Tsinghua team takes the lead in reaching the final node of Moore's Law, and the research on 0.34nm gate length transistor is published in Nature, breaking the Stanford record

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How small can a transistor be? Recently, Professor Ren Tianling ’s team from the School of Integrated Circuits at Tsinghua University gave the answer.

The team used a single layer of graphene as the gate to create a "sidewall" transistor, setting a record for gate length of 0.34nm . This research was published in the latest issue of Nature.

In 2016, Stanford University used carbon nanotubes to create a 1nm gate length transistor, and the record was finally broken six years later.

Ren Tianling pointed out that this may be the last node of Moore's Law.

In the future, it will be almost impossible to manufacture gate lengths less than 0.34 nm.

So, what is so special about the sidewall transistor of the Tsinghua University team?

Sidewall transistor

Let us first introduce the basic principles of transistors.

The basic unit transistor in modern chips, more precisely the field effect transistor (MOS), consists of two conductive electrodes, the source and the drain , separated by a semiconductor.

Whether the source and drain can be connected is controlled by the gate . The gate length is one of the most important indicators for measuring the size of the transistor.

In order to cram more transistors onto a chip, transistor size, especially gate size, must be reduced.

For example, the FinFET currently used in the 5nm process stands up the gate to reduce the area of ​​a single transistor.

In 2016, Stanford University used single-walled carbon nanotubes to make a gate with a size of 1nm, and the research results were published in Science.

How can we further reduce the gate length? A single-layer graphene sheet, which is thinner than a carbon nanotube, can do this. Its thickness is only the size of a carbon atom - 0.34nm.

To create this transistor, the researchers redesigned the structure of the transistor.

△ Schematic diagram of the structure of the sidewall transistor

First, graphene was deposited on a silicon dioxide substrate, and then a layer of metallic aluminum was deposited on the graphene surface. Although aluminum is a conductor, the researchers let it sit in the air for a few days to form aluminum oxide on the surface.

In this way, the graphene has silicon dioxide underneath and aluminum oxide on top, which is insulated from the upper and lower layers.

The researchers then etched along the edge of the aluminum down into the silicon dioxide layer, exposing the edge of the graphene that served as a gate just one atom thick.

△ The process flow of sidewall transistor

The entire device is first covered with a thin layer of hafnium oxide insulation, followed by a layer of molybdenum disulfide semiconductor as the channel between the source and drain.

The source and drain, made of titanium-palladium metal, are placed on both sides of the gate, with a vertical wall between them, so it is called a sidewall transistor.

△ Microstructure of the sidewall transistor

The study found that because a single-layer two-dimensional molybdenum disulfide film has a larger effective electron mass and a lower dielectric constant than bulk silicon materials, the transistor can be effectively turned on and off under the control of a sub-1 nanometer physical gate length, and its off-state current is in the pA range.

Since this transistor has very low leakage current, it can be used for low power operation.

However, the process of making sidewall transistors is still relatively complicated. Researchers have only made dozens of transistors, and there is still a long way to go before practical application.

Team Profile

The corresponding authors of this article are Professor Ren Tianling and Associate Professor Tian He from the School of Integrated Circuits, Tsinghua University.

Wu Fan, a 2018 doctoral student from the School of Integrated Circuits at Tsinghua University, Associate Professor Tian He, and Shen Yang, a 2019 doctoral student, are the co-first authors.

△ Professor Ren Tianling and Associate Professor Tian He

Ren Tianling received his Ph.D. from the Department of Modern Applied Physics of Tsinghua University in 1997. He has been a professor at the Institute of Microelectronics of Tsinghua University since 2003. His main research areas are intelligent micro-nanoelectronic devices, chips and systems.

Tian He received his Ph.D. from Tsinghua University in 2015, with his supervisor being Ren Tianling. He is currently an associate professor and doctoral supervisor at the Institute of Microelectronics and the School of Integrated Circuits at Tsinghua University.

Reference links:
[1] https://www.nature.com/articles/s41586-021-04323-3
[2] https://arstechnica.com/science/2022/03/a-transistor-made-using-two-atomically-thin-materials-sets-size-record/
[3] https://spectrum.ieee.org/smallest-transistor-one-carbon-atom
[4] https://www.tsinghua.edu.cn/info/1175/92075.htm
[5] https://www.sic.tsinghua.edu.cn/info/1015/1035.htm