Effects On Power Transistors Of Terrestrial Cosmic Rays: Study, Experimental Results and Analysis G. Consentino, M. Laudani, G. Privitera
C.Pace, C.Giordano, J.Hernandez and M.Mazzeo
Power Transistor Division STMicroelectronics Catania, Italy
[email protected]
Department of Computer Science, Modeling, Electronics and Systems (DIMES) University of Calabria Rende (CS), Italy
[email protected]
Abstract— High voltage power transistors used in inverters for photovoltaic panels and avionic applications are naturally exposed to Terrestrial Cosmic Rays, or Atmospheric Neutrons, which are known to induce catastrophic failures such as burnouts or gate ruptures. Accelerated tests on different power MOSFETs and IGBTs technologies were performed in ANITA neutron facility, at Uppsala, Sweden. Experimental details and results will be presented and discussed.
I.
INTRODUCTION
Nowadays, the most common components involved in energy conversion are the power transistors. In recent years, a huge increment of the demand was observed in the segment of applications for renewable energy such as photovoltaic, eolic, cogeneration, etc. In photovoltaic, power MOSFETs are largely utilized in inverters managing up to 3kW of electric power and the reliability of the systems need to be guaranteed for 20 to 25 years based on international and regional regulations. Typical devices used in such inverters are High Voltage Super Junction (SJ) power MOSFETs or Trench Field Stop (TFS) IGBT, with breakdown voltage between 600V and 650V. When used in these applications, converters are located outdoors, between sea level and 2000m of altitude. In these conditions, the power transistors are exposed to a natural radiation environment. In fact, when galactic cosmic rays coming from the outer space reach the earth’s atmosphere, they collide with the nuclei of nitrogen and oxygen atoms and create cascades of secondary radiation, i.e. terrestrial cosmic rays (see Fig. 1) which are composed of different kind of particles including protons, muons, pions and neutrons. On the contrary of charged particles, neutrons do not undergo to Coulomb scattering and represent more than 95% of total particles at sea level [1, 2]. Literature measurements state that the flux of neutrons is around 0.004 cm-2 s-1, spread in a wide range of energies, but concentrated especially around 1MeV (fast neutrons) and around 100 MeV (ultra-fast neutrons) [3]. However, flux and spectral distribution of terrestrial neutrons can change depending on altitude (one order factor higher at 2000m altitude), longitude, latitude, and solar activity.
978-1-4799-2325-0/14/$31.00 ©2014 IEEE
Figure 1. Terrestrial neutrons cascade
Today, the effect of the terrestrial neutrons on power transistors functionality are becoming a big concern for its impact on operating life of power conversion systems. In fact, under specific operating conditions, a single neutron-silicon nucleus interaction may induce catastrophic failures, as Single Event Burnout (SEB), Single Event Gate Rupture (SEGR) or both [4-6]. These phenomena are generally classified as Single Event Effects (SEEs) and they have been studied a lot since the late 80s, thanks to the great interest of the aerospace community into heavy ions effects on memories, digital ICs and power transistors. Neutron and heavy ion SEEs are conceptually similar (therefore, detailed information can be found here [7-11]). The only difference is in the ionization mechanism. In fact, neutrons are able to ionize in an indirect manner, by means of charged recoils generated as a result of
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nuclear reactions with the silicon nuclei. Several reaction have been identified as hypothetical responsible for neutron SEEs [12]. For example, in the energy range between 1MeV and 10MeV, the silicon nuclei can react with neutrons, giving an alpha particle and magnesium recoil or a proton and aluminum recoil [13]. Of course, given the large energy distribution of the neutron flux, many other interactions can occur, such as spallation, radioactive decay, and may also involve other materials inside the devices, in addition to silicon. Both for heavy ions and neutrons, the ionization produces a track, around which it develops an electron-hole pair distributions. SEB and SEGR normally affect power MOSFETs operating in switching mode, polarized in the off-state, with applied bias near to the rated breakdown voltage. After ionization, electron-hole pairs are produced and then separated by the electric field. Electrons drift toward to drain and holes to the body-source contacts respectively. Many other carriers are generated by impact ionization, enhancing the resulting filament current. In SEB, body-source current is so high, in a reduced area to turn on the parasitic bipolar transistor that is an inherent part of the vertical power MOSFET structure. A positive feedback is established because electrons are injected from emitter (source) into collector (epi-drain), new holes are generated by impact ionization and drifted toward the bodysource enhancing the base current and the base-emitter voltage difference. The result is a strong drain current increase and thermal runaway. Considering SEGR, holes accumulate in the neck region under the gate causing a localized electric field increment in the oxide over its dielectric breakdown value. All this also applies to IGBTs with the difference that SEB is caused by the parasitic thyristor turn on [14]. Besides SEB and SEGR, neutrons can also induce nondestructive cumulative damages, e.g. on-resistance, breakdown, threshold voltages variations and leakage currents increase [15]. In this paper, several accelerated neutron irradiation tests were performed on power MOSFETs and IGBTs to assess their terrestrial cosmic rays vulnerability in terms of Mean Time To Failure (MTTF) and SEB threshold voltages, i.e. the maximum “safe” bias in off condition. II. A.
EXPERIMENTAL DETAILS
Devices Under Test (DUTs) Two families of ST Microelectronics power transistors, with different technologies, packages and breakdown voltages, were tested: HV SJ power MOSFETs and TFS IGBTs. The complete list of DUTs is shown in Tab. 1. A brief description is as follows. Regarding power MOSFETs, SJ II and SJ V are standard technologies that differ in terms of cell/stripe densities. In particular, SJ V has a higher density and, therefore, a greater current capability and a lower on resistance. SJ II fast is derived from SJ II, but also has an internal fast diode, suited for high switching frequency applications. As the name suggests, SJ II improved is the newest generation of SJ II standard technology. The improvements consist in the specific on resistance halving. Mesh new gen devices have a hybrid technology which combines some SJ and planar features. Moreover, they have
higher breakdown voltages compared to SJ standard devices. As for IGBTs, TFS 1 and TFS 2 differ in breakdown voltages (650 V and 1200 V, respectively). TABLE I.
LIST OF TESTED POWER TRANSISTORS Normalized die size
Package
0.22
T0-220
0.49
T0-247
0.49
T0-247
1
T0-247
MOS 800 V SuperMesh, newest generation
0.49
T0-220
MOS 950 V SuperMesh, newest generation
0.49
T0-220
IGBT 650 V TFS
0.44
T0-247
IGBT 1200 V TFS
0.41
T0-247
Technology
Description
MOS SJ II improved MOS SJ II std MOS SJ II fast MOS SJ V std MOS MESH new gen 1 MOS MESH new gen 2 IGBT TFS 1 IGBT TFS 2
MOS 600 V MDMesh II Plus, newest generation MOS 600 V MDMesh II, standard generation MOS 600 V Fast diode (FD) MDMesh II MOS 650 V MDMesh V, standard generation
A 24h high Temperature burn-in [16] test was carried out for all DUT to prevent possible early failures due infant mortality. Furthermore, electrical characterization was performed before and after (on the survived devices) the experiments in order to monitor the cumulative effects of neutrons. B. Neutron beam facility The experiments were conducted at The Svedberg Laboratory (Uppsala, Sweden), using the ANITA facility [17]. As it is possible to see in Fig. 2, it can provide a neutron beam with atmospheric-like spectrum in the range 1-150 MeV, with a maximum acceleration factor of 300 million. This feature makes the ANITA facility particularly suited to investigate terrestrial cosmic rays effects on electronic devices. Beam operations were controlled remotely in the counting room with the BoRN [15] system, which allowed us to start the beam and set a fluence value: thanks to several real-time monitors, the system was able to determine if the set point fluence had been reached and to automatically stop the beam. The fluence was set to 2.8·107 cm-2, i.e. the same value of terrestrial neutron fluence with E > 10MeV at sea level for 250 years or, equivalently, at 2000m altitude for 25 years [3]. Finally, we used an average flux of 1.01·105 cm-2 s-1, corresponding to an acceleration factor of 2.84·107. C. Test circuit SEB measurements represent a non-trivial task because they are inherently destructive and need a large number of samples to obtain a reasonable statistics. In 1987, a “circumvention” technique had been proposed [18] (and recently adopted in [19]) in which the event of SEB is quenched before degenerating into a destructive failure, treated as a pulse and counted. At the end of the experiment, the cross section is calculated as the ratio between the number
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of pulses and the fluence of the run depending on the test voltage, while the failure rate is estimated as the product of the cross section and the predicted flux in service. It should be noted that a constant failure rate implies an exponential statistic of failures. Basing on this assumption, it is possible to calculate the MTTF as the failure rate reciprocal. Although this technique is recognized as a standard [20] circumvention network design is empirical because strict criteria have not been defined yet. In addition, it’s somewhat difficult to distinguish a “real” SEB pulse, i.e. a phenomenon which would cause catastrophic failure without circumvention from another pulse that would not cause SEB just arising from the injected charge collection. For these reasons, in our experiment, we chose to design the test circuit in order to obtain destructive failures with the goal of testing 8 DUT simultaneously.
A typical SEB event is detected as a sudden drop in the drain (collector) voltage, because of drain-source (collectoremitter) resistance collapse and thermal runaway triggering [4, 5, 7, 8]. As the radiation-caused switch-on starts, the capacitor C discharges releasing the previously stored energy and causing the transistor burnout. After that, the resistor R limits the post-SEB current to a few milliamps avoiding its over-damaging (explosion) and the shutdown of the power supply. Capacitance values of 5µF were chosen to guarantee an adequate available SEB energy in order to identify clearly visible damaging traces on the die surface with the microscope without resorting to the liquid crystal technique. TABLE II.
CUMULATIVE PDF LOOKUP TABLE t
F(t)
t1
1/Ntot
t2
2/Ntot
tn
n/Ntot
Finally, the test circuit has been splitted in two separate boards: the first one, where only DUTs were arranged, was placed in the neutron beam path and the second one, with all other components, was kept protected in a borated paraffin shield cabinet, at a “safe” distance from the DUTs, in order to minimize the exposure to scattered neutrons. D. MTTF evaluation To assess the MTTF of various experiments, statistical analysis was conducted according to the Weibull model, which provides a cumulative Probability Density Function (PDF) equal to: Figure 2. Differential flux comparison between atmospheric neutrons (according to std JESD89A) and ANITA facility
(1) where, the constants k and β are the scale and the shape parameter, respectively. Depending on the scale parameter value, the Weibull model describes a specific portion of the well-known bathtub curve, representing the failure rate versus time. In fact:
Figure 3. Basic element with 1 DUT of test circuit schematic
As depicted in Fig. 3, DUTs are polarized in off condition with gate and source (emitter for IGBT) electrodes shorted, while drain (collector) voltages are supplied by a High Voltage Source Measurement Unit (SMU), through separate RC filters. Drain (collector) voltages are also attenuated with a voltage divider and continuously measured by a DAQ board, with a 100 Hz sampling frequency.
•
for β1, failure rate increases with time (wear out period).
When k and β are known, the MTTF can be calculated with the following expression to: (2) where, Γ(x) is the gamma function [21]. Operationally, we first evaluated the failure times t1, t2,..., tN according to the criterion laid down in section 2C; subsequently, we calculated
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the discrete functions F(t) as in Tab. 2, where Ntot is the total number of devices tested in one run. Then we plotted and fitted data with (1), obtaining k and β. Some fitting examples can be found in Fig. 4 and Fig. 5. 1.0
(4) where, Γ ’ is the first derivative of the gamma function.
MOS SJ II improved Vtest=480V (80%BVdss)
0.9 0.8 0.7
F(t)
0.6 0.5 0.4 0.3 0.2
Cumulative failure fraction
0.1
Best Weibull fit
0.0 0
25
50
75
100 125 150 t ·AF[years]
175
200
225
250
Figure 6. Typical burnout caused by SEB
Figure 4. Fitted data for MOS SJ II improved
Figure 7. Minimum voltage at which the devices under test fails
III. Figure 5. Fitted data for IGBT TFS 2
Finally we computed MTTF with equation (2) and assessed the propagation of fit parameters uncertainty Δk and Δβ on MTTF, under the assumption of uncorrelated variables (verified with correlation factors given by the fit): (3)
RESULTS AND DISCUSSION
Several failures were observed during the experiment. In the failed DUTs, post irradiation electrical characterizations evidenced a strong increase in both Idss and Igss leakage, as if they were practically shorted. The post-failure microscope analysis clearly evidenced burnout point sites, with a random distribution over the entire active area of the devices, as it is possible to see in Fig. 6. This scenario is consistent with primary SEB damages, followed by SEGR. On the other side, no significant variations were noticed into electrical parameters of survived DUTs. In order to compare different technologies, we can use 2 criteria: SEB threshold voltage (the minimum experimental voltage at which failures are detected, the higher the better)
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TABLE IV.
and MTTF values (the higher the better) at the same percentage of the breakdown voltage. TABLE III. Device
Vtest [V]
MOS SJ II improved
480 (80% BVdss) 480 (80% BVdss) 480 (80% BVdss) 455 (70% BVdss) 600 (75% BVdss) 760 (80% BVdss) 585 (90% BVdss) 960 (80% BVdss)
MOS SJ II std MOS SJ II fast MOS SJ V std MOS MESH new gen 1 MOS MESH new gen 2 IGBT TFS 1 IGBT TFS 2
Device
Vtest [V]
480 (80% BVdss) 480 (80% BVdss) 600 (75% BVdss) 585 (90% BVdss) 960 (80% BVdss)
Failed DUTs
MTTF [years]
β
6/7
167±23
0,91±0,26
MOS SJ II improved MOS SJ II fast MOS MESH new gen 1
5/7
253±24
0,66±0,09
IGBT TFS 1
6/7
89,7±4,1
1,22±0,09
IGBT TFS 2
5/6
89,8±7,7
0,43±0,07
5/8
24,4±1,8
1,07±0,13
7/7
20,6±1,0
1,95±0,26
6/7
27,4±2,0
1,36±0,17
8/8
35,8±2,6
1,27±0,17
MTTF E WEIBULL PARAMETER β
FAILURE RATE Failure Rate [FIT]
700±94 1272±58 4678±345 4166±304 3188±231
A further performance indicator for the device reliability is the failure rate, expressed in FIT (failures per billion device hours), which can be calculated as the MTTF reciprocal for those experiments with a β value close to unit. Approximate results are listed in Tab. 4. These values should be compared with a maximum acceptable failure rate of 100 FIT, as in [19]. IV.
CONCLUSIONS
Even at sea level, power transistors are subjected to a natural neutron exposition, due terrestrial cosmic rays. Under specific bias conditions they can induce catastrophic failures as SEB and SEGR which can bring serious issues in these application fields, e.g. photovoltaics, where reliable HV of SJ power transistors need. In this work, power MOSFETs and IGBTs of different technologies were irradiated and tested at ANITA neutron facility. By virtue of the good energy spectrum correspondence between this source and the atmospheric neutrons, it is possible to assert that the testing performed is particularly significant.
Figure 8. MTTF values with a test voltage equal to 80% of the breakdown voltage
As for SEB threshold, most of the irradiated devices started to fail with a test voltage between 70% (MOS SJ V std) and 90% (IGBT TFS 1) of breakdown voltage, as showed in Fig. 7. MTTF values are listed in Tab. 3. In the worst cases, we have values of about 20 years. DUTs performances, in terms of MTTF at the 80% of the breakdown voltage, are compared in Fig. 8. Considering power MOSFETs, the most rugged technology is the MOS SJ II standard technology. In particular, the fast and the improved versions present significantly lower MTTF values. Therefore, the inherent fast diode seems to worsen performances. Considering IGBTs, however, most rugged technology is TFS 1.
Regarding the experimental results, the majority of failures occurred for test voltages ranging between 70% and 90% of the breakdown voltages. Microscopic analysis revealed evident burnouts, randomly located inside the device active area, while electrical characterization tests evidenced that the DUT’s terminals were practically shorted. Therefore, these failures were classified as SEB and/or SEGR. Weibull statistic analysis of failures was also made in order to evaluate MTTF and failure rate. In a first approximation, the more rugged device is the IGBT TFS 1 with the breakdown voltage of 650 V. Concerning power MOSFETs, standard technologies has superior reliability in terms of MTTF and failure rate referring to the new generation ones. On the other hand, MESH devices have interesting features, which combine the best of planar and SJ technologies.
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