Scott C. Moulzolf

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.

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).

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.

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.

Capacitively coupled IDT for high temperature SAW devices

Harsh environment surface acoustic wave (SAW) sensors are being researched and developed jointly by the University of Maine (UMaine) and Environetix Technologies Corp. for wireless and wired sensor applications, such as those found in gas turbines and power plant combustors. One goal of this work is to extend the operational temperature range of SAW sensors above 1000°C, potentially up to near the melting point of piezoelectric langasite crystals at 1400°C. To achieve stable performance at 1000°C and above, UMaine has developed nanocomposite thin film electrode materials, such as PtRh/HfO2, and protecting capping layers, such as SiAlON and Al2O3. However, these protective top layers, which aid in extending the life of the electrodes, are electrical insulators that prevent direct bonding to the electrodes. The UMaine team also found evidence of accelerated thin-film degradation close to the SAW interdigital transducer (IDT) bond pad welds at these extreme temperatures. This paper introduces a high temperature capacitive approach to electrically couple to the IDT, thus allowing electrical access to the SAW device. The capacitive coupling approach also avoids premature failure of the nanocomposite film caused by interdiffusion between the bond wires and the SAW IDT bond pads. The technique has been successfully implemented and SAW device operation at 1000°C has been achieved.

Langasite SAW pressure sensor for harsh environments

Wireless pressure sensing in harsh environments at temperatures in excess of 200°C has important applications in energy generation, aerospace, and industrial processing. Such environments restrict or prevent the use of silicon based technology and battery powered sensor devices. Microwave acoustics, in particular surface acoustic wave (SAW) technology, allows the design of wireless pressure sensors to operate battery-free in a harsh environment. Current SAW-based pressure sensors operate up to approximately 200°C and 150 psi and typically employ quartz substrates. The Langasite (LGS) family of crystals retains their piezoelectric properties up to their melting point of approximately 1470°C, thus opening up the possibility of designing and implementing very high temperature and harsh environment LGS SAW pressure sensors. A square shaped sealed cavity has been used in this work with sensors strategically placed at the edges of the cavity for improved differential sensitivity and temperature compensated response. The fabricated devices were tested up to 225 psia and 500°C.

Thin film electrodes and passivation coatings for harsh environment microwave acoustic sensors

°Stable nanostructured ultra-thin electrodes and protective passivation coatings have been developed for langasite (La3Ga5SiO14) based surface acoustic wave (SAW) sensors that can successfully operate in harsh high temperature environments up to 1000°C. Ultrathin (<100nm) nanocomposite Pt-Rh/ZrO2 electrode structures were fabricated by electron beam co-evaporation and characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), 4-point resistivity, and scanning electron microscopy (SEM) studies. It was found that the incorporation of ZrO2 into the Pt-Rh electrode films retards recrystallization and de-wetting, thereby maintaining film continuity and low resistivity up to at least 1000°C. XPS results show that with heating at 800°C, the stoichiometry of the bare langasite SAW sensor surface becomes depleted of Ga in a reducing (vacuum) environment, but remains close to the bulk composition when heated in an oxidizing (air) environment. The incorporation of a thin oxynitride (SiAlON or SiZrON) coating over the entire sensor diminishes high temperature roughening and degradation of both the electrode and bare langasite surfaces. The viability and performance of these sensors was validated by experiments in which the SAW devices were tested in a controlled atmosphere laboratory furnace and also attached to rotating turbine blades within a small turbine engine operating with centripetal acceleration loads and temperatures in excess of 52,000g and 650°C, and under cyclical temperature shock conditions.

Electrically conductive Pt-Rh/ZrO2 and Pt-Rh/HfO2 nanocomposite electrodes for high temperature harsh environment sensors

Nanocomposite films comprised of either Pt-Rh/ZrO2 or Pt-Rh/HfO2 materials were co-deposited using multiple e-beam evaporation sources onto langasite (La3Ga5SiO14) substrates, both as blanket films and patterned interdigital transducer electrodes for surface acoustic wave (SAW) sensor devices. The films and devices were tested after different thermal treatments in a tube furnace up to 1200°C. X-ray diffraction and electron microscopy results indicate that Pt-Rh/HfO2 films are stabilized by the formation of monoclinic HfO2 precipitates after high temperature exposure, which act as pinning sites to retard grain growth and prevent agglomeration of the conductive cubic Pt-Rh phase. The Pt-Rh/ZrO2 films were found to be slightly less stable, and contain both tetragonal and monoclinic ZrO2 precipitates that also help prevent Pt-Rh agglomeration. Film conductivities were measured versus temperature for Pt-Rh/HfO2 films on a variety of substrates, and it was concluded that La and/or Ga diffusion from the langasite substrate into the nanocomposite films is detrimental to film stability. An Al2O3 diffusion barrier grown on langasite using atomic layer deposition (ALD) was found to be effective in minimizing interdiffusion between the nanocomposite film and the langasite crystal.

Electronic Transport Mechanisms in WO 3 -Based Ultra-Thin Film Chemiresistive Sensors

A variety of WO3 thin film structures were produced on sapphire substrates under controlled physical vapor deposition. Direct correlations were found between deposition parameters, microstructure, and electrical response both in vacuum and in the presence of test gases. Carrier concentration data indicate that WO3 becomes substoichiometric in vacuum and electronic transport is dominated by oxygen vacancy states, while in atmospheric air and/or oxidizing gases, the vacancy concentration becomes smaller and other defect states become important. Hall mobility data reveal that charge scattering mechanisms (hopping vs. impurity vs. phonon) depend on the crystallographic phase and microstructure of the WO3 film.