Introduction
The trend to system miniaturization is moving the semiconductor technology toward creation of micro- and nanostructured light sources (i.e., micro- and nanoLEDs). To increase the efficiency of LED light extraction and outcoupling, high-precision microoptical elements are required. Thus, in this work, precisely positioned microlens arrays were designed and manufactured using a thermal reflow method to be combined with LED devices. Even though this lens fabrication technique had been well known, its processing reproducibility and output quality were not yet analyzed in detail. Further optimization and characterization therefore was combined with nanometrological methods in order to obtain replicable manufacturing processes of microlens arrays.
Methodology and Results
The fabrication process starts with defining the parameters of the spincoating and photolithography steps in accordance with the required geometrical and optical properties (e.g., focal length, diameter, and maximum center thickness) of the microlenses. The spincoating parameters should be controlled very precisely to obtain a photoresist layer with a constant thickness and required aspect ratio of resist thickness and lens radius, otherwise the lens morphology would be different from the desired semi-spherical shape (Figs. 1(a)-(c)). The semi-spherical microlenses were obtained by thermal reflow of the photoresist cylinders formed at the photolithography step. To estimate their performance, optical characterization was carried out by observing and analyzing the shape of the light beam, which was formed by each lens (Figs. 2(a) and (b)). From the measurement results, the microlens arrays have exhibited high uniformity over the whole substrates with parameters agreeing to the calculated values. Besides, integration of the microlens arrays into structured LED wafers is currently being executed using spin-on-glass, which is more mechanically and chemically stable, and its results will be presented.
Discussion
Semi-spherical microlens arrays with high quality and precision were designed and fabricated by combining photolithography and enhanced thermal reflow processes. 3D confocal laser scanning microscopy and optical metrology were carried out for ensuring the quality of their shapes, focal lengths, surface profiles and outcoupling performance, respectively. Further integration of these microoptical elements with structured light-emitting diode wafers will pave the way to realize ultra-compact optoelectronic sensing and manipulation devices.
