Point light sources based on pinhole InGaN/GaN microLED arrays for lensless cell monitoring systems

For evolving applications in life sciences (e.g., cell counting and imaging), a compact system based on digital inline holographic microscopy provides a lightweight and cost-effective platform, which can be integrated in an incubator set-up enabling a real-time, in-situ and continuous biological cell monitoring. Although commercial surface mounted device (SMD) -packaged LEDs can be employed, they still have limitations in terms of size, integration flexibility, spatial illumination coherency and therefore achievable image resolution, especially when extension to 3D imaging and capabilities of pixel super-resolution are required. Therefore, smaller point light sources arrays, which can be specifically designed and fully integrated into a miniaturized imaging system, would offer tremendous advantages.  

In this work, pinhole-shaped microLED arrays with openings ranging from 100 µm down to 5 µm have been designed and fabricated from planar InGaN/GaN LED wafers using processing flow steps shown in Fig. 1. Firstly, an insulating SiO₂ layer is deposited on the p-GaN, which is followed by sequential photolithography and etching processes to realize openings for transparent p-contacts and larger n-contact areas. Different thin metal layers, such as Ti/Au and Pd/Au, are deposited and subsequently annealed to produce the contacts. Finally, metallization defines both electrical connection and dimensions of the pinhole LEDs (Fig. 2).

Prior to integration into lensless holographic microscope, the LED devices are characterized by electroluminescence (EL) measurements at room temperature. The I‑V curves (Fig. 3) indicate that the LED devices can be operated under normal bias of ~3 V. To test their capability as an enhanced point light source, a single 90 µm pinhole LED is assembled into a lensless holographic microscope to image polystyrene microbeads with diameters of 5 µm. From the captured images, we have observed that the interference and diffraction effects from the dense microbead clusters can be reduced by using the pinhole LED instead of a large area LED with a size of 230 × 230 µm². This results in images with better resolution and quality (based on their focus value) than those obtained by commercial blue SMD LEDs (Fig. 4).