UV-LED Photolithography Imparts Micro Features

Technologies like integrated signal distributing, processing, and sensing networks require basic optical elements such as waveguides, splitters, gratings, and optical switches to be miniaturized. This poses challenges, especially for curved elements such as bends and ring resonators, which require an especially high resolution and lower sidewall roughness.

There are a number of methods to achieve subwavelength high-resolution manufacturing, though they tend to be costly, complex, and time-consuming. Nanoimprint lithography is promising as an approach to efficient high-resolution manufacturing, but it requires high-quality master stamps often produced with electron beam lithography.

An illustration of the UV-LED-based microscope projection photolithography (MPP) system (a). A schematic illustration of the process chain, including the steps from structure design to the final projection lithography (b). High-resolution gratings fabricated using MPP (c). Feature sizes below 200 nm achieved by MPP (d). The lines shown in the upper part and lower part were fabricated using a costly objective and an economical objective, respectively. Courtesy of Lei Zheng, Tobias Birr, Urs Zywietz, Carsten Reinhardt, and Bernhard Roth.

To enable rapid, high-resolution manufacturing of optical elements, a Leibniz University research group developed a low-cost, UV-LED-based microscope projection photolithography (MPP) system, capable of producing optical elements within seconds.

Instead of using a mercury lamp or a laser as its light source, the MPP system uses an inexpensive UV-LED with a wavelength of 365 nm. The system, which was created using off-the-shelf components, allows for micro- and nanostructuring within seconds, demonstrating that it has the potential to efficiently produce low-cost structures for optical sensing, optoelectronics, nanophotonics, and other applications.

The MPP system implements structuring by transferring structure patterns on a photomask to a photoresist-coated substrate under UV illumination. To obtain the structure-patterned chromium photomask required by MPP, the researchers performed several preprocessing steps. These included developing a structure design, printing it on a transparent foil, and using lithography to prepare the chromium photomask. The team also established a lithography setup for preparing photomasks. Using this setup and a wet etching process, the researchers transferred structure patterns printed on a transparent foil to a chromium photomask.

The researchers optimized each processing step. They used inkjet printing and laser plotter printing to print the designed structure patterns onto a transparent foil and compared the printing quality of each approach. For the chromium photomask preparation, they incorporated two different lenses into the setup. They used the lenses to transfer structure patterns from the transparent foil to the chromium and the photoresist-coated substrate and compared and analyzed the structuring accuracy. To minimize the imaging aberrations, the team used tube lenses in the established setup, allowing for an infinity-corrected optical system combined with a microscope objective.

By optimizing the printing approach and developing optical elements with minimized field-dependent aberrations, the researchers were able to fabricate high-resolution structures with a feature size down to about 85 nm, using a 100× objective with a numerical aperture (NA) of 1.4. To confirm the stability and functionality of the MPP system, the team showed that structures with the same chromium photomask could be achieved using an economical, 100× microscope objective with an NA of 1.25. The researchers realized a minimum feature size of ~100 nm when using the economical objective.

The results show that the MPP fabrication approach can achieve high-resolution structuring with sub-100-nm feature size, which is below the diffraction limit, and demonstrate the capability of the MPP system to achieve high-resolution fabrication.

The MPP system, which is based on standard optical and optomechanical elements, could advance the use of lithography for the rapid, high-resolution structuring of optical elements. It could be particularly well suited for applications where rapid prototyping and low-cost fabrication are important. For example, it could be used to develop new optical devices for biomedical research or to prototype new MEMS devices for consumer electronics applications.

The research was published in Light: Advanced Manufacturing (www.doi.org/10.37188/lam.2023.033).