Study On The Reduction Of Dielectric Charging In Capacitive RF MEMS Switches

RF MEMS switches are a kind of micro-mechanical switches on transmission line which uses the mechanical movement working in the frequency range of RF to millimeter wave.It is the basis of MEMS variable capacitor,filter,resonator and phase shifter.At present,the main factor restricting MEMS switch application is the reliability of the switch,especially the charge injection problem that the switch generates during the working process.The purpose of this paper is to analyze the charge injection mechanism of MEMS switch,at the same time,the distribution of electric field and the main factors such as driving voltage that influence the switch charge injection,in order to reduce the switch charge injection effect.The main work of this paper are as follow:1.The critical failure state of MEMS capacitor switch is analyzed.And than,deduces the charge injection that causes the switch to fail and the switch life time.By using MATLAB to simulate and analyze the physical quantities of the charge injection and the life of the switch,the change trend of charge injection and switch life with the physical quantity is obtained.The current density increases with the increase of voltage and temperature and decreases exponentially with the increase of relative permittivity.So,the charge injection of MEMS capacitor switch can be reduced by reducing the electric field intensity of the dielectric layer and the elastic coefficient of the switch beam.2.The structure of the MEMS capacitor switch is optimized,and the electric field intensity in the contact area of the switch capacitor can be reduced by separating the signal wire and the drive electrode.The switch structure separating the signal line and the drive electrode includes the structure of the lateral pull electrode and the structure of the hollow electrode.By using HFSS to simulate the switches of the two structures,the electric field of the traditional structure is 1.07×10~8V/m,the electric field of the lateral pull electrode structure is 1.19×10~7V/m and the electric field of the hollow electrode structure is 4.58×10~6~1.38×10~7V/m.So the electric field intensity of the switch capacitor contact area can be effectively reduced by two kinds of switches.There is a slight increase in the insertion loss of the switch up state of the lateral pull electrode structure,and the isolation degree of the switch down state is not very different,but the resonant frequency of the switch will increase.There is little change in the insertion loss of the switch up state of the hollow electrode structure.Due to the decrease of capacitance in the capacitor contact area,there is a increase of the switching resonant frequency and a decrease of the switch down state isolation degree.3.Monolayer graphene membrane is used to design the structure of the gold-graphene-gold to enhance the mechanical strength of the switch beam,to make up for the switch beam elastic coefficient to reduce the impact of low mechanical strength,and reduce the residual stress of switch beam.The change of driving voltage of switch beam before and after adding graphene was analyzed.The process flow of design switch beam is designed and optimized by experiment.The tested residual stress of the beam is 52.58MPa.And the simulated elastic coefficient is 35.33N/m.The life time of the switch using the beam is 1.11×10~6s.