Small and affordable CO2 sensors are in high demand for modern applications, such as smart buildings, smartphones, electrical cars or medicine. The thin-film bulk acoustic resonator (FBAR) presents a promising platform to fulfil these demands by functionalising its surface with materials that reversibly interact with CO2. In this thesis, aminopolysiloxane-coated FBARs are prepared and analysed regarding their CO2-sensing performance. It is found that they can reach high CO2 sensitivity with resolutions up to 50 ppm in a dynamic range between 400 ppm and 5000 ppm. It is also shown that common cross-sensitivities, such as changing humidity, can be separated from the CO2 signal. These are promising results on the way to develop a new generation of CO2 sensors. However, it is also found that the sensor sensitivity decreases over time. Analytical examinations show that the main degradation product in aminopolysiloxanes is urea, which forms preferrably in softer polymers and at temperatures above 80 °C. This degradation is found in all analysed compositions of aminopolysiloxanes that were aged for more than one year showing the stability limitations of this sensor concept.:1 Introduction 9 1.1 Motivation for new CO2 sensors . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.2 The FBAR as a high-potential sensor device . . . . . . . . . . . . . . . . . . . 10 1.3 Content of this work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2 Fundamentals 13 2.1 Gas Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 2.1.1 De_nition, History and Classi_cation . . . . . . . . . . . . . . . . . . . 13 2.1.2 Gas sensor characteristics . . . . . . . . . . . . . . . . . . . . . . . . . 15 2.1.3 State-of-the-art CO2 sensors . . . . . . . . . . . . . . . . . . . . . . . . 16 2.2 FBAR Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.2.1 Acoustic resonator theory . . . . . . . . . . . . . . . . . . . . . . . . . 20 2.2.2 The Mason Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 2.2.3 Sensing theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 2.2.4 Film resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 2.3 CO2-sensitive materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.3.1 General Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 2.3.2 Classes of CO2 sorbents . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2.3.3 Suitabe materials for the functionalisation of the FBAR . . . . . . . . 35 3 Experimental details 37 3.1 FBAR designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.1.1 Passive FBARs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.1.2 Active FBARs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.2 Gas measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.2.1 Passive FBARs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.2.2 Active FBARs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.3 Development of the sensitive layer . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3.1 Material choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 3.3.2 Material preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.3.3 Deposition methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 3.3.4 Annealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 3.4 Analytical techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 3.4.1 Fourier-transform infrared spectroscopy . . . . . . . . . . . . . . . . . 46 3.4.2 Raman spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 3.4.3 X-ray photoelectron spectroscopy . . . . . . . . . . . . . . . . . . . . . 47 3.4.4 Nuclear magnetic resonance spectroscopy . . . . . . . . . . . . . . . . 47 3.4.5 Acoustic measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 47 4 Results and discussion 49 4.1 FBARs functionalised with ethyl cellulose . . . . . . . . . . . . . . . . . . . . 49 4.1.1 Acoustic characterisation . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.1.2 Humidity sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 4.1.3 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 6 Contents 4.2 FBARs with aminopolysiloxanes . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2.1 Acoustic characterisation . . . . . . . . . . . . . . . . . . . . . . . . . 59 4.2.2 Humidity and CO2 sensing . . . . . . . . . . . . . . . . . . . . . . . . 63 4.2.3 Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 4.3 Degradation mechanisms in aminopolysiloxanes . . . . . . . . . . . . . . . . . 69 4.3.1 Stability evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 4.3.2 Analytical characterisation . . . . . . . . . . . . . . . . . . . . . . . . 70 4.3.3 Degradation hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . 78 4.4 CO2 sensing with an active FBAR array . . . . . . . . . . . . . . . . . . . . . 82 4.4.1 Presentation of the functionalised sensor chip . . . . . . . . . . . . . . 82 4.4.2 Sensor characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 4.4.3 Selective Multigassensing . . . . . . . . . . . . . . . . . . . . . . . . . 89 5 Summary 93 6 Outlook 95 7 Appendix 97 7.1 Additional tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 7.2 Additional pictures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 Bibliography 103