Filament Poisoning of IG Tungsten Filament in Freeman Ion Source in Working

Freeman ion source has been 30 years of history, it is still one of the most widely scope of application and the most commonly used ion source. Because it can make most of the elements generate a very small focus and Ma level ion beam, and it has a wide energy range so that this kind of ion source becomes a common choice. On semiconductor ion implantation applications, the source has a good performance and a simple operation and other excellent characteristics, which is widely used as a doped of silicon devices. Freeman ion source has been successfully applied in many aspects of ion implantation apparatus for decades, including the production of a milliampere level of a variety of metal ion beam. Throughout the ion implantation apparatus for industrial using, approximately 95% are using it.

While working, it is possible to have the phenomenon of poisoning. On the basis of a chemical treatment to remove the surface oxide layer and the chemical cleaning, load a diameter of 1.5mm industrial tungsten wire in ion source injection machine. Adopting the 99.9% purity of MoCl5 as working substance which is easy to absorb moisture, on the other hand, it is easy to be broken down. During the test, it is found that IG tungsten filament would poison in Working. Whiling work, the insulation of insulator is breakdown, causing the short-circuited of electrodes between the filament and the arc chamber, resulting in the filament poisoning. During the MoCl₅ injection process, if the amount of intake air is too high, the ionization rate of the arc chamber would be reduced, a part of MoCl₅ will be deposited on the directly heated cathode, causing poisoning phenomenon. Because the temperature of electron-tube heater which is near the both ends of the arc chamber wall is relatively low, it is easier to deposit MoCl₅ at its ends. Therefore, poisoning of this process often occurs at both ends, it can be found through the thicker filament. But the poisoning of the middle part of the filament is not so obvious so it also has the ability to emit electrons. Most reasons for filament poisoning are the higher vapor pressure under the low temperatures. In addition, molybdenum and tungsten are two completely miscible metals, it is conducive to the growth of the film at the initial stage, but the amount of intake air also makes a great impact of filamentary cathode.

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