Initial Oxynitridation of A Si(001)-2xl Surface by No
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INTRODUCTION Si oxynitride is a leading candidate as a material for ultrathin gate dielectric films and tunnel dielectric films, because of its improved electrical reliability and resistance to boron penetration. 1"3 The nitrogen incorporation is usually achieved by using a NH 3, N20, or NO gas. However, it was reported that NH 3 nitridation causes electron traps due to the incorporation of hydrogen.4 And the N concentration of the N2 0 oxynitride is lower than that of the NO oxynitride. 5 Therefore, the most useful method is thought to be NO oxynitridation. Direct NO-oxynitridation of Si surface and NO-annealing of SiO2 film have been proposed. In particular, the direct oxynitridation is useful to grow a thin oxynitride layer, because it takes place in a self-limiting manner.
On the other hand, the atomic-scale roughness at the dielectric/Si interface is a crucial issue affecting the electrical properties. Many researchers have investigated the conventional SiO2/Si interface, and reported that the roughness changes the interfacial state density, the dielectric breakdown, and so on. 6' 7 In addition, the SiO2 /Si interface strongly depends on the flatness of the initial Si surface, because the oxidation proceeds in a layer-by-layer manner."' 9 Therefore, we think that the oxynitridation manner and its interfacial structure are also important to produce a good oxynitride layer. However, few studies have dealt with the growth mechanism of the oxynitridation, although oxidation by 02 has been studied extensively by many researchers. In this work, we used scanning reflection electron microscopy (SREM) and reflection highenergy electron diffraction (RHEED) to investigate the ultrathin-oxynitride/Si interface formed by NO oxynitridation of an atomically-flat Si(001)-2xl surface. We also used x-ray photoelectron spectroscopy (XPS) to characterize its chemical structure.
EXPERIMENT Oxynitridation and characterization were carried out using an ultrahigh-vacuum (UHV) system.10 An atomically flat Si(001)-2xl surface was prepared by flash heating at 1200°C under UHV conditions. We introduced NO gas into the UHV chamber, and oxynitridation was performed at substrate temperatures ranging from room temperature to 900"C under reduced NO pressure. The oxynitrided Si surfaces were investigated without exposing the sample to air. We used SREM to investigate the structural change of the oxynitride/Si interface. A 30233 Mat. Res. Soc. Symp. Proc. Vol. 592 0 2000 Materials Research Society
0.8
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NO-20L
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spectra Fig. 1. Auger electron of the Si(001)-2xl surface exposed by NO of 20 L. (a) at and 600*C under lxl0 Torr NO (b) at 900"C under 1x10 4 Torr NO.
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Fig. 2. Compositional and structural change due to the NO reaction. (a) Dependence of the (NKLL)/(NXLL+OKuL) ratio on the temperature and NO pressure. (b) and (c) RHEED patterns after the NO reaction shown by arrows in (a).
keV electron beam with a 3-nm diameter was
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