Spectroscopic Analysis of Silica Atomic Layers
Marie C. CHEYNET 1, T. EPICIER 2, M.N. Séméria 3, P. Mur 3, A.M.Papon 3
e-mail: mcheynet@ltpcm.inpg.fr
1 LTPCM-UMR CNRS 5614, BP75, 1130 rue de la Piscine, 38402 St Martin d'Hères France
2 GEMPPM-UMR:INSA-CNRS 5510, Bât 502, INSA, 69621 Villeurbanne Cedex France
3 CEA-LETI-DTS Centre d'Etudes Nucléaires de Grenoble 38054 Grenoble Cedex 9
In nano-technology research the knowledge of the local structure and chemistry of materials is of prime importance to understand and improve their physical properties. This is especially true in the microelectronic industry where conventional field-effect transistors now face physical limitations and are on the way to be replaced by quantum structures. These nanostructures have different properties than bulk materials; this means that structural, chemical or electronic properties have to be investigated at the nanoscale. If HREM coupled with image simulations is one of the best technique to record structural informations at the atomic scale, Field Emission Scanning Transmission Electron Microscopy (FE-STEM) combined with Electron Energy Loss Spectroscopy (EELS) becomes more and more an alternative or a complementary way to study nanostructures.
This work reports results obtained by this latest technique on nanoscale silica layers. Comparison with HREM investigations performed concurrently on these samples[1] or reported in the litterature[2] will be carried out.
The experiments were done on model samples. Those consist of monocrystalline silicon wafer oxidized under two different conditions and encapsulated with polycrystalline silicon. (110) cross-sections were cut and thinned for TEM[3] and EELS analysis. The investigations were performed using either a JEOL 2010-FEG equiped with a Gatan-PEELS (CLYME-Lyon), or a PHILIPS Tecnai-FEG equiped with a Gatan Image Filter and an HAADF detector (ST-Microelectronics Grenoble). Both low-loss and core-loss (Si-L2,3 and O-K edges) linescans were recorded with subnanometric probes (4 Å or 2 Å) across the oxide layer with a line-scan-step equal to 2 Å. The analysis of the profiles and the interpretation of the signatures observed on the low-loss spectra and on the near edge fine structures were correlated with the properties of the layers; they show that:
* the structure changes across the layer, from crystalline in contact with the Si substrate to "amorphous or disorganized" up to the Si cap. The width of the crystalline region is estimated to 0.6±0.3 nm, corresponding to two or three crystalline planes.
* a difference in oxygen content between the crystalline and the amorphous region is measured.
* the dielectric properties depend on the characteristics of the oxide layer.
These results demonstrate that highly resolved EELS is really competitive with HREM since morphological and structural informations deduced from both techniques well agree; it however supplies additional information on the chemical and electronic properties.
[1] M.C. Schouler, P. Donnadieu, E. Blanquet, P. Mur rapport interne LTPCM-LETI.
[2] A. Ouzmard, D.W. Taylor, J.A. Rentschler, J. Bevk Phys.Rev.B vol.59, n°2, P.213 (1987)
[3] A.M. Papon, P. Mur CEA-LETI-Grenoble France