Renesas Electronics Corporation announced that it has developed a circuit technology for embedded spin transfer torque magnetoresistive random access memory (STT-MRAM) test chips. The chip features fast read and write operations and is fabricated on a 22nm process. The test chip includes a 32-megabit (Mbit) array of embedded MRAM memory cells capable of random read accesses of 5.9 nanoseconds (ns) and writes of 5.8 megabytes per second at a maximum junction temperature of 150°C Throughput (MB/sec).
Renesas is presenting these achievements on June 16 at the 2022 IEEE VLSI Technology and Circuits Symposium in Hawaii, June 12-17.
As IoT and AI technologies continue to advance, microcontroller units (MCUs) used in endpoint devices are expected to deliver higher performance than ever before, requiring fabrication at finer process nodes. MRAM fabricated in BEOL has advantages for sub-22 nm processes because it is compatible with existing CMOS logic process technology and requires fewer additional mask layers than flash fabricated in FEOL. However, MRAM has less read margin than flash memory, which slows down read speeds. The large gap between the CPU operating frequency and the read frequency of non-volatile memory is also a challenge, as it degrades MCU performance.
MRAM can also achieve shorter write times than flash because it does not require an erase operation before a write operation. However, further speed improvements are needed to reduce system downtime for over-the-air (OTA) updates required by endpoint devices and reduce the cost of writing control code for MCUs by end-product manufacturers.
To address these challenges and respond to market demands for higher MCU performance, Renesas has developed the following new circuit technologies to enable faster read and write operations in MRAM.
1.Fast read technology using high precision sense amplifier circuit
MRAM uses memory cells that include magnetic tunnel junction (MTJ) devices, where high and low resistance states correspond to data values of 1 and 0, respectively, to store information. Differential sense amplifiers distinguish these two states by reading the voltage difference of the discharge rate between the memory cell current and the reference current. However, since the cell current difference between the 1 and 0 states of MRAM is smaller than that of flash memory, the voltage difference read by the sense amplifier is smaller. Even if the discharge time is extended to a large voltage difference between the differential input nodes of the sense amplifier, both input nodes are prone to be fully discharged before the necessary voltage difference is ensured. This problem is especially severe at high temperatures.
To solve this problem, Renesas has introduced a new technology that uses capacitive coupling to boost the voltage level of the differential input node. Even when the current difference between the memory cells is small, the differential amplifier can sense the voltage difference, thereby Enables high precision and fast read surgery.
2.Fast write technology with simultaneous write bit optimization to shorten mode transition time
Following the embedded STT-MRAM high-speed write technology announced in December 2021, the new technology achieves higher speeds by reducing mode transition times during write operations.
This technique divides the area where the write voltage is applied, and selectively applies the voltage only to the necessary area by inputting the write address before the write voltage setting. This approach reduces parasitic capacitive loading on regions where voltage is applied during write operations, thereby reducing voltage settling time. As a result, the mode transition time for write operations is reduced by approximately 30%, resulting in faster write operations.
Renesas Electronics continues to develop technologies aimed at applying embedded MRAM technology to MCU products. These new technologies have the potential to significantly increase memory access speeds, a challenge for MRAM today, beyond 100 MHz, enabling higher performance MCUs with embedded MRAM. Faster write speeds will help write code to endpoint devices more efficiently. Renesas Electronics is committed to further improving the capacity, speed, and power efficiency of MCUs for a range of new applications.
