Atomic Scale Nitridation of Silicon Oxide Surfaces by Remote-Plasma-Excited Nitrogen
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ATOMIC SCALE NITRIDATION OF SILICON OXIDE SURFACES BY REMOTE-PLASMA-EXCITED NITROGEN Yoji Saito* and Koichi Tokuda* *Department of Electrical Engineering and Electronics, Seikei University, 3-3-1 Kichijoji-Kitamachi, Musashino, Tokyo 180-8633, Japan ABSTRACT We have incorporated several percent of nitrogen only near the top surfaces of thermally grown oxides by exposure to fluorine gas at room temperature followed by an atomic nitrogen treatment at 550oC. The depth profiles and the bonding of incorporated nitrogen atoms have been studied by angle-resolved x-ray photoelectron spectroscopy. MOS devices were fabricated using the nitrided oxide with boron doped polycrystalline silicon gate. From the capacitance-voltage measurements we confirmed that the nitrided oxide would prevent the boron penetration in comparison with the conventional oxide films. The proposed technique identifies a unique process for obtaining high quality ultrathin dielectrics. INTRODUCTION It is difficult to obtain sufficient reliability for the conventional SiO2 for MOS gate dielectric in the greatly scaled ICs. One of the serious problems will be the boron-penetration from p+ polysilicon gate into the Si substrate through oxide, which can change threshold voltage in p-channel p-MOSFET [1,2]. Several researchers reported that nitrogen incorporation into oxide surfaces is important to block the diffusion of boron without the film degradation. Ion implantation technique or low-pressure plasma technique were reported [3-5]. Generally it is hard to introduce nitrogen to the surface without ion impact or hydrogen, because the Si-O bonds are very stable [6]. We feel that the ion impact or high-energy-species from the plasma would induce the nitridation in the conventional works using the low-pressure-plasma. Recently we have successfully nitrided only the top surfaces of the oxide by combination of fluorination and remote plasma generated atomic nitrogen exposure at 550oC [7]. The process-induced damage is also estimated to be negligible [8]. In this report, we investigated the nitrogen-depth-profiles in the films with angle-resolved x-ray photoelectron spectroscopy (AR-XPS) measurements, after the film growth with various experimental conditions, to investigate this surface-nitridation -process and control more extensive nitrogen concentration. Moreover, we fabricated MOS devices using the nitrided oxide films with boron-doped polysilicon gates, and measured C-V (capacitance-voltage) characteristics to confirm the suppressed boronpenetration through the nitrided oxide films. EXPERIMENTAL DETAILS Surface density of incorporated nitrogen We used the (100) oriented n-type single crystalline silicon substrates with resistivity of 8-12 ohm cm. The substrates were thermally oxidized at 1000oC in dry oxygen by a conventional furnace. The thickness of the oxide was typically 10nm. The K7.9.1
oxide surfaces were, then, nitrided by the following procedure. Figure 1 shows a schematic diagram of the experimental system. The substrate was mounted on the suscept
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