Renesas Electronics Corporation, a provider of advanced semiconductor solutions, recently announced that it has developed circuit technologies for an integrated spin transfer torque magnetoresistive random access memory (STT-MRAM) test chip with fast read operations and fabricated recording – using a 22nm process.
The test chip includes an embedded 32-megabit (Mbit) MRAM memory cell array and achieves 5.9 nanoseconds (ns) random read access at a maximum junction temperature of 150°C, including a throughput of recording 5.8 megabytes per second (MB/s).
Renesas presented these achievements on June 16 at the 2022 IEEE Symposium on VLSI Technology and Circuits, held June 12-17 in Hawaii.
As advancements in IoT and AI technologies continue, microcontroller units (MCUs) used in endpoint devices are expected to deliver higher performance than ever before and are expected to be manufactured with more refined process nodes.
BEOL-fabricated MRAM (Note 1) is advantageous compared to FEOL-fabricated flash memory (Note 2) for sub-22nm 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 non-volatile memory read frequency is also a challenge as it can degrade MCU performance.
MRAM can also achieve a shorter write time than flash memory because it does not require an erase operation before the write operation. However, additional speed improvements are needed to reduce system downtime for over-the-air (OTA) updates required for terminal devices and reduce costs for end product manufacturers in writing control codes to 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 operations in MRAM.
1. Fast reading technology employing high-precision detection 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 detection amplifier distinguishes between the two states by reading the voltage difference in discharge speed between the memory cell current and the reference current.
However, because the memory cell current difference between states 1 and 0 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 for wider voltage differences between the differential input nodes of the sense amplifier, both input nodes are susceptible to being completely discharged before they guarantee a required voltage difference. This problem is particularly acute at high temperatures.
To solve this problem, Renesas introduced a new technology using capacitive coupling to increase the voltage level of the differential input nodes, allowing the differential amplifier to detect a voltage difference even when the memory cell current difference is small, achieving high precision and fast reading. Operation.
2. Fast Write technology with simultaneous optimization of the number of recording bits and reduced mode transition time
Following the high-speed write technologies for embedded STT-MRAM announced in December 2021, the new technology achieves even greater speed by reducing the mode transition time during the write operation.
This technology divides the areas to which the write voltage is applied and, by entering the write address before setting the write voltage, selectively applies the voltage only to the required area. This method reduces parasitic capacitive load in the area where voltage is applied during the recording operation, reducing voltage setup time. As a result, the mode transition time for write operation is reduced by approximately 30%, speeding up the write operation.
Renesas continues to develop technologies aimed at applying MRAM technology embedded in MCU products. These new technologies have the potential to dramatically increase memory access speed, which is currently a challenge with MRAM, to exceed 100 MHz, enabling higher performance MCUs with embedded MRAM. The faster write speed will contribute to more efficient code writing on terminal devices.
Renesas is committed to further increasing the capacity, speed and power efficiency of MCUs to accommodate a variety of new applications.
