Few mobile ions can be transported through an oxynitride film (SiON) because of the formation of Si-N bonds instead of the broken Si-H bonds at the Si/SiO 2 interface. 1,2 The SiON films can be used as good insulators in ultralarge-scale integrated (ULSI) circuits, and they are less permeable to moisture and contaminants than silicon dioxide. In addition, an oxynitride film incorporated at the gate oxide/silicon interface can render it immune from hot carrier and radiation damage and forms a good barrier against boron penetration from the p ϩ -polysilicon gate in metal-oxide semiconductor field effect transistors (MOSFETs). 2 Oxynitride is usually prepared by the nitridation of SiO 2 in a N 2 O, NO, or NH 3 atmosphere and plasma-enhanced chemical vapor deposition (PECVD). 2-5 Liquid phase deposition (LPD) is a newly developed technology for growing silicon dioxide. 6 From our previous study, 7 a new LPD method of growing oxynitride films was proposed in which an aqueous solution mixture of hydrosilicofluoric acid, boric acid, and ammonia hydroxide aqua was used. It has the advantages of low-temperature growth, simplicity, and low cost.The deposition rate, refractive index, leakage current, and breakdown voltage as a function of mole concentration of ammonia hydroxide aqua are studied in the range 0-2 M in this paper. The thickness and refractive index of the oxynitride film as a function of deposition time are discussed in detail and a deposition model of LPDSiON is proposed. ExperimentalThe deposition system contains a temperature-controlled water bath, which offers a uniform temperature environment with an accuracy of Ϯ0.1ЊC, and a Teflon vessel.The substrate used in our work is p-type, boron-doped (100)-oriented silicon with a resistivity of 15-25 ⍀ cm. The preparation procedures of the treatment solution used to deposit LPD films are (i) dissolution of high-purity (99.99%) silica-gel in a hydrosilicofluoric acid (H 2 SiF 6 ) aqueous solution (3.89 mol/L), (ii) stirring for 17 h at room temperature, (iii) filtration to remove undissolved silica-gel. 0.1 M boric acid (2.8 mL) was added to the treatment solution (32 mL) for LPD-SiO 2 growth at a temperature of 40ЊC. Ammonia hydroxide aqua in the range of 0.1-2 M (4.2 mL) was added to the treatment solution for the LPD-SiON growth. Denser oxynitride films can be obtained by a subsequent N 2 annealing.The thickness and refractive index of LPD-SiON were directly measured by ellipsometry using a Gaertner model L116C auto-gain ellipsometer at a wavelength of 632.8 nm. MOS capacitors with LPD-SiON as a dielectric were prepared for electrical characterization. A high-frequency (1 MHz) Hewlett-Packard 4280A capacitance-voltage (C-V) meter with a 30 mV signal amplitude was used for the C-V measurement. An HP4145B semiconductor-parameter analyzer was used for the current-voltage (I-V) characterization. Results and DiscussionThe chemical reaction for depositing LPD-SiO 2 films has been proposed by Nagayama et al. 6 and is in equilibrium Eq. 1 given byIn equilibrium 1 (Eq. ...
We have studied the chemical bonding in thin films of SiOx (x<2) by Auger electron spectroscopy (AES). We have compared the AES Si LVV spectra of suboxides of silicon with the AES Si LVV spectrum of stoichiometric SiO2 and have observed that the spectra of the suboxides are characterized by the emergence of a feature associated with Si–Si bonds that are present in the suboxides but not in stoichiometric SiO2. We have taken care to distinguish between spectral features that are associated with departures from SiO2 bulk stoichiometry, and features that are generated by ion and electron beam induced surface damage. Ion and electron beam induced damage produces changes in the surface composition that also generates AES spectral features associated with Si–Si bonds. We have based our interpretation of the additional AES spectral feature in the silicon suboxides on an empirical tight-binding model calculation for the electronic structure. This calculation shows that Si–Si bonding states in the suboxides appear near the middle of the energy gap of stoichiometric SiO2. The position of the new spectral feature in the suboxides is in agreement with the predictions of the model calculation.
Using an aqueous solution of hydrosilicofluoric acid, boric acid, and ammonium hydroxide, fluorine-doped oxynitride films can be deposited on a silicon substrate. The fluorine and nitrogen atoms are piled up at about 160 Å from the interface. The electrical characteristics were improved by decreasing the film thickness toward 160 Å due to the increase of fluorine and nitrogen concentrations. The electrical characteristics were degraded by further decreasing the film thickness from 160 to 110 Å due to the interfacial nonstoichiometric oxide. The quality of fluorine-doped oxynitride films with a prepared thickness thinner than 160 Å can be improved under photoillumination. Thus higher nitrogen and fluorine concentrations of the films can be obtained by photoillumination.
The flow and performance for a micro-channel reformer of a fuel cell is studied numerically. Methanol with water flows into a micro-channel with catalyst layer and is reformed to provide hydrogen for the fuel cell. The channel length is varied from 800 to 6000 μm and the channel height varied from 50 to 400 μm. The inlet flow velocity varies from 0.00001 to 0.002 m/s, and the inlet temperature varies from 250 to 350 °C. The density (area fraction on the wall) of catalyst varies from 25 to 100%. A reference case is chosen to have 100 μm channel height, 0.0001 m/s inlet velocity, and 100% catalyst density. The results indicate that the higher the inlet velocity, the lower the methanol conversion mass fraction. The variation of the conversion ratio is almost linear with the flow rate. As expected, the higher the catalyst desity, the higher the conversion effieciency. However, the present results indicate that the reduction of the catalyst density by a half only sacrifices 5% performance. In addition, the conversion efficiency decreases with the increase of the channel height, but increases with the increase of the inlet temperature. The present results can provide design reference for fuel cells with a micro-channel reformer.
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