Volume 4, Issue 1 - Micro-optics for industrial applications / Guest Editors: Robert Brunner and Tasso R.M. Sales

Diffraction gratings are key components for spectroscopic systems. For high-end applications, they have to meet advanced requirements as, e.g., maximum efficiency, lowest possible scattered light level, high numerical aperture, and minimal aberrations. Diffraction gratings are demanded to allow spectrometer designs with highest resolution, a maximal étendue, and minimal stray light, built within a minimal volume. This tutorial is intended to provide an overview of different high-end spectroscopic gratings, their theoretical design and manufacturing technologies.

Illumination systems for various industrial applications, e.g., in semiconductor industries and medical treatment techniques, require a specific and well-adapted intensity distribution with maximum efficiency, best achievable accuracy, and highest flexibility in the selection of the target light field. Additionally, for particular applications, all these aspects have to be fulfilled simultaneously for different wavelengths. In most of these situations, the challenging optical specifications are accompanied by the requirement of a cost-efficient fabrication technique, suitable at least for batch or small series production. In the present contribution, we first retrace the development steps and performance progress of micro-optical-based beam shaping setups. Secondly, we discuss the possibilities resulting from a combination of both refractive and diffractive optical characteristics. Especially, we show that a well-adapted combination of refractive and diffractive properties in a single element allows us to create achromatic diffusers with exceptional flexibility and high conversion efficiency.

Mask aligners are lithographic tools used to transfer a pattern of microstructures by shadow printing lithography onto a planar wafer. Contact lithography allows us to print large mask fields with sub-micron resolution, but requires frequent mask cleaning. Thus, contact lithography is used for small series of wafer production. Proximity lithography, where the mask is located at a distance of typically 30–100 μm above the wafer, provides a resolution of approximately 3–5 μm, limited by diffraction effects. Proximity lithography in mask aligners is a very cost-efficient method widely used in semiconductor, packaging and MEMS manufacturing industry for high-volume production. Micro-optics plays a key role in improving the performance of shadow printing lithography in mask aligners. Refractive or diffractive micro-optics allows us to efficiently collect the light from the light source and to precisely shape the illumination light (customized illumination). Optical proximity correction and phase shift mask technology allow us to influence the diffraction effects in the aerial image and to enhance resolution and critical dimension. The paper describes the status and future trends of shadow printing lithography in mask aligners and the decisive role of micro-optics as key enabling technology.

Subwavelength gratings comprising a stack of two thin metallic wire gratings embedded in a dielectric exhibit distinctive color properties in transmittance. Its angle-dependent coloring can be easily noticed by human observers in ambient light. Moreover, it has pronounced polarizing properties in the visible wavelength range. The optical properties of these gratings have been numerically studied. The results have been confirmed by manufactured samples replicated by a simple nanoimprint process on plastic foils. The manufacturing process is predestined for industrial mass production. Applications as authentication devices are given.

The intention of this article is to give a concise overview on current applications of micro-optical components in lighting, including general lighting, automotive lighting, projection, and display backlighting. Regarding the light sources, the focus of this paper is on inorganic light-emitting diodes (LEDs) and the characteristic problems encountered with them. Lasers, laser diodes, and organic light-emitting diodes (OLEDs) are out of scope of this paper. Micro-optical components for current applications of inorganic LEDs may be categorized essentially into three classes: First, components for light shaping, i.e., adjusting the intensity distribution to a desired target; second, components for light homogenization with respect to space and color, and third, large-area micro-optical elements. These large-area elements comprise micro-optical slabs and sheets for guiding, reflection, and refraction of light and are designed without regard to particular details of type, design, arrangement, and layout of the individual light emitters. References are given to textbooks and review articles to guide the interested reader to further and more detailed studies on the problems discussed here.

We present techniques and results of optical simulations of microstructured surfaces using BSDF measurements.

Several design examples show how correcting Petzval curvature in high NA lithographic designs leads to much of the design complexity. There are two cases in which much simpler designs are possible, neither of them practical with today’s technology. One is that of a curved object and/or image system, where no attempt at all is made to control Petzval curvature. The other case is where a diffractive surface is used in the design, which allows Petzval to be easily corrected in relatively simple designs. A very simple 1.1 NA all-refractive immersion design is shown that uses one diffractive surface.

Optimizing angular or spatial colour homogeneity has become an important task in many general lighting applications and first requires a valid description of illumination colour homogeneity. We analyse different frequently used methods to describe colour distributions in theory and with measurement data. It is described why information about chromaticity coordinates, correlated colour temperature and global chromaticity coordinate distances are not sufficient for describing colour homogeneity perception of light distributions. We present local chromaticity coordinate distances as expandable and easy implementable method for describing colour homogeneity distributions that is adaptable to the field of view by only one intuitive, physiological meaningful parameter.