Although there are many types of ion source, the purpose is nothing more than on-line cleaning, improvement of the energy distribution of the coated surface and increase of the reactive gas energy. The ion source can greatly improve the bonding strength between the coating and the substrate, at the same time the hardness, wear resistance and corrosion resistance of the coating itself will also be improved. For tools wear-resistant coating, as the general thickness is large and the uniformity of the film thickness is not high, the ion sources with higher ion current levels, such as Hall ion source or anode layer ion source can be used.
The principle of anode layer ion source is similar to that of Hall ion source. A strong magnetic field is applied in an annular (rectangular or circular) slot, and the working gas is ionized under the action of the anode and the ions are directed at the workpiece. The anode layer ion source can be made in a very large and long size to especially suitable for large workpieces such as architectural glass. The ion current of it is more divergent and the energy level distribution is too wide, so it is generally suitable for large workpieces, glass, wear, decorative workpieces and so on. However, it is not used too much for advanced optical coatings.
The Kaufman ion source is a kind of ion source that applied earlier. It belongs to grid ion source. The plasma is first generated by the cathode in the ion source chamber and the ions are extracted from the plasma chamber by two or three anode grids. The ion that produced by this ion source has strong directionality and the energy bandwidth is concentrated, so it can be widely used in vacuum coating. The disadvantage of the Kaufman ion source is that the cathode (usually a tungsten filament) burns off rapidly in the reaction gas. In addition, there is a limit to the ion flow, which may be unsuitable for users who requiring large ion fluxes.
For the principle of the Hall ion source, an anode makes the process gas to be plasmas in cooperation with a strong axial magnetic field. This strong imbalance of the axial magnetic field separates gas ions and forms ion beams. Due to the strong effect of the axial magnetic field, the ion beam of Hall ion source needs to supplement electrons to neutralize the ion current. A common source of neutralization is tungsten (cathode). Hall ion sources are characterized by:
1. Simple operation and durable.
2. The ion current is almost proportional to the gas flow, and a larger ion current can be obtained.
3. The tungsten wire generally crosses the exit, and it will be eroded away by the ion beam impact. So it generally needs to be replaced in more than ten hours especially for the reaction gas. Besides, tungsten also has some pollution problems. To solve the shortcomings of tungsten wire, the neutralizers with longer life, such as a small hollow cathode source can be used.
Hall ion source can be said to be the most widely used ion source.
When coating the wear-resistant decorative film that has large thickness is large strong bending strength while the uniformity is not required to be high, the Hall ion source is available as its ion current is large and its energy level is high. When coating an optical film, the ion current level is mainly required to concentrate, and the uniformity of the ion current should be good. Therefore, Kaufman in source or RF ion sources are preferably used, and the best choice are ECR (electron cyclotron) or ICP (inductively coupled) ion sources.