Research Alert
Electronics started at the beginning of the 20th century with the invention of vacuum tubes. It brought about modern technologies such as telecommunication, broadcast (radio/TV), computer, etc., and changed our society revolutionarily. Then, vacuum tubes were gradually replaced by semiconductor devices in the 1960’s, because of the advantage in cost, size and performance. Since then, the semiconductor devices have been continuously miniaturized. At the beginning of 1970’s, their feature device-size decreased to 10 μm and the number of the MOSFETs in a chip exceeded 1,000. This was the beginning of micro-electronics offering us Large-Scale Integrated Circuits (LSIs) such as microprocessors. Now, the feature device-size has reached the 10 nm-range and we call it nano-electronics. Micro-/nano-electronics has brought up new technologies such as internet, smart phones, artificial intelligence, virtual reality, robots, etc., and is changing our society towards the ultimately intelligent one.
Miniaturization or the downsizing of electron devices such as the MOSFETs has been the key to improve the cost and performance. Today, 1T-bit (or 128G-Bite) SD cards composed of semiconductor memories are available with reasonable price. Let us imagine the world only with old-style vacuum tubes. Then, a 1T-bit memory with 1 trillion vacuum tubes would cost 100 trillion Japanese yen (or 1 trillion US dollars); the amount of Japan’s national budget, occupy the volume of 0.5 X 0.5 X 1.0 km3; larger than the world-tallest building, weigh 100M ton; the weight of 20 million elephants, and consume 5T Watt; the generated power of 5,000 nuclear reactors, and 4 X 1020 calorie energy in a year; 2,000 times as large as the energy of yearly global human consumption. So, without the development of micro-/nano-electronics, today’s smart society with internet, artificial intelligence, etc. does not exist, and thus, the invention of micro-/nano-electronics is the biggest technological revolution after that of electronics.
Limit of miniaturization
What is the limit of miniaturization? There are 3 stages of the limit. The first one is the ‘ultimate limit’, which is defined by the distance of atoms in the material; about 0.3 nm in the Si substrate case. Changing material does not make a big difference. No one can make the device structures less than the atomic distance. So, it should be noted that there is no ‘pico-electronics’ anymore next to the nano-electronics.
Before reaching the ultimate limit, there is another limit, named ‘fundamental limit’ which is defined by the direct-tunneling distance; about 3 nm. When the distance between the structures becomes 3 nm, the wave function of the materials penetrates the potential barrier between the structures, and huge direct-tunneling current starts to flow. So, there is no electrical insulation or switching-off of the devices. Thus, 3 nm is the limit for the device operation.
The third limit is a ‘practical limit’ defined by the demerit of downsizing; about 10 ~ 20 nm. When the demerit of the downsizing becomes larger than the merit, then the downsizing will stop. The demerit may be the degradations in off-leakage current, production cost, reliability, and variability of the device characteristics, and so on. Recently, logic devices with the commercial names of ‘7 nm’ or ‘5 nm’ technology are in production. However, it should be noted that those sizes used in the commercial names of the logic devices have nothing to do with the real sizes, which are typically 30 ~ 15 nm for the minimum line widths. Then, what would be the limit of the downsizing? Off-leakage current or subthreshold leakage current between the source and drain of the MOSFET is regarded as the one of the main causes of the limit for the downsizing, because we cannot suppress the diffusion of electrons from the source to the channel (which is defined by Boltzmann statistics) unless changing the device operation temperature to extremely low such as 77K. However, it is difficult for the majority of the market. So, it is the consensus that the channel length is difficult to reach sub-10 nm for the major market devices.
Future technology development in next 10 years and after that
Although we are very close to the limit, we have to continue to develop the new technologies towards its limit, because nano-electronics is the key for today’s smart society and the demand from the market is so strong. The new technologies are 3D integration, new materials for interconnects, resolution improvement of EUV lithography, etc. Even, the limit of the downsizing is approaching, we will have so many things to do in the next 10 years and even after that.
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