Sensing Materials for Surface Acoustic Wave Chemical Sensors

Online real‐time monitoring of gases requires a miniaturized, passive, and accurate gas sensor. Surface acoustic wave (SAW) devices possess these properties which make them suitable for gas‐sensing applications. They have shown remarkable results in sensing of different gases in terms of sensitivity, selectivity, response, and recovery times. One of the important prerequisites a designer should know is to have knowledge on the different types of sensing material suitable for gas‐sensing applications, prior to design and fabrication of the sensor. Different sensing materials, including metal oxides, polymers, carbon nanotubes, graphene, nanocomposites, etc. have been used for SAW gas sensors. In this article, different sensing materials for SAW gas sensors will be discussed

Electrostatically driven vacuum-encapsulated polysilicon resonators part I. design and fabrication

Basic design issues and a fabrication process based on surface-micromachining techniques for electrostatically driven vacuum-encapsulated polysilicon resonators are presented. A novel freeze-drying method that does not require vacuum equipment is presented. Reactive sealing with LPCVD silicon nitride is used to create the evacuated cavity, resulting in cavity pressures close to the deposition pressure. Design issues regarding choice of materials, technology and layout are discussed. First experimental results, including an admittance plot of the one-port resonator and a plot indicating the dependence of the Q-factor on the resonator geometry and ambient pressure, are presented

SST: Integrated Fluorocarbon Microsensor System Using Catalytic Modification

Selective, sensitive, and reliable sensors are urgently needed to detect air-borne halogenated volatile organic compounds (VOCs). This broad class of compounds includes chlorine, fluorine, bromine, and iodine containing hydrocarbons used as solvents, refrigerants, herbicides, and more recently as chemical warfare agents (CWAs). It is important to be able to detect very low concentrations of halocarbon solvents and insecticides because of their acute health effects even in very low concentrations. For instance, the nerve agent sarin (isopropyl methylphosphonofluoridate), first developed as an insecticide by German chemists in 1938, is so toxic that a ten minute exposure at an airborne concentration of only 65 parts per billion (ppb) can be fatal. Sarin became a household term when religious cult members on Tokyo subway trains poisoned over 5,500 people, killing 12. Sarin and other CWAs remain a significant threat to the health and safety of the general public. The goal of this project is to design a sensor system to detect and identify the composition and concentration of fluorinated VOCs. The system should be small, robust, compatible with metal oxide semiconductor (MOS) technology, cheap, if produced in large scale, and has the potential to be versatile in terms of low power consumption, detection of other gases, and integration in a portable system. The proposed VOC sensor system has three major elements that will be integrated into a microreactor flow cell: a temperature-programmable microhotplate array/reactor system which serves as the basic sensor platform; an innovative acoustic wave sensor, which detects material removal (instead of deposition) to verify and quantify the presence of fluorine; and an intelligent method, support vector machines, that will analyze the complex and high dimensional data furnished by the sensor system. The superior and complementary aspects of the three elements will be carefully integrated to create a system which is more sensitive and selective than other CWA detection systems that are commercially available or described in the research literature. While our sensor system will be developed to detect fluorinated VOCs, it can be adapted for other applications in which a target analyte can be catalytically converted for selective detection. Therefore, this investigation will examine the relationships between individual sensor element performance and joint sensor platform performance, integrated with state-of-the-art data analysis techniques. During development of the sensor system, the investigators will consider traditional reactor design concepts such as mass transfer and residence time effects, and will apply them to the emerging field of microsystems. The proposed research will provide the fundamental basis and understanding for examining multifunctional sensor platforms designed to provide extreme selectivity to targeted molecules. The project will involve interdisciplinary researchers and students and will connect to K-12 and RET programs for underrepresented students from rural areas

Development and Test of High Temperature Surface Acoustic Wave Gas Sensors

The demand for sensors in hostile environments, such as power plant environments, exhaust systems and high-temperature metallurgy environments, has risen over the past decades in a continuous attempt to increase process control, improve energy and process efficiency in production, reduce operational and maintenance costs, increase safety, and perform condition-based maintenance in equipment and structures operating in high-temperature, harsh-environment conditions. The increased reliability, improved performance, and development of new sensors and networks with a multitude of components, especially wireless networks, are the target for operation in harsh environments. Gas sensors, in particular hydrogen gas sensors, operating above 200°C are required in the instrumentation, process control and general safety of a number of industries including coal, natural gas, and nuclear power generation facilities, the aerospace and automotive industries, metallurgical production and defense-related applications. The surface acoustic wave (SAW) platform is a particularly promising option for high-temperature, harsh-environment gas sensing applications since the platform exhibits advantages, such as battery-free and wireless operation, small size, possibility for scale production using well-developed technologies from the semiconductor industry, and low cost of installation and operation. In this work, one-port SAW resonators (SAWRs) operating along five different orientations on a commercially available langasite (LGS) wafer were designed, fabricated, and used as high-temperature H2 sensors. Two of the selected orientations were predicted and confirmed to have temperature-compensated operation above 150°C. A gas sensor test setup was developed, capable of gas cycling between N2, O2 and N2/H2 mixtures under extended high-temperature periods (up to 650°C for over 20 hours). Thin film Pt-Al2O3 was used as the electrode material for transducers and reflectors capable of high-temperature operation, and also as H2 sensing film. In addition, yttria-stabilized zirconia (YSZ) thin films with Pt decoration were tested as sensing films aimed to enhance the SAWR sensor response to H2. The SAW devices were monitored in excess of 1700 hours in real-time during gas cycling sequences up to 600°C, leading to the following findings: i) the Pt-Al2O3 electrodes performed better for H2 sensing than the Pt-decorated YSZ sensing film, showing as much as 50% higher frequency response variation in the 200°C to 400°C range; ii) different crystallographic orientations operating on the same LGS wafer experienced different responses to H2 exposures up to 500°C; iii) the surface oxidation state of the SAWR sensors was shown to have an important impact on subsequent H2 exposure responses. Additionally, the feasibility of a sensor system capable of detecting H2 and determining the ambient temperature simultaneously by employing two different SAWR sensors operating along different LGS orientations was examined. Finally, wireless interrogation of a SAWR sensor was successful within the gas cycling test fixture, and successful wireless H2 detection was achieved above 400°C

Field Programmable Gate Array based Readout for Surface Acoustic Wave Portable Gas Detector

Surface acoustic wave (SAW) is one of the most promising technology in the field of gas sensing at low concentrations. Field deployable portable SAW detectors are, however, prone to noise, there by limiting the detection at low concentrations. To meet the current requirements of gas detection at low concentrations, the readout methodology needs to be based on minimal hardware and better noise management. In this paper we describe a readout scheme for portable SAW gas detectors incorporating a field programmable gate array (FPGA). The developed readout system includes a modified reciprocal frequency counter for differential SAW sensor, median noise filtering and moving averages smoothing for noise management, peak detection and interfacing with external display, all implemented in FPGA. The developed readout was tested against VOCs using a lab developed vapour generator and the results have been presented in the paper. The readout system is compact, low power consuming and expandable through software thus ideal for portable handheld applications

AlN ja Sc0.2Al0.8N ohutkalvojen märkäkemiallinen etsaus

Aluminium nitride is a piezoelectric material commonly used in piezoelectric microelectromechanical systems (MEMS) in the form of thin films deposited by sputtering. AlN-based devices are found in wireless electronics in the form of acoustic filters, but they also have prospective applications in a wide variety of sensor systems. To enhance the piezoelectric properties of AlN, some of the Al can be replaced with scandium, which is required for next-generation devices. However, addition of Sc makes both the deposition and patterning of the film more difficult. This work focuses on patterning of AlN and Sc0.2Al0.8N thin films with wet etching. Both materials are etched anisotropically, which in theory enables etching the materials with little deviation from the mask dimensions. However, in practise, undercutting at the mask edges occurs easily making the structures narrower compared to the etch mask. This work investigates and compares the mechanisms and etch rates of AlN and Sc0.2Al0.8N. Tetramethyl ammonium hydroxide was mostly used for etching, but also H3PO4 and H2SO4 were tested. Addition of 20 atom-% Sc lowered the etch rate of the material and resulted in more undercutting. The causes behind mask undercutting were examined by using 11 differently deposited etch masks, and the undercutting was minimized by optimizing the mask deposition, using thermal annealing, and optimizing the etching temperature. Finally, the work identifies and discusses the relevant factors in depositing and patterning the AlN, ScxAl1-xN and mask films

Influence of Platinum Nanoparticles on Ionic Transport and Hydrogen Reactivity of Yttria-Stabilized Zirconia Thin Films

Yttria-stabilized zirconia (YSZ) is a widely used ceramic material in solid oxide fuel cells, oxygen sensors, and sensing applications due to its high ionic conductivity, chemical inertness, and thermal stability. YSZ is promising active coating for use in miniaturized harsh environment wireless surface acoustic sensors to monitor gases such as H2. Adding catalytic Pt nanoparticles can enhance gas reactivity and lead to associated film conductivity changes. In this work, thin films with an (8% Y2O3 - 92% ZrO2) composition were deposited onto piezoelectric langasite substrates using RF magnetron sputtering in Ar:O2 - 95:5 gas mixture. Films were grown using growth temperatures (30 - 7000C), deposition rates (0.03 - 0.07 nm/s), and substrate bias (-300 - +300 V). Platinum was deposited in-situ via e-beam evaporation at either 30oC or 400oC and then subsequently annealed to cause nanoparticle formation. YSZ and Pt/YSZ films ionic conductivities were measured and characterized with electrochemical impedance spectroscopy (EIS) in pure N2, or in a 4% H2 - 96% N2 mixture. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also used to analyze the surface composition, crystal structure and nanoparticles morphology, respectively. By manipulating the deposition parameters, either (111) or mixed (111)/(200) YSZ film crystallographic texture can be achieved. Post-deposition annealing up to 1000oC in air causes grain growth, strain relief and yttria segregation. EIS measurements from YSZ films over the range 400oC - 600oC indicate that ionic conductivities are strongly dependent on yttria segregation and film nanostructure. For YSZ films decorated with Pt nanoparticles, the surface becomes reactive towards hydrogen. Pt nanoparticles form (111) oriented crystallites, and the amount of yttria segregation is less than that for Pt-free films. Ionic conductivities and sensitivities towards hydrogen depend on the nanoparticle size and film nanostructure. Pt nanoparticles lower the H2 adsorption energy and facilitate the interaction. The conductivity changes that occur corresponding to pure N2 versus exposure to 4% H2 - 96% N2 were found to be reversible. These results indicate that Pt/YSZ films hold promise as hydrogen sensing films that can be incorporated onto a variety of sensing platforms for H2 gas detection and management

Flexible Strain Detection Using Surface Acoustic Waves: Fabrication and Tests

Over the last couple of decades, smart transducers based on piezoelectric materials have been used as sensors in a wide range of structural health monitoring applications. Among them, a Surface Acoustic Wave sensor (SAW) offers an overwhelming advantage over other commercial sensing technologies due to its passive, small size, fast response time, cost-effectiveness, and wireless capabilities. Development of SAW sensors allows investigation of their potential not only for measuring less-time dependent parameters, such as pressure and temperature, but also dynamic parameters like mechanical strains. The objective of this study is to develop a passive flexible SAW sensor with optimized piezoelectric properties that can detect and measure mechanical strains occurred in aerospace structures. This research consists of two phases. First, a flexible thin SAW substrate fabrication using hot-press made of polyvinylidene fluoride (PVDF) as a polymer matrix, with lead zirconate titanate (PZT), calcium copper titanate (CCTO), and carbon nanotubes (CNTs) as micro and nanofillers’ structural, thermal and electrical properties are investigated. Piezoelectric property measurements are carried out for different filler combinations to optimize the suitable materials, examining flexibility and favorable characteristics. Electromechanical properties are enhanced through a noncontact corona poling technique, resulting in effective electrical coupling. Second, the two-port interdigital transducers (IDTs) deposition made of conductive paste onto the fabricated substrate through additive manufacturing is studied. Design parameters of SAW IDTs are optimized using a second-order transmission matrix approach. An RF input signal excites IDTs and generates Rayleigh waves that propagate through the delay line. By analyzing the changes in wave characteristics, such as frequency shift and phase response, the developed passive strain sensor can measure mechanical strains

A Surface Acoustic Wave Mercury Vapor Sensor

An ST cut quartz 261 MHz surface acoustic wave (SAW) delay line mercury vapor sensor was designed, fabricated, and tested. A gold sensing film was sputtered onto the delay path of a SAW device to collect mercury for detection. The sensor\u27s ability to detect mercury was due to the strong interaction between gold and mercury, known as amalgamation. In the present work, a large number of gold films of various thicknesses were exposed to low concentrations of gaseous mercury. It was shown for thinner films (i.e. 25A) the total amount of mercury that could be absorbed was limited; however, the response time for this film was fast and the response slope was linear with respect to mercury concentration. It was also shown that the slope responses for the thicker films (i.e. 500A) were linear with respect to mercury concentration, but the response time and the response magnitude were significantly reduced. In the case of the thinner film, the decrease in frequency was attributed primarily to mass loading, while for the thicker film the changes in frequency were due to both mass loading and elastic stiffening. For the thick film the mass loading response (frequency decrease) was offset by the elastic stiffening response (frequency increase). This resulted in a decrease in response slope resolution with respect to mercury concentration. In conclusion the present work clearly indicates that a judicious choice of gold film thickness between these two thickness extremes results in a mercury sensor capable of very rapid detection of low mercury concentration levels with high resolution over a wide dynamic range