Optimization of high tunability barium strontium titanate thin films grown by RF magnetron sputtering

Barium strontium titanate is a solid solution perovskite with a field-dependent permittivity. At microwave frequencies, its tunable dielectric constant and low loss make it a competitive choice for varactors and other tunable circuit elements. Much attention has been focused on the production of low-loss films for such applications, with little emphasis on how the film properties contribute to the circuit loss. When tunable elements are implemented in circuits, electrode loss dominates over film loss. In applications such as phase shifters, where cascaded tuning elements provide a predetermined amount of tuning, circuit designs using high tunability films minimize the number of tuning elements required, resulting in an overall reduction in circuit loss. When growth conditions are optimized for superior electrical properties, tunability and film loss are the two quantities of interest. By changing the oxygen partial pressure during growth, the amount of excess Ti incorporated into the film is changed. Films with higher excess Ti contents exhibit lower losses, higher breakdown voltages, and lower permittivities than more stoichiometric films. While all of the films approach the same high-field capacitance limit, the total tunability is determined not only by the zero-field permittivity but also by the breakdown voltage; a device must be able to tolerate sufficient applied bias to reach its high-field capacitance limit. By balancing these factors, we have produced capacitors with an unprecedented 13.71:1 (92.7%) tuning ratio at an applied field of 4.7 MV/cm.