradiation on dif-ferent types of silicon substrates (standard and oxygenated float-zone, Czochralski, and epitaxial silicon) have been experimentallyinvestigated. Irradiations up to a fluence of 2.1 1015 e/cm2 havebeen performed with the electron beam of the LINAC injector atthe synchrotron light facility Elettra in Trieste (Italy). Irradiateddevices have been electrically characterized by reverse I–V andC–V measurements. Substrate type inversion has been observedfor standard and oxygenated float-zone but not for Czochralskiand epitaxial devices. The effects of isothermal annealing cycles at80 C have also been studied, and the hardness factor of 900 MeVelectrons, with respect to 1 MeV neutrons, has been experimen-tally estimated from the measurement of the reverse leakage cur-rent after annealing.

Index Terms—Electron radiation effects, radiation hardening,semiconductor device radiation effects, silicon.

I. INTRODUCTION

S ILICON detectors are widely employed in high-energyphysics experiments operating at high luminosity hadron

and electron colliders. They are an even more important com-ponent of the experiments under construction for the very highluminosity Large Hadron Collider (LHC) at CERN. In addition,they are foreseen to be extensively used at future machines suchas second-generation b-factories and linear electron-positroncolliders. For most of these applications the detector survivalafter years of operation in a harsh radiation environment is anessential requirement.

In recent years much effort has been devoted to the studyof the radiation hardness of silicon detectors against differentparticle types, and to the study of the improvements possiblyachievable by using different silicon substrate materials. Ex-tensive investigations have been performed in particular byirradiating detectors, test structures and raw materials withcharged hadrons (pions and protons) and neutrons [1]. Severalstudies have also been conducted with ray photons [2], [3]. Bycontrast, very few contributions have been devoted to the study

Manuscript received September 17, 2003; revised May 21, 2004. This workwas supported by the Istituto Nazionale di Fisica Nucleare, Italy, under ProjectElda, by the German Ministry of Economy BMWi under Contract R&D 289/00within the framework of CiS-SRD Project 642/06/00, and by the German Re-search Foundation DFG under Contract FR1547/1-1.

S. Dittongo and L. Bosisio are with Istituto Nazionale di Fisica Nucleare,Sezione di Trieste, I-34012 Trieste, Italy, and with the Dipartimento di Fisica,Università di Trieste, I-34127 Trieste, Italy (e-mail: dittongo@ts.infn.it).

M. Ciacchi is with the Dipartimento di Fisica, Università di Trieste, I-34127Trieste, Italy.

D. Contarato, E. Fretwurst, and G. Lindström are with Institut für Experimen-talphysik, Universität Hamburg, D-22761 Hamburg, Germany.

G. D’Auria is with Area di Ricerca, Sincrotrone Trieste S.C.p.A., I-34012Trieste, Italy.

Digital Object Identifier 10.1109/TNS.2004.835117

of the damage produced by high-energy (GeV) electrons insilicon. Previous results, obtained with 500 and 900 MeV elec-tron irradiation of silicon devices fabricated on high-resistivityfloat-zone material, have shown that high-energy electrons,like neutrons and protons, are very effective in creating bulkdamage in silicon [4]–[6]. In a recent work, extending the studyto oxygen enriched silicon substrates [7], no significant effectof the oxygen has been observed up to an electron fluence ofabout 5 10 e/cm .

In this paper we further extend these investigations by con-sidering a wider range of substrate materials, and by reachinghigher fluence levels (up to 2.1 10 e/cm ).

The different irradiated devices and the experimental condi-tions will be described in Section II. Section III will be devotedto the discussion of the experimental results: firstly we willaddress the radiation-induced change of the substrate effectivedopant concentration, then results on the increase of the reverseleakage current with fluence will be reported, together with anestimation of the hardness factor of 900 MeV electrons with re-spect to 1 MeV neutrons; results from thermal annealing studiesat 80 C will also be analyzed, for annealing times up to around100 h. Section IV will report the summary and conclusions.

II. DEVICES AND EXPERIMENTAL CONDITIONS

Tested devices are vertical p n n diodes fabricated ondifferent silicon substrates, namely, standard and diffusion-oxy-genated float-zone (which in the following we will address asStFZ and DOFZ, respectively), Czochralski (CZ), and epitaxial(EPI) silicon.

A set of standard and oxygenated float-zone devices has beenmanufactured by CiS (Erfurt, Germany) on Wacker (111) sub-strates of typical resistivity of 3–4 k cm. Oxygen diffusion forDOFZ devices has been performed in an N environment for72 h at 1150 C. A second set of StFZ diodes has been fab-ricated by ITC-irst (Trento, Italy) on high purity wafers fromTopsil, with a resistivity of 10–20 k cm. Some of the substrateshad been previously converted to DOFZ by a 12 h oxidation at1150 C followed by a 36 h diffusion in N at the same temper-ature, resulting in an oxygen concentration of 1-3 10 cmacross the substrate.

Czochralski and epitaxial devices have also been processedby CiS. CZ diodes are manufactured on (100) wafers of resis-tivity 1.2 k cm from Sumitomo; EPI diodes are processed ona 50 m thick epitaxial layer (of resistivity 50 cm) grownby ITME (Warsaw, Poland) on a 300 m thick low-resistivity(0.01 cm) Czochralski (111) substrate.

The junction area of all CiS diodes is of 0.5 0.5 cm , whiledevices fabricated by ITC-irst have an area of 0.35 0.35 cm .

0018-9499/04$20.00 © 2004 IEEE

DITTONGO et al.: HIGH-ENERGY ELECTRON IRRADIATION OF DIFFERENT SILICON MATERIALS 2795

Junction depth is of the order of 1 m. All diodes are providedwith a 100 m wide guard-ring, surrounded by several addi-tional floating guard-rings. For all devices, the n base contactis fabricated on the back surface of the wafer, whose total thick-ness is of about 300 m.

Irradiations up to a fluence of 10 e/cm have beenperformed with the electron beam of the LINAC injector at thesynchrotron light facility Elettra in Trieste (Italy). Devices havebeen kept unbiased during irradiation, at the room temperatureof the LINAC (25 C). To ensure a uniform irradiation of thewhole area covered by the devices, these were moved along aserpentine path in a plane perpendicular to the beam, by meansof a remotely controlled translation stage. The electron fluencewas measured by means of a toroidal coil coaxial with the beam,allowing the electric charge of the individual beam pulses to bemonitored. The fluences released after each irradiation step arereported in Table I. The first error associated with these valuestakes into account the fluctuations of the beam intensity ob-served during the irradiation, while the second one accounts forthe systematic uncertainty on the calibration of the toroidal coil.For each irradiation step, different devices have been used.

Irradiated devices have been electrically characterized by re-verse bias I–V and C–V measurements, performed on bare diesfirst about one day after irradiation and then after thermal an-nealing cycles at 80 C. During the measurements, the deviceshave been biased by applying a positive voltage to the back-sidecontact while connecting to ground the central diode and the firstguard-ring.

Between irradiation and the first measurements the sampleshave been kept at room temperature. After the first series of mea-surements, devices have been stored at about 7 C. The C–Vmeasurements have been performed at 10 kHz frequency, andthe depletion voltage , defined as the minimum reverse biasnecessary to extend the space charge region over the whole sub-strate thickness, has been estimated from the saturation of theC –V or of the C– V curves. The temperature in the lab-oratory during the measurements varied between 22 and 24 C.All measured currents have been normalized to 20 C.

III. EXPERIMENTAL RESULTS

A. Effective Dopant Concentration

The values of the substrate effective dopant concentration, defined as the difference between the concentration of

positively charged donors and the concentration of negativelycharged acceptors, have been calculated for all devices from themeasured values of the depletion voltage , according to thestandard expression valid in the depletion approximation and foruniform doping

(1)

where is the relative dielectric constant of silicon,10 F/cm is the permittivity of vacuum,

10 C is the elementary electric charge, and is thesubstrate thickness, evaluated for each device from the satura-tion value of the diode capacitance above full depletion voltage.

TABLE ICUMULATIVE BEAM FLUENCES RELEASED AFTER EACH IRRADIATION STEP

We report both the results of the measurements performedsoon after irradiation (i.e., one day later) and after thermal an-nealing for 8 min at 80 C. According to [8], an annealing stateof 8 min at 80 C corresponds to a minimum in the radiation-in-duced change of the substrate effective doping concentration,after the irradiated silicon material has undergone initial shortterm beneficial annealing; this is therefore taken as a generalcriterion for comparison with preirradiation measurements, al-though the existence of the minimum has only been verified fortype-inverted StFZ and DOFZ materials.

Figs. 1 and 2 show the values of as a function of theelectron fluence for the StFZ and DOFZ devices manufacturedrespectively by ITC-irst and CiS. A negative sign has been as-signed to those values of that we consider likely to cor-respond to an inverted ( -type) substrate, based on the crite-rion of obtaining a smooth curve. Type inversion appears around2.5 10 e/cm for the ITC-irst devices (fabricated on higherresistivity material), and at about 5 10 e/cm for the CiSdevices.

After type inversion, the slope of versus fluence is about1.0 10 cm for ITC-irst StFZ devices, 1.3 10

cm for CiS StFZ diodes, 0.6 10 cm for ITC-irstDOFZ structures, and 0.5 10 cm for CiS DOFZdevices. A lower slope appears to be associated with DOFZsubstrates. Differences between diodes fabricated by ITC-irstand by CiS could be due to different starting materials andoxygen diffusion treatments. Similar results had also beenobserved after charged hadron irradiation [1].

In Fig. 3 we report the values obtained for the CZsubstrates. Due to the fact that the samples used for thevarious irradiations had non negligibly differing values ofinitial resistivity, the following “equalization” procedure hasbeen applied, in order to obtain a meaningful dependence onfluence: for each device, the variation in due to irradiation

has been added to the average value ofthe initial effective dopant concentration, taking all devicesinto account.

After the first steps of irradiation, the curve appears to be ap-proximately linear, with a slope of about 1.0 10 cm .Nevertheless, it does not appear to be possible, within this lim-ited range of fluences, to separate the relative contributions tothe observed rate of change of coming from a) the initialpart of an exponential decay of the donor concentration or b) alinear introduction of acceptors.

2796 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004

Fig. 1. Effective dopant concentration for StFZ and DOFZ devices fromITC-irst measured soon after irradiation (closed symbols) and after 8 min ofthermal annealing at 80 C (open symbols).

Fig. 2. Effective dopant concentration for StFZ and DOFZ devices from CiSmeasured soon after irradiation (closed symbols) and after 8 min of thermalannealing at 80 C (open symbols).

Fig. 3. Effective dopant concentration for CZ devices measured soon afterirradiation (closed symbols) and after 8 minutes of thermal annealing at 80 C(open symbols). The values reported have been equalized according to theprocedure described in the text.

Previous irradiations of the same type of devices with chargedhadrons (24 GeV/ protons and 190 MeV/ pions) have shown

Fig. 4. Effective dopant concentration for two different sets of EPI devicesmeasured soon after irradiation (closed symbols) and after 8 min of thermalannealing at 80 C (open symbols). The values reported have been equalizedaccording to the procedure described in the text.

no type inversion even at fluences up to 10 particle/cm [9].In our case, a simple linear extrapolation of the observed flu-ence dependence would suggest that type inversion will occurat a fluence of about 4.5 10 e/cm . Such values of fluenceare feasible with our experimental setup and we plan to reachthem in future irradiations, in order to distinguish between thepreviously mentioned contributions to the slope of and tofind out whether the observed linear trend continues up to typeinversion or rather a different behavior appears.

Fig. 4 shows the corresponding plot for the EPI devices.Again, the above-described “equalization” procedure hasbeen applied in order to account for differing initial dopantconcentrations. For these devices, the absolute variations in

are of the same order of magnitude ( 10 cm ) asfor other materials. Nevertheless, thanks to the higher initialdopant concentration, the relative changes in are quitesmall and comparable with the uncertainty in the evaluationof the depletion voltage. Substrate type inversion is not evenapproached at the fluences considered, in agreement with whatalready observed after irradiation with 24 GeV/ protons [10],[11], where an initial decrease in is followed by a moderateincrease at higher fluences. For these reasons we consider ourpresent measurements on EPI devices as an intermediate steptoward heavier irradiations, from which a clearer trend shouldemerge.

B. Leakage Current and Damage Constant

In Fig. 5 we report the observed fluence dependence of thevolume density of the reverse leakage current, as measured forall devices after thermal annealing for 8 min at 80 C. Thischoice of the annealing state at which to refer the leakage cur-rent values, besides being consistent with that adopted for ,has the purpose of giving a value which is less dependent onthe detailed initial thermal history of the irradiated device (e.g.,duration of the irradiation itself, time elapsed from the end ofirradiation to first measurements) [1]. A satisfactory uniformityis obtained between all devices considered, indicating that theleakage current increase does not depend on the substrate mate-rial, as already observed after hadron irradiation [12].

DITTONGO et al.: HIGH-ENERGY ELECTRON IRRADIATION OF DIFFERENT SILICON MATERIALS 2797

Fig. 5. Leakage current density measured for all devices after thermalannealing for 8 min at 80 C. We consider the slope of the linear fit(9.06� 10 A/cm, see text) as an estimation of the damage constant �.

The value of the slope of the linear fit is 9.06 10 A/cm,and can be considered as an experimental estimation of thedamage constant , referred to the actual electron fluence. Thisvalue, obtained after annealing for 8 minutes at 80 C, is com-patible with previously measured values [5]–[7], if we take intoaccount that the latter did refer to measurements performed soonafter irradiation and before any intentional thermal annealing.

The hardness factor of 900 MeV electrons with respect to1 MeV neutrons can be estimated from the ratio of the damageconstant thus measured to that reported in the literaturefor 1 MeV neutrons (after the same annealing cycle), namely4.0 10 A/cm [12]. This yields a value .It is interesting to note that the ratio of the nonionizing energyloss (NIEL) values for high-energy electrons and for 1 MeVneutrons is about 8.1 10 (we have used the results of NIELcalculation relative to 200 MeV electrons [13], the highestavailable energy in the literature). This indicates that, at equalNIEL values, high-energy electrons are about 3.5 times lesseffective than 1 MeV neutrons in degrading the carrier gener-ation lifetime of the material, providing further evidence thatthe NIEL scaling hypothesis is not adequate when comparingelectrons with hadrons, even in the GeV electron energy range.

C. Annealing Studies

For two series of EPI devices and for StFZ and DOFZ diodesfabricated by ITC-irst, irradiated at the two highest fluences, wecarried out a study of the time evolution of the effective dopantconcentration and of the leakage current, by means of ac-celerated isothermal annealing above room temperature. Sev-eral cumulative annealing steps at 80 C have been performedon the same devices for each of the two considered fluences, upto a total time of about 100 h.

Figs. 6 and 7 show the annealing behavior of the effectivedopant concentration for the ITC-irst StFZ and DOFZ devicesand for the EPI diodes, respectively. In order to underline andto compare the time evolution of for the different devicesand the two irradiation fluences, we have plotted the normalizedquantity , where is the first value measuredafter irradiation, before any annealing step. In all the annealingplots, the preannealing value has been arbitrarily displayed at

Fig. 6. Normalized effective dopant concentration for StFZ and DOFZ devicesfrom ITC-irst, irradiated at the two highest fluences, as a function of annealingtime at 80 C.

Fig. 7. Normalized effective dopant concentration for EPI devices from CiS,irradiated at the two highest fluences, as a function of annealing time at 80 C.

an abscissa value of 0.1 min. It is likely that the time elapsedat room temperature (22 to 25 C) before the first measurementcould be performed is equivalent to an annealing time longerthan 0.1 min at 80 C; nevertheless, given the fact that this timeis variable and the difficulty in properly accounting for it, wepreferred to use a conventional value of 0.1 min in order to in-troduce a minimum of bias in the interpretation of the plot.

For the StFZ and DOFZ devices (inverted) an initial beneficialannealing can be observed, leading to a minimum of afterabout 8 min, followed by an increase (reverse annealing). Withinthe limited accuracy of these data, meaningful differences inthe annealing behavior between the various devices and the twoirradiation fluences are not apparent.

EPI devices (noninverted) show an increase in the thatamounts to approximately 2%–3% after about 100 h at 80 C).Here again the data do not suggest a clear difference betweenthe two irradiation fluences.

Finally, in Figs. 8 and 9 we report the annealing behavior ofthe leakage current density for the ITC-irst StFZ and DOFZ de-vices and for the EPI diodes, respectively. The annealing curvesof devices made on different substrates and irradiated at dif-ferent fluences appear to follow a common functional depen-dence on the annealing time.

2798 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 51, NO. 5, OCTOBER 2004

Fig. 8. Evolution of the leakage current density for StFZ and DOFZ devicesfrom ITC-irst, irradiated at the two highest fluences, as a function of annealingtime at 80 C.

Fig. 9. Evolution of the leakage current density for EPI devices from CiS,irradiated at the two highest fluences, as a function of annealing time at 80 C.

IV. CONCLUSION

Silicon test structures manufactured on different substratematerials have been irradiated with 900 MeV electrons. Theradiation-induced change in the substrate effective dopant con-centration has been studied as a function of the electron fluence,showing bulk type inversion for float-zone (both standard andoxygenated) but not for Czochralski and epitaxial materials.For float-zone materials, a slightly beneficial effect of oxygendiffusion is observed.

Isothermal annealing studies at 80 C, performed on float-zone and on epitaxial devices, have shown different behaviors:epitaxial substrates (noninverted) show an increase of theeffective donor concentration as a function of the annealingtime, while both standard and oxygenated float-zone substrates(which are inverted at the fluences considered) show an initial

beneficial annealing, reaching a minimum in the effectiveacceptor concentration after about 8 min at 80 C, followed byreverse annealing at longer annealing times.

Finally, the increase of the reverse leakage current after ir-radiation has shown a uniform linear trend as a function of theelectron fluence for all devices, and has allowed an estimate of

for the hardness factor of 900 MeV electronswith respect to 1 MeV neutrons.

ACKNOWLEDGMENT

The authors are indebted to the operators of the LINAC in-jector at the Elettra synchrotron light source for carefully tuningthe beam and patiently overseeing the machine operation duringthe long hours of irradiation.

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