Oxynitridation of Si with Nitrogen Plasma for Flash Memory and its High Frequency C-V Characteristics

Si (100) substrates were oxynitrided with nitrogen plasma exposure, at different conditions, followed by thermal oxidation in dry O 2 without using any harmful or global warming gas. Atomic concentration of N to Si was 7.5% for plasma discharge power of 0.5kW and exposure time of 1min. While, it was 17.6% for plasma power of 2.0kW and 3min, as depicted from X-ray photoelectron spectroscopy (XPS) measurements. A progressive reduction of the oxidation rate with the increase of N concentration associated with the increase of N plasma power was observed. Uniformity of film thickness across 4 inch wafer was improved with nitrogen plasma exposure followed by rapid thermal oxidation (RTO) with discrepancy less than 2.6% compared with 13.2% for N2O thermal oxidation process. High frequency C-V measurements investigated the advantage of using N plasma exposure to Si for device application. A distortion in the C-V curve was observed for samples without nitridation, which is not observed for nitrided samples. The distortion in C-V curves was found to be substantially reduced as a result of Si nitridation, under the condition of reduced plasma power. Key-Words: Silicon oxynitride, Flash memory, Nitrogen plasma, C-V characteristics, Interface states AKIHIRO IKEDA, M. ABD ELNABY *, Tsuyoshi Fujimura, REIJI HATTORI, YUKINORI KUROKI Graduate School of Information Science and Electrical Engineering, Kyushu University, Japan. * Department of Electronics and Communications, Tanta university, Egypt. C for 10min, and then dipped in BHF water to remove the native oxide. After drying with nitrogen gas, the wafer for nitrided sample was immediately transferred into the load lock chamber attached to the plasma apparatus. Inductively coupled plasma (ICP) was used for a nitrogen plasma generation. Single turn loop antenna was set around the quartz discharge tube. Since a clean process without any metal contamination is needed for gate oxide process. Therefore, in this ICP system designed in our laboratory, the discharge chamber was formed by quartz tube, as same as a furnace oxidation equipment. Hence, metal contamination generated by chamber wall sputtering was negligible and clean nitrogen plasma could be produced. Base pressure in the discharge chamber was 5 * 10Pa. The processed wafer was put under 150mm from the loop antenna. Discharge pressure was 0.6Pa and Pure N2 gas was inlet to the discharge chamber from the upper end plate on the quartz tube. Gas flow rate was 40sccm. Wafer temperature during plasma exposure was kept at room temperature, monitored by thermocouple attached in the wafer holder. Discharge power and exposure time were changed to evaluate the plasma effect on the nitridation degree of the Si surface. Two nitridation conditions, ‘lightly’ and 'heavily’ nitridation, were examined under the plasma conditions shown in Table 1. Electron temperature, electron density, and incident ion energy were determined by Langmuire probe. The ‘non-nitridation’ samples were also fabricated to clarify the immunity of stress reliability as a results of using nitridations. After the N plasma exposure, wafer was oxidized using RTO or FO equipment. Pure 100% dry O2 or 1% dry O2 diluted with N2 was introduced into the RTO or the FO, respectively. Total film thickness was estimated to be 50Angstrom using ellipsometry technique. Electrode area about 1.225 * 10cm for MOS capacitor was formed by photolithography and wet etching of Al deposited on wafer with DC magnetron sputtering technique. 3 Results and Discussions 3. 1 Nitrogen concentrations and the thermal oxidation characteristics of the oxide films In order to identify the chemical bonding state of nitrogen in our samples, we measured the N1s binding energy. Figure 1 shows the N1s X-ray photoelectron spectroscopy (XPS) spectra of the films with plasma nitridation followed by the RTO. Table 1. Nitrogen plasma conditions corresponding to ‘heavily’ and ‘lightly’nitridation conditions. Fig. 1. The N1s XPS spectra with the atomic concentration of N to Si. Fig. 2. Total film thickness as a function of oxidation time with the RTO. 392 394 396 398 400 402 404 Binding energy (eV) In te ns ity ( A rb . u ni ts ) Lightly nitridation (N/Si=7.5 at. %) Heavily nitridation (N/Si=17.6 at. %) N Si 3 0 10 20 30 40 50 60 70 80 90 0 20 40 60 80 100 120 140 Oxidation time(sec) Fi lm th ic kn es s( ) Non-nitridation