Thorough study of the local structure of LiPON thin films to better understand the influence of a solder-reflow type thermal treatment on their performances

Microbatteries are energy sources well adapted to power various microsystems. To be considered as a microelectronic component, the latters must be compatible with the solder-reflow process which reaches a temperature of 260 degrees C during few seconds. In this study, we have investigated the effect of a solder-reflow type thermal treatment on the composition, local structure and electrical performances of the most used solid electrolyte, a lithium phosphorus oxynitride (LiPON). Amorphous thin films of LiPON were prepared by radio-frequency magnetron sputtering from a Li3PO4 target with a pure nitrogen discharge gas. Ionic conductivity increases from 3.0 x 10(-6) S cm(-1) to 7.2 x 10(-6) S cm(-1) with the thermal treatment at 260 degrees C. However, this treatment induces an increase of the activation energy from 0.57 eV to 0.80 eV which is detrimental for a use of the microbattery at low temperature. The study of the local structure by X-ray photoelectron spectroscopy (XPS) shows the break of Li+...O--P ionic bond and the formation of two new type of species such as Li2O and a Li-O-P environment with a covalent bond between lithium and oxygen atoms. This induces a decrease of the charge carrier mobility and so an increase of the activation energy. Nevertheless, as the ionic conductivity is improved, we can expect an increase of the mobile charge carrier density. (C) 2011 Elsevier B.V. All rights reserved.