The sputtering process has emerged as one of the major deposition techniques for thin film coating practices in research and industrial production. The process is limited by low deposition rates and low kinetic energy of the sputtered atoms. This not only slows the time required to sputter a film, but lowers the purity of the sputtered film.
The Molten Target Sputtering (MTS) method can increase the kinetic energy, the energy latency, and the flux density of sputtered atoms by combining the benefits of both magnetron sputtering and evaporation systems. It does this by a clever, but simple mechanical modification to the magnetron sputtering gun. For the MTS method, a simple 1~2-mm (depth and width) ring-shaped groove is cut between the magnets in order to keep a gap between the copper plate and the sputtering target. This effectively is an efficiency increase to a current magnetron-based sputtering machine. The ring enhances the magnetic field intensity and increases the temperature of the target material.
A key difference in design between a conventional sputtering gun and the MTS gun seems very minor, but the ring-shaped groove between magnets allows for trapping a portion of magnetic field within the groove. A trapped field creates an additive force to expel the ionized particles (atoms and molecules combined within plasma) by increasing their kinetic energy. The ring groove also serves to increase the target material temperature because the conduction passage of thermal energy to the water-cooled copper sink is interrupted by the empty space of a ring groove below the target. Accordingly, the target surface material is heated to a higher temperature, which in turn increases the flux density by more easily liberating the atoms from the target.
The MTS enables the growth of high-quality thin (<100 nm) and thick (>100 m) films with the molten target and plasma. The degree of precise formation of rhombohedral film structures on a sapphire substrate is greatly improved. This leads to higher-quality crystals with higher yield. In conventional techniques, a preprocessor is required to elevate the sapphire wafer temperature through a 2- to 4- hour thermal soak process. These high temperatures, typically 890 °C, are costly and burdensome in a production facility. The MTS process can enable the sputtering temperatures to operate at 500 °C and still produce superior-quality films.
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