Thermisch geregenereerde sensoren voor vluchtige organische verbindingen op basis van IGZO-transistoren, gefunctionaliseerd met metaal-organische raamwerken

Detection of volatile organic compounds is of significant importance in numerous domains, including air quality control, environmental monitoring, food freshness assessment, and early diagnostics. However, existing gas sensors and electronic noses fail to meet ideal characteristics, which include high sensitivity, high selectivity, portability, low costs, ease of integration, and low power consumption, among others. In this context, we have demonstrated a proof-of-concept sensor based on the combination of microheaters, a-IGZO thin film transistors and metal-organic frameworks. Whereas the field effect transistor provides a scalable, low-powered and cost-effective electronic readout, the metal-organic frameworks enable gas sensitivity and selectivity. In this sensor, a gold-based permeable gate allows the adsorption of the gas analytes in the metal-organic framework layer, acting as part of the dielectric. This configuration resembles a suspended-gate gas-sensitive field-effect transistor with the benefit of ease in fabrication resulting from the absence of the airgap. This sensor is fabricated on top of a microheater platform, enabling the regeneration of the sensor through thermally assisted gas desorption. Although variations in the metal work function are also possible, we have established as predominant sensing mechanism the changes in the dielectric permittivity of the gate insulator induced by gas adsorption. The exposure of methanol leads to variations in the drain current, threshold voltage and subthreshold slope. In comparison, devices in absence of metal-organic framework do not exhibit any apparent response. The relative drain current sensor response (AIDS/IDS) agrees with the type V adsorption isotherm expected for methanol in ZIF-8 and reaches 450% for methanol concentrations of 37.1%. The limit of detection established as ~64 ppm. While the sensor exhibits its highest sensitivity toward methanol, it also displays a strong response to water, whereas the response to ethanol and 2-propanol is limited. Typically, a-IGZO TFTs exhibit a clockwise hysteresis resulting from reversible electron trapping. Remarkably, the adsorption of analytes induces a counterclockwise hysteresis, attributed to the reorientation of guest molecules within the dielectric layer according to the applied field. This counterclockwise hysteresis is key to explaining the behaviour response of the threshold voltage and subthreshold slope. Sensors suffer from drain current baseline drift due to accumulation of adsorbed residuals, which strongly limits their applicability in real-world applications. The integration of a microheater effectively mitigates this issue, decreasing the drift from 20.5% to 2.0% via thermally assisted methanol desorption achieved with 103 °C pulses of 20 s duration. The elimination of gas residuals with the temperature is also found to improve the threshold voltage, and more significantly, the subthreshold slope response and baseline. Furthermore, the application of thermal pulses together with pre-measurement runs appears to enhance the limit of detection. Prior to the fabrication of the proof-of-concept sensor, the three key components (i.e. microheater, thin-film transistor and metal-organic framework) were studied in detail. Regarding microheaters, a thorough analysis of the influence of geometrical parameters on meander structures was performed. The response time is predominantly affected by the microheater size, whereas power consumption depends on both size and aspect ratio. For microheaters with the same active area, different resistances modulate the temperature range achieved at a given voltage. Additionally, thermal uniformity was evaluated using both finite element method simulations and experimentally employing novel localized thermal sensors. The good agreement between simulations and experimental readings further allows to validate the simulation model. Two optimized microheater geometries (i.e. big microheater and small microheater) were designed to minimize thermal inhomogeneities due to hotspots and edge effects, achieved by limiting the line spacing, and progressive reducing the linewidth in the microheater periphery. In relation with thin-film transistors, an overview on the existing technologies and semiconductor physics was provided, with emphasis on a-IGZO transistors. Two geometries were compared, with a top-contact top-gate exhibiting a better performance in terms of drain current, mobility, and device-to-device variability. This results from a better carrier injection and other morphological effects. The simulation of a-IGZO TFTs, based on a TCAD reference model, was performed using COMSOL Multiphysics. This reference model was replicated with high fidelity, highlighting the potential of COMSOL as a satisfactory tool for semiconductor simulations. The integration of zeolitic imidazolate-8 through metal-organic framework chemical vapor deposition was explored at 72C. Although significant sample variability was found, a general trend in film refractive index and thickness increase factor was observed. Films were characterized by ellipsometric porosimetry, with the samples with the highest adsorption exhibiting an increase in relative index of ~0.07 (at p/p0 65.6% of methanol). Although the proof-of-concept sensor shows promising results, there are some limitations and considerations that need attention. There is a strong device-to-device variability arising variations from the quality of both the metal-organic framework layer and a-IGZO transistors. Due to the prolonged temperatures for the synthesis of the metal-organic framework, metastable oxygen vacancies are created in the a-IGZO layer, resulting in variations of the response over time. In addition to this, a limited adsorption due to the aging of zeolitic imidazolate-8 is observed. Future research focus is suggested, including possible routes for the valorization of this device.