Total Dose Response of HfSiON MOS Capacitors

Total Dose Response of HfSiON MOS Capacitors

D. K. Chen, F. E. Mamouni, R. D. Schrimpf, D. M. Fleetwood, and K. F. Galloway

Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN

G. Lucovsky and S. LeeDept of Physics and Material Science,

North Carolina State University

B. Jun and J. D. CresslerDept of Electrical Engineering and Computer Science,

Georgia Institute of Technology

Introduction

Quantum mechanical tunneling significant in SiO2

Charge trapping in HfSiON (Amorphous and Crystalline HfO2) [G. Luc

ovsky et al., IEEE Trans. Nucl. Sci., vol. 53, pp. 3644-3648, 2006.]

Total dose irradiation and constant voltage stress

TID-induced charge trapping compared to previous Hf silicate devices

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Studies by Lucovsky et al. have shown that Hf silicates phase separate after ~900C anneal. The charge trapping properties of amorphous hafnium silicate and crystalline HfO2 are drastically different. We report on the charge trapping characteristics of two types of HfSiON films. An amorphous film and one which have been shown by Lucovsky's group to contain nano-crystalline HfO2.

Device Description

(HfO2)0.4(SiO2)0.4(Si3N4)0.2 Low-Si3N4 K = 14.6 tox = 15 nm

(HfO2)0.3(SiO2)0.3(Si3N4)0.4 High-Si3N4 K = 12.7 tox = 13 nm

Equivalent oxide thickness ~ 4 nm

The samples were prepared by remote plasma enhanced chemical vapor deposition (RPECVD), with ~0.6 nm SiON layer; Rapid thermal anneal done at 900 oC

HfSiON

Al Gate

Silicon Substrate

Al Substrate Contact

Vg

FOX FOX

SiON

Experimental Details

1-MHz C-V measurement at a rate of 0.05 V/s; Irradiated with 10 keV X-ray at a rate of 31.5 krad(SiO2)/min

Interface trapped charge density (Nit) unchanged after radiation; prerad Nit ~ 3.8 x 1012 cm-2

Relatively small gate leakage current (<10 pA @ Vfb + 1 V) unaffected by radiation

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The gate leakage current was also measured using an Agilent 4156B Parameter Analyzer. The gate leakage currents were approximately less than 10 pA at flatband + 1V. They were unaffected by radiation. The devices were irradiated incrementally to 5 Mrad(SiO2) at a dose rate of 31.5 krad/min, and C-V characteristics were taken after each dose. We use high freq capacitance-voltage measurements to characterize these devices before and after irradiation and stress. We estimate the net oxide-trap and interface-trap charges from the changes in flatband and midgap voltages. This figure shows a device irradiated to 5 Mrad, with slight shift to the left. The interfacial traps remained unchanged after radiation; this is evident in the shape of the CV curve, where a stretchout will be observed in the presence of TID-induced interfacial charge trapping. We’ve estimated the prerad Nit~… This relatively large interface state density is likely the cause for the lack of change after radiation.

Improvement Relative to Hf Silicate Devices

TID-induced charge trapping factor of 17 less relative to previous Hf silicate devices [J. A. Felix et al., IEEE Trans. Nucl. Sci., vol. 49, pp. 3191-3196, 2002]

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After irradiating with various gate bias conditions, we found that these devices show very small levels of charge trapping. Here we compare the radiation response of a device that showed the largest -ΔVmg to the worst case for Hf Silicates of previous work. We found ~ a factor of 17 smaller Not.

Midgap Voltage Shifts

Weak bias dependence Relative low mobility of holes in HfSiON similar to hafnium silicates

[J. A. Felix et al., IEEE Trans. Nucl. Sci., vol. 49, pp. 3191-3196, 2002]

Electron trapping TID-induced and substrate injection at large positive biases

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Now we look at the radiation response in more detail. The left figure summarizes the ΔVmg for different bias conditions. First you’ll notice that from gate voltages -1.5 to 1 V, there is very small difference in the midgap voltage shifts. This weak bias dependence has been observed before in Hf Silicates, and is caused by the insensitivity of the charge centroid to the applied bias. This is attributed to the relative low mobility of holes in hafnium silicates. Also the high-k films like hafnium silicates naturally have a much higher defect densities and fixed charges from processing compared to SiO2. So these effects possibly are the reasons for the weak bias dependence. The only exceptions are for larger positive voltages (1.5 and 2 V). Where we found large positive shifts in midgap voltages, which indicate electron trapping. The figure on the right shows the changes in Vmg during each radiation dose. It also compares the trend in midgap voltage shifts for two bias conditions. Whereas in the negative bias case, the ΔVmg decreases monotonically with total dose, the positive bias exhibits increasing electron trapping. Finally the trapped holes balance and outnumber the trapped electrons which results in the rebound. Through constant voltage stress experiments we believe that the electron trapping is not only radiation-induced, which has been observed in HfO2 devices, but also from substrate injection.

Comparison of Low- and High-Si3N4 Devices

Crystalline HfO2 in the low-Si3N4 film contains grain boundaries

Negative gate bias show similar charge trapping levels

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Next we compare the two types of films. The figure on the left compares the positive bias case and the right compares the negative bias case. Clearly for positive gate biases, the high silicon nitride film does not exhibit the elevated levels of electron trapping as in the low silicon nitride film. We believe that this is attributed to the difference in film composition. Dr. Lucovsky’s group has found that during high temperatures (~900C) anneals, the hafnium silicon oxynitride film chemically phase separate, forming nano-crystalline HfO2, which contains grain boundaries. O vacancies and interstitials can gather at the grain boundaries and are favorable trap sites for electrons and holes. However the larger defect densities only seem to have a significant effect for large positive biases. The negative bias conditions produced similar levels of charge trapping, as shown in the right figure.

Constant Voltage Stress (Substrate Injection)

O vacancies and interstitials in the Low-Si3N4 film favorable for electron trapping from substrate injection

Defects located near the interface an important source for electron trap sites

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We further examine the charge trapping nature of these films by constant voltage stress tests. For these tests, we apply a constant +1.5 V to the gate and stressed for times comparable to the times required to reach each radiation dose. The CV is swept after each stress interval. I should also note that sweeping the CV repeatedly produces curves with negligible changes. Both films show increasing electron trapping with increase in stress time, but the low silicon nitride film which contains crystalline HfO2 still exhibits elevated levels of electron trapping. Under the positive gate voltage, the p-type silicon surface is biased to inversion. The electrons that accumulate near the oxide/Si interface can easily tunnel through to traps located near the interface. We always find a sudden increase in midgap voltage after the first stress followed by more gradual increases. This suggests that the electron trap sites are likely located mostly near the interface, possibly the interface between the high-k dielectric and the SiON interfacial layer. So these sites are filled quickly after the initial stress. The radiation+stress also showed larger electron trapping especially after the initial dose. So we believe that the combination of radiation-induced electrons and electrons from substrate injection take place in the low silicon nitride films.

Constant Voltage Stress (Gate Injection)

Level of charge trapping much less compared to substrate injection

Saturation occurs much earlier than substrate injection case

Electron injection from the gate and movement of charge centroid

Comparison to Hf Silicate Devices

Electron injection balances positive charge trapping for positive and negative biases

∆Not ~ factor of 17 smaller at 1 Mrad(SiO2)

1.2 x 1012 cm-2 Hf silicate 7.1 x 1010 cm-2 HfSiON

Reduced charge trapping indicates superior HfSiON film qualities (smaller bulk trap density) and improvements in processing

Conclusions

HfSiON show drastic improvements in total dose irradiation hardness relative to Hf silicate devices

Crystallization creates grain boundary defects especially vulnerable to stress-induced electron trapping

Reduced charge trapping indicates smaller bulk defect density in HfSiON film relative to Hf silicates

Acknowledgements

Dr. Lucovsky’s group at NC State University for providing the samples

Air Force Office of Scientific Research (AFOSR) for supporting this work

Total Dose Response of HfSiON MOS Capacitors

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