X photon 3D nanolithography

Virtual and Physical Prototypes: X-ray laser direct writing 3D nanolithography.

Multi-photon polymerization (MPP), also known as 3D nanoprinting, has been investigated using wavelength-tunable femtosecond lasers. At a fixed pulse width of 100 fs, any spectral color in the range of 500nm to 1200nm can be used, which reveals the interaction of more subtle photophysical mechanisms than two-photon photopolymerization.

The effective absorption sequence of the photosensitized and pure SZ2080TM prepolymers (i.e., X-ray photon absorption) and the optimal exposure conditions were evaluated. The tunability of the wavelength greatly affects the dynamic manufacturing window (DFW), which increases by a factor of 10 under optimized conditions.

In addition, at longer wavelengths, the energy deposition from X-ray photon absorption is not obvious, and the transverse size begins to increase, which can be understood as the condition of ε near zero is reached. This control, and the resulting volume of photopolymerization, could potentially improve the efficiency of 3D nanocrinting.

In summary, the results show that wavelength is an important degree of freedom for custom MPP processes, which, if optimized, will be conducive to a wide range of applications in micro-optics, nano-photonic devices, metamaterials, and tissue engineering.

Figure 1:3D Resolution bridge (RB) printing and linear analysis.

Multiphoton lithography (MPL) is a technique that uses ultra-short laser pulses to fabricate complex three-dimensional (3D) structures at the micro - and nanoscale. It is based on the principle of multi-photon absorption (MPA), a nonlinear optical process that occurs when two or more photons are absorbed by a molecule at the same time.

Figure 2: Experimental design.

By focusing a laser beam on a photosensitive material, such as a photoresist or prepolymer, multi-photon absorption causes a local chemical reaction that changes the properties of the material. By scanning the laser beam or translating the sample in three dimensions, the desired shape can be manufactured with high resolution and precision without any geometric limitations. This enables laser 3D nanocprinting as an additive manufacturing technology.

MPL has been widely used in micro-optics, nano-photonic devices, metamaterials, integrated chips and tissue engineering. It can create structures that are impossible or difficult to achieve with traditional lithography methods, such as curved surfaces, hollow structures, and functional gradients. It can also manufacture new materials with customized optical, mechanical and biological properties.

Although MPL devices are commercially available, the understanding of the photophysics and photochemical mechanisms is still controversial, as the choice of most common laser sources is 800nm wavelength, while others such as 515nm or 1064 nm wavelength have also been shown to be suitable.

Figure 3: Energy deposited at the focal point.

The researchers also investigated the evolution of the polymerization volume during laser direct writing (DLW) through different energy transfer mechanisms: one/two/three photon absorption, avalanche ionization, and thermal diffusion lead to controlled photopolymerization. The study found that 3D nanolithography of ultrashort pulses in the wide visible to near-infrared spectrum range of 400-1200 nm is performed by multi-photon excitation defined by effective absorption sequence.

Figure 4: Relationship between voxel size and average power at different λ.

The researchers noticed that the lateral voxel size deviated from the analysis curve and had a distinct stepped start, mainly manifested in longer wavelengths and higher power. The researchers attribute this to the formation of an ENZ state in the focal region, which causes most of the incident light intensity to be absorbed, producing a large transverse cross-section of photopolymerized monomers.

Figure 5: Voxel size: Minimum voxel size obtained at a fixed power.

The researchers present some examples of controlled refractive index, high transparency and elasticity, and active microoptical elements achieved by X-ray photon lithography combined with calcination and atomic layer deposition techniques. These results can be directly applied to open space sensing in harsh conditions, including unmanned aerial vehicles (UAVs).

The researchers say more in-depth studies are needed to investigate the mechanism of heat accumulation, which depends on the scanning speed and laser repetition rate, as well as the size of the focal spot. Tunable wavelengths, as well as pulse chirps, durations and burst mode operation that are becoming standard in commercial fiber lasers, allow for further improvements.

Considering the trend of Moore's Law over the past 20 years, the average femtosecond laser power is doubling every two years, and high-throughput applications will benefit from parametric optimization of 3D nanoprinting technology.

Links to related articles:
https://doi.org/10.1080/17452759.2023.2228324
https://phys.org/news/2023-08-x-photon-3d-nanolithography.html

Source: Sohu-Yangtze River Delta Laser Alliance