Influence of sputtering pressure on the structure and ionic conductivity of thin film amorphous electrolyte

Ionic conducting thin film amorphous electrolytes are promising candidates for microelectronics applications. This study presents an investigation into the structure and composition of lithium phosphorus oxynitride (LiPON) thin film electrolyte prepared using radio frequency (RF) sputtering on Li₃PO₄ target. The ionic conductivity of LiPON thin films has been dramatically improved by decreasing N₂ pressure. X-ray photoelectron spectra (XPS) were used to determine the structure and composition of LiPON thin films. It was found that increasing the N₂ pressure during the deposition process resulted in a greatly decreased formation of triply coordinated –N<(N_t) as compared to doubly coordinated –N=(N_d) in LiPON thin films. These results indicate that the N_t structural unit plays an important role in the improvement of ionic conductivity as compared to the N_d structural unit. It also shows that PO₂N₂ tetrahedra with two N_t structural units exist in LiPON thin films at low N₂ pressures. Consequently, the improved ionic conductivity of the LiPON thin film deposited at low pressure results from the existence of PO₂N₂ tetrahedra with two N_t structural units in LiPON thin film. PO₂N₂ tetrahedra with two N_t structural units provides higher cross-linking density of the glass network and lower electrostatic energy than with two N_d structural units.