High dynamic grayscale lithography with an LED-based micro-image stepper

Abstract

We developed a novel LED projection based direct write grayscale lithography system for the generation of optical surface profiles such as micro-lenses, diffractive elements, diffusors, and micro freeforms. The image formation is realized by a LCoS micro-display which is illuminated by a 405 nm UV High Power LED. The image on the display can be demagnified from factors 5x to 100x with an exchangeable lens. By controlling exposure time and LED power, the presented technique enables a highly dynamic dosage control for the exposure of h-line sensitive photo resist. In addition, the LCoS micro-display allows for an intensity control within the micro-image which is particularly advantageous to eliminate surface profile errors from stitching and limited homogeneity from LED illumination. Together with an accurate calibration of the resist response this leads to a superior low surface error of realized profiles below <0.2% RMS. The micro-display is mounted on a 3-axis (XYθ) stage for precise alignment. The substrate is brought into position with an air bearing stage which addresses an area of 500 × 500 mm2 with a positioning accuracy of <100 nm. As the exposure setup performs controlled motion in the z-direction the system to maintain the focal distance and lithographic patterning on non-planar surfaces to some extent. The exposure concept allows a high structure depth of more than 100 μm and a spatial resolution below 1 μm as well as the possibility of very steep sidewalls with angles larger than >80°. Another benefit of the approach is a patterning speed up to 100 cm2/h, which allows fabricating large-scale optics and microstructures in an acceptable time. We present the setup and show examples of micro-structures to demonstrate the performance of the system, namely a refractive freeform array, where the RMS surface deviation does not exceed 0.2% of the total structure depth of 75 μm. Furthermore, we show that this exposure tool is suitable to generate diffractive optical elements as well as freeform optics and arrays with a high aspect ratio and structure depth showing a superior optical performance. Lastly we demonstrate a multi-level diffraction grating on a curved substrate

Concept, manufacturing and challenges of ultra-compact snapshot multi-spectral multi-aperture imaging systems

“…However, the sag height and shapes of the micro-lenses achieved by those technologies limit the spatial resolution and the f-number of each channel, but there are emerging technologies to overcome these restrictions for multi-aperture imaging systems. The manufacturing of the filter tile array is performed by a grayscale photolithography process [5] in resist and a subsequent proportional dry-etching process (RIE) into fused silica of the filter's cavity layer. However, the realization of the differences in cavity height between the filter tiles as well as the final absolute height are ambitious due to a varying etch rate factor over time and the selectivity during the dry-etching process.…”

High-order-helix point spread functions for monocular three-dimensional imaging with superior aberration robustness

“…The three phase element designs studied in section 3 (double-, tripleand tetra-helix) are implemented using state-of-the-art micro-optics manufacturing processes on wafer-scale. A thin photoresist layer is initially spun on a borosilicate glass substrate and subsequently structured using the LED writing lithography approach presented by Eckstein et al in [27]. The next steps provide a significant advancement in the manufacturing of phase elements for monocular depth estimation compared to previous elements [9,15].…”

“…(26) and Eq. (27) in Eq. (30) and considering only Zernike aberrations that fulfill condition (25) results in:…”

Development of an adaptive template for fast detection of lithographic patterns of light-emitting diode chips

“…The light-emitting diode (LED) has the advantages of low energy consumption, high brightness and long service life [2], which are a new development focus in lighting systems [3]. To form lithographic patterns on LED chips, as shown in Figure 1(a), lithographic optical and chemical etching procedures are required, with a variety of light masks [4] and optical path projection systems [5].…”