Tantalum and Niobium Oxide

Films of TaaOs have been deposited by LPCVD at temperatures in the ranges of 340-400°C [118], and 470-650°C [119], with growth rates in the range of 40 Amin . The precise control of residence time of the precursor, temperature, pressure, and the Ta(OEt)s/02 ratio each was found to be an important factor in controlling conformality and uniformity. Film uniformity can be obtained that is better than 1.5% (SD lo) over a single 15 cm wafer and better than 4.5% (SD 3o) from wafer to wafer, run to run. 

While films grown by thermal CVD appear to show potential for commercialization, lower deposition temperatures are still desirable. Studies indicate that films grown by PECVD are porous and show high current leakage. Amorphous Ta20s films 

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Tantalum and niobium oxides dissolve very slowly in HF solutions. Thus, it is recommended to use a high concentration of HF or a mixture of HF and H2SO4 at a temperature of about 70-90°C. The best precursors for the preparation of fluoride solutions are hydroxides. Both tantalum hydroxide, Ta205 nH20, and niobium hydroxide, M Os-nHjO, dissolve well, even in diluted HF solutions. 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

It was proposed [445 - 447] that the dissolution of tantalum and niobium oxides in mixtures of hydrofluoric and sulfuric acids takes place through the formation of fluoride-sulfate complexes, at least during the initial steps of the interaction and at relatively low acid concentrations. Nevertheless, it was also assumed that both tantalum and niobium fluoride-sulfate complexes are prone to hydrolysis yielding pure fluoride complexes and sulfuric acid. No data was provided, however, to confirm the formation of fluoride sulfate complexes of tantalum and niobium in the solutions. 

Table 62. Typical purity of tantalum and niobium oxides prepared from strip solutions after extraction with 2-octanol. Impurity level is given in ppm.

Preparation of tantalum and niobium oxides 8.4.1. General notes... 

Tantalum and niobium oxides, Ta2Os and Nb2Os, are among the final products obtained from tantalum and niobium strip solutions following liquid-liquid extraction processes. The strip solutions of tantalum and niobium consist of solutions of fluorotantalic and oxyfluoroniobic acids, H2TaF7 and H2NbOF5, respectively. 

Several possibilities exist for the conversion of the liquid acids into solid tantalum and niobium oxides. The most common steps performed in all such methods are ... 

The defluorination of the complex acids is a key step in the production of tantalum and niobium oxides as it defines the quality of the products and durability of the production equipment. 

In some cases, the degree of fluorine contamination of tantalum and niobium oxides containing increased fluorine levels is not very critical to the later application of the oxides. Applications related to the manufacturing of optic and electronic devices, however, require significant limitations of the fluorine content of tantalum and niobium oxides. 

Preparation of tantalum and niobium oxides based on the precipitation by ammonium solution of tantalum or niobium hydroxides from strip solutions is the most frequently used method in the industry and consists of several steps. Fig. 135 presents a flow chart of the process. 

Different procedures for the precipitation, washing and thermal treatment of hydroxides result in different fluorine contamination levels in the final products - tantalum and niobium oxides. Laboratory and industrial experience confirms some correlation between the initial concentration of fluorine in the dried hydroxides and the fluorine content in the final oxides obtained after appropriate thermal treatment. For instance, it is reported in [499] that if the initial concentration of fluorine in niobium hydroxide equals A%, then the fluorine content in the final niobium oxide can be estimated according to the thermal treatment temperature as follows ... 

Other methods for precipitation of tantalum and niobium oxide precursors... 

The formation and stability of peroxoniobates and peroxotantalates can be used successfully in the technology of tantalum and niobium oxide production. Belov, Avdonina and Mikhlin [512] investigated processes of precipitation and thermal decomposition of high-purity ammonium tetraperoxoniobate and tetraperoxotantalate as precursors for the production of tantalum and niobium... 

Leaving aside the incompleteness of the equilibriums, Equations (154) and (155) indicate a clear possibility of producing tantalum and niobium oxides based on the precipitation of peroxometalate precursors. 

To avoid explosion, the compounds can be decomposed via hydrolysis in liquid solution. Ultra-fine particles are obtained in water and water-ammonia media. Hydrolysis in HC1 and HN03 solutions leads to the precipitation of an agglomerated powder of both tantalum and niobium oxides. Agglomerates obtained are up to 12 pm in diameter, while the estimated diameter of the smallest ciystalline particles varies in the range of 0.01-0.5 pm [512]. 

The method emphasizes perspective directions in the development and improvement of the technology of tantalum and niobium oxides. 

The first experiments on the plasma chemical decomposition of fluoride solutions containing tantalum or niobium to obtain tantalum and niobium oxides were reported about fifteen years ago [524]. Subsequent publications were devoted to further development and expansion of the method for other refractory rare metals such as titanium and zirconium [525 - 532]. 

Table 66 presents a comparative analysis of different methods for the production of tantalum and niobium oxides. 

Table 66. Comparative analysis of different possible methods of the production of tantalum and niobium oxide powders from strip solutions.

S Fluorination of tantalum and niobium oxides by hydrofluorides of ammonium or alkali metals yields fluorotantalate or monooxy-fluoroniobate compounds. Fluorination of tantalum or niobium oxides in the presence of oxides of other metals yields complex fluoride compounds containing both tantalum or niobium and added metals. 

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