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Evaluation of liquid properties using a silicon lamb wave sensor
175
Sensorsand Actuators A, 43 (1994) 175-180
Evaluation of liquid properties using a silicon Lamb wave sensor M.J. Vellekoop”, G.W. Lubkingb, P.M. Sarro’ and A. Venemab ‘Xensor Integration bv, PO Box 3233,2601DE De& (Netherian@ %elft Universi@ of Technology, Labomtory for Ekctmnic hbwnentation, ‘DIMES, PO Box 5053, 2600 GB De& (Netherlands)
PO Box 5031, 2600 GA De@ (Netherlands)
Abstract The design and realization of a Lamb wave oscillator sensor system are described. Both the oscillation frequency and the amplifier gain level are monitored, giving information about the Lamb wave velocity and amplitude, respectively. The system is fabricated in two separate silicon chips (hybrid set-up) or in one chip (monolithic set-up), and can be used for different sensor applications, in liquid or gas environment. We demonstrate the sensor for detection of the density and viscosity of a liquid, and for sensing of the adsorption of human serum albumin (HSA) at the sensor surface.
Introduction based on the acoustic zero-order antisymmetric (A,) Lamb wave mode have gained interest because of their inherent high sensitivity and broad application possibilities. The slow A, Lamb wave shows low acoustic losses when contacted by a liquid and the frequency of operation is relatively low, typically between 5 and 20 MHz. In addition, Lamb wave devices can be fabricated in silicon by using micromachinmg techniques [l, 21. The monolithic integration of acoustic device and electronic circuitry is then a logical sequel, resulting in a smart sensor system, givmg all the advantages of the IC technology; reproducibility, reliability, small size and low production costs. Both physical and (bio)chemical sensors based on acoustic Lamb waves have been reported, e.g. for liquid viscosity and density, and particle detection in gases as well as in liquids [3-71. Two quantities of the wave are (independently) sensitive for changes at the surface of the plate in which it propagates: the phase velocity and the amplitude. By using the Lamb wave device in a gain controlled oscillator, both wave quantities are monitored simultaneously, allowing measurement of several fluid properties at the same time (e.g. viscosity and density of a liquid).
(1)
Sensors
Tbe & Lamb wave velocity and propagation loss The phase velocity of a stress-free A, Lamb wave in a thin plate is given by:
where p is the wavelength, B the bending stiffness of the plate: B =d%412(1v’)], d is the plate thickness, E is Young’s modulus, v the Poisson constant and M,, is the effective mass per unit area. When the plate is loaded at one side by a liquid, Me, is given by: M,,=m,,+m,+p6+
g im,,, is the plate mass per unit area and madr the mass of adsorbed particles at the sensor surface, p is the liquid density, S the thickness of the evanescent-moved fluid layer: 6=p/2n[l -(v,,,/ct)~‘“, ct is the compressional wave velocity in the liquid. When loaded by a liquid, eqn. (1) is valid for dap and v~,
A=Cfi
0924X47/94/$07.00 0 1994 Elsevier Science S.A. All rights reserved SSDI
0924-4247(93)00689-Z
(3)
where f is the operating frequency of the slow Lamb wave and C a constant depending on design parameters
176
sensingside
such as the plate thickness and plate material [3, 11, 121.
PGA
uII
/ SE&OR bonding wire
Fig. 2. Sensor in pin grid array (PGA).
Oscillator description The Lamb wave oscillator consists of two main parts, the amplifier and the frequency-determining Iamb wave delay line (Fig. 1). B(W)] and ]B(o)( are the amplifier gain and the delay line losses, respectively. The oscillation conditions are a loop gain of unity and a loop phase of 2rm (m= integer): IA(~)l~P(~)l=l
(4)
+I,, = - 27Vn= - &l - $J*amp - #u - +,12
(5)
Fig. 3. Layered
structure
u-
silicon
(bulk)
of the Lamb wave delay line.
f+‘JL++qL-gqGL Cr,
where &, is the phase shift caused by the delay line, &,,,P the phase shift caused by the amplifier and & and & are the phase shifts of each of the two interdigital transducers (IDTs). The loop phase is mainly determined by the (acoustic) contribution of the delay line: &, = or; o is the angular frequency, r is the acoustic delay time. The delay time depends on the length of the propagation path, T=L~~,&,,,; Lpath is the centerto-center distance of the two IDTs. If &>&,,,,+ & + &., the zero-phase condition is given by fo=mh and the frequency difference between two phase nulls (modes) is l/x Because of the use of a gain controlled amplifier, the oscillator-loop signals remain linear and the gain level (which is a measure for the acoustic losses in the Lamb wave device) can be monitored. The frequency mode at which the delay line losses are the smallest will become the oscillation frequency. After switchingon, the amplitude of this mode will increase until the first oscillation condition is reached. The electronics are integrated on a silicon chip, separate or combined with the acoustic device. Only the backside of the sensor is in contact with the fluid so that the front side, where the electric connections are located, can be protected against the measuring environment (Fig. 2).
B(O) Fig. 1. Acoustic wave oscillator system consisting of an amplifier and a Lamb wave delay line in the feedback loop.
IDT2
IDTl
Fig. 4. The equivalent
electrical
circuits of both ITDs.
Design considerations
Lamb wave d&y line The Lamb waves are generated and detected directly in the plate by means of the interdigital transducer. Transducers fabricated in a ZnO/Al/SiOJSi layered structure as shown in Fig. 3 show a high piezoelectric coupling coefficient, when the layer thickness are chosen properly. A high coupling coefficient results in low conversion losses. The performance of an acoustic wave oscillator is prominently determined by the design of the transducer and the complete delay line. The Lamb wave delay line has to meet requirements concerning the& wave velocity, frequency, wavelength, transducer conversion losses, propagation loss, transducer capacitance, bandwidth, delay time, and sensitivity to the measurand. The design parameters are the layered transducer configuration, the transducer dimensions and the propagation path length of the delay line. By using the transducers equivalent electrical circuits [13] (Fig. 4), and assuming that G,%- ]G,, +j+&] and G,> ]Gn2+j~Cn], the transfer from sending to receiving transducer can be written as:
where iGL is the current through the load G,_, G,, and G,* are the conductances of the sending and receiving transducer, respectively. The minimum required gain of a transimpedance amplifier accordingly is {2/(G,, G,)}‘” V7.
177
The conductance of a single-electrode transducer of equal electrode width and electrode distance, is given by G. = S?NfoCT
(7)
where Kz is the piezoelectric coupling coe5cient, N is the number of finger pairs of the transducer and C, is the static transducer capacitance with N fmger pairs and aperture W. It is clear that a high piezoelectric coupling coeflicient is required. The piezoelectric coupling coefficient and the phase velocity of layered plate structures have been calculated by using dedicated software [14]. Increasing the transducer capacitance also increases the conductance, but it cannot be made too large because of unacceptable high capacitive loading of the amplifier which would deteriorate the oscillator performance. The wavelength is determined by the finger spacing in the transducer, the wave velocity mainly by the plate material(s) and the ratio of plate thickness to wavelength. A low velocity is required in order to avoid acoustic radiation into a loading liquid [B]. Therefore, the frequency will be restricted to relatively low values. The typical required dimensions of the Lamb wave delay lines are collected in Table 1.
Electronics Electronic measurement of the wave attenuation in an acoustic delay line is realized by using a nonsaturating oscillator set-up. This oscillator contains an amplitude determining element (gain control) which avoids clipping or over-steering of the amplifier. The amplifier has a 20 dB dynamic range and a maximum gain of about 50 dB. The -3 dB bandwidth is about 30 MHz. Devices with different acoustic losses and different operating frequencies can all be operated with the same amplifier because of the large control range. Also extra losses which occur during measurements are (automatically) compensated. In Fig. 5 the control TABLE 1. ‘Apical dimensions of Lamb wave delay lines Layer
thickness (pm) silicon silicon dioxide aluminum zinc oxide total Finger period, p (q) No. finger pairs, N Aperture width (mm) Path length (wavelength) Path length (mm) Plate width (mm) Plate length (mm) Sensitive area (mm’)
278 0.1 0.2-0.6 0.7-2.0 3.0-10.7 Wl20 25-50 2-3 30-60 2.4-5.0 2.8 8 20
30
34
38 Insertion
42 46 Loss [dFJ]
50
Fig. 5. Amplifier gain control voltage vs. insertion loss in the acoustic device.
voltage versus the propagation loss in the acoustic device is given. The application of a non-saturating amplifier has in general two other advantages. First, the oscillator will always oscillate at the frequency with the lowest attenuation which prevents mode-hopping. Secondly, a non-saturating oscillator adds less noise to the output signal. Using the equivalent circuit of Fig. 4 the receiving transducer can be considered a current source shunted by capacitances. If current sensing is used at the amplifier input, the influence of instabilities in the capacitances between the fingers and the aluminum plate underneath the zinc oxide is decreased since, in the ideal case, they carry no charge. Another advantage of using current sensing at the ampliier input is the reduction of regeneration of waves caused by voltages generated at the receiving IDT. The triple transit signal is an example of this effect. If the receiving transducer is short circuited (current sensing) the regeneration of waves is eliminated (however, mechanical reflections remain). The input impedance must be low (ideal: zero). The output impedance of the amplifier is also chosen low, so that voltage steering is applied at the sending transducer. In this way, regeneration of waves at the IDT is suppressed since the voltage source determines the voltage, while in the case of current steering the amount of current through the capacitances would codetermine the voltage over the IDT. As a consequence of the foregoing considerations, a transimpedance amplifier concept was chosen (voltage steering, current sensing). The electronic circuitry is schematically shown in Fig. 6. The Lamb wave delay line and the electronic circuitry are designed in such a way that they can be implemented in a hybrid setup (two separate chips) or monolithically on a single chip. Fabrication
technology
The sensors are fabricated in a 2 km bipolar integrated circuit process, extended with two extra processing
178
amplifier
1level reference
Fig. 6. Scheme of tbe total Iamb wave oscillator.
modules for the monolithic integration of the Lamb wave device [2, 151: the zinc oxide technology module and the micromachining module. The first module comprises the deposition and the etching of zinc oxide and the etching of aluminum on top of the zinc oxide. The electrical connection of the interdigital electrodes on top of the zinc oxide to the metal leads of the electronic circuitry underneath the zinc oxide is achieved by adding an extra aluminum deposition and etching step. In the micromachining module the bulk etching of silicon takes place, using an electrochemical etch stop. The monolithic oscillator system consists of two thermally coupled but electrically non-coupled oscillators fabricated in one 10 X 10 mm chip. A dual configuration serves for suppression of unwanted common signals such as fluctuations in temperature, pressure or fluid flow. The transducers have 38 finger pairs, an aperture of 2.4 mm and a period of 80 m. A photograph of the monolithic sensor system is shown in Fig. 7. The transfer characteristics of the Lamb wave delay line are given in Fig. 8.
Expriments The measurements presented are obtained from both the hybrid and the monolithic systems, using several devices with operating frequencies in water of between 9.5 and 15.5 MHz. Measurements were performed at 20 “C. The sensor was exposed to acetone, methanol, chloroform and several water-glycerol solutions. In Fig. 9 the simultaneous frequency and gain response of the system is shown. Increasing the weight-percentage glycerol yields an increase of both density and viscosity resulting in a decreasing frequency and an increasing (automatic) gain control voltage. The frequency instability of the sensor system depends strongly on the packaging of the device, especially when
Fig, 7. The monolithic dual Lamb wave oscillator sensor in silicon, diiensiohs 10X 10 mm.
Fig. 8. ‘I)rpical transfer characteristics of the Lamb wave delay line. loaded by air and water, respectively.
loaded by a liquid. If the sensor is carefully mounted and temperature stabilized the short term stability in air amounts to OS-l.0 Hz (standard deviation of 100 measurements at 50 ms gate, HF’5335A counter). When loaded by a liquid, the short term stability decreases somewhat, to about 0.7-1.5 Hz. We found that liquid loading increases the phase noise of the Lamb wave delay line, which explains the decrease of the short term stability. The exact cause of this increased phase noise is not known but the fact that the frequency lowers with more than 25% makes it likely that liquid loading can have some influence on the stability of the
179
IL [dB]
9.2' 0
10
20 t
30
I 50
40
bnl
Fig. 9. Simultaneous frequency and gain level response of the Lamb wave sensor due to loading by various water+lycerol solutions.
0
5 (fprl)’
10 [x10’
15
Ns/mS]
Fig. 11. Insertion loss due to loading by several liquids. 2
stand.dev.[Hz]
I
1.5+ 1
0
0.5
1.5
I t
2
lhrl
Fig. 10. Short term stability (defined as the standard deviation, 100 measurements, 50 ms gate) of a Lamb wave oscillator loaded by water. TABLE 2. Experimental and calculated oscillator frequencies due to loading by various liquids Fluid
Air Methanol Acetone chloroform Water-glycerol solutions 0% glycerol 30% 50% 80% 98%
f
f
(ahjhIz)G-W
DcX (%)
18.090 12.940 13.400 11.170
18.165 12.923 13.375 11.197
+ 0.4 - 0.13 -0.19 + 0.24
13.881 13.879 13.862 13.752 13.470
13.884 13.874 13.861 13.760 13.660
+ 0.02 -0.04 0 + 0.06 +1.4
frequency. In Fig. 10 the short term stability during a 2 h test is shown. In Table 2 experimental and calculated frequencies of a sensor loaded by various fluids are given. The calculated values have been corrected for stress oc curring in the plate due to the zinc oxide deposition process. The insertion losses in the propagation path due to liquid loading are shown in Fig. 11 (losses are referenced to the insertion loss in air). The result for chloroform is not given because it has a very low compressional
wave velocity (990 m/s) which comes so close to the Lamb wave velocity that (extra) radiation losses were observed. Instead of a proportional relation to Cfpq)lR (eqn. (3)), the attenuation tends to saturate at higher viscosities. In addition, for small values of we observe a relatively strong increase of cfP4”, insertion loss compared to the loss in air, which appears as a ‘bias’. Loss measurements performed with Love wave devices (which are purely horizontally polarized waves), gave similar results [16]. The saturation cannot be caused by the relaxation time of the liquid [ll] because the operating frequency is low. It might be due to unknown interaction mechanisms that take place at the interface of the solid and the liquid. However, possible effects such as slip, the amount of hydrophilicity, or increasing effective viscosity at rough surfaces [17, 181 do not give satisfactory explanations. For the determination of the mass-loading sensitivity we deposited polymethyl methacrylate (PMMA) layers on top of the thin plates. The experimentally found frequency shift corresponds to the theory, about 3.5 Hz/rig/cm2 (measurements in air). Preliminary experiments were performed to investigate immunosensor applications. The sensor was exposed to diierent concentrations of human serum albumin (HSA) in phosphate buffered saline (PBS). In Fig. 12 the experimental results and the adsorption isotherm obtained from radioactive labelled adsorption at silicon test samples are shown. The calculated mass sensitivity of the device under test in a watery environment amounts to 16 mZ/ kg (or 2.2 Hz/n&m2). However, the experimentally found sensitivity is about three times lower: 0.8 Hz/
Conclusions A silicon implemented Iamb wave sensor provided with both velocity and amplitude detection has been
200 *en*or O0.1
response 1
Cb hrhll
Fig. 12. Sensor response to HSA adsorption, compared to the HSA-to-silicon adsorption isotherm. Cb is the HSA concentration in the solution, Cs is the adsorbed HSA at the sensor surface.
designed and fabricated. Roth the hybrid version and the monolithic version show stable oscillation and gain output. The frequency response to liquid loading is in agreement with calculated values. The results with respect to the propagation losses show a deviation with theory. Various possible explanations are discussed but give no complete understanding, The mass sensitivity of the sensor was confirmed by the deposition of a thin layer. However, the adsorption of HSA shows a lower sensitivity than expected.
For the fabrication of the devices we thank Jan Groeneweg and Wim van der Vlist (DIMES) and Ruth Ytsma (Xensor Integration). For the adsorption tests we acknowledge Dr Maarten Nieuwenhuizen and Hans Harteveld of PMLTNO, Rijswijk. This research is financially supported by the Dutch Technology Foundation (STW).
References 1 R.M. White, P.J. Wither, S.W. Wenael and E.T: Zellers, Plate-mode ultrasonic oscillator sensors, IEEE 7bm.r. Lnrrason. Ferroekcfi Freq. Contml$ UFFC-34 (1987) 162-171.
2 M.J. Vellekoop, A.J. van Rhijn, G.W. Lubking and A. Venema, All-silicon plate wave oscillator for sensors, Sensors and Actuators A, 25-27 (1991) 699-703. 3 B.A. Martin, SW. Wenael and R.M. White, Viscosity and density sensing with plate waves, Sensors and Ach~~tars, A21423 (1990) 704-708. 4 B.J. Costello, B.A. Martin and R.M. White, Ultrasonic plate waves for biochemical measurements, Proc. Ultrasonics Syrnp., Montreal, Canada, IRIS, pp. 977-981. 5 M.J. Vellekoop, G.W. Lubking, P.M. Sarro and A. Venema, A multi-purpose smart acoustic Lamb wave Sensor system, Rot Transducers ‘93, Yokohama, Japan, 1993, pp. 1052-1055. 6 B.J. Costello, SW. Wenzel, R.M. White, Density and viscosity sensing with ultrasonic fleaural plate waves, %. Transducem 93, Yokohama, Japan, 1993, pp. 704-707. 7 V. Rajendran, M. Koike, F. Saito, K. Hashimoto and M. Yamaguchi, Mass density sensor for liquids using ZnO-fiM Al-foil Lamb wave device, Fmc. Transducers 93, Yokohama, Japan, 1993, pp. 708-711. 8 G. Kurtae and RH. Bolt, On the interaction between plate bending waves and their radiation load, Acurrica, 9 (1959) 238-242. 9 K.K. Kanazawa and J.G. Gordon II, The oscillation frequency of a quarts resonator in contact with a liquid, Anal. Chim. Acta, 175 (1985) 99-105. 10 H.E. Hager, Fluid property evaluation by piezoelectric crystals operating in the thickness shear mode, Chem. Eng. Commun, 43 (1986) 2.538. 11 A.J. Ricco and S.J. Martin, Acoustic wave viscosity sensor, AppL Phys. Lett., 50 (1987) 1474-1476. 12 G. Kovacs, Mechanical perturbations of acoustic surface waves and plate waves, Infer&Rep., Delft University of Technology, Netherlands, Feb. 1993. 13 W.R. Smith, H.M. Gerard, J.H. Collins, T.M. Reeder and H.J. Shaw, Analysis of interdigital surface wave transducers by use of an equivalent circuit model, IEEE Trans. Microwave Theory Tech., MIT-17 (1969) 856-864. 14 E.L. Adler, J.K. Slaboszewicz, G.W. Fame11 and C. Kuie Jen, PC software for SAW propagation in anisotropic multilayers, IEEE Tmns Ultrason. Fenvelectr. Freq. Control, VFFC37 (1990) 215-222. 15 P.M. Sarro, Sensor technology strategy in silicon, Sensors and Actuators A, 31 (1992) 138-143. 16 G. Kovacs, M.J. Vellekoop, G.W. Lubking and A. Venema, A Love wave sensor for @io)chemical sensing in liquids, Pnx. Transducers 93, Yokohama, Japan, 1993, pp. 510-513. 17 M. Thompson, A.L. Kipling, W.C. Duncan-Hewitt, L.V. Rajakovic and BA. Cavic-Vlasak, Thickness-shear-mode acoustic wave sensors in the liquid phase, a review,Analyst, 116 (1991) 881-890. 18 R. Schumacher, The quarts microbalance: a novel approach to the in situ investigation of interfacial phenomena in the solid/liquid junction. Anger. C/tent_, Int. Ed. EngL, 29 (1990) 329-343.
Sensorsand Actuators A, 43 (1994) 175-180
Evaluation of liquid properties using a silicon Lamb wave sensor M.J. Vellekoop”, G.W. Lubkingb, P.M. Sarro’ and A. Venemab ‘Xensor Integration bv, PO Box 3233,2601DE De& (Netherian@ %elft Universi@ of Technology, Labomtory for Ekctmnic hbwnentation, ‘DIMES, PO Box 5053, 2600 GB De& (Netherlands)
PO Box 5031, 2600 GA De@ (Netherlands)
Abstract The design and realization of a Lamb wave oscillator sensor system are described. Both the oscillation frequency and the amplifier gain level are monitored, giving information about the Lamb wave velocity and amplitude, respectively. The system is fabricated in two separate silicon chips (hybrid set-up) or in one chip (monolithic set-up), and can be used for different sensor applications, in liquid or gas environment. We demonstrate the sensor for detection of the density and viscosity of a liquid, and for sensing of the adsorption of human serum albumin (HSA) at the sensor surface.
Introduction based on the acoustic zero-order antisymmetric (A,) Lamb wave mode have gained interest because of their inherent high sensitivity and broad application possibilities. The slow A, Lamb wave shows low acoustic losses when contacted by a liquid and the frequency of operation is relatively low, typically between 5 and 20 MHz. In addition, Lamb wave devices can be fabricated in silicon by using micromachinmg techniques [l, 21. The monolithic integration of acoustic device and electronic circuitry is then a logical sequel, resulting in a smart sensor system, givmg all the advantages of the IC technology; reproducibility, reliability, small size and low production costs. Both physical and (bio)chemical sensors based on acoustic Lamb waves have been reported, e.g. for liquid viscosity and density, and particle detection in gases as well as in liquids [3-71. Two quantities of the wave are (independently) sensitive for changes at the surface of the plate in which it propagates: the phase velocity and the amplitude. By using the Lamb wave device in a gain controlled oscillator, both wave quantities are monitored simultaneously, allowing measurement of several fluid properties at the same time (e.g. viscosity and density of a liquid).
(1)
Sensors
Tbe & Lamb wave velocity and propagation loss The phase velocity of a stress-free A, Lamb wave in a thin plate is given by:
where p is the wavelength, B the bending stiffness of the plate: B =d%412(1v’)], d is the plate thickness, E is Young’s modulus, v the Poisson constant and M,, is the effective mass per unit area. When the plate is loaded at one side by a liquid, Me, is given by: M,,=m,,+m,+p6+
g im,,, is the plate mass per unit area and madr the mass of adsorbed particles at the sensor surface, p is the liquid density, S the thickness of the evanescent-moved fluid layer: 6=p/2n[l -(v,,,/ct)~‘“, ct is the compressional wave velocity in the liquid. When loaded by a liquid, eqn. (1) is valid for dap and v~,
A=Cfi
0924X47/94/$07.00 0 1994 Elsevier Science S.A. All rights reserved SSDI
0924-4247(93)00689-Z
(3)
where f is the operating frequency of the slow Lamb wave and C a constant depending on design parameters
176
sensingside
such as the plate thickness and plate material [3, 11, 121.
PGA
uII
/ SE&OR bonding wire
Fig. 2. Sensor in pin grid array (PGA).
Oscillator description The Lamb wave oscillator consists of two main parts, the amplifier and the frequency-determining Iamb wave delay line (Fig. 1). B(W)] and ]B(o)( are the amplifier gain and the delay line losses, respectively. The oscillation conditions are a loop gain of unity and a loop phase of 2rm (m= integer): IA(~)l~P(~)l=l
(4)
+I,, = - 27Vn= - &l - $J*amp - #u - +,12
(5)
Fig. 3. Layered
structure
u-
silicon
(bulk)
of the Lamb wave delay line.
f+‘JL++qL-gqGL Cr,
where &, is the phase shift caused by the delay line, &,,,P the phase shift caused by the amplifier and & and & are the phase shifts of each of the two interdigital transducers (IDTs). The loop phase is mainly determined by the (acoustic) contribution of the delay line: &, = or; o is the angular frequency, r is the acoustic delay time. The delay time depends on the length of the propagation path, T=L~~,&,,,; Lpath is the centerto-center distance of the two IDTs. If &>&,,,,+ & + &., the zero-phase condition is given by fo=mh and the frequency difference between two phase nulls (modes) is l/x Because of the use of a gain controlled amplifier, the oscillator-loop signals remain linear and the gain level (which is a measure for the acoustic losses in the Lamb wave device) can be monitored. The frequency mode at which the delay line losses are the smallest will become the oscillation frequency. After switchingon, the amplitude of this mode will increase until the first oscillation condition is reached. The electronics are integrated on a silicon chip, separate or combined with the acoustic device. Only the backside of the sensor is in contact with the fluid so that the front side, where the electric connections are located, can be protected against the measuring environment (Fig. 2).
B(O) Fig. 1. Acoustic wave oscillator system consisting of an amplifier and a Lamb wave delay line in the feedback loop.
IDT2
IDTl
Fig. 4. The equivalent
electrical
circuits of both ITDs.
Design considerations
Lamb wave d&y line The Lamb waves are generated and detected directly in the plate by means of the interdigital transducer. Transducers fabricated in a ZnO/Al/SiOJSi layered structure as shown in Fig. 3 show a high piezoelectric coupling coefficient, when the layer thickness are chosen properly. A high coupling coefficient results in low conversion losses. The performance of an acoustic wave oscillator is prominently determined by the design of the transducer and the complete delay line. The Lamb wave delay line has to meet requirements concerning the& wave velocity, frequency, wavelength, transducer conversion losses, propagation loss, transducer capacitance, bandwidth, delay time, and sensitivity to the measurand. The design parameters are the layered transducer configuration, the transducer dimensions and the propagation path length of the delay line. By using the transducers equivalent electrical circuits [13] (Fig. 4), and assuming that G,%- ]G,, +j+&] and G,> ]Gn2+j~Cn], the transfer from sending to receiving transducer can be written as:
where iGL is the current through the load G,_, G,, and G,* are the conductances of the sending and receiving transducer, respectively. The minimum required gain of a transimpedance amplifier accordingly is {2/(G,, G,)}‘” V7.
177
The conductance of a single-electrode transducer of equal electrode width and electrode distance, is given by G. = S?NfoCT
(7)
where Kz is the piezoelectric coupling coe5cient, N is the number of finger pairs of the transducer and C, is the static transducer capacitance with N fmger pairs and aperture W. It is clear that a high piezoelectric coupling coeflicient is required. The piezoelectric coupling coefficient and the phase velocity of layered plate structures have been calculated by using dedicated software [14]. Increasing the transducer capacitance also increases the conductance, but it cannot be made too large because of unacceptable high capacitive loading of the amplifier which would deteriorate the oscillator performance. The wavelength is determined by the finger spacing in the transducer, the wave velocity mainly by the plate material(s) and the ratio of plate thickness to wavelength. A low velocity is required in order to avoid acoustic radiation into a loading liquid [B]. Therefore, the frequency will be restricted to relatively low values. The typical required dimensions of the Lamb wave delay lines are collected in Table 1.
Electronics Electronic measurement of the wave attenuation in an acoustic delay line is realized by using a nonsaturating oscillator set-up. This oscillator contains an amplitude determining element (gain control) which avoids clipping or over-steering of the amplifier. The amplifier has a 20 dB dynamic range and a maximum gain of about 50 dB. The -3 dB bandwidth is about 30 MHz. Devices with different acoustic losses and different operating frequencies can all be operated with the same amplifier because of the large control range. Also extra losses which occur during measurements are (automatically) compensated. In Fig. 5 the control TABLE 1. ‘Apical dimensions of Lamb wave delay lines Layer
thickness (pm) silicon silicon dioxide aluminum zinc oxide total Finger period, p (q) No. finger pairs, N Aperture width (mm) Path length (wavelength) Path length (mm) Plate width (mm) Plate length (mm) Sensitive area (mm’)
278 0.1 0.2-0.6 0.7-2.0 3.0-10.7 Wl20 25-50 2-3 30-60 2.4-5.0 2.8 8 20
30
34
38 Insertion
42 46 Loss [dFJ]
50
Fig. 5. Amplifier gain control voltage vs. insertion loss in the acoustic device.
voltage versus the propagation loss in the acoustic device is given. The application of a non-saturating amplifier has in general two other advantages. First, the oscillator will always oscillate at the frequency with the lowest attenuation which prevents mode-hopping. Secondly, a non-saturating oscillator adds less noise to the output signal. Using the equivalent circuit of Fig. 4 the receiving transducer can be considered a current source shunted by capacitances. If current sensing is used at the amplifier input, the influence of instabilities in the capacitances between the fingers and the aluminum plate underneath the zinc oxide is decreased since, in the ideal case, they carry no charge. Another advantage of using current sensing at the ampliier input is the reduction of regeneration of waves caused by voltages generated at the receiving IDT. The triple transit signal is an example of this effect. If the receiving transducer is short circuited (current sensing) the regeneration of waves is eliminated (however, mechanical reflections remain). The input impedance must be low (ideal: zero). The output impedance of the amplifier is also chosen low, so that voltage steering is applied at the sending transducer. In this way, regeneration of waves at the IDT is suppressed since the voltage source determines the voltage, while in the case of current steering the amount of current through the capacitances would codetermine the voltage over the IDT. As a consequence of the foregoing considerations, a transimpedance amplifier concept was chosen (voltage steering, current sensing). The electronic circuitry is schematically shown in Fig. 6. The Lamb wave delay line and the electronic circuitry are designed in such a way that they can be implemented in a hybrid setup (two separate chips) or monolithically on a single chip. Fabrication
technology
The sensors are fabricated in a 2 km bipolar integrated circuit process, extended with two extra processing
178
amplifier
1level reference
Fig. 6. Scheme of tbe total Iamb wave oscillator.
modules for the monolithic integration of the Lamb wave device [2, 151: the zinc oxide technology module and the micromachining module. The first module comprises the deposition and the etching of zinc oxide and the etching of aluminum on top of the zinc oxide. The electrical connection of the interdigital electrodes on top of the zinc oxide to the metal leads of the electronic circuitry underneath the zinc oxide is achieved by adding an extra aluminum deposition and etching step. In the micromachining module the bulk etching of silicon takes place, using an electrochemical etch stop. The monolithic oscillator system consists of two thermally coupled but electrically non-coupled oscillators fabricated in one 10 X 10 mm chip. A dual configuration serves for suppression of unwanted common signals such as fluctuations in temperature, pressure or fluid flow. The transducers have 38 finger pairs, an aperture of 2.4 mm and a period of 80 m. A photograph of the monolithic sensor system is shown in Fig. 7. The transfer characteristics of the Lamb wave delay line are given in Fig. 8.
Expriments The measurements presented are obtained from both the hybrid and the monolithic systems, using several devices with operating frequencies in water of between 9.5 and 15.5 MHz. Measurements were performed at 20 “C. The sensor was exposed to acetone, methanol, chloroform and several water-glycerol solutions. In Fig. 9 the simultaneous frequency and gain response of the system is shown. Increasing the weight-percentage glycerol yields an increase of both density and viscosity resulting in a decreasing frequency and an increasing (automatic) gain control voltage. The frequency instability of the sensor system depends strongly on the packaging of the device, especially when
Fig, 7. The monolithic dual Lamb wave oscillator sensor in silicon, diiensiohs 10X 10 mm.
Fig. 8. ‘I)rpical transfer characteristics of the Lamb wave delay line. loaded by air and water, respectively.
loaded by a liquid. If the sensor is carefully mounted and temperature stabilized the short term stability in air amounts to OS-l.0 Hz (standard deviation of 100 measurements at 50 ms gate, HF’5335A counter). When loaded by a liquid, the short term stability decreases somewhat, to about 0.7-1.5 Hz. We found that liquid loading increases the phase noise of the Lamb wave delay line, which explains the decrease of the short term stability. The exact cause of this increased phase noise is not known but the fact that the frequency lowers with more than 25% makes it likely that liquid loading can have some influence on the stability of the
179
IL [dB]
9.2' 0
10
20 t
30
I 50
40
bnl
Fig. 9. Simultaneous frequency and gain level response of the Lamb wave sensor due to loading by various water+lycerol solutions.
0
5 (fprl)’
10 [x10’
15
Ns/mS]
Fig. 11. Insertion loss due to loading by several liquids. 2
stand.dev.[Hz]
I
1.5+ 1
0
0.5
1.5
I t
2
lhrl
Fig. 10. Short term stability (defined as the standard deviation, 100 measurements, 50 ms gate) of a Lamb wave oscillator loaded by water. TABLE 2. Experimental and calculated oscillator frequencies due to loading by various liquids Fluid
Air Methanol Acetone chloroform Water-glycerol solutions 0% glycerol 30% 50% 80% 98%
f
f
(ahjhIz)G-W
DcX (%)
18.090 12.940 13.400 11.170
18.165 12.923 13.375 11.197
+ 0.4 - 0.13 -0.19 + 0.24
13.881 13.879 13.862 13.752 13.470
13.884 13.874 13.861 13.760 13.660
+ 0.02 -0.04 0 + 0.06 +1.4
frequency. In Fig. 10 the short term stability during a 2 h test is shown. In Table 2 experimental and calculated frequencies of a sensor loaded by various fluids are given. The calculated values have been corrected for stress oc curring in the plate due to the zinc oxide deposition process. The insertion losses in the propagation path due to liquid loading are shown in Fig. 11 (losses are referenced to the insertion loss in air). The result for chloroform is not given because it has a very low compressional
wave velocity (990 m/s) which comes so close to the Lamb wave velocity that (extra) radiation losses were observed. Instead of a proportional relation to Cfpq)lR (eqn. (3)), the attenuation tends to saturate at higher viscosities. In addition, for small values of we observe a relatively strong increase of cfP4”, insertion loss compared to the loss in air, which appears as a ‘bias’. Loss measurements performed with Love wave devices (which are purely horizontally polarized waves), gave similar results [16]. The saturation cannot be caused by the relaxation time of the liquid [ll] because the operating frequency is low. It might be due to unknown interaction mechanisms that take place at the interface of the solid and the liquid. However, possible effects such as slip, the amount of hydrophilicity, or increasing effective viscosity at rough surfaces [17, 181 do not give satisfactory explanations. For the determination of the mass-loading sensitivity we deposited polymethyl methacrylate (PMMA) layers on top of the thin plates. The experimentally found frequency shift corresponds to the theory, about 3.5 Hz/rig/cm2 (measurements in air). Preliminary experiments were performed to investigate immunosensor applications. The sensor was exposed to diierent concentrations of human serum albumin (HSA) in phosphate buffered saline (PBS). In Fig. 12 the experimental results and the adsorption isotherm obtained from radioactive labelled adsorption at silicon test samples are shown. The calculated mass sensitivity of the device under test in a watery environment amounts to 16 mZ/ kg (or 2.2 Hz/n&m2). However, the experimentally found sensitivity is about three times lower: 0.8 Hz/
Conclusions A silicon implemented Iamb wave sensor provided with both velocity and amplitude detection has been
200 *en*or O0.1
response 1
Cb hrhll
Fig. 12. Sensor response to HSA adsorption, compared to the HSA-to-silicon adsorption isotherm. Cb is the HSA concentration in the solution, Cs is the adsorbed HSA at the sensor surface.
designed and fabricated. Roth the hybrid version and the monolithic version show stable oscillation and gain output. The frequency response to liquid loading is in agreement with calculated values. The results with respect to the propagation losses show a deviation with theory. Various possible explanations are discussed but give no complete understanding, The mass sensitivity of the sensor was confirmed by the deposition of a thin layer. However, the adsorption of HSA shows a lower sensitivity than expected.
For the fabrication of the devices we thank Jan Groeneweg and Wim van der Vlist (DIMES) and Ruth Ytsma (Xensor Integration). For the adsorption tests we acknowledge Dr Maarten Nieuwenhuizen and Hans Harteveld of PMLTNO, Rijswijk. This research is financially supported by the Dutch Technology Foundation (STW).
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