Jay W. Grate
Acoustic wavf iicrosensors are being used for broader applications in chemical sensing and interfacial studies as more devices are invented and new sensor response mechanisms are elucidated. In this two-part REPORT, Jay W. Grate, Stephen J. Martin, and Richard M. White compare and contrast the more commonly used acoustic wave devices and provide a current description of the physical origins of observed sensor responses in both gasand liquid-phase studies. In Part I, the types of waves and devices as well as information on operation and measurement methods, mass sensitivity, and detection will be discussed. Part II, which will appear in the November 15 issue, will focus on viscoelastic films, polymer-coated vapor sensors, acoustoelectric and dielectric effects, and liquid-phase sensing.
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Molecular Science Research Center Pacific Northwest Laboratory Battelle Boulevard Richland, WA 99352
Stephen J. Martin Microsensor Research and Development Department Sandia National Laboratories Albuquerque, NM 87185
Richard M. White Berkeley Sensor and Actuator Center Department of Electrical Engineeringand Computer Sciences and the Electronic Research Laboratory University of California Berkeley, CA 94720
Microsensors t h a t u s e a c o u s t i c waves comprise a very versatile class of sensors. Because they are highly sensitive to surface mass changes, m a n y applications as chemical sensors. However, they can also determine a variety of other properties of solid or fluid media i n contact with their surfaces, including liquid density, liquid viscosity, polymer modulus, and electrical conductivity. Previously as
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1,199
sors have been found to respond to a combination of factors. The in-
write this REPORT. We will focus primarily on t h i c k n e s s - s h e a r mode (TSM),surface acoustic wave (SAW), f l e x u r a l p l a t e wave (FPW), a n d acoustic plate mode (APM) devices. These devices are illustrated in Figure 1. The areas of the sensing surfaces are typically 1 cm2 or less. Although these sensors a l l u s e piezoelectric substrates, piezoelectricity is not the sole means for generating acoustic waves. The waves can also be generated magnetostrictively, electrostrictively, and photothermally. Therefore, many opportu nities exist for t h e design of new acoustic devices and instrumentation that may be used in sensing applications. Depending on the device and the type of wave generated, it is possible to measure properties, processes, or chemical species in the gas phase, the liquid phase, in a vacuum, or in thin solid films. The u aves for sens-
applications. Lithium niobate, for example, h a s higher acoustoelectric coupling. Thin films of zinc oxide,
cates tr larized face, w
TSM devices (see Figure 1) are bulk transverse waves that travel in a didecreases the film. Both th wave velocities. layers on sensors spa
plexes, electroactive materials, met films, and metal oxide films. A wealth of information on this topic has been published, including
APM acoustic sensors (30-36).We will not address mechanisms for chemical selectivity or chemically selective layers, which have been reviewed elsewhere ( 8 , 9 , I I ,18,20,21, 37). Nor will we discuss the use of
rection, as in sound waves. The d vices described here generate waves that are transverse or have a transverse component, although we note that a microfabricated acoustic sensor using compressional waves was
trast the most commonly used acoustic microsensor devices and to provide an up-to-date description of the physical bases of observed sensor responses i n both g a s a n d liquid phases.
Shear horizontal wave motion indi-
Acoustic waves and devices The typical acoustic microdevice con sists of a piezoelectric material with one or more metal transducers on its surface(s). These transducers launch acoustic waves into the material a t ultrasonic frequencies, which may range from one to hundreds of megahertz. The transducer metal is usually selected for either chemical in‘ertness (e.g., gold) or for its acoustic match to the piezoelectric material (e.g., aluminum on q u a r t z ) . The piezoelectric material may consist a polished plate or a n oriented t h film. Because of its temperature stability, quartz is the most
t
used i n a detection application.
SAW
ws illustrating the structures of TSM, SAW, FPW, and Side views are cross sections. Lower diagrams in each column illustrate the wave motion; double- headed arrows indicate directions of surface particle displacements. Shaded areas illustr e wave motion or indicate the depth of wave penetration r the TSM and APM devices.
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cle motions of Lamb wa lates oriented for Rayleigh wav ion, most of the energy IDT is transmitted as aves provided that the ss is much greater t h a c wavelength (e.g., at Alternatively, selection of a diffe
elliptical with both and surface - paralle where t h e surface ent is transverse to t
tion. In shear horizontal plate wa
plate material and its thickness.
established when the surface to prevent wave diffraction into the bulk of the plate. The mass of the thin film or grating slightly reduces wave velocity at the surface and guides the wave along the surface. Wave-guiding is more effective length of the device to the out transducer 5 orders of
se are all surcenter of the device. These waves radiate outward and are reflected back toward the center by microfabricated grooves or ridges on the device surface, creating a “resonant cavity” in the center. A resonator device can be fabricated with one or two IDTs in ANALYTICAL CHEMISTRY, VOL
ese waves pro
have wave velocities greater than that of a SAW in the same medium, and their velocities increase with decreasing plate thickness. By contrast, the lowest order antisymmetric mode has a wave velocity less than t h a t of t h e SAW i n t h e same medium, and its velocity decreases with decreasing plate thickness. T h i unique Lamb wave h a s flexu character, hence the name “flex plate wave,” or FPW. T h e FPW device develo White and co-workers (see F is microfabricated in a silic strate to create a very thin co plate with a sputtered zinc 0x1 piezoelectric layer (22-29). T thickness is only a few percent of the acoustic wavelength. Because t h e plate is only about 2-3 pm thick, it is often referred to as a membrane. I t is supported by the surrounding silicon substrate. IDTs in a delayline configuration generate flexural waves that set the entire thickness of t h e membrane in motion. Seen i n cross section, these waves a r e like the ripples in a flag waving in the wind. The waves wave the I mem mine At a constant transducer periodicity, plate thickness de-
s
contact with th base is bonded electric ultraso
With TSM and plate mode devices the wave motion occurs throu
Table I.
ANALYTICAL CHEMIST
are mismatched co
ic wave devices that use pitricity are operated by excitinput transducer with a n aling voltage at a radio (rf). A distinction can be ne-port devices, such ice, and two-port de-
ticularly important on highe quency devices. On two-port devices, wave c teristics can change a s t h e propagates between the two ducers. Changes in the wave v occur when mass is deposited device surface or when phys changes occur i n the medium c
plitude to that of the input, i ecibels. I t measures the total
the liquid. Rayleigh
used in liquid-phas ion losses in order to sustain o
liquids such as water. Therefore, the flexural wave does not couple t compressional waves in the liqui and it can also be used in liqui phase sensing applications. T h e FPW device also functio with gels such as poly(acry1amide) 944 A
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quency. When using oscillators in
form. A refinement on the oscillator technique t h a t includes automatic
ing a signal generator and a vector voltmeter, as shown in Figure 4b, provides both velocity and amplitude
ctor voltmeter. e indicates the
Figure 4. Three methods of operating piezoelectric devices. (a) Oscillator circuit provides a single-frequency signal. (b) Vector voltmeter provides phase and amplitude information. (c) Network analyzers are connected to one- and two-port devices. M: matching network; T: 50 R termination.
tion frequency and wave velocity, respectively, a n d Af a n d Av a r e t h e shifts i n frequency and velocity. Counting t h e oscillator frequency with a digital frequency counter provides a very precise indirect measurement of the acoustic wave velocities. Oscillators can also be designed to work with one-port devices. Oscillator circuits a r e relatively simple and inexpensive to fabricate, especially at low frequencies, and are suitable for field instruments. The device and the oscillator can be placed on a printed circuit board or in a hybrid electronic package, or (in the case of thin-film devices) integrated onto the same substrate as t h e device. D i g i t a l f r e q u e n c y counters to measure the oscillator signal are equally suitable for fabrication as lightweight components of field instruments. The disadvantages of the oscillator met provide info plitude,
plitude changes provide information about the attenuation of the wave by the medium contacting the surface. This information is particularly useful when monitoring thin -film properties and can help to identify when observed velocity changes are not solely due to mass loading. The vector voltmeter method also has the advantage that analytical signals are not lost when large increases in signal attenuation occur during a n experiment. However, phase measurem e n t s w i t h commercial vector voltmeters a r e 10-100 times less sensitive to velocity changes t h a n frequency measurements by the oscillator method. Therefore, lower limits of detection can be achieved with the oscillator method than with the vector voltmeter method when sensing chemical species at very low concentrations. However, p h a s e measurement systems with sensitivities equal to those of oscillators can be devised (54, 55). It is also possible to use a vector voltmeter in a phase-locked loop. In this case the phase is maintained constant by adjusting the frequency, and the changes in frequency are recorded. This method is somewhat similar to an oscillator, which maintains a constant phase automatically and provides a frequency signal. A network analyzer (Figure 4c)
ure sensor responses of either one- or two-port port devices, t h e measures the signal reflected from the input transducer to obtain impedance characteristics of t h e device being tested, including the magnitude and phase angle of the impedance as a function of frequency. Characteristic resonant frequency, phase, and impedance values can be extracted from these data, together with equivalent electrical circuit parameters (56-58). The latter can be related to both the electrical and the acoustic properties of the sensor. For two-port devices, the network analyzer monitors the transmitted signal to obtain the impedance characteristics and characterize the device. In this case, it functions essentially as the source/vector voltmeter combination of Figure 4b, measuring the amplitude and phase information as a function of input frequency. The value of the network analyzer is that it allows complete characterization of acoustic sensor devices under all conditions, including those for whi t h e oscillator method fails. Fr quency scans can be made periodically during experiments to extract the sensor response as a function of time.
Mass sensitivity and detection limits Although mass loading is not t h e only mechanism of sensor response, it is conceptually one of the simplest response mechanisms and is therefore a useful benchmark for characterizing and comparing devices. We define mass sensitivity as the incremental signal change occurring i n response to a n incremental change in mass per unit area on one surface of the device. This parameter be confused with sensitivi context of detecting a n analyte. Analyte sensitivity is the incremental signal change occurring in response to a n incremental change in analyte concentration i n the medium being lyte sensitivity i n t h a t influence t h e transfer of the analyte from the medium to the device surface in addition to the inherent device sensitivities t h a t influence t h e size of the resulting signal (i.e., mass ity, modulus sensitivity, etc.). Sensor responses can be either in absolute terms, for example, as the frequency shift in hertz, or as the shift relative to the deviceoperating frequency. I n the latter case, t h e r e s p o n s e woul
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REPORT pressed a s a fractional frequency shift in units such as h e r t d m e g a hertz or parts per million. (Units of parts per million for frequency response should not be confused with units of ppm for analyte concentrations.) To compare mass sensitivities, we define a mass sensitivity factor, Sm,such that
Sm = limit (Av/V0)/Am
(1)
Am40
where A m is the uniformly distributed mass per unit area added to the surface of the device, Vo is the unper turbed (i.e., without mass- or liquidloading) phase velocity of the device, and Av is the change in phase velocity that occurs on mass loading (26). Because velocities and frequencies a r e directly related, it is possible (subject to certain conditions) to express S, in terms of oscillator frequencies
Sm = limit (Af / FO)/Am
(2)
Am+O
where Fo is the unperturbed fundamental frequency of the oscillator in air and Af is the frequency change that occurs on mass loading. Thus S , expresses mass sensitivity as the fractional frequency change per in-
cremental change in mass loading. Expressions for absolute sensor responses to added mass and sensitivities in Sm are given in Table 11. I n general, mass sensitivity i n creases as the effective mass per unit area of the sensing plate decreases. Consequently, the mass sensitivities of quartz TSM and SH-APM devices are not as great as those of FPW and surface-wave devices. The FPW device owes its high mass sensitivity to the thinness of its membrane. The thickness of the sensing plate of a surface wave device is effectively the penetration depth of the waves. On SAW devices this depth is less than a n acoustic wavelength, so the effective mass per unit area of the sensing plate is small and mass sensitivi t y is high. Decreasing t h e wavelength decreases wave penetration, increases the mass sensitivity, and increases the frequency at the same time. On STW devices, increasing the grating thickness decreases the wave penetration and increases the mass sensitivity. Quartz TSM and SH-APM devices can be made more mass -sensitive by decreasing t h e i r plate thicknesses, b u t t h i s leads to increasingly fragile devices t h a t a r e difficult to support. De-
creasing the TSM plate thickness increases the device frequency. In the past, it was customary to discuss mass sensitivities in terms of device frequencies. This approach made sense with TSM and SAW devices because absolute mass sensitivities in Hz/(ng/cm2) increase with the square of device frequency. How ever, the idea that mass sensitivities are higher with higher frequency devices does not apply uniformly to plate wave devices. In these cases mass sensitivities are increased by decreasing t h e plate thickness, which decreases the frequencies of FPW devices but increases the frequencies of higher plate modes (n > 0) of SH-APM devices. Frequency is of no value for mass sensitivity comparisons among different types of devices. For example, the relatively low-frequency FPW device at 5 MHz can have the same mass sensitivity as a 40-MHz SAW device i n absolute terms, Hz/(ng/ cm2), or the same mass sensitivity as a 300-MHz SAW device in terms of fractional frequencies, (Hz/MHz)/ (ng/cm2) (22). Mass detection limits depend on the sensor’s noise (i.e., instabilities) and drift in addition to the sensor’s
Tablc 1. Mass sensi iities of five types of acoustic wave devices
-
De ._tvpe
15.5 MHz, -3mpos
0.019
Key: F, device fre uency; At, frequency shirt; Am,change in mass per unit area; LT,a constant depenaent on tne properties OT tne pleZOeleCtrlC plate Or the TSM device; &, a constant dependent on the properties of the piezoelectric plate of the SAW device; C ,, a constant dependent on the properties of the piezoelectric plate and grating of the STW device; K(a),a factor dependent on the properties of the piezoelectric plate of the SAW device; K‘ ((3, I factor dependent on the DroDerties of the Diezoelectric date and aratina of the STW device; M, the mass Der unit area of the FPW membrane; P, the sity of the plate material;’d, ihe plat mass sensitivity factor. a Assumed to be evenly distribuL3are device in gas or vacuu HzlMHz)/(ng/cm*) can be c -PW composite membrane FPW membrane. /here .I = 1 /3 for n : e SH-APM expressions are approximak. ._ for an isotropic plate and are fnr inrlividiml “ i n 101 MHz is the frequency for the n = 0 mode, and the mass sensitivity i
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sensit Noise the short-term fluctuatio e signal. Noise i s a complicated subject, and comparisons of one device with another are difficult to make because measurements in the literature are made under widely varying conditions. The observed noise can depend on environmental factors such as minute temperature fluctuations, the measurement system, and the time interval of t h e measurement (gate time). In a well-designed system, the acoustic device determines the noise, not the other electronic components. Attempts have been made to compare noise levels and mass detection limits of different devices (22,43,55). I n analytical applications, t h e limit of detection is often defined as the analyte concentration that pro duces a signal three times larger than the noise. Most devices can be configured as sensors with noise levels of 1 Hz or less, meaning t h a t a signal of only 3 Hz could be resolved, in principle. The real question, however, is whether such a small signal has chemical significance. Can it be confidently assigned to a particular analyte detected by a particular mechanism from a potentially complex environment? The real detection limit of interest is the concentration at which the analyte can be detected without significant interference from other species. In addition, drift may contribute to small signals that might be misinterpreted as indicating analyte. At this point, device engineering to reduce noise is much less important than addressing issues of chemical selectivity in sensors and sensor systems. I n addition, mechanisms of slow drift in signal require elucidation so that they can be compensated for or eliminated. Drift is also not a simple issue of device engineering, because factors such as the adhesion of chemically selective layers, slow leaking of entrained reagents, and environmental impacts such as h u midity can contribute to the drift of chemical sensors.
Mass detectors The high mass sensitivities of acoustic wave devices have prompted the development of a variety of chemical sensors for use in the gas and liquid phases. Until recently, the observed responses have been assumed to be due to mass loading; other mechanisms that might also give rise to the analytical signals have not been investigated. Below we present a few examples of mass detectors. Subse-
11 show t h a t many other mechanisms can contribute to and even dominate sensor responses. The most straightforward example of mass detection with a n acoustic microsensor is the measurement of the deposition of thin metal films in vacuum systems (10).Quartz TSM devices are used, and the method has been widely commercialized. The measured frequency shift is propor tional to the mass of the film and, via the film density and acoustic impedance, gives the film thickness. This method is accurate, provided that the film is thin (ideally no more t h a n a few percent of the acoustic wavelength). It is also significant t h a t ' e metal film adheres well to the s u face and is quite stiff. Thus the added mass moves synchronously with the shear motion of the surface. The mass sensitivities of new acous-
T
tic devices can be conveniently c brated by depositing known amounts of mass in a vacuum metal evaporation system equipped with a caliquartz TSM thickness monitor. u a r t z TSM devices have also been used as detection elements in cascade impactor systems for measuring airborne particulates (10). When t h e sample air is d r a w n through orifices, the particles are accelerated toward the surfaces of the TSM devices, which then measure the mass that is deposited. It is customary to first coat the devices with a thin film of nonvolatile grease to ensure that the particles stick when they make their impact. Mass t h a t does not firmly adhere cannot be accurately measured. Sampling is usually limited to time periods that deposit l e s s than a single l a y e r of particles. Recently, 200-MHz SAW resonators were used to enhance the sensitivity of these systems to very
small parti T h e h i g h m a s s sensitivities of SAW devices also permit the measurement of the surface adsorption of vapor molecules at submonolayer coverages. This capability was demonstrated by monitoring vapor adsorption on the bare device surface in a vacuum system (59). As the partial pressure of the vapor increases, surface coverage increases and the SAW frequency shifts proportionately downward. However, t h e s e straightforward results are observed only with polar vapors t h a t bind tightly to the surface. Less polar vapors with lower binding energies produce frequency shifts that are not proportional to the vapor concentration. Inadequate binding allows slippage or other effects that complicate t h e i n t e r p r e t a t i o n of frequency shifts. I t has been shown t h a t a d sorbed gases and vapors on quartz TSM device surfaces do not always move synchronously with the shear motion of the surface (60). The ability to measure mass deposition at submonolayer surface coverages h a s also been exploited to monitor self-assembly and supramolecular assembly processes and to characterize solid thin- film microstructures. Self-assembly of thiols from the gas phase onto gold s u r faces has been measured with SAW devices, and assembly of silanes from the gas phase onto aluminum oxide surfaces h a s been measured with TSM devices (61,62).The resulting monolayers are tightly bound, and t h e observed frequency decreases were in agreement with calculated masses per unit area of the films. Sequential surface reactions have also been demonstrated (62,63). The microstructures of thin solid oxide films deposited on SAW devices have been characterized by measuring nitrogen adsorption isotherms at liquid nitrogen temperatures (19,64).The isotherms were obtained from t h e SAW frequency decreases, and parameters such as surface area, pore volume, and pore size distribution were determined by using the BET model. Experimental surface areas and fractional porosi ties of films prepared from colloidal silicate particles of known diameters were compared with values calculated from the particle sizes, film thicknesses, and refractive indices. Good agreement confirmed the accuracy of this mass - sensing technique. Mass detection with acoustic sensors is usually straightforward if the device is operating in the gas or the vacuum phase, if t h e added mass
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served frequency shifts provide simple measure of added mass. References
R. M.; White, R. M. . Lett. 1991,59,77 M.; Arthur, C. L.,
Trends and Approaches in Electrochemical Technology; Kidansha Scientific Ltd.: Tokyo, 1993; Chapter 7, pp. 151-65. (34) Ricco, A. J.; Martin, S. J. Lett. 1987,50,1474-76.
ompson, M. Anal. Ch
tors 1990, A2I-A23,
. G.; Bein, T. Angew.
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