Researchers at Tohoku University have demonstrated a high-speed spin-orbit-torque (SOT) magnetoresistive random access memory cell compatible with 300 mm Si CMOS technology.

The development, announced at the 2019 IEEE International Electron Devices Meeting in San Francisco, pave the way for the replacement of SRAM with low-power magnetoresistive random access memories (MRAMs).

Purely CMOS-based memories such as embedded Flash memory (eFlash) and static random-access memory (SRAM) are consuming much power. In order to lower the power consumption but also maintain a high performance, MRAMs have been proposed and developed. Spin-transfer torque MRAMs (STT-MRAMs) are the most popular MRAMs. Major semiconductor companies have announced that they are ready for mass production of STT-MRAM as alternative to eFlash.

Researchers are aiming to replace SRAM with MRAM. For SRAM replacement, MRAM must achieve high-speed operation above 500 MHz. To meet the demand, an alternative MRAM, so-called spin-orbit torque MRAM (SOT-MRAM) was proposed, which has several advantages for high-speed operation. Because of these advantages, SOT-MRAM has also been developed; however, most laboratory studies focus on the fundamentals of SOT devices.

To realize SRAM replacement by SOT-MRAM, it is required to demonstrate high-performance of SOT-MRAM memory cell on 300mm CMOS substrate. In addition, it is necessary to develop the integration process for SOT-MRAM, e.g., thermal tolerance against 400°C annealing, which is a requirement of the standard CMOS back-end-of-line process.

The research team led by Professors Tetsuo Endoh and Hideo Ohno - the current president of Tohoku University - has developed an integration process for SOT devices compatible with 55nm CMOS technology and fabricated SOT devices on 300mm CMOS substrates. The newly developed SOT device has simultaneously achieved high-speed switching down to 0.35 ns and a sufficiently high thermal stability factor (E/kBT＝70) for the high-speed non-volatile memory applications with robustness against annealing at 400°C. Based on this achievement, the research team has integrated the SOT device with CMOS transistors and finally demonstrated high-speed operation in complete SOT-MRAM memory cells.

These achievements bring SOT-MRAM closer to practical implementation to commercial applications, paving the way for the replacement of SRAM with SOT-MRAM.