1-4244-0449-5/06/$20.00 ©2006 IEEE IPEMC 2006

Influence of Proton Irradiation dose on the Performance of Local

Lifetime Controlled Power Diode with

Proximity Gettering of Platinum B.D. Han, D.Q. Hu, S.S. Xie, Y.P. Jia, B.W. Kang

College of Electronic & Control Engineering, Beijing University of Technology, Beijing 100022, China

Abstract－ Based on proximity gettering of platinum by vacancy defects which are induced by proton irradiation, local platinum doping is obtained. It is used as a local lifetime control technology in high-power diodes. The theoretical dependence of electrical active Pt concentration Cpts on irradiation induced defects concentration Cv is also studied. The diodes’ reverse performance parameters are measured. They are functions of irradiation dose. For low proton irradiation dose, the gettered quantities of platinum by irradiation induced defects are enhanced when the irradiation dose increases. This can improve the performances of the device. But when the proton irradiation dose is high enough, the peak concentration of gettered platinum tends saturation. Further more, for deep junction device, the side effect brought by high dose irradiation will decrease the platinum gettering efficiency and the performances of the device degenerated under higher irradiation dose. On the base of theoretical study, we improved the device structure and manufacture process, a higher peak concentration is obtained. The recovery speed has been improved further. Keywords: localized platinum doping; platinum gettering; irradiation dose; gettering efficiency; power diode;

1．INTRODUCTION

High power P-i-N diodes play an important role in most power circuits. The criteria for suitable diodes in various applications are low static and dynamic losses, low recovery charge and a soft recovery behavior. For these purpose, various structure and process methods are developed to optimize the devices’ performances [1-2]. It is generally recognized that one of the most efficient concepts for an improved diode performance is the appropriate design of the on-state excess carrier distribution.

Theoretically, the most efficient method for plasma distribution control is local lifetime engineering [3-5].

The local lifetime control is realized mainly by light ion irradiation[6].Normally, the effective defect level induced by irradiation defects is at ET=EC-042eV. (ET and EC are conduct band level and any trap level, respectively).It’s quite near the center of the band gap. Thus, the device with local lifetime controlled by such trap level has the disadvantage of high reverse leakage current and bad thermal stability[7, 8]. Comparatively, the recombination center generated by platinum is localize at ET=EC-0.23eV. It is far from the center of the band gap. The device which uses platinum as a lifetime killer has the advantage of low leakage current and good high temperature stability and reliability. It is the key process to obtain localized platinum doping for the manufacture of fast soft recovery performance diode with low leakage current.

On the base of proximity gettering of platinum[9, 10], we realize localized platinum doping in the local lifetime control power diode. The influence of proton irradiation dose on the performances of the device is examined. With this understanding, the device structure and platinum doping are improved.

2．EXPERIMENTS

The first group of sample diodes has P+-N--N+ structure with 72 µm depth junction. On P+ side, PtSi layer is produced by sputtering and sintering (450ºC, 60 min). Then the devices were irradiated with proton at doses ranging from 5×1011~2.7×1014cm-2 through the anode. The energies 2.8 MeV was chosen. The sample devices were annealed at 700ºC for 15min to promote the platinum diffusion from the PtSi contact to the position of the

maximal damage. To improve the switch speed, the emitter efficiency

controlled structure is used in the second group samples. The diodes have P+PNN+ structure. The P+ region has surface concentration of 1.8×1019cm-3 with 0.9µm deep. P region are 4.43×1017cm-3 and 4.5µm respectively. On P+ side PtSi layer is produced by sputtering and sintering (300ºC, 60 min). Then the devices were irradiated with proton doses range of 1×1013～5×1014cm-2 through the anode. The implantation energies 550 KeV was chosen. The sample devices were annealed at 700ºC for 15min.

3． RESULTS AND DISCUSS

(1) For the First Sample Diodes

Fig. 1 Dependence of S and trr on irradiation dose

for first group samples The dependence of S and trr on irradiation dose

for the first group of sample is depicted in Fig.1. Where VRR is reverse recovery voltage, IF is forward leakage current, trr is reverse recovery time and S is softness factor. From fig.1, one can see that the reverse recovery time decreases greatly as the irradiation dose increases at low proton irradiation dose. As the proton irradiation dose approaches 1013cm-2, the reverse recovery time change gently with the irradiation dose. When the proton irradiation dose is more than 2×1013cm-2, the variation tendency reverses——the reverse recovery time increases as proton irradiation dose increases. Normally, trr is determined by the carrier lifetime and the lifetime is determined by localized active platinum concentration. Dependence of trr on irradiation dose embodies the

dependence of gettered active platinum on proton irradiation dose. That is the gettered active platinum concentration increases with proton irradiation dose at low irradiation dose and decreases with proton irradiation dose at high irradiation dose.

Generally, there are two configurations of platinum in silicon, the interstitial configuration (PtI) and the substitutional (PtS) one. The PtI guides the transport until it is transformed into PtS. In the later configuration, platinum is electrically active with an acceptor level at ET=EC-0.23 eV and a donor level at ET=EV +0.32 eV.(EC , EV , and ET denote the energy positions of the conduction band, valence band, and any deep level, respectively.) They are good recombination centers as a lifetime killer. The transformation to the preferred substitutional configuration can be realized by the Frank–Turnbull mechanism:

SPtPt ⇔+ VI （1）

where the interstitial platinum recombines with a vacancy (V), or via the kick-out mechanism:

IPtPt SI +⇔ （2）

where the interstitial platinum kicks out a silicon lattice atom thereby creating a silicon self-interstitial(I).

In irradiation silicon, there are various void defects (include void-void pair, void-O, void-impurity and so on). The Frank–Turnbull mechanism dominates the transformation [13-14]. For reaction（1）, there exists a equilibrium for the reactant concentration, that is:

PtSVPtI CTKCC )(=× （3）

Where CPtI, CV and CPtS are concentration of interstitial Pt, voids and substitution Pt, K(T) is a temperature related parameter.

Also:

PtSPtPtI CCC −= （4）

where CPt is total concentration of Pt, it relates to platinum solubility and voids degree in silicon under certain temperature. Using（3）and（4）, we obtain:

+

−=V

PtPtS CTKTKCC

)()(1 （5）

Equation (5) shows that the concentration of substitutional PtS is mainly determined by the void concentration when CPt is determined. The more CV is, the more CPtS is. If CV is high enough, CPtS tends to CPt, but not decreases as CV increases.

The experiment result of active Pt degree decreases as irradiation dose increases has been observed in alpha particles implantation[12]. It was attributed to the fact that in-diffusing platinum interstitials were transformed into the substitutional configuration on their way to the region of maximal damage. As the concentration of defects towards the surface increased, so did the number of interstitial platinum atoms that was transformed within this region. The degradation of softness factor also supports this point. Another possible reason is that the high dose irradiation implantation may cause voids-Pt complexes which have different activity from substitutional Pt. We discussed this in other place.

To validate equation (5) and above analysis, we design second group of diodes with shallower junction. Meanwhile, high temperature Pt pre-diffusion is made to increase CPt.

Fig. 2 Dose dependence of the peak concentration

of the active Pt with H+ irradiation [9] Reference [9] gave the dose dependence of peak

concentration of platinum with H+ irradiation followed by

700ºC annealing (see Fig.2). We can extract the value of parameter K(T) at T=700ºC. It is about 5×1017cm-3. Put it in equation（5）, we can see that when voids concentration reaches 5×1018cm-3（the corresponding proton irradiation dose is 1013cm-2）, the substitutional Pt is at 0.90 peak value. Under such consideration, the proton irradiation dose range 1×1013～5×1014cm-2 is chosen for second experiment.

(2) For the Second Sample Diode

(a)

(b)

FIG. 3 Concentration vs. depth profiles of the platinum acceptor trap at ET=EC -0.23eV

(a) 550keV/ 1×1013cm-2 proton irradiation followed by 700°C/15min annealing process.

(b) 2.8MeV/2.7×1013cm-2 proton irradiation followed by 700ºC/15min annealing process.

First, the influence of pre-diffusion of Pt on the

gettered peak concentration of active Pt is compared. It is depicted in Fig.3. It’s very clear the pre-diffusion sample has higher gettered peak concentration. That is the existence of voids make CPt be higher than the platinum solubility in silicon. It can be improved by platinum pre-diffusion before proton irradiation.

The measured results of sample performance parameter are list in table-1. It’s very clear that when the irradiation dose is more than 1×1013 cm-2 , its increasing influences little on the improvement of reverse recovery time. The gettering saturation tendency is very obvious. Comparing the data with Fig.1 one can see that performance of the second group sample is superior than the fist group’ s. (1) Under the same irradiation dose, the reverse recovery time of second group sample is almost half of the first group’s. (2) The phenomenon what reverse recovery time increases with irradiation dose under high irradiation dose eliminates. Shallower junction and platinum pre-diffusion are effective. (3) Lower reverse recovery time trr with higher softness factor indicates that the reduction of trr is due to the improvement of gettered platinum peak concentration caused by platinum pre-diffusion, not pre-diffusion itself.

Tab.1 Datasheets of diode performance parameters for

the second samples

5．CONCLUSION

On the proximity gettering of platinum by proton irradiation induced defects, we realize localized platinum

doping in the local lifetime control power diode. The relationship of substitution platinum concentration, total platinum concentration and void concentration is given. The variation tendency of substitutional platinum with irradiation dose is studied. The experiments indicates that when irradiation dose approach 1×1013cm-2 the gettered substitution platinum has saturation tendency. This limits the improvement of switch speed using such local lifetime control. Using shallow junction anode and platinum pre-diffusion, the faster and softer diodes are obtained.

REFERENCES

1 H. Schlangenotto, J. Serafin, F. Sawitzki, and H. Maeder, “Improved recovery of fast power diodes with self-adjusting p emitter efficiency,” IEEE Electron Device Letters, vol. 10, pp. 322-324, July 1989.

2 S. Sawant and B. J. Baliga, “Comparative study of high voltage (4kV) power rectifiers PiN/MPS/SSD/SPEED” , IEEE International Symposium on Power Semiconductor Devices and ICs (ISPSD), Toronto, Canada, 1999: pp.153-156

3 O. Humbel, N. Galster, T. Dalibor, T. Wikstrom, F.D. Bauer, and W. Fichtner, “Why is Fast Recovery Diode Plasma-Engineering With Ion-Irradiation Superior to That With Emitter Efficiency Reduction,” IEEE Transactions on Power Electronics, Vol. 18, pp. 23-29, Jan. 2003.

4 E. Napoli, A. G. M. Strollo, and P. Spirito, “Numerical Analysis of Local Lifetime Control for High-Speed Low-Loss P-i-N Diode Design,” IEEE Transactions on Power Electronics, Vol. 14, pp. 615-620, April 1999.

5 J. Vobecky, P. Hazdra, and J. Homola, “Optimization of Power Diode Characteristics by means of Ion Irradiation,” IEEE Transactions On Electron Devices, Vol. 43, pp. 2283-2289, Dec. 1996.

6 P. Hazddra, J. Vobecky, and K. Brand, “Optimum Lifetime Structuring in Silicon Power Diodes by means of Various Irradiation Techniques,” Nuclear Instruments and Methods in Physics Research, Vol. B186, pp. 414-418, Jan. 2002.

7 R. Siemieniec and J. Lutz, “Possibilities and Limits of Axial Lifetime Control by Radiation Induced Centers

parameters Proton Doses

IR

(µA) trr

(ns) ta

(ns) S

1×1013cm-2 0.564 252 126 1.00

8×1013cm-2 0.838 246 148 0.66

5×1014cm-2 1.04 242 140 0.73

Note: ta is reverse falling time; IR is reverse leakage current. Test condition: IR@VR=100(V), T=125°C; trr@IF=1A, VR=30V, di / dt=-20A/µs, RT

in Fast Recovery Diodes,” Microelectronics Journal, Vol. 35, pp. 259-267, March 2004.

8 P. Hazdra, J. Vobecky, N. Galster, and O. Humbel, “New Degree of Freedom in Diode Optimization: Arbitrary Axial Lifetime Profiles by means of Ion Irradiation” Proceeding of the 12th Intern. Symp. on Power Semiconductor Devices & ICs, pp. 123-127, 2000.

9 D.C. Schmidt, B. G. Svensson, N. Keskitalo, S. Godey, E. Ntsoenzok, J. F, Barbot, and C. Blanchard, “Proximity Gettering of Platinum in Proton Irradiated Silicon,” Journal of Applied Physics, vol. 84, pp. 4214-4218, Oct. 1998.

10 A. Cacciato, C. M. Camalleri, G. Franco, V. Raineri, and S. Coffa, “Efficiency and Thermal

Stability of Pt Gettering in Crystalline Si,” Journal of Applied Physics, Vol. 80, pp. 4322-4327, Oct. 1996.

11 D. C. Schmidt, B. G. Svensson, J. F. Barbot, and C. Blanchard, “Stability of Proximity Gettering of Platinum in Silicon Implanted with Alpha Particles at Low Doses, ”Appl. Phys. Lett. Vol. 75, pp. 364-366, 1999.

12 D. C. Schmidt, B. G. Svensson, S. Godey, E. Ntsoenzok, J. F. Barbot, and C. Blanchard, “The Influence of Diffusion Temperature and Ion Dose on Proximity Gettering of Platinum in Silicon Implanted with Alpha Particles at Low Doses,” Appl. Phys. Lett. Vol. 74, pp. 3329-3331, May 1999.