Tuneable RF MEMS components using SU-8

Abstract

With the rapid progress in the wireless communication field, radio frequency microelectro- mechanical systems (RF MEMS) are seen as one of the promising technologies to replace the existing high power communication systems. MEMS based tuneable devices such as varactors and phase shifters offer many advantages over their conventional diode-based counterparts including low loss, low power consumption and high linearity. MEMS varactors in particular can be integrated into many reconfigurable modules such as switching and reconfigurable matching networks. Moreover, distributed MEMS transmission line (DMTL) phase shifters with their linear phase characteristic can be applied to wideband phased array antennas for microwave medical imaging which requires beam steering and high gain antenna systems. This thesis focuses on the design and development of two RF MEMS devices which are a high tuning ratio digital MEMS varactor and a low frequency DMTL phase shifter using SU-8 polymer. The design and simulation of a 4-bit and a 5-bit digital MEMS varactors have been carried out in the first phase of this study. One of the limitations of the digital MEMS varactors fabricated on silicon substrates is the high fringing field capacitance that reduces the overall capacitance ratios of the devices. To reduce the effect of the fringing fields, two methods have been proposed to elevate the varactors from the silicon substrate. In the first method, a 26.35 μm deep trench is etched in the silicon substrate under the 4-bit digital MEMS varactor which is able to achieve a high capacitance ratio of 35.7. In the 5-bit digital MEMS varactor design, SU-8 material is used to form a 20 μm thick separation layer between the varactor and the silicon substrate instead of the deep trench method applied in the 4-bit MEMS varactor. The simulated capacitance ratio of the 5-bit digital MEMS varactor is 34.8. Additionally, the SU-8 also serves as a sacrificial layer to release the MEMS bridges on the devices hence reducing the fabrication process compared to the conventional MEMS release process that uses oxide as the sacrificial material. To verify the performance of using the thick SU-8 dielectric layer in reducing the fringing field capacitance in the varactor design, single-bridge varactors with different lengths and widths have been fabricated and analysed. A novel truss bridge structure has been proposed in order to reduce the pull-in voltage of the varactors. It is found that by using the truss structure, the measured pull-in voltage of the bridge can be reduced by 12.5% compared to the conventional solid fixed-fixed bridge structure. However, due to the high residual stress from the fabrication process which causes the bridge to warp over its width, the achievable average down-state capacitance of the fabricated single-bridge varactor is limited to 211 fF compared to the simulated value of 1.28 pF. Nevertheless, the capacitance ratio of the device fabricated on the SU-8 layer increases by 56.75% over a similar device fabricated without the polymer which proves that the fringing field capacitance has been reduced. Furthermore, fabrication of the single-bridge MEMS varactors on low-resistivity silicon has been carried out with the use of SU-8 as the passivation layer without affecting the performances of the varactors. This finding can lead to the realisation of low-cost MEMS varactors in the future. The second part of this thesis investigates the development of distributed MEMS transmission line (DMTL) phase shifters for operation in the frequency range of 2 GHz to 4 GHz (S-band). The proposed phase shifters are a 2-bit and 3-bit digital DMTL phase shifters. One of the potential applications of the proposed phase shifters is for phased array antenna systems for microwave head imaging that requires wideband performance. The 2-bit and 3-bit DMTL phase shifters have been designed and simulated with 41 MEMS bridges and 105 MEMS bridges respectively. The simulated phase shifts of the 2-bit phase shifter design are 00, 900, 1800 and 2700 whereas for the 3-bit phase shifter, 8 phase shifts have been achieved namely 00, 450, 900, 1350, 1800, 2250, 2700 and 3150. To validate the performance of the proposed low frequency DMTL phase shifter, the 2-bit phase shifter design has been fabricated and analysed. The measured impedance matching of the phase shifter shows good performance with reflection coefficients of less than -10 dB across the operating frequency range for all the states of the phase shifter. The measured differential phase shifts of the device are 00, 17.890, 34.510 and 52.390. The lower measured differential phase shifts compared to the simulated values can be attributed to the warping of the bridges over their width which causes a formation of an air gap between the bridge and dielectric layer hence reducing the down-state capacitance of the varactors in the phase shifter. Nevertheless, this is the first DMTL phase shifter to achieve a maximum differential phase shift of 52.390 at 2.45 GHz. Based on the measured differential phase shifts, the phase shifter can provide a maximum steering angle of ±5.730 for a 4-element phased array antenna at 2.45 GHz. The maximum measured transmission loss of the phase shifter is -10.51 dB at 2.45 GHz. The high loss of the phase shifter is due to the skin depth effect since the co-planar waveguide (CPW) transmission line of the phase shifter is fabricated using 300 nm thick aluminium. Therefore, further investigation has been carried out to provide better estimation of the transmission loss of the phase shifter by fabricating a CPW transmission line with the same configuration to that of the transmission line in the fabricated phase shifter by using 2 μm thick aluminium. The measured loss of the transmission line is -2.39 dB which shows significant improvement over the loss obtained from the phase shifter. Moreover, several CPW transmission lines with different centre conductor’s widths have been fabricated and analysed to further reduce the losses of the transmission lines. An attenuation loss of only 0.122 dB/cm has been achieved using a 500 μm-width centre conductor in the fabricated CPW transmission line which can lead to the realisation of a low-loss DMTL phase shifter for low microwave frequency range. The characterisation and optimisation of the varactors and phase shifters using SU-8 provide the initial step towards the development of tuneable RF MEMS devices for wide range of applications including wireless communications and radar systems. Moreover, the proposed DMTL phase shifters for operation at the lower end of microwave spectrum particularly in the frequency range of 2 GHz to 4 GHz are vital for the realisation of wideband phased array antennas for microwave medical imaging applications.