Balanced magnetron sputtering is a kind of conventional magnetron sputtering. A permanent magnet or a magnetic coil that has a certain magnetic field equal to or close to the magnetic field of the core is placed behind the cathode target, and a magnetic field perpendicular to the direction of the electric field is formed on the surface of the target. The deposition chamber is filled with a certain amount of working gas, and usually it is Ar. Ar atom is ionized into Ar+ ions and electrons under high pressure to generate glow discharge. The Ar+ ion bombard the target after accelerated by the electric field and then sputters out the target atoms, ions and ions, secondary electrons, etc.
Under the action of mutually perpendicular electromagnetic fields, electrons move in a cycloidal manner and they are trapped on the surface of the target to prolong their trajectory in the plasma, and increase their participation in collisions and ionization of gas molecules, and ionizing more ions to increase the ionization rate of the gas. The discharge can be maintained at a lower gas pressure. Therefore, the magnetron sputtering not only reduces the gas pressure during the sputtering process, but also improves the sputtering efficiency and the deposition rate.
However, balanced magnetron sputtering also has disadvantages. For example, due to the magnetic field, the electrons generated by the glow discharge and the secondary electrons sputtered are tightly confined near the target surface by the parallel magnetic field, and the plasma region is strongly confined in the area of about 60 mm on the target surface, the plasma concentration rapidly decreases as the distance from the target surface increases. At this time, the workpiece can only be placed within a range of 50 to 100 mm of targets to enhance the effect of ion bombardment. Such a short effective coating area limits the geometric dimensions of the workpiece to be plated, so it is not suitable for large workpieces or batch loading, which limits the application of magnetron sputtering technology. In the case of balanced magnetron sputtering, the energy of the ejected target particles is low, and the bonding strength of the film base is poor. Low-energy deposition atoms have a low mobility on the surface of the substrate and easily generate film with a porous and rough columnar structure. Increasing the temperature of the workpiece can certainly improve the structure and performance of the film, but in many cases, the workpiece material itself cannot withstand the required high temperatures.
Fig. 1 (a) Balanced magnetron sputtering (b) Unbalanced magnetron sputtering
The emergence of unbalanced magnetron sputtering partly overcomes the above drawbacks by introducing the plasma at the cathode target surface into the range of 200-300 mm in front of the sputtering target, so that the substrate is immersed in the plasma, as shown in Fig. 1 . In this way, on the one hand, the sputtered atoms and particles are deposited on the surface of the substrate to form a thin film. On the other hand, the plasma bombards the substrate with a certain energy, which acts as an ion beam assisted deposition and greatly improves the quality of the film.