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A study of Love-wave acoustic sensors
ELSEVIER
Sensorsand ActuatorsA 56 (1996) 21 !-219
A study of Love-wave acoustic sensors J. D u , G . L . H a r d i n g , J . A . O g i l v y , P . R . D c n c h c r , M . L a k e CSIRO Division of Applied Physics, PO Box 218, Lindfield, NSW 2070, Australia
Received19 December1995;t~zvised5 March 1996;accepted19 Match 1996
Abstract Love-mode acoustic devices are very promising as biosensors in gaseous and liquid environments because of their high sensitivity. An experimental study of Love-wave devices based on SiO2/ST-cat quartz, over a wide range of SiO2 thickness, is presemed it, this paper. Devices with up to 7.3/~m thick SiO2 guiding layers have been successfully manufactured via an r.f. sputtering technique. Mass sensitivity, velocity, insertion loss, oscillation frequency s:ability and temperature coefficient of the frequency have been studied as a function of layer thickness. The sensitivity increases with increasing layer thickness and reaches a maximum at around 5.5 tam, for a wavelength of 40/an, in accordance with theory. Further increasing the thickness decreases the sensitivity dramatically. High sensitivity ( >300 cmz g-~) can be achieved at thicknesses between 3.5 and 6.5/.tm. The Love-wave dual-channel delay-fine osciilators also demonstrate high frequency stability and low noise levels. The frequencies of the two channels track each other extremely well. Keywords: Acousticsensors;Lovewaves;Masssensitivity;Biosensors
1. Introduction In recent years, there has been great interest in using surface acoustic wave (SAW) devices for detection of both physical and chemical analytes. The devices offer a simple and inexpensive technique for sensing applications, since interactions between the acoustic waves and the mass density, elastic stiffness and electric/dielectric properties of the propagation medium can give rise to a sensing response. Any change in these properties leads to changes in the velocity and amplitude of aco'.~stic wave modes. For detection of the velocity change, the acoustic wave sensing element is usually realized as a delay line or resonator and serves as a positive feedback element in an oscillator, with the oscillation frequency changing as the velocity changes. As the acoustic energy is confined to a thin near-surface region of substrata, SAWs are highly sensitiv~ to surface perturbation of the propagation medium. One of the most important detection quantities for SAW sensors is the surface mass change. A small mass loading on the surface of the acoustic propagation medium can be detected by a highly sensitive SAW device. A (bio)chemical sensor is usually obtained by coating a selective chemical interface or biological material on the surface of the acoustic element. The adsorption of a chemical compound or biological species at the active surface changes the surface mass density, leading to a change in wave velocity which is detected as a frequency or insertion-loss change (see, for example, Refs. [ I-5] ). 0924-4247/96/$15.00 © 1996ElsevierScienceS.A.All fightsn:se~ed P l i S 0 9 2 4 - 4 2 4 7 ( 9 6 ) 0 1 3 1 I-8
For sensing in liquid environments, there is a strong radiation loss into the liquid for longitudinal bulk modes, Rayleigh surface waves and most Lamb-wave modes. Shear horizontal (SH) polarized wave modes are preferred since they do not couple elastically with ideal (inviscid) liquids. For increased sensitivity, Love waves, which are SH-polarized guided waves, may be used. These waves propagate in a layered structure consisting of a substrata and a layer on top of it. The layer acts as a guide, with the elastic waves generated in the substrata being coupled to the surface guiding layer. Because of the wavegniding effect, these waves can be very sensitive to surface perturbations, and high sensitivity to surface loading can be achieved. Since Love waves do not have elastic coupling loss in liquids and the guiding layer also protects the interdigital transducers (IDTs) from liquid or chemical environments, they are particularly attractive for sensing in liquids. Love-wave sensors will most likely find applications in chemical analysis, food technology, environment protection and clinical diagnosis. Several Lovemode sensors operating in both gaseous and liquid media have been demonslrated [ 3,6-9]. A condition for the existence of Love-wave modes is that the shear velocity in the layer is smaller than the shear velocity in the substrata. It is important to choose an overlay material which has low shear velocity, low density and low acoustic absorption. Fused silica (SiO2) [7-9] and polymers, typically polymethyl-methacrylate (PMMA) [3,6], have been
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J. Du et aL /Sensors and ActuatorsA 56 (1996) 211-219
used to construct the layered structure for Love-mode sensors. Polymers, though having low density and low shear velocity, are usually lossy. The acoustic absorption increases quickly with increasing layer thickness [6] and this limits the sensitivity gain. For the quartz substrate being considered here, SiO2 satisfies the condition for the existence of a Love wave and has excellent elastic and thermal properties and, therefore, is considered to be an ideal overlay material. Acoustic losses in SiO2 are very low in comparison with polymers. SiO2 is also resistant to water and most chemical degradation and has excellent abrasion resistance. Relatively high mass sensitivity has been achieved u~ing a SiO2/quartz configuration [7,8]. Theoretical calculations [7,10-12] have shown that the sensitivity of Love modes strongly depends on the thickness of the guiding layer and there exists an optimal thickness, for a given wavelength, to achieve maximum sensitivity. However, experimental realization of the optimal thickness for the guiding layer has not yet, to our knowledge, been achieved. The reported maximum thicknesses are far from the calculated optimal thickness. Kovacs et al. [7,8] obtained up to 1.46/~m for the SiO2 layer on quartz, for wavelengths in the range 40-16/~m (normalized thicknesses in the range 0.037-0.091). Deposition of a thicker SiO2 film had failed due to technical problems probably related to film adhesion and stresses. Gizeli et al. [3,6] obtained 1.6/zm fo~ PMMA overlay material. Further increases in the thickness led to rapid increases in the acoustic absorption, and therefore in the insertion loss, and hence limited the maximum usable film thickness. It is desirable to obtain thicker layers for higher sensitivity. Experimental confirmation of the relationship of sensitivity versus layer thickness is also necessary to optimize the sensor design to give maximum detection sensitivity. In the present work, we have successfully obtained Lovewave devices based on SiO2/ST-cut quartz structures, with the SiO2 layer ranging from 0 to 7.3/tm, which covers the optimal thickness for maximum sensitivity for the wavelength we are using (40 ftm). Mass sensitivity, velocity, insertion loss, frequency stability and temperature coefficient have been studied as functions of the layer thickness. To the best of our knowledge, there is no report in the literature on successful manufacture, operation and measurement of mass sensitivity and other related properties for Love-wave devices with such a wide range of layer thickness covering the optimal thickness for peak sensitivity. The fabrication, mass-loading technique and the experimental results on mass sensitivity, velocity, insertion loss, frequency stability and temperature coefficient are described in this paper.
2. Theory Fig. I shows schematically the geometry of the Love-wave devices considered in this paper. A substrate of ST-cutquartz, 0.5 mm thick, is overlayed by sputtered SiO2. A Love wave propagates in this structure, travelling perpendicular to the
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crystallographic X axis, which is itself parallel to the surface, with the wave polarized parallel to this axis (the waves are purely shear in nature for this orientation of ST-quartz). For a detailed description of Love waves and their properties, see, for example, Auld [ 13], chapter 10. The IDTs (not shown in the Figure) are at the interface between the SiO2 and the quartz. The distribution of Love-wave energy through the depth of the plate is dependent on the ratio of the overlay thickness, h, to the wavelength, ~o. For small values of this ratio (h/~o < 0.06) the wave is partially confined within the layer, but also has considerable energy within the quartz. As the ratio increases (to h/~o ~ O. 14), the wave is increasingly confined in the layer, with a large normalized wave amplitude (normalized relative to the total energy in the wave) at the surface. Further increases in the layer thickness lead to further confinement of energy within the layer, but with a smaller normalized amplitude at the top surface. (We see below that it is this normalized amplitude which determines the sensitivity of a Love wave to mass loading.) For thicker layers, more than one Love-wave mode may be supported. All of the measurements reported in this paper are for the zeroth-order Love mode. Fig. 1 also shows a thin, uniform, solid film on the SiO2 layer, taken to be a mass-loading layer (this may represent, for example, a chemical species that binds to a chemically selective surface). The thickness of the film is assumed to be much less than the acoustic wavelength, so that first-order perturbation theory may be used to calculate the effect of the film on the Love-wave velocity. In this approximation, the acoustic field within the film is assumed to be uniform through the depth of the film, allowing an explicit expression to be derived for the fractional change in wave velocity (see Auld [13], chapter 12). This takes the following form for Love waves: _ _ = _ Vp,~V. 2 2 Af 1 _ (_V__~._m~1113i, =o fo
4 eUL
(I)
~v,] I
where Af ( = f - - f o ) is the change in frequency due to the mass loading of the thin film, d is the film thickness, p is the film density, Vmis the bulk shear wave velocity in the film and Vp is the phase velocity of the Love wave without mass loading. The quantity I ~1~.o is the particle velocity at the surface of the SiO2, norraalized with the total energy density (per unit width of wave) throughout the plate thickness. We see that the sensitivity of a wave to a mass-loading layer is dependent on the mass loading per unit area (pd), the normalized par-
J. Du et al. /Sensors and Actuators A 56 (1996) 211-219
ticle velocity at the surface (so that the greater the confinement of the wave energy close to the surface, the larger the sensitivity), and on the o,~.~tity 1 - (Vm/]/'p) 2. This latter represents a reduction in m-',~s-loading sensitivity because of the elastic effect of the mass-loading layer itself. If this layer does not support acoustic shear waves (because, for example, it is macroscopically discontinuous) then Vmffi0 and this quantity is unity. The Love-wave velocity for the system without the massloading layer, Vp, may be determined from the dispersion equation for these waves (this equation relates the wave velocity to the frequency). We have assumed that the system may be modelled as an isotropic layer on an isotropic substrate of infinite extent in the - z direction of Fig. 1. An explicit expression may be derived for the dispersion equation of this system, solution of which provides us with the required value for Vp and hence for the mass sensitivity. The details of this calculation are not given here, as they may be found in, for example, Ref. [ 11 ] and Ref. [ 13], chapter 10. The sensitivity to mass loading is defined in this paper to be Sm = L i m - - ~ aM~ofo AM/A
(2)
where AM/A is the mass per unit area deposited on the surface. Using Eq. (1) we see that the sensitivity may be written $m = - -
1--
I01~-0
(3)
since Od ffi ~MIA. We see that according to first-order perturbation theory the sensitivity of a given Love wave to mass loading is independent of the mass of the loading layer, and independent of the area over which the m ~ s is loaded, as the film is assumed to be uniform over the whole area under which the wave is propagating. If the elastic effects of the mass-loading layer are ignored (Vmffi0) then the sensitivity is also independent of the material being deposited, and becomes an intrinsic property of the Love wave being considered. In practice, the concept of uniform mass loading over the whole acoustic wave path is difficult to apply to a biosensor device of conventional design, because of the finite size of the IDTs and of the 'sweet spot' ( the area between the IDTs). If a layer of uniform thickness is applied to the 'sweet spot' only, then the effective mass per unit area for the whole wave path is less than the true mass per unit area, as when the wave is travelling under the IDTs it will not experience mass loading. However, the total effective area is less than the sum of the 'sweet spot' and IDT areas, as we would expect some reduction in mass-loading sensitivity towards the rear edges of the IDT areas. There is therefore some ambiguity in the definition of the area to be used in determining the quantity Sm for practical biosensor devices. We have defined the effective area to be the sum of the 'sweet-spot' area and one half of the area under the IDTs. If the mass is applied only to the
213
'sweet spot' or a fractional effective area, it is necessary to use the full effective area when calculating the sensitivity S,~. In this way, the obtained device sensitivity should be independent of the area that is mass loaded. The exact definition of the effective area is a complicated issue and further work is needed to determine this quantity.
3. Experlmenbd
3.1. Devicefabrication The Love-wave devices were fabricated on 20 m m × 19 mm and 0.5 mm thick ST-cut quartz subswates, with propagation perpendicular to the crystallographic X axis. The transmit and receive IDTs consisted of 75 finger pairs wi~:h l 0 / a n width of electrode and l 0 / m l separation, i.e., a periodicity of 40/~m. The finger pairs were obtained by d.c. sputtering = 240 nm chromium onto the quartz substratc, followed by patterning with photolithographic techniques. The IDT aperture was 3 nun and the IDT centre-to-centre separation was 6 mm. This device supports an SH mode at a frequency of 124 MHz. The initia~ insertion losses of SH devices were around 23 dB. An SiO2 layer was then deposited on each SH device via an r.f. magnetron sputtering technique, with a silicon target reacting in the mixed gases of oxygen and argon. The area of SiOz was 12 ram× 12 mm, defined by a mask made from thin stainless-steel sheet. A specimen of quartz was attached to each device for thickness measurement. The thicknesses of the SiO2 layers were measured with a Dektak 3030 Surface Profile Measurement System. A range of different SiO2 thicknesses up to 7 . 3 / ~ n were obUdned. The films adhered to the IDTs and quartz substra'e very well and appeared to have low internal stresses. There were no delamination problems even for very thick films, suggesting that thicker films could be made using this technique. We believe that using chromium as the electrode material is a key factor in ensuring good adhesion between the SiO2 film and IDTs/ substrate, since it generally has excellent adhesion properties, particularly to materials containing oxygen.
3.2. Delay-line oscillator configuration Two identical Love-wave delay lines were fabricated on the same quartz substrate to form a dual-channel delay-line oscillator as shown in Fig. 2, one channel to be used as sensing element and the other as the reference channel. Two broadband amplifiers were used to feed energy back from the receive IDT to the transmit IDT to compensate for the loss of the delay lines and to maintain oscillation. The amplifiers were built into a small module into which the quartz substrate and associated heater unit were plugged. This system was then plugged into a unit that provides power and temperature control, mixing of the two channel frequencies to produce a difference frequency, and buffering of all three frequencies to minimize the effects of measuring equipment on the oscil-
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J. Du et al. /Sensors and Actuators A 56 (1996) 211-219
Amplifier
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lation frequencies. The difference frequency from the control unit provides a signal that is compensated for ambient temperature change and easier to measure to high resolution than the oscillation frequencies.
3.3. Measurement techniques The operation frequencies were obtained either by measuring the oscillation frequency with a frequency counter or by recording the peak resonance frequency of the delay line with a signal spectrum analyser. The propagation velocities are given by v =foAo, where Ao is the wavelength defined by the periodicity of the IDTs. Insertion losses were obtained by monitoring the resonance frequency of the transmission signal and recording the signal loss with a Tektronix 2710 Spectrum Analyscr. To determine the mass sensitivity, a convenient and reproducible technique for surface mass loading has been developed. The delay-line oscillator was set up in a vacuum chamber and an ultra-thin ( < 2 nm) gold film was deposited on the 'sweet spot' of each device using a d.c. sputtering technique. The area over which mass was deposited was 2.5 mm × 3 mm, defined by a mask made of thin polyimide plastic. The frequency change due to mass loading was measured by recording the oscillation frequencies immediately before and after gold deposition and the sensitivity was then calculated using Eq. (2). By calibrating the gold deposition rate and the area coated, the amount of mass deposited on th.~ devices could be precisely controlled. SEM and AFM microstructure examinations of the ultra-thin Au films suggested that the films were formed as microscopic uniformly distributed gold islands and therefore were not electrically conducting and were not likely to act as eiastically guiding layers themselves. The deposited gold was easily removed by gentle abrasion with cotton tips after measurement without damage to the surface of the SiO2 and the experiment could be repeated. In comparison with the mass-loading techniques of spin-coating photoresist [7,8,14] and Langmuir-Blodgett (LB) films [ 1,3,6] used by other authors, this technique has the advantage of ease of control of small amounts of mass loading and is highly reproducible. The added layer is very
thin compared with the SiO2 and quartz thicknesses and can be regarded as a small perturbation, as assumed in the theoretical calculations. However, this technique is not suitable for studying mass sensitivity in a liquid medium. Measurements were usually carded out by making three consecutive gold depositions for each delay line and the mean values of the frequency changes were taken. The frequency change versus the added mass showed a linear relationship for the Love-wave devices, for up to six consecutive depositions. Separate runs also gave reproducible results with small standard deviations. The frequency stability was determined by monitoring the frequency shift over a period of time (e.g., 30 rain) during operation. The measurement temperature was usually set between 28 and 40°C via a specially designed temperaturecontrol unit and the devices were heated up and stabilized at this temperature. This minimized frequency drift due to heating from the oscillator electronics. The frequencies of both channels and the difference frequency were measured. The measurement procedure was controlled and the data were recorded using a computer. The noise level was determined from the difference frequency versus time by calculating the r.m.s, fluctuation in the frequency.
4. Results and discussion The Love-wave propagation velocity as a function of the thickness of SiO2 layer is shown in Fig. 3. The individual data points are marked, together with a linear fit to the data. The data at zero thickness are for an SH device and are close to the expected shear wave velocity in quartz, along the x direction of Fig. 1 (5060 m s - t). The velocity decreases with increasing SiO2 thickness as expected, since the shear wave velocity in SiO2 (2850 m s - t ) is smaller than the shear velocity in the substrate and the SiO2 overlay slows down the
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J. Du et aL I Sensors and Actuators A 56 f1996) 21 i-219
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wave propagation. If the thickness is further increased, the velocity would approach the velocity in SiO2. The variation of velocity with SiO2 thickness is not expected to be linear over a wide range of thickness values (see, for example, Ref. [7] ) but is closely linear over the range we have examined. It is interesting to note that the measured velocity decrease for a given film thickness/wavelength ratio is less than the corresponding decrease measured and predicted in Ref. [7], Figure 5, suggesting that the SiO2 films which we have manufactured have a larger shear velocity and/or density than the film manufactured in Ref. [7]. A separate measurement of the film density gave the value 2.34 (+0.05) × 103 kg m -3. 6% higher than the value quoted in Ref. [7]. Measurement Oi
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of the shear wave velocity of the SiO2 film, which has not yet been pursued, would provide more information. The insertion losses for Love-wave delay lines with different layer thicknesses are given in Fig. 4. With increasing thickness, the insertion loss decreases quickly first, reaches a minimum and then increases again with further increasing thickness. The insertion losses for all Love-wave devices were smaller than those of SH devices. The decrease of the insertion loss for Love-wave modes results from effic~.enz guiding of acoustic energy in the overlay and enhancement of the electrom¢chanical coupling of elastic waves by IDTs. However, strong coupling can only be obtained when the maximum acoustic field is near the substratc-laycr inc.'face [7] where the IDTs are located. At larger thickness ( > 2.5 /zm), more acoustic energy is guided in the top layer and the maximum field distribution is shifted away from the interface. This leads to a decease in coupling again and the insertion loss increases. Minimum insertion loss occurs at h ffi2-2.5 /zm for our Love-wave devices with a wavelength of 40 pln, agreeing qualitatively with theory (Ref. [7], Figure 3), and the minimum insertion loss was = 10 dB. However, the position of minimum insertion loss also depends on the material of the IDTs. Fig. 5 shows the mass sensitivity defined according to Eq. (2) versus SiO2 thickness. The sensitivity is negative because the velocity always decreases due to mass loading. Two sets of data were for the devices in two separate batches, each with an SiO2 layer formed from one continuous deposition, and one set of data was from a single device with the SiO2 layer built up in steps. The same trend is obtained for either method of SiO2 deposition. The spread of data at each nominal SiO2 thickness is likely to be caused by the difference in actual SiO2 thickness for the two channels on the same sub[
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-5O0 SlO, ~ l c k n ~ m (urn) Fig. 5. Mass-loading sensitivity vs. SiO2 thickness. The solid and dashed lines are the theoretical predictions ignoring the elasticity o f the gold film and with elasticity included, respectively.
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J. Du et al. /Sensors and Actuators A 56 (1996) 211-219
strate, as the distance between the device and the silicon target was small ( = 4 0 mm) and therefore a small gradient in the film thickness could have occurred during the deposition process if the device was not well centred. The solid and dashed lines are the theoretical predictions, as described in Section 2. The solid line is the prediction ignoring the elasticity of the gold film, the dashed line is with elasticity included (assuming a uniform gold film). The material properties used in the calculations were taken from Wang et al. [ 11 ] except for the film density, which was increased to 2.34 x i03 kg m -3 (our measurement data). The datum at h = O is for an SH device. The sensitivity increases quickly with increasing layer thickness, reaches a maximum at -- 5.5 /~m (a normalized thickness = 0.14 ) and then decreases with further increase in thickness. High sensitivities ( ~ 300 cm 2 g - ;) were obtained between 3.5 and 6.5/~m for the wavelength of 40/~m, i.e., for normalized thickness of 0.09-0.16. The maximum mass-loading sensitivity ( -- 380 cm 2 g - ~), compared with = 15 cm 2 g - t for SH devices, is 25 times better than that of the SH devices. To our knowledge, it is the highest mass sensitivity reported among various SAW and guided-wave sensors, at this wavelength. The high sensitivity for the Love modes is due to the strong waveguiding effect of the layer. More acoustic energy propagates near the surface and this increases the sensitivity to surface mass perturbation. At larger thicknesses ( ~ 15% of the wavelength), the sensitivity decreases again as the wave becomes more evenly distributed through the thickness of the layer, so that surface displacements are reduced. We note here that the high sensitivities for SiO2/quartz reported in Refs. [7,8] were actually for devices with a waveguide consisting of SiO2 and photoresist layers. In this case, the photoresist acts as an efficient waveguide and thus increases the sensitivity. The experimental data agree reasonably well with theoretical prediction for SiO2 layer thickness up to = 5/zm. After the peak, however, the sensitivity seems to decrease with thickness significantly faster than the theoretical prediction. The reason for this is not clear. It muv~ be remembered that the theory is for isotropic solids, and t~erefore neglects anisotropic and piezoelectric effects in calculating the wave profiles in the layer and substrate, and also assumes that the substrate is of infinite thickness. A~ :he SiO2 layer thickness increases, this latter assumption will become decreasingly valid. This may account for some of the observed discrepancy. A model based on the solution of the full piezoelectric equations is being developed [ 15] and will be used to study this discrepancy. We measured the short-term stability of the oscillation frequency for Love-wave sensors. This can be used to assess the frequency stability, channel tracking, noise level and mass detection limit. Fig. 6(a) shows a typical result of the frequency shift with time during warm-up to a set temperature (37°C in this case) and for a period of time at constant temperature. Fig. 6(b) is an expansion of the difference frequency, which gives a clearer picture of the noise level. After initial switching-on, the frequency increased quickly with
temperature and usually stabilized at the set temperature within five minutes. From the Figure, the oscillation frequency was very stable and showed almost no drift after the temperature stabilized. The frequencies of the two channels tracked each other extremely well during the temperature change, which is clearly shown by the very small shift ( < 3 ppm) of the difference frequency. The noise level measured from the difference frequency shift was 0.02 ppm (r.m.s.) or 2.4 Hz. Our measurements have shown that all of the Lovewave delay lines for which we made measurements, with different SiO2 thicknesses, have similar frequency stability and noise level to those shown in Fig. 6. In all cases, the two channels tracked each other very well and showed little frequency drift. The noise levels were in the range 0.01-0.02 ppm (r.m.s.), which correspond to the frequency change in the range 1.2-2.4 Hz. The high frequency stability and low noise are due to a stable laboratory temperature combined with substrate temperature control and the deposition of an SiO2 overlay. The SiO2 layer on top of the IDTs and quartz appears to reduce the frequency drift with temperature. The measurements on SH devices sometimes showed a slight drift for the oscillation frequencies. However, the differential frequencies were still very stable due to the dual-channel tracking and gave a similar level of noise. A parameter of great interest is the mass detection limit ( d l ) of the sensor. It can be estimated from the frequency noise level and the mass sensitivity. If we consider the mass loading required to create a frequency response three times greater than the r.m.s, noise level in the measurement system, we have dl=
(3 × noise level) (sensitivity × operation frequency)
For the noise of 2.4 Hz and the maximum sensitivity 380 cm 2 g - t which corresponds to an operation frequency = 110 MHz, the mass detection limit is dl
(3 × 2.4 Hz) (380cm2g_l×l10MHz)
1 7 2 p g c m -2
Any mass loading smaller than this figure could not be unambiguously resolved. (Note that some authors estimated the detection limit to be equal [9] to the noise level or twice [ 14] the noise level.) This detection limit is around three orders of magnitude smaller than the mass per unit area of a monolayer of gold ( = 400 ng cm-2) or of a typical organic layer coated using Langmuir-Blodgett techniques (e.g., cadmium arachidate has a mass per unit area = 300 ng cm-2 [ 16]). This suggests that the Love-wave devices reported here should be well able to detect a monolayer of most substances. It should be noted, however, that this mass-detection limit is relevant to a uniform mass-loading layer and cannot be used to determine the smallest number of molecules of a given species that would be detectable using these devices, when the number of molecules is insufficient to form a uniformly distributed covering across the width of the area being sensed by the acoustic wave.
J. Du et aL /Sensors and Actuators A 56 (1996) 211-219
217
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Fig. 6. (a) Measurementof the frequencyshih with time for both oscillationfrequencyand the difference:frequencyduring warm-up to = 3"PC. (b) An expansion of the differencefrequencyin (a). We measured the static temperature coefficient of frequency for the Love-wave devices with different SiO2 thicknesses. This was obtained by changing the temperature from room temperature to - - 40°C in steps and measuring the corresponding frequency shift. Each time, the temperature was altered and left to stabilize and then the frequency shift was measured. Fig. 7 shows the frequency shift versus temperature change at different layer thicknesses with linear fits to the data for each SiO2 film thickness. A linear and reversible relationship was found for temperatures in the range 2"7-40 °C. The coefficients of the frequency shift versus temperature (the slopes) were in the range 27-31 ppm ° C - I which included = 5 ppm ° C - t contributed by the electronics. It is noted that the differences in the coefficient of frequency shift versus temperature for the Love-wave devices with different ~icknesses of SiO2 are small, but with a trend towards an i,,crease in the coefficient as the film thickness increases. The SFI devices have temperature coefficients of frequency shift
in the same range but with slightly scattered values. The results showed that fairly uniform and low temperature coefficients of frequency shift could be obtained for various SiO2 thicknesses. It should be noted, however, that the chemical sensing layers applied to a SAW device can dramatically alter the temperature coefficient (see, for example, Ref. [ 17] ), so that the small coefficients measured here might be increased in any practical biosensing device that incorporates a selective coating.
5. Conclusions Love-wave acoustic devices based on an SiO2/ST-cut quartz system have been successfully manufactured and operated with the thickness of SiO2 ranging from 0 to "7.3/zm (0 to 0.18 of the wavelength). The mass sensitivity, velocity, insertion loss, frequency stability and noise level were stud-
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J. Du et al. / Sensors and Actuators A 56 (1996) 211-219
35o 300
s t ~" 150, 100,
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2
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6
8
10
Temperaturechange(C) Fig. 7. Frequency shift vs. temperature change at different layer thicknesses for the Love-wave devices. The slopes give the static temperature coefficient of frequency shift. ied. T h e sensitivity increased with layer thickness up to = 5.5 /Lm for a wavelength o f 40 /~m (normalized thickaless = 0 . 1 4 ) and then decreased dramatically with further increases in the thickness. The sensitivity agrees reasonably well with first-order perturbation theory for isotropic solids, before reaching the optimal thickness, and decreases m u c h faster than the theoretical prediction after the peak. A maxim u m sensitivity o f 380 c m 2 g - i was obtained for the wavelength o f 4 0 / t m and a sensitivity over 300 c m 2 g - i could be achieved at SiO2 thickness between 3.5 and 6.5/~m. In general, adding the SiO2 overlay increases the sensitivity to a large degree, enhances the electromechanical coupling and therefore reduces the insertion loss. A high stability o f freq u e n c y and low noise level were achieved for the whole range o f SiO2 thicknesses studied. O n e o f the potential benefits o f Love*wave devices is their usefulness for operating in liquid environments. W e therefore intend to extend the above work to include a similar systematic study o f Love-wave devices operating in liquids.
Acknowledgements T h e authors would like to thank Dr A.F. Collings and Dr D.C. Price for useful suggestions and c o m m e n t s on the manuscript.
References [ i ] H. Wohhjen, A.W. Snow, W.R. Barger and D.S. Ballantine, Trace chemical vapor detection using SAW delay line oscillators, IEEE Trans. Ultrasonics, Ferroelectrics Freq. Control, UFFC-34 (1987) 172-178.
[2] SJ. Martin. A,J. Ricco, T,M. Niemczyk and G.C. Frye, Cheractedsation of SH acoustic plate mode liquid sensors, Sensors and Actuators, 20 (1989) 253-268. [3] E. Gizeli, N.J. Goddard, C.R. Lowe and A.C. Stevenson, A Love plate biosensor utilising a polymer layer, Sensors and Actuators B, 6 ( 1992 ) 131-137. [4] J.C. Andle and J.F. Vetelino, Acoustic wave biosensors, Sensors and Actuators A, 44 (1994) 167-176. [5] J.C. Andle, J.T. Weaver, J.F. Vetelino and DJ. McAIlister, Selective acoustic plate mode DNA sensor, Sensors and Actuators B, 24-25 (1995) 129-133. [ 6] E. Gizeli, A.C. Stevenson,N.J. Goddard and C.R. Lowe, A novel Loveplate acoustic sensor utilizing polymer overlayers, 1EEE Trans. Ultrasonics, Ferroelectrics Freq. Control, UFFC- 39 ( ! 992 ) 657.-659. [7] G. Kovacs, G.W. Lubking, MJ. Vellekoob and A. Venema, Love waves for (bio)chemical sensing in liquids, Proc. IEEE Ultrasonics Syrup., Tucson, AZ, USA, 20-23 Oct., 1992, pp. 281-285. [ 8 ] G. Kovacs, MJ. Vellekoop, R. Haueis,G.W. Lubking and A. Venema" A Love wave sensor for (hio) chemical sensing in liquids, Sensors and Actuators A, 43 (1994) 38-43. [ 9] R. Haueis, MJ. Vellekoop, G. Kovacs, G.W. Lubking and A. Venema, A Love-wave based oscillator for sensing in liquids. Tech. Digest, 5th lnt. Meeting Chemical Sensors, Rome, 11-14 July, 1994, Vol. 1, pp. 126-129. [ 10l G. Kovacs and A. Venema. Theoretical comparison of sensitivities of acoustic shear wave modes for (bio)chemlcal sensing in liquids,Appl. Phys. Left., 61 (1992) 639--641. [ I I ] Z. Wang, J.D.N. Cheeke and C.K. Jan, Sensitivity analysis for Love mode acoestic gravimatric sensors, Appl. Phys. Left., 64 (1994) 29402942. [ 12] J. Endedein, E. Chilla and H-J. FrOhlich, Comparison of the mass sensitivityof Love and Rayleigh waves in a three-layer system.Sensors ond Actuators A~ 41-42 (1994) 472-475. [13] B.A. Auld, Acoustic Fields and Waves in Solids, Vol.ll. Krieger, Florida, 1990. [ 14] F. Josse, J.C. Andle, J.F. Vetelino, R. Dahint and M. Granse, Theoretical and experimental study of mass sensitivity of PSAWAPMs on ZX-LiNbO3, IEEE Trans. Ultrasonics, Ferroelectrics Freq. Control, UFFC-42 (1995) 517-524.
J. Du et al. /Sensors and Acmators A 56 (1996) 211-219
[ 15] J.A. Ogilvy,Approximateanalysisof wavesin layeredpiezoelectric plates froman interdigitalsourcetransducer,J. Phys. D: Appl. Phys.. (1995) acceptedfor publication. [ 16] R. Urguhart,personalcommunication(1995). [ 17] K. Bodenh6fer,A. Hierlemann,G. Noetzel,U. WeimarandW.G6pel, Comparisonof mass-sensitivedevicesfor gas sensing:bulkacoustic wave (BAW) and surfaceacousticwave (SAW) tmusducets,Tech. Digest, 9th Int. Conf. Solid-State Sensors and Actuators (Transducers '95)/ Eurosensors IX, Stockholm, Sweden, 25-29 June, 1995, pp. 728-
731.
Biographies Yia Du obtained her B.Sc. (1982) and M.Sc. (1984) in China, and received a Ph.D. (1993) from the University of Technology, Sydney, Australia. She worked as a research scientist and lecturer at the University of Electronic Science and Technology of China for four years and as an STA research fellow at Japan National Institute of Materials and Chemical Research for one year. She joined the CSIRO Division of Applied Physics in 1995 and is now working on surface acoustic wave (SAW) devices and biosensing applications. Geoffrey L. Harding is a graduate of Monash University, Australia (B.Sc. Hons., 1969) and the University of Cambridge, UK (Ph.D., 1973). Between 1974 and 1985, he worked as a lecturer at the University of Sydney, Australia, developing an advanced evacuated glass solar energy collector. He subsequently joined the CSIRO Division of Applied
219
Physics, where he is currently a priucipal research scientist, leading a group undertaking research and development of SAW transducers for bioseusing applications. .till Ogilvy is a theoretical physicist whose principal research interests are the physics of SAW devices, and ultrasonic non-destructivetesting (NDT). She obtained a physics degree from Oxford University and a Ph.D in mathematics from the University of Bath (UK). She has been working as a senior research scientist at the CSIRO Division of Applied Physics for three years. Prior to this she was engaged on research for I I years at Harwell Labormory (UK), principally concerned with woblems related to NDT. Peter R. Dencher is a senior technical officer at the CSIRO Division of Applied Physics. His current work is in the design and construction of electronics for research purposes in the division's ultrasonic project groups. MichaelLake obtained his B.Sc. ( 1981 ) and Ph.D. (1989) from the University of Sydney, Australia. After graduation, he worked in AWA MicroElectronics Pry Ltd for three years, where he was reponsible for plasma oxide deposition and ionimplantation processes. He then worked in CSIRO Division of Applied Physics for three years. His research interests included surface texturing of metals by sputtering, plasma deposition of amorphous silicon and SAW sensors. He is currently at the University of Technology, Sydney, Australia, working on sensor applications of conducting polymers.
Sensorsand ActuatorsA 56 (1996) 21 !-219
A study of Love-wave acoustic sensors J. D u , G . L . H a r d i n g , J . A . O g i l v y , P . R . D c n c h c r , M . L a k e CSIRO Division of Applied Physics, PO Box 218, Lindfield, NSW 2070, Australia
Received19 December1995;t~zvised5 March 1996;accepted19 Match 1996
Abstract Love-mode acoustic devices are very promising as biosensors in gaseous and liquid environments because of their high sensitivity. An experimental study of Love-wave devices based on SiO2/ST-cat quartz, over a wide range of SiO2 thickness, is presemed it, this paper. Devices with up to 7.3/~m thick SiO2 guiding layers have been successfully manufactured via an r.f. sputtering technique. Mass sensitivity, velocity, insertion loss, oscillation frequency s:ability and temperature coefficient of the frequency have been studied as a function of layer thickness. The sensitivity increases with increasing layer thickness and reaches a maximum at around 5.5 tam, for a wavelength of 40/an, in accordance with theory. Further increasing the thickness decreases the sensitivity dramatically. High sensitivity ( >300 cmz g-~) can be achieved at thicknesses between 3.5 and 6.5/.tm. The Love-wave dual-channel delay-fine osciilators also demonstrate high frequency stability and low noise levels. The frequencies of the two channels track each other extremely well. Keywords: Acousticsensors;Lovewaves;Masssensitivity;Biosensors
1. Introduction In recent years, there has been great interest in using surface acoustic wave (SAW) devices for detection of both physical and chemical analytes. The devices offer a simple and inexpensive technique for sensing applications, since interactions between the acoustic waves and the mass density, elastic stiffness and electric/dielectric properties of the propagation medium can give rise to a sensing response. Any change in these properties leads to changes in the velocity and amplitude of aco'.~stic wave modes. For detection of the velocity change, the acoustic wave sensing element is usually realized as a delay line or resonator and serves as a positive feedback element in an oscillator, with the oscillation frequency changing as the velocity changes. As the acoustic energy is confined to a thin near-surface region of substrata, SAWs are highly sensitiv~ to surface perturbation of the propagation medium. One of the most important detection quantities for SAW sensors is the surface mass change. A small mass loading on the surface of the acoustic propagation medium can be detected by a highly sensitive SAW device. A (bio)chemical sensor is usually obtained by coating a selective chemical interface or biological material on the surface of the acoustic element. The adsorption of a chemical compound or biological species at the active surface changes the surface mass density, leading to a change in wave velocity which is detected as a frequency or insertion-loss change (see, for example, Refs. [ I-5] ). 0924-4247/96/$15.00 © 1996ElsevierScienceS.A.All fightsn:se~ed P l i S 0 9 2 4 - 4 2 4 7 ( 9 6 ) 0 1 3 1 I-8
For sensing in liquid environments, there is a strong radiation loss into the liquid for longitudinal bulk modes, Rayleigh surface waves and most Lamb-wave modes. Shear horizontal (SH) polarized wave modes are preferred since they do not couple elastically with ideal (inviscid) liquids. For increased sensitivity, Love waves, which are SH-polarized guided waves, may be used. These waves propagate in a layered structure consisting of a substrata and a layer on top of it. The layer acts as a guide, with the elastic waves generated in the substrata being coupled to the surface guiding layer. Because of the wavegniding effect, these waves can be very sensitive to surface perturbations, and high sensitivity to surface loading can be achieved. Since Love waves do not have elastic coupling loss in liquids and the guiding layer also protects the interdigital transducers (IDTs) from liquid or chemical environments, they are particularly attractive for sensing in liquids. Love-wave sensors will most likely find applications in chemical analysis, food technology, environment protection and clinical diagnosis. Several Lovemode sensors operating in both gaseous and liquid media have been demonslrated [ 3,6-9]. A condition for the existence of Love-wave modes is that the shear velocity in the layer is smaller than the shear velocity in the substrata. It is important to choose an overlay material which has low shear velocity, low density and low acoustic absorption. Fused silica (SiO2) [7-9] and polymers, typically polymethyl-methacrylate (PMMA) [3,6], have been
212
J. Du et aL /Sensors and ActuatorsA 56 (1996) 211-219
used to construct the layered structure for Love-mode sensors. Polymers, though having low density and low shear velocity, are usually lossy. The acoustic absorption increases quickly with increasing layer thickness [6] and this limits the sensitivity gain. For the quartz substrate being considered here, SiO2 satisfies the condition for the existence of a Love wave and has excellent elastic and thermal properties and, therefore, is considered to be an ideal overlay material. Acoustic losses in SiO2 are very low in comparison with polymers. SiO2 is also resistant to water and most chemical degradation and has excellent abrasion resistance. Relatively high mass sensitivity has been achieved u~ing a SiO2/quartz configuration [7,8]. Theoretical calculations [7,10-12] have shown that the sensitivity of Love modes strongly depends on the thickness of the guiding layer and there exists an optimal thickness, for a given wavelength, to achieve maximum sensitivity. However, experimental realization of the optimal thickness for the guiding layer has not yet, to our knowledge, been achieved. The reported maximum thicknesses are far from the calculated optimal thickness. Kovacs et al. [7,8] obtained up to 1.46/~m for the SiO2 layer on quartz, for wavelengths in the range 40-16/~m (normalized thicknesses in the range 0.037-0.091). Deposition of a thicker SiO2 film had failed due to technical problems probably related to film adhesion and stresses. Gizeli et al. [3,6] obtained 1.6/zm fo~ PMMA overlay material. Further increases in the thickness led to rapid increases in the acoustic absorption, and therefore in the insertion loss, and hence limited the maximum usable film thickness. It is desirable to obtain thicker layers for higher sensitivity. Experimental confirmation of the relationship of sensitivity versus layer thickness is also necessary to optimize the sensor design to give maximum detection sensitivity. In the present work, we have successfully obtained Lovewave devices based on SiO2/ST-cut quartz structures, with the SiO2 layer ranging from 0 to 7.3/tm, which covers the optimal thickness for maximum sensitivity for the wavelength we are using (40 ftm). Mass sensitivity, velocity, insertion loss, frequency stability and temperature coefficient have been studied as functions of the layer thickness. To the best of our knowledge, there is no report in the literature on successful manufacture, operation and measurement of mass sensitivity and other related properties for Love-wave devices with such a wide range of layer thickness covering the optimal thickness for peak sensitivity. The fabrication, mass-loading technique and the experimental results on mass sensitivity, velocity, insertion loss, frequency stability and temperature coefficient are described in this paper.
2. Theory Fig. I shows schematically the geometry of the Love-wave devices considered in this paper. A substrate of ST-cutquartz, 0.5 mm thick, is overlayed by sputtered SiO2. A Love wave propagates in this structure, travelling perpendicular to the
h
/'~, " ~ - ~ : ~VM.ss,oad,.S - ' ~ ........ ~,~ I /,," j ' - " /I .,"
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crystallographic X axis, which is itself parallel to the surface, with the wave polarized parallel to this axis (the waves are purely shear in nature for this orientation of ST-quartz). For a detailed description of Love waves and their properties, see, for example, Auld [ 13], chapter 10. The IDTs (not shown in the Figure) are at the interface between the SiO2 and the quartz. The distribution of Love-wave energy through the depth of the plate is dependent on the ratio of the overlay thickness, h, to the wavelength, ~o. For small values of this ratio (h/~o < 0.06) the wave is partially confined within the layer, but also has considerable energy within the quartz. As the ratio increases (to h/~o ~ O. 14), the wave is increasingly confined in the layer, with a large normalized wave amplitude (normalized relative to the total energy in the wave) at the surface. Further increases in the layer thickness lead to further confinement of energy within the layer, but with a smaller normalized amplitude at the top surface. (We see below that it is this normalized amplitude which determines the sensitivity of a Love wave to mass loading.) For thicker layers, more than one Love-wave mode may be supported. All of the measurements reported in this paper are for the zeroth-order Love mode. Fig. 1 also shows a thin, uniform, solid film on the SiO2 layer, taken to be a mass-loading layer (this may represent, for example, a chemical species that binds to a chemically selective surface). The thickness of the film is assumed to be much less than the acoustic wavelength, so that first-order perturbation theory may be used to calculate the effect of the film on the Love-wave velocity. In this approximation, the acoustic field within the film is assumed to be uniform through the depth of the film, allowing an explicit expression to be derived for the fractional change in wave velocity (see Auld [13], chapter 12). This takes the following form for Love waves: _ _ = _ Vp,~V. 2 2 Af 1 _ (_V__~._m~1113i, =o fo
4 eUL
(I)
~v,] I
where Af ( = f - - f o ) is the change in frequency due to the mass loading of the thin film, d is the film thickness, p is the film density, Vmis the bulk shear wave velocity in the film and Vp is the phase velocity of the Love wave without mass loading. The quantity I ~1~.o is the particle velocity at the surface of the SiO2, norraalized with the total energy density (per unit width of wave) throughout the plate thickness. We see that the sensitivity of a wave to a mass-loading layer is dependent on the mass loading per unit area (pd), the normalized par-
J. Du et al. /Sensors and Actuators A 56 (1996) 211-219
ticle velocity at the surface (so that the greater the confinement of the wave energy close to the surface, the larger the sensitivity), and on the o,~.~tity 1 - (Vm/]/'p) 2. This latter represents a reduction in m-',~s-loading sensitivity because of the elastic effect of the mass-loading layer itself. If this layer does not support acoustic shear waves (because, for example, it is macroscopically discontinuous) then Vmffi0 and this quantity is unity. The Love-wave velocity for the system without the massloading layer, Vp, may be determined from the dispersion equation for these waves (this equation relates the wave velocity to the frequency). We have assumed that the system may be modelled as an isotropic layer on an isotropic substrate of infinite extent in the - z direction of Fig. 1. An explicit expression may be derived for the dispersion equation of this system, solution of which provides us with the required value for Vp and hence for the mass sensitivity. The details of this calculation are not given here, as they may be found in, for example, Ref. [ 11 ] and Ref. [ 13], chapter 10. The sensitivity to mass loading is defined in this paper to be Sm = L i m - - ~ aM~ofo AM/A
(2)
where AM/A is the mass per unit area deposited on the surface. Using Eq. (1) we see that the sensitivity may be written $m = - -
1--
I01~-0
(3)
since Od ffi ~MIA. We see that according to first-order perturbation theory the sensitivity of a given Love wave to mass loading is independent of the mass of the loading layer, and independent of the area over which the m ~ s is loaded, as the film is assumed to be uniform over the whole area under which the wave is propagating. If the elastic effects of the mass-loading layer are ignored (Vmffi0) then the sensitivity is also independent of the material being deposited, and becomes an intrinsic property of the Love wave being considered. In practice, the concept of uniform mass loading over the whole acoustic wave path is difficult to apply to a biosensor device of conventional design, because of the finite size of the IDTs and of the 'sweet spot' ( the area between the IDTs). If a layer of uniform thickness is applied to the 'sweet spot' only, then the effective mass per unit area for the whole wave path is less than the true mass per unit area, as when the wave is travelling under the IDTs it will not experience mass loading. However, the total effective area is less than the sum of the 'sweet spot' and IDT areas, as we would expect some reduction in mass-loading sensitivity towards the rear edges of the IDT areas. There is therefore some ambiguity in the definition of the area to be used in determining the quantity Sm for practical biosensor devices. We have defined the effective area to be the sum of the 'sweet-spot' area and one half of the area under the IDTs. If the mass is applied only to the
213
'sweet spot' or a fractional effective area, it is necessary to use the full effective area when calculating the sensitivity S,~. In this way, the obtained device sensitivity should be independent of the area that is mass loaded. The exact definition of the effective area is a complicated issue and further work is needed to determine this quantity.
3. Experlmenbd
3.1. Devicefabrication The Love-wave devices were fabricated on 20 m m × 19 mm and 0.5 mm thick ST-cut quartz subswates, with propagation perpendicular to the crystallographic X axis. The transmit and receive IDTs consisted of 75 finger pairs wi~:h l 0 / a n width of electrode and l 0 / m l separation, i.e., a periodicity of 40/~m. The finger pairs were obtained by d.c. sputtering = 240 nm chromium onto the quartz substratc, followed by patterning with photolithographic techniques. The IDT aperture was 3 nun and the IDT centre-to-centre separation was 6 mm. This device supports an SH mode at a frequency of 124 MHz. The initia~ insertion losses of SH devices were around 23 dB. An SiO2 layer was then deposited on each SH device via an r.f. magnetron sputtering technique, with a silicon target reacting in the mixed gases of oxygen and argon. The area of SiOz was 12 ram× 12 mm, defined by a mask made from thin stainless-steel sheet. A specimen of quartz was attached to each device for thickness measurement. The thicknesses of the SiO2 layers were measured with a Dektak 3030 Surface Profile Measurement System. A range of different SiO2 thicknesses up to 7 . 3 / ~ n were obUdned. The films adhered to the IDTs and quartz substra'e very well and appeared to have low internal stresses. There were no delamination problems even for very thick films, suggesting that thicker films could be made using this technique. We believe that using chromium as the electrode material is a key factor in ensuring good adhesion between the SiO2 film and IDTs/ substrate, since it generally has excellent adhesion properties, particularly to materials containing oxygen.
3.2. Delay-line oscillator configuration Two identical Love-wave delay lines were fabricated on the same quartz substrate to form a dual-channel delay-line oscillator as shown in Fig. 2, one channel to be used as sensing element and the other as the reference channel. Two broadband amplifiers were used to feed energy back from the receive IDT to the transmit IDT to compensate for the loss of the delay lines and to maintain oscillation. The amplifiers were built into a small module into which the quartz substrate and associated heater unit were plugged. This system was then plugged into a unit that provides power and temperature control, mixing of the two channel frequencies to produce a difference frequency, and buffering of all three frequencies to minimize the effects of measuring equipment on the oscil-
214
J. Du et al. /Sensors and Actuators A 56 (1996) 211-219
Amplifier
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lation frequencies. The difference frequency from the control unit provides a signal that is compensated for ambient temperature change and easier to measure to high resolution than the oscillation frequencies.
3.3. Measurement techniques The operation frequencies were obtained either by measuring the oscillation frequency with a frequency counter or by recording the peak resonance frequency of the delay line with a signal spectrum analyser. The propagation velocities are given by v =foAo, where Ao is the wavelength defined by the periodicity of the IDTs. Insertion losses were obtained by monitoring the resonance frequency of the transmission signal and recording the signal loss with a Tektronix 2710 Spectrum Analyscr. To determine the mass sensitivity, a convenient and reproducible technique for surface mass loading has been developed. The delay-line oscillator was set up in a vacuum chamber and an ultra-thin ( < 2 nm) gold film was deposited on the 'sweet spot' of each device using a d.c. sputtering technique. The area over which mass was deposited was 2.5 mm × 3 mm, defined by a mask made of thin polyimide plastic. The frequency change due to mass loading was measured by recording the oscillation frequencies immediately before and after gold deposition and the sensitivity was then calculated using Eq. (2). By calibrating the gold deposition rate and the area coated, the amount of mass deposited on th.~ devices could be precisely controlled. SEM and AFM microstructure examinations of the ultra-thin Au films suggested that the films were formed as microscopic uniformly distributed gold islands and therefore were not electrically conducting and were not likely to act as eiastically guiding layers themselves. The deposited gold was easily removed by gentle abrasion with cotton tips after measurement without damage to the surface of the SiO2 and the experiment could be repeated. In comparison with the mass-loading techniques of spin-coating photoresist [7,8,14] and Langmuir-Blodgett (LB) films [ 1,3,6] used by other authors, this technique has the advantage of ease of control of small amounts of mass loading and is highly reproducible. The added layer is very
thin compared with the SiO2 and quartz thicknesses and can be regarded as a small perturbation, as assumed in the theoretical calculations. However, this technique is not suitable for studying mass sensitivity in a liquid medium. Measurements were usually carded out by making three consecutive gold depositions for each delay line and the mean values of the frequency changes were taken. The frequency change versus the added mass showed a linear relationship for the Love-wave devices, for up to six consecutive depositions. Separate runs also gave reproducible results with small standard deviations. The frequency stability was determined by monitoring the frequency shift over a period of time (e.g., 30 rain) during operation. The measurement temperature was usually set between 28 and 40°C via a specially designed temperaturecontrol unit and the devices were heated up and stabilized at this temperature. This minimized frequency drift due to heating from the oscillator electronics. The frequencies of both channels and the difference frequency were measured. The measurement procedure was controlled and the data were recorded using a computer. The noise level was determined from the difference frequency versus time by calculating the r.m.s, fluctuation in the frequency.
4. Results and discussion The Love-wave propagation velocity as a function of the thickness of SiO2 layer is shown in Fig. 3. The individual data points are marked, together with a linear fit to the data. The data at zero thickness are for an SH device and are close to the expected shear wave velocity in quartz, along the x direction of Fig. 1 (5060 m s - t). The velocity decreases with increasing SiO2 thickness as expected, since the shear wave velocity in SiO2 (2850 m s - t ) is smaller than the shear velocity in the substrate and the SiO2 overlay slows down the
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2
3
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4
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Fig. 3. Love-wave propagation velocity vs. SiO~ thickness.
J. Du et aL I Sensors and Actuators A 56 f1996) 21 i-219
II
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]
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wave propagation. If the thickness is further increased, the velocity would approach the velocity in SiO2. The variation of velocity with SiO2 thickness is not expected to be linear over a wide range of thickness values (see, for example, Ref. [7] ) but is closely linear over the range we have examined. It is interesting to note that the measured velocity decrease for a given film thickness/wavelength ratio is less than the corresponding decrease measured and predicted in Ref. [7], Figure 5, suggesting that the SiO2 films which we have manufactured have a larger shear velocity and/or density than the film manufactured in Ref. [7]. A separate measurement of the film density gave the value 2.34 (+0.05) × 103 kg m -3. 6% higher than the value quoted in Ref. [7]. Measurement Oi
I i
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/
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of the shear wave velocity of the SiO2 film, which has not yet been pursued, would provide more information. The insertion losses for Love-wave delay lines with different layer thicknesses are given in Fig. 4. With increasing thickness, the insertion loss decreases quickly first, reaches a minimum and then increases again with further increasing thickness. The insertion losses for all Love-wave devices were smaller than those of SH devices. The decrease of the insertion loss for Love-wave modes results from effic~.enz guiding of acoustic energy in the overlay and enhancement of the electrom¢chanical coupling of elastic waves by IDTs. However, strong coupling can only be obtained when the maximum acoustic field is near the substratc-laycr inc.'face [7] where the IDTs are located. At larger thickness ( > 2.5 /zm), more acoustic energy is guided in the top layer and the maximum field distribution is shifted away from the interface. This leads to a decease in coupling again and the insertion loss increases. Minimum insertion loss occurs at h ffi2-2.5 /zm for our Love-wave devices with a wavelength of 40 pln, agreeing qualitatively with theory (Ref. [7], Figure 3), and the minimum insertion loss was = 10 dB. However, the position of minimum insertion loss also depends on the material of the IDTs. Fig. 5 shows the mass sensitivity defined according to Eq. (2) versus SiO2 thickness. The sensitivity is negative because the velocity always decreases due to mass loading. Two sets of data were for the devices in two separate batches, each with an SiO2 layer formed from one continuous deposition, and one set of data was from a single device with the SiO2 layer built up in steps. The same trend is obtained for either method of SiO2 deposition. The spread of data at each nominal SiO2 thickness is likely to be caused by the difference in actual SiO2 thickness for the two channels on the same sub[
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-5O0 SlO, ~ l c k n ~ m (urn) Fig. 5. Mass-loading sensitivity vs. SiO2 thickness. The solid and dashed lines are the theoretical predictions ignoring the elasticity o f the gold film and with elasticity included, respectively.
216
J. Du et al. /Sensors and Actuators A 56 (1996) 211-219
strate, as the distance between the device and the silicon target was small ( = 4 0 mm) and therefore a small gradient in the film thickness could have occurred during the deposition process if the device was not well centred. The solid and dashed lines are the theoretical predictions, as described in Section 2. The solid line is the prediction ignoring the elasticity of the gold film, the dashed line is with elasticity included (assuming a uniform gold film). The material properties used in the calculations were taken from Wang et al. [ 11 ] except for the film density, which was increased to 2.34 x i03 kg m -3 (our measurement data). The datum at h = O is for an SH device. The sensitivity increases quickly with increasing layer thickness, reaches a maximum at -- 5.5 /~m (a normalized thickness = 0.14 ) and then decreases with further increase in thickness. High sensitivities ( ~ 300 cm 2 g - ;) were obtained between 3.5 and 6.5/~m for the wavelength of 40/~m, i.e., for normalized thickness of 0.09-0.16. The maximum mass-loading sensitivity ( -- 380 cm 2 g - ~), compared with = 15 cm 2 g - t for SH devices, is 25 times better than that of the SH devices. To our knowledge, it is the highest mass sensitivity reported among various SAW and guided-wave sensors, at this wavelength. The high sensitivity for the Love modes is due to the strong waveguiding effect of the layer. More acoustic energy propagates near the surface and this increases the sensitivity to surface mass perturbation. At larger thicknesses ( ~ 15% of the wavelength), the sensitivity decreases again as the wave becomes more evenly distributed through the thickness of the layer, so that surface displacements are reduced. We note here that the high sensitivities for SiO2/quartz reported in Refs. [7,8] were actually for devices with a waveguide consisting of SiO2 and photoresist layers. In this case, the photoresist acts as an efficient waveguide and thus increases the sensitivity. The experimental data agree reasonably well with theoretical prediction for SiO2 layer thickness up to = 5/zm. After the peak, however, the sensitivity seems to decrease with thickness significantly faster than the theoretical prediction. The reason for this is not clear. It muv~ be remembered that the theory is for isotropic solids, and t~erefore neglects anisotropic and piezoelectric effects in calculating the wave profiles in the layer and substrate, and also assumes that the substrate is of infinite thickness. A~ :he SiO2 layer thickness increases, this latter assumption will become decreasingly valid. This may account for some of the observed discrepancy. A model based on the solution of the full piezoelectric equations is being developed [ 15] and will be used to study this discrepancy. We measured the short-term stability of the oscillation frequency for Love-wave sensors. This can be used to assess the frequency stability, channel tracking, noise level and mass detection limit. Fig. 6(a) shows a typical result of the frequency shift with time during warm-up to a set temperature (37°C in this case) and for a period of time at constant temperature. Fig. 6(b) is an expansion of the difference frequency, which gives a clearer picture of the noise level. After initial switching-on, the frequency increased quickly with
temperature and usually stabilized at the set temperature within five minutes. From the Figure, the oscillation frequency was very stable and showed almost no drift after the temperature stabilized. The frequencies of the two channels tracked each other extremely well during the temperature change, which is clearly shown by the very small shift ( < 3 ppm) of the difference frequency. The noise level measured from the difference frequency shift was 0.02 ppm (r.m.s.) or 2.4 Hz. Our measurements have shown that all of the Lovewave delay lines for which we made measurements, with different SiO2 thicknesses, have similar frequency stability and noise level to those shown in Fig. 6. In all cases, the two channels tracked each other very well and showed little frequency drift. The noise levels were in the range 0.01-0.02 ppm (r.m.s.), which correspond to the frequency change in the range 1.2-2.4 Hz. The high frequency stability and low noise are due to a stable laboratory temperature combined with substrate temperature control and the deposition of an SiO2 overlay. The SiO2 layer on top of the IDTs and quartz appears to reduce the frequency drift with temperature. The measurements on SH devices sometimes showed a slight drift for the oscillation frequencies. However, the differential frequencies were still very stable due to the dual-channel tracking and gave a similar level of noise. A parameter of great interest is the mass detection limit ( d l ) of the sensor. It can be estimated from the frequency noise level and the mass sensitivity. If we consider the mass loading required to create a frequency response three times greater than the r.m.s, noise level in the measurement system, we have dl=
(3 × noise level) (sensitivity × operation frequency)
For the noise of 2.4 Hz and the maximum sensitivity 380 cm 2 g - t which corresponds to an operation frequency = 110 MHz, the mass detection limit is dl
(3 × 2.4 Hz) (380cm2g_l×l10MHz)
1 7 2 p g c m -2
Any mass loading smaller than this figure could not be unambiguously resolved. (Note that some authors estimated the detection limit to be equal [9] to the noise level or twice [ 14] the noise level.) This detection limit is around three orders of magnitude smaller than the mass per unit area of a monolayer of gold ( = 400 ng cm-2) or of a typical organic layer coated using Langmuir-Blodgett techniques (e.g., cadmium arachidate has a mass per unit area = 300 ng cm-2 [ 16]). This suggests that the Love-wave devices reported here should be well able to detect a monolayer of most substances. It should be noted, however, that this mass-detection limit is relevant to a uniform mass-loading layer and cannot be used to determine the smallest number of molecules of a given species that would be detectable using these devices, when the number of molecules is insufficient to form a uniformly distributed covering across the width of the area being sensed by the acoustic wave.
J. Du et aL /Sensors and Actuators A 56 (1996) 211-219
217
4oo
(a) 3SO
'!l
¢ ~ n : e faKwe~y
J 1o
o
is
3o
2O
s
":t 1.s
~
/--
3
frr~y
/
1 o.s
o
i s
) lo
i is
i 2o
i 2s
3o
Tin, (r~)
Fig. 6. (a) Measurementof the frequencyshih with time for both oscillationfrequencyand the difference:frequencyduring warm-up to = 3"PC. (b) An expansion of the differencefrequencyin (a). We measured the static temperature coefficient of frequency for the Love-wave devices with different SiO2 thicknesses. This was obtained by changing the temperature from room temperature to - - 40°C in steps and measuring the corresponding frequency shift. Each time, the temperature was altered and left to stabilize and then the frequency shift was measured. Fig. 7 shows the frequency shift versus temperature change at different layer thicknesses with linear fits to the data for each SiO2 film thickness. A linear and reversible relationship was found for temperatures in the range 2"7-40 °C. The coefficients of the frequency shift versus temperature (the slopes) were in the range 27-31 ppm ° C - I which included = 5 ppm ° C - t contributed by the electronics. It is noted that the differences in the coefficient of frequency shift versus temperature for the Love-wave devices with different ~icknesses of SiO2 are small, but with a trend towards an i,,crease in the coefficient as the film thickness increases. The SFI devices have temperature coefficients of frequency shift
in the same range but with slightly scattered values. The results showed that fairly uniform and low temperature coefficients of frequency shift could be obtained for various SiO2 thicknesses. It should be noted, however, that the chemical sensing layers applied to a SAW device can dramatically alter the temperature coefficient (see, for example, Ref. [ 17] ), so that the small coefficients measured here might be increased in any practical biosensing device that incorporates a selective coating.
5. Conclusions Love-wave acoustic devices based on an SiO2/ST-cut quartz system have been successfully manufactured and operated with the thickness of SiO2 ranging from 0 to "7.3/zm (0 to 0.18 of the wavelength). The mass sensitivity, velocity, insertion loss, frequency stability and noise level were stud-
218
J. Du et al. / Sensors and Actuators A 56 (1996) 211-219
35o 300
s t ~" 150, 100,
50.
o~
2
4
6
8
10
Temperaturechange(C) Fig. 7. Frequency shift vs. temperature change at different layer thicknesses for the Love-wave devices. The slopes give the static temperature coefficient of frequency shift. ied. T h e sensitivity increased with layer thickness up to = 5.5 /Lm for a wavelength o f 40 /~m (normalized thickaless = 0 . 1 4 ) and then decreased dramatically with further increases in the thickness. The sensitivity agrees reasonably well with first-order perturbation theory for isotropic solids, before reaching the optimal thickness, and decreases m u c h faster than the theoretical prediction after the peak. A maxim u m sensitivity o f 380 c m 2 g - i was obtained for the wavelength o f 4 0 / t m and a sensitivity over 300 c m 2 g - i could be achieved at SiO2 thickness between 3.5 and 6.5/~m. In general, adding the SiO2 overlay increases the sensitivity to a large degree, enhances the electromechanical coupling and therefore reduces the insertion loss. A high stability o f freq u e n c y and low noise level were achieved for the whole range o f SiO2 thicknesses studied. O n e o f the potential benefits o f Love*wave devices is their usefulness for operating in liquid environments. W e therefore intend to extend the above work to include a similar systematic study o f Love-wave devices operating in liquids.
Acknowledgements T h e authors would like to thank Dr A.F. Collings and Dr D.C. Price for useful suggestions and c o m m e n t s on the manuscript.
References [ i ] H. Wohhjen, A.W. Snow, W.R. Barger and D.S. Ballantine, Trace chemical vapor detection using SAW delay line oscillators, IEEE Trans. Ultrasonics, Ferroelectrics Freq. Control, UFFC-34 (1987) 172-178.
[2] SJ. Martin. A,J. Ricco, T,M. Niemczyk and G.C. Frye, Cheractedsation of SH acoustic plate mode liquid sensors, Sensors and Actuators, 20 (1989) 253-268. [3] E. Gizeli, N.J. Goddard, C.R. Lowe and A.C. Stevenson, A Love plate biosensor utilising a polymer layer, Sensors and Actuators B, 6 ( 1992 ) 131-137. [4] J.C. Andle and J.F. Vetelino, Acoustic wave biosensors, Sensors and Actuators A, 44 (1994) 167-176. [5] J.C. Andle, J.T. Weaver, J.F. Vetelino and DJ. McAIlister, Selective acoustic plate mode DNA sensor, Sensors and Actuators B, 24-25 (1995) 129-133. [ 6] E. Gizeli, A.C. Stevenson,N.J. Goddard and C.R. Lowe, A novel Loveplate acoustic sensor utilizing polymer overlayers, 1EEE Trans. Ultrasonics, Ferroelectrics Freq. Control, UFFC- 39 ( ! 992 ) 657.-659. [7] G. Kovacs, G.W. Lubking, MJ. Vellekoob and A. Venema, Love waves for (bio)chemical sensing in liquids, Proc. IEEE Ultrasonics Syrup., Tucson, AZ, USA, 20-23 Oct., 1992, pp. 281-285. [ 8 ] G. Kovacs, MJ. Vellekoop, R. Haueis,G.W. Lubking and A. Venema" A Love wave sensor for (hio) chemical sensing in liquids, Sensors and Actuators A, 43 (1994) 38-43. [ 9] R. Haueis, MJ. Vellekoop, G. Kovacs, G.W. Lubking and A. Venema, A Love-wave based oscillator for sensing in liquids. Tech. Digest, 5th lnt. Meeting Chemical Sensors, Rome, 11-14 July, 1994, Vol. 1, pp. 126-129. [ 10l G. Kovacs and A. Venema. Theoretical comparison of sensitivities of acoustic shear wave modes for (bio)chemlcal sensing in liquids,Appl. Phys. Left., 61 (1992) 639--641. [ I I ] Z. Wang, J.D.N. Cheeke and C.K. Jan, Sensitivity analysis for Love mode acoestic gravimatric sensors, Appl. Phys. Left., 64 (1994) 29402942. [ 12] J. Endedein, E. Chilla and H-J. FrOhlich, Comparison of the mass sensitivityof Love and Rayleigh waves in a three-layer system.Sensors ond Actuators A~ 41-42 (1994) 472-475. [13] B.A. Auld, Acoustic Fields and Waves in Solids, Vol.ll. Krieger, Florida, 1990. [ 14] F. Josse, J.C. Andle, J.F. Vetelino, R. Dahint and M. Granse, Theoretical and experimental study of mass sensitivity of PSAWAPMs on ZX-LiNbO3, IEEE Trans. Ultrasonics, Ferroelectrics Freq. Control, UFFC-42 (1995) 517-524.
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[ 15] J.A. Ogilvy,Approximateanalysisof wavesin layeredpiezoelectric plates froman interdigitalsourcetransducer,J. Phys. D: Appl. Phys.. (1995) acceptedfor publication. [ 16] R. Urguhart,personalcommunication(1995). [ 17] K. Bodenh6fer,A. Hierlemann,G. Noetzel,U. WeimarandW.G6pel, Comparisonof mass-sensitivedevicesfor gas sensing:bulkacoustic wave (BAW) and surfaceacousticwave (SAW) tmusducets,Tech. Digest, 9th Int. Conf. Solid-State Sensors and Actuators (Transducers '95)/ Eurosensors IX, Stockholm, Sweden, 25-29 June, 1995, pp. 728-
731.
Biographies Yia Du obtained her B.Sc. (1982) and M.Sc. (1984) in China, and received a Ph.D. (1993) from the University of Technology, Sydney, Australia. She worked as a research scientist and lecturer at the University of Electronic Science and Technology of China for four years and as an STA research fellow at Japan National Institute of Materials and Chemical Research for one year. She joined the CSIRO Division of Applied Physics in 1995 and is now working on surface acoustic wave (SAW) devices and biosensing applications. Geoffrey L. Harding is a graduate of Monash University, Australia (B.Sc. Hons., 1969) and the University of Cambridge, UK (Ph.D., 1973). Between 1974 and 1985, he worked as a lecturer at the University of Sydney, Australia, developing an advanced evacuated glass solar energy collector. He subsequently joined the CSIRO Division of Applied
219
Physics, where he is currently a priucipal research scientist, leading a group undertaking research and development of SAW transducers for bioseusing applications. .till Ogilvy is a theoretical physicist whose principal research interests are the physics of SAW devices, and ultrasonic non-destructivetesting (NDT). She obtained a physics degree from Oxford University and a Ph.D in mathematics from the University of Bath (UK). She has been working as a senior research scientist at the CSIRO Division of Applied Physics for three years. Prior to this she was engaged on research for I I years at Harwell Labormory (UK), principally concerned with woblems related to NDT. Peter R. Dencher is a senior technical officer at the CSIRO Division of Applied Physics. His current work is in the design and construction of electronics for research purposes in the division's ultrasonic project groups. MichaelLake obtained his B.Sc. ( 1981 ) and Ph.D. (1989) from the University of Sydney, Australia. After graduation, he worked in AWA MicroElectronics Pry Ltd for three years, where he was reponsible for plasma oxide deposition and ionimplantation processes. He then worked in CSIRO Division of Applied Physics for three years. His research interests included surface texturing of metals by sputtering, plasma deposition of amorphous silicon and SAW sensors. He is currently at the University of Technology, Sydney, Australia, working on sensor applications of conducting polymers.