Microlens Array Fabrication Method Reduces Device Costs

Though aspheric microlens arrays (AMLAs) play an important role in advanced optical sensing, communications, imaging, and other industrial and biomedical applications, the fabrication of the elements remains a challenge. Even as MLAs stepped into diverse emerging applications, including AR/VR display and beam shaping, it is difficult using traditional MLA fabrication approaches, such as the hot reflow, ink-jet, and self-assembly, to fabricate AMLAs directly with a desired arrangement and profile, which determines the AMLA’s optical performances.

Further, the drawbacks, such as the debris induced by the top-down writing, difficulties in the topography control, and the process complexity, hinder these methods from large-scale commercialization.

Researchers at the Institute of Technological Sciences at Wuhan University have addressed this challenge. The researchers developed a technique for fabricating and characterizing AMLAs that is based on a 12-bit laser direct writing lithography (DLWL) technology with single beam exposure.

According to the researchers, the DLWL technique exhibited a high fabrication speed over a large area and provides perfect lens shape control through a 3D optical proximity correction.

It also exhibited average surface roughness lower than 6 nm. The fabrication of AMLAs based on the DLWL technique could reduce the complexity of AMLA fabrication while increasing AMLA performance.

To control the AMLA profile, the researchers used an optimization method to reduce deviation from the desired profile. The relative profile deviation of a perfect microlens shape was compressed to 0.28% through the researchers’ gradient convergence optimization method.

The team used parallel and scattered light sources to test optical performance. They fabricated an autostereoscopic-display thin film that demonstrated excellent performance and that could be used for flexible, low-cost holographic displays.

(a) Schematic diagram of an off-axis microlens array. (b) Three-dimensional topography of a fabricated off-axis microlens array. (c) Experimentally captured focused spot arrays with the operating wavelength of 635 nm. (d) Off-axis microlens array characterized via the SEM. (e-f) SEM photos in partial views of the microlens arrays with filling factors of 90.7% and 100%. Courtesy of Shiyi Luan et al.

The team showed that an AMLA with dimensions of 30 × 30 mm²could be fabricated in approximately 8.5 hours, which, according to the researchers, is comparable to a high-speed writing exceeding 100 mm²/h.

The high-quality AMLA fabricated by the team demonstrated high-performance optical imaging, light focusing, and focal length consistency. The highly flexible DLWL technique could be used to design AMLAs with different filling factors. For example, an arbitrary off-axis AMLA could be fabricated using one-step photolithography.

Compared to traditional fabrication methods, the DLWL technique provided a high degree of design flexibility that could make the preparation of microlens arrays with complex morphology easier, and make the technique itself more suitable for industrial production than traditional methods.

According to the researchers, the fabrication method points to many application prospects, such as laser beam shaping and wavefront sensing. For example, to realize a freeform beam shaper, the microlenses inside a microlens array should be aligned irregularly, in such a way that the focused spot arrays are randomly distributed. This requires a complex grayscale mask for other approaches, according to the researchers. However, with the laser direct writing lithography technology with a high degree of manufacturing freedom, a user could directly fabricate an off-axis microlens array to generate irregular spot arrays without the requirement of a complex grayscale mask.

The advanced photolithography technique could additionally improve the performance of many functional devices based on microlens arrays and could reduce the cost of devices that are built using microlenses, such as endoscopes, infrared detectors, holographic displays, and optical couplers.

The research was published in Light: Advanced Manufacturing (www.light-am.com/article/doi/10.37188/lam.2022.047).