Ion Sources Used In Vacuum Coating

There are many different types of ion sources used in vacuum coating. The main ones are: high frequency ion sources, arc discharge ion sources, Kaufmann ion sources, radio frequency ion sources, Hall ion sources, cold cathode ion sources, electron cyclotron ion sources, anode layer ion sources, inductively coupled ion sources and probably many other types of ion sources that have not been mentioned.

Although there are many types of ion source, the objectives are in-line cleaning, improved energy distribution on the plated surface and modulation to increase the energy of the reaction gas. The ion source can greatly improve the bonding strength of the film to the substrate, as well as the hardness and wear and corrosion resistance of the film itself.

High frequency ion sources

The ionisation of gases by means of high-frequency discharges in dilute gases is generally used to produce positive ions in a low-charge state, but sometimes negative ions are also induced from them and used as a negative ion source.

In a high frequency electric field, free electrons collide with atoms (or molecules) in the gas and ionise them. As a result of the multiplication of charged particles, an induction discharge is formed and a large amount of plasma is generated. The discharge tube of a high-frequency ion source is generally made of Pyrex glass or quartz tubes. The high frequency field can be generated either by a coil of solenoid outside the tube or by a ring electrode set outside the tube. The former is called inductive coupling, the latter is called capacitive coupling high frequency oscillator frequency of 10 ~ 10 Hz, the output power in the hundreds of watts or more.

There are two ways to draw ions from the high frequency ion source. One is to insert a tungsten wire at the top of the discharge tube as the positive electrode, at the end of the discharge tube with a hole in the negative electrode, and the hole made of tube-shaped, from which the ion flow. The other way is to make the positive electrode cap-shaped and mount it near the lead electrode, with the discharge area on the other side of it. Whichever lead-in method is used, the metal electrode is encased in quartz or glass to reduce ion compounding on the metal surface.

When a constant magnetic field is added to the high frequency discharge area, the ion concentration in the discharge area can be increased due to resonance phenomena. Sometimes, a non-uniform magnetic field is also added to the lead-out region to improve the lead-out.

Arc discharge ion source

An ion source in which the gas discharge is maintained by the thermal emission of electrons from the cathode in a uniform magnetic field. To reduce gas consumption, the discharge area is often enclosed. The anode is made in the shape of a cylinder with the axis parallel to the direction of the magnetic field. The magnetic field confines the flow of electrons emitted by the cathode well and ionises the atoms (or molecules) of the gas in the anode cavity, forming an arc column with a high plasma density. The ion beam can be led laterally perpendicular to the direction of the axis or in the direction of the axis.

Anode layer ion source

If the tool is plated with a wear-resistant layer, which is generally thicker and does not require high uniformity of film thickness, an ion source with a higher ion current and higher energy level can be used, such as a Hall ion source or an anode layer ion source. Anode layer ion source, and Hall ion source principle is similar. A strong magnetic field is applied to a narrow slit in a ring (rectangular or circular), and the working gas is ionised and directed towards the workpiece by the anode. The anode layer ion source can be made very large and long and is particularly suitable for plating large workpieces, such as architectural glass. The ion current of the anode layer ion source is also high. However, the ion flow is diffuse and the energy level distribution is too wide. It is generally suitable for large workpieces, glass, wear and tear, and decorative workpieces. However, not many applications are available for advanced optical coatings.

Kaufmann ion sources

The Kaufmann ion source is an early ion source. It is a grid-type ion source. The plasma is first generated by a cathode in the inner cavity of the ion source and then extracted from the plasma cavity by a two- or three-layer anode grid. This type of ion source produces highly directional ions with a concentrated ion energy bandwidth and can be widely used in vacuum coating. The disadvantages are that the cathode (often a tungsten filament) burns out quickly in the reaction gas, and also that there is a limit to the ion flux, which may not be suitable for users who require a large ion flux.

Hall ion source

A Hall ion source is a system where the anode ionises the process gas with the help of a strong axial magnetic field. The strong imbalance of this axial magnetic field separates the gas ions and forms an ion beam. Because the axial magnetic field is so strong, the Hall ion source ion beam needs to be supplemented with electrons to neutralise the ion flow.