Some properties of tungsten

Oxidation Behavior Tungsten is susceptible to oxidation by oxygen at temperatures above 300°C. The reaction is not self-limiting and is enhanced because its forms W03 which is volatile at higher temperatures. Therefore, when tungsten is used as an interconnect material, precaution is needed to prevent oxidation especially during subsequent dielectric depositions. 

Tungsten can be oxidized at room temperature by (violent) reaction with fluorine under formation of WF6. This is in fact the chemical route for the synthesis of WF6 from raw tungsten. 

Wet Chemistry For sub-micron interconnect systems wet etching of tungsten is not a viable way to pattern tungsten lines. However, to clean reactor parts, or to reclaim or repair (8 ) precious wafers, a good wet etch chemistry is of importance. 

Tungsten dissolves very slowly in inorganic acids. The following are examples of wet tungsten etch solutions with acceptable etch rates  

The reaction can be vigorously accelerated by the addition of NH4OH, however, extreme precautions should be taken since the reaction is exothermic. Therefore, appropriate cooling of the reaction vessel is necessary when using this solution. Etch rates depend on exact conditions but can be as fast as several microns per hour. 

---

White H, R Feicht, C Huber, F Lottspeich, H Simon (1991) Purification and some properties of the tungsten-containing carboxylic acid reductase from Clostridium formicoaceticum. Biol Chem Hoppe-Seyler 372 999-1005. 

Some of the physical properties of tungsten are given in Table 3 further property data ate available (12-14). For thermodynamic values, References 5,15, and 16 should he consulted. Two values ate given for the melting point. The value of 3660 K was selected as a secondary reference for the 1968 international practical temperature scale. However, since 1961, the four values that have heen reported ranged from 3680 to 3695 and averaged 3688 K. 

In addition, some physical and chemical properties of tungsten will... 

At least three different tungsten sources have been reported to give CVD tungsten films WF6, WC16 and W(CO)6. Some properties of these sources are listed in table 6.1. 

Finally, in table 6.4 some relevant properties of tungsten and silicon are listed. An important fact is that meta-stable j8-W has quite different properties than a-W. (Refer to chapter II about the occurrence of /3-W in CVD-W films). 

Some properties of the six tungsten enzymes known so far are smnmarized in Table 14.2. It is seen that some are very complex, having molecular weights of 300,000 and more, and contain high concentrations of iron (CAR and FDH2 from Chlostridium... 

Properties. — The properties of tungsten show material variation, depending on the degree of purity and the methods of production and treatment. The following statements are taken from some of the best recent publications and refer for the greater part to the ductile form of the metal. 

Che] Chemienko, V.V., Martyukhin, I.D., Effect of Niobium, Molybdenum, and Tungsten on Some Properties of Iron-Boride Materials , Sov. Powder Metall. Met.Ceram., 19(2), 135-138 (1980) (Experimental, 5)... 

Acherman, W. L., Carter, J. P., Kenahan, C. B. and Schlan, D., Corrosion Properties of Molybdenum, Tungsten, Vanadium and some Vanadium Alloys, Report of Investigations No. 6 715, US Bureau of Mines (1966)... 

Because several of the superalloys contain very little iron, they are closely related to some of the non-ferrous alloys. Some of the second- and third-row transition metals possess many of the desirable properties of superalloys. They maintain their strength at high temperatures, but they may be somewhat reactive with oxygen under these conditions. These metals are known as refractory metals, and they include niobium, molybdenum, tantalum, tungsten, and rhenium. 

Iron oxide (Fe Oj) and tungsten oxide (WO ) films have been studied and developed as candidate semiconductor materials for the PEC junction (photoanode). High-temperature synthesis methods, as reported for some high-performance metal oxides, have been found incompatible with multijunction device fabricatioa A low-temperature reactive sputtering process has been developed instead. In the parameter space investigated so far, the optoelectronic properties of WO3 films were superior to those of Fe Oj films, which showed high recombination of photogenerated carriers (Miller et al., 2004). 

At IREQ, besides the participation in the field tests run by the engineers of Hydro-Quebec (12), the main effort has been to tackle fundamental problems in the field of electrocatalysis (18-22) and of anodic oxidation of different potential fuels (23-26). A careful and extensive study of the electrochemical properties of the tungsten bronze has been carried out (18-20) the reported activity of these materials in acid media for the oxygen reduction could not be reproduced and this claim by other workers has been traced back to some platinum impurities in the electrodes. Some novel techniques in the area of electrode preparation are also under study (21,22) the metallic deposition of certain metals on oriented graphite show some interesting catalytic features for the oxygen reduction and also for the oxygen evolution reaction. 

---