In the CAPD process, material is evaporated under the action of one or more vacuum arcs, the source material being the cathode in the arc circuit (Figure 33.1). A basic coating system consists of a vacuum chamber, cathode and arc power supplies, arc igniter, anode and substrate bias power supplies. The arc is maintained by a voltage in the range of 15 to 50 V, depending on the source material; a typical arc uses a current in the range of 30 to 400 A. When high current is used, one arc spot splits into multiple spots on the cathode surface, the number depends on the cathode material. This illustrates the titanium source in Figure 33.2. In this case, the average arc current/arc point is about 75 A. The arc spot moves randomly on the surface of the cathode, usually at a speed of tens of meters per second. The movement and speed of the arc spot may be further influenced by external means such as magnetic field, air pressure and electrostatic field during coating process.
Removal of material from the source occurs as a series of rapid flash events, such as arc spot migration to the cathode surface. Persisting arc points due to material plasmas are created by the arc itself and can be controlled by appropriate boundary shielding and/or magnetic fields.
Figure 33.1 Schematic diagram of cathodic arc deposition system.
Figure 33.2 The relationship between the number of arc points on the titanium cathode arc source and the arc current
CAPD differs significantly from physical vapor deposition processes. Some of its features are as follows:
1. The material plasma is generated by one or more arc spots.
2. A high percentage (30 to 100%) of the evaporated material is ionized.
3. Ions exist in multiple charge states (for example, in the case of Ti, Ti+, Ti2+, Ti3+, etc.).
4. Ions have very high kinetic energy (10 to 100 eV).
These properties of CAPD result in deposits of superior quality to other deposition plasma processes. Some of these advantages are as follows:
1. Good thin films under various deposition conditions (for example, stoichiometrically achieved thin films can be obtained with enhanced adhesion and film density and evaporation rate over a wide range of reactant gas pressures)
2. High deposition rate of metals, alloys and compounds with excellent coating uniformity
3. Substrate temperature is low
4. Retention of alloy composition from source to deposit
5. Thin films of compounds that are prone to reactions
