Renesas Developed Low-Power Technology for Use in Embedded Flash Memory

Renesas Electronics Corporation announced the development of low-power technology for use in embedded flash memory based on a 65nm SOTB (Silicon On Thin Buried Oxide) process.

Available with 1.5MB capacity, it is a first embedded 2T-MONOS (Two Transistors-Metal Oxide Nitride Oxide Silicon) flash memory based on 65nm SOTB technology ⁽¹⁾.

With the addition of a new circuit technology that reduces the power consumption of the peripheral circuits on flash memory, the company achieves read energy of 0.22 picojoules per bit (pJ/bit) at an operating frequency of 64MHz – among a lowest levels for embedded flash memory on an MCU.

This developed low-power technology for peripheral circuits comprises circuit technology that (1) reduces energy consumption when sensing data in memory and (2) reduces the amount of transmission energy consumed when read data is transmitted to an external destination. Together, these advances substantially reduce energy consumption when reading data from the memory.

The company presented these results on June 12 at the 2019 Symposia on VLSI Technology and Circuits in Kyoto, Japan, June 9-14, 2019.

The SOTB-based technology has been implemented in the firm’s R7F0E embedded controller, which is intended for energy harvesting applications. The company’ SOTB process technology reduces power consumption in both the active and standby states. Power consumption in these two states had previously been a tradeoff: Lower power consumption in one generally meant higher power consumption in the other. This technology substantially reduces power consumption when reading data from the flash memory. In contrast to non-SOTB 2T-MONOS flash memory, which requires a memory read current of about 50µA/MHz, the read current is reduced to a mere approximately 6µA/MHz. This is equivalent to a read energy level of 0.22pJ/bit, one of the lowest levels for embedded flash memory on an MCU. This technology also contributes to the achievement of a low-active read current of 20µA/MHz on the R7F0E.

Features of embedded flash memory technology:

• Low-power 2T-MONOS flash memory suited to SOTB process
  The 2T-MONOS embedded flash memory using the SOTB process has a two-transistor structure comprising electrically isolated elements. Unlike a single-transistor structure, there is no need for negative voltage during read operation, and this reduces power consumption when reading data. In addition, compared with other memory processes, MONOS uses fewer masks during the production process, and it is possible to store data with a discrete charge-trapping scheme. This enables low power consumption and high rewrite reliability without increasing the production cost.

• Sense amplifier circuit and regulator circuit technology for ultra-low energy consumption
  Most of the energy consumption during memory read operations occurs during sensing operation to identify data and transmit operation to output the identified data to an external destination. To address the former, a single-ended sense amplifier substantially reduces the bit line pre-charge energy during sensing operation, employing a new charge-transfer technology that boosts pre-charging speed and energy efficiency. In addition, a newly developed regulator circuit technology ses leak monitoring to perform optimal intermittent control of the sense amplifier’s reference voltage which consumes energy in a constant manner. These advances make it possible to speed up sensing operation while substantially reducing energy consumption.

• Circuit technology that cuts data transmission energy consumption
  One characteristic of the SOTB process is minimal variation in the transistor threshold (Vth), and this developed circuit technology takes advantage of this to achieve data transmission using an small voltage amplitude. This advance results in a substantial decrease in the transmission energy consumed when read data transmitted to an external destination.

By helping to make endpoint devices more intelligent, the company is accelerating the trend toward a ‘smart society.’ The firm considers energy harvesting systems, which do not need batteries and therefore never require battery replacement, to be a step toward this goal, and plans to continue developing technology that will help to realize an environmentally friendly smart society.

⁽¹⁾ MONOS: Metal-oxide-nitride-oxide-silicon. Renesas has achieved a substantial track record with MONOS technology over the past 20 years in EEPROM, secure MCU, and other products. The technology is used in the embedded flash memory of Renesas MCUs.