Latent Reliability Degradation of Ultra-thin Oxides After Heavy Ion and Fray irradiation

B. Wang'*, J.S. Suehle', E. M. Vogel', J.R. Conley, Jr.3, C.E. Weintraub4, A.H. Johnston3, and J.B. Bernstein'

' Center for Reliability Engineering University of Maryland, College Park, MD 20742

Phone: 206-834-1 107, Fax: 206-5 17-5262 binwang@impinj.com

* Semiconductor Electronics Division, National Institute of Standards and Technology Gaithersburg, MD 20899

NASA Jet Propulsion Laboratories, California Institute of Technology, Pasadena, CA 9 1 109

Department of Electrical Engineering, North Carolina State University, Raleigh, NC 27695

Abstract-We studied the effects of heavy ion and y-ray irradiation on radiation-induced leakage current (RILC) and time-dependent dielectric breakdown (TDDB) life distributions of ultra-thin oxides (<3.5 nm). Thermal annealing experiments were carried out on irradiated devices to study the nature of RILC. TDDB experiments were also performed on irradiated devices with thermal annealing. Our results show that gamma irradiation had a minimal effect on intrinsic TDDB lifetime of ultra-thin oxides. However, heavy ion irradiation induced RILC and substantially reduced the lifetime of ultra-thin oxide films. Thermal annealing experiments suggests that RILC is due to trapped holes. Removal of RILC (and holes) do not improve TDDB lifetime.

Keywords- reliability, TDDB, RILC, Heavy ion irradiation, gamma irradiation

1. INTRODUCTION

The space radiation environment poses a threat to microelectronics in in-orbit space systems. Long-term exposure to ionization can reduce the operational lifetime of microelectronics as a result of energetic carriers or atomic displacement. The basic effect of ionizing radiation in SiO, films is understood in terms of dielectric charging effects resulting in threshold voltage and transconductance

shifts of the MOSFET transistor. Recently, it was reported that the breakdown of thin oxides could be triggered by a single impinging ionizing particle [ 1-21. Single Event Gate Rupture (SEGR) of irradiated thin gate oxides with heavy ions was also investigated extensively by Johnston, et al. [2]. Many groups reported an increase in leakage current of irradiated oxide films and Ceschia [3] reported radiation- induced soft breakdown (RSB) of 4-nm thick oxides. The effects of ionizing radiation in ultra-thin oxides is becoming a concern, particularly due to the observation of the effects described above. RILC can be modeled as inelastic Trap Assisted Tunneling (TAT) via neutral traps in the oxide similar to that found after electrical stress. Many conclusions have been reached by studying the correlation between RILC and stress-induced leakage current (SILC). The nature of RILC after heavy ion irradiation has not been well studied, though some researchers [4] believe that it was due to defects created by trapped holes. The effect of heavy ion irradiation on the wear-out and long- term reliability of ultra-thin oxides has not been well characterized. In this work, radiation-induced leakage current (RILC) of ultra-thin oxides was studied for heavy ion irradiation as a hnction of dose. Constant voltage time- dependent dielectric breakdown (TDDB) experiments were also performed to study if the heavy ion irradiation affected the long-term reliability of the films.

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Latent Reliability Degradation of Ultra.thin Oxides After Heavy Ion and g-ray irradiation Wang et al.

2. EXPERIMENTAL DETAILS

Test capacitors were provided by two sources. The gate electrodes were phosphorus-doped polysilicon. The back of the wafers were coated with aluminum to reduce series resistance. All of the tests were conducted at the wafer level using a probe station. MOS n-channel test capacitors had an oxide thickness of 3.2 nm, an area of 1x10” cm2, and were used in the y-ray irradiation study. MOS n-channel test capacitors had an oxide thickness of 3 .Onm, an area of 1x1 0-4 cm2, and were used in the heavy ion irradiation study. Oxide thickness was determined by high frequency C-V curves with quantum simulation of substrate quantization and poly gate depletion effects.

The NIST gamma cell radiation source was used and provided a dose rate of approximately 6.5 KGray/hr. Irradiations were performed with the gate electrode unbiased. Heavy ion beams were supplied by the Texas A&M University Cyclotron. Testing was conducted using 823 MeV 12’Xe (LET=60 MeV-cm2/mg) with a total dose up to 1 x 1 Os ions/cm2. Exposures were conducted at normal incidence with the capacitor gates tied to ground. TDDB tests of devices after y-ray and heavy ion irradiation were performed at a voltage of-5.0 V and -4.9 V respectively. Weibull distributions for TDDB tests were obtained for both the non-irradiated and irradiated devices. Weibull statistics were used to determine the time-failure (t )for a population of samples usually having 16 capacltors. With the initial breakdown event defined as oxide breakdown, breakdown was determined by the combination of a step increase in the I-t curve and a sudden jump of the current noise.

Thermal annealing experiments following irradiation were also carried out to study the properties and origin of the defects induced by heavy ion irradiation. The irradiated devices with an oxide thickness of 3 .O nm and an area of 4 x 1 0-4 cm2 were placed on a hot plate at a temperature of 350 “C for 90 minutes. TDDB experiments were then performed at a gate bias of -4.7 V.

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3. RESULTS AND DISCUSSIONS

A. Radiation Induced Leakage Current (RILC) Figure 1 shows the RILC following the gamma

exposure for the 3.2 nm thick oxides. Note that the leakage current does not significantly increase until a total dose of 15 Mrad (Si) is achieved.

le-7 i I

to,= 3.2 nm, p-well, A=lX10-3 cm2 1 le-8 -

le-9 -

2 ie-10

le-11- a 0 le-1% Pre Irradiation c Q) 0 Post 1 Mrad(Si) 8 7 Post 5 Mrad(Si)

le-1 v Post 15 Mrad(Si) rn Post 20 Mrad(Si)

le-15 0 Post 30 Mrad(Si)

h

e

1 e-1 d I I

-4 -3 -2 -1 0 Gate Voltage (V)

Fig. 1: Radiation-induced Leakage Current (RILC) characteristics following y-ray irradiation for the 3.2 nm oxides.

Figure 2 shows the RILC of the 3.0 nm thick oxides following the heavy ion irradiation. The RILC is clearly increasing as a function of dose. At the dose level of los ions/cm2 the RILC produced is almost 2 orders of magnitude higher than the non-irradiated devices. The dramatic increase in RILC as a function of dose may suggest the occurrence of RSB.

lm-7 , .--. le-8 ~

le-9 . le-10

3 le-11-

S‘ le-12

le-15

1 e-1 4-

le-15

to, = 3.0 nm, Area = 1 x IO4 cm2

023 MeV Xe

Fig. 2: RILC characteristics following heavy ion irradiation for the 3.0 nm thick oxides.

B. TDDB

Figure 3 shows the Weibull distributions after y- ray irradiation as a function of dose for the 3.2 nm thick oxides. These breakdown statistics indicates no trend or statistical difference for the lifetime distributions.

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Wana et al.

1

e o

5 7- v z +

-2

-3

le15 -

- - 20 Mrad(Si) 3 - 30 Mrad(Si)

10 100

. tbd (S)

Fig 3: TDDB Weibull distributions for the 3.2 nm oxide stressed at 5.0 V following gamma irradiation

Figure 4 shows typical stress current versus time (I-t) curves for devices subjected to different doses of heavy ion irradiation. Note the reduction in breakdown time as the dose is increased. One also needs to notice that the I-t curve for the device after 1x10' ions/cm2 exhibited noisy characteristics immediately when the stress was applied. Both RILC and I-t indicated that some devices experienced RSB at the highest dose of 1x10' ions/ cm2.

Latent Reliability Degradation of Ultra-thin Oxides After Heavy Ion and g-ray irradiation

tOX=3.0 nm A = 1 xl0-4cm2 12'Xe (LET=BO) Vg=4.9 V

1 e-6

+- 1x10~ ions/cm2 0 1x10~ ions/cm2 9- 1 XI o7 ions/cm2

1 10 100 1000

Time (s) Fig. 4: Typical I-t curves for irradiated devices with heavy ion, under CVS as a function of does.

Figure 5 shows the Weibull lifetime distributions for the heavy ion irradiated devices. As shown for the typical I-t curves, it was observed that there was a significant effect of heavy ion irradiation on the reliability of thin gate oxides. The same phenomena was observed for a different device area under a different stress bias (see Figure 7).

Area=lxl0'cmZ stressvoltage=4.9v 1

10 loo lam loo00

nme (s) Fig 5: Weibull plots of 3.0 nm oxides following heavy ion irradiation

C. Thermal Annealing

Figure 6 shows the effect of thermal annealing on the RILC of irradiated samples with a heavy ion exposure of 1x106 ions/cm2. It was observed that the EULC was removed after thermal annealing. It should be mentioned here that devices receiving this level of dose did not exhibit soft breakdown characteristics. The thermal annealing suggests RILC is due to trapped holes [ 6 ] . Additional TDDB experiments were performed to investigate if the thermal annealing removed latent reliability damage induced by the heavy ions.

Q3.0 nm A=lx104cmQ

823 "%e (LET = 60)

v. Post-Irradiation -U- - Post-Thermal Anneal

Fig. 6: Effect of thermal annealing on RILC for 3.0 nm oxides

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. Latent Reliability Degradation of Ultrathin Oxides After Heavy Ion and g-ray irradiation Wan! et al.

1 ,

Figure 7 shows the Weibull lifetime distributions of non-irradiated and irradiated devices after thermal annealing. A reduction in kD with an increasing irradiation dose was observed similar to figure 5. The results shown in figure 7 indicate that removal of RILC does not improve reliability.

Area = 4 x lo4 cm2 Stress Voltage = 4.7 V.

-3 ~

I

4 + 1 x10*ions/cm2 1 x io7 ions/cm2 ... o

--F non-Irradiated

0

Time (5)

Fig. 7: Weibull plots of irradiated devices with thermal annealing under CVS at a bias of -4.7 V.

CONCLUSIONS

The effect of gamma and heavy ion irradiation on the reliability of ultra-thin oxide films was investigated. It was found that gamma irradiation had a minimal effect on intrinsic TDDB lifetime of ultra-thin oxides. However, heavy ion irradiation induced RILC and substantially reduced the lifetime of ultra-thin oxide films. A fluence of 1x1 Os ions/cm2 is only 10 “hits” for 1x1 0-4 cm2 device area. Thermal annealing experiments suggests that RILC is due to trapped holes. Removal of RILC (and holes) do not improve TDDB lifetime. When ionizing radiation occurs inside the oxide, the excitation of valence-band electrons to the conduction band creates electron-hole pairs that are mobile in the electric field. The difference between the effect of y-ray and heavy ion irradiation on oxide lifetime may lie in the fact that heavy ions lose more energy per nanometer in the oxide than gamma radiation. As a result, more energetic electrons are created to contribute to generating wear-our defects. The results of this work indicate a potential latent reliability hazard in a radiation environment even when devices are powered off.

REFERENCES

[ 13 F.W. Sexton, D. M. Fleetwood, M.R. Shaneyfelt, P. E. Dodd, and G. L. Hash, “Single Event Gate Rupture in Thin Gate Oxides,” IEEE Trans. Nucl. Sci., vo1.44, pp. 2345-2352, 1997.

[2] A. H. Johnston, G. M. Swift, T. Miyahira and L. D. Edmonds “Breakdown of gate Oxides During Irradiation with Heavy Ions,” IEEE Trans. Nucl. Sci., vol. 45, pp. 2500-2508, 1998.

[3] M. Ceschia,A. Paccagnella, S. Sandrin, G. Ghidini, J. Wyss, M. Lavale, and 0. Flament “Low field Leakage Current and Soft Breakdown in Ultra-Thin Gate Oxides After Heavy Tons, Electrons or X-ray Irradiation,” IEEE Trans. On Nucl. Sci., 47, pp. 566-573, 2000.

[4] L. Larcher, A. Paccagnella, M. Ceschia, and G. Ghidini, “A Model of Radiation Induced Leakage Current (RILC) in Ultra- Thin Gate Oxides,” IEEE, Tran. On Nuc. Sci. vol. 46, no. 6, 1999.

[SI J.S.Suehle, E.M. Vogel, B. Wang, and J. B. Bernstein, “Temperature Dependence of Soft Breakdown and Wear-out in Sub-3 nm SiO, Films,” in Proc. IRPS, pp. 33-39,2000.

[6] C-H. Ang, C-H. Ling, Z-Y Cheng, S-J. Kim, and B-J. Cho, “A Comparative Study of Radiation- and Stress -Induced Leakage Currents in Thin Gate Oxides,” Semicond. Sci. Technol., 15, pp. 961-964,2000.

*Bin Wang is currently with Impinj, Inc., Seattle, WA

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