Microwave properties of tunable capacitors basee on magnetron sputtered ferroelectric Na_0.5K_0.5NbO₃ film on low and high resistivity silicon substrates

Abstract

In this work, small signal DC voltage dependent dielectric permittivity, loss tangent, and tuneability of magnetron sputtered epitaxial Na_0.5K_0.5NO₃ films are studied experimentally. (100)-oriented Na_0.5K_0.5NbO₃ films are deposited onto SiO₂-buffered CMOS grade low resistivity (p = 10–20 Ωcm) and high resistivity (p = 15–45 kΩcm) silicon substrates. Planar capacitors with 2 or 4 μm gaps between electrodes have been fabricated on top of ferroelectric films. These devices have been characterized in the frequency range 1.0 MHz to 50 GHz at temperatures 30 – 300K. Na_0.5K_0.5NbO₃/SiO₂/Si structures on high resistivity silicon substrate exhibit C-V performances typical for Metal-Insulator-Semiconductor (MIS) capacitors. At low frequencies, f< 1.0 GHz, the large tuneability and large losses are associated with the MIS structure, while at higher microwave frequencies the tuneability is mainly associated with the ferroelectric, film. At 1.0 MHz and room temperature, the tuneability of Na_0.5K_0.5NbO₃/SiO₂/Si structures more than 90%, reducing to 10–15 % at 50 GHz. The losses decrease with increasing the DC bias and frequency. A Q-factor more than 15 at 50 GHz is observed. The dielectric permittivity of the Na_0.5K_0.5NbO₃ film is in the range 50–150 at frequencies 0.045–50 GHz. On low resistivity substrate the performance of Na_0.5K_0.5NbO₃ films is completely screened by the high losses in silicon, and the tuneability is negligible.

Key Words:

• Ferroelectric
• dielectric permittivity
• microwave loss
• tuneability
• Na_0.5K_0.5NbO₃