Under the influence of mutually perpendicular electromagnetic fields, electrons move in a cycloidal manner and are bound to the target surface, which prolongs their trajectory in the plasma and increases their participation in the collision and ionization process of gas molecules, ionizing more ions and improving the ionization rate of the gas. Discharge can be maintained even under lower gas pressure. Therefore, magnetron sputtering not only reduces the gas pressure during the sputtering process, but also improves the sputtering efficiency and deposition rate.
However, balanced magnetron sputtering also has its drawbacks. For example, due to the magnetic field, electrons generated by glow discharge and sputtered secondary electrons are tightly confined to the vicinity of the target surface by the parallel magnetic field. The plasma region is strongly confined to an area of approximately 60 mm on the target surface. As the distance from the target surface increases, the plasma concentration decreases rapidly. At this point, the workpiece can only be placed within a range of 50–100 mm on the magnetron target surface to enhance the effect of ion bombardment.
This short effective coating area limits the geometric dimensions of the workpiece to be coated, making it unsuitable for larger workpieces or furnace loads, thus restricting the application of magnetron sputtering technology. Furthermore, during balanced magnetron sputtering, the ejected target particles have lower energy, resulting in poor film-substrate bonding strength. Low-energy deposited atoms have low mobility on the substrate surface, easily forming porous, rough, columnar thin films. While increasing the temperature of the workpiece can improve the structure and properties of the film, in many cases, the workpiece material itself cannot withstand the required high temperature.
The emergence of unbalanced magnetron sputtering partially overcomes the above-mentioned shortcomings. It directs the plasma from the cathode target surface to a range of 200–300 mm in front of the sputtering target, immersing the substrate in the plasma, as shown in the figure. In this way, on the one hand, sputtered atoms and particles deposit on the substrate surface to form a thin film; on the other hand, the plasma bombards the substrate with a certain energy, acting as an ion beam-assisted deposition agent, greatly improving the quality of the film.
Unbalanced magnetron sputtering systems have two structures. One type has a higher magnetic field strength in the core than in the outer ring, and the magnetic field lines are not closed, being drawn towards the vacuum chamber wall, resulting in a low plasma density on the substrate surface. Therefore, this method is rarely used. Another method involves an outer ring magnetic field strength higher than the core magnetic field strength. The magnetic field lines do not form a completely closed loop, with some of the outer ring's magnetic field lines extending to the substrate surface. This allows some secondary electrons to escape from the target surface region along the magnetic field lines and collide with neutral particles, ionizing them.
The plasma is no longer completely confined to the target surface region but can reach the substrate surface, further increasing the ion concentration in the deposition area and raising the substrate ion current density, typically reaching above 5 mA/cm². In this way, the sputtering source also acts as an ion source bombarding the substrate surface. The substrate ion beam current density is proportional to the target current density. Increased target current density leads to a higher deposition rate, while the increased substrate ion beam current density provides a certain bombardment effect on the deposited film surface.
Unbalanced magnetron sputtering ion bombardment can clean the oxide layer and other impurities on the workpiece before coating, activate the workpiece surface, and form a pseudo-diffusion layer on the workpiece surface, which helps to improve the adhesion between the film and the workpiece surface. During the coating process, the bombardment of energetic charged particles can achieve the purpose of modifying the film. For example, ion bombardment tends to peel off loosely bonded and protruding particles from the film, interrupting the dominant growth of the crystalline or condensed state of the film, thereby producing a denser, more uniform, and more uniform film, and can deposit high-performance coatings at lower temperatures.
The application of unbalanced magnetron sputtering vacuum deposition technology has solved the problem of depositing dense and complex films encountered in balanced magnetron sputtering. However, it is difficult to deposit uniform films on complex substrates using a single unbalanced magnetron target. Moreover, as electrons fly towards the substrate, some electrons are adsorbed onto the vacuum chamber walls as the magnetic field strength weakens, leading to a decrease in electron and ion concentrations. To address this, researchers have developed multi-target unbalanced magnetron sputtering systems to overcome the shortcomings of single-target unbalanced magnetron sputtering. Multi-target unbalanced magnetron sputtering systems can be divided into closed magnetic field unbalanced magnetron sputtering with adjacent magnetic poles opposite to each other and mirror magnetic field unbalanced magnetron sputtering with adjacent magnetic poles identical to each other, as shown in the figure for dual-target closed magnetic field and dual-target mirror magnetic field.
Comparing the magnetic field distributions of closed-field non-equilibrium target pairs and mirror target pairs, it can be seen that the magnetic field difference is not significant near the target surface. The transverse magnetic field between the inner and outer magnetic poles confines electrons, forming a highly ionized plasma cathode region. Within this region, positive ions strongly sputter and etch the target surface, sputtering out a large number of target particles that fly towards the substrate surface. At the inner and outer ring magnetic poles, especially at the stronger outer ring magnetic poles, the longitudinal magnetic field dominates, becoming the main channel for secondary electrons to escape the target surface.
This becomes the main channel for transporting charged particles to the coating area. Comparing the magnetic field distribution of closed magnetic fields and mirror magnetic fields within the coating area reveals a significant difference. For mirror target pairs, due to the mutual repulsion between the two target magnetic fields, the longitudinal magnetic fields are forced to bend outwards from the coating area (vacuum chamber wall), causing electrons to be guided to the vacuum chamber wall and lose, thus reducing the overall number of electrons and subsequently ions.
Because the mirror magnetic field method cannot effectively confine electrons, the plasma sputtering efficiency is not improved. In contrast, the longitudinal magnetic field of a closed magnetic field non-equilibrium target pair is closed within the coating area. As long as the magnetic field strength is sufficient, electrons can only move between the coating area and the two targets, avoiding electron loss and thus increasing the ion concentration in the coating area, significantly improving sputtering efficiency.
