Herstellung und Charakterisierung hocheffizienter, mikromechanischer NIR-Beugungsgitter für den Einsatz in miniaturisierten Spektrometern

Optical procedures play an increasingly important role in today's field of analytics and are used in a variety of applications. In this context the non-contact and non-destructive measurement with light, which greatly simplifies the analysis of critical substances, is advantageous. Optical spectroscopy, therefore, is an important part of analytics. In particular near-infrared (NIR) spectroscopy with a wave length range from 780 nm to 3000 nm exhibits numerous applications, ranging from chemical reactions and process analyses, examinations of polymers and textile to quality control of fruits and seeds as well as medical technology. The NIR spectrometers used in this context are primarily designed as spectrographs while the dispersive element is firmly installed and the spectrum is measured via a diode array based on gallium arsenide. Diode arrays in the NIR range are costly but contribute significantly to the spectral resolution of the spectrograph. An alternative approach uses monochromators with a flexible dispersive element as the spectrum swipes across the detector at a specific time. A single diode as a detector is sufficient for this purpose. However, the spectral resolution is a function of the time-dependent position of the dispersive element, which must be controlled and guided with high precision. A macroscopic realization in combination with quick spectrum measurement times in the millisecond range is very elaborate. The present work alternatively deals with the manufacturing and characterization of a micromechanical NIR diffraction grating with which a new, miniaturized NIR spectrometer, based on a monochromator configuration, could be realized. The diffraction grating is based on a resonantly driven micro-mirror, which has been manufactured as a micro-electromechanical system (MEMS) with silicon-based, near-surface volume micromechanics. In this process an extremely compact and robust NIR spectrometer (10 x 8 x 7.5 cm³) was developed by using a micromechanical diffraction grating. The spectrometer shows a decisive cost advantage in comparison with current NIR spectrographs. In this regard it was absolutely necessary to mirror the requirements for the micromechanical diffraction grating with the marginal conditions of the NIR spectrometer and bring these in agreement with each other. A spectral resolution of the NIR spectrometer below 10 nm with a wave length range of 900 - 2500 nm was achieved with a targeted dimensioning and optimization of the mechanical and optical properties of the micromechanical diffraction grating; this also included among other things the size, diffraction efficiency, vibration frequency, vibration stability, shock resistance and a procedure to detect the time-dependent torsion angle. Concurrently 3 different grating structures were successfully examined, manufactured and measured in order to maximize the diffraction efficiency