Design of a RF MEMS tunable inductor

Abstract

The continuous development and improvement of Integrated Circuit (IC) fabrication processes along with the decreasing feature sizes, enables the miniaturisation of electronic devices, thus permitting to integrate more wireless communication standards within the same device. This in turn requires that the components employed within the circuitry are able to function properly across multiple frequency bands. In addition, these circuits need to consume less power while obtaining a higher performance at a lower cost. A possible approach to meet these demanding requirements is to use high performance tunable passive devices within circuits. In this way a single component can be utilised across a wider frequency range and allow for more exibility. In particular, the work presented in this dissertation focuses on the design and simulation of a Micro Electro Mechanical Systems (MEMS) thermally actuated tunable inductor using the MetalMUMPs process. A variety of applications can utilize such a device since it would allow for a circuit to operate in a wider range of frequencies. The inductor structure considered in this dissertation was aimed to be tunable such that its value is controlled according to the actuation applied. Initially, the mechanical aspects of the structure need to be considered in order to ensure that the mechanical properties of the various materials used in the fabrication process are not altered permanently by the actuation applied. In so doing, the range of actuation applied is determined. Three main mechanical properties to be taken into consideration are the temperature such that the melting temperature of the materials is not exceeded, the displacement achieved through the actuation applied, and the stresses in the materials. After determining the limits due to the mechanical aspects, the electrical properties such as the inductance, the quality factor, and the tuning range can be determined at the di erent applicable actuations. In this dissertation an inductor structure based on a single loop formed from two beams was analysed and the variation of the inductance and how well it operates at di erent frequency values were found after having determined the steady state mechanical response of the system. Simulations were performed and the results were compared to the applicable analytical models both for the mechanical and electrical counterparts so as to check out the performance for the structure implemented in both aspects. In this way the range of voltages applied on the thermal actuator implemented was rst determined, after which the displacement of the beams was found. From all these, the electrical parameters of the structure, particularly of the inductance, the tuning range, and the quality factor were determined across a wide frequency range and so the frequency of operation was also determined.