In order to improve the mechanical reliability and lifetime of a series contact Radio Frequency Microelectro-mechanical System(RF MEMS) switch with dielectric bridge,the fabrication process was studied.The switch structure was analyzed,the results illustrated that the mechanical reliability of the switch was debased with regular process because of the stress concentration of the bridge film.After that,an isotropic undercutting process for forming the upper control electrodes and contact bar under the bridge was proposed to obtain plane bridge film.Finally,the whole process of the switch was presented.Compared with the regular process,the proposed method can avoid the problem of stress concentration,and can improve the mechanical reliability of the switch.As a result,the yield rate of the switch increases from 10% up to 95%,the lifetime is improved from 1 000 cycles up to 2.5×10⁷ cycles.In addition,under the actuation voltage of 23.3 V,the insertion loss is less than 0.55 dB@DC-10 GHz,and the isolation is more than 53.2 dB@DC-10 GHz.These results show that proposed process can satisfy the modern wireless communication system requirements of yield rate,lifetime and microwave performance.