D.J. Frankel

High temperature stability of langasite surface acoustic wave devices

High temperature acoustic wave (AW) devices capable of operating above 600degC and in hostile environments have opened potential applications for monitoring industrial processes, power plants, and aerospace systems. The authors have reported on the development of thin film electrodes and protective ceramic layers to allow surface acoustic wave (SAW) device operation up to 800degC on langasite (LGS) crystals. This success motivated further study of the electrode material and protective ceramic overlayer, as well as investigations of long term performance, temperature cycling and shock behavior, which are reported in this work. Among the results reported are: behavior of a co-deposited Pt/Rh/ZrO2 composite electrode structure up to 1000degC; investigation of oxygen rich and nitrogen rich SiAlON protective ceramic layers; long term (4080 hours, or about 5frac12 months) operation of a two-port SAW resonator at 800degC; cyclical thermal tests between room temperature and 850degC; and thermal shock tests of crystals between 700degC and room temperature.

Performance of Zr and Ti adhesion layers for bonding of platinum metallization to sapphire substrates

Single crystal sapphire wafers with <1 nm root mean square (RMS) roughness are ideal substrates for chemiresistive sensors that utilize ultra-thin (<50 nm thick) semiconducting metal oxide (SMO) films. Platinum metallization on a highly polished sapphire platform to form electrodes, heater, and a resistive temperature device (RTD) requires the use of a very thin (<20 nm) buffer layer, such as Ti or Zr, to achieve good adhesion at the Pt/sapphire interface. Using AES, secondary ion mass spectroscopy (SIMS), XRD, and wire bond tests before and after annealing treatments, we have found that Zr has superior performance as an adhesion layer compared to Ti. At temperatures of 200–700°C, required for RTD and SMO film stabilization as well as prolonged sensor operation, there is significant migration of Ti through the Pt film, whereas the Zr layer is less mobile. The Pt/Zr/sapphire architecture also minimizes delamination failure of wire bonds to the sensor device.

A lateral-field-excited LiTaO 3 high-frequency bulk acoustic wave sensor

The most popular bulk acoustic wave (BAW) sensor is the quartz crystal microbalance (QCM), which has electrodes on both the top and bottom surfaces of an AT-cut quartz wafer. In the QCM, the exciting electric field is primarily perpendicular to the crystal surface, resulting in a thickness field excitation (TFE) of a resonant temperature compensated transverse shear mode (TSM). The TSM, however, can also be excited by lateral field excitation (LFE) in which electrodes are placed on one side of the wafer leaving a bare sensing surface exposed directly to a liquid or a chemi/bio selective layer allowing the detection of both mechanical and electrical property changes caused by a target analyte. The use of LFE sensors has motivated an investigation to identify other piezoelectric crystal orientations that can support temperature-compensated TSMs and operate efficiently at high frequencies resulting in increased sensitivity. In this work, theoretical search and experimental measurements are performed to identify the existence of high-frequency temperature-compensated TSMs in LiTaO3. Prototype LFE LiTaO3 sensors were fabricated and found to operate at frequencies in excess of 1 GHz and sensitively detect viscosity, conductivity, and dielectric constant changes in liquids.

High temperature sensing technology for applications up to 1000°C

This work reports on the research and development of high temperature (HT) thin electrodes for use in acoustic wave sensor platforms up to 1000degC. Previously developed thin film platinum (Pt) electrodes limits HT operation due to a de-wetting phenomena, which results in loss of Pt film electrical continuity and device failure above 650mnplus750degC. To address the problem, co-deposition of Pt/rhodium (Pt-Rh) alloys with zirconia (ZrO2) was used to fabricate HT-stable thin film surface acoustic wave interdigitated electrodes on langasite substrates. The resulting devices showed stable operation for over five months at 800degC. As temperature sensors, these devices have a sensitivity of 9.1 KHz/degC @800degC. In addition, ceramic silicon-alumina-nitrogen (SiAlON) overlayers were investigated to protect the sensor surface. The resulting devices enable harsh environment temperature and pressure sensors, for applications such as monitoring of jet engines, atmospheric re-entrance, and space exploration.

Recent advances in harsh environment acoustic wave sensors for contemporary applications

There is a significant need for wireless sensor systems capable of operation up to 1100°C and beyond, in abrasive or corrosive harsh environments, in particular for the energy, steel, aerospace, oil and gas exploration industries. These environments and applications preclude the use of batteries and normally require wireless and multiple sensor interrogation. The University of Maine and Environetix Technologies have successfully responded to these needs by researching and developing surface acoustic wave (SAW) sensors based on the langasite family of crystals and co-deposited Pt/Rh/ZrO2 thin-film electrode technology. This paper reports on the recent achievements, which include: long term operation in furnace and technology validation in jet-engine static and rotating parts up to 53,000 gs; stable and repetitive wired and wireless responses of temperature sensors; multiple wireless sensor interrogation; and associated packaging (tests run in the 200°C to 1000°C range).

A lateral field excited acoustic wave pesticide sensor

Excessive use of pesticides such as organophosphates (OPs) on fruits and vegetables can have adverse effects on the environment and jeopardize the health of the consumer. As a result a need exists for an accurate, low cost, portable sensor to detect harmful pesticide levels. In the present work a lateral field excited (LFE) sensor (1), which has a bare sensing surface that allows the measurement of mechanical and electrical property changes in a target analyte selective film, has been used to detect phosmet, a commonly used OP. The acoustic energy distribution of this LFE sensor has been found to exhibit a circular pattern with maximum sensitivity at the center of the sensor. The LFE pesticide sensor is shown to be more sensitive than the standard QCM. Also, it is shown that the response time of the sensor can be drastically shortened by using the derivative of the frequency response. I. INTRODUCTION Organophosphates (OPs) are widely used in agriculture for pest control in fruits and vegetables, with about 25,000 brands of pesticides sold in the United States (2). Categorized as neurotoxins or cholinesterase inhibitors, they can affect neuromuscular transmission (2). This is especially critical for young children who consume large amounts of fruits and vegetables and have a lower tolerance than adults (3). In order to guard against the adverse effects of OPs, the Environmental Protection Agency (EPA) has determined the allowable concentration of pesticides. Depending on the crop and pesticide used, tolerances are normally restricted to the 0.1 - 100 ppm range (4). Since many countries do not have such regulations, a need exists to detect pesticides on imported fruits and vegetables. Currently, the two standard methods of testing for the presence of pesticides are gas chromatography/mass spectroscopy (GC/MS) and immunoassay. GC/MS is the testing procedure approved by the EPA. Although very precise and accurate, these tests are expensive to run, time consuming and have to be performed in a laboratory environment. Also significant training is required to operate the GC/MS machine. Recently, immunoassay tests have been introduced as a cheaper, quicker, and portable alternative to GC/MS. This test method is however not reusable and is qualitative in that it indicates only whether the measurand is above or below a particular level. Also, cross reactivity and reaction to broken down pesticides in these tests may lead to false positive tests (5). Therefore a need exists for a sensor that would combine the quantative and reusable properties of the GC/MS with the low cost, quick, and portable properties of the immunoassay tests.

In situ four-point conductivity and Hall effect apparatus for vacuum and controlled atmosphere measurements of thin film materials

An ultrahigh vacuum (UHV) chamber equipped with a fixture for in situ four-point Van Der Pauw conductivity and Hall effect measurements has been constructed and attached to a multichamber thin film synthesis and characterization system. The combined systems allow for film synthesis and characterization of microstructure, chemical composition, morphology, and electronic transport properties without air exposure. The four-point measurement fixture features spring-loaded probes for electrical contacts and temperature measurement and a sample docking mechanism designed to minimize probe damage to the films. The electronics were designed for measurement of high resistance samples. Measurements can be made at sample temperatures from 25 to 450 °C in selected gas environments from UHV to atmospheric pressure. The design and performance of the system are reported, and representative results on the electronic transport properties of n-type Si (100) and tungsten oxide films on sapphire are presented.

Wireless acoustic wave sensors and systems for harsh environment applications

This paper reviews current progress in the area of wireless microwave acoustic sensor technology, and discusses advances in wireless interrogation systems that can operate in harsh environments. The use of wireless, battery-free, low maintenance surface acoustic wave (SAW) sensors has been successfully demonstrated in applications including high temperature turbine engines and inflatable aerospace structures. Wireless interrogation of multiple sensors up to 910°C has been established and sensor tests in gas turbine engine are reported. This paper elaborates on several aspects of the technology, including: high-temperature thin-film electrode and sensor development, temperature cycling, thermal-shock behavior, testing in turbine engine environments, sensor packaging and attachment, wireless operation, and adaptation to energy and industrial applications.

Stable electrodes and ultrathin passivation coatings for high temperature sensors in harsh environments

Sensor operation in harsh environments up to 1000degC requires robust packages including stable electrodes and protective coatings. We have developed nanostructured ultra-thin (< 100 nm) Pt-10%Rh / ZrO2 electrode structures grown by e-beam co-evaporation that operate at temperatures approaching 1000degC. X-ray diffraction (XRD), resistivity, and electron microscopy (EM) studies indicate incorporation of ZrO2 within the film delays recrystallization, maintaining a stable morphology. We have also developed ultra-thin (< 50 nm) SiAlON passivation coatings that mechanically protect the sensor surfaces, yet allow interaction with the environment. Different SiAlON stoichiometeries were produced by rf magnetron sputtering of Al and Si targets in O2/N2/Ar mixtures. The SiAlON films are amorphous and extremely smooth (< 1 nm rms) and remain so even after extended annealing at 1000degC. Our results are applicable to a wide range of high temperature sensor configurations.

Thin films and techniques for SAW sensor operation above 1000°C

High temperature (300°C to 1400°C) wireless sensors have applications in energy exploration and generation, harsh environment industrial processing, and aerospace engineering. Existing technology developed at the University of Maine allows the fabrication of surface acoustic wave (SAW) langasite (LGS) sensors with Pt-Rh/ZrO2 electrodes that can deliver long-term stable operation up to 850°C. Since LGS remains piezoelectric up to its melting point of ~1400°C, it is desirable to extend the current SAW sensor temperature range of operation. In addition, it is desirable to diminish the SAW interdigital transducer (IDT) electrode dimensions to increase the wireless frequency of operation towards the GHz range. In this work, new thin film electrode materials have been investigated to allow the operation of SAW one-port resonators up to 1000°C and beyond. In particular, alternative Pt/Al2O3 and Pt-Rh/HfO2 thin film electrode compositions are presented, which yield operation of SAW resonator sensors up to 1100°C. In addition to a previously used capping layer, an interfacial layer has been added between the LGS and the electrodes to delay any interdiffusion between the materials and extend the temperature and/or time of sensor performance. Finally, it is also reported in this work that exposure of untreated SAW device electrodes with 120 nm thick and 2μm wide Pt-Rh/ZrO2 co-deposited IDT fingers to temperatures above 850°C can create long platinum-rich nano-whiskers. These structures short-circuit the SAW interdigital (IDT) fingers, rendering the device unusable. The short-circuit problem was solved by the use of multilayered electrode structures and the used of the capping layer.