Date of Award


Document Type

Master Thesis

Degree Name

Master of Engineering (Research)


Electronic Engineering

First Advisor

Mr. Martin Hill


The circuits used in telecommunications are made of two types of components; active integrated circuits that have continuously been improved together with the evolution of the Si technology and off-chip and on-chip passive components. The latter did not profit from much development and now constitute the bottleneck for further integration of RF (Radio Frequency) integrated circuits. MicroElectroMechanical Systems (MEMS) is rapidly emerging as an enabling technology to yield a new generation of high performance components to replace off-chip (not integrated) and on-chip (poor performance) counterparts. The RF components (RF-MEMS) that can be fabricated with MEMS technology are mainly the resonator, capacitor, inductor, switches, transmission lines and antennas. They can be fabricated using 1C compatible techniques, which makes them suitable for integration with other passive and/or active elements. One of the most important elements in the RF domain is a tunable capacitors, which find applications in tunable matching networks, tunable filters and as the frequency controlling element in the LC-tank of a Voltage Controlled Oscillator. The tuning range {TR) of the capacitor determines the frequency range of these circuits; a large TR allows circuits to operate over several frequency bands, making an RF system more efficient. Electrostatically actuated parallel plate, single gap configurations as MEMS tunable capacitors are the most commonly used, but the theoretical tuning range of such structures is limited to around 50% by the pull-in effect.

This thesis describes the characterisation, modelling and evaluation of multi-electrode tunable capacitor structures. In order for the operational characteristic (electromechanical and frequency) of these devices to be predicted and modeled, it is essential that the manufacturing process is known very well and an accurate mathematical model is available. A wide range of aluminium test structures were designed and fabricated on a common silicon substrate in the NMRC Cffl25 surface micromachining process. These included single electrode fixed-fixed beams for process assessment and multi-electrode tunable capacitor structures for RF applications.

Firstly, the results of white-light interferometry inspection and electrostatic measurements of the fixed-fixed beams with various anchor design are presented and used to investigate process quality. This data, combined with Finite Element Method analysis of anchor compliance is used to extract the elastic modulus and residual stress for the thin film aluminium structural layer.

Secondly, the Multielectrode Tunable Capacitor (MTC) is presented. The devices consist of a suspended top plate and split bottom electrode. The “leverage bending” method is examined for extending the TR of such capacitor. An analytical model for the pull-in voltage of the MTC taking account of partial and/or multi-electrode configuration, non-ideal anchors, field fringing and plane strain is presented. The analytical and FEM results of pull-in voltage are validated with measured data. The RF performances of the MTC have been simulated using Ansoft HFSS. The original design has been modified to improve the RF properties, while keeping previous achieved electromechanical characteristic. The MTC design over CPW (Co-planar Waveguide) has been simulated up to 55GHz.

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