Single Event Effects in Power MOSFETs during Heavy Ions Irradiations performedafter -Ray degradation

G. Busattoa,b, V. De Lucaa,b, F. Iannuzzoa,b, A. Sansevernoa,b, F. Velardia,b

a Università degli studi di Cassino e del Lazio Meridionale, Via G. di Biasio 43,03043 – Cassino, Italyb INFN sezione di Roma, P.le Aldo Moro 2, 00185 – Roma, Italy

Work founded by I.N.F.N. – Italy, under project APOLLO

Corresponding author and perspective presenting author:

Giovanni BusattoD.A.E.I.M.I., Università degli studi di Cassino e del Lazio Meridionalevia G. di Biasio 43,03043 Cassino, ItalyPhone: +39 0776 2993699Fax: +39 0776 2994325e-mail: busatto@unicas.it

Abstract

An experimental study about the combined effects of gamma-ray irradiation followed by heavy ionirradiation shows that TID degradation of the gate oxide severely worsen heavy ions SEE robustnessof commercial power MOSFETs.

Work motivation and purposePower MOSFETs are very important electron devices in power converters and their reliableemployment in harsh radiation environments, like aerospace missions or high energy nuclear physicsexperiments [1], is a goal to reach. When energetic photons or particles impinge on the devicestructure they loss energy by ionization or by collision causing defects and changes in the electricalbehavior which can lead the device to a premature failure. Ionizing mechanisms include total ionizingdose (TID) effects and single event effects (SEE).Although TID and SEE have been widely studied in the literature [2-11], not much work has beendone so far to study the combined effects of the phenomena involved in TID and SEE. Thesecombined effects may become particularly significant not only in space applications but also in highenergy nuclear physics experiments where high energy neutrons or protons, which aren’t able todirectly induce a SEE, can originate by spallation a recoil nucleus heavier than the original particle,able to generate a sufficient amount of electron-hole pairs to induce a SEE [12].The purpose of this work is to study, for the first time, the degradation of the SEE sensitivity ofcommercial power MOSFETs after TID irradiation. Experimental results show that the surfacechanges caused by γ irradiation severely worsen the SEE robustness and drastically modify thephysical behavior of the device under test after the impact of a heavy ion. A new failuremechanism is proposed to take place. It involves the activation of the parasitic bipolar transistorassisted by the gate structure whose degraded characteristics permits a current to flow throughthe distributed base resistance thus causing the emitter junction to be forward biased after theheavy ion impact.

Experimental resultsThe combined experiments were developed in two phases. In the first phase commercial powerMOSFETs were irradiated at different total doses up to 10KGy[SI] with the Calliope 60Co γ-ray sourceof the ENEA Casaccia Research Center, Rome, Italy. In the second phase, the same samples wereirradiated with 155 MeV bromine ions, able to stimulate failure mechanisms in the MOSFETstructure. The heavy ion irradiation was performed at the Laboratori Nazionali del Sud, INFN,Catania, Italy.The tested devices belong to a commercial 200V n-channel power MOSFETs family. The sampleswere irradiated with four different γ-doses: 1.60, 3.20, 5.89 and 9.60kGy[SI] with a dose rate of10Gy/h (about 0.28rad/s). The highest dose was chosen in order to comply with the specification ofthe ATLAS Liquid Argon calorimeter (LAr) after the LHC phase 2 upgrade [1]. For each dose, at leastthree different samples were tested. During the γ-irradiation the drain and the source of the testeddevice were grounded and VGS was set to the 80% of the maximum nominal gate voltage. After theirradiation all the samples were annealed for about 1 week at a temperature of 100°C. For each device,threshold voltage shift, breakdown voltage shift, on-resistance and oxide current leakage wereacquired at the above mentioned doses. The TID has effect on the threshold voltage shift, on the slopeof the sub-threshold current and on the degradation of the carriers mobility at the Si-SiO2 interface.The gradual reduction of both threshold and breakdown voltage measured after the annealing, atincreasing doses are depicted in Fig.1.a) and b), respectively.Our results confirm what is normally observed on power MOSFETs after -irradiation: ionizationdamage affects the SiO2 layer of the MOS structure causing the build-up of trapped charge, a growthof the amount of interface traps and an increase in the number of bulk oxide traps [2-7]. Ionizationcreates electron-hole pairs in the dioxide; electrons quickly move under the electric field influence to

Fig. 1: The sub-threshold transcharacteristics a) and the IDSS-VDSS curves for VGS= -10V b), at increasing doses.

the contacts, on the other hand, the holes, having a very low effective mobility, may be trapped in theoxide. Some holes slowly diffuse towards the Si-SiO2 interface, where they accumulate and captureelectrons and create interface defects which work as traps for electrical charges depending on thepolarity of the applied gate bias. The fixed charges trapped in the oxide alter the promptness of theinversion layer build-up. The monotonic decrease in the threshold voltage reported in Fig.1.a) denotesa positive net trapped charge. The slope variation of sub-threshold current indicates also the trappingof charges at oxide-silicon interface [8]. The reduction of the breakdown voltage, observed in Fig.1.b)at increasing doses, is consistent with the mechanism described in [5] which attributes to the positivecharges trapped in the field oxide the modification of the surface potential at the body-drain junctiontermination and the consequent reduction of the curvature radius of the depletion region.In Fig.2, we report the mean value of the on-resistance measured on each group of samples as afunction of the dose. The figure shows no significant changes in the resistance values. Indeed, TIDdegrades the carriers surface mobility and causes the increase of the channel resistance which is asmall amount of the total on-resistance in our devices. Instead, the main component of the on-resistance, which is due to the bulk region, is not affected by TID. In fact, at the maximum dose of10KGy[SI] used in our experiments, the amount of displacement damages induced by γ-irradiation,which cause the reduction of the carriers mobility in the bulk region, can be neglected as compared tothe surface changes [13].

Fig. 2: The mean value of the Ron vs -ray dose. Tab. 1: Bias voltages at which the failure appears.

In the second experimental phase, both fresh and γ-irradiated devices were exposed to a beam of 79Brions at 155 MeV.The charge generated into the device as a consequence of an ion impact is partly collected at theexternal leads where it produces a current pulse that is recorded by a fast sampling oscilloscope; thedrain current waveforms are acquired and the related charges are numerically computed. A statisticalprocedure, allows us to obtain compact information regarding the acquired data in terms of their meanand variance. A detailed description of the heavy ion irradiation procedure is reported in [11].During each irradiation, the device under test (DUT) is biased at constant VDS and VGS. For-irradiated devices, VGS was set to a negative value for compensating the effects of the threshold shift(see Fig. 1.a). We used the minimum VGS, in absolute value, able to guarantee normally off operationof the DUT. For each device several tests were repeated at increasing VDS until Single Event failurehad been detected. The VDS at which damages or failures were observed at the drain and gate sides arereported in the second and third columns of Tab. 1, respectively.All the fresh devices tested experienced a single-event burnout (SEB) at VDS between 100V and 110V.The SEB nature of the failure is confirmed by the registration during the irradiation of a single verylarge current pulse like the red one in Fig.3. In this case, the SEB pulse was clipped in order to protectthe instrumentation. In the figure the typical safe pulses (black curves) normally recorded at the samebias condition are also reported for comparison.Table 1 shows that the increase of the dose, previously absorbed by DUT during -irradiation, causes areduction of the Single Event failure voltage which goes down to 40V-45V for the 9.6kGy dose. Forthese devices no large drain current SEB pulse was registered during the irradiation. Moreover, largeincreases of both drain and gate leakage currents measured after the failure indicates serious damagesboth at drain and gate sides of the DUTs thus making difficult to distinguish if the failures must beassociated to a SEB or a SEGR (single-event gate rupture).Let us first consider the behavior of the γ-irradiated devices observed during the heavy ions exposuresat VDS lower than the SE failure voltage. This behavior is very similar to the one of the fresh samples.In fact, the amounts of charge collected at the Br ions impacts are practically independent of theabsorbed dose as indicated in Fig.4, where the mean values of the collected charge are reported as afunction of the VDS applied during the irradiation. In the figure, we report five groups of points eachreferring to a single device exposed at a different γ-ray dose. Each point of each group corresponds to

TID Drain Damage Gate Damage

0Gy Vds=100V-110V Vds=100V-110V1600Gy Vds=60V-70V Vds=60V-70V3200Gy Vds=40V-50V Vds=40V-50V5600Gy Vds=45V-50V Vds=40V-45V9600Gy Vds=40V-45V Vds=40V-45V

a different value of VDS applied on the device during the irradiation and indicates the mean value ofthe collected charge measured on a population of about 1500 current pulses recorded during the Brirradiation. The figure shows that TID induces no substantial changes in the charge collected during Brimpacts. This result, together with the invariance of the on Ron (see the comments on Fig.2), indicatesthat the silicon part of the device (source, body, drain, parasitic BJT, etc.) is weakly affected by γ-irradiation. 3-D finite element SEE simulations, not reported here for brevity, have confirmed thisconclusion.Let us now consider the behavior of the device at the biasing conditions where failures occur. Thescatter plots of the collected charge vs. the current peaks associated to 1500 events registered duringBr irradiation, at VDS=40V, are reported in Fig. 5 for a fresh device (a), 3.2 (b), 5.86 (c) and 9.6 (d)kGy -irradiated samples. After the Br irradiations, at the test conditions of Fig. 5, the fresh devicesurvived whilst the other samples failed.The scatter plot of Fig.5 (a) obtained for irradiation of fresh devices shows two populations of events.The shapes of the typical current pulses are depicted by the two insets of the figure. Both of them havea faster rise up and a slower fall down. The more probable population, with pulses with larger peaks(the light blue curve on the right side of the plot), though, has a current fall faster than the other one(the dark blue curve on the higher-left side of the plot). The scatter plots of Figs. 5 (b), (c) and (d)show that the second population of events on the lower-left side of the plot becomes much moreprobable for -irradiated devices. Moreover, pulses belonging to such population exhibit a current tailwhich becomes more pronounced at increasing doses that, in turns, causes the increase of the averagecollected charge. Finally, in Figs. 5 (c) and (d) one red current curve is also reported. We attributethese curves to the single events that have caused the devices destruction. They are characterized by a

Fig. 3: The SEB current pulse (red curve). Fig. 4: The mean value of the generated charge vs. VDS

Fig.5: Scatter plots of the collected charge vs. current peaks associated to the events registered during Br irradiation,at VDS=40V, for a fresh device (a), 3.2 (b), 5.86 (c) and 9.6 kGy (d) -irradiated samples

relatively small value of the current which, though, remains practically constant after the impact thusindicating that a regenerative effect has been induced in the device.

The proposed failure mechanismWe propose here a new failure mechanism to interpret theexperimental data presented in the previous section.The elementary cell of a power MOSFET is reported inFig. 6 where the equivalent circuit including the parasiticelements of the device is depicted too.The appearance of the current tail evidenced in Fig. 5 for -irradiated devices is consistent with the activation of theparasitic bipolar transistor Q1. We propose here that suchactivation is caused by the conduction of the portion of thegate over the body P+ region where the capacitance CGB islocated. In fact, after an ion impact, a large electric field isapplied across the gate oxide [11]. The duration of thisstimulus is prolonged by the gate capacitance so that a largevoltage is applied for a quite long time on the gate oxide. The reduced isolation characteristics of thisoxide due to -irradiation permits a current to flow through the oxide and then through the distributedbase resistance RB1-RB2 of the parasitic transistor. The emitter junction becomes forward-biased andcauses electrons to be injected in the drain region for the whole duration of the gate capacitancedischarge thus justifying the current tails registered in the second population current pulses. Furtherdetails will be supplied in the final paper.

ConclusionsAn experimental study is presented dealing with the effects of the combined -ray and heavy ionirradiation on the sensitivity to Single Event Effects of 200V power MOSFETs. It is demonstrated thatthe voltage at which SE failures take place during 79Br irradiation in -irradiated devices is much lowerthan the SEB voltage observed on the fresh device. This indicates that the degradation of the gateoxide due to -irradiation severely worsen the SEE sensitivity of the commercial power MOSFETtested. A new physical mechanism has been proposed to explain the SE failure of the tested samples.

AcknowledgmentsThe work is done on behalf of the INFN Apollo collaboration, and supported by the INFN, Istituto Nazionale diFisica Nucleare. The authors wish to thank all the other members of the INFN Apollo collaboration: M.Alderighi, M.Citterio, M. Lazzaroni, M. Riva, INFN Milano, Milan, Italy, A. Costabeber, A. Paccagnella, F.Sichirollo, G. Spiazzi, M. Stellini, P. Tenti, INFN Padova, Padua, Italy, S. Baccaro, INFN Roma, Rome, Italy, P.Cova, N. Delmonte, A. Lanza, R. Menozzi, INFN Pavia, Pavia, Italy. Moreover, the authors wish to explicitlythank Dr. S. Baccaro, from ENEA, Ente per le Nuove Tecnologie, l'Energia e l'Ambiente, for the extraordinarysupport for the experimental irradiations. Special thanks to the staff of the heavy beam accelerating facility of theLaboratori Nazionali del Sud, INFN, Catania, Italy and its director, Dr. Giacomo Cuttone.

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Fig. 6: Elementary cell of a power MOSFETwith the equivalent circuit including the

parasitic elements of the structure