Enhancing the <b> <i>β</i> </b> phase thickness window and spin injection efficiency of tungsten films for wafer-scale spin–orbit-torque magnetic random access memory

Abstract

Spin–orbit-torque magnetic random access memory (SOT-MRAM) is widely considered as the next-generation nonvolatile storage technique with great potential for cache and embedded memory applications, offering the unique advantages of high endurance, ultra-fast writing speed, and low writing latency. Among various SOT channel materials for SOT-MRAM, the β-phase tungsten thin films (β-W) are intensively explored owing to their relatively high charge-to-spin conversion ratio, i.e., the SOT efficiency. However, sputtered tungsten thin films typically show a structural phase transition from the β- to α-phases when reaching a critical thickness of 3.5–6 nm, beyond which the SOT efficiency degrades substantially. As a result, the corresponding etching process window for β-W-based SOT-MRAM fabrication becomes extremely narrow, restricting the bit-cell yield of SOT-MRAM in mass production. Herein, we demonstrate an atomic migration and relaxation (AMR) deposition approach that effectively extends the critical thickness of β-W without introducing detrimental doping elements. Compared to commonly used continuous deposition, the critical thickness of AMR-deposited β-W readily reaches a maximum value of 8 nm, a 30% increase when compared to the upper limit in the literature reports. As a performance gauge, we demonstrate SOT efficiencies of 0.43 ± 0.04 for 5 nm thick β-W and 0.31 ± 0.02 for 7 nm thick films, based on 12-in. production line equipment. Equally important, the AMR method efficiently optimizes the surface roughness of the β-W films, which is critically important for high-performance SOT-MRAM. These results provide a simple yet effective solution for improving β-W properties toward the mass production of SOT-MRAM.