Thin-film batteries (TFBs) are produced by thin-film deposition technology which has special advantages that are not only intrinsically safe as their being all-solid-state but also thin, small, lightweight, and flexible [1], and also have the features such as low self-discharge and long cycle-life. As MEMS and high-density packaging technology grows, the device size becomes smaller and there is a growing demand for miniaturized devices with stand-alone power supply supported by TFBs and energy harvesting devices [2, 3]. However, Li metal that is commonly used as anode in TFBs, has low melting point as around 180 deg.C and cannot be employed for packaging process that include solder reflow, epoxy molding, and even through-silicon via (TSV). Si is promising anode material due to high specific capacity and low electrode potential, but there are the issuers such as low resistance and electrode breakage due to drastic volume change during cycles.

In our previous work, TFBs with high-temperature tolerance was fabricated by all sputtering; Pt/Ti as cathode current collector, 3μm-thick-LiCoO₂ with post-annealed at around 600 deg.C as cathode, 2 μm-thick-LiPON as solid electrolyte, 600 nm-thick-Si_1-xTi_x (x=0, 0.03, and 0.12) as anode, and Ni as anode current collector [4], and finally completed by encapsulation with UV curable resin and inorganic lid. The active area of fabricated TFBs was 1.4 mm². Ti which is inactive material against Li, was used as dopant for not only mitigating mechanical stress but also decreasing resistivity. Cycling was performed between 2.5 and 4.1 V with standard current rate condition, and cycle performance resulted well during 100 cycles. In this work, microstructure of Si-Ti alloy film was adjusted by changing sputtering condition such as process pressure and gas species. Figure 1 shows cross-sectional SEM image of each Si_1-xTi_x films. The effect of Si_1-xTi_x anode microstructure on electrochemical and cycle performance of full cell will be discussed.
References

[1] T. Jimbo, P. Kim, and K. Suu, Production technology for thin-film lithium secondary battery, Energy Procedia, 14, 1574-1579 (2012).
[2] M. Fojtik, D. Kim, G. Chen, Y. S. Lin, D. Fick, J. Park, M. Seok, M. T. Chen, Z. Foo, D. Blaauw, and D. Sylverster, A millimeter-scale energy-autonomous sensor system with stacked battery and solor cells, IEEE Journal of Solid-State Circuits, 48 (no. 3), 801-813 (2013).
[3] T. Kuriyama, A. Suzuki, Y. Okamoto, I. Kimura, Y. Morikawa, and Y. Mita, A micromachined all-solid on-chip thin-film battery towards uninterruptible photovoltaic cells, 2018 Symposium on Design, Test, Integration and Packaging of MEMS and MOEMS (DTIP), 153-156 (2018).
[4] A. Suzuki, S. Sasaki, and T. Jimbo, Development of All-solid-state Thin-film Secondary Battery for MEMS and IoT Device, The 18th International Conference on PowerMEMS 2018, Proceedings, T5A-03 (2018).