An Investigation in Optimising RF MEMS Switching for Integrated Mobile Wireless Systems

Radio Frequency Micro Electromechanical Systems (RF MEMS) technology is used to help switch, filter or tune signals from Direct Current to RF. RF MEMS switches are known to provide good isolation with low insertion loss and can be applied to a wide range of frequencies with almost no power consumed to drive them, which provides an advantage for integrated mobile wireless systems. Despite the benefits, RF MEMS switches have not seen a rapid development for commercialisation in the integrated mobile wireless systems market because of reliability issues and high actuation voltage requirements, which makes additional voltage drive circuitry necessary. This in turn makes the overall switch larger in size. Due to these shortfalls, this research undertakes an investigation by optimising RF MEMS switches. This thesis presents an RF MEMS switch for use in mobile systems, based on the optimisation of a cantilever, ohmic type switch. The optimisation resulted in five major iterations, each providing improvements over the cantilever. The final optimised iteration researched is the ‘S’ Shaped, Split Pivot, Seesaw, Double-Pole Double- Throw (DPDT) switch. This optimised design provides capabilities of high RF isolation. It provides low actuation voltages (by using ‘Delta Plates’). The switch takes advantage of an ‘S’ Shaped, Split Pivot, allowing the pivot to flex with less force, which reduces actuation voltage. The optimised designs provide a selection of switches, which take advantage of low voltages used by mobile systems (i.e. ≤ 5V). The ‘S’ shaped pivot has a lower von mises stress (15MPa), which is below the yield strength of copper (70 MPa), allowing the design to return to its original state without deforming. This proved a 97.8% reduction in von mises stress over the cantilever and a reduction of actuation voltage from 30V down to 1.13V. The functionality of the switch is increased by 4 times to provide DPDT switching over the cantilever’s Single- Pole Single-Throw (SPST) switching. Also, the ‘S’ Shaped, Split Pivot, Seesaw DPDT switch, maintains a higher isolation over its predecessor (cantilever) with an average isolation of -102.89 dB over a frequency range from 5GHz to 45GHz. This provides a 246% improvement to that of the cantilever’s isolation. A Finite Element Analysis approach was used, with mathematical analysis to validate the Intellisuite simulation tool. A secondary validation was conducted, with known practical cantilever results against the Intellisuite Simulation tool for the electromechanical characteristics of the switch. The electromagnetics (EM) characteristics of the switch were also validated, with the Computer Simulation Technology (CST) electromagnetic simulation tool, against the cantilever’s practical results. The optimisation followed a linear approach, with each component of the switch having incremental improvements, such as: Contacts, Beam, Electrostatic Parallel Plates and Pivots. The research has discovered that RF MEMS switches (i.e. cantilevers) are larger than the micro size, require high actuation voltages and have increased von mises stress. The optimisations focused on four areas of improvements to the characteristics of the switch and addressed them as follows: reducing actuation voltage, decreasing von mises stress, increasing isolation of the contacts and expanding functionality.