On the mass sensitivity of acoustic-plate-mode sensors

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Sensors and Actuators A 53 (1996) 243-248

PHYSICAL

On the mass sensitivity of acoustic-plate-mode sensors Fabien Josse a,., Reiner Dahint b, Jtirgen Schumacher b, Michael Grunze b, Jeffrey C. Andle c,a, John F. Vetelino c,,J a Department of Electrical and Computer Engineering, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881, USA b Angewandte Physikalische Chemie, Universitiit Heidelberg, lm Neuenheimer Feld 253, 69120 Heidelberg, Germany Department of Electrical and Computer Engineering, University of Maine, Orono, ME 04469-5764, USA d Biode lnc, 18 Mussey Rd, Scarborough, ME 04074, USA

Abstract The mass sensitivity of acoustic plate modes (APMs) in general and in particular APMs on ZX-LiNbO3 is investigated for gas and liquid phase-sensing applications. It is shown that, unlike pure shear horizontal APMs, the modes on ZX-LiNbO3 consist of both propagating bulkwave components (with a linear frequency dependence of mass sensitivity) and evanescently trapped surface components from the pseudo surface acoustic wave (PSAW) (withf 2 frequency dependence of mass sensitivity). Thus, for a given plate thickness, the overall frequency dependence of mass sensitivity depends on the frequency range of operation. Theoretical and experimental results are in good agreement. Results are also given for actual immunosensor experiments. The results show that, in sensor applications, anf 2 dependence at relatively high frequencies, and hence higher mass sensitivity, is achievable. This will require using thinner plates and the dominant PSAW-derived APM. Keywords: Mass sensitivity; Acousticplate modes; Sensors

1. Introduction Acoustic-wave sensors are being investigated in both gas and liquid phase-sensing applications. One of the primary interaction mechanisms in these devices for the detection of molecules is mass loading caused by the added mass bound or adsorbed at the layer being used as the chemical or biological interface. However, because the added mass is usually very small, increasing sensitivity requires increasing the frequency of operation for the commonly used quartz-crystal microbalances or surface acoustic wave (SAW) devices. While this may be easily achieved for SAW devices, SAWs are known to suffer high propagation loss in liquid-phase applications, caused by mode conversion and viscous coupling. An alternative method is to use acoustic plate modes (APMs) [ 1,2]. The use of APMs has proved to be a promising concept for the detection of corrosive analytes as the device electrodes and analyte are strictly separated. While the mass sensitivity of quartz-crystal microbalances and SAW devices has been extensively studied theoretically and experimentally, there exist only few data on the mass sensitivity of APMs. More recently, the mass sensitivity of APMs * Corresponding author. Tel: 414-288-6789. Fax: 414-288-5579. E-mail: JOSSEF@ vms.csd.mu.edu. 0924-4247/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved

P11S0924-4247(96)01134-X

on Z-cut, X-propagating (ZX) lithium niobate (LiNbO3) has been investigated for liquid phase-sensing applications [ 3 ]. ZX-LiNbO3 is known to support a family of quasi-shear horizontal (SH) APMs which are related to the pseudo-SAW (PSAW) of the semi-infinite plate. The acoustic displacements and high piezoelectric coupling (k 2 > 2%) and high mass sensitivity ( ~-200 pg mm -2 at 150 MHz on a 0.5 mm thick plate) make this substrate attractive for fluid phasesensing applications. Ideally, one would like to employ a plate approaching zero thickness at as high a frequency as possible. Under these operating conditions, one could excite a single quasi-shear APM with a velocity at or near the PSAW velocity with excellent piezoelectric coupling and mass sensitivity. In order to provide additional data on mass sensitivity for the design and implementation of APM sensors in general, and in particular, on ZX-LiNbO3, mass-sensitivity experiments are performed using devices at different frequencies of operation. Theoretical calculations are also performed in which special attention is given to the selected acoustic mode.

2. Experimental In order to provide mass-sensitivity data, experiments are performed using various ZX-LiNbO3 APM devices in a chem-

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ical-vapor deposition (CVD) system. This technique allows one to observe the change in sensor response while a thin layer of polyamic acid (PAA) is being deposited on the sensor surface. PAA is a polymer obtained as the product of a surface-moderated reaction between pyromellitic dianhydride (PMDA) and 4,4'-diamino-diphenylether (ODA). To study the frequency dependence of mass sensitivity, various APM devices with interdigital transducer (IDT) periods of 334, 169, 130, 106 and 88 Ixm, corresponding to 13.2, 26.5, 34.0, 41.7 and 50.0 MHz, respectively, were designed. In addition to these frequencies, higher-frequency devices were obtained by operating at the third harmonic. All sensors were metallized with 20 nm of aluminum in order to eliminate acoustoelectric interactions. Dual-delay-line devices were used to compensate for temperature and other secondary effects. The sensor responses, which include changes in attenuation and phase shift due to film deposition, were measured using a network analyzer in a phase-measurement technique. The corresponding frequency shifts were obtained by using experimentally determined phase-frequency characteristics of the devices. Phase distortion resulting from crosstalk, triple transit, etc. was minimized. Assuming a constant deposition rate, the mass sensitivity, S = limAh~o(1/p) (Af/Ah), was obtained by determining the slope of the frequency shift-film thickness curve at zero film thickness ( p = 1.42 g cm -3 denotes the density of the PAA coating, f the frequency and h the film thickness). The film thickness evaluated from the deposition rate and time was also measured using a profilometer. Fig. 1 (a) and (b) shows the measured mass-sensitivity data for the dominant mode at each of the selected frequencies. In Fig. 1 (a) the results are compared to liquid-phase measurements where a n f 2 frequency dependence was found in the measured frequency range of 13 to 50 MHz [3]. Reasonable agreement between sensitivities in gas and liquid environments is observed at low frequencies. The measured frequency dependence of mass sensitivity is shown in Fig. 1 (b) for all experiments, including those with high-frequency devices. At higher frequencies, the mass sensitivity values are substantially lower than the f2 dependence predicted from the liquid-phase measurements. The overall trend of the curve suggests a linear frequency dependence of mass sensitivity rather than an f2 relationship. A linear fit to the data points is S = - 1.4 lf, where S is measured in Hz n g mm 2 and f i n MHz.

3. T h e o r y In order to explain the above measured data and discrepancies, especially at higher frequencies, the mass-sensitivity data of the various modes utilized were theoretically determined. First, the spectrum of modes was examined for different wavelengths on a 0.5 mm thick plate. It should be recalled that on ZX-LiNbO3, bulk acoustic wave (BAW)

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radiation is enhanced by the presence o f a PSAW [4--6]. The energy is mostly radiated into horizontally (SH) polarized shear waves. For a thin parallel plate, the radiated BAW energy sets up plate modes at various discrete velocities with different amplitudes, and hence different mass sensitivity. At these discrete velocities, the plate modes propagate, satisfying the boundary conditions at the top and bottom surfaces of the crystal [4-6]. Fig. 2 shows calculated plate-mode spectra for a 0.5 mm thick plate and for two values of h/A (h = plate thickness and A = wavelength). Note that the highest-coupling APM occurs approximately at the PSAW velocity, which indicates the PSAW-APM characteristics of this mode. Also, from Fig. 2, it is seen that at smaller wavelengths (h/A = 17.0) corresponding to higher frequencies of excitation, adjacent mode spacing and rejection is reduced and single-mode device operation becomes difficult. As will be shown later, each mode has a different characteristic, and hence different mass sensitivity.

F. Josse et al. / Sensors and Actuators A 53 (1996) 243-248

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f for low frequencies. At approximately 80 MHz, the slope of the mass-sensitivity curve begins to approach a linear behavior. This behavior occurs because the anisotropic quasishear APMs that propagate in ZX-LiNbO3 consist of both propagating bulk-wave components (with a linear mass-sensitivity dependence) and evanescently trapped surface components from the PSAW (with a frequency-squared dependence of sensitivity). When the plate is thin compared to the wavelength, the evanescent terms carry a significant fraction of the acoustic energy. However, as the plate becomes thick compared to the wavelength, the effects of the propagating bulk wave dominate. While isotropic theory [7] applied to pure SH APMs indicates that all the modes should have a constant and equal mass sensitivity (linear with f), it is apparent that the anomalous behavior in ZX-LiNbO3 is due to the evanescent fields of the PSAW. This behavior is physically justified and in very good agreement with the experimental results of Fig. 1 and the fluid-loaded experiments previously reported in Ref. [3]. Also shown in Fig. 3, in addition to the peak values of mass sensitivity, are the sensitivity values at adjacent modes or adjacent frequencies. Thus, an observation of the theoretical data indicates that the discrete nature of the modes and/or device operation at frequencies other than the peak frequencies can be assumed to account for the scatter in the experimental data of Fig. 1. Thus, questions arise as to whether some of the responses represent the intended designed peak modes. Finally, it is also seen from Fig. 3 that the highest sensitivity is observed for the PSAW-APM. This can be explained by evaluating the respective mode profiles. The results of such calculations, which require the mode profiles, i.e., amplitude variations of particle displacements throughout the plate, to be analyzed, are shown in Fig. 4. The amplitude variations with depth of the longitudinal (u,), shear horizontal (u2) and shear vertical (u3) components are shown for the PSAW-APM and an adjacent mode in

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The excitation of a single mode will require many electrodes. However, a large number of electrodes leads to undesirable distortion in conventional device designs. This limits the frequency at which conventional transducer design may be employed to excite a single acoustic mode. To evaluate the mass sensitivity in an APM chemical sensor, a complete model would include many layers ranging from the piezoelectric plate to the chemical fluid [5]. In between are various films that allow the target species from the fluid to be 'seen' by the propagating acoustic wave. To calculate, for example, the mass sensitivity, this model can be treated with varying degrees of accuracy depending on the number of layers and the availability of the material data. However, using perturbation theory (flat-field approximation) combined with exact numerical computations of the particle displacement amplitudes, the mass sensitivity was obtained [4,6] for each selected mode. Fig. 3 shows the theoretical mass sensitivity of the device to added mass per unit area (ng m m - 2 ) , Sm= IOf/Om (Hz mm 2 n g - 1) I, of the dominant APM and the nearest adjacent APMs plotted as a function of the device frequency for a plate thickness h of 0.5 mm. The plate is assumed to have an infinitesimally thin shorting plane at the upper and lower vacuum-bounded surfaces. The modes have a peak of sensitivity, the values of which follow a quadratic dependence on

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Fig. 4. Theoretically calculated mode profiles for (a) the dominant PSAWAPM and (b) an adjacent APM for a wavelength of 90/zm on a 0.5 mm thick plate loaded with water. The amplitude variations through the plate of the longitudinal (u~), shear horizontal (u2) and shear vertical (u3) components of the mode are shown.

Fig. 4(a) and (b), respectively. The results are for a wavelength of 90 I~m on a 0.5 mm thick plate loaded with water. For the dominant PSAW-APM mode, the mode profile in Fig. 4(a) shows energy concentration near the surfaces, within a fraction of the wavelength. Note that this energy is primarily associated with a shear horizontal component of the displacement. On the other hand, the adjacent mode whose profile is shown in Fig. 4(b) appears to be a pure APM with no energy concentration near the surfaces, which explains the relatively lower mass sensitivity.

ondary effects by restricting the immobilization of the antibodies to the sensing line. Note that the antibodies are linked to the non-electroded surface of the APM device. A liquid cell is fixed on the sensor surface and filled with about 1.6 ml of phosphate buffered saline (PBS). After monitoring the stability of the system, target antigen is injected into the liquid cell. Fig. 5 shows the frequency response of a non-metallized APM sensor, successively operated at 50.3 and 151.1 MHz. The thickness of the crystal plate is 0.5 mm. A first injection of target antigen yields signal changes of about 0.4 and 1.2 MHz, respectively. For a second injection, no further reduction of frequency was observed, indicating that the binding sites of the immobilized antibodies have been saturated. Comparison of the responses clearly shows a linear rather than a quadratic frequency dependence of mass sensitivity. Following the results of the present study, this may be explained by the comparatively small PSAW characteristics of the excited wave at high frequencies. However, assuming a linear frequency dependence for the mass sensitivity as shown in Fig. 1 (b), the results of the immunosensor still yield larger than expected frequency shifts at higher frequencies. Note that the same linear behavior and relatively high response has been observed for sensor devices in which the sensing surface has been metallized with a thin chromium/ gold layer (400 and 800 .~) to eliminate acoustoelectric interactions (Fig. 6). The results are compared with the measured mass sensitivities presented in Fig. 1 (b). From the linearity coefficients of the curves, it can be seen that mass loading alone cannot explain the sensor response. Furthermore, the assumption of a strictlyf 2 dependence of mass sensitivity at all frequencies will lead to erroneous interpretations of the observed results [8]. Such interpretation concludes that viscoelasticity of the binding layer may have contributed to the 'lower' sensitivity at higher frequencies. Other effects may be due to a possible false estimation of the I

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F. Josse et al. / Sensors and Actuators A 53 (1996) 243-248

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[5] T. Zhou, Theoretical modeling of acoustic waves in layered structure chemical sensors and biosensors, M.S. Thesis, Marquette University, 1992. [6] J.C. Andle, An experimental and theoretical analysis of acoustic plate mode devices for biosensor applications, Ph.D. Dissertation, University of Maine, 1993. [7] S.J. Martin, A.J. Ricco, T.M. Niemczyk and G.C. Frye, Characterization of SH acoustic plate mode liquid sensors, Sensors and Actuators, 20 (1989) 253-268. [8] J. Renken, R. Dahint, M. Grunze and F. Josse, Multi-frequency evaluation of different immunosorbents on acoustic plate mode sensors, Anal. Chem., 68 (1996) 176-182.

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actual added mass. All these factors are still being investigated to provide a better interpretation of the observed results.

5. Conclusions The mass sensitivity of acoustic plate modes (APMs) on ZX-LiNbO3 has been investigated. For a given plate thickness (typically 0.5 mm), sensitivity data indicate a quadratic frequency dependence at low frequencies (f< 60 MHz) and a linear frequency dependence in the high-frequency regime (f> 60 MHz). T h e f 2 dependence is due to the evanescently trapped surface components of the pseudo-SAW associated with this crystal orientation, while the linear frequency dependence can be associated with the propagating bulkwave components of the mode. Both contributions result in an overall frequency dependence of mass sensitivity that is linear rather than quadratic. However, the linearity coefficient is still higher than that of APMs on quartz. Moreover, t h e f 2 dependence at higher frequencies can still be achieved by using thinner plates (0.1-0.2 mm) and the dominant PSAWderived APM. Such thinner plates, and hence higher sensitivity, are being investigated to improve applicability.

References [ 1] J.C. Andle, J.F. Vetelino, M.W. Lade and D.J. McAllister, An acoustic plate mode biosensor, Sensors and Actuators B, 8 (1992) 191-198. [2] R. Dahint, M. Grunze, F. Josse and J.C. Andle, Probing of strong and weak electrolytes with acoustic wave fields, Sensors and Actuators B, 9 (1992) 155-162. [3] F. Bender, R. Dahint, F. Josse, M. Grunze and M. v. Schickfus, Mass sensitivity of acoustic plate mode liquid sensors on ZX-LiNbO3, J. Acoust. Soc. Am., 95 (1994) 1386-1389. [4] F. Josse, J.C. Andle, J.F. Vetelino, R. Dahint and M. Grunze, Theoretical and experimental study of mass sensitivity of PSAW-APMs on ZXLiNbO3, IEEE Trans. Ultrason., Ferroelec., Freq. Contr., UFFC-42 (July) ( 1995 ).

Fabien Josse received the License de Math6matique et Physique from the Universit6 du Benin in 1976 and the M.S. and Ph.D. degrees in electrical engineering from the University of Maine at Orono in 1979 and 1982, respectively. He has been with Marquette University, Milwaukee, WI, since 1982 and is currently professor in the Department of Electrical and Computer Engineering. He is an adjunct professor in the Department of Electrical and Computer Engineering, Laboratory for Surface Science and Technology (LASST), University of Maine, and has been a visiting professor at the University of Heidelberg since 1990. His primary research interest is in microwave acoustics and solid-state device sensors. His current research also involves electrooptic sensors. Dr Josse is a member of Eta Kappa Nu, Sigma Xi and the New York Academy of Sciences. Reiner Dahint was born in Coburg, Germany. He received the Diploma and Ph.D. degrees in physics in 1990 and 1994, respectively, from the University of Heidelberg. Currently he is with the Institute of Applied Physical Chemistry at the University of Heidelberg, where his research involves chemical sensors and biosensors utilizing acoustic wave devices. Jiirgen Schumacher was born in Mannheim, Germany. He received the Diploma degree in physics in 1995 from the University of Heidelberg. Currently he is a Ph.D. candidate at the Institute of Applied Physical Chemistry at the University of Heidelberg, where his research involves biosensors utilizing acoustic wave devices. Michael Grunze was born in Germany. He received the Diploma Thesis in chemistry and physics in 1972 and the Ph.D. degree in 1974 from the Freie Universit~it Berlin, Berlin, Germany. He also attended Knox College, Galesburg, IL, as a Fulbright Fellow. He has held several research, teaching and scientist positions since 1972 at various institutions, including the Freie Universit~it Berlin, the University of Munich, the University of London, the Fritz-Haber-Institut of the Max-PlanckGesellschaft, Berlin, the IBM Research Laboratory, San Jose, CA, the University of Osnabriick, Germany, and the Laboratory of Surface Science and Technology (LASST), Uni-

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versity of Maine at Orono. From 1985-1987 he was also the director of LASST. Presently, he is professor of physical chemistry and director of the Institute of Applied Physical Chemistry at the University of Heidelberg, Heidelberg, Germany. Dr Grunze has been the recipient of several awards including a Fulbright Fellowship (1968 and 1977), Liebig-Habilitations-Stipendium ( 1975-1977), and the Otto-Klung-Preis for outstanding contributions in physical chemistry (1976). He is a member of the American Vacuum Society and the Deutsche Bunsengesellschaft. J e f f r e y C. A n d l e received his B.S., M.S. and Ph.D. degrees in electrical engineering from the University of Maine in 1984, 1989 and 1993, respectively. He was employed at RF

Monolithics from 1985 to 1988 as a research engineer. Currently he is a research scientist at the University of Maine and is associated with Biode Inc., Cape Elizabeth, ME and Sensor Research and Development, Inc. in Bangor, ME. His major research interest is acoustic wave sensors. J o h n F. Vetelino received his Ph.D. degree in electrical engineering from the University of Rhode Island in 1969. He joined the University of Maine in 1969 as an assistant professor of electrical engineering. He was the electrical engineering chairperson from 1983 to 1987 and is currently a professor of electrical engineering. His major research interests are in the areas of solid-state and microwave acoustics. Currently, he is directing research programs in the area of microsensors for applications such as gas sensing, biosensing and thin-film characterization.