Chinese researchers are exploring an old idea, the use of an electron accelerator as a light source for lithography.
However, the argument that such a development could be used to bypass US export controls on extreme ultra violet lithography equipment is misplaced. An electron synchrotron only provides an alternative light source for lithography and the complexities of resists, masking and directing and focusing the light to produce nanometer details – so far mastered only by ASML Holding NV – would remain.
It will still take many years of work to produce a reliable system for the mass production of chips at single-digit nanometer nodes. In addition, the same disadvantage that beset and eventually thwarted IBM’s x-ray lithography synchrotron back in the 1990s is still there. That is that synchrotron-based optical sources are enormous – more than 20 meters in diameter – and enormously expensive.
Such systems are typically based on electrons circling in storage ring, because electrons are the lightest sub-atomic particle and therefore easiest to accelerate. The synchrotron radiation is produced by the acceleration of electrons to relativistic speeds.
A big ring
However, the smaller the ring the more energy must be expended to accelerate the electrons and keep them circling. Such systems can only approach viability by having multiple lithography units arranged tangentially to make use of the light/x-rays generated by the electrons. Such a ring could have a great number of lithography machines located tangentially to make use of the single optical source.
Researchers at Tsinghua University are making use of a refinement called steady-state microbunching (SSMB) to try and improve synchrotron optical sources. There is hope for the technique – in the long-term – because established laser-produced plasma (LPP) light sources are struggling to produce enough power to achieve wafer throughput.
In 2021 a group from Tsinghua University, Helmholtz-Zentrum Berlin and Physikalisch-Technische Bundesanstalt experimentally tested steady-state microbunching at the Metrology Light Source synchrotron in Berlin.
“A high-power EUV source is of crucial importance for high-volume manufacturing using EUV lithography,” said Professor Chuanxiang Tang, of the Department of Engineering Physics at Tsinghua University, in a statement at the time. A major issue within EUVL is that the optical system is reflective with power loss at each of 11 reflections of about 30 percent, he added. At the time he said he hoped to build a SSMB synchrotron in Beijing within the next five to six years.
Researchers at Tsinghua University are now in discussions with authorities in the Xiongan New Area about finding a site to build a synchrotron with a circumference of between 100 and 150 meters, according to the South China Morning Post. This would equate to a diameter of between about 16 and 25 meters. The electron beam would be used to generate light for lithography and for scientific inquiry, the SCMP said.
The fact that such a system would be used for lithography AND scientific enquiry provides the clue that this would an academic research machine rather than a commercial manufacturing unit.
The Tsinghua University research can also be related to a paper published in Nature Scientific Reports in February 2022 (see A synchrotron-based kilowatt-level radiation source for EUV lithography)
This paper reported on the use of a compact damping ring with a circumference of about 160 meters to produce kilowatt-level coherent EUV radiation at 13.5nm wavelength. This uses the micro-bunching method to produce coherent power with an average power of about 2.5kW. However, the authors of this paper are predominantly from the Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai.
The Shanghai Synchrotron Radiation Facility (SSRF) already exists and has 15 beamlines in the first phase of the project, with plans for 26 beamlines after completion of a second phase.
The research paper does highlight a couple of advantages of the synchrotron as an optical source. The first is that unlike laser-produced plasma (LPP) sources they do not produce debris contaminating the optics and requiring downtime to clean the system and replace the target. And it is comparatively easy to tune the optical wavelength of synchrotron-based sources. This holds out the possibility of tuning a synchrotron radiation source down to lower wavelengths than 13.5nm without a major technical challenge, the paper states.