Michael J. Vellekoop

Acoustic wave sensors and their technology

Abstract In the past two decades, acoustic-wave devices have gained enormous interest for sensor applications. The delay line device, where a transmitting and a receiving interdigital transducer are realized on a (piezoelectric) substrate is the most common structure used. The sensitive part is the surface between the two transducers. By placing the device in the feedback loop of an amplifier, an acoustic-wave oscillator is formed with properties such as inherent high sensitivity, high resolution, high stability and a frequency output signal which is easy to process. A very interesting development is the large amount of wave types now available for sensor applications. Sensors have been published using Rayleigh waves, Lamb waves, Love waves, acoustic plate modes, and surface transverse waves (STW). Each of these wave types have their special advantages and disadvantages with respect to sensitivity, stability, usability in liquids or gases, and fabrication complexity. For the fabrication of the acoustic-wave devices, planar technologies are used, which will be discussed in the paper. Examples will be given of gas sensors, biochemical sensors in liquids, viscosity and density sensing and high-voltage sensing. A comparison of the usability of the different wave types will be presented.

Properties of Love waves: applications in sensors

Love wave sensors are highly sensitive microacoustic devices which are especially suited for sensing in liquids. In this paper we review the basic properties of Love waves and their utilization in sensor devices for (bio)chemical as well as for density and viscosity measurements. These properties are first discussed in general and then illustrated by means of numerical sample results for an -quartz layered structure. Furthermore the technology needed to produce such a Love wave device is discussed. Finally we present an overview on the current state of the art in Love wave sensor research.

Viscosity sensing using a Love-wave device

Abstract In this contribution we report an effective way of measuring the viscosity of liquids by using a microacoustic Love-wave device. Lovewave devices provide one of the highest sensitivities among microacoustic devices and they are moreover well suited for liquid-sensing applications. We discuss the possibility of viscosity sensing with Love-wave devices using the viscosity-induced damping and velocity change of the wave. We present experimental results together with a suitable electronic sensor set-up which also allows us to determine whether the test liquid is actually measured in the Newtonian range.

A love wave sensor for (bio)chemical sensing in liquids

Abstract It has been shown recently that a properly designed Love wave sensor is very promising for (bio)chemical sensing because of its high sensitivity. Furthermore, it is well known that a Love wave has a pure shear horizontal polarization and therefore no elastic interaction with an ideal liquid. This property makes it particularly interesting for (bio)chemical sensing in a liquid environment. However, interaction with a viscous fluid can never be avoided. It causes a small frequency shift which slightly distorts the sensor response and increases the insertion loss of the device. A theoretical and experimental study of this viscous fluid loading is presented here.

Love waves for (bio)-chemical sensing in liquids

It has been shown that a properly designed Love wave sensor is very promising for (bio)chemical sensing in liquids. Some fundamental properties of a Love wave delay line, such as dispersion relation, electromechanical coupling factor, and, in particular, sensitivity to mass loading on its surface are discussed. Theoretical predictions are compared with experimental data obtained from measurements on eight different devices built in ST-quartz with a sputtered SiO/sub 2/-layer.<<ETX>>

Compatibility of zinc oxide with silicon IC processing

Abstract In this paper we present the passivation of zinc oxide by a thin silicon nitride layer. With this passivation, silicon wafers covered with zinc oxide can be further processed without contamination of the process chambers of the subsequent processes, and without damaging the zinc oxide layer. In addition, we review some process technology concerning zinc oxide: the cleaning and etching of zinc oxide and the etching of aluminium on zinc oxide.

Design aspects of saw gas sensors

Abstract General information is presented with respect to the design of gas sensors using surface acoustic waves as the detection principle. Useful configurations use the SAW delay-line oscillator and the SAW resonator oscillator as the detecting element. Some recent experimental results with a SAW NO 2 sensor, developed in our laboratories, are presented.

NO 2 Gas-Concentration Measurement with a SAW-Chemosensor

A highly stable and selective SAW-chemosensor has been realized. The sensor operation is based upon the interaction of SAW fields with the simultaneous changes of mass and electrical conductivity of a chemical interface originating from changes in the gas concentra- tion. The sensor system is based upon a dual delay-line oscillator. The difference between the two oscillator frequencies is a measure of the gas concentration. The delay lines are implemented in an STX-quartz substrate. One delay line is provided with the chemical interface ob- tained by vacuum sublimation of an organic semiconductor, metal-free phthalocyanine. The oscillator is stabilized by means of automatic gain control. The oscillator operates without tuning inductors and with symmetrical driving of the transducers, aiming at a (total) monolithic integration of this sensor in a ZnO-SiOz-Si layered structure. A spe- cial feature of the sensor is that the delay lines and the electronic sys- tem are constructed in separate modules. This simplifies the inter- change of different delay lines and allows measurements to be performed when only the delay lines are at an elevated temperature (e.g., environmental monitoring and automobile emission control). The operating frequencies of the sensors are between 30 and 110 MHz. To date one sensor has a sensitivity of about 100 Hz/ppm NOZ at an op- erating frequency of 39.48 MHz with a threshold sensitivity of about 0.5 ppm.

Preliminary results with a silicon-based surface acoustic wave chemical sensor for NO2

Abstract A surface acoustic wave chemical sensor consisting of two identical ZnOSiO2Si layered delay lines has been realized on silicon. Together with a dual automatic gain-controlled amplifier, a dual delay-line oscillator system is formed. Preliminary results of the system when used as a sensor for NO2 are presented. One of the delay lines is covered with copper phthalocyanine as the chemical interface, grown by a physical vapour deposition technique. The experimental performance has been compared with previous results obtained with a surface acoustic wave chemical sensor system based on quartz. Parameters such as sensitivity, selectivity, drift, response time and noise have been examined.

Analysis and optimization of Love wave liquid sensors

Love wave sensors are highly sensitive microacoustic devices, which are well suited for liquid sensing applications thanks to the shear polarization of the wave. The sensing mechanism thereby relies on the mechanical (or acoustic) interaction of the device with the liquid. The successful utilization of Love wave devices for this purpose requires proper shielding to avoid unwanted electric interaction of the liquid with the wave and the transducers. In this work we describe the effects of this electric interaction and the proper design of a shield to prevent it. We present analysis methods, which illustrate the impact of the interaction and which help to obtain an optimized design of the proposed shield. We also present experimental results for devices that have been fabricated according to these design rules.