Electronics began at the start of the 20th century with the invention of vacuum tubes. He brought modern technologies such as telecommunications, broadcasting (radio / TV), IT, etc., and changed our society in a revolutionary way. Then, vacuum tubes were gradually replaced by semiconductor devices in the 1960s, due to the advantage in cost, size and performance. Since then, semiconductor devices have been continually miniaturized. In the early 1970s, their device size shrank to 10m and the number of MOSFETs in a chip exceeded 1000. It was the start of microelectronics that offered us large-scale integrated circuits (LSI ) such as microprocessors. Now the size of the device has reached the range of 10nm and we call it nanoelectronics. Micro- / nano-electronics have brought about the emergence of new technologies such as the Internet, smart phones, artificial intelligence, virtual reality, robots, etc., and made our society evolve towards an ultimately intelligent society.
Miniaturization or downsizing of electronic devices such as MOSFETs has been the key to improving cost and performance. Today, 1T-bit (or 128G-Bite) SD cards made up of solid-state memories are available at a reasonable price. Imagine the world just with old-fashioned vacuum tubes. Then, a 1T-bit memory with 1,000 billion vacuum tubes would cost 100,000 billion Japanese yen (or 1,000 billion US dollars); the size of the national budget of Japan, occupy the volume of 0.5 X 0.5 X 1.0 km3; taller than the tallest building in the world, weighing 100 million tons; the weight of 20 million elephants, and consumes 5T Watt; the power produced by 5,000 nuclear reactors and 4 X 1,020 calories of energy per year; 2,000 times more than the energy of the annual global human consumption. Thus, without the development of micro- / nano-electronics, today’s intelligent society with the internet, artificial intelligence, etc. does not exist, and thus, the invention of micro- / nano-electronics is the greatest technological revolution after that of electronics.
Limit of miniaturization
What is the limit of miniaturization? There are 3 stages of the limit. The first is the “ultimate limit”, which is defined by the distance of the atoms in the material; about 0.3 nm in the case of the Si substrate. Changing the material doesn’t make a big difference. No one can make the structures of the device less than atomic distance. Thus, it should be noted that there is no longer any “pico-electronics” next to nano-electronics.
Before reaching the ultimate limit, there is another limit, called the “fundamental limit” which is defined by the direct tunnel distance; about 3 nm. When the distance between structures reaches 3nm, the wave function of the materials penetrates the potential barrier between the structures, and a huge direct tunnel current begins to flow. Thus, there is no electrical insulation or shutdown of the devices. Thus, 3 nm is the limit for the operation of the device.
The third limit is a “practical limit” defined by the disadvantage of downsizing; about 10 ~ 20nm. When the demerit of downsizing becomes more important than the merit, downsizing will stop. The disadvantage can be the degradation of the leakage current, the cost of production, the reliability and the variability of the characteristics of the device, etc. Recently, logic devices with the trade names of “7nm” or “5nm” technology have been in production. However, it should be noted that the sizes used in the trade names of logic devices have nothing to do with the actual sizes, which are generally 30 to 15 nm for minimum line widths. So what would be the limit on downsizing? The leakage current or the leakage current below the threshold between the source and the drain of the MOSFET is considered to be one of the main causes of the limit of downsizing, since one cannot suppress the diffusion of electrons from the source to channel (which is defined by Boltzmann statistics) unless changing the operating temperature of the device to an extremely low temperature such as 77K. However, it is difficult for the majority of the market. So, everyone agrees that it is difficult to achieve a channel length of less than 10nm for the main devices on the market.
Future technological development in the next 10 years and beyond
Although we are very close to the limit, we have to keep developing new technologies to the limit, because nano-electronics is the key to today’s smart society and the market demand is so strong. New technologies are 3D integration, new materials for interconnects, improved resolution of EUV lithography, etc. Even as the downsizing limit approaches, we will have so much to do in the next 10 years and beyond.