All-Oxide Thin-Film Varactors: RF Characterization - Modeling - Integration

Abstract

This work depicts the pioneer steps on investigating the novel all-oxide technology for its application in radio frequency (RF) varactors. The platinum bottom electrode of conventional ferroelectric varactors is replaced with the highly conducting oxide strontium molybdate SrMoO3 (SMO). Its structural similarity to the ferroelectric barium strontium titanate BaxSr1-xTiO3 (BST) helps to overcome the most severe drawbacks of the conventional approach: Increased defect concentration in BST and the formation of an untunable deadlayer at the platinum/BST interface. In all-oxide varactors, BST grows epitaxially on top of the bottom electrode and, hence, an ideal crystal forms instead of the polycrystalline BST on top of platinum. Due to the scarce preliminary knowledge on the properties of jointly integrated SMO and BST layers, a substantial portion of RF characterization and material parameter extraction is required. The first major result of this work is the frequency extension of a widely used on-wafer test structure for dielectric characterization from lower GHz frequencies up to the self-resonant frequency. This allows for the correct extraction of impedance data in the frequency range of interest and is a crucial prerequisite for comprehensive modeling. Furthermore, RF-characterization-based optimization of the deposition process is key to the fabrication of high-performance varactors with unprecedented SMO and BST thicknesses greater than 5μm and smaller than 50 nm, respectively. The greatest emphasis is put on highly accurate analytic models. Present models are not sufficiently accurate for thin-film devices with electrodes often thinner than skin depth. By incorporating the significant contribution of the substrate, a novel schematic model is proposed. It reacts to all significant material properties and yields the true RF properties of the varactor, based on predicting reasonably exact impedance data. This high accuracy further allows for the derivation of a bias-dependent model that considers the electromechanic excitation of acoustic waves. The final model tracks the impedance over a wide range of frequency and electric bias field up to 12GHz and 100V/μm, respectively. Based only on a simple RF characterization, this model enables a confident estimation of both electric and mechanic properties of the integrated varactor. On the one hand, this allows for the precise design of high-performing circuits. On the other hand, it helps to evaluate the great potential of all-oxide varactors to overcome a severe frequency limitation of conventional varactors by reducing the impact of acoustic resonances. To its end, this work addresses the transfer from material identification and optimization towards an industrial realization of the all-oxide technology. Sample reproducibility and uniformity are evaluated as hitherto insufficient, which is, however, a known drawback of the deployed pulsed laser deposition (PLD). Furthermore, the functional varactor deposition on silicon as an industrially established substrate is demonstrated. Finally, the integrability of the novel all-oxide varactors into circuits is demonstrated in two configurations; as a standalone surface-mount device and via integrated growth in an exemplary phase shifter. An outstanding performance of all-oxide devices is predicted after two enhancements in future work. Firstly, the transfer to a method other than PLD is recommended to allow larger substrates and reliably yield high-quality materials. Secondly, a significant improvement was demonstrated by doping BST to increase the Q factor and decrease the acoustic activity. This greatly increases the applicable frequency range beyond the limits of conventional ferroelectric varactors and predicts low loss of all-oxide devices in the full 5G sub-6GHz range and beyond.