For the deposition of tungsten layers from the vapor phase, an easily evaporable tungsten compound is mixed with a carrier gas and/or reducing gas and decomposed or reduced at a higher temperature on a suitable. Possible tungsten compounds are halides (preferably WF6 and WCl6), tungsten hexacarbonyl [W(CO)6], and organometallics. Until now, only the halides are technically important. Extensive investigations in the direction of organometallics have been undertaken in order to overcome the disadvantages of the halides: the high-temperature necessary for reduction and the difficulties caused by corrosion.
Sheets, produced via CVD(chemical vapor deposition), exhibit a quite different microstructure as compared to wrought P/M products in terms of grain size and grain orientation. Both parameters can be influenced by the deposition conditions. Very fine grained CVD(chemical vapor deposition) tungsten can be obtained by changing the growth conditions during deposition, for example, by brushing, which increases the nucleation rate and disrupts the columnar grain growth. The method also offers the possibility to deposit layers of desired crystal orientation or even single crystal layers.
CVD(chemical vapor deposition) of tungsten and tungsten-rhenium (co-deposition of WF6 and ReF6) is of commercial importance for the production of X-ray targets. W-Re layers of up to 1mm are deposited onto graphite disks with diameters between 100 and 150 mm. Furthermore, CVD can be used to produce cylindrical or conical shells, cones (such as shaped charge liners), crucibles, and thin-walled tubes.
Low-pressure CVD of tungsten is tested for via plugs, low-resistance contact barriers, and interconnects for high density silicon integrated circuits. Depending on the substrate and the deposition conditions, either α-or β-tungsten, or a mixture of both, forms. This is of interest, because the resistivity ofβ-W is about 10 times higher than that ofα-W.
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