Microelectronics Reliability xxx (2014) xxx–xxx

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Microelectronics Reliability

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Mechanism of anomalous recovery in advanced SiGe bipolar transistorsafter low dose rate irradiation for very high total doses

http://dx.doi.org/10.1016/j.microrel.2014.04.0090026-2714/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: Centro Nacional de Microelectrónica (CNM-CSIC), Campus Universitario de Bellaterra, 08193 Barcelona, Spain. Tel.: +34 93 59477 00; fax: +34 93 580 14 96.

E-mail address: Miguel.Ullan@imb-cnm.csic.es (M. Ullán).

Please cite this article in press as: Pershenkov VS et al. Mechanism of anomalous recovery in advanced SiGe bipolar transistors after low dose radiation for very high total doses. Microelectron Reliab (2014), http://dx.doi.org/10.1016/j.microrel.2014.04.009

V.S. Pershenkov a, M. Ullán b,⇑, M. Wilder c, H. Spieler d, E. Spencer c, S. Rescia e, F.M. Newcomer f,F. Martinez-McKinney c, W. Kononenko f, A.A. Grillo c, S. Díez b

a Moscow Engineering Physics Institute (NRNU ‘‘MEPhI’’), Moscow, Russiab Centro Nacional de Microelectrónica (CNM, CSIC), Barcelona, Spainc Santa Cruz Institute for Particle Physics (SCIPP, UCSC), Santa Cruz, CA, USAd Lawrence Berkeley National Laboratory (LBNL), Physics Division, Berkeley, CA, USAe Brookhaven National Laboratory (BNL), Upton, NY, USAf University of Pennsylvania, Philadelphia, PA, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 December 2013Received in revised form 16 April 2014Accepted 17 April 2014Available online xxxx

Keywords:Radiation effectsELDRSGamma irradiationSiGe bipolar transistorsAnomalous recovery

The possible physical mechanism of the anomalous recovery effect in SiGe bipolar transistors isdescribed. The qualitative analysis of saturated oxide trapped charge and interface trap densities at veryhigh total doses as a function of dose rate affords an explain of decreasing excess base current andincreasing current gain during further low dose rate irradiation.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction and background

Silicon–germanium heterojunction bipolar transistors (SiGeHBT) are an attractive candidate for operation in extreme environ-ments including front-end electronics for high energy physicsexperiments such as at the Large Hadron Collider at CERN [1,2].It makes use of their advantages of high gain and high speed at rel-atively low power consumption. A significant radiation tolerance isone of the main requirements for using these devices for Physicsexperiments. Front-end electronics for the ATLAS Upgrade detectorat CERN must operate during 10 years (3�108 s) at a total absorbeddose of 50 Mrad(SiO2) that corresponds to a dose rate near0.25 rad(SiO2)/s, taking into account the shutdown and inactiveperiods. At this relatively low dose rate, the effect of enhancedlow dose rate sensitivity (ELDRS) can lead to significantly reducedelectronics useful operation time. Authors of [3] have presentedexperimental data concerning current gain degradation of SiGeheterojunction bipolar transistors for total absorbed dose of

50 Mrad(SiO2) 60Co gamma irradiation in dose rates from0.25 rad(SiO2)/s to 200 rad(SiO2)/s. It was shown that for relativelysmall total dose, 0.65 Mrad(SiO2), gain degradation is constant inthis range of dose rates. This is an indication that SiGe devices donot suffer from ELDRS in a low dose range. But increasing the irra-diation time for absorption of a dose of 50 Mrad(SiO2) can lead tothe manifestation of this effect. Therefore, in [3] an additionalexperiment was performed using an accelerated ELDRS test basedon ‘‘switched experiments’’ [4]. Devices were irradiated at a highdose rate (HDR) of 25 rad(SiO2)/s up to 0.64, 5, 10, 30 and50 Mrad(SiO2). Later, these devices were irradiated again, but thistime at a low dose rate (LDR) of 0.25 rad(SiO2)/s up to an additionaltotal dose of 3.5 Mrad(SiO2) with intermediate tests conducted at0.3, 0.65 and 1.4 Mrad(SiO2). Figs. 1 and 2 show the results of thisexperiment in terms of normalized current gain bN (bN = bF/bI,where bF = ICF/IBF is ratio of the post-irradiation collector ICF andbase IBF currents; bI is the pre-irradiation current gain) and interms of the excess base current density DjB(DjB = (IBF�IBI)/AF,where IBF and IBI are post- and pre-irradiation base currents respec-tively; AF is the transistor emitter area). The thin short lines inFig. 1 and thick kinked lines in Fig. 2 show the results for additionalLDR irradiations on devices that have already been irradiated atHDR. It can be seen that at the highest total doses (30 and

te irra-

Fig. 1. Results from the switched experiment for SiGe bipolar transistors, wherehigh dose rate irradiation (solid line) is followed by low dose rate irradiation (thinshort lines) [3].

Fig. 2. Details of the anomalous recovery effect in SiGe bipolar transistors in term ofthe excess base current density for switching experiment [3]. Recovery for low doserate irradiation is shown by the thick kinked lines below the smooth line for highdose rate irradiation.

2 V.S. Pershenkov et al. / Microelectronics Reliability xxx (2014) xxx–xxx

50 Mrad(SiO2)) radiation damage of transistors seems to bereduced when they are submitted to further irradiation at LDR.At the highest total doses, the LDR curves do not continue to followthe HDR interpolation curves but diverge from them indicating arecovery in radiation damage on devices when they are irradiatedat LDR. In [3] the effect was denoted as anomalous damage recov-ery. The possibility of reducing degradation at low dose rate wasformerly shown in [5]. Those were simulation results for differentinitial concentrations of the recombination centers in the oxide. In[3] the experimental confirmation of this prediction was observedfor case of very high total doses in bipolar transistors.

The goal of this work is to present a qualitative physical modelfor the anomalous recovery effect. The model is based on the con-version model of the low dose rate effect in bipolar devices [6].

2. The mechanism of anomalous recovery

The degradation of the bipolar transistor current gain and theincrease in base current density JB is caused by the accumulation

Please cite this article in press as: Pershenkov VS et al. Mechanism of anomalodiation for very high total doses. Microelectron Reliab (2014), http://dx.doi.org

of radiation-induced positive oxide trapped charge Qot and inter-face trap densities Nit in the silicon base-emitter spacer-oxide.For qualitative analysis it can be supposed that excess base currentdensity DJB increases (thus normalized current gain bN decreases)with increasing oxide trapped charge Qot and interface trap densi-ties Nit. The value of DJB and bN saturate at very high total dosewhen oxide charge Qot and trap densities Nit reach a maximumvalue.

2.1. Oxide trapped charge

The positive oxide trapped charge saturation is ruled by thecompeting annealing phenomena. Annealing is caused by radiationinduced electrons in the oxide at high charge concentration. Therate of variation of trapped charge in time can be written in sim-plest form as

dQot=dt ¼ Kacc � PY � Qot=ðsannÞot; ð1Þ

where Kacc is a coefficient characterizing trapped charge accumula-tion; PY is dose rate; (sann)ot is the time constant of positive chargeannealing process by substrate and radiation induced elec-trons.From (1) can be obtained

Qot ¼ ðQ otÞsatð1� e�t=ðsannÞotÞ; ð2Þ

where (Qot)sat is maximum trapped charge in saturation

ðQ otÞsat ¼ Kacc � PY � ðsannÞot: ð3Þ

From Eq. (3)it is possible to conclude that the saturation of oxidetrapped charges is reached with very high total doses over an irra-diation time longer than the time constant of the annealing pro-cess.For low total dose (t� (sann)ot) trapped charge is linearfunction of dose

Qot ¼ Kacc � PY � t ¼ Kacc � D; ð4Þ

where D is total dose.For high total dose, that is particularly interesting for the anom-

alous damage recovery effect, the oxide trapped charge saturatesand reaches the maximum value (Qot)sat. As follows from (3), thevalue of saturated trapped charge is proportional to the productof dose rate PY and the time constant of annealing process (sann)ot.Therefore, in the switched experiment when the dose ratedecreases from a high to a low value, the maximum oxide trappedcharge tends to a smaller value.

2.2. Interface trap densities

In this paper we use the conversion model of interface trapbuildup [6]. This model is based on the assumption that the gener-ation of interface traps is caused by the neutralization of positivecharge by electrons. An interaction of captured positive charge(may be hydrogen-related species) and electrons is of crucialimportance for interface buildup. It means that the Nit generationrate is proportional to the value of oxide trapped charge Koi�Qot,where Koi is a coefficient characterizing conversion of the oxidetrapped charge to interface traps.

The interface trap density would saturate at high doses. For-mally, it can be considered as a reduction of the interface trap den-sity proportional to Nit/(sann)it, where (sann)it is the time constantfor reducing Nit concentration due to the exhaustion of precursordefects in the oxide, to space charge effects, or to interaction effectsbetween interface traps [7–9].

In analogy with the oxide trapped charge case, the dependenceof Nit versus time can be written as

dNit=dt ¼ Koi � Q ot � Nit=ðsannÞit; ð5Þ

where Koi and (sann)it are some constants.

us recovery in advanced SiGe bipolar transistors after low dose rate irra-/10.1016/j.microrel.2014.04.009

Fig. 4. The dependence of the excess base current versus total dose in the switchedexperiment for high dose rate (HDR) and low dose rate (LDH). Points a, b, c and dcorrespond to the dose rate switching from HDR to LDR. After switching the devicesare irradiated to total additional dose of 3.5 Mrad(SiO2).

Fig. 5. The dependence of the normalized current gain versus total dose in switchedexperiment. Points a, b, c and d correspond to the switching dose rate from HDR toLDR. After switching the devices are irradiated to total dose 3.5 Mrad(SiO2) (bN0 isthe current gain near start point of switching).

V.S. Pershenkov et al. / Microelectronics Reliability xxx (2014) xxx–xxx 3

At high dose the oxide trapped charge is saturated at lowerdoses relative to the saturation of interface traps. In saturation,dNit/dt = 0 and because Qot reaches a saturated value,

ðNitÞsat ¼ Koi � ðQ otÞsat � ðsannÞit: ð6Þ

As follows from (6), reducing the dose rate PY also reduces thevalue of (Nit)sat because (Qot)sat reduces as well.

2.3. Switched experiment

Fig. 3 shows the qualitative dependence of the oxide trappedcharge (or interface trap densities) versus total dose for high doserate (HDR) and low dose rate (LDH). Note, that the value of satu-rated oxide charge and interface traps is larger for HDR than forLDR.

In the switched experiment, the dose rate is switched from highto low dose rate after total doses of 5 Mrad(SiO2), 10 Mrad(SiO2),30 Mrad(SiO2) and 50 Mrad(SiO2). At low dose rate the devicesare additionally irradiated to a total dose 3.5 Mrad(SiO2). The pointsa, b, c and d in Fig. 3 correspond to changing the dose rate. After anaverage total dose of 5 Mrad(SiO2) (point a) switching leads to someincreased oxide trapped charge (interface trap densities) duringabsorption of an additional dose of 3.5 Mrad(SiO2). In the range ofhigh total doses (30 Mrad(SiO2): point c and 50 Mrad(SiO2): pointd in Fig. 3), the oxide trapped charge (interface trap densities)reduces after switching. At point b (switching after 10 Mrad(SiO2)total dose) the values for Qot and Nit change non-monotonicly: aftera small reduction, the oxide charge and interface traps increase fol-lowing the curve LDR. All these behaviors are connected withreducing the maximum saturated oxide charge (interface trap den-sities) after switching dose rates from high to low.

Fig. 4 shows the qualitative dependence of the excess base cur-rent versus total dose in the switched experiment. This depen-dence repeats the dependence of oxide trapped charge (interfacetraps) versus total dose because the excess base current is propor-tional to Qot and Nit. Switching the dose rate after 5 Mrad(SiO2)causes the excess base current to continue to increase; after30 Mrad(SiO2) and 50 Mrad(SiO2) the excess base current mono-tonically decreases by moving from the HDR to LDH curves; after10 Mrad(SiO2) at point b the dependence of the excess base currentduring additional LDR irradiation is not monotonic: increasingafter a small reduction.

2.4. The current gain

The dependence of the normalized current gain bN versus totaldose is shown in Fig. 5. This dependence follows from Fig. 4 (reduc-

Fig. 3. The dependence of oxide trapped charge (interface trap densities) versustotal dose in the switched experiment for high dose rate (HDR) and low dose rate(LDH). Points a, b, c and d correspond to the dose rate switching from HDR to LDR.After switching the devices are irradiated to an extra total dose of 3.5 Mrad(SiO2).

Please cite this article in press as: Pershenkov VS et al. Mechanism of anomalodiation for very high total doses. Microelectron Reliab (2014), http://dx.doi.org

ing excess base current leads to increasing current gain). At point a,current gain decreases after dose rate switching. At points c and d,the value of bN monotonically increases. At point b, current gaindecreases after a small increase , repeating the non-monotonicdependence of excess base current versus dose in Fig. 4 (point b).More clearly it can be seen in Fig. 6 where the dependence of(bN–bN0) (the change current gain near the start point of switching)versus dose during the additional irradiation to 3.5 Mrad(SiO2) is

Fig. 6. The dependence of changing current gain (bN–bN0) near the start point ofswitching dose rates versus total dose during additional low dose rate irradiation toan additional 3.5 Mrad(SiO2) [3].

us recovery in advanced SiGe bipolar transistors after low dose rate irra-/10.1016/j.microrel.2014.04.009

Fig. 7. The possible dependence of excess base current versus dose when ELDRS cantake place for some dose ranges.

4 V.S. Pershenkov et al. / Microelectronics Reliability xxx (2014) xxx–xxx

shown. The data of Fig. 6 are presented in [3]. Switching after50 Mrad(SiO2) and 30 Mrad(SiO2) leads to a monotonic increaseof current gain and the value of (bN–bN0) as well. Switching after5 Mrad(SiO2) leads to monotonic decrease of current gain and thevalue of (bN–bN0). Switching after total a dose 10 Mrad(SiO2) leadsto small increase in current gain and (bN–bN0) followed by a netdecrease. It completely agrees with the qualitative analysis onFig. 4.

The qualitative data of Figs. 4 and 5 qualitatively correspond toexperimental data [3] (Figs. 1 and 2) and more precise observationin Fig. 6, which illustrate the anomalous recovery effect.

The mechanism of anomalous recovery is based on the fact thatthe saturated values of oxide trapped charge and interface trapdensities at very high total doses are proportional to dose rate.The reduction of the saturated values of oxide charge and interfacetraps with decreasing dose rate leads to the decrease of the excessbase current and increase of current gain during additional lowdose rate irradiation. A modification of the radiation damage dur-ing recovery after dose rate switching is very a complicated prob-lem and demands special physical investigations.

3. Future directions

Two orders of magnitude reduction of dose rate in [3] (from25 rad(SiO2)/s to 0.25 rad(SiO2)/s) leads to relatively small reduc-tion of maximum oxide charge, interface traps and excess base cur-rent. It can be connected with the Radiation Induced ChargeNeutralization (RICN) effect [10] when the time constant of posi-tive charge annealing can be significantly reduced due to positivecharge annealing by radiation induced electrons. The problem ofthe RICN effect demands special analysis.

In Fig. 4 the start slope of HDR and LDR curves for small dose(less than 1.0 Mrad(SiO2)) is the same. It is correct for tested SiGebipolar devices and follows from experimental data [3]: for rela-tively small total dose (up to 0.65 Mrad(SiO2) gain degradation isconstant for dose rates from 0.25 rad(SiO2)/s to 200 rad(SiO2)/s.For a typical case, this dependence can be as shown in Fig. 7. Exper-imental data in [11] confirm these possibilities. For doses less thanDcrit, devices suffer from ELDRS: the degradation of the excess basecurrent at low dose rate is higher than for high dose rate. Note, thata coefficient of enhanced degradation (ratio of excess base currentat LDR and HDR) is not constant in this dose range and has a max-

Please cite this article in press as: Pershenkov VS et al. Mechanism of anomalodiation for very high total doses. Microelectron Reliab (2014), http://dx.doi.org

imum near dose Dmax. For doses higher than Dcrit the anomalousrecovery effect can be observed. The consideration of the high totaldose feature can lead to a revision of some basic principal ELDRSeffects [12]. The device behavior for high total dose applicationscan be investigated in the future.

4. Conclusion

The physical mechanism of SiGe bipolar transistor current gainrecovery during low dose rate irradiation after absorption of high60Co gamma dose in the range of 30 Mrad(SiO2) to 50 Mrad(SiO2)is presented. Anomalous recovery is related to the dependence ofmaximum saturated oxide trapped charge and interface trap den-sities on dose rate. Using a qualitative model of accumulationand annealing of oxide charge Qot and interface traps Nit, it wasshown that reducing dose rate leads to reducing the maximum sat-urated values of Qot and Nit. After switching from high dose rate tolow dose rate, excess base current and current gain correspond tothe saturation values of lower base current and greater currentgain. This change is referred to as anomalous recovery. The pro-posed qualitative mechanism agrees with the experimental datapresented in the original work [3].

Acknowledgement

The authors thank A.T. Yastrebov for technical support. Thiswork is supported and financed in part by the Spanish Ministryof Economy and Competitiveness through the Particle PhysicsNational Program (ref. FPA2012-39055-C02-02) and co-financedwith FEDER funds.

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