A Study On Reliability Of Silicon Nitride For Space Applications In MEMS Devices

MEMS(Micro-electro-mechanical-system)refers to one kind of device or system which consists of micro-structures,micro-sensors,micro-actuators as well as drive circuits.The feature size of MEMS varies from sub-microns to microns.The features of miniaturization,integration and informatization make MEMS suitable for space applications,especially for micro-satellites and pico-satellites.According to the complex space environments,the reliability of MEMS devices used in mid-to-long-term space applications have to be evaluated.The researches on the reliability of MEMS for space applications have been focused on the performance degradation and failure phenomena in irradiation environments,but the theories on the radiation induced failure mechanism are still lacking.Various types of dielectric materials are used as isolation layer in MEMS devices fabricated with surface micromachining processes.The radiation effects in space environments may significantly influence the lifetime of the MEMS devices.Therefore,the study on reliability of dielectric materials such as silicon nitride in MEMS for space applications is of significance to the reliability of MEMS devices.Firstly,the basic concept and principle of MEMS are introduced.The MEMS devices which have been applied in space applications are sorted out.The space environments,the basic damage mechanisms and degradation effects of accelerators,comb drivers and RF switches are reviewed.The conduction characteristics of leakage current of SiN in irradiation environments are focused in this paper.Secondly,the designs and principles of two main RF MEMS switches are introduced,followed by the failure processes and mechanisms.The charging effects in the dielectric film and its impacts on capacitive RF switches are introduced in detail.Thirdly,the experimental schemes for the study of dielectric reliability for space applications are introduced including the design and fabrication of metal-insulator-metal capacitor,the specific irradiation conditions,and the measuring methods of semiconductor parameter analyzer.Finally,the I-V characteristics of MIM capacitor before and after different doses of ? irradiation are obtained using semiconductor parameter analyzer.The Ohmic conduction,space charge limited current and Frenkel-Poole emission mechanisms are verified under different electric fields.A quantitative model in low electric field implying the irradiation effects on MIM capacitor is abstracted while the ? irradiation effects in medium and higher electric field are discussed on the base of the band diagram of the conduction processes respectively.The reliability of silicon nitride for space applications in MEMS devices is studied both theoretically and experimentally using? irradiation source of high dose rate.The experimental conclusions are of value to MEMS devices with dielectric materials including RF switches.