|
Micro-optical systems, that utilize multiple channels for imaging instead of a single one, are frequently discussed for
ultra-compact applications such as digital cameras. The strategy of their fabrication differs due to different concepts of
image formation. Illustrated by recently implemented systems for multi-aperture imaging, typical steps of wafer-level
fabrication are discussed in detail. In turn, the made progress may allow for additional degrees of freedom in optical
design. Pressing ahead with very short overall lengths and multiple diaphragm array layers, results in the use of
extremely thin glass substrates down to 100 microns in thickness. The desire for a wide field of view for imaging has led
to chirped arrays of microlenses and diaphragms. Focusing on imaging quality, aberrations were corrected by
introducing toroidal lenslets and elliptical apertures. Such lenslets had been generated by thermal reflow of lithographic
patterned photoresist and subsequent molding. Where useful, the system's performance can be further increased by
applying aspheric microlenses from reactive ion etching (RIE) transfer or by achromatic doublets from superimposing
two moldings with different polymers. Multiple diaphragm arrays prevent channel crosstalk. But using simple metal
layers may lead to multiple reflections and an increased appearance of ghost images. A way out are low reflecting black
matrix polymers that can be directly patterned by lithography. But in case of environmental stability and high resolution,
organic coatings should be replaced by patterned metal coatings that exhibit matched antireflective layers like the
prominent black chromium. The mentioned components give an insight into the fabrication process of multi-aperture
imaging systems. Finally, the competence in each step decides on the overall image quality.
|
