Microfabricated Solid State Thin Film Lithium Batteries
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Microfabricated Solid State Thin Film Lithium Batteries Sami OUKASSI1,2, Raphael Salot2, and Jean-Pierre Pereira-Ramos3 1 STMicroelectronics, 850 rue Jean Monnet, Crolles, 38054, France 2 CEA, 17 rue des martyrs, Grenoble, 38054, France 3 CNRS, 2 rue Henri Dunant, Thiais, 94320, France ABSTRACT This work presents recent advances in the development and the integration of a solid state thin film battery, to work as a high voltage energy source for RF-MEMS powering. Microelectro-mechanical systems require similarly miniaturized power sources. Up to day, microbatteries are realized with mechanical masks, this method doesn’t allow dimensions below several decades of mm² of active area, and besides the whole process flow is done under controlled atmosphere so as to ensure materials chemical stability (mainly lithiated materials). Within this context, Microelectronics micro-fabrication procedures (photolithography, Reactive Ion Etching…) are used to reach both miniaturisation (100x100 µm² targeted unit cell active area) and microelectronic IC technological compatibility. The whole process is realized in clean room environment. The thin film battery is composed of three active layers. First the positive electrode layer of crystalline vanadium pentoxide cV2O5, the next level presents then the solid state electrolyte, a glassy ionic conducting material commonly known as “LiPON”. Finally, a negative electrode top level is realized by the evaporation of metallic lithium. The total stack thickness is of about 10 µm. A final wafer level packaging step is then realized to avoid reactivity with air and moisture. Specific attention will be put on the microfabrication processes developed for the positive electrode and the electrolyte (etching chemistry, resist stripping…). Several electrochemical characterizations (spectroscopic electrochemical impedance, charge-discharge cycling) were performed before and after micro-fabrication process steps so as to evaluate any possible effect on the electrochemical behaviour of the different studied layers. INTRODUCTION The development of microdevices has accelerated over the last few years. These microdevices present various advantages including better performances, low cost and better integration due to miniaturized dimensions. Yet, one of the limitations toward a full realization is the miniaturisation of the power supply system [1-4], which could be especially a critical issue for autonomous Microsystems. In this context, thin film microbatteries are considered as suitable solution for a miniaturized power supply system for microdevices. In fact, fabrication process is mostly compatible with standard IC technology, but dimensions are still several times larger in size than devices they would power in this particular case. This paper describes the results of our investigations on the development of solid state thin film microbatteries microfabrication realized in clean room atmosphere and using standard microelectronic technology (photolithography, etching etc...).
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