Hybrid Microsensors for Displacement and Acceleration (HYMIDIAC)

We aim at the exploration of light transmission modulation by relatively moving apertures for novel transducers of displacement or inertia. The aperture arrangement will be illuminated by means of light emitting diodes and the transmitted flux is measured with a suitable photo detector. Both, the sending and receiving element can be attached to the aperture assembly forming a miniature, lightweight device. Prior work reveals that this approach is capable for sub-picometer sensitivity. The transduction characteristic can be optimized for diverse applications owing to the freedom given for aperture design. The employed apertures comprise suitably shaped single openings, regular arrays of such items as well as more complex structures such as Fresnel lenses. Moreover, new application fields are opened by the flexibility of the proposed transduction technique. An acceleration sensor can be built on the proposed mode of transduction if the relative displacement between the apertures is due to inertial forces. Such devices are well suited to investigate the potential of the proposed technology in depth and to give a proof of the achievable capabilities with respect to the state of the art. One aperture (array) is stationary and the other adheres to a spring suspended mass produced by silicon micromachining. Optionally, micromachined capacitive actuators will be integrated to the proof mass. This feature allows closed- loop control for nulling the relative deviation of apertures enabling inertial sensors with a very high dynamic range and self-test capability. For research purposes, such actuators facilitate high-resolution investigations of the optoelectronic displacement to intensity conversion. We aim at pushing the displacement resolution to at least 1 pm/sqrt(Hz) for 2D positioning systems utilizing Fresnel lenses in combination with circular apertures. Comparable sensitivities are envisioned for devices utilizing large arrays of slot apertures featuring a few m width. This corresponds to inertial sensitivities better than 1 g for a bandwidth below 1 kHz. Finally, with combinations of appropriately shaped apertures and suitable MEMS, example devices featuring advanced functionality like dynamic compression or frequency specific threshold detectors for vibration monitoring will be investigated. The project tasks cover the manufacturing of these proposed devices including the development of novel processing steps, especially the precision bonding of aperture holders. Regarding device characterization, a thorough investigation of the mechanical transfer function of the microstructures is of primary interest. Furthermore, the spectral dependence of the light modulation efficiency and the impact of temporal and spatial coherence of the light source on the sensor performance will be examined in depth. The influence of undesired reflections as well as mechanical, optical and electronic noise on the detection limit will be studied in order to further improve the sensitivity.

Inertial sensors are used in many different fields of application. Measuring motions and accelerations is a relevant issue for advanced features and novel services in consumer electronics or mobile devices as well as for the monitoring of bridges and buildings. Additionally, they have seen decades of service for safety applications in the automotive industry. However, the majority of commercially available inertial sensing products rely on capacitive sensing methods which quickly reach technological limitations when sensitivity is to be improved significantly. The project investigated novel, compact optomechanical transduction methods for measuring displacement, vibration and inertia based on the modulation of a light flux through the relative movement of a pair of micromechanical apertures. One of these apertures is fixed, the other consists of holes in a spring suspended mass made from mono-crystalline silicon that is deflected by inertial forces acting from the outside. The light flux originates, for instance, from an attached LED and hits a photodetector after passing the modulator. The motion of the apertures changes the flux, and the change is thus a measure of the deflection. The optical readout also entails an electrical separation of force actuation and signal derivation, which makes the transduction free of feedback. Besides fundamental technological questions, the project also investigated the achievable sensitivity of the sensing principle as well as the possibilities of designing the shape of the apertures almost freely, which enables the implementation of a wide range of passive transfer characteristics. Despite the use of low-cost optoelectronic components, the characterized prototypes achieved displacement ranges of several micrometers and resolutions below one picometer for one sample per second, which is equivalent to an acceleration of less than 1 g for an inertial sensor. Despite these outstanding results, the experiments showed that the method still has potential for increasing the resolution. Apart from inertial forces, the transduction principle can be applied to any physical quantity that can be translated into spatial displacement. This includes gravitation, electromagnetic field related forces, pressure, or thermal expansion of materials. Thus, there is a multitude of potential applications.