This work presents recent advances in the development and the integration of an electrochemical (chemicalelectrical energy conversion) micro power generator used as a high voltage energy source for RF-MEMS powering. Autonomous MEMS require similarly miniaturized power sources. Up to day, solid state thin film batteries are realized with mechanical masks. This method doesn't allow dimensions below a few mm^2 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 $\mu$m^2 targeted unit cell active area) and Above IC technological compatibility. All process steps developed here are realized in clean room environment.
Microelectromechanical systems (MEMS) are one of the most promising enabling technologies for developing low-cost miniaturized RF components for high-frequency applications. During the last decades, several achievements were realized in terms of technological development of RF-MEMS and integration with ICs. One important remaining integration challenge is the incorporation of the energy source on the silicon chip. Most of RF-MEMS in the literature, and mainly electrostatically driven ones require a low dc power but a high actuation voltage. The utilisation of solid state thin film batteries as a MEMS-RF power generator seems to be very adequate to the cited requirements. In fact, solid state batteries even with small footprints could provide the targeted electrical specifications. High power or voltage could then be obtained through either use of multilayered structure or a simple planar connection in parallel or in series respectively. On chip integration of such devices should be compatible with standard microelectronic technology. Within this context, major efforts concern materials selection and process development. Currently, deposition processes are already compatible with standard microelectronic technology (RF magnetron sputtering, evaporation). Thin film batteries are generally realized with mechanical masking, allowing reaching dimensions as low as a few mm² of active area. On chip integration requires a miniaturization of these devices, by means of processes such as photolithography and etching. Very few studies in literature are dedicated to this subject [1][2][3], which is probably due to the evident difficulties associated with the processing of the materials constituting the thin film batteries.
It is the purpose of this paper to present the results of our investigations on the development of solid state thin film batteries microfabrication realized in clean room environment and using standard microelectronic technology. Results in this work concern the microfabrication process development for both cathode and solid state electrolyte thin films.
Cathode and solid electrolyte materials in this study were both realized by RF magnetron sputtering deposition. Conditions are listed in Table .1.
S. Oukassi, X. Gagnard, R. Salot, S. Bancel, J.P. Pereira-Ramos 3 Above IC micro-power generators for RF-MEMS Cathode material in this study is crystalline vanadium pentoxide. V 2 O 5 was selected for its good electrochemical performances (3V oxide, high energy storage capacity and cycle life) [4]. Another selection argument resides in the low temperature deposition process (T<200°C), which is compatible with the back-end thermal budget of current generation CMOS semiconductor technology, whereas conventional cathode materials, especially 4V oxides require higher temperature deposition and/or post annealing treatments.
A wide range of solid state materials were developed and studied as solid state electrolyte for lithium thin film batteries. For our study, glassy inorganic solid state Lithium Phosphorus Oxynitride, commonly known as LiPON [5] was selected since it presents one of the most interesting electrochemical properties (high ionic conductivity, chemical stability toward lithium up to 5.5V/Li) and also for its process compatibility (low temperature during deposition).
To provide electrical isolation for thin film batteries, SiO 2 and Si 3 N 4 films were deposited respectively by thermal oxidation and low pressure chemical vapor deposition. Cathode and anode current collectors were then realized in DC sputtered Ti (50 nm)/Au (200 nm). These films were then patterned with positive resist (SJR 335, TMA 238 developer, DI rinse) and etched respectively with (KI+I 2 ) and HF solutions. Resist stripping was finally performed with HNO3 solution.
Cathode V 2 O 5 thin film was patterned with negative photoresist (SC180 FUJIFILM electronics, Waycoat Negative Resist Developer, n-butyl acetate rinse). After development, V 2 O 5 etching was performed in a nextral 110 RIE reactor, etch parameters were: 15 sccm SF 6 gas flow, 15 mTorr total pressure, and 10 W power. Spectroscopic ellipsometry characterization was used to evaluate V 2 O 5 etch rate and conformity, and Si 3 N 4 overetch. Roughness Measurements (Rms and Ra) were realized with a digital instruments atomic force microscope on a 5x5 µm² window. A NEXUS FT-IR spectrometer was used to monitor residual resist evolution and V 2 O 5 chemical stability before and after stripping step.
While investigations and results on LiPON etching using ion milling were performed, we chose to present here only results obtained with wet chemical etching. Solid state electrolyte LiPON thin film was patterned, as V 2 O 5 with negative photoresist (SC180 FUJIFILM electronics, Waycoat Negative Resist Developer, n-butyl acetate rinse). After development, LiPON etching was performed in a tetramethyl ammonium hydroxide solution (TMAH) at room temperature.
Spectroscopic elli
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