Spin Transfer Technologies, Inc., the developer of advanced STT-MRAM for embedded SRAM and stand-alone DRAM applications, today announced results of its unique Precessional Spin Current (PSC™) structure. The results from advanced testing of the PSC structure confirm that it will increase the spin-torque efficiency of any MRAM device by 40-70 percent — enabling dramatically higher data retention while consuming less power. This gain translates to retention times lengthening by a factor of over 10,000 (e.g., 1 hour retention becomes more than 1 year retention) while reducing write current. Improved efficiency is critical for enabling MRAM to replace SRAM and DRAM in mobile, datacenter and AI applications, as well as for improving retention and performance in high-temperature automotive applications. The company reported these results at the prestigious Intermag 2018 Conference.

Spin-torque efficiency is one of the core performance metrics of the pMTJ (perpendicular magnetic tunnel junction — the “bit” that stores the memory state in an MRAM memory) and is defined by the ratio between the thermal retention barrier, measuring how long data can be reliably stored in the memory, and the switching current necessary to change the value of the bit. In previous MRAM implementations, increasing the energy barrier to increase retention would require a proportional increase in write current — leading to higher power consumption and much faster wear-out of the pMTJ devices (lower endurance). The PSC structure is a breakthrough because it effectively decouples the static energy barrier that determines retention from the dynamic switching processes that govern the switching current. As a result, when the PSC structure is added to any pMTJ, benefits include:

• A higher energy barrier when the pMTJ does not have current flowing through it, which is ideal for retaining data for long periods
• An increased spin polarization when current is flowing and the device is writing a new state, which is ideal for minimizing switching current and extending the life of the device by many orders of magnitude

The PSC structure was designed from the outset to be modular and fabricated with any pMTJ — either the company’s own pMTJs, or a pMTJ from other sources. The PSC structure is fabricated during the pMTJ deposition process and adds approximately 4nm to the height of the pMTJ stack. The structure is compatible with a wide range of standard MRAM manufacturing processes, materials and tool sets — enabling any foundry to readily incorporate the PSC structure into existing pMTJ stacks without adding significant complexity or manufacturing costs.

“MRAM is attracting a lot of attention as an embedded memory for ASICs and MCUs, but issues of write current and data retention have caused concern,” said Jim Handy, general director of Objective Analysis. “Spin Transfer Technologies’ new PSC structure shows a lot of promise to solve a number of those issues and pave the path for MRAM to take a significant share of the embedded memory market.”

Spin Transfer Technologies’ testing of the PSC structure involved comparing the performance of the same pMTJ devices with and without PSC for a large number of devices within CMOS test chip arrays at various temperatures and device diameters. The tests exhibited a robust performance advantage due to the PSC structure, both during writing of the low-resistance (“0”) and the high-resistance (“1”) memory states. Some specific examples of the advantages that the data have shown are as follows:

• Increase of the spin-torque efficiency by up to 70 percent
• Demonstration of the efficiency gain across a range of sizes (40-60nm) and temperatures (30°C to 125°C)
• Increase of the thermal energy barriers by 50 percent corresponding to an increase in data retention time of greater than four orders of magnitude while reducing the switching current
• Reduction of read disturb error rate up to five orders of magnitude

These advantages have come without degradation to other performance parameters. The data for the PSC structure indicate significant potential for enabling high-speed applications as well as high-temperature automotive and other applications. Furthermore, since the data shows that the PSC structure’s efficiency gains actually increase as the pMTJ get smaller, the PSC structure opens new pathways to achieving embedded SRAMs in the latest 7nm and 5nm generations.

“There is a huge demand for a memory with the endurance of SRAM, but with higher density, lower operating power and with non-volatility. We believe the improvements the PSC structure brings to STT-MRAM technology will make it a highly attractive alternative to SRAM for these reasons,” said Mustafa Pinarbasi, CTO and SVP of Magnetics Technology at Spin Transfer Technologies. “We are excited to enable the next generation of STT-MRAM and to shake up the status quo of the memory industry through our innovation.”