Antimony (Sb) is an intriguing material for advanced electronics, with thickness-dependent properties at the nanoscale offering new functionalities. However, conventional methods for depositing Sb thin films cannot produce continuous ultrathin films with conformality in complex nanoscale structures. This study introduces a novel sacrificial atomic layer deposition (s-ALD) approach that overcomes these limitations by using chemical substitution between the antimony precursor and the pre-deposited Sb₂Te₃. The structural similarity between Sb₂Te₃ and Sb enables local epitaxial growth of a uniform, (00l)-oriented Sb film with exceptional surface smoothness (root-mean-squared roughness << 1 nm) at a 4-nm thickness. Highly pure Sb films with excellent wafer-scale uniformity and conformality are achieved on high-aspect-ratio structures. The mechanism involves substitution reactions driven by the preferential Te-(CH₃)₃Si bonding, along with enhanced atomic diffusion through the aligned crystal structure. Phase change memory devices using 5-nm-thick s-ALD Sb films demonstrate ultrafast switching with femtosecond laser pulses (≈220 fs) with high device-to-device uniformity (coefficient of variation < 5 %) and ultralow drift coefficients (0.0013 for the on state and 0.0073 for the off state). This s-ALD technique offers a promising pathway for depositing ultrathin, uniform Sb films, enabling full utilization of Sb's unique nanoscale properties.