Renesas Electronics has successfully developed the world's first separation gate metal nitrous oxide silicon oxide (SG-MONOS) flash memory unit

Renesas Electronics, a leading global supplier of semiconductor solutions, has announced the successful development of the world's first separating-gate metal nitrous oxide silicon oxide (SG-MONOS) flash memory unit with a finned transistor for microcontrollers (MCUS) equipped with on-chip flash memory with an electrical path width of 16 to 14 nanometers (nm) or smaller. SG-MONOS technology can be reliably used in automotive applications, and Renesas is currently using the technology in mass production of 40nm MCUS, and 28nm MCUS are also in the process of development. This successful development demonstrates the excellent scalability of SG-MONOS technology for process nodes 16/14 nm and above.

Advances in automotive automation such as Advanced Driver Assistance Systems (ADAS) and the development of an intelligent society connected by the Internet of Things (IoT) have created the need to assemble advanced MCUS using more refined process technologies. To meet this demand, Renesas has developed embedded flash memory based on 16/14 nm technology, successfully replacing the latest 40/28 nm technology. In the 16/14nm logic process, a transistor with a fin-like structure, the fin-type field-effect transistor (FinFET), is widely used to improve performance and reduce power consumption to overcome the scaling limitations of traditional flat transistors.

However, depending on the structure of the flash memory, the use of fins in embedded flash memory may face a major challenge. At present, two types of embedded flash memory are proposed and implemented: floating gate and charge capture. Charge-capture flash memory, which Renesas has been using in recent years, has better charge-retention characteristics than floating gate flash memory and consistently performs well in automotive MCUS where reliability is required. In addition, because the memory function material is formed on the surface of the silicon substrate, it is relatively easy to extend into a three-dimensional fin structure. In contrast, the structure of the floating gate flash unit is complex, so it is difficult to integrate it into the fin structure.

Another advantage of SG-MONOS over floating gate structures is that the memory cell structure remains unchanged after the pseudo-polycrystalline silicon gate electrode is replaced with a metal gate electrode, and the process is also used to produce advanced logic CMOS devices with high dielectric gate insulation and metal gate electrodes.

Renesas Electronics is the first company in the world to successfully develop SG-MONOS flash memory with a highly scalable fin structure for high-performance and high-reliability MCUS for process nodes 16/14 nm and above.

Key features of newly developed embedded flash technology:

(1) The fin structure significantly improves memory operation and transistor characteristics

Renesas confirmed that the change in threshold voltage during programming/erasing and the programming/erasing speed of the newly developed fin structure SG-MONOS memory unit were within the expected range. In a transistor with a fin structure, the gate closes the channel and thus maintains a large drive current, even if the working area is significantly reduced for greater integration. In addition, by increasing the controllability of the gate, the variability of the threshold voltage is significantly increased. The above results show that the fin structure SG-MONOS memory unit has excellent characteristics, which can achieve high-speed random access reading at more than 200MHz required by the next generation of flash memory, and can greatly improve the on-chip storage capacity.

(2) Develop programming methods that can mitigate the performance degradation caused by fin structures

When fin-like structures are used, due to the enhancement of the electric field at the tip of the fin, certain degradation or deterioration of the equipment characteristics may occur over time. The electric field enhancement effect is most pronounced at the beginning and after the programming operation is completed, so Renesas has investigated the feasibility of "step pulse" programming (gradually increasing the programming voltage). This technique has been used in the past for memory with a planar structure, but it is currently proving to be particularly effective at mitigating the fin tip electric field enhancement in fin structure memory. It has been confirmed that the technology can effectively reduce degradation for long-used fin-shaped SG-MONOS storage units, and the number of programming/erase cycles in the data storage flash memory can reach 250,000 times.

(3) Provide the same high temperature data retention characteristics

The fin structure is ideal for charge capture MONOS flash memory has excellent charge retention characteristics. Data retention time, which is very important for automotive applications, can still be achieved for a decade or more after 250,000 programming/erase cycles. This level of reliability is the same as that achieved by earlier memory.

These results show that by using 16/14 nm nodes and high dielectric gate insulators and metal gate electrodes above, SG-MONOS flash memory can be easily integrated into advanced fin-structure logic processes, enabling high-capacity chip storage in the 100 Megabyte (MB) range, while also enabling highly reliable MCUS. Its processing performance can reach more than four times that of 28nm devices. Renesas will continue to confirm the operation of high-capacity flash memory based on this technology and advance research and development work, aiming to enter practical use around 2023.