Photoresist Enables High-Throughput, High-Resolution Nanolithography

A new cationic-based photoresist can improve the production speed of two-photon lithography (TPL) without sacrificing resolution. The highly sensitive photoresist, called TP-EO (short for two-photon epoxy oligomer), was developed by a team at Zhejiang University.

TPL is a powerful tool for creating complex 3D shapes at the nanoscale. It uses photoresists — materials that change their chemical properties when exposed to light — to allow precise control during the nanofabrication process.

However, high-throughput TPL is problematic, especially for cationic-based photoresists. Cationic photoresists, like the classic SU-8 epoxy photoresist, often take longer to fabricate and result in less detailed structures, compared to free-radical-based photoresists. To speed up the TPL process, highly sensitive photoresists are essential.

Researchers at Zhejiang University developed a cationic-based photoresist named TP-EO for high-throughput, high-resolution two-photon lithography (TPL) nanofabrication. Courtesy of Pixabay/CC0 Public Domain.

In demonstrations, the TP-EO photoresist exhibited around 600 times greater sensitivity to two-photon laser exposure than SU-8, making it an attractive alternative to SU-8 for high-throughput 3D nanomanufacturing.

TP-EO achieves high-speed fabrication through a bimolecular sensitization system. It uses a bimolecular photosensitizer-photo acid generator (PS-PAG) pair to resolve the photosensitivity bottleneck common in cationic-based photoresists.

The bimolecular photosensitized initiation system separates the processes of light absorption and energy transfer, enhancing the material’s ability to absorb light. The researchers used 5-nitroacenaphthene (5 Nan), a photosensitizer that broadens the absorption spectrum, to enable the material to capture of light wavelengths down to 430 nm.

To create TP-EO, the team combined the 5 Nan photosensitizer with pyrazoline-based sulfonium salt, which served as a photoacid generator (PAG), and polyfunctional epoxy, which acted as a building block.

TP-EO enables high-resolution nanofabrication by limiting the photoacid diffusion, which is done by tuning the monomer’s intra- and intermolecular stereostructure.

To demonstrate potential applications of the TP-EO resin, the researchers fabricated a topological liquid diode with nanoscale features. TP-EO demonstrated a lithography speed of 100 millimeters per second (100 mm s^-1) and produced fine features with a minimum width of about 170 nm. The researchers used TP-EO to fabricate 3D structures with low shrinkage and show the rapid fabrication of centimeter-scale nanodevices possible with the new photoresist.

“We demonstrated the 3D fabricated structures with fine features of less than 200 nanometers (nm) and a fast writing speed of 100 millimeters/second (mm/s) using nanolattices, to show potential application for high-throughput nanofabrication of microscopic 3D devices,” professor Cuifang Kuang, who led the research, said.

The researchers showed that the TPL fabrication speed and resolution of TP-EO is a significant improvement over traditional cationic photoresist materials, and that TP-EO could facilitate high-throughput TPL for creating high-resolution nanostructures.

“Such a high-performance TP-EO photoresist is suitable for the scalable fabrication of complex architectures for various applications, such as optical gratings, diffraction elements, microelectromechanical systems, microfluidic devices, and tissue engineering scaffolds,” Kuang said.

The research was published in Advanced Functional Materials (www.doi.org/10.1002/adfm.202409859).