SEL Develops CPU Including Non-volatile Memory without Deterioration in Principle
June 4, 2012
Semiconductor Energy Laboratory Co., Ltd.
Semiconductor Energy Laboratory Co., Ltd. (SEL) (President: Shunpei Yamazaki) has developed a CPU including a non-volatile memory which does not deteriorate in principle and is formed with the use of a crystalline oxide semiconductor.
In this technology, a non-volatile memory is formed using a crystalline oxide semiconductor which offers extremely low off-state current characteristics, and this memory which can retain data even when the power is turned off is incorporated into a CPU.
SEL has already developed mass-production technology for oxide semiconductor devices in the field of displays for the first time in the world in collaboration with Sharp Corporation (Sharp) and Sharp began production this year. The oxide semiconductor devices are promising devices.
A crystalline oxide semiconductor was first reported by Dr. Noboru Kimizuka and his colleagues at National Institute for Research in Inorganic Materials in 1985, and they actively studied it for more than 10 years.
SEL discovered an IGZO material which has a crystalline structure in the form of a thin film, CAAC-IGZO （CAAC: C-Axis Aligned Crystal）. SEL also found for the first time in the world that, in a thin film transistor (TFT) using the CAAC-IGZO, off-state current at 85℃ was 50 ｙA/μm （50 ｘ 10-24A/μm）, which is extremely low.
SEL has applied CAAC-IGZO to a non-volatile memory element in order to make use of this extremely low off-state current, and made some presentations on this element at academic conferences last year and this year. Unlike conventional non-volatile memory elements such as a flash memory, this memory element does not deteriorate in principle and does not have a limitation on the number of write operations. SEL also presented an image sensor utilizing data retention characteristics of such an element last year.
Along with the wide spread use of mobile electronic devices, reduction in power consumption of a CPU is being demanded. Last year, a project for normally-off computing was launched by New Energy and Industrial Technology Development Organization (NEDO) and research was started to develop a normally-off CPU by retaining data in a non-volatile memory such as a MRAM in the CPU. However, a MRAM and a ReRAM, which are known as non-volatile memories, have the problems of high power consumption in writing and accordingly an increase in overhead power.
On the other hand, the memory using oxide semiconductors developed this time does not deteriorate in principle and consumes extremely low power in writing, which can meet the needs and solve the issues in this age of energy conservation
A normally-off CPU including this memory in which power supply is off except for operation time can reduce power consumption to 1/100 (when the CPU is applied to a music program.)
This normally off CPU is going to be presented at SID 2012 in Boston, from June 3 to 8, 2012.
Many oxide semiconductors have a wide band gap. InGaZnO (IGZO) is known as the typical example. TFTs using IGZO have been intensively researched and developed because of its high potential.
・Project for Development of Infrastructure for Normally-off Computing Technology by NEDO
The Normally-off Computing is a computing technology to achieve ultralow power consumption compared to existing devices by (actively) using non-volatile elements. NEDO launched this project last year.
・CAAC（C-Axis Aligned Crystal）
CAAC is a crystal in a thin oxide semiconductor film which SEL found for the first time in the world. CAAC is aligned in the c-axis direction and not necessarily aligned in the a-b plane direction. TFTs using CAAC have high reliability.
1-1．Classification of Oxide Semiconductors
1-2．Application of Oxide Semiconductors
2-1．Summary of Normally Off CPU
3-1．Application to 1Mbit NOR Memory（Presented at IMW2011）/span>
3-2．Application to DRAM Memory（Presented at IMW2012）
3-3．Application to Image Sensor （Presented at 2011VLSI Technology）
【Comparison of Memories】
1) D. Halupka et al., ISSCC, pp. 256-258 (2010)
2) Z. Wei et al., IEDM Tech. Dig., pp. 293-296 (2008)