In-Situ Thermo-Mechanical Characterization and Strain Engineering of Lipon Thin Films

High temperature multi-optical-stress sensor (HTMOSS) has been used to characterize the coefficient of thermal expansion (CTE) and yield stress of 1-micron thick LiPON films. Six fully dense, amorphous films were deposited on glass and sapphire substrates. The films were then annealed at temperatures ranging from 80 to 200 C for 3 hours. The CTE of LiPON is found to be approximately 4.1e-6, and argued to be independent of the substrate type. Because of this intermediate CTE value, by varying the substrate, we could impose either tension and compression due to thermal mismatch to the film. We observed further that the yield stress of the film is approximately 60 MPa under compression and 100 MPa under tension. Using constant-load hold at and beyond yield point, the films were found to relieve the stress developed during heating with visco-plastic deformation, which led to permanent residual stress during cooling as high as 120 MPa in either tension or compression depending on substrate type. We also found that at annealing temperature higher than 140 C LiPON lost ductility which could be due to composition changes as indicated by XPS measurement. The stress-relief mechanism at constant-load indicates that LiPON may be beneficial as a protective layer against dendrites penetration. Moreover our experimental platform proves that it's an effective method to engineer strain into thin-film solid electrolytes that could be extended to other materials like LLZO or sulfide electrolyte.