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Low-power technology for embedded flash based on SOTB

Renesas is developing a new low-power technology for use in embedded flash memory based on a 65 nanometer (nm) SOTB (Silicon On Thin Buried Oxide) process.

Available with 1.5 MB capacity, this product is, according to Renesas, the world’s first embedded 2T-MONOS (2 Transistors-Metal Oxide Nitride Oxide Silicon) flash memory based on 65nm SOTB technology.

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

The new SOTB-based technology has already been implemented in the Renesas R7F0E embedded controller, which is intended specifically for energy harvesting applications. Renesas’ exclusive 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.

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 contrast to non-SOTB 2T-MONOS flash memory, which requires a memory read current of about 50 µA/MHz, the read current is reduced to approximately 6 µA/MHz. This is equivalent to a read energy level of 0.22 pJ/bit.

MONOS also uses fewer masks during the production process, when compared to other memory devices; it is possible to store data with a discrete charge-trapping scheme also. This enables low power consumption and high rewrite reliability without increasing the production cost.

The new technology also contributes greatly to the achievement of a low-active read current of 20 µA/MHz on the R7F0E.

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 uses 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.

One characteristic of the SOTB process is minimal variation in the transistor threshold (Vth), and the newly developed circuit technology takes advantage of this to achieve data transmission using an extremely 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, Renesas is accelerating the trend toward a “smart society.” Renesas considers energy harvesting systems, which do not need batteries and therefore never require battery replacement, to be an essential step toward this goal, and plans to continue developing technology that will help to realise an environmentally friendly smart society.

Author
Bethan Grylls

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