High-purity or ultrahigh-purity tungsten (5N or 6N) is used in the form of thin films in the electronic industry as the preferred material for gate electrodes, interconnects, and barrier metal because of its high resistance to electron migration, high temperature stability, an d tendency to form stable silicides. In principle, tungsten thin films can be produced either by CVD or by PVD. For PVD-produced tungsten thin films, sputter targets of high purity (6N) are commercially available today.
This is partly due to improvements in production methods, which in fact caused higher purity. But it is also somewhat a consequence of increased sensitivity of the analytical methods and the equipment used nowadays. For example, some years ago it was only possible to say that the concentration is less than 1µg/g or less than 0.1µg/g. Meanwhile, application of more advanced techniques has changed the situation drastically. In high-purity tungsten, metal levels of trace elements are in the ng/g level or even below, which affords special analytical requirements such as working in clean room laboratories, prevention of any other contamination, and correct calibration. The most important analytical methods applied today for that purpose are glow discharge mass spectrometry, secondary ion mass spectrometry, isotope dilution, and trace matrix separation combined with inductively coupled plasma atomic emission spectro-metry.
As an example, it was possible by isotope dilution analysis to determine 15ppq of uranium in an ultrapure tungsten sample. This corresponds to /5vx 10-15%. These methods are generally very complex and expensive. Comparative analyses often show significant scatter.
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