The key idea of steady-state micro-bunching is to introduce laser modulation into the electron storage ring of traditional synchrotron radiation accelerators. Originally, the electron bunching in the storage ring was achieved using "RF cavities" for microwave radio frequency, but this was replaced by a much more complex laser modulation system, combined with wigglers and magnets for transverse and longitudinal modulation. This cleverly brought the electron beam together even more perfectly. SSMB was able to publish an article in "Nature," which demonstrated how to practically achieve this and proved that the shape of the electron beam was indeed more perfect.

The figure shows that waveform in Figure AB is broad and has not been modulated by laser, while the waveform in Figure CD is the result of modulation by laser and magnet, with five beams emerging in the middle. The result of adding a filter is shown in Figure EF, which makes the pattern even more apparent. Of course, the experiment only conducted one modulation, and continued modulation may face technological difficulties as future work.

Similar to FEL, SSMB generates "micro bunches," but the key is to add "steady state." FEL is not steady state, and the electron group freely interacts with each other in the wiggler before emitting strong light. In contrast, SSMB allows the electron beam to circle in the storage ring, making it potentially steady state, which is important for repeated light emission. This combines two characteristics: coherent radiation from micro bunches produces strong light and a high repetition frequency in the storage ring.

According to a research paper from Tsinghua University, these two characteristics combined make SSMB-EUV light source very promising for lithography. It appears to be better than the linear SRF-FEL and easier to control. When the electron beam circles in the storage ring and produces strong light, the micro bunches emit coherent radiation, exporting the EUV light source for lithography.

According to a video presentation by Zhao Wu in 2021 at Yang Zhenning Academic Thought Seminar, the advantage of using SSMB-EUV light source for lithography is that it only requires three reflectors (since the SSMB-EUV light source is much purer than LPP-EUV light source), and the mirror area requirement is much smaller, only one-tenth. This appears to be a huge advantage, and the quality of the light source is superior to that of ASML's EUV lithography machine. The overall difficulty of development definitely decreased, and the requirements for mirrors were also reduced.

However, this huge advantage depends on the successful development of SSMB-EUV light source. There are many difficulties in the subsequent development of SSMB, which are clearly stated in a literature review in "Acta Physica Sinica," and the technical details are difficult to understand. One type is the problem of generating and maintaining micro bunches in the storage ring, and the other is the problem of coherent radiation emission from SSMB, both requiring a lot of follow-up research.

Although the use of electron micro bunches sounds good, electrons slip longitudinally when turning, making it impossible to maintain the beam's focusing. It is also very difficult to keep a good angle when modulating laser and electron micro bunches. These practical problems will become huge trouble in engineering, causing apparent results to be delayed and facing inevitable major problems.

This is a common phenomenon in scientific research, where actual R&D workers sweat over difficulties and numerous problems waiting to be solved, working overtime and racking their brains. When outsiders see a glimmer of hope, they may assume success is imminent, and the US technology blockade is about to be finished. Some people even pull up images of accelerators in Beijing and make them appear as lithography factories, which have absolutely no relevance.

In my opinion, SSMB-EUV is a good direction with great potential in theory, and has obvious advantages compared to LPP-EUV light sources. If the SSMB accelerator is ultimately built and produces high-quality EUV light sources, creating a research platform based on large-scale scientific equipment, it will indeed be a breakthrough from small-scale EUV lithography machines.

However, there are still two major steps to cross before achieving success. One is the landing of the SSMB accelerator in Xiong'an and generating high-quality EUV light sources, building a research platform based on large-scale scientific equipment. The second step is to develop an EUV lithography machine that is compatible with high-quality SSMB-EUV light sources and aims for mass production, with a lower difficulty than the LPP-EUV light source lithography machine of ASML but still very challenging.

EUV lithography machines have key components such as light sources, workbench platforms, objective lenses, and laser interferometers, all of which are extremely difficult to develop. Moreover, it will be difficult to match all components into a complete system without conflict.

One research choice is not to develop an EUV lithography machine directly after SSMB accelerator shines, but rather to first dock with a DUV lithography machine, as the energy loss of the beam is smaller, and to accomplish intermediate tasks on a less difficult platform.

Even if these major steps are successful, the time required will not be short. However, China has begun to seek various solutions to extremely difficult scientific and engineering problems due to the U.S. pressure. Many people have combined clever ideas with engineering implementation, and this process will be exciting. The more difficult the problem, the greater the rewards of success, and we can learn about scientific principles and technical backgrounds and wait patiently.