Quartz microresonators for the high-resolution detection of infrared radiation

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Frequency-dependent, resonant sensors are suitable for the high-resolution detection of a variety of measurands, e.g. for measuring smallest changes in mass by means of quartz microbalances or for time using quartz resonators. Various publications have shown that this principle is also suitable for the temperature measurement and, hence, for the detection of infrared radiation. The high accuracy results from the interference independence of the quasi-digital output frequency and the excellent, long-term stable electromechanical properties of quartz as resonator material. For that reason, noise and other interfering effects are low compared to other IR detector types. The responsibility (i.e. the change in frequency caused by temperature change) of such resonance detectors is proportional to the resonance frequency, which increases with decreasing size of the resonating structure. However, a limiting factor is the large dimension of the monocrystalline quartz (in particular quartz thickness). This results in low resonance frequencies and, thus, in large thermal conductance values (compared to other IR detectors) from the detector element to the surroundings. Although the use of piezoelectric thin films could solve this miniaturization problem, the polycrystalline properties lead to huge contributions processes so that the achievable detectivity would remain below that of other thermal IR sensors.Therefore, the goal of the project proposed here is to create 3D-structured, ultra-thin (quartz thickness < 5 µm) quartz resonators that have typical dimensions of thin-film devices but avoid the disadvantages of polycrystalline thin films. As a result, such quartz elements have significantly smaller heat capacities and thermal conductivities, which increases both the thermal resolution and the sensitivity as well as the detectivity. To reduce the thermal conductance, the detector element is designed to be self-supporting and thermally insulated by an etching trench. A further increase in sensitivity of the detector system is achieved by means of trenches which are introduced into the crystal. As a result, the resonant frequency of the quartz oscillator increases due to the lower mass. The production takes place by means of ion beam etching. Finally, the absorptivity of the sensor element should be increased by ultra-thin nanostructured black layers. These are produced by glancing angle evaporation of corresponding black layer materials. By means of analytical models and numerical simulations design guidelines are to be derived with which the greatest possible detectivity values can be achieved. To evaluate the novel approach, demonstrators should be constructed and characterized regarding their sensitivity and their noise behavior.