Mikako Takeda

Characterization of Silicon Native Oxide Formed in SC-1, H2O2 and Wet Ozone Processes

The structures of silicon native oxides formed in the SC-1, H2O2 and wet ozone processes were characterized using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR). Spectral simulation was performed to clarify the FT-IR spectra, assuming that the native oxide was pure silicon dioxide. Effective medium theories were applied to understand deviations of the observed spectra from the calculated ones. The deviations between the native oxide thickness evaluated by XPS and the absolute thickness obtained by TEM were also discussed. These deviations can be explained if the void is incorporated in the native oxides and the interface between the native oxide and the basal silicon obtained by the wet ozone process has a relatively smooth surface and a structure more similar to that of pure silicon dioxide, compared with that obtained by SC-1 or H2O2 treatment.

Hardness of Oxide Scales on Fe-Si Alloys at Room- and High-Temperatures

Hardness of oxide scales on Fe-(0, 0.5, 1.5, 3.0)Si alloys was studied at room temperature after oxidation at 1273 K for 18 ks in oxygen, and at 1073 and 1273 K for 180 and 1080 ks in dry air, by micro-Vickers hardness measurements. After oxidation at 1273 K for 18 ks, high-temperature hardness of oxide scales on Fe-(0, 1.5, 3.0)Si alloys was also measured at 1273 K. Oxide scales on Fe-Si alloys were mainly Fe2O3, Fe3O4, FeO and Fe2SiO4. Hardness of Fe2O3, Fe3O4 and FeO on Fe was 6.7, 4.0 and 3.5 (GPa), respectively, and hardness of Fe2O3 on Fe-Si alloys slightly increased with increasing silicon content at room temperature. At 1273 K, hardness of Fe3O4 and FeO on Fe was 0.08 and 0.05 (GPa), respectively, and hardness of Fe2O3 on Fe-1.5Si alloy was 0.32 (GPa), and that of Fe2O3 and Fe2SiO4 on Fe-3.0Si alloy was 0.53 and 0.63 (GPa), respectively.

Oxidation Behavior and Scale Properties on the Si Containing Steels

The adhesion and micro-structure of the primary scale formed in LNG combustion gas on steels containing Si, up to 3.0mass%, have been studied using a hot-compression test at 1273K, Raman spectroscopy, and X-Ray absorption fine structure analysis (XAFS). The Fe2SiO4 at the scale/steel interface is increased as the Si content increases, suppressing the diffusion of Fe ions from the steel and forming dense and highly-adhesive subscale, when oxidized below the temperature at which the Fe2SiO4 melts. The behavior of the secondary scale growth in air up to 1173K on these has been studied using in-situ XRD. It is apparent that the formation behavior of Fe2SiO4 ,FeO and Fe2O3 is influenced by the Si content.

Measurement of Young’s modulus of oxides at high temperature related to the oxidation study

Abstract α-Al2O3, Cr2O3, and α-Fe2O3 specimens were prepared by a sintering process. A 400 – 1000-Hz sine wave was applied to the specimen at 290 – 1273 K. The applied and respond waves were monitored by using force and acceleration sensors. The intensity ratio and phase shift between the applied and respond waves were analysed, and the anti-resonance frequency was obtained. Young’s moduli of α-Al2O3, Cr2O3, and α-Fe2O3 are estimated to be 386, 286, and 220 GPa at 298 K, respectively. The temperature dependence values of these oxides are estimated to be 54.3, 46.9, and 42.0 MPa K-1, respectively. The temperature dependence of Young’s modulus can be classified on the basis of the crystal structure of solids. The estimation of Young’s modulus at 1273K is possible with an error range of 10 – 30 GPa for a crystalline solid if the crystal structure of the solid is known. It is found that the temperature dependence of Young’s modulus depends on the density of the oxides, and an experiment in which well-characterized crystalline solids are used must be conducted to minimize the error range.

High Temperature Oxidation of Fe-3Si Alloy

The high temperature oxidation behavior of Fe-3Si alloy was studied in oxygen for 0.3 ~ 18 ks at 873 ~ 1273 K, by mass gain measurements, X-ray diffraction (XRD), scanning electron microscopy (SEM), optical microscopy and electron probe microanalysis (EPMA). Mass gain of the alloy increased with increasing time and temperature of oxidation. Oxide nodules were formed on the alloy after oxidation at more than 973 K. After oxidation at 1273 K, the number of oxide nodules increased with inc reasing oxidation time, and these nodules for 0.3, 1.8, 7.2 and 18 ks were 20, 120, 250 and 350 μm in diameter, respectively. Oxide nodules consisted of the outer scale (Fe2O3, Fe3O4) and the inner scale (Fe2SiO4, SiO2). Subscale was observed at the inner scale/substrate for 0.3 and 1.8 ks, however, the subscale disappeared after 7.2 ks oxidation at 1273 K.

Calculation of Boundary Conditions between Internal and External Oxidation of Silicon or Chromium Containing Steels

The boundary constants between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were calculated in order to clarify the formation mechanism of fayalite scale (Fe2SiO4) or chromite scale (FeCr2O4), which can form as a “sub-scale” in Si or Cr containing steels. The diffusion coefficient of oxygen in the alloy, Do, and the oxygen concentration at the specimen surface, NO(s), which are constituents of the internal oxidation rate constant, (2DONO(s)/NB(O)n), were calculated for various oxidation conditions, and the rate equation for internal oxidation was derived. By comparing the calculated and measured values of (2DONO(s)/NB(O)n), we confirmed that the rate equation determined for internal oxidation was reasonable. The boundary condition between internal and external oxidation of Si or Cr containing steels (Fe-Si alloys or Fe-Cr alloys) at 850°C were also calculated by substituting the calculated values of DO and NO(s) into the rate equation.

Influence of Native Oxides on the Reliability of Ultrathin Gate Oxide.

The influence of wet cleaning processes, such as the SC-1 and H2O2 processes, on the time dependent dielectric breakdown (TDDB) of ultrathin gate oxide was investigated. A large difference in the reliability by wet cleaning processes was observed, especially when an electrode is an anode. The reliability of the gate oxide by the H2O2 process was worse than SC-1. It was found by Fourier transformed infrared attenuated total reflection (FT-IR-ATR) analysis that the amount of structural imperfection of native oxides formed in H2O2 was larger than SC-1. Since stress-induced positive charges which affect the TDDB properties are generated near the anode- side oxide interface, a large amount of structural imperfection in the native oxides formed in H2O2 probably results in a defective thermal oxide surface, leading to an increase in the generation and trapping of positive charge.

Recent theoretical development on solidification processing with high temperature gradient

A overview of recent theoretical development on solidification with high temperature gradient is given. Growth morphologics and dimensions of dendrite/cell near the absolute stability are discussed experimentally and theoretically. Some models predicting quantitatively those dimensions near the absolute stability are introduced.