Yasunobu Akiyama

Formation of hydrogen-permselective SiO2 membrane in macropores of α-alumina support tube by thermal decomposition of TEOS

Abstract A porous α-alumina tube of 2.5 mm O.D. and 1.9 mm I.D. was used as the support of an inorganic membrane and was modified for the purpose of separating small-molecule gases. Macropores of the tube, about 150 nm in size, were plugged with silica formed by thermal decomposition of tetraethylorthosilicate at 600–650°C with no special pretreatment. To improve the step coverage of the deposition, the reactant was evacuated through the porous wall of the support. Silica was decomposed inside the macropores, forming a layer of 500–1000 nm. At a permeation temperature of 600°C, the H 2 permeance of the modified membrane was of the order of 10 −8 mol·m −2 ·s −1 ·Pa −1 , while the N 2 permeance was below 10 −11 mol·m −2 · s −1 · Pa −1 . When the membrane was exposed in mixture of nitrogen and steam at 500°C for 24 h, the H 2 permeance was decreased to half the initial value but was little reduced by a further exposure of 48 h. The membrane was resistant to cyclic temperature changes.

Reaction analysis for ZrO2 and Y2O3 thin film growth by low-pressure metalorganic chemical vapor deposition using β-diketonate complexes

A mathematical model was developed for low-pressure metalorganic chemical vapor deposition (LPMOCVD) of ZrO 2 and Y203 film growth. Zr(DPM)a(tetrakis-2,2,6,6-tetramethyl-3,5-heptandionate zirconium (/3-diketonate complex)) and Y(DPM) 3 were used as source materials. The surface reaction rate constant (or the reactive sticking coefficient) was determined by comparing the experimentally observed step coverages on micro-scale trenches with those predicted by a simplified Monte Carlo simulation. A gas-phase reaction rate constant was taken as a disposable parameter to fit the growth rate distributions along the reactor tube by a diffusion reaction transport model. Arrhenius-type equations were proposed for both surface and gas phase reactions. For the surface reactions, the activation energies were 188 kJ/mol (T 909 K) for ZrO 2 and 133 kJ/mol (T < 870 K) for YzO3. For the gas phase reactions, they were 140 and 123 kJ/mol for ZrO 2 and Y203, respectively. The scanning electron microscopy (SEM) micrographs and X-ray diffraction (XRD) patterns revealed that the crystallographic orientation and morphology of the grown films depend on the growth temperature.

Numerical simulation of oscillatory Marangoni flow in half-zone liquid bridge of low Prandtl number fluid

Abstract A set of numerical simulations was conducted using a finite difference method to understand the general feature of oscillatory Marangoni convection in half-zone liquid bridges of low Prandtl number fluids (Pr=0, 0.01, 0.02) with a cylindrical liquid surface over a wide aspect ratio range (As=0.6 to 2.2). The simulation results indicated that under smaller temperature differences the flow in the liquid bridge was axisymmetric but it became unstable against three-dimensional (3-D) disturbances beyond a certain threshold value of temperature difference, i.e., the flow became steady 3-D flow at and beyond the first critical Reynolds number Rec1. This steady 3-D flow became unstable against time-dependent 3-D disturbances beyond a second critical condition, Rec2. The numerical simulations revealed the critical conditions and flow structures in detail for Pr=0 fluid and rough sketches for fluids of Pr=0.01 and 0.02. The present results of Rec1 and Rec2 showed good agreements with those of linear stability analyses and with available numerical results. A stability map for low Pr fluids (Pr=0, 0.01 and 0.02) was proposed.

Infrared diode laser spectroscopy of the SiF radical

The vibration-rotation spectrum of the SiF radical produced in a glow discharge of silicon tetrafluoride (SiF4) was observed using an infrared diode laser spectrometer. The SiF lines were discriminated from much stronger SiF2 lines by the discharge current modulation and Zeeman modulation techniques. Twelve and 15 lines belonging to the 2Π12 and 2Π32 spin states, respectively, were identified for the fundamental band in the ground electronic state. Most of the lines in the 2Π12 state were observed with splitting due to the Λ-type doubling. Precise molecular constants for the v = 1 state, including the rotational, centrifugal distortion, and spin-orbit coupling constants, were obtained from the least-squares analysis, in which the ground state constants were constrained as determined by microwave spectroscopy. The fundamental band origin thus obtained is 847.7205 ± 0.0005 cm−1, where the uncertainty corresponds to three standard deviations. The equilibrium bond length re is derived as 1.601018 ± 0.000003 A from the equilibrium rotational constant, Be = 0.581241 ± 0.000002 cm−1.

Infrared diode laser spectroscopy of the SiF+ ion

Abstract The infrared diode laser spectrum of the SiF + ion generated in a SiF 4 plasma has been observed. Twenty-one and ten lines in the R and P branches, respectively, of the fundamental band and three R branch lines in the v =2←1 hot band were recorded using the hollow cathode discharge modulation technique. A sharp decrease of the intensity caused by the magnetic field, which is characteristic of ion lines, was observed. The band origins are 1040.4833±0.0006 and 1030.5908±0.0005 cm −1 for the fundamental and hot bands, respectively, from which ω e = 1050.3757 ±0.0013 cm −1 and ω e χ e =4.9462±0.0004 cm −1 are derived. The rotational constants for the ground and first excited vibrational states are 0.637010±0.000015 and 0.632311±0.000015 cm −1 , respectively, and the centrifugal distortion constant in the ground state is (0.9466±0.0046) × 10 −6 cm −1 . The equilibrium bond length is r e =1.52652±0.00008 A.

Shape of Film Grown on Microsize Trenches and Holes by Chemical Vapor Deposition: 3-Dimensional Monte Carlo Simulation

A semi-microscale 3-dimensional film growth simulation code using a simple Monte Carlo method was developed. This code predicts the step coverage on a trench and a hole of arbitrary shapes and requires much smaller computer memory size and less calculation time than the Direct Simulation Monte Carlo (DSMC) method. The simulation code was evaluated by comparing the results with experimental results of a zirconia ( ZrO2) film grown on a hole. The experiments and/or 3-dimensional simulations indicated that the film grown on the side and bottom walls of a hole is thinner than that of a 2-dimensional trench, and complete occlusion of a hole is more difficult compared with a trench with opening width equal to the hole diameter. The surface reaction rate constant is the most important factor in the occlusion process. When the reaction rate constant is small, the hole is occluded with a thin film. However, when the reaction rate constant is large, there remain a void inside and a small unfilled pinhole through the thick film grown on the top.

Micro/macro modeling of CVD synthesis

We propose a CVD reaction scheme in which a source material undergoes a gas-phase reaction to produce an intermediate, and then the intermediate diffuses to the solid surface and changes into a solid film through a surface reaction. A series of simple Monte Carlo (SMC) codes has been developed to simulate the observed film shape on micro-trenches and holes. By using these codes, surface reaction rate constants were determined so as to reproduce the experimentally observed film shape. By means of a macro-scale reactive transport analysis of a hot wall tubular reactor, gas-phase reaction rate constants for single component systems were determined to simulate the experimental growth rate distributions. The composition and growth rate of Yttria stabilized Zirconia (YSZ) film, a solid solution of Yttria and Zirconia, were qualitatively explained by a sum of single components growth rates. As an application of these reaction models, we simulated a rotating-disk CVD reactor under low pressure. The simulations based on a quasi three-dimensional model revealed that the susceptor rotation suppresses the buoyancy convection and forms steeper gradients in temperature and concentration near the susceptor uniformly over wide area. At higher temperatures, the growth rate increased with rotation speed, but at lower temperatures the growth rate decreased with increasing rotation speed because the reduced retention time in the high-temperature region suppressed the gas-phase reaction.

Millimeter-wave spectroscopy of the GeF+ ion

Abstract The millimeter-wave spectrum of the GeF + ion in the ground vibrational state was observed for four major isotopic species. The ion was generated in a free-space absorption cell by a hollow cathode discharge in GeF 4 mixed with Ar. The magnetic field effect was used to discriminate the absorption lines of the ion from those of neutral species; the intensity of the ion lines decreased markedly on applicatin of an axial magnetic field of about 170 G. The observed millimeter-wave spectrum was combined with a previously reported diode laser spectrum in an analysis to determine mass-independent Dunham coefficients. The equilibrium bond length is r e = 1.664821 A.

Infrared diode laser spectroscopy of the GeF+ ion

Abstract The first high-resolution spectroscopic observation of the GeF + ion is reported. The ion was generated in a hollow cathode discharge of GeF 4 mixed with He. The spectrum was measured using an infrared diode laser combined with the discharge modulation technique. Absorption lines belonging to the fundamental band were identified for five Ge isotopes. A sharp decrease of signal caused by a magnetic field confirmed that the spectrum was due to a charged species. The spectroscopic constants derived for the 74 GeF + species are ω e =815.60(12), ω e x e =3.223(62), B e =0.40212(14), and α e =0.0025060(53) cm −1 . The equilibrium bond length is r e =1.66549(29) A. The figures in the parentheses are 3σ uncertainties in the last digit.

MODELING OF TUNGSTEN THERMAL CHEMICAL VAPOR DEPOSITION

Low-pressure chemical vapor deposition (LPCVD) of tungsten (W) film on silicon (Si) substrate was performed by reducting hexafluoride (WF6) with hydrogen. This CVD system is known for its nonlinear dependence of growth rate on WF6 concentration. This study adopted a simple surface-reaction model which assumes that the precursor, i.e., WF6, in the gas phase adsorbs on solid surfaces and then the adsorbed WF6 molecule is converted into tungsten solid film. The two kinetic parameters involved in the model are derived from the experimental results. The solidification rate constant (ks) is equal to the growth rate at very high WF6 concentrations. The adsorption rate constant (ka) is derived from profile analyses of films grown in microtrenches under very low WF6 concentrations by applying the conventional Monte Carlo simulation code, which is valid for linear surface-reaction systems. In the temperature range of 623 to 823 K, ka and ks have activation energies of 82 kJmol-1, 66.1 kJmol-1, respectively. A newly proposed Monte Carlo simulation for nonlinear reaction systems, in combination with the two kinetic parameters, can quantitatively predict the shape of film in microtrenches for a wide range of temperatures and WF6 concentrations.