Specht, Jan Peter (2022) Thermally modulated solidly mounted resonators for air quality monitoring. PhD thesis, University of Warwick.

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Abstract

The effect of air pollution on the environment and human health is a cause of major concern. Each year millions of deaths are attributed to poor air quality, and it is estimated that its economic cost runs into the trillions of pounds. Especially the pollutant particulate matter has been identified as one of the main contributors to poor health. Hence there is much activity that attempts to reduce the concentration of small particles in air. To better understand the effect of particulate matter on the world and for the effective mitigation of the problems it causes and exacerbates, it is necessary to acquire reliable air quality data. Readily available particle sensing equipment is thus required to expand existing air quality monitoring systems that can deliver meaningful results.

To this end, a range of particle sensing technologies have been studied. Resonator particle sensors based on microelectromechanical systems are one promising example of this because of their potential to provide an affordable solution that can be mass manufactured and use very little power or space compared to many currently available particle monitoring devices. In this thesis a novel particle sensor based on a solidly mounted resonator with an integrated microheater that is compatible with a standard integrated circuit fabrication process is developed and tested experimentally. The main objective of this work is to demonstrate for the first time that temperature modulation applied to a solidly mounted resonator could increase its sensitivity to particles, while targeted particle deposition could increase the effective sensitivity of the system to aerosolised particles and that the application of both could thus help to make this type of sensor more suitable for real world air quality monitoring applications.

The design of the sensor is based upon a complementary metal oxide semiconductor process that includes the deposition of a piezoelectric bulk acoustic wave resonator on top of the standard layer stack. It is verified in an extensive set of simulations and the fabricated sensor is subsequently characterised. In the characterisation study the resonator had a resonant frequency around 2 GHz and a Q factor of approximately 200. The device was found to be capable of handling temperatures induced through the application of an electric current to the integrated microheater of up to 598 K. Experimentally the device’s resonant frequency, S-parameter value and its temperature for different applied currents were found to be within approximately 6 % of the sensor simulations.

A custom particle test rig was built to evaluate the sensors performance as a particle sensor. One of the main obstacles remaining with these types of sensors is the reliability of particle measurements, which is reduced by difficulties to achieve repeatable particle sampling. To resolve this issue a thermophoretic particle deposition channel was added to a commercial FBAR device and experimental tests were carried out that showed it could reduce the variation in measurement results between repeat tests from 71 % to 14 %.

The novel solidly mounted resonator particle sensor device was tested inside the particle test rig and found to have a sensitivity to particle deposition of approximately 40 Hz/ng. Temperature modulation was applied to the sensor through the integrated microheater and this was found to increase the sensitivity of the device by a factor of almost five to 190 Hz/ng. It also reduced the sensor’s detection limit from approximately 100 ng to 50 ng. The thermophoretic microchannel was added and found to approximately double the sensitivity of the novel sensor to airborne particles through increased particle sampling efficiency. The novel thermally modulated SMR particle sensor was found to have significant potential for low-cost quality monitoring applications.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QC Physics T Technology > TD Environmental technology. Sanitary engineering T Technology > TK Electrical engineering. Electronics Nuclear engineering T Technology > TP Chemical technology
Library of Congress Subject Headings (LCSH):	Air -- Pollution -- Measurement, Particulate matter, Chemical detectors, Electric resonators
Official Date:	March 2022
Dates:	Date Event March 2022 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Cole, Marina ; Gardner, J. W. (Julian W.), 1958-
Sponsors:	COMET IC-MPPE (Programme)
Format of File:	pdf
Extent:	xv, 186 leaves : colour illustrations, charts, photographs
Language:	eng

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