research article proton irradiations on sj hv power ...advances in electricalengineering t : main...

Research ArticleProton Irradiations on SJ HV Power MOSFETs toRealize Fast Diode Devices

Ignazio Bertuglia, Giuseppe Consentino, and Michele Laudani

Power Transistors Division, STMicroelectronics, Catania, Italy

Correspondence should be addressed to Michele Laudani; [email protected]

Received 4 June 2014; Revised 31 July 2014; Accepted 19 August 2014; Published 3 November 2014

Academic Editor: Changhwan Shin

Copyright © 2014 Ignazio Bertuglia et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

This paper studies the effects of proton irradiations on Super Junction High Voltage power MOSFETs to realize transistors withfast diode. Experiments were performed on a sample of 600V power MOSFETs and achieved results were compared to standardirradiated devices by electrons.

1. Introduction

In many modern applications and, in particular, when usingfull bridge converters, intrinsic diode of HV power MOSFETis even utilized to free wheel the current in the circuitwithout using any other external component [1–7]. Intrinsicdiode of a power MOSFET is implemented by consideringthe body-drain junction of the same device. In fact, whenthe device is switched off and the transistor is polarizedin reverse mode, the current flows from the source to thedrain terminals working in third quadrant configuration. Insuch operating conditions, intrinsic diode needs to guaranteeparticular performances and features. In particular, diodeneeds to be fast in order to reduce switching losses whenoperating switching frequencies increase. A diode becomesfast when chargesmoved during rapid variation of𝑉ds againsttime are quite low as shown in Figure 1. In fact, in the exampleshown in Figure 1,MOSFET is rapidly switched off.When𝑉dsincreases, drain current decreases down to negative valuesbecause charges stored in the body-drain junction need tobe discharged. Standard devices (red curve) reach the lowestpeak of drain current (𝐼rm) because the charge stored inthe junction (𝑄std) is higher than fast devices (green curve).That turns in a higher time to discharge (𝑡rr) and, thus, itmakes the devices slower during commutation. To improvethe performances of intrinsic diode, special processes need tobe implemented.The typical action consists in the irradiationof the body-drain junction by energetic electrons in order to

create suitable damage in the reticle. Such damage introducesdeep energetic level traps in the silicon band-gap. Such trapscapture the carriers lowering the quantity of charges movedduring the fast transition from on to off states as describedabove. Typical irradiation doses are in the range of 5–50MRad when considering power MOSFETs. Afterwards, athermal process needs to be implemented in order to activatethe traps. However, irradiation by considering electrons canbring issues related, for example, to the quality of gate oxide.In fact, interface traps states can be created increasing drain-source leakage current.

Recently, a new technique was implemented to realizedevices with fast diodes based on proton irradiation. In fact,protons can be introduced in the body-drain junction of apower MOSFET’s structure only concentrating the effect onthe body-drain border junction differently from the electronswhich are implanted on the entire structure.

2. Protons Irradiation, Experiment Setup,and Test Vehicles

A series of experiments were implemented by changing thedose of protons irradiation and acting on thermal processto evaluate the best solution and to compare the resultswith the standard electron irradiation process. The testswere performed by considering a tandem accelerator withprotons at energies in the ranges of some MeV useful to

Hindawi Publishing CorporationAdvances in Electrical EngineeringVolume 2014, Article ID 423803, 5 pageshttp://dx.doi.org/10.1155/2014/423803

2 Advances in Electrical Engineering

tQirr

Qstd

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Ids irrIds std

Figure 1: Example of commutation of the intrinsic diode and com-parison with a fast diode.

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Figure 2: Example of cross section of a Super Junction HV powerMOSFET. (1) Gate planar oxide; (2) body region; (3) EPY driftregion; (4) back-drain contact; (5) source well-source contact; (6)polysilicon; (7) current flow; (8) metal; (9) drain column; and (10)region to be damaged by proton.

Table 1: Main characteristics of test vehicle.

Device type SJ HV power MOSFETs usedBVdss [V] 𝐼𝑑 continuous @ 25

∘C [A] 𝑉th [V]n-ch 600 35 4

reach the depth inside of body-drain junction of the devices.The irradiation has been performed in air where at 1 cm ofdistance is located the wafer under test.Wafers weremountedon a frame connected to a precise step by step motor utilizedto move it. To perform these experiments, a family of SJ HVpower MOSFETs of 600V was considered (see in Table 1 thepeculiarities of the adopted test vehicle). Typical cross sectionof a Super Junction device is shown in Figure 2 where thedepth of implant is even highlighted.

Irradiation of protons was implemented either in thefront or in the back side of the wafer considered, reachingdifferent depths in the region included between the body-drain junction and the same epitaxial layer under the pillars.Based on simulation activity, it was established that fluencyof protons on the silicon needs to be in the range of 1011 cm−2and 5∗1012 cm−2 in order to compare with effects of electronsin silicon. It is necessary to observe that protons’ irradiation

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03 green 1011 proton/cm2Wf03 yellow 5 × 1011 proton/cm2Wf03 purple nonimplantationWf03 orange 1012 proton/cm2Wf03 red 5 × 1012 proton/cm2Wf

Body diode recovery waveforms2F6B on wafer with proton implantation

Wafer ID = 3302758 03F7,energy = 2.17MeV, side = front, depth = 40𝜇m

VDD = 60V, di/dt = 50A/𝜇s, Isd = 10A, tIsd = 50𝜇s

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Figure 3: Example referred to data of Table 3.

implies a certain tail of damage along the initial path of thetrajectory that could affect the electrical performances of thedevices. For example, if the implantation is performed by thetop of the wafer, the tail of damage involves essentially thefirst micrometers of silicon layers in the top of the die. Suchvacancies could even affect the reliability of the gate oxide.These considerations will be more evident after studying theexperimental results on the wafer irradiated on the frontof the wafers. After the proton implantations, wafers undertests were thermally treated in furnaces and, afterwards, theyfinished the process (back end). Before packaging the dices,wafers were tested to evaluate the performances in terms of𝑡rr (diode recovery time), 𝑄rr (diode recovery charge), and𝐼rm (reverse peak current during the recovery time of diode).Figure 3 and Tables 2, 3, 4, and 5 report the data when apolarization which equals 60V is applied.

As it is possible to see from the above data, by increasingproton dose 𝐼rm, 𝑡rr, and𝑄rr decrease and the diode switchingperformances improve. However, results are different if com-pared to several trials and with standard irradiated deviceswith electrons. In fact, considering the devices irradiatedaround the body-drain junction with a concentration of1012 cm−2, 𝑄rr is comparable to standard irradiated transis-tors. Instead, considering devices with an implanted dose of5∗1012 cm−2,𝑄rr is lower than standard irradiated transistors.

Considering devices irradiated in the epitaxial region with aconcentration of 5∗1012 cm−2,𝑄rr is quite similar to standardone. It is necessary to highlight that when the dose increases,a lowering of breakdown voltage is observed together withan increasing of drain-source leakage currents. Such resultsare more evident when dice implanted from the top areconsidered. This phenomenon can be explained taking into

Advances in Electrical Engineering 3

Table 2: Main results on wafer bench—implantation on the frontEPI layer.

Dose [cm−2] SJ HV power MOSFETs results𝐼rm [A] 𝑡rr [ns] 𝑄rr [nC] BVdss [V]

5 ∗ 1012 5.1 120 354 5001 ∗ 1012 9.2 189 936 6105 ∗ 1011 11.5 237 1466 6501 ∗ 1011 21.1 400 4475 680No irradiation 29.6 534 7582 680Std. electrons 8.7 184 877 660

Table 3: Main results on wafer bench—implantation on the frontbody-drain junction.



Table 4: Main results on wafer bench—implantation on the backbody-drain junction.



account that tails of the damage, located near the body-drainjunction, change the charges balance inside the space chargeregion.

Comparable results can be achieved by considering pack-aged devices when current is switched off at around 27A (dif-ferent operating condition compared to the previous test). InTables 6–9 and Figures 4, 5, 6, and 7 data regarding dynamicmeasurements of packaged devices are shown.

3. Conclusions

This paper has analyzed the effects on SJ HV power MOS-FETs of proton irradiations to realize intrinsic “fast diode”components. Intrinsic diode needs to be fast in order todecrease switching losses and to increase operating switchingfrequencies without any issue related to dV/dt. In order toimprove the performances of intrinsic diode, typical actionconsists in the irradiation of the body-drain junction byenergetic electrons. Electronic irradiations can sometimesbring issues related, for example, to the quality of gate oxide

Table 5: Main results on wafer bench—implantation on the backEPI layer.



Table 6: Main results on packaged powerMOSFETs—implantationon the front EPY layer.

Dose [cm−2] SJ HV power MOSFETs results𝐼rm [A] 𝑡rr [ns] 𝑄rr [nC]

5 ∗ 1012 13 162 11801 ∗ 1012 18 203 18705 ∗ 1011 24 267 34601 ∗ 1011 36 448 8612Std. electrons 18 180 1800

Table 7: Main results on packaged power MOSFETs—implantationon the front body-drain border.


5 ∗ 1012 10 116 6301 ∗ 1012 19 233 23705 ∗ 1011 23 254 31331 ∗ 1011 35 414 7626Std. electrons 18 180 1800

Table 8: Main results on packaged power MOSFETs—implantationon the back body-drain border.


5 ∗ 1012 18 190 17561 ∗ 1012 20 228 24105 ∗ 1011 26 286 40561 ∗ 1011 34 406 7400Std. electrons 18 180 1800

Table 9: Main results on packaged powerMOSFETs—implantationon the back EPY layer.


5 ∗ 1012 19 205 20581 ∗ 1012 23 253 31105 ∗ 1011 27 290 42001 ∗ 1011 35 431 8330Std. electrons 18 180 1800

4 Advances in Electrical Engineering

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because they are spread out in the entire wafer volume.Instead, protons are localized in a specific part of the die and,in particular, in the epitaxial layer slightly below the body-drain junction. A series of experiments was performed byconsidering a suitable proton irradiator on samples of SuperJunction High Voltage power MOSFETs with a breakdownvoltage of 600V. Irradiations with protons were implementedeither in the front or in the back side of the wafers with

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projected range in the region between the body-drain junc-tion and the same epitaxial layer under the pillars. Basedon simulations, it was established that fluency of protons onthe silicon needs to be in the range of 1011 cm−2 and 5 ∗1012 cm−2 to compare with effects of electrons irradiation

on silicon. After the proton implantations, wafers undertests were thermally treated by considering dedicated processflow.Therefore, several measurements were performed either

Advances in Electrical Engineering 5

considering tested dice on wafers or considering static anddynamic characterizations after the packaging. Experimentalresults show that 𝐼rm, 𝑡rr, and 𝑄rr decrease by increasingproton doses. However, it is necessary to highlight thatwhen the dose increases, a lowering of breakdown voltage isobserved together with an increasing of drain-source leakagecurrents.

Therefore, it is possible to assume that the best trial, interms of reverse recovery performance, was the one with pro-tons implanted on the back side with a dose of 5 ∗ 1012 cm−2.Nevertheless, breakdown voltage degradation implies that amore optimized solution still has to be found by investigatingdifferent doses combined with thermal annealing processfine tuning, just to improve𝑄rr/𝑡rr performances without BVdegradation.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

References

[1] P. Hazdra, J. Vobecký, and K. Brand, “Optimum lifetime struc-turing in silicon power diodes by means of various irradia-tion techniques,” Nuclear Instruments and Methods in PhysicsResearch B: Beam Interactions with Materials and Atoms, vol.186, no. 1–4, pp. 414–418, 2002.

[2] J. Lutz, R. Siemieniec, F. J. Niedernostheide, and H. J. Schulze,“Characteristics of irradiation induced deep levels in silicon,” inProceeding of the Solid State Chemistry and Modern Micro- andNanotechnologies Conference, Kislovodsk, Russia, 2005.

[3] M. Schmitt, H.-J. Schulze, A. Schlogl et al., “A Comparison ofElectron, Proton and Helium Ion Irradiation for the Optimiza-tion of the CoolMOSTM Body Diode,” Infineon TechnologiesAG, AI PS DD, Balanstr. 59, D-81541 Munich, Germany, 2011.

[4] V. Benda, “Design considerations for fast soft reverse recoverydiodes,” inProceedings of the 15th EuropeanConference on PowerElectronics and Applications, Brighton, UK, September 1993.

[5] P. Lévêque, P. Pellegrino, A. Hallén, B. G. Svensson, and V.Privitera, “Hydrogen-related defect centers in float-zone andepitaxial n-type proton implanted silicon,” Nuclear Instrumentsand Methods in Physics Research, Section B: Beam Interactionswith Materials and Atoms, vol. 174, no. 3, pp. 297–303, 2001.

[6] T. Mizushima, M. Nemoto, H. Kuribayashi, T. Yoshimura, andH. Nakazawa, “Inhibiting effect of middle broad buffer layerdiode using hydrogen-related shallow donor on reverse recov-ery oscillation,” in Proceedings of the 22nd International Sym-posium on Power Semiconductor Devices & IC’s (ISPSD ’10), pp.115–118, June 2010.

[7] R. Siemieniec, F.-J. Niedernostheide, H.-J. Schulze, W.Südkamp, U. Kellner-Werdehausen, and J. Lutz, “Irradiation-induced deep levels in silicon for power device tailoring,”Journal of the Electrochemical Society, vol. 153, no. 2, pp.G108–G118, 2006.

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