Characterization of (Ba,Sr)TiO3 thin films for multiplier applications

Doktorsavhandling, 2006

(Ba,Sr)TiO3 (BSTO) and SrTiO3 (STO) parallel plate ferroelectric varactors are investigated for microwave multiplier applications. STO and BSTO exhibit low losses, high permittivity and large tunability of both permittivity and losses. All films are deposited using Pulsed Laser Deposition, magnetron sputtering and E-beam evaporation. Detailed microstructural analysis of the deposited films is carried out including X-Ray Diffraction, Transmission Electron Microscopy (TEM) and Energy Dispersive X-rays. The growth modes, film defects and preferred orientation depend heavily on for instance substrate, former films and deposition temperatures. The BSTO/STO films in this work are grown SiO2, Pt, Pt/Au and SrRuO3 (SRO) on Si substrate. Scanning Electron Microscopy (SEM) and Atomic Force Microscope are used to investigate the surface roughness. SEM is also used to shown the granular structure of the ferroelectric film. The TEM is also used for interfacial studies and microstructural analysis. The electrical measurable material properties (i.e. permittivity and losses) are correlated to the measured material properties (i.e. strain, surface roughness, film growth etc.). The ferroelectric films are characterized both in terms of small and large signal dielectric properties at RF (1 - 30 MHz) and microwave frequencies (45 MHz - 45 GHz) at both room or cryogenic temperatures. The ferroelectric exhibit very low losses (0.01 to 0.04 at 10 GHz), high permittivity (200 and 1000) and tunability (65 % for 25 V). The losses increase as f(1/3), the permittivity and tunability are practically frequency independent up to 3 THz. By using these reliable models intended for a large frequency span, equivalent circuits with approximately frequency independent parameters could be developed. In order to keep the parameters frequency independent different equivalent circuits are suggested for the ferroelectric varactor, depending on the frequency range. The small signal equivalent circuit is transformed into a large signal ditto by considering the factors influencing the power handling capability. The power handling for the varactor intended for multiplier applications is limited by heat conversion and self-biasing. The large signal equivalent circuit is used in non-linear simulations. The non-linearities are simulated using both Agilent ADS, Microwave office and an in-house harmonic balance simulator. For preliminary estimations of the varactor for multiplier (tripler) applications, a two-port teststructure was designed to investigate the non-linear properties and harmonic generation. The corresponding losses from first to third order harmonic was less than 20 dB and the IP3 point was measured to be 22 dBm.