How do carbon-based semiconductors break through?

Over the past half a century, the semiconductor industry has been following the trajectory of Moore's Law to develop at a high speed. Nowadays, the method of improving chip performance simply by upgrading the process can no longer fully meet the needs of the times, and the semiconductor industry has gradually entered the "post-Moore era." The advent of the post-Moore era has brought new development opportunities to the development of China’s integrated circuit industry. China Electronics News has launched a series of reports on “Exploring the Disruptive Technologies of Integrated Circuits in the Post-Moore Era” to sort out the potential disruptive technologies of integrated circuits. Discuss the development status, industrial problems, and future prospects of each technology.

With the chip manufacturing process approaching 2 nanometers, the potential of silicon-based chip materials has basically been tapped, and it cannot meet the needs of the industry's future development. The use of new materials is recognized as a fundamental solution to chip performance problems. If Moore's Law really fails, silicon-based chips that are gradually approaching physical limits are likely to be in a situation where there is no way out of doubt. In this case, will carbon-based semiconductors be the savior that will show us the scene of "a new village"? At this stage, how carbon-based semiconductors can get out of the "glass room" of the laboratory and truly realize their potential is still the focus of attention and difficulties faced by the industry.

Carbon-based semiconductors have unique advantages

Following Moore's Law, the "golden rule" of the semiconductor industry, the performance of silicon-based semiconductor chips will double every 18 to 24 months. However, as chip sizes continue to shrink, especially when the level of chip manufacturing technology enters the 5-nanometer node, the development of silicon chips begins to face many physical constraints, and the industry has gradually emerged "Moore's Law is dead" and "Silicon-based technology has come to an end." And other views. Carbon-based semiconductors are considered to be one of the disruptive technologies in the post-Moore era.

Carbon-based semiconductor is a semiconductor material developed on the basis of carbon-based nanomaterials, represented by carbon nanotubes (CNT) and graphene. The ITRS research report has clearly pointed out that the future research focus of the semiconductor industry should focus on carbon-based electronics.

In order to continue Moore's Law, researchers have continued to explore new materials and new device structures. Compared with traditional silicon-based technology, what advantages does carbon-based semiconductors, which have attracted countless scientific researchers "compete"?

The technical staff of Beijing Carbon-based Integrated Circuit Research Institute previously told the reporter of "China Electronics News" that carbon-based technology has better performance and lower power consumption than silicon-based technology. For example, carbon-based chips using the 90-nanometer process are expected to produce silicon-based chips with performance and integration equivalent to the 28-nanometer technology node, and carbon-based chips using the 28-nanometer process can achieve silicon-based chips equivalent to the 7-nanometer technology node.

Ma Yaobin, a researcher at the Institute of Integrated Circuits, CCID Think Tank, took carbon nanotubes as an example to show reporters from China Electronics News about the technological advantages of carbon-based semiconductors. "CNT (carbon nanotube) has extremely high carrier mobility, very thin body size and excellent thermal conductivity. Compared with silicon-based processors, the operating speed and energy consumption of CNFET-based processors can both have about 3%. The advantage of two times, that is, the advantage of the energy delay product (EDP) of about 9 times." Ma Yaobin told reporters.

The use of graphene materials is also a strong proof of the advantages of carbon-based semiconductors. Ma Yaobin pointed out to reporters that graphene has excellent characteristics such as high carrier mobility and good thermal conductivity, which allows graphene transistors to obtain high signal transmission speed and good heat dissipation. In the future, graphene is expected to play an important role in realizing smaller size chips, 3D package interconnection, and optimizing chip heat dissipation.

The dawn of carbon nanotube technology

In fact, people's pursuit and exploration of carbon-based semiconductor materials has not only begun in recent years. Carbon nanotube technology using new materials has always attracted the attention of countless scientists. In 1991, Japanese physicist Sumio Iijima, who has been elected as a foreign academician of the Chinese Academy of Sciences, unexpectedly discovered carbon nanotubes when he used a high-resolution transmission electron microscope to observe carbon fiber products produced by the arc method. According to his observations, carbon nanotubes are made of carbon molecules arranged in a tubular shape, which can be regarded as a single layer of graphite rolled into a "cylinder", which needs to be prepared from carbon materials such as graphite rods by a special method.

In August 2019, a research result of carbon nanotubes once again made the "Hello, World" string of characters familiar to every programmer a global sensation. A paper published in the journal Nature showed that Max Shulaker and his colleagues at the Massachusetts Institute of Technology in the United States managed to design and construct a carbon nanotube microprocessor. This microprocessor is a 16-bit microprocessor manufactured using more than 14,000 carbon nanotube (CNT) transistors. Its design and manufacturing method overcomes the previous challenges related to carbon nanotubes and is expected to be the silicon in advanced microelectronic devices. Bring a high-performance alternative. This microprocessor was named "RV16X-NANO" and successfully executed a program in the test, generating a message: "Hello, world! I am RV16XNano, made of carbon nanotubes."

Ma Yaobin told reporters that researchers from TSMC, Stanford University and the University of California, San Diego have also jointly developed a top-gate CNFET (carbon nanotube field effect transistor) with a gate length of 10nm and a subthreshold swing of 68mV/dec.

Last year, the team of academicians of the Chinese Academy of Sciences and professors of the Department of Electronics of Peking University Peng Lianmao and Professor Zhang Zhiyong's major research results on carbon-based semiconductor materials also gave the industry new hope in the post-Moor era. On May 22, 2020, the team published a paper "High-density semiconductor carbon nanotube parallel arrays for high-performance electronics" in Science magazine, introducing the team’s latest development of multiple purification and dimension-constrained self-assembly methods. The method solves the problems of material purity, density and area that have long plagued the preparation of carbon-based semiconductor materials.

The progress of research on carbon-based semiconductors is not always smooth sailing. After the "big waves", research in this field by some institutions and companies has stalled. As early as 2014, IBM made bold rhetoric, saying that it would use carbon nanotubes to produce chips 5 times faster than then by 2020, but there is no further progress in research and development.

Carbon-based and silicon-based circuits need to be developed differently

Although carbon-based semiconductors represented by carbon nanotubes and graphene have many technical advantages and the market potential is obvious to all, there are still many difficulties in high-quality, mass-production and practical application of carbon-based semiconductors.

It is not easy for carbon tubes to form thin films for processing VLSI circuits. Professor Wan Qing of the School of Electronic Science and Engineering of Nanjing University expressed his opinion to the reporter of "China Electronics News": If it is direct directional growth, it is difficult to obtain a high-density perfect semiconductor carbon tube film; To meet the needs of large-area (12-inch) nano-scale very large-scale integrated circuit technology, mass manufacturing and product yield may become challenges.

Wan Qing believes that, although a single carbon-based device is already doing well, compared with silicon integrated circuits, carbon-based semiconductors still have certain problems in nano-scale ultra-large-scale integration and industrial yield. In terms of conventional integrated circuit applications, carbon tube circuits may not be able to compete with silicon-based circuits at present, so carbon-based semiconductors may need differentiated development. In the future, it is expected to find a way out in new fields such as sensing and flexible systems.

Ma Yaobin told reporters that the batch preparation of ultra-high semiconductor purity (99.9999%), in-line (orientation angle <9°), high density (100-200/μm), and large-area uniform CNT array film currently exists. Difficulties, which hinder the rapid application of CNFET in the field of integrated circuits. "In terms of purity, the current prepared CNTs will have the symbiosis phenomenon of semiconductors and metal CNTs, and the appearance of metal CNTs will cause serious degradation of the electrical performance of devices and chips." Ma Yaobin said.

As far as graphene is concerned, Ma Yaobin said that the zero band gap feature of intrinsic graphene makes the graphene transistor switch ratio very small, which will also limit the application of carbon-based semiconductors in logic circuits.

From the "ideal value" of the laboratory to the large-scale application in the market, the road to industrialization of carbon-based semiconductors is long and difficult. Xu Zheng, a professor at the School of Science of Beijing Jiaotong University, told the reporter of "China Electronics News" that at present, carbon-based semiconductor materials have achieved physical properties, but to make devices, they need to undergo a lot of process polishing. "Technology realization and the guarantee of cost performance are the prerequisites for the industrialization of carbon-based semiconductors." Despite the difficulties and challenges, Xu Zheng is still full of hope for the future of carbon-based semiconductors. "If the development level of related equipment is improved, the carbon-based semiconductor industry can With the support of industrialized equipment, it is possible for carbon-based semiconductors to achieve large-scale and industrialized development."

In the long journey of tackling carbon-based semiconductors, the industry needs to cultivate internal skills and form a systematic accumulation of technology. A few days ago, the academician Peng Lianmao team also expressed the willingness to do things low-key to reporters. This may be able to explain from the side that if the carbon-based semiconductor industry is to achieve further development, the industry still needs to concentrate on research and development and be down-to-earth.