In recent years, significant research efforts have been made to optimize the lithography processes. Liu et al.^[1] (Nat. Commun, 2024, https://doi.org/10.1038/s41467-024-46743-5) pioneered a new multi-photon lithography technology in which light field and matter are co-confined, significantly exceeding the limitations of traditional lithography technology. In this news and views, we introduce this work to readers.

Aiming at the current predicament, Liu et al.^[1], introduced a new multi-photon lithography technology in which light field and matter are co-confined. By skillfully introducing efficient free quencher as well as photo-inhibition into the material system, this technology reduces the adverse influence of proximity effect on CD and LR, and achieves a CD of 30 nm and a LR of 100 nm, which significantly surpasses the limitations of traditional lithography technology, and accurately realizes the high-fidelity transfer of patterns on the wafer. This research achievement not only shows the enormous potential of multi-photon lithography, but also lays a solid foundation to compete with the cutting-edge technologies such as electron beam lithography (EBL) and extreme ultraviolet lithography (EUV). To some extent, it will further promote the technological leap in the micro-nano processing field.

In order to clearly understand the gap between this groundbreaking innovation and the established framework of traditional lithography, it is necessary to study the challenges faced by the latter in depth. In the practice of traditional lithography technology, diffraction limit^[7] constitutes a big obstacle to realize the optimal LR from a perspective of underlying mechanism, the diffraction limit is primarily attributed to the long wavelength of the excitation laser. When propagating through space, long-wavelength lasers undergo diffraction, leading to an increase in beam width and a larger laser focus spot size, which ultimately limits the resolution of photolithography technology. For an optical imaging system consisting of an objective lens or a lens, the diffraction limit resolution is

On this basis, the team adopted three lithography strategies: matter-confined multi-photon lithography (MC-MPL), light-confined multi-photon lithography (LC-MPL), and LMC-MPL, respectively, and successfully manufactured the three-dimensional woodpile structure as shown in Fig. 2 and the Nezha model with a height of 40 μm as shown in Fig. 3. Among them, LMC-MPL showed the best performance in terms of structural clarity and manufacturing capability. At the same time, the research team also established corresponding mathematical models to qualitatively analyze the distribution of free radicals in the lithography process of the three strategies. The simulation data are highly consistent with the experimental data, which further demonstrates the advantages of LMC-MPL technology and allows us to intuitively see how LMC-MPL technology can achieve high-precision lithography. Besides, through the pattern transfer experiment on silicon substrate, they verified that the photoresist prepared by LMC-MPL technology has excellent etching resistance, which indicated that this technology is expected to play a vital role in the practical manufacturing fields such as integrated circuits in the future.

However, it is undeniable that LMC-MPL technology still has some limitations. First of all, the resolution is improved at the expense of the photosensitivity of photoresist. Actually, the decrease of photoresist sensitivity means that more light energy or longer exposure time is required to achieve the same lithography effect, which will directly affect the production efficiency. Therefore, our future research work should be devoted to exploring new photoresist materials or refining the existing photoresist formula, in order to improve the lithography accuracy while maintaining or improving the sensitivity of photoresist. Secondly, LMC-MPL technology requires high stability of the lithography system, aiming to realize accurate control of the excitation beam and the suppression beam. In practice, the lithography system may be affected by various external factors, such as temperature fluctuation, mechanical vibration and electromagnetic interference, thus affecting the stability of the equipment system. Hence improving the stability of lithography system is also of great significance for developing multi-photon lithography technology. Last but not least, although LMC-MPL technology has demonstrated excellent performance in laboratory environment, its applicability in mass production has not been fully verified. High cost and complicated operation process may limit its wide application in industrial production. With the continuous progress of nanotechnology, the requirements for lithography technology will be higher and higher. Therefore, the future development of multi-photon lithography technology needs to pay attention to how to further improve resolution, enhance stability, reduce manufacturing costs and improve production efficiency.

1δ=(0.61λ)/(n⋅sinθ).

However, for MPL, continuing to push the limits of critical dimension (CD) and lateral resolution (LR) remains a significant challenge. Up to now, many efforts have been made to improve the CD and LR of MPL, including shortening the wavelength of excitation laser^[3], introducing photo-inhibited multi-photon lithography^[4] and developing new multiphoton photoresists^{[5, 6]}. Although these methods have gained some progress, there are still some obstacles to be overcome, such as the difficulty in obtaining shorter femtosecond laser, the hardship in developing suitable multiphoton photoresist and the existence of proximity effect.

In summary, the LMC-MPL strategy presents an innovative manufacturing approach that achieves high-precision structural fabrication at the three-dimensional nanoscale by integrating optimized pathways of photo-inhibition and chemical quenchers. This breakthrough surpasses the traditional photolithography's limits in accuracy and resolution, while mitigating the reliance on high-energy radiation and sophisticated equipment, thereby effectively reducing manufacturing costs. Furthermore, LMC-MPL's demonstrated exceptional pattern transfer capability is poised to propel the integrated circuit manufacturing industry towards higher density integration, laying a solid foundation for the development and application of novel materials and devices.

The core advantage of this strategy is that it can use the dual mechanism of photoinhibition and chemical quenching at the same time to significantly limit the free radical distribution in the exposed area. The light suppression mechanism is inspired by the stimulated emission depletion (STED) microscope, and the suppression effect is produced at the tail of the laser focal spot by introducing the suppression beam, thus limiting the polymerization area to a smaller space. Compared to the method of shortening the excitation wavelength, the light inhibition strategy has several advantages. Firstly, it eliminates the need to develop shorter-wavelength femtosecond lasers, thus avoiding complicated engineering challenges. Secondly, utilizing lasers with shorter wavelengths is costly, whereas the light inhibition strategy is relatively inexpensive and more feasible to implement. Thirdly, the light inhibition strategy is not limited to specific wavelength lasers, it can be applied to photolithography systems with different wavelengths, indicating its higher flexibility and applicability. As for chemical quenchers, such as 2,2,6,6- tetramethylpiperidine nitroxide radical (TEMPO) screened in the essay, effectively inhibit the diffusion and accumulation of free radicals through three paths: static quenching, dynamic quenching and direct reaction with free radicals. What’s more, on the basis of technological innovation, they also innovatively suggest a new mechanism of two-step STED process as shown in Fig. 1, suitable for light and matter co-confined multi-photon lithography (LMC-MPL), which further enriched our understanding of photoinhibition mechanism. According to the traditional STED mechanism, suppressing the light beam can make the excited initiator return to the ground state, thus inhibiting photopolymerization. However, in the experiment of LMC-MPL, the authors observed that even when the initiator was completely quenched by the quencher, the polymerization reaction could still occur, indicating the traditional STED mechanism is no longer applicable. Therefore, they propose a two-step STED process as an important supplement to the traditional STED mechanism. This new mechanism not only explains why the light suppression effect in LMC-MPL is still remarkable, but also provides a new idea for the subsequent design of light suppression strategy.

In a word, light and matter co-confined multi-photon lithography technology effectively overcomes the limitations of optical diffraction limit and proximity effect faced by traditional lithography technology through combining photoinhibition and chemical quencher, and achieves a critical dimension breakthrough of 30 nm and a lateral resolution breakthrough of 100 nm. This technology not only significantly narrows the gap with high-end lithography technologies such as EBL and EUV, but also shows great potential in the field of integrated circuit manufacturing. Through continuous technological innovation and optimization, we have reason to believe that multi-photon lithography technology will play a more important role in the future micro-nano manufacturing field and contribute more to the scientific and technological progress of mankind.

Multi-photon lithography (MPL)^[2], an innovative three-dimensional nanomanufacturing technology, leverages its multi-photon absorption mechanism to precisely trigger localized photochemical reactions in photosensitive materials, enabling the direct writing of three-dimensional structures. Recently, this technology has shown great potential in the application of micro-nano processing, biomedicine, photonics and other fields because of its fantastic characteristics: no mask, high resolution and flexible construction of arbitrary three-dimensional structures, as well as the advantages of relatively lower cost and easy implementation.

δ is related to the wavelength λ of incident light in vacuum, the refractive index n of the object and the half aperture angle θ of the object. n⋅sinθ is also commonly called numerical aperture (n⋅sinθ = NA), and Eq. (1) is called Rayleigh criterion^[8] of optical imaging system. With the continuous reduction of required feature size, the diffraction limit has become more and more significant in limiting lithography resolution, which has become the bottleneck of technological progress.

Furthermore, mitigating the impact of the proximity effect constitutes a pivotal challenge in traditional lithography technology. The proximity effect refers to the phenomenon where light diffraction and scattering induce a blurring effect in the transitional zone delineating the exposed and unexposed regions during the lithographic process. From a microscopic perspective, this blurring can be understood as the result of the diffusion and accumulation of free radicals generated by laser irradiation of the photoresist in the non-irradiated regions during the photolithography process. Actually, this blurring not only compromises the precision of pattern edges, but also impedes the accurate replication of the mask pattern onto the wafer, thereby directly influencing the fidelity of the pattern transfer. Especially in the process of sub-micron or even nano-scale lithography, the proximity effect is particularly prominent, which not only leads to serious distortion of patterns, but also greatly increases the difficulty of size control, posing a severe challenge to high-precision manufacturing.