Reverse Con Igbt

Proceedings of 2004 International Symposium on Power Semiconductor Devices & ICs, Kitakyushu

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1200V Reverse Conducting IGBT Hideki Takahashi, Aya Yamamoto, Shinji Aono and Tadaharu Minato Mitsubisbi Electric Corporation, Power Device Works 1-1-1 Imajuku-Higashi, Fukuoka-City, Fukuoka, 819-01 Japan Phone : +SI-92-805-3332 Fax : +81-92-805-3779 Email : [email protected]

Abstract This report premieres to reveal the newly developed 12OOV Reverse Conducting IGBT (RC-IGBT) by using our thin wafer pmcess technology. The fabricated RC-IGBT operates as both IGBT and free wheeling diode (FWD). Adopting a Helium-in?ldiation carrier lifetime conhulling techno log^ to our RC-IGBT, the essential characteristics of the fabricated 12oOV/lOOA chip can achieve a level comparable to those of conventional 3rd generation PT-type IGBT and FWD pai. The d e off with Vce(sat) and tum-off loss and the correlation between Vf and Err is almost same to the conventional 3rd generation F'T-type IGBT and FWD pair. Introduction From the time of the advent of IGBT, a Free wheeling Diode (FWD) has been desired to build into one IGBT chip. A conventional IGBT with an opfimizea trade-off comlation between fonvard voltage drop Vce (sat)and lumoff energy loss Eoff required a hvo-layered thick epitaxial punch through type collector shudure in a thick wafer PIW%S,so the wafer procffsing of the backside collector side was very di5cult Therefore, one-chip IGBT with built-in FWD was impossible to realize. Recently, as the development of thin wafer process technology pmgresses, the wafer pnress for the collector side becomes feasible. We investigated the IGBT with built-in FWD, and we call it a R e v m Conducting IGBT (RC-IGBT). Then we fabricated 12OOV RC-IGBT based upon Mitsubishi's commercial 1200V Light h c h Through type Carrier Stored T m c b gate Bipolar Transistor (LPTCSTBT) wafer process [I]. We found that the characteristics of 12OOV one-chip RC-IGBT are enough to replace a pair of IGBT and anti-parallel FWD.

conduction loss (VF), and FWD's revme recavq (Em). We cannot discuss all the candidates in detail, hut will focus on the CS typedevice up and explain the process to ensure each fundamental step by step via both simulationand experiment ch&stic First of all, in this collector shorted deice structure, we did device simulation to estimate the function of an IGBT and a built-in FWD in the wafer t!ickness of 2oOpm. As the simulation referencefor an ullimate IGBT and FWD function, a diode having a d o d e s i d e uniform N'emitter layer is the goal for IGBT, and another diode having an anodeside uniform P-base layer is the goal for FWD. We simulated the fonvard voltage drop of a diode (Vg in each direction by vwing both the d o and widths of the backside CS-Zebra-pattemed N-layer and P-layer. When positive voltage is biased to the backside, a diode having uniform front-side N' emitter conducts the forward direction current from backside h p e P layer to bnt-side N^ emitter, and Vf of this diode corresponds to Vce(sat) of 1GBT. When negative voltage is biased to the backside, an another diode having a uniform front-side P-base conducts the backward CUITent h m the front-side P-base to the backside h p e N-layer, and Vf of this diode corresponds to Vf of built-in FWD. Fig.1 shows the dependenceof Vf of these two diodes upon the ratio and width of P-region and N-region. In this simulation, as long as the ratio of P region and N region of backside is equal, IGBT and built-in FWD can operate without problems, for all the size ofN and P region.

Device Concept and S t ~ c t u ~ t A h several shuctures were investigated as RC-IGBT candidates before this papa, we chose a collector-shorted (CS) device shucture, in which an N and P Zebra-striped-pattem is formed on a backside of wafer [Z], instead of the conventional uniform plane two-layered N-buffer and P-collector shudure. This was chosen over other candidates because of a g d agreement for four directionstrade-off relationship among IGBT's forward voltage drop, 1GBT's turn-off capability, FWD's forward

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Fig. 1 The dependence of Vf of two directions (forward and backward) diodes on the ratio and width of P region and N region. In the case of backside P-region and N-region widths are set to

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l o p , the Vf of each direction showed high value. In the case of 2 0 total ~ cell pitch, the backside shorted P-region and N- region cancel the carrier injection each other, so as to increase of Vf beyond acceptable level. But the P-region and N-region widths are set to 4Opm and more, Vf of each direction can be kept the same low level. In the case of 8Opm total cell pitch, the increase ofVf is around 0.5V compared to Vf of the diode with backside 100% P layer and Vf of the diode with backside IOG% N-region. Fig.2 depicts the RC-IGBT device shuchue.

direaion by applying a negative bias voltage to the collector side. In the forward IGBT operation mode, ow fabricated RC-IGBT shows a snapback current-voltage curve, but the snapback voltage (Vsp) is small enough below 3V at 25degC only in a very low current density region, so it seemed to be acceptable. Furthermore, on about 1 an2active area of the chip, and under tht: current density of 100 A/cm2, Vce(sat) of the fabricated RC-IGBT was 2.1V at 25degC in the first quadrant operation, and Vl‘of the fabricatedRC-IGBT in the thiid q u a d “ operation was 1.4V.

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We choose our commercial 12OOV LFT-CSTBT chip for basic structure. The stripe N-region and stripe P-region are independently formed on the wafer backside in the orthogonalassing direction to the wafer h n t side trench-gate stripe direction, instead of forming conventional stacking N-buffer and P-collector layers. The LPT s ” r e wafer thickness is set to 170pm. Both the backside P-region and N-region widths are set to 4Opm each, and the total cell pitch is 8Opm. As shown in Fig.3, the P-region and N-region are formed side by side in the backside structure. N- region P- region

Fig.4 RC-IGBTs forward and reverses output I-V characteristics Tj=25degC

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Fig. 3 cross sectional SCM photograph ofwafer backside structure S C M :ScauningCapacitance Microscope

Experimental Results Fig.4 indicates that the fabricated RC-IGBT operates as both IGBT in the fust quadrant and FWD in the third quadrant, k a m e our RC-IGBT can conduct backward current in the reverse

Fig9 shows the forward output I-V charaaeristia of IGBT fimction of RC-IGBT at +degC, 25degC and 125degC. Fig.5 also shows the result of Non Punch Through IGBT (NPT-IGBT) without an n-buffer layer. The difference of Vce(sat) between RC-IGBT and NPT-IGBT is 0.4 V, which is within the predicted value in the numerical device simulation. As shown in Fig. 5, snapback phenomena were m m d in our fabricated RC-IGBT structure in all the temperatme ranges, but the snapback voltage was 3.6 V. It was almost Same at the (Vsp) even at +de& 25degC condition, and was reduced to less than 1.5V at 125degC condition. It, however, should be emphasized that Vsp might be acceptable for the standard inverter operation because it was existent only in the very low current density operation region and our fabricated RC-IGBT’s Vce(sat) is still low and is just slightly deferent fmm the NPT-IGBT described in detail above, whose Vce(sat) is the lowest h i t of the IGBT operation. On the other hand, Figd shows the forward output I-V characteristics of a built-in FWD function of RC-IGBT sbuctures at +de& 25degC and 125degC. Fig.6 also shows the result of the “uniform plane cathode” diode, which has a uniform backside cathode N-layer for the convenienceof pilot fabrication, i.e. not IGBT but

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a simple MOSFET sbucture by backside Phosphorus ion implantation. In those FWD cases,just same as in tbe f o d IGBT ose,there is not so large a difference, i.e. about 0.2V in the Vf between RC-IGBT’s built-in FWD and the refmced %niform plane cathode” diode. This slight differens was within tbe predicted value in the simulation.

(a) RC-IGBT inductive load tum off

Fig. 5 The forward output characteristicsof IGBT function of RC-IGBT and NPT-IGBT a t 4 de&, 25 degC and 125 de& 0

V,,:2OO[V/div]l,:3O[A/di~]:5OO[ns/div] @) NPT-IGBT inductive load tum off

Fig. 7 Measured inductive load hun-off waveforms at two kind of IGBTs. (a)RC-IGBT (b)NPT-IGBT

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Under the opxation of reverse recovery mode of a built-in FWD bction of fabricated RC-IGBT, the peak revme recovery current (Im) is two times larger than the rated cunent IOOA as shown in Fig.% The measured chip has a uniform backside cathodeN layer as described above. In this measuremenf the gate electrode is shorted to the emitter electrode, to avoid chip deshuction resulting h m an unexpected very large revem m v q current. Since the pbase of IGBT is used as an anode of the diode, this large reverse m v e y current comes 6um the high impurity conmlr&ion of the P-bass Without carrier lifetime control. Then we examined Helium-irradiation technology to reduce the reverse m v e y current as a carrier lifetime wntrol technique.

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Fig.6 The foMard output ChraaCtaiSti~sof D i d fundon of RC-IGBT and uniform &odes d i d a 4 0 de& 25 degC and 125 degC The tibricated RC-IGBT’s fundamental trade-off relationship between Vce(sat) and Eoff can be discussed here. Fig.7 shows measured inductive load m-off waveforms at 125degC without any carrier lifetime control for both RC-IGBT and NTP-IGBT as the reference. This RC-IGBT has shorter tum-off tail and lower tum-off loss compared to conventional NPT-IGBT. 7he trade-off relationship between Vce(sat) and Eoff of tbis RC-IGBT is beaer than that of NPT-IGBT.

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Fig. 8 Reverse w v e v current waveform of an RC-IGBT (a) RC-IGBT inductive load tum on

The correlations between Vf and Irr are shown in Fig.9, for both with and without Helium ion (He? irradiation chip having uniform plane backside cathode N layer, and our commercial -base FWD. He’-inadiated chip having uniform plane backside cathode N layer, which has about 1.0 volt larger Vf compared to OUT commercial FWD, achieves the same level Irr as our commercial -base FWD. 250 200

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so (a) RC-IGBT inductive load turn off Fig. 10 inductive load turn-offand turn-on waveforms of Irr reduced by H e t - M a t e d RC-IGBT

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Conclusion Fig. 9 The correlationsbetween Vf and In

Finally we discuss how to compromise fabricated RC-IGBT’s four directions characteristics, which are Vce(sat) and Eoff in the IGBT mode and Vf and Irr in the FWD mode. Fig.10 shows inductive load turn-off and tum-on waveforms of Im reduced by He+-irradiatedRC-IGBT that are put into both an upper-ann and a lower-ann of a half-bridge circuit Afta He’ had been inadiated, Vce(sat) was i n m e d fiom 2.1 volt to 3.lvolf and Vf was i n m a d h m 1.4 volt to 2.3 volt That increase of l.Ovolt is the same value as the inneased Vf of Het irradiated uniform planar cathode chip. The switching ch;nactaiStics are as follows; turn-m loss of IOOA chip is 142ml, and turn-off loss is 8.99 ml. These values are ahnost same as our commercial-base F’T-type previous 3”le third generation planar gated IGBT, even though its n-body thickness is as large as an NPT type.

We fabricated the 12oOV class RC-IGBT by using our thin wafm process technology and confirmed the fundamental device fundons of RC-IGBT. Adopting a Helium-inadialion carrier lifetime controlling technology to our RC-IGBT, the fabricated RC-IGBT’s fundamental characteristics reached a level comparable to those of conventional PT-tYpe IGBT and FWD pair. We are aiming to f i b e r improve our RC-IGBT to have still bigger Vce(sat) and Irr than the conventional IGBT and FWD, and are aimiig mass-production in near future.

References [I] KNakakirmura et al “Advanced Wide Cell Pitch CSTBTs Having Light

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punch s m 9 ’ pp2n-280, m D of ISPSD2002. [2] W y a m a et al ‘Effects of Shorted CoUeculr on charadaish‘csof IGBTS.” pp. I3 1-136., P” . gs of ISPSD’PO

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