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3D all-solid-state microbatteries are promising onboard power systems for autonomous devices. The fabrication of 3D microbatteries needs deposition of active materials, especially solid-state electrolytes, as conformal and pinhole free thin films in 3D architectures, which has proven very difficult for conventional deposition techniques, such as chemical vapor deposition and physical vapor deposition. Herein, we report an alternative technique, atomic layer deposition (ALD), for achieving ideal solid-state electrolyte thin films for 3D microbatteries. Lithium tantalate solid-state electrolytes, with well-controlled film composition and film thickness, were grown by ALD at 225 °C using subcycle combination of 1 × Li₂O + n × Ta₂O₅ (1 ≤ n ≤ 10). The film composition was tunable by varying Ta₂O₅ subcycles (n), whereas the film thickness displayed a linear relationship with ALD cycle number, due to the self-limiting nature …