The wake-up procedure that demands prolonged time and a high electric field poses a significant obstacle to the aggressive scaling of the ferroelectric (FE) Hf 0.5 Zr 0.5 O 2 (HZO) thickness. A comprehensive understanding of its origin is thus imperative. A new mechanism, referred to as interfacial-layer soft breakdown (IL-SBD), is proposed to elucidate the wakeup behavior in the ultrathin HZO capacitor. Compelling experimental evidence is presented to support the critical role of IL and its SBD. A multi-domain FE wake-up model is developed that incorporates defect generation, trap-assisted tunneling within the IL, and charge screening at the IL/HZO interface. Remarkably, this model accurately reproduces the trend of thickness-dependent wake-up behavior, emphasizing the utmost significance of IL optimization in ultrathin HZO.