<i>In situ</i> surface etching-assisted deposition for module-level optimization of pSOT-MRAM with sub-nanometer perpendicular magnetic anisotropy

Abstract

In perpendicular spin–orbit torque (SOT) magnetic random-access memory, the SOT-track and the perpendicular magnetic tunnel junction (pMTJ) stack are typically deposited sequentially without vacuum break to minimize interfacial contamination. However, this requirement fundamentally limits interface engineering between the SOT-track and pMTJ, restricting independent optimization of each module and reducing process flexibility. This study demonstrates an in situ surface etching-assisted deposition (ISED) process that enables efficient interface engineering without sacrificing interfacial integrity. By integrating ion beam etching (IBE) and magnetron sputtering within a single vacuum system, the ISED process decouples the deposition of the SOT-track and pMTJ stack, enabling integrated module-level optimization. We demonstrate the ISED process using a β-W/CoFeB/MgO/Ta heterostructure. The IBE process effectively smoothed the β-W surface under optimized etching conditions, reducing surface roughness by 30.1%. Consequently, the ISED stack revealed a clear and smooth interface, leading to improved MgO crystallinity compared with the stack without ISED. The x-ray-based analyses confirmed that the ISED process modifies crystallinity and bonding of the β-W surface, reducing interface contamination and enhancing β-W/CoFeB bonding. The magnetic characterizations confirmed perpendicular magnetic anisotropy (PMA) in ISED-processed stacks, both in the as-deposited sample and after annealing, even at sub-nanometer (down to 5 Å) CoFeB thicknesses. Moreover, ISED-processed stacks exhibited PMA after the β-W layer was exposed to ambient air prior to CoFeB/MgO deposition, underscoring its effective surface plasma cleaning capability.