Micro-hotplate based CMOS sensor for smart gas and odour detection

Low cost, highly sensitive, miniature CMOS micro-hotplate based gas sensors have received great attention recently. The global sensor market is expanding rapidly with an expected increase of 5 ~ 8% grow thin the next five years. The application areas for a gas sensor include but are not limited to, air quality monitoring, industrial and laboratory conditions, military, and biomedical sectors. It is the key hardware component of an electronic nose, as well as the signal processing on the software side. In this thesis, both aspects of such a system were studied with new sensor technologies and improved signal processing algorithms. In addition, this thesis also described different applications and research projects using these sensor technologies and algorithms. A novel plasmonic structure was employed as an infrared source for anon- dispersive infrared gas sensor. This structure was based on a CMOS micro hot plate with three metal layers and periodic cylindrical dots to induce plasmon resonance, that allowed a tunable narrow band infrared radiation with high sensitivity and selectivity. Five gases were studied as target gases, namely, carbon monoxide, carbon dioxide, acetone, ammonia and hydrogen sulfide. These emitter sources were fabricated and characterised with a gascell, optical filters and commercial detectors under different gas concentrations and humidity levels. The results were promising with the lowest detection limit for ammonia at 10 ppm with 5 ppm resolution. On the data processing side, various signal processing methods were explored both on-board and on-board. Temperature modulation was the on-board method by switching the operating temperatures of a micro hotplate. This technique was proven to over come and reduce some typical sensor issues, such as drift, slow re-sponse/recovery speed (from tens of seconds to a few seconds) and even cross sensitivities. Off-board post processing methods were also studied, including principal component analysis, k-nearest neighbours, self-organising maps and shallow/deep neural networks. The results from these algorithms were compared and overall an 85% or higher classification accuracy could be achieved. This work showed the potential to discriminate gases/odours, which could lead to the development of a real-time discrimination algorithm for low cost wearable devices

Determination of pressure dependent gas properties by MEMS-structures for vacuum measurement applications

In der Vakuumtechnik müssen unterschiedliche Messverfahren eingesetzt werden, um den Druck in einem großen Druckbereich zu bestimmen, womit Bedarf sowohl an einer Messbereichserweiterung, als auch an einer Reduzierung der Gasartabhängigkeit der einzelnen Verfahren besteht. Da MEMS-basierte Sensorelemente zudem in weiteren Anwendungsbereichen eingesetzt werden könnten, wurden für die gängigsten Messverfahren die Auswirkungen einer Miniaturisierung theoretisch untersucht. Aus einem vereinfachten eindimensionalen Modell der druckabhängigen Wärmeleitfähigkeit eines Gases konnte analytisch abgeleitet werden, dass die Sensitivität eines Wärmetransport-Vakuummeters von der Oberfläche des beheizten Filaments und dessen Abstand zur Wärmesenke abhängt, und nicht von den parasitären Wärmeverlusten. Diese Vorhersagen konnten an Sensorelementen mit Filamentoberflächen von 0,3mm² bis 0,8mm² und einer bzw. zwei Wärmesenken im Abstand von 150µm, welche mittels Si-Bulk-Mikrotechnologie realisiert wurden, bestätigt werden. Im Konstant-Temperatur-Betriebsmodus umfasste der auflösbare, druckempfindliche Bereich bis zu acht Dekaden von 1e-5 mbar bis 1e3 mbar. Die Grundlage für ein miniaturisiertes Ionisations-Vakuummeter ist eine kalte Elektronenquelle, welche die durch Glühkathoden verursachten thermischen Störeffekte reduzieren würde. Als Basis hierfür wurden durch Kombination von Trockenätzung und thermischer Oxidation Feldemitter aus Si mit Verrundungsradien von etwa 20nm und Höhen von 1,1µm bis 2,5µm realisiert. Die Emitterfelder wiesen eine große Anzahl an funktionsfähigen Emittern (>90%), eine homogene Verteilung der Einsatzspannungen (Standardabweichung ~25%) und geringe Stromschwankungen (<1,5%) beim Betrieb im für p-dotierte Si-Emitter typischen Sättigungsbereich auf. Basierend auf einem System aus zwei Vakuummetern mit sich überlappenden Messbereichen, wobei einer der Messwerte gasartunabhängig ist, wurde ein Verfahren entwickelt, das es ermöglicht, Wärmetransport-Vakuummeter während der Messung softwaregestützt zu kalibrieren und somit die Gasartabhängigkeit von bis zu 50% im Bereich von 1e-3 mbar bis 10 mbar auf unter 10% zu reduzieren.In vacuum technology different measurement principles are necessary to determine pressure in a large pressure regime. Hence, an enlargement of the measuring range as well as a reduction of the gas-species dependency of the particular principles is needed. Due to the additional application areas of MEMS-based sensing elements, the effects of miniaturization on the most established principles were theoretically studied. From a simplified one-dimensional model for the pressure dependent thermal conductivity of a gas was analytically derived that the sensitivity of thermal conductivity vacuum gauges is dependent on the surface of the heated filament and its distance to the heat sink, but not to the parasitic heat losses. These predictions were experimentally proven with sensor elements with filament surfaces ranging from 0.3mm² to 0.8mm² facing one or two heat sinks in a distance of 150µm. These chips, which were fabricated by silicon technology, showed in a constant temperature operating mode a sensitivity on pressure that was detectable within up to eight orders of magnitude in a range from 1e-5 mbar to 1e3 mbar. The basic structure for a miniaturized ionization gauge is a cold electron source that reduces thermal perturbation effects caused by the conventional thermionic cathode. Therefore Si-based field emission cathodes with apex radii of about 20nm and heights of 1.1µm to 2.5µm were realized by a combination of reactive ion etching and thermal oxidation. The emitter arrays showed a high number of working emitters (>90%), a homogeneous distribution of the turn-on voltages (standard deviation ~25%) and low current fluctuations (<1.5%) when operated in a saturation regime that is typical for p-doped silicon field emitter. Based on a system of two vacuum gauges with overlapping measurement ranges, a procedure was developed to calibrate a thermal conductivity gauge during the measurement on the signal of a gas-species independent gauge. By this automated calibration procedure the gas-species dependency of up to 50% in a pressure regime of 1e-3 mbar to 10 mbar was reduced to less than 10%