Tsinghua News Network2moon25Japan News(reporterWen Xingyu) On February 25, the research group of Tang Chuanxiang, a professor at the Department of Engineering Physics at Tsinghua University, and a collaborative team from the Helmholtz Center for Materials and Energy Berlin (HZB) and the German Federal Institute for Physics and Technology (PTB) published a paper in Nature (Nature) published a research paper titled "Experimental demonstration of the mechanism of steady-state microbunching", reporting the first principle verification experiment of a new type of particle accelerator light source "Steady-state microbunching (SSMB)".

Based on the SSMB principle, high-power, high-repetition frequency, narrow-bandwidth coherent radiation can be obtained, and the wavelength can cover the terahertz to extreme ultraviolet (EUV) bands, which is expected to provide broad new opportunities for photon scientific research. The reviewer of "Nature" spoke highly of this research, saying that it "demonstrates a new methodology" and "will certainly arouse interest in the fields of particle accelerators and synchrotron radiation." A related review article in "Nature" wrote: "This experiment demonstrates how to combine the characteristics of the two main types of existing accelerator light sources - synchrotron radiation light sources and free electron lasers. The SSMB light source is expected to be used in fields such as EUV lithography and angle-resolved photoelectron spectroscopy in the future." Once the paper was published, it immediately attracted great attention from domestic and foreign academic and industrial circles.

Figure 1. Schematic diagram of SSMB principle verification experiment (Picture source: "Nature")

Figure 2. SSMB principle verification experimental results (Picture source: "Nature")

In the experiment, the research team used a laser with a wavelength of 1064 nanometers to control the electron beam in the storage ring MLS in Berlin, so that the electron beam made a full circle (circumference 48 meters) to form a fine microstructure, that is, a micro-bunch. Micro-bunching radiates high-intensity narrow-bandwidth coherent light at the laser wavelength and its higher harmonics. The experiment verified the formation of micro-bunching by detecting this radiation. The formation of micro-bunching proves that the optical phases of electrons can be correlated circle by circle with a precision shorter than the laser wavelength, so that electrons can be stably bound in the optical potential well formed by the laser, verifying the working mechanism of SSMB. The experimental schematic is shown in Figure 1, and some experimental results are shown in Figure 2.

The SSMB concept was proposed in 2010 by Zhao Wu, a professor at Stanford University and a distinguished visiting professor at Tsinghua University, and his doctoral student Daniel Ratner. Zhao Wu continues to promote research and international cooperation at SSMB. In 2017, Tang Chuanxiang and Zhao Wu initiated the experiment. Tang Chuanxiang's research group led the completion of the theoretical analysis and physical design of the experiment, developed the laser system for the test experiment, conducted experiments with cooperative units, and completed experimental data analysis and article writing.

is expected to beEUVLithography light source provides new technology routeattracting attention from the international community

"One of the potential applications of SSMB light source is as a light source for future EUV lithography machines. This is an important reason why the international community pays close attention to Tsinghua University's SSMB research." Tang Chuanxiang told reporters.

In the chip manufacturing industry chain, photolithography machines are indispensable precision equipment and the most complex and critical process step in integrated circuit chip manufacturing. The exposure resolution of the lithography machine is directly related to the wavelength. For more than half a century, the wavelength of the light source of the lithography machine has been continuously shrinking. The new generation of mainstream lithography technology recognized by the chip industry is EUV (extreme ultraviolet light source) lithography with a wavelength of 13.5 nanometers. The EUV lithography machine's work is equivalent to using extreme ultraviolet light with a wavelength of only one ten thousandth of the diameter of a hair to "carve" circuits on the wafer. Eventually, a chip the size of a fingernail will contain tens of billions of transistors. This equipment and technology demonstrates the top level of human technology development. The Dutch ASML company is currently the only supplier of EUV lithography machines in the world, with each EUV lithography machine selling for more than US$100 million.

High-power EUV light source is the core foundation of EUV lithography machine. ASML currently uses a high-energy pulse laser to bombard a liquid tin target to form a plasma and then generate an EUV light source with a wavelength of 13.5 nanometers and a power of about 250 watts. As chip process nodes continue to shrink, the requirements for EUV light source power are expected to continue to increase, reaching the kilowatt level.

"In short, the EUV light required by the lithography machine requires short wavelength and high power." Tang Chuanxiang said. Breakthroughs in high-power EUV light sources are crucial to the further application and development of EUV lithography. Tang Chuanxiang said: "EUV light sources based on SSMB are expected to achieve large average power and have the potential to expand to shorter wavelengths, providing a new solution for breakthroughs in high-power EUV light sources."

The independent research and development of EUV lithography machines still has a long way to go. EUV light sources based on SSMB are expected to solve the core "stuck neck" problem in independent research and development of lithography machines. This requires continuous technological research on SSMB EUV light sources and the cooperation of the upstream and downstream industry chains to achieve real success.

It’s the right time to tackle key problemsDemonstrate the pattern of international cooperation

The Tsinghua SSMB team started the theoretical analysis and numerical simulation of the SSMB principle verification experiment in April 2017. On July 21 of that year, Tang Chuanxiang and Zhao Wu organized the first SSMB cooperation meeting in Tsinghua, and took the lead in establishing an international SSMB research group. They joined forces with scientific researchers from China, Germany, the United States and other countries to start promoting various studies including SSMB principle verification experiments. After four years of research, the SSMB research team has made a number of important advances, with results leading the world.

"SSMB uses lasers to focus electrons. Compared with microwaves commonly used in synchrotron radiation sources, the wavelength of the focusing system is shortened by 5 to 6 orders of magnitude. Therefore, to verify the principle of SSMB, an accelerator is required to have very high control accuracy for the change of the longitudinal position (phase) of electrons from circle to circle, and the M of PTB in Germany The LS storage ring is closest to the experimental needs of SSMB in this aspect. After preliminary contact and communication among teachers, two institutions in Germany, HZB and PTB, actively joined the research team and carried out cooperative research with us," said Deng Xiujie, a 2015 doctoral student in the Department of Engineering and Physics at Tsinghua University who participated in the entire experiment in Germany.

Since 2017, Tsinghua team members have traveled to Berlin eight times to participate in all aspects of the experiment from preparation to operation. After a long period of hard work, the experiment was successful on August 31, 2019. Deng Xiujie said: "SSMB involves many physical effects and the experiments are difficult. The team went through many failed attempts and continued to deepen their understanding of physical problems and actual accelerator operations during the experiment until they finally solved the problems one by one. When it was impossible to conduct on-site experiments, we did not stop working and would conduct theoretical analysis on the experimental data collected previously. , hold regular working meetings, and conduct email or online discussions. "In addition, the SSMB experimental team is an international cooperation team. From the initial run-in to the gradual familiarity and understanding to the gradual improvement, the entire team agreed that we have truly achieved '1+1>>2', and everyone is full of confidence in further cooperation in the future."

Solve the "stuck neck" problemTsinghua University bears the heavy responsibility bravely

“my country’s universities must bravely shoulder heavy responsibilities and release the potential of basic research and scientific and technological innovation in universities.” On September 22, 2020, General Secretary Xi Jinping placed high hopes on universities to strengthen innovation and break through key core technologies at a symposium of expert representatives in the field of education, culture, health and sports.

Tsinghua University inherits and carries forward the Tsinghua scientific research tradition of "holding the sky, standing on the ground, and cultivating people", and enhances the sense of responsibility, mission and urgency to serve the country's self-reliance in science and technology. Deepen the reform of scientific research systems and mechanisms, and innovate scientific research organizational models. Strengthen basic research "from 0 to 1" and speed up the research on key core technologies, especially "stuck neck" problems.

Aim at the forefront of world science and technology and prescribe the right medicine. This time, Tang Chuanxiang's research group in the Department of Engineering Physics of Tsinghua University and the international cooperation team have made great efforts in "Steady State Micro-bunching" (SSMB), a key area that is expected to solve key areas and needs to solve "stuck neck" issues. They continue to increase the intensity of key core technology research and innovation in forward-looking and strategic fields, strive to enhance independent innovation capabilities, and serve the national innovation-driven development strategy.

Currently, Tsinghua University is actively supporting and promoting the project establishment of SSMB EUV light source at the national level. The Tsinghua SSMB research group has submitted a project proposal for the "Steady-state Micro-bunching Extreme Ultraviolet Light Source Research Device" to the National Development and Reform Commission, applying for a major national science and technology infrastructure in the "14th Five-Year Plan".

Professor Tang Chuanxiang from the Department of Engineering and Engineering of Tsinghua University and Dr. Jörg Feikes from HZB are the corresponding authors of this article, and Deng Xiujie, a 2015 doctoral student in the Department of Engineering and Engineering at Tsinghua University, is the first author. This research was supported by the independent scientific research project of Tsinghua University.