Study On The Electrosynthesis And Purification Of Tantalum Alkoxides

With the fast development of very large scale integration (VLSI), the device feature size scales down, resulting in the thinning of the gate dielectric. The tunneling current increases exponentially as the gate dielectric thickness decreases, which leads to unbearable power consumption and device performance problems. A solution is to use materials with large dielectric constants to replace conventional SiO2 dielectric layer. Tantalum pentoxide (Ta2O5) is considered one of the promising candidates as a capacitor insulator in dynamic random access memories (DRAM) and as an ultra large scale integrated circuits (ULSI) gate dielectric due to its high dielectric constant (about 26) and compatibility with ULSI processing. Tantalum alkoxides are widely used precursors in chemical vapor depositon (CVD) and atomic layer deposition (ALD) processes of oxides. Aimed at some shortages such as long flow path, bad work environment, high cost and low recovery rate, a new preparation technology of tantalum alkoxides using electrosynthesis and purification was proposed in this paper. The electrochemical behaviors of tantalum in ethanol containing supporting electrolytes, the electrosynthesis of tantalum ethoxide and its technology and theory of distillation purification were systematically investigated, and some meaningful results were obtained.The electrochemical behaviors of tantalum in ethanol solutions containing Et4NOl, Et4NBr and BU4NHSO4 were investigated using cyclic voltammetry, linear polarization, potentiostatic current-time transient and impedance techniques. The results revealed that, when an anodic potential was imposed on the tantalum, it suffered from pitting corrosion in Et4NCl or Et4NBr ethanol solution, while uniform corrosion occurred in BU4NHSO4 ethanol solution. The pitting potential of tantalum in ethanol solution containing Cl- and Br- increased with the increase of scan rate, while current density of tantalum in ethanol solution containing HSO4 was proportional to the square root of the scan rate. The increase of solution temperature could accelerate the dissolution of tantalum, and the apparent activation energy of tantalum in ethanol solution containing Cl-, Br- and HSO4 ions were 36.25,44.17 and 43.39 kJ/mol, respectively. The pitting potential of tantalum in ethanol solution decreased with the increase of supporting electrolyte concentrations, while the passive film thickness of tantalum in ethanol solution containing HSO4 began to thin with the increase of HSO4 concentrations. In the ethanol solution containing Cl- or Br-, the pitting corrosion occurred when the potential was higher than 2.0 V, the pitting potential and repassivation potential increased but the current density decreased with the increase of water content, while in the ethanol solution containing HSO4, tantalum was still in the incubation period and the current density increased with the increase of water content.The cell voltage sharply decreased at the early stage of electrolysis, and then began to stabilize in the medium term, finally increased gradually. The current density was basically linear with the cell voltage. Under the same conditions, the cell voltage and current density of tantalum in the solution containing BU4NHSO4 were the highest, followed by Et4NCl, Et4NBr was the lowest. The cell voltage increased with the increase of electrode distance and current density, but decreased with the increasing BU4NHSO4 concentration and solution temperature. All the current efficiency was higher than 100%, which was related to low valence of tantalum.Saturated hydrocarbon was distilled when gas phase temperature was 46.7?at 200 Pa pressure. The distillation time had little effect on the product quality, and the quality of the middle stage was somewhat better than that of the early and late stage. The yield of tantalum ethoxide decreased and the distillation time was prolonged with the increase of calcium weight. When increasing the reflux ratio, the purity of tantalum ethoxide was enhanced, the direct recovery rate of the product sharply decreased, and the residue rate rose obviously. The recovery rate significantly increased and it could rise to 90%when using the ethanol recovered from distillation as solvent in the electrosyntheis of tantalum ethoxide. The purity of tantalum ethoxide could reach 99.999%by using high purity tantalum plates as raw materials and purifying in the distillation device with excellent sealing performance, and the impurity contents such as main group elements, reactive metals, transition metals, aluminium and niobium were superior to that of 5N standard of SAFC Hitech company. Infraed spectrum conformed to chemical bonds excellently, which was consistent with the results reported in literature. The peak area ratio of nuclear magnetic resonance coincided with number ratio of hydrogen atoms.Tantalum propoxide (Ta(OPrn)5), tantalum isopropoxide (Ta(OPr')5) and tantalum butoxide (Ta(OBun)5) were successfully prepared by electrosynthesis and distillation purification, using tantalum plate as anode, stainless steel plate as cathode and solutions of quaternary ammonium salt in the corresponding alcohol as electrolytes. Measured by fourier transform infrared spectra (FT-IR) and raman spectra, the absorption peaks of the product were excellently consistent with chemical bonds. The peak area ratio of nuclear magnetic resonance coincided with number ratio of hydrogen atoms. The analysis results of ICP-Mass revealed that the purity of Ta(OPrn)5, Ta(OPrl)5 and Ta(OBun)5 was 99.998%,99.998% and 99.997%, respectively. The volatility of Ta(OPrn)5 was the best, followed by Ta(OBun)5, and Ta(OPr')5 was the poorest, and the total weight loss rate(heat treatment at 800?) was 95%,88% and 75%, respectively. The value of k was obtained by calculating the weight loss rate of benzoic acid, and the vapor pressure of tantalum alkoxides was calculated using Langmuir equation and k value aforementioned. The data of vapor pressure were basically consistent with the results reported in literature. Enthalpies of vaporization for tantalum alkoxides were calculated from the vapor press-temperature data using the Clausius-Clapeyron equation.In this paper, the electrochemical behaviors of tantalum in the electrosynthesis of tantalum ethoxide were investigated, which could provide theoretical guidance for the electrosynthesis of tantalum alkoxides; the influence of various factors on the electrosynthesis and distillation purification of tantalum ethoxide was studied, which could provide technical guidance for the small scale production of high purity tantalum alkoxides, and could provide technical prototype for the equipment selection of industrialization; the thermal decomposition characteristics of tantalum alkoxides were also studied, which could provide theoretical basis for the selection of MOCVD precursors.