135-baumvol-apl-1998-450 - On the behavior of deuterium in...

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On the behavior of deuterium in ultrathin SiO 2 films upon thermal annealing I. J. R. Baumvol Instituto de Fisica—UFRGS, Porto Alegre, RS, 91540-000, Brazil E. P. Gusev a) Departments of Chemistry and Physics, and Laboratory for Surface Modification, Rutgers University, Piscataway, New Jersey 08855 F. C. Stedile Instituto de Quimica-UFRGS, Porto Alegre, RS, 91540-000, Brazil F. L. Freire, Jr. Departamento de Fı´sica, PUC-Rio, Brasil M. L. Green and D. Brasen Bell Laboratories/Lucent Technologies, Murray Hill, New Jersey 07974 ~ Received 12 September 1997; accepted for publication 25 November 1997 ! Following the observation of the large isotopic effect in D 2 passivated gate dielectrics @ J. Lyding, K. Hess, and I. C. Kizilyalli, Appl. Phys. Lett. 68 , 2526 ~ 1996 !# , we studied the behavior of deuterium in ultrathin SiO 2 films by nuclear reaction analysis techniques. Accurate concentrations of deuterium in the films, deuterium depth distributions, and deuterium removal from the film upon thermal annealing in vacuum have been examined. For D 2 passivated films, we found rather high concentrations of deuterium near the SiO 2 /Si interface, well above both the solubility of deuterium in silica and the maximum concentration of electrically active defects at the interface. Our results suggest a complex multistep mechanism of thermally activated deuterium removal from the film, which probably consists of D detrapping, diffusion, and desorption steps. © 1998 American Institute of Physics. @ S0003-6951 ~ 98 ! 03004-6 # Recent time-dependent degradation studies 1 of deuter- ated gate ~ 5.5 nm ! oxides have inspired a new wave of in- terest to the problem 2–7 of hydrogen ~ and its isotope deute- rium ! in the SiO 2 /Si system. In particular, it was shown that the buildup of interface traps upon irradiation is significantly retarded in deuterium-passivated samples, and that transistor lifetime improvements ~ due to reduced hot-electron degrada- tion effects ! by factors of 10–50 can be achieved. 1 Subse- quently, these results were reproduced by Devine et al. who showed that the lifetime for a 10% reduction in the transcon- ductance is enhanced by ; 10 times for devices annealed in D 2 as compared to H 2 . 8 Several models have been suggested to account for this giant isotopic effect, although the behav- ior of deuterium in ultrathin oxides and its role in improving the properties of metal–oxide–semiconductor ~ MOS ! de- vices are still not well understood. 1,9 Due to its chemical similarity to hydrogen and very low background level in processing environments, it is very con- venient to use deuterium as a ‘‘marker’’ to elucidate basic chemical and physical properties of hydrogen in SiO 2 films. Previous secondary ion mass spectroscopy 5,6,10,11 and nuclear reaction analysis ~ NRA ! experiments 3,12–15 made use of this fact to investigate various aspects of hydrogen in relatively thick ( . 100 nm) oxide films. Ultrathin ~ sub-10 nm ! films of interest for near-future generation of MOS devices are much less studied. In this letter, we use a highly sensitive and selective nuclear reaction analysis technique 3,12,14,16 to study
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