High-Performance and Miniature Greenhouse Gas Sensor for Drone-based Remote Sensing

Human activities have significantly increased the concentration of greenhouse gases in the atmosphere, intensifying the greenhouse effect and contributing to global climate change. Since CO2 is the most abundant greenhouse gas to be released into the atmosphere, the effect of the others is calculated with respect to it through the global warming potential, which estimates the energy-trapping capability of a gas with respect to CO2 over a 100-years period. Among all the main other greenhouse gases, N2O has a global warming potential of around 270, while its concentration is about 3 orders of magnitude smaller than the one of CO2, leading to a contribution to heat-trapping approximately equal to the 25% of the one of CO2. This makes an accurate assessment of N2O emissions essential. Unlike CO2, the main source of N2O released into the atmosphere is agricultural land use. However, many technical challenges associated with the capability of achieving reliable emission estimates with low uncertainties at the field level come out due to the significant variations of N2O flux in space and time and they are ultimately attributed to the lack of suitable sensors and platforms that can precisely capture the in-field variations in a large area while remaining low cost and non-invasive to farming activities. Researchers have been striving to overcome these limitations and a novel approach combining piezoelectric MEMS resonators with plasmonic metamaterials has been recently proposed, offering advantages including ultra-miniaturized footprint, low power consumption, and high detection accuracy. This innovative approach typically involves an oscillator loop that includes a resonator covered by a spectrally selective metamaterial layer. When the incident IR light is absorbed by the metamaterial and converted into heat, the resulting temperature change causes a shift in the admittance response of the resonator, thereby altering the oscillator frequency. The purpose of this thesis is to design through numerical simulations, fabricate, and test a sensor achieving a measurement sensitivity of N2O molecules’ concentration at the ppb level by combining a MEMS-based spectrally-selective IR detector with an IR spectroscopic gas cell. The enabling technology is laser absorption spectroscopy, which allows the detection of trace gases molecules through the absorption of highly coherent IR light at wavelengths that corresponds to their roto-vibrational states: the optical power of the IR laser beam passing through the gas cell is modulated by the concentration of N2O molecules and sensed by the detector. This thesis eventually provides the experimental demonstration of the functioning of all the sub-parts of the designed system. In particular, it presents the first implementation of a high temperature coefficient of frequency (TCF around - 55 ppm/C) and good electromechanical properties (Qm around 800 and kt2 around 3.3%) 30%-doped ScAlN resonator on a patterned interdigitated bottom electrode with an enhanced absorption peak of 80% at a wavelength of 4.8 μm. This result has been achieved by replacing SiO2 with AlN as the dielectric layer of the metal-insulator-metal metamaterial absorbers without compromising the IR absorbance.