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

Freeman ion source is a kind of ion source which is drew forth by electron collision. Its filament and the narrow slit of ion extraction are parallel and located directly behind ion source. The length of the filament is corresponding to length of narrow slit. Under the interaction on magnetic field of filament and the external magnetic field, forcing the electrons generated by the heat of the cathode do the screw motion in the non electric field plasma. About 100 Oe variable magnetic field can be generated by a magnetic field. The breakdown of ion source is that due to some problems which would cause the ion source is not working properly. In particular, when the metal ions are implanted, the working substance is mostly metal chlorides, easily deposited on the ion source insulator, causing a short circuit between the electrodes. In addition, filament poisoning, intake duct blockage and corrosion damage is common problems of ion sources.

While using an ion source injection machine for modification process of material surface, the ion source may not work properly so that it needs to be shut down for inspection and maintenance. If there exists operation materials that deposited on the filament during inspection which will lead to a partial or entire filament become thicken slowly. The filament resistance will be weakened, reducing its ability to emit electrons which would cause arc blowout that is also known as arc extinction. When the voltage exceeds a certain value, it would lead to an arc discharge. For example, when the breaker is disconnected under the circumstances of electricity, however, due to the high voltage on the fracture, it will generate result in arc extinction, and the circuit continues to conduct which is very dangerous. This phenomenon is referred to as filament poisoning.

After a chemical treatment and the chemical cleaning, tungsten filament is put in ion source. Adopt MoCl₅ whose purity is 99.9% as the working substance to analyze poisoning of an IG tungsten filament. In the process of degassing, heating the filaments slowly, and then start to vaporization, through the heat conduction of intake-tube to make the part of the work material be gasified. After entering the arc chamber, most of MoCl₅ gas will be extracted in beam induced exit of arc chamber. The remaining part is deposited on the tungsten which would decompose into Mo and Cl₂ quickly at a high temperature. Cl₂ will be pulled out, but Mo is still deposited on the filament, leading poisoning. During the inspection, find that filament become thicker evenly and has a serious poisoning.

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