Tungsten vapor pressure, high temperature

Another high-temperature cell (Fig. 8 g, up to 2400 °C, can be produced from tungsten. Tungsten Knudsen cells are used primarily for high-temperature vapor pressure measurements, e.g. for metal oxides. They are suitable also for metals when graphite linings are applied to the inner surface. The vapor pressure can be determined... 

From this we see that WF6 is the most convenient source in terms of vapor pressure and physical state. Generally, solid sources are much more difficult to deliver to the reactor in a reproducible way. The halides are very stable and decompose only at high temperatures whereas the carbonyl compound will decompose readily at temperatures above 200°C. In the following section we will discuss tungsten deposition results obtained with each source. 

Tungsten is the most important metal for thermoemission applications, not only because of its high emissivity but because of its high thermal and chemical stability (extremely low vapor pressure at service temperature high hot strength and rigidity excellent corrosion resistance against metal and oxide vapors). 

At 10 torr, a vapor pressure of 10 torr is required to produce an acceptable evaporation rate. For most metals, this temperature is in excess of 1000°C. The refractory metals, or metals with a high melting point such as tungsten, titanium, or molybdenum, are frequently used as carriers, or boats, to hold other metals during the evaporation process. To prevent reactions with the metals being evaporated, the boats may be coated with alumina or other ceramic materials. 

The second method is chemical vapor deposition (CVD). As suggested by the name, unlike PVD, chemical reactions are involved in CVD. Precursor materials in gas phases are introduced into heated furnaces and react at the substrate surface to deposit the desired thin film. For example, CVD is typically performed in low pressure conditions (< 1 Torr) this technique is called LPCVD and usually requires an inert diluent gas such as nitrogen. CVD processes typically involve high temperatures (above 500°C). This is a very important factor to consider in a designing a fabrication process. For example, no metal except tungsten (W) is allowed into CVD furnaces. LPCVD usually has very slow deposition rate. Plasma-enhanced CVD (PECVD) can deposit dielectric films much faster. It also allows deposition at lower temperatures (<400°C). This is very useful when a substrate has already been metalized. 

Knudsen effusion cells are used to determine vapor pressures of high-temperature materials. For example, a Knudsen cell is filled with tungsten and heated to 4500 K in a vacuum. Measurements show that the cell loses mass— assumed to be W vapor—at the rate of 2.113 grams per hour out of a hole that is 1.00 mm in area. Calculate the vapor pressure of W at 4500 K. 

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