Professor Tang Chuanxiang's team from the Department of Engineering Physics at Tsinghua University has made significant progress in the research of extreme ultraviolet (EUV) lithography machine light sources. This light source project is called SSMB-EUV (Steady-State Micro-Focused Extreme Ultraviolet Light Source).

Professor Tang Chuanxiang published a comprehensive review article in the Chinese Physics B journal in 2022. The article provides a clear overview, addressing all possible questions regarding EUV light sources that the project team has already considered. The full text of the paper spans 16 pages. Here, I will provide a brief summary and outline of several key questions you may be interested in.

Is SSMB-EUV a PPT?

No, it is not! Tsinghua University has conducted experimental verification of the physical theoretical principles of the SSMB-EUV project from 0 to 1, and this was done under limited experimental conditions. Therefore, the physical theory is highly reliable. However, if the ultimate goal of this project is to be used in advanced chip manufacturing, the successful experimental verification of the physical principles does not necessarily mean it can be commercially operational. This is because large-scale and reliable production of advanced chips requires the development of additional technologies and cost control, which are separate dimensions.

Currently, Tsinghua University has formed the SSMB-EUV team and has undertaken three major technical challenges for the SSMB-EUV light source:

1. Laser modulator: From the concept introduction of SSMB, it can be seen that the laser modulator corresponds to the microwave RF cavity and is the most important difference between SSMB and traditional storage rings. To achieve SSMB, it is necessary to modulate the laser power and phase lock it. To achieve a high beam duty cycle and thereby enhance the average power of SSMB radiation, continuous wave or high-duty-cycle modulated lasers need to be used. To meet these requirements, an optical gain cavity is planned for the laser modulation system of SSMB.

2. Long-pulse injection system: To achieve high radiation power, SSMB has a high average current density, approximately 1A. Achieving the injection of a large charge amount with long pulses (in the range of hundreds of nanoseconds) requires specialized design. In order to reduce power fluctuations during SSMB operation, it is desired for SSMB to work in a top-up mode with a nearly constant beam current. Furthermore, the top-up mode can also reduce the requirements for the intensity of a single injection beam.

3. Linear induction accelerator: In order to improve the beam duty cycle of the SSMB storage ring, in addition to using continuous lasers, different requirements for energy replenishment of the long-pulse electron beam compared to traditional storage rings have been proposed. A linear induction accelerator operating at the MHz repetition rate is one feasible option to achieve energy replenishment of the SSMB beam.

In February of this year, the project conducted site selection investigations in Xiong'an.

Potential Advantages of SSMB-EUV Light Source for EUV Lithography

1) High Average Power: The SSMB storage ring can support the installation of multiple EUV beamlines, serving as both a high-power illumination source for lithography and a detection source for masks and optical devices. It can also provide support for research on EUV resist materials.

2) Narrow Bandwidth and High Collimation: The SSMB light source can easily meet the narrow bandwidth requirement of less than 2% for EUV lithography and achieve radiation concentrated within an angle range of ≲0.1 mrad. The narrow bandwidth and high collimation characteristics enable innovative designs for EUV lithography optical systems based on SSMB technology while reducing the process difficulty of EUV optical mirrors.

3) High Stability Continuous Wave Output: The SSMB output is in the form of continuous or quasi-continuous wave radiation, which can avoid significant fluctuations in radiation power that may cause damage to the chips. The storage ring light source has good stability, and the top-up operating mode of the SSMB storage ring further improves the long-term availability of the light source.

4) Clean Radiation: Compared to LPP-EUV light sources, the high vacuum environment of the oscillator radiation in SSMB does not contaminate the optical system mirrors used in lithography. This significantly extends the lifespan of the mirrors.

5)Scalability: The SSMB principle allows for easy expansion towards shorter wavelengths, leaving the possibility for next-generation Blue-X lithography technology using wavelengths of 6.xnm.

Conclusion

The EUV lithography solution developed by Tsinghua University is indeed an alternative approach. However, the application of this new physical principle necessitates extensive and long-term technological development and trial-and-error in various technical fields, including engineering technology, materials science, chemistry, computational lithography, mask fabrication, research on optical lenses and mirrors, and ultra-precision inspection, for mass production of advanced chips using this approach. Moreover, substantial research and development investments are required, making the journey challenging and time-consuming.

We hope to see substantial progress in the industrial implementation of Tsinghua's SSMB-EUV solution at an early date.