Why is the Target Material the Cathode?

In magnetron sputtering, the target material is typically designed as the cathode. This design is not coincidental; it is based on various physical principles and practical needs, playing a crucial role in the efficient and stable operation of the sputtering process.

Ion Acceleration and Sputtering Process

In magnetron sputtering, the negative potential of the electric field makes the target material a cathode, which attracts positive ions from the plasma. When the target is at a negative potential, the generated electric field directs positive ions to accelerate directly toward the target surface. These positively charged ions strike the target surface with a certain kinetic energy, releasing target atoms, which are subsequently sputtered onto the substrate to form a thin film. Thus, the negative voltage configuration of the target ensures that positive ions can accelerate to the target surface, facilitating an effective sputtering process.

Ensuring Stability of Plasma Generation

The introduction of a magnetic field further optimizes the role of the target as a cathode. In the magnetron sputtering process, a transverse magnetic field is set near the surface of the target, inducing electrons in the plasma to spiral along the magnetic field. Because the electrons are confined by the magnetic field, their paths are significantly elongated, greatly increasing the collision probability between electrons and gas molecules. This enhances the ionization degree of gas molecules in the vicinity of the target surface, resulting in a high-density plasma. This plasma enhancement effect creates a high concentration of positive ions at the target surface, providing a sufficient source of positive ions for sputtering. Compared to traditional sputtering, magnetron sputtering can achieve high sputtering efficiency under low gas pressure conditions, reducing the system's demand for working gas and significantly improving sputtering stability and economy.

Maintaining Stability of the Sputtering Process

As a cathode in magnetron sputtering, the target not only facilitates ion acceleration and promotes plasma generation but also plays a key role in maintaining the stability of the sputtering process. The negative potential of the target continuously attracts positive ions, causing them to strike the target and produce a stable sputtering effect. If the target loses its negative potential, positive ions will lack acceleration and will be unable to bombard the target, leading to a cessation of the sputtering process. Furthermore, the stability of the target's negative potential is crucial for the uniformity of the thin film deposition process. Under the influence of the negative potential, the frequency of positive ion bombardment remains high, and the angles of impact tend to be consistent. This stability allows for more uniform release of target atoms, enhancing the density and structural uniformity of the thin film and ensuring the continuity of the deposition process, which helps optimize the surface flatness and thickness uniformity of the film.

Reasons Why the Anode is Not Suitable

In contrast to the cathode role, the target is not suitable as an anode. From the electrochemical characteristics of the anode, the anode is a positively charged electrode that primarily attracts electrons and is used for oxidation reactions in chemical batteries. However, in the plasma environment of magnetron sputtering, sputtering relies on positive ions to bombard the target surface, while an anode attracts electrons instead of positive ions, hindering the acceleration of positive ions. The accumulation of electrons on the target surface not only obstructs the release of sputtered atoms but also leads to plasma instability, thereby reducing overall sputtering efficiency.

From a theoretical perspective, if the target is at a positive potential, it can cause instability in the sputtering current. A positively charged target attracts electrons, and electron bombardment cannot achieve sputtering, leading to a decline in the sputtering effectiveness of the entire system. Additionally, a positively charged target will not provide sufficient acceleration for positive ions, preventing effective bombardment of the target and directly affecting the release rate of sputtered atoms, thus reducing sputtering efficiency. Particularly under high current conditions, a positively charged target can trigger abnormal arc discharge phenomena, further destabilizing the sputtering process.

In practical operations, particularly in RF sputtering, the periodic reversal of the electric field means that the target briefly becomes the anode in each cycle. This periodic positive potential can cause fluctuations in the sputtering rate, affecting the quality of film deposition. The configuration of a positively charged anode in practice may also lead to plasma instability, resulting in phenomena such as arc discharge or incomplete discharge, which causes uneven plasma density and weakens the uniform deposition of films. Therefore, the configuration of the target as an anode presents numerous theoretical and practical obstacles, hindering the continuous and stable operation of magnetron sputtering.