Low-Temperature Deposition of SrTiO3 Thin Films by Electron-Cyclotron-Resonance Sputtering for Monolithic Microwave Integrated Circuits Operating in the mm-Wave Band

Electron-cyclotron-resonance (ECR) plasma assisted deposition of strontium titanate (STO) thin films is discussed. Films having a high dielectric constant (ε r) can be deposited at a temperature as low as 100°C by controlling the ECR plasma energy and ion current density. There exists a pressure that gives the maximum refractive index. The dependence of the refractive index and dielectric constant on the deposition temperature can be minimized at that pressure, where the ECR plasma energy and ion current density strongly assist the STO deposition. The pressure is approximately 0.05 Pa in an ECR sputter apparatus with a plasma energy of 10 eV and an ion current density of 5 mA/cm2. 100-nm-thick STO thin films, having ε r of 70, a Qf product greater than 1000 in the mm-wave band up to 50 GHz, and dc leakage current less than 10-8 A/cm2 at 0.5 MV/cm, are realized by deposition at 0.05 Pa and 100°C. Deposition at other pressures results in a lower dielectric constant due to poor ECR plasma assistance. The properties of the STO thin films presented here are essential to process on-chip capacitors for monolithic microwave integrated circuits (MMICs).