Zinc Oxide Sputtering Targets
Zinc oxide is an inorganic compound with the chemical formula of ZnO. Zinc oxide is a white powder that is insoluble in water, and it is widely used as an additive in numerous materials and products including rubbers, plastics, ceramics, glass, cement, lubricants, paints, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants, and first-aid tapes.
Zinc oxide is a material that possesses versatile mechanical, electrical, chemical, and optical properties. Because of its unique physical properties, zinc oxide has drawn broad attentions in different fields and been utilized in a wide range of applications. In particular, using piezoelectric zinc oxide thin films to design sensors and actuators has been studied with demonstrated results, including surface acoustic wave device, acousto-optical device, acoustic microscopy, electromechanical filters, and bulk acoustic wave devices.
A strategic advantage of deposited zinc oxide thin film is that it is amenable to be integrated with IC fabrication process, enabling a single-chip system of the transducers with signal processing electronics. The performance of piezoelectric sensors and actuators critically depends on the quality of the piezoelectric zinc oxide thin films. Substantial research efforts have been devoted to the studies of various deposition methods and parameters for zinc oxide thin films during the last two decades. Among various approaches, RF sputtering is the most studied method. The deposition parameters that could influence the quality of the resulting films include RF power, the ratio of argon to oxygen as the feed gases, target-to-substrate distance, deposition temperature and pressure, substrate, and post-deposition annealing temperature.
To achieve a zinc oxide thin film with high piezoelectric effect, the polycrystalline structures within the thin film should exhibit a strong dominance of c-axis orientation. In particular, high deposition rates usually result in randomized polycrystalline structures within the thin film, and thus compromising piezoelectric performance.
The most important parameter determining the quality of the sputtered ZnO thin film is the substrate on which the film is deposited. Various substrates had been used in different studies but their influences on the film quality were not clearly identified. In a study was conducted comparing the qualities of different zinc oxide thin films sputtered on glass, (100) silicon, silicon nitride (Si3N4), borosilicate glass (BSG), aluminum (Al), and chromium (Cr). The important finding was that the amorphous substrates such as glass, BSG, and Si3N4, could produce high quality zinc oxide films. Other studies focused on metalized silicon surfaces and the role of the adhesion layer for noble metals on silicon. It was found that titanium could form a better zinc oxide film than chromium as the adhesion layer for gold. Also, platinum was a good surface for zinc oxide growth.