Renesas develops circuit tech for 22-nm Embedded STT-MRAM

The test chip includes a 32-megabit (Mbit) embedded MRAM memory cell array and achieves 5.9-nanosecond (ns) random read access at a maximum junction temperature of 150°C, and a write throughput of 5.8-megabyte-per-second (MB/s).

Renesas presented these achievements on June 16 at the 2022 IEEE Symposium on VLSI Technology and Circuits, held between June 12 and 17 in Hawaii.

As the advances of IoT and AI technologies continue, microcontroller units (MCUs) used in endpoint devices are expected to deliver higher performance than ever, and therefore need to be fabricated with finer process nodes. MRAM fabricated in BEOL (Note 1) is advantageous compared to flash memory fabricated in FEOL (Note 2) for sub-22 nm processes because it is compatible with existing CMOS logic process technology and requires fewer additional mask layers. However, MRAM has a smaller read margin than flash memory, which degrades read speed. A large gap between the CPU operating frequency and the read frequency of the non-volatile memory is also a challenge since it can degrade MCU performance.

MRAM can also achieve shorter write timethan flash memory because it requires no erase operation before write operation. However, further speed improvements are needed to shorten system downtime for over-the-air (OTA) updates required for endpoint devices and reduce costs for end product manufacturers in writing control codes for MCUs.

To address these challenges and respond to market demand for higher MCU performance, Renesas has developed the following new circuit technologies to achieve faster read and write operation in MRAM.

Fast Read Technology Employing High-Precision Sense Amplifier Circuit

MRAM uses memory cells including magnetic tunnel junction (MTJ) devices in which high and low-resistance states correspond to data values of 1 and 0 respectively to store information. A differential sense amplifier distinguishes between the two states by reading the voltage difference in discharge speed between the memory cell current and reference current. However, since the memory cell current difference between the 1 and 0 states is smaller for MRAM than for flash memory, the voltage difference read by the sense amplifier is smaller. Even if the discharge time is extended to wider voltage differences between the differential input nodes of the sense amplifier, both of the input nodes are susceptible to being completely discharged before securing a necessary voltage difference. This problem is particularly acute at high temperatures.

To resolve this issue, Renesas introduced a new technology utilizing capacitive coupling to boost the voltage level of the differential input nodes, allowing the differential amplifier to sense a voltagedifference even when the memory cell current difference is small, achieving high-precision and fast read operation.

Fast Write Technology with Simultaneous Write Bit Number Optimization and Shortened Mode Transition Time

Following the high-speed write technologies for embedded STT-MRAM announced in December 2021, the new technology achieves even higher speed by shortening the mode transition time during write operation.

This technology divides up the areas to which write voltage is applied and, by inputting the write address before the write voltage setup, it selectively applies voltage only to the necessary area. This method reduces the parasitic capacitive load on the area where the voltage is applied during the write operation, reducing the voltage setup time. As a result, the mode transition time to write operation is reduced by approximately 30%, speeding up write operation.

Renesas continues to develop technologies aimed at the application of embedded MRAM technology in MCU products. These new technologies have the potential to dramatically boost memory access speed, which is currently a challenge with MRAM, to exceed 100 MHz, enabling higher-performance MCUs with embedded MRAM. Faster write speed will contribute to more efficient code writing to endpoint devices. Renesas is committed to further increasing capacity, speed, and power efficiency for MCUs to accommodate a range of new applications.