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UV Light - 5 Pages

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Catalogue excerpts

OPTICS Engineer’s know-how UV Light DEMANDS ON OPTICAL SYSTEMS Ultraviolet light is being used more and more in many high-tech applications such as semiconductor processing. Optical components and systems designed specifically for the UV are essential in order to use its significant potential benefits. Optical engineers must therefore have design knowledge appropriate to the UV and also be proficient in the technologies used to manufacture and test UV optics. THOMAS THOENISS STEFAN MEWES ptical systems working with ultraviolet (UV) light below approximately 400 nm have typically been used for the optical detection of electric discharge (solar-blind method), criminalistics (crime scene inspection) or spectroscopic analytics. Over the past few years, however, UV light has also taken over other classic optical system applications, for example, industrial inspection or laser material processing. Another high-tech field, microchip processing, could not be imagined without UV light. The search for larger storage capacities using smaller semiconductor structures is intimately linked with the use of light of ever shorter wavelengths (Figure 1). For all these reasons, it is worth taking a closer look at the ultraviolet spectrum. O Broadband and narrow-band systems Optical systems for the UV field can be sub-divided into two categories: Broada b band and narrow-band systems. Broadband systems are used when broadband light sources such as Xenon lamps are employed. The light source spectrum can extend itself far beyond the actual UV field from visible light up to infrared light. A typical application is the inspection of wafer surfaces using stray light evaluation. For this purpose, high-resolution optical imaging systems are used that – with regard to their optical requirements – are often similar to classic microscope lenses. The second category comprises systems that have been developed for specific (laser) wavelengths. Here, the field of application for narrow-band and high-resolution systems is (narrow-band) wafer inspection. A further important field of application for UV optics is laser material processing. In electronics, printed circuit boards (PCBs) are structured using suitable laser light sources and optical systems such as F-Theta lenses and beam expansion systems. The desire to use light with even shorter wavelengths is justified by the increase in resolution of the processes, in c 1 Simulation of the resolution increase by reducing the wavelength: Illustration of a test structure at (a) 1064 nm, (b) 532 nm and (c) 266 nm in an aberration-free optical system 18 Laser+Photonics order to detect or generate increasingly fine structures. Diffraction theory shows that higher resolution can only be achieved by an increase in aperture or by a reduction of wavelength. However, from a technical point of view, the option of higher aperture has either been used already or cannot be implemented. Thus, the only way forward is to reduce the wavelength. Examples of UV lenses are shown in Figure 2. Challenges for the optical designer As with all lens systems, the UV-optical systems achieve high performance through the appropriate combination of optical materials with different refractive indexes and dispersions. In contrast to visible-light systems, only a few crystal materials are available to the optical designer for use in the UV system. The reason for this is that the transmission limit of the optical glasses is approximately 330 nm. Furthermore, typical UV materials, such as quartz (SiO2) and calcium fluoride (CaF2) have a very low refraction index. Therefore, to generate the same diverging or converging effect as with a lens made of a highly refractive flint glass, the surface of the lens must be highly curved or several components have to be used. Correcting chromatic aberrations is another significant problem, and not only when designing the system for broad spectral range. Even with high-resolution narrow-band laser systems, the laser band- 3 | 2008

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Engineer’s know-how width or picometer deviations in the center wavelength may decrease the resolution significantly. Therefore, color correction is not uncommon for these systems. Effective mirror systems The use of pilot or auto focus wavelengths in the visible or infrared spectrum also makes the color correction necessary. Unfortunately, with their dispersion properties, both the main UV materials, quartz and CaF2, limit the options for chromatic aberration correction considerably, especially with broadband systems. While, due to their material properties, other UV transparent materials, such...

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Engineer’s know-how a b The demands on the coating process itself are more stringent, as that process has a considerable influence on the surface quality (cleanliness, micro roughness) and even on the actual shape of the surface of the optical components. The extreme performance requirements of the UV systems mandate optical components that, in some cases, have a surface accuracy way below λ/10. The coating process must be optimized so that this surface quality is also maintained after the coating. Precise mounting technologies 3 UV-VIS mirror lens (a); evaluation of the stray light distribution...

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Engineer's know-how anti-reflection coat- defined Lateral or Longitudinal shift can be carried out. The elements can then be fixed after the adjustment (Figure 5a). In some cases, these adjustment methods A further method is widely used in mi- croscope lens production. The individual lenses of a system are mounted in their own sub-mountings (Figure 5b). These are optically aligned on specialized ma- chines and the mounting diameters and lengths are very accurately machined us- ing diamond turning. The individually sub-mounted optical elements now have an accurately defined geometry. If these elements...

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OPTICS Engineer’s know-how Wavefront measurement setup Wave front measuring system Beam expander Beam splitter Calibration plate Entrance/ exit pupil Collimating lens Focusing lens Fiber collimator Fiber/pinhole Lens under test Concave reference mirror Conjugated pupil plane Shack-Hartman sensor © Laser+Photonics 6 Diagram of the wavefront measuring system implemented by Linos for measuring high-resolution lenses for UV applications Metrology for the ultraviolet spectrum With imaging systems, the optical imaging performance is generally found by calculating the modulation transfer function (MTF)...

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