Extreme ultraviolet (EUV) lithography technology is critical for realizing high-end chip manufacturing at the 7 nm node and below. Currently, EUV lithography machines mainly use laser plasma (LPP) light sources. The maximum EUV power achieved by an LPP light source is approximately 500 W. For nodes smaller than 3 nm, satisfying the power requirements of LPP light sources is difficult. The development of EUV lithography in the future will require more powerful light sources.
A free-electron laser based on energy recovery linacs (ERL-FEL) can achieve a laser output with high repetition frequency, high average power, and high energy efficiency. With the development of FEL and ERL technologies, an ERL-FEL light source can achieve an output power of more than 10 kilowatts at a wavelength of 13.5 nm and is thus a promising high-power EUV lithography light source.
A free-electron laser is a type of radiation laser based on free electrons in vacuum. Compared with those of traditional lasers, the radiation wavelength does not depend on the excited medium but is related only to the electron beam energy and undulator magnetic field. An energy recovery linac accelerates the electron beams in the acceleration phase. After application, the accelerated electron beams return to the main accelerator during the deceleration phase and the power of the high-energy electron beam is converted into the microwave acceleration field power to accelerate the subsequent injected electron beams, which can achieve high-efficiency energy recovery and utilization. The FEL light source based on ERL technology provides a new technical route for the development of high-power EUV lithography.
Since Madey first proposed the concept of free-electron lasers in 1971, at least 50 FEL facilities have been built worldwide, and at least 20 FEL facilities are currently under construction or planned. In 1965, Tigner first proposed the concept of energy-recovery linacs. In recent decades, ERL technology has been regularly applied in different fields, and countries worldwide have conducted research and construction work on ERL facilities. One of the most important applications of ERL is in the generation of high-power FELs.
Global ERL-FEL light sources that have been constructed mainly include the JLAb FEL in the United States, Novosibirsk FEL in Russia, ALICE in the United Kingdom, and JAEA FEL and cERL in Japan. In addition, Peking University, the Institute of High Energy Physics of the Chinese Academy of Sciences, the Shanghai Institute of Applied Physics of the Chinese Academy of Sciences, and China Academy of Engineering Physics have conducted physical design and theoretical research studies on ERL-FELs. However, no ERL-FEL facilities have been fully constructed in China.
In 2015, KEK proposed an ERL-FEL plan for EUV lithography light sources based on a cERL, which can generate an EUV laser power greater than 10 kW. Russia, Germany, and Israel have proposed a compact EUV-FEL light source with an output power of approximately 5 kW. The Shanghai Advanced Research Institute of the Chinese Academy of Sciences has also proposed a fully coherent EUV light source plan based on the ERL.
Although ERL-FEL light sources for EUV lithography have significant development potential, many key technical problems must still be solved. To obtain a kilowatt-level EUV-FEL output, the facility must operate in a state of high average current and high beam power for a long period, which places higher requirements on photocathode injectors, superconducting accelerators, and energy recovery technology.
Traditional LPP technology encounters bottlenecks below the 3 nm node. In the future, the development of EUV lithography will require kilowatt-level high-power light sources. An ERL-FEL light source can achieve an output power above the kilowatt level and is considered to be a highly promising next-generation lithography light source. This study introduces the working principles, development status, and key technical challenges of high-power ERL-FEL light sources.
