Decohesion and rupture mechanisms of a multilayered microbattery studied by 4-point bending

This paper presents tests to characterize the mechanical resistance of the multilayers set which forms an all-solid-state lithium-ion micro battery. The system under investigation consists in five layers that were deposited successively onto a crystalline silicon substrate (100): Ti, TiOS, LiPON, aSi, Ti. The layer thicknesses are between 65 nm (amorphous silicon layer aSi) and 1400 nm (LiPON, TiOS). First, simple water immersion experiments allow internal stress to be put into evidence in the two external layers (aSi and Ti), basically a strong compressive stress in the aSi layer (at least −0.8 GPa). Afterwards, well-controlled 4-point bending tests lead to delamination of the weakest parts of the multilayer, which are the TiOS-LiPON and LiPON-aSi interfaces. The TiOS-LiPON interface is the weakest, with an adhesion energy around 2.5 J/m2. The TiOS layer exhibits a rather low cohesion energy, scarcely higher than 2.5 J/m2. The brittle character of LiPON clearly appears. By using samples of another kind (-Ti-TiOS-Ti on Si), local adhesion flaws are identified between the TiOS and underlying Ti layer. In our experiment, these local adhesion flaws lead to characteristic phenomena (formation of comet-shaped scales), which are explained in the framework of a simple mechanical model.