Research On Key Technologies Of RF MEMS Switch For Coplanar Waveguide Tunable Filter Based On Silicon

Radio frequency microelectromechanical systems (RF MEMS)tunable filter technology can replace many filters in the traditional communication system, and has the advantages of small size, light weight,low power consumption and low cost. The key technologies of RF MEMS switch, which is the core device for the tunable filter, have become an advanced research hotspot in the world.This dissertation aims to the urgent requirements for high performance RF MEMS switches from tunable filters, and focuses on the problems of "mechanical failure caused by stress concentration of the folded and bending MEMS beam with a low spring coefficient" and"characteristic impedance matching of high on/off capacitance ratio MEMS switch based on coplanar waveguide (CPW) discontinuous structure". The mechanics model and the equivalent circuit model of the MEMS switch with a low actuation voltage have been established.Design methods of the MEMS switches and the tunable filters with high performances have been proposed. Therefore, several kinds of MEMS switches and tunable filters have been developed based on these methods.Finally, the measured results with thoroughly analysis of these devices have been given, and the modified models of the MEMS switches and the filters also have been established.The main innovative achievements of this dissertation as follows:1. Mechanical failure is easily caused by stress concentration of the folded and bending MEMS beam with a low spring coefficient, and residual stress results in great influences on the spring constant of MEMS switch. To resolve these issues, an elliptical ring MEMS beam structure with uniform distribution of stress was creatively designed, and as a result,the effect of residual stress on the spring coefficient of MEMS beam was decreased. Compared with the four-cantilever MEMS switch with low sensitivity to stress, simulation results show that the spring contant growth rate of 100MPa stress is decreased from 284% to 58%. The Ka band MEMS capactive switch was fabricated according to this beam structure. Measured results indicated that the MEMS switch has worked more than 122.4 million without fracture, collapse and dielectric charging.The proposed beam structure can improve the lifetime of Ka band switch effectivity.2. A design method of high on/off capacitance ratio and low loss RF MEMS switch with a low gap was proposed. The MEMS switch based on CPW singal discontinuous structure was designed, and up-state capacitance of the switch was decreased effectively. Meanwhiles, the change law of characteristic impedance of CPW discontinuous structure was thoroughly studied and analyzed, and the characteristic impedance of the switch at operating frequency was realized. Several types of MEMS switches were developed based on this method. Measured results show that, the on/off capacitance ratio of the MEMS switch with 1.5?m gap is 313, which is 2.84 times higher than the switch based on a circuit model with a single variable capacitance.3. The mathematical relationship among circuit parameters, the center frequency and the frequency tuning range of the resonator was studied in depth. A design method of a CPW resonator based on an independent short line with a large inductance and an elliptical annular MEMS switch was proposed, and the method above can improve the flexibility of the design of a tunable filter. The mathematical calculation model of the inductance parameter of a CPW short line was established,and the equivalent conversion between the physical size and the inductance value of the short line was achieved. A K band tunable bandpass filter based on the resonator was designed.4. The relationship between the center frequency and the slot structure for a half wave coupled bandpass filter was thoroughly analyzed.A filter with an open-tree-slot structure, wide-frequency tunable range and miniature size, based on MEMS switches, was designed. Plenty of tunable bandpass filters at K and Ka band have been fabricated based on the proposed structure. Measured results show that, compared with the Ka band CPW half-wave coupled tunable bandpass filter, the circuit size of the proposed Ka band filter is decreased by 61.6%, and the tunable range is increased by 2.98%.