Plasma-Assisted Growth and Characterization of Piezoelectric AlN and Sc(x)Al(1-x)N Films for Microwave Acoustic Sensor Applications

The use of surface acoustic wave (SAW) sensors in high temperature harsh environments such as those found in power plants, industrial manufacturing, or aerospace applications allows for monitoring of internal conditions at locations where traditional sensors do not operate or are unreliable. Surface acoustic wave resonator (SAWR) sensors are based on piezoelectric materials and feature a small passive low-profile self-powered design that can operate and wirelessly transmit data to monitor parameters such as temperature, pressure, or strain. SAWR sensors typically consist of a series of metal electrodes fabricated onto a bulk crystal piezoelectric such as langasite (La3Ga5SiO14). However, there are major advantages in using thin film piezoelectrics such as AlN and ScxAl1-xN rather than bulk single crystal piezoelectrics, including the ability to fabricate devices on a wider range of substrates allowing for greater tuning of devices properties. This thesis investigates the film growth, materials characterization, and surface acoustic wave resonator (SAWR) device behavior of AlN and ScxAl1-xN thin film piezoelectric materials. AlN has many properties that make it an ideal candidate for harsh environment SAW sensors, including the ability to remain piezoelectric up to 1200oC, stability in air up to 700oC, and relatively high phase velocity and low acoustic loss. In this work, piezoelectric AlN and ScxAl1-xN films were synthesized at 930oC using a nitrogen plasma-assisted e-beam evaporation growth method, and the influence of substrate preparation, Al flux, Sc flux, N-plasma flux, and the use of a TiN (111) seed layer were investigated. The films contain epitaxial (0002) oriented grains that yield piezoelectric coupling when integrated into SAWR devices, and the specific film growth parameters that determine epitaxial film quality are correlated with SAWR response and the film electromechanical coupling coefficient (k2). The piezoelectric strength of AlN can be enhanced by alloying with Sc to form a ScxAl1-xN film and this increases the magnitude of electromechanical coupling by up to 400%. ScxAl1-xN films were grown with Sc compositions ranging from 8% to 57% and the electromechanical coupling constant, k2, extracted from SAWR device measurements was found to be significantly increased compared to AlN. A prototype Sc0.13Al0.87N-based SAWR temperature sensor was fabricated and packaged at the Frontier Institute for Research in Sensor Technologies (FIRST) and tested on an exhaust baffle in the UMaine Steam Plant for over 1000 hours, demonstrating the transition of the research from a Technology Readiness Level of ‘experimental proof of concept’ to ‘system prototype demonstration in an operational environment’

Microwave Acoustic SAW Resonators for Stable High-temperature Harsh-Environment Static and Dynamic Strain Sensing Applications

High-temperature, harsh-environment static and dynamic strain sensors are needed for industrial process monitoring and control, fault detection, structural health monitoring in power plant environments, steel and refractory material manufacturing, aerospace, and defense applications. Sensor operation in the aforementioned extreme environments require robust devices capable of sustaining the targeted high temperatures, while maintaining a stable sensor response. Current technologies face challenges regarding device or system size, complexity, operational temperature, or stability. Surface acoustic wave (SAW) sensor technology using high temperature capable piezoelectric substrates and thin film technology has favorable properties such as robustness; miniature size; capability of mass production; reduced installation costs; battery-free operation; maintenance-free; and offer the potential for wireless, multi-sensor interrogation. These characteristics are very attractive for static and dynamic strain sensors targeted to operate in high-temperature harsh-environment conditions. The investigation of harsh-environment static and dynamic SAW strain sensors requires addressing the issues of: (i) sensor platform endurance and stability; (ii) development of durable packaging and attachment techniques; (iii) temperature compensation techniques, to mitigate temperature cross-sensing; and (iv) methods of sensor interrogation and calibration at high temperatures. In this work, langasite-based SAW resonator (SAWR) sensors have been investigated. A stable sensor platform was verified for two types of thin-film electrode configurations, namely: co-deposited Pt/Al2O3 (up to 750oC) and multilayered PtNi|PtZr (up to 1000oC). High-temperature sensor attachment solutions for strain sensor applications were developed for temperatures up to 500oC. The developed SAWR sensors were tested and calibrated for both static and dynamic strain up to 400oC. A temperature compensation technique and a novel finite element analysis was used to perform high-temperature static strain calibration. A high-temperature dynamic strain test rig using a constant stress beam was designed, implemented and used to characterize the SAWR strain sensor performance in measuring dynamic strain. Using the in-phase and quadrature strain sensor signal analysis technique proposed and developed in this study, the existence of both amplitude and frequency modulations of the SAWR RF signal by the dynamic strain signal was confirmed, and the two types of modulations separated and quantified

Acoustic Wave Based MEMS Devices, Development and Applications

Acoustic waves based MEMS devices offer a promising technology platform for a wide range of applications due to their high sensitivity and the capability to operate wirelessly. These devices utilize acoustic waves propagating through or on the surface of a piezoelectric material. An acoustic wave device typically consists of two layers, metal transducers on top of piezoelectric substrate or thin films. The piezoelectric material has inherent capabilities of generating acoustic waves related to the input electrical sinusoidal signals placed on the transducers. Using this characteristic, different transducer designs can be placed on top of the piezoelectric material to create acoustic wave based filters, resonators or sensors. Historically, acoustic wave devices have been and are still widely used in telecommunications industry, primarily in mobile cell phones and base stations. Surface Acoustic Wave (SAW) devices are capable of performing powerful signal processing and have been successfully functioning as filters, resonators and duplexers for the past 60 years. Although SAW devices are technological mature and have served the telecommunication industry for several decades, these devices are typically fabricated on piezoelectric substrates and are packaged as discrete components. Considering the wide flexibility and capabilities of the SAW device to form filters, resonators there has been motivation to integrate such devices on silicon substrates as demonstrated in (Nordin et al., 2007; M. J. Vellekoop et al., 1987; Visser et al., 1989). One such example is illustrated in (Nordin et al., 2007) where a CMOS SAW resonator was fabricated using 0.6 m AMIs CMOS technology process with additional MEMS post-processing. The traditional SAW structure of having the piezoelectric at the bottom was inverted. Instead, the IDTs were cleverly manufactured using standard complementary-metal-oxide-semiconductor (CMOS) process and the piezoelectric layer was placed on the top. Active circuitry can be placed adjacent to the CMOS resonator and can be connected using the integrated metal layers. A SAW device can also be designed to have a long propagation path between the input and output transducer. The propagating acoustic waves will then be very sensitive to ambient changes, allowing the device to act as a sensor. Any variations to the characteristics of the propagation path affect the velocity or amplitude of the wave. Important application for acoustic wave devices as sensors include torque and tire pressure sensors (Cullen et al., 1980; Cullen et al., 1975; Pohl et al., 1997), gas sensors (Levit et al., 2002; Nakamoto et al., 1996; Staples, 1999; Wohltjen et al., 1979), biosensors for medical applications (Andle et al., 1995; Ballantine et al., 1996; Cavic et al., 1999; Janshoff et al., 2000), and industrial and commercial applications (vapor, humidity, temperature, and mass sensors) (Bowers et al., 1991; Cheeke et al., 1996; Smith, 2001; N. J. Vellekoop et al., 1999; Vetelino et al., 1996; Weld et al., 1999). In recent years, the interest in the development of highly sensitive acoustic wave devices as biosensor platforms has grown. For biological applications the acoustic wave device is integrated in a microfluidic system and the sensing area is coated with a biospecific layer. When a bioanalyte interacts with this sensing layer, physical, chemical, and/or biochemical changes are produced. Typically, mass and viscosity changes of the biospecific layer can be detected by analyzing changes in the acoustic wave properties such as velocity, attenuation and resonant frequency of the sensor. An important advantage of the acoustic wave biosensors is simple electronic readout that characterizes these sensors. The measurement of the resonant frequency or time delay can be performed with high degree of precision using conventional electronics. This chapter is focused on two important applications of the acoustic-wave based MEMS devices; (1) biosensors and (2) telecommunications. For biological applications these devices are integrated in a microfluidic system and the sensing area is coated with a biospecific layer. When a bioanalyte interacts with this sensing layer, physical, chemical, and/or biochemical changes are produced. Typically, mass and viscosity changes of the biospecific layer can be detected by analyzing changes in the acoustic wave properties such as velocity, attenuation and resonant frequency of the sensor. An important advantage of the acoustic wave biosensors is simple electronic readout that characterizes these sensors. The measurement of the resonant frequency and time delay can be performed with high degree of precision using conventional electronics. Only few types of acoustic wave devices could be integrated in microfluidic systems without significant degradation of the quality factor. The acoustic wave based MEMS devices reported in the literature as biosensors are film bulk acoustic wave resonators (FBAR) and surface acoustic waves (SAW) resonators and SAW delay lines. Different approaches to the realization of FBARs and SAW resonators and SAW delay lines used for various biochemical applications are presented. Next, acoustic wave MEMS devices used in telecommunications applications are presented. Telecommunication devices have different requirements compared to sensors, where acoustic wave devices operating as a filter or resonator are expected to operate at high frequencies (GHz), have high quality factors and low insertion losses. Traditionally, SAW devices have been widely used in the telecommunications industry, however with advancement in lithographic techniques, FBARs are rapidly gaining popularity. FBARs have the advantage of meeting the stringent requirement of telecommunication industry of having Qs in the 10,000 range and silicon compatibility

Implementation and Testing of Surface Acoustic Wave Strain Sensors for Harsh Environment Applications

Static and dynamic strain sensing is needed in high-temperature, harsh environment applications for structural health monitoring, condition-based maintenance, process efficiency monitoring, and operator safety in power plants, oil wells, metallurgy, aerospace, and automotive industries. Some challenges for sensors in these environments include device integrity, stability, mounting, packaging, and data acquisition techniques. In addition, it is desirable for sensors in high-temperature harsh-environments to be compact, operate without a battery, and have wireless interrogation capabilities so that they can be installed in small, hard-to-reach locations that otherwise could not be monitored. Surface acoustic wave resonator (SAWR) sensors can respond to the demands of high-temperature, harsh-environment applications due to: (i) the existence of piezoelectric substrates and thin film electrode technology capable of operating at high temperatures (above 1000°C); (ii) sensor response to static and dynamic strain components; (iii) small sensor size; (iv) wireless interrogation capability; (v) and battery-free operation. SAWR strain sensing for harsh-environment applications needs to address some of the issues inherent to these environments, such as: (i) sensor mounting techniques to metal parts, (ii) stability of the sensor and sensor mounting technique, (iii) packaging of the sensor, and (iv) cross-sensitivity between strain and temperature. In this work, langasite (LGS) SAWR sensors were used, due to the proven performance of these devices at high temperature at UMaine, for static and dynamic strain measurements. Simulation of the strain due to thermal expansion and mechanical loads was performed to determine where there were concentrations of high strain at the adhesive/LGS and adhesive/metal interfaces as well as adhesive shaping designs aimed at minimizing this strain. Wireless interrogation of SAWR static and dynamic strain sensors using inductive coupling techniques was achieved up to 400°C. After temperature cycling, it was determined that cracking was taking place within the ceramic adhesive layer and along the borders of the SAWR sensor chip that causes degradation and inconsistency in the SAWR strain response. Based on these results, further investigation of static and dynamic strain sensors using alternative adhesives was done limited to 200°C. Two polymer epoxy adhesives showed stability after temperature cycling between 50°C and 250°C. Using the polymer epoxy that showed greater stability for the static strain, dynamic strain was measured. The test setup implementation was investigated towards improving the stability of dynamic strain sensor measurements after temperature cycling. Finally, a method for extracting temperature and the dynamic strain magnitude and spectral components was devised and implemented using a single SAWR sensor

Towards a cell-based chemo receiver for artificial insect olfaction

Infochemical communication is ubiquitous amongst all living organisms, and particularly important in insects. Because smell being the most common basic means of chemical communication, infochemical blends must be constantly decoded in order to proclaim their readiness to mate, to mark out territorial boundaries, to warn off intruders and predators or, in some cases, to locate food or predators with millisecond precision. The central challenge of the thesis was to mimic nature in both cellular and molecular levels on to a technological platform that aids in the development of a new class of technology employing chemicals alone to communicate over space and time. This thesis describes a body of work conducted in the development of a miniaturised, smart and label-free cell-based chemoreceiver for artificial insect olfaction, as part of the development of a novel biomimetic infochemical communication system. A surface acoustic wave based microsensor has been utilized to engineer and develop a chemoreceiver system that mimics the cellular and molecular mechanisms occurring during infochemical detection and decoding in insects. Successful recovery of ratiometric information with the aid of polymer-based gas-phase measurements, established the concept of chemical communication. Thus, small scale, high-throughput infochemical communication has been realized by a combination of precise spatiotemporal signal generation using fruit volatiles and insect sex pheromones with highly sensitive detection and signal processing. This was followed by the investigation of the feasibility of using the prototype cell-based biosensor system in a static mode for artificial insect olfaction applications, mimicking the cellular detection in the receptor/antenna apparatus of insects. Finally, as part of the development of a compact and low-power portable chemoreceiver system, the discrete sensor drive and interface circuitry was deployed in an analogue VLSI chip, thereby overcoming the associated measurement complexity and equipment cost, in addition to extending the reach and functionality of point of use technologie

Microelectromechanical Systems and Devices

The advances of microelectromechanical systems (MEMS) and devices have been instrumental in the demonstration of new devices and applications, and even in the creation of new fields of research and development: bioMEMS, actuators, microfluidic devices, RF and optical MEMS. Experience indicates a need for MEMS book covering these materials as well as the most important process steps in bulk micro-machining and modeling. We are very pleased to present this book that contains 18 chapters, written by the experts in the field of MEMS. These chapters are groups into four broad sections of BioMEMS Devices, MEMS characterization and micromachining, RF and Optical MEMS, and MEMS based Actuators. The book starts with the emerging field of bioMEMS, including MEMS coil for retinal prostheses, DNA extraction by micro/bio-fluidics devices and acoustic biosensors. MEMS characterization, micromachining, macromodels, RF and Optical MEMS switches are discussed in next sections. The book concludes with the emphasis on MEMS based actuators

Modeling and Experimental Analysis on the Temperature Response of AlN-Film Based SAWRs

The temperature responses of aluminum nitride (AlN) based surface acoustic wave resonator (SAWR) are modeled and tested. The modeling of the electrical performance is based on a modified equivalent circuit model introduced in this work. For SAWR consisting of piezoelectric film and semiconducting substrate, parasitic parameters from the substrate is taken into consideration for the modeling. By utilizing the modified model, the high temperature electrical performance of the AlN/Si and AlN/6H-SiC based SAWRs can be predicted, indicating that a substrate with a wider band gap will lead to a more stable high temperature behavior, which is further confirmed experimentally by high temperature testing from 300 K to 725 K with SAWRs having a wavelength of 12 μm. Temperature responses of SAWR’s center frequency are also calculated and tested, with experimental temperature coefficient factors (TCF) of center frequency being −29 ppm/K and −26 ppm/K for the AlN/Si and AlN/6H-SiC based SAWRs, which are close to the predicted values