- Email: [email protected]
Optimization of sensitivity induced by surface conductivity and sorbed mass in surface acoustic wave gas sensors
Sensors and Actuators B 161 (2012) 114–123
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Optimization of sensitivity induced by surface conductivity and sorbed mass in surface acoustic wave gas sensors Li Fan a,∗ , Huan Ge a , Shu-yi Zhang a , Hui Zhang a , Jian Zhu b a b
Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, China Electronics Technology Group Corporation of China, No. 55 Research Institute, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 4 May 2011 Received in revised form 26 September 2011 Accepted 26 September 2011 Available online 20 October 2011 Keywords: Gas sensor Surface acoustic wave Piezoelectric film Sensitivity optimization
a b s t r a c t Acoustic transmissions in surface acoustic wave (SAW) gas sensors based on multilayered structures consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate are studied with the transfer matrix method, in which the shifts of the central frequencies of SAW sensors induced by the variations of surface conductivities and sorbed masses are evaluated. According to the results, the sensitivities of SAW gas sensors using different piezoelectric films are optimized by simultaneously considering the variations of surface conductivities and masses caused by sorbates on the surfaces of the sensors. It is found that the frequency shift induced by the variation of surface conductivity is closely related with the initial conductivity and relative dielectric constant of the sensitive layer, while the frequency shift induced by the sorbed mass is almost independent on them. Furthermore, the operating frequencies, SAW modes and characteristics of sensitive layers are optimized in order to achieve high sensitivities according to the responses of the SAW gas sensors to different types of the detected gases. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Surface acoustic wave (SAW) chemical sensors have been widely studied and applied in the detection of toxic, inflammable and explosive gases [1–7] in virtue of high sensitivity, small size, and potential for wireless sensing [8–10]. In the development of SAW sensors, the multilayered structures consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate become the focus of research, in which piezoelectric films take the place of the bulk ones to excite the SAW. In this type of SAW sensors, when silicon wafers are used as the substrates [1], the sensors have the potential to be integrated with electric circuits for micro-electro-mechanical systems (MEMS), which can realize the miniaturization, portability and long distance wireless data communication [10–12]. In the researches of SAW chemical sensors, it has been known that in addition to changing the sorbed masses, the matters sorbed by the sensitive layers can change the conductivities of the sensitive layers [13,14], which also influence the SAW velocities or center frequencies of the sensors. Then, similar to the mass sensitivity Sm [15], the conductivity sensitivity Sc can be defined to
Abbreviations: SAW, surface acoustic wave; MEMS, micro-electro-mechanical system; ECC(s), electromechanical coupling coefficient(s). ∗ Corresponding author. Tel.: +86 25 83597484; fax: +86 25 83593690. E-mail address: tx [email protected] (L. Fan). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.077
be the relative variation of the central frequency of the SAW sensor caused by the variation of the conductivity. According to the related theory [15–17] and numerous experiments, Sm is always negative, i.e., the center frequency of the SAW sensor is always decreased by the sorbed mass. However, the conductivity of the sensitive layer can be increased or decreased by the sorbates, which decreases or increases the center frequency of the SAW sensor, respectively. If the center frequency is increased by the variation of the conductivity, an “abnormal” response of the sensor can occur [18–21]. Especially, for the SAW gas sensor, the influence of sorbed mass induced by the gas is weak and then the influence of the variation of the surface conductivity becomes important [22,23]. Therefore, both Sc and Sm must be considered simultaneously in the performance optimizations of SAW gas sensors. In this paper, the transfer matrix method [24–27] is used to evaluate and optimize both Sc and Sm of the SAW sensor based on the multilayered structure consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate. The influences of the initial conductivities, relative dielectric constants and thicknesses of the sensitive layers, the properties of piezoelectric films, modes of SAW waves and operating frequencies on the sensitivities of SAW sensors are evaluated. Based on the results, the optimized sensitivities of the SAW gas sensors can be obtained by using proper SAW modes, sensitive layers, piezoelectric films and operating frequencies according to different detected gases.
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
115
Fig. 1. Multilayered structure and dispersion curve of sensors: (a), (c) Rayleigh wave; (b), (d) Lamb wave.
2. Theory The sorption of gaseous matters by the sensitive layers can be classified into two types: adsorption and absorption [28]. Adsorption is sorption onto a surface and absorption can be seen as dissolving into the absorbent material, the latter often happens when a sensitive layer sorbs the detected gas [28,29], which changes the conductivity and increases the mass of the layer. Fig. 1 shows the models of SAW sensors based on multilayered structures, in which piezoelectric films are deposited on nonpiezoelectric substrates and covered by sensitive layers, x1 is the propagation direction of the SAW and x2 is the normal direction. Fig. 1(a) shows the model of the sensor based on the Rayleigh wave, in which the substrate is semi-infinite and the Rayleigh wave is dispersive due to the multilayer. Fig. 1(b) shows the model based on the Lamb wave, in which the multilayered structure forms a thin plate. The corresponding dispersion curves of the Rayleigh wave and the A0 and S0 modes of the Lamb wave are shown in Fig. 1(c) and (d) respectively. The acoustic and electromagnetic waves in the rth layer can be expressed as [24–27]:
As the conductivity of the sensitive layer (r = 1) is considered, the electromagnetic wave equation in the sensitive layer must contain the dissipation item as [18]: [1] [1] [1] ¨ [1] [1] eikl u¨ k,il − εik − J˙ k,k = 0 ,ik [1]
[1]
(3)
[1]
[1]
in which Jk,k = ik ,ik is the conduction current and ik is the conductivity of the material. For the simple harmonic wave, Eq. (3) can be rewritten as:
[1] [1] eikl uk,il
−
[1] εik
[1]
−
ik
[1]
jω
,ik = 0
(4)
Comparing Eq. (4) with Eq. (2), it shows that Eq. (4) can be obtained by replacing εik of Eq. (2) with an equivalent dielectric constant εik − ik /jω. The solutions of equation set (1) and (2) or (1) and (4) for the SAW wave (Rayleigh and Lamb waves) can be expressed in the same forms as: uj =
8
˛jq Bq exp j k1 x1 + k2q x2 − ωt
(5)
q=1 [r]
[r]
[r]
[r]
[r]
cijkl uk,il + ekij ,ik = [r] u¨ j
(1) =
[r] [r] eikl uk,il
[r] [r] − εik ,ik
=0
[r]
and εik are the elastic, piezoelectric and dielectric matrices of the rth material, respectively, i, j, k, l = 1, 2, 3 indicate the directions of [r] x1 , x2 and x3 . [r] is the density, uj and [r] are the particle displacement and electric potential, respectively.
˛4q Bq exp j k1 x1 + k2q x2 − ωt
(6)
q=1
(2)
where the superscript r = 1, 2, 3 indicates the sensitive layer, piezo[r] [r] electric film and non-piezoelectric substrate, respectively. cijkl , ekij
8
in which ˛jq and ˛4q are the amplitudes of the partial wave solutions and Bq is the undetermined coefficients; k1 and k2q = bq k1 are the wave numbers in the x1 and x2 directions, respectively; ω and t are the angle frequency and time, respectively. The stress and electric displacement of the normal direction x2 can be expressed as: [r]
[r]
[r]
[r]
[r]
T2j = c2jkl uk,l + ek2j ,k
(7)
116
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 3. Influence of 0 and ε of sensitive layer on Sc of sensor with ZnO piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
Defining = D˙ + J, then the normal component of the vector must be continuous at surfaces and interfaces of the multilayered structure. For the simple harmonic wave, Eq. (10) becomes:
Fig. 2. Frequency responses of Sc in sensors with different piezoelectric films: (a) ZnO; (b) AlN; (c) PZT.
[r]
[r]
[r]
[r]
[r]
D2 = e2kl uk,l − ε2k ,k
(8)
Substituting Eqs. (5) and (6) into Eqs. (7) and (8) results in the equation set of the stress and electric displacement. The mechanical boundary conditions are independent on the electric loss, while the electric boundary conditions are dependent on it. When the material is electrically lossless and no charge exists at the surfaces and interfaces, the Gaussian equation is: Di,i = 0
(9)
Therefore, the normal component of the electric displacement D is continuous at the surfaces and interfaces. While when the conduction current is considered, the Gaussian equation becomes: D˙ i,i + Ji,i = 0
(10)
Di,i −
Ji,i jω
=0
(11)
Therefore, similar to Eq. (4), the new electric boundary condition can also be obtained by replacing εik in the boundary condition of the lossless material with the equivalent dielectric constant εik − ik /jω. Then the dispersion curves of the Rayleigh and Lamb waves can be obtained from the coefficient matrix of the equation set of the boundary conditions with or without the electrical loss, and the influence of the conductivity on the SAW velocity or the central frequency of the sensor can be evaluated. Generally, the sensitive layer is non-piezoelectric, however, due to the existence of the piezoelectric film, the acoustic and electric fields in the multilayered material are coupled with each other and the fields are connected at the interfaces of the multilayer material, then the variation of the conductivity of the sensitive layer changes the acoustic velocity of the SAW in the multilayered structure, which can be expressed by the piezoelectric stiffened elastic constant: CKL + jCKL = CKL +
eKj Kj [Ki eiL ]
Ki εij − ij /jω Kj
(12)
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 4. Influence of 0 and ε of sensitive layer on Sc of sensor with AlN piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
where CKL is the equivalent elastic matrix without the piezoelecT , tric effect, eKj is the equivalent piezoelectric matrix and eiL = eKj the superscript T indicates the transposition; CKL is the real part of is the the equivalent piezoelectric stiffened elastic matrix and CKL imaginary part indicating the electric loss caused by the conduction current. They are all expressed with the contracted subscript for convenience. Ki = [k1 , k2 , 0] and Kj = KiT . Then the influence of the conductivity on the acoustic velocity is related to the real part of the stiffened part of the equivalent elastic constant:
=− dCKL
2 eKj Kj [Ki eiL ] εij ij
ε2ij ω + ij2 /ω
2 dij
(13)
Eq. (13) shows that a positive shift of the conductivity dij results , and vice versa. in a negative shift of equivalent elastic constant dCKL Therefore, if the sorbate increases the conductivity of the sensitive layer, the equivalent elastic constant is decreased, which decreases the SAW velocity or the central frequency of the sensor. On the other hand, if the sorbate decreases the conductivity of the sensitive layer, the SAW velocity and central frequency of the sensor increase. In this case, when the frequency shift induced by the conductivity variation is larger than that coming from the sorbed mass, an “abnormal” response can occur, in which the central frequency of the sensor increases after sorbing gases [19–21].
117
Fig. 5. Influence of 0 and ε of sensitive layer on Sc of sensor with PZT piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
Eq. (13) shows that several factors dominate the influences of the surface conductivity on the SAW velocity, i.e., the Sc of the sensor. First, an optimized angle frequency ω brings the maximum , then the maximum S can be obtained at the optimized fredCKL c quency. Meanwhile, the frequency response of the Sc is similar to that of the electromechanical coupling coefficient (ECC) because they are both related with the coupling between the acoustic field and electromagnetic field [18]. Second, the piezoelectric constant eKj influences the Sc , which is mainly related with the piezoelectric film in the sensor. Finally, the equivalent conductivity ij and relative dielectric constant εij also have the influences on the Sc , therefore, the multilayered materials with the proper conductivities and relative dielectric constants must be selected in the SAW sensors. The numerical simulations are based on the multilayered structures shown in Fig. 1, in which a silicon wafer and a WO3 layer are taken as the substrate and sensitive layer respectively, and three types of piezoelectric films of ZnO, AlN and PZT are used to excite the SAW, the materials are usually used in SAW gas sensors. It is assumed that the thickness of the piezoelectric film is 1 m, the silicon substrate is semi-infinite for the Rayleigh wave in Fig. 1(a) and 3 m for the Lamb wave in Fig. 1(b), the thickness of the sensitive layer is 0.2 m except that the influences of different thicknesses of the sensitive layers are evaluated.
118
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 6. Maximum Sc of sensor with ZnO film and different 0 and hs of sensitive layer: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
3. Conductivity sensitivity Sc 3.1. Frequency response of Sc In order to optimize the Sc of the SAW sensor, the relative frequency shift induced by the variation of the conductivity of sensitive layer is obtained with the transfer matrix method. According to practical experiments of gas sensors, the initial conductivities 0 of sensitive layers before the sorption of any gases vary in a wide range of about 0.01–100 S/m3 [29,30]. Meanwhile, the relative variations of the conductivities induced by the sorbed gases also vary from several per mille [31] to several percents [13]. Therefore, in the simulations, the initial conductivities of the sensitive layers are chosen in a wide range of 0.1–60 S/m3 . Furthermore, in order to compare the results, the variations of the conductivities induced by the sorbed gases are assumed to be 10 mS/m3 , then the relative variations of the conductivities are in the range of about 0.01–10%. Fig. 2 shows the relative frequency shifts in the sensors with different piezoelectric films and SAW modes. As shown in Fig. 2(a) and (b), the frequency responses of the frequency shifts in the sensors with ZnO and AlN piezoelectric films are similar, in which the optimized frequencies for the Sc of the Rayleigh wave and the A0 mode of Lamb wave are in the low frequency ranges and the optimized frequency for that of the S0 mode of Lamb wave is higher. However, in the sensor with PZT piezoelectric film, as shown in Fig. 2(c), high Sc of the Rayleigh wave and A0 mode can be obtained at frequencies higher than the optimized frequency for the S0 mode.
Fig. 7. Maximum Sc of sensor with AlN film and different 0 and hs of sensitive layer: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
with the sheet conductivity of M = Vε0 1 + εp , where ε0 is the dielectric constant of air and εp is the relative dielectric constant of the bulk piezoelectric material. Similarly, in the multilayered structure, a large relative dielectric constant of the piezoelectric film also increases the optimized initial conductivity of the sensitive layer for high Sc . PZT is a type of piezoelectric material with high relative dielectric constant ε, which is 50–100 times larger than that of ZnO or AlN, so the initial conductivities of sensitive layers are much larger than those with the ZnO or AlN films for optimizing Sc of the sensors. Furthermore, it can be seen that the Sc keeps constant while ε of the sensitive layer varies in a large range, but Sc decrease when ε exceeds a value εc [as indicated in Fig. 3(a)]. As shown in Figs. 3 and 4, for the ZnO and AlN films, the Sc of the three wave modes decrease when ε increases to the order of the relative dielectric constants (about 10) of ZnO or AlN. While as shown in Fig. 5, with the PZT film, the Sc of the three wave modes decrease when ε increase to about 500, which is also the same order as the relative dielectric constants of PZT. 3.3. Optimization of thickness of sensitive layer Another important factor determining the Sc is the thickness of the sensitive layer hs , which is evaluated in Figs. 6–8 according
3.2. Influence of initial conductivity and relative dielectric constant of sensitive layer Figs. 3–5 show the influences of the initial conductivity 0 and relative dielectric constant ε of the sensitive layer on the Sc . As shown in the three figures, Sc decreases to zero if 0 becomes too small or too large, which means that regardless of the piezoelectric films and the wave modes, the initial conductivity of the sensitive layer must be set in a narrow “window” [as indicated in Fig. 3(a)] for obtaining high Sc . Furthermore, the conductivity window for high Sc is dependent on the piezoelectric film. For ZnO or AlN film, the high Sc can be obtained with the initial conductivity in the range of about 0.5–5.0 S/m3 , while for the PZT piezoelectric film, the conductivity window shifts to about 10–100 S/m3 . In a homogenous bulk piezoelectric material [32,33], the maximum SAW velocity shift induced by the variation of surface conductivity is obtained
Fig. 8. Maximum Sc of sensor with PZT film and different 0 and hs of sensitive layers: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 9. Influence of 0 and ε of sensitive layer on Sm of sensor with ZnO piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
to different initial conductivities of the sensitive layer. It can be seen that with smaller 0 , larger hs of the sensitive layers are required for obtaining high Sc . As shown in Figs. 6 and 7, similar results can be obtained in the sensors with ZnO or AlN piezoelectric films: by optimizing the thicknesses of the sensitive layers, high Sc of Rayleigh wave and S0 mode of the Lamb wave are achieved with 0 in the range of 0.5–3.0 S/m3 , and besides, high Sc of the A0 mode of the Lamb wave can also be obtained with a smaller 0 = 0.1 S/m3 . For the PZT film, to achieve high Sc , a sensitive layer with large initial conductivity 0 must be selected, as shown in Fig. 8, high Sc is obtained with the 0 larger than 10.0 S/m3 . The results are also attributed to the high relative dielectric constant of PZT, which makes the optimized sheet conductivity of the sensitive layer increase greatly, and so larger 0 is required for optimizing Sc . 4. Mass sensitivity Sm As described above, the sorption of gas into the sensitive layer increases the mass. Then the frequency shift (decrease) induced by the sorbed gas can also be evaluated by the transfer matrix method. Based on experiments, the detection limits of SAW gas sensors can be less than 100 pg/cm3 , and typically the mass sensitivities of 10–100 Hz/(ng cm2 ) can be achieved [9], so in the simulation, a sorbed mass of 20 ng/cm2 is assumed.
119
Fig. 10. Influence of 0 and ε of sensitive layer on Sm of sensor with AlN piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
4.1. Influence of initial conductivity and relative dielectric constant of sensitive layer Figs. 9–11 shows the relative frequency shifts induced by the sorbed masses in the SAW sensors with three types of piezoelectric films of ZnO, AlN and PZT, in which the influences of the initial conductivity 0 and relative dielectric constant ε of the sensitive layer on the Sm are evaluated. Different from the Sc , the Sm is almost independent on 0 and ε, in which the largest variation is only a decrease of 15% in the Sm of the S0 mode of the Lamb wave in the sensor using PZT as the piezoelectric film. It is reasonable that the electrical parameters 0 and ε have more obvious influences on the Sc than those on the Sm . 4.2. Optimization of operating frequency and thickness of sensitive layer The frequency responses of the Sm in the sensors with different thicknesses of sensitive layers hs are shown in Figs. 12–14, in which 0 and ε of the sensitive layers are set as 1 S/m3 and 10, respectively, since they hardly influence the Sm . It can be seen that in the three types of sensors with different piezoelectric films, similar frequency responses of the Sm are obtained. For the Rayleigh wave and A0 mode of the Lamb wave, high Sm can be obtained by increasing the operating frequency, while for the S0 mode of the
120
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 11. Influence of 0 and ε of sensitive layer on Sm of sensor with PZT piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave. Fig. 12. Frequency response of Sm of sensor with ZnO film and different hs of sensitive layer: (a) Rayleigh mode; (b) A0 mode and (c) S0 mode of Lamb wave.
Lamb wave, high Sm is obtained at one optimized frequency. On the other hand, as shown in Figs. 12–14, the influences of hs on the Sm of the Rayleigh wave and A0 mode of the Lamb wave are slight, especially in the sensor using ZnO as the piezoelectric film, the Sm of the Rayleigh wave and A0 mode of the Lamb wave are almost independent on hs . While for the S0 mode of the Lamb wave, the Sm at the optimized frequency is rapidly increased by decreasing the thickness of sensitive layer. Furthermore, in the sensor with the AlN piezoelectric film, a peak of the Sm occurs at a lower frequency when the thickness of the sensitive layer increases to 0.3 m, as shown in Fig. 13(c). 5. Optimization of performance Both the Sc and Sm must be considered simultaneously in the optimizations of the performances of the SAW gas sensor. Additionally, high ECC must also be assured for obtaining high signal-to-noise ratios in the applications of SAW sensors. According to the responses of the sensor to different sorbed gases, two conditions are considered. First, when the sorbed gas extracts and consumes electrons from the sensitive layer, which decreases the conductivity of the sensitive layer and increases the central frequency of the sensor, the frequency shifts induced by both conductivity variation and sorbed mass counteract each other and decrease the sensitivity of the
sensor. Fortunately, the frequency shift induced by the variation of the conductivity reduces to zero when the initial conductivity of the sensitive layer is outside the range of the conductivity window, and meanwhile, the initial conductivity hardly influences the Sm . Therefore, the influence of the conductivity variation on the sensitivity of the sensor can be easily eliminated by choosing a sensitive layer with an initial conductivity outside the conductivity window. On the other hand, if the sorbed gas produces electrons into the sensitive layer, which increases the surface conductivity, both the variation of conductivity and sorbed mass decreases the central frequency of the sensor. In this case, it is favorable if high Sc and Sm can be obtained simultaneously. Therefore, the initial conductivity of the sensitive layer must be set in the conductivity window and a small relative dielectric constant of the sensitive layer must be selected simultaneously. Furthermore, in the performance optimizations of the SAW sensors, high ECCs are also required. Since the Sc and the ECC are closely related with each other, they can be optimized simultaneously [18], then the optimized operating frequencies of the sensors can be chosen according to the frequency responses of the Sc in Fig. 2 and those of the Sm in Figs. 12–14.
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
121
Fig. 13. Frequency response of Sm of sensor with AlN film and different hs of sensitive layer: (a) Rayleigh mode; (b) A0 mode and (c) S0 mode of Lamb wave.
Fig. 14. Frequency response of Sm of sensor with PZT film and different hs of sensitive layer: (a) Rayleigh mode; (b) A0 mode and (c) S0 mode of Lamb wave.
As shown in Figs. 12–14, high Sm of the Rayleigh wave and A0 mode of the Lamb wave are obtained with high operating frequencies. Comparing them with Fig. 2(a) and (b), in the sensors with ZnO or AlN films, the optimized frequencies for high Sc of the Rayleigh wave and A0 mode of the Lamb wave are in the range of low frequency. In this case, Sc and Sm cannot be optimized simultaneously with the same operating frequency. Then, by considering both sensitivities and ECCs, in the sensors with ZnO or AlN films and based on the Rayleigh wave or A0 mode of the Lamb wave, the operating frequencies are chosen to be the optimized frequencies for high Sc and ECC, but at some expense of Sm . On the other hand, as shown in Fig. 2(c), in the sensor with the PZT piezoelectric film, the high Sc of the Rayleigh wave and A0 mode of the Lamb wave can be obtained with high frequencies, which is basically in agreement with the optimized frequencies for the Sm . Furthermore, as shown in Figs. 12–14, in three types of sensors, the Sm for the S0 mode of the Lamb wave reaches the maximum in the frequency range of 500–700 MHz, with which a high Sc can also be obtained, as shown in Fig. 2. Therefore, using the S0 mode, the Sc , Sm and ECC can be optimized simultaneously in the frequency range of 500–700 MHz, which is favorable to the SAW gas sensors.
6. Conclusion 1. The transfer matrix method is used to theoretically simulate the sensitivity of the SAW gas sensors fabricated by the multilayered structures consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate by simultaneously considering the Sc and Sm . Based on the results, the sensitivities of the sensors can be optimized by proper selection of the operating frequencies, structural parameters and properties of the multilayered materials in the sensors. 2. It is found that the central frequencies of the sensors are generally decreased by sorbates, but sometimes they are increased by the sorbates, i.e., the “abnormal response” phenomena, which is attributed to the decrease of the conductivity by the sorbates, and in this case, the frequency shifts induced by the variation of conductivity and the sorbed mass counteract each other. If the frequency shift induced by the variation of conductivity dominates over that induced by the sorbed mass, the sensitivity of the sensor is mainly determined only by Sc , which can be optimized according to the results obtained above. Otherwise, the frequency shift induced by the variation of conductivity has adverse effects on the sensitivity, fortunately, the influence can
122
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
be eliminated by setting the initial conductivity of the sensitive layer out of the “conductivity window”. However, it is favorable if a proper sensitive layer with the conductivity increased by the sorbate can be achieved, in this case, the frequency shifts induced by both effects superpose with each other, then Sc , Sm and ECC can be optimized simultaneously. 3. At last, although the influences of the Sc have been proved by “abnormal response” in the experiments [19–21], it is not easy to evaluate the contributions of Sc and Sm of the sensors separately due to the coaction of both effects. In order to achieve a check of the simulations, two experimental projects are suggested. First, when the initial conductivity of the sensitive layer is set out of the “conductivity window” or the relative dielectric constant is large, the influence of the Sc can be eliminated. In this case, the Sm can be independently evaluated by selecting the structural parameters and operating frequencies of the sensors. However, when the conductivity sensitivity is evaluated, the influence of the sorbed mass cannot be separated from the response of the sensor. It was found that in the gas sensor detecting a light gas, such as the hydrogen, the influence of the surface conductivity becomes more obvious [22], then in the type of sensors, by neglecting the influence of the sorbed mass, the conductivity sensitivity can be approximately evaluated.
Acknowledgements This work is supported by the National Basic Research Program of China No. 2012CB921504, National Natural Science Foundation of China, Nos. 10904067, 11174142, Ph.D. Programs Foundation of Ministry of Education of China, No. 20090091120050 and the PAPD of Jiangsu Higher Education Institutions.
References [1] C. Caliendo, P. Verardi, E. Verona, A. D’Amico, C.D. Natale, G. Saggio, M. Serafini, R. Paolesse, S.E. Huq, Advances in SAW-based gas sensor, Smart Mater. Struct. 6 (1997) 689–699. [2] W.P. Jakubik, M.W. Urbanczyk, S. Kochowski, J. Bodzenta, Palladium and phthalocyanine bilayer films for hydrogen detection in a surface acoustic wave sensor system, Sens. Actuators B 96 (2003) 321–328. [3] W.P. Jakubik, M.W. Urbanczyk, SAW hydrogen sensor with a bilayer structure based on interaction speed, Sens. Actuators B 106 (2005) 602–608. [4] S.J. Ippolito, S. Kandasamy, K. Kalantar-Zadeh, W. Wlodarski, Layered SAW hydrogen sensor with modified tungsten trioxide selective layer, Sens. Actuators B 108 (2005) 553–557. [5] A.C. Fechete, W. Wlodarski, K. Kalantar-Zadeh, A.S. Holland, J. Antoszewski, S. Kaciulis, L. Pandolfi, SAW-based gas sensors with rf sputtered InOx and PECVD SiNx films: Response to H2 and O3 gases, Sens. Actuators B 118 (2006) 362–367. [6] H.C. Hao, K.T. Tang, P.H. Ku, J.S. Chao, C.H. Li, C.M. Yang, D.J. Yao, Development of a portable electronic nose based on chemical surface acoustic wave array with multiplexed oscillator and readout electronics, Sens. Actuators B 146 (2010) 545–553. [7] W. Wang, S.T. He, S.Z. Li, M.H. Liu, Y. Pan, Advances in SXFA-coated SAW chemical sensors for organophosphorous compound detection, Sensors 11 (2011) 1526–1541. [8] D.S. Ballantine, S.J. Martin, H. Wohltjen, R.M. White, E.T. Zellers, Acoustic Wave Sensors Theory, Design, and Physico-chemical Applications, Academic Press, New York, 1997, pp. 3–7. [9] M.J. Vellekoop, Acoustic wave sensors and their technology, Ultrasonics 36 (1998) 7–14. [10] A. Pohl, A review of wireless SAW sensors, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 47 (2000) 317–332. [11] E.T. Zellers, R.M. White, S.W. Wenzel, Computer modelling of polymer-coated ZnO/Si surface-acoustic-wave and lamb-wave chemical sensors, Sens. Actuators A 14 (1988) 35–45. [12] W. Wang, K. Lee, T. Kim, I. Park, S. Yang, A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line, Smart Mater. Struct. 16 (2007) 1382–1389. [13] W. Lim, J.S. Wright, B.P. Gila, J.L. Johnson, A. Ural, T. Anderson, F. Ren, S.J. Pearton, Room temperature hydrogen detection using Pd-coated GaN nanowires, Appl. Phys. Lett. 93 (2008) 072109. [14] N. Al-Hardan, M.J. Abdullah, A. Abdul Aziz, The gas response enhancement from ZnO film for H2 gas detection, Appl. Surf. Sci. 255 (2009) 7794–7797.
[15] S.W. Wenzel, R.M. White, Analytic comparison of the sensitivities of bulkwave, surface-wave and flexural plate-wave ultrasonic gravimetric sensors, Appl. Phys. Lett. 54 (1989) 1976–1978. [16] J. Enderlein, S. Makarov, E. Chilla, H.J. Fröhlich, Mass sensitivity of temperaturestabilized surface acoustic wave delay line on GaAs, Sens. Actuators B 24-25 (1995) 65–68. [17] C. Zimmermann, D. Rebiere, C. Dejous, J. Pistre, E. Chastaing, R. Planade, A love-wave gas sensor coated with functionalized polysiloxane for sensing organophosphorus compounds, Sens. Actuators B 76 (2001) 86–94. [18] L. Fan, F.M. Zhou, C. Wang, H.D. Gao, S.Y. Zhang, H. Zhang, X.B. Wang, Influence of surface conductivity on sensitivity of acoustic wave gas sensors based on multilayered structures, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 58 (2011) 451–460. [19] R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K. Kalantar-zadeh, Hydrogen gas sensor based on highly ordered polyaniline nanofibers, Sens. Actuators B 137 (2009) 529–532. [20] M.Z. Atashbar, A.Z. Sadek, W. Wlodarski, S. Sriram, M. Bhaskaran, C.J. Cheng, R.B. Kaner, K. Kalantar-zadeh, Layered SAW gas sensor based on CSA synthesized polyaniline nanofiber on AlN on 64 degrees YX LiNbO3 for H2 sensing, Sens. Actuators B 138 (2009) 85–89. [21] S.J. Ippolito, S. Kandasamy, K. Kalantar-zadeh, W. Wlodarski, K. Galatsis, G. Kiriakidis, N. Katsarakis, M. Suchea, Highly sensitive layered ZnO/LiNbO3 SAW device with InOx selective layer for NO2 and H2 gas sensing, Sens. Actuators B 111-112 (2005) 207–212. [22] A.Z. Sadek, C.O. Baker, D.A. Powell, W. Wlodarski, R.B. Kaner, K. Kalantar-zadeh, Polyaniline nanofiber based surface acoustic wave gas sensors—effect of nanofiber diameter on H2 response, IEEE Sens. J. 7 (2007) 213–218. [23] R. Arsat, M. Breedon, M. Shafiei, P.G. Spizziri, S. Gilje, R.B. Kaner, K. Kalantarzadeh, W. Wlodarski, Graphene-like nano-sheets for surface acoustic wave gas sensor applications, Chem. Phys. Lett. 467 (2009) 244–267. [24] S.G. Joshi, Y. Jin, Propagation of ultrasonic Lamb waves in piezoelectric plates, J. Appl. Phys. 70 (1991) 4113–4120. [25] J.T. Stewart, Y. Yong, Exact analysis of the propagation of acoustic waves in multilayered anisotropic piezoelectric plate, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 41 (1994) 375–390. [26] H.D. Gao, S.Y. Zhang, X. Qi, K. Wasa, H.D. Wu, Influence of material parameters on acoustic wave propagation modes in ZnO/Si bi-layered structures, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 52 (2005) 2361–2369. [27] L. Fan, S.Y. Zhang, K. Zheng, W. Lin, H.D. Gao, Calculation of electromechanical coupling coefficient of Lamb waves in multilayered plates, Ultrasonics 44 (2006) e849–e852. [28] D.S. Ballantine, S.J. Martin, H. Wohltjen, R.M. White, E.T. Zellers, Acoustic Wave Sensors Theory, Design, and Physico-chemical Applications, Academic Press, New York, 1997, pp. 272–292. [29] H. Nakagawa, N. Yamamoto, S. Okazaki, T. Chinzei, S. Asakur, A roomtemperature operated hydrogen leak sensor, Sens. Actuators B 93 (2003) 468–474. [30] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sensing characteristics of tin-doped ZnO thin films as NO2 gas sensor, Sens. Actuators B 107 (2005) 379–386. [31] S. Virji, R.B. Kaner, B.H. Weiller, Hydrogen sensors based on conductivity changes in polyaniline nanofibers, J. Phys. Chem. B 110 (2006) 22266–22270. [32] A. Wixforth, J.P. Kotthaus, Quantum Oscillations in the surface-acoustic-wave attenuation caused by a two-dimensional electron system, Phys. Rev. Lett. 56 (1986) 2104–2106. [33] J. Pedros, F. Calle, R. Cuerdo, J. Grajal, Z. Bougrioua, Voltage tunable surface acoustic wave phase shifter on AlGaN/GaN, Appl. Phys. Lett. 96 (2010) 123505.
Biographies Li Fan received his B.S. and Ph.D. degrees in acoustics from Institute of Acoustics, Nanjing University, Nanjing, P.R. China, in 2002 and 2007, respectively. Since then he has been with the Lab of Modern Acoustics, Institute of Acoustics of Nanjing University, where he focuses on the research of thermoacoustic refrigeration and SAW sensors. Now he is an associate professor of the School of Physics, Nanjing University. As the author and co-author, he has published more than 20 papers. Huan Ge is now an undergraduate student of School of Physics, Nanjing University, and will start to pursue his master degree in Acoustics from Institute of Acoustics, School of Physics, Nanjing University, in September, 2011. Shu-yi Zhang graduated from the Department of Physics, Nanjing University, Nanjing, China, and then from the graduate school of the same University with a speciality in acoustics. Since then, she has been with the Department of Physics and Institute of Acoustics, Nanjing University. Since 1986, she has been appointed as a Professor of the Department of Electronic Science and Engineering, Nanjing University (1986), and then as a Member of Chinese Academy of Sciences (1991). Now she is a Professor of School of Physics, Nanjing University (2010). She has also been a Visiting Associate Professor of Wayne State University, USA, (1985), a Visiting Professor of ESPCI, Paris, France, (1988), and also The University of Tokyo, Japan, (1991). During 1992–2001, she has been the Director of the Institute of Acoustics, Nanjing University, in the period as a Chairperson, she organized 9th Int. Conf. on Photoacoustic and Photothermal Phenomena, and 7th and 8th Int. Workshops on Acoustics, in
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123 Nanjing, and then edited 3 Proceedings of the Conferences (Workshops). Her research fields include molecular acoustics, surface acoustic waves, acousto-optic interaction, photoacoustics and ultrasonic physics, devices and NDE, etc. As the author and co-author, she has published more than 300 papers and 4 chapters included in 4 monographs on photoacoustics and acoustics published in USA and Europe. Hui Zhang received his B.E. degree from Dalian Jiaotong University, China in 2000, M.E. degree from Southeast University, Nanjing, China in 2003, and Ph.D. degree in acoustics from the Institute of Acoustics, Nanjing University (NJU), China, in
123
2006. Since then, he has worked in the Lab of Modern Acoustics, at the Institute of Acoustics of Nanjing University. Now he is an associate professor of the School of Physics of NJU. His research interests are in numerical modeling of bulk acoustic wave resonators, acoustic actuators, and acoustic sensors. Jian Zhu was born in 1962 and received her Ph.D. degree from the School of Instrument Science and Engineering in Southeast University, Nanjing, China. Now she is the director of the second center in the Electronics Technology Group Corporation of China, No.55 Research Institute, Nanjing, P.R. China.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Optimization of sensitivity induced by surface conductivity and sorbed mass in surface acoustic wave gas sensors Li Fan a,∗ , Huan Ge a , Shu-yi Zhang a , Hui Zhang a , Jian Zhu b a b
Laboratory of Modern Acoustics, Institute of Acoustics, Nanjing University, Nanjing 210093, China Electronics Technology Group Corporation of China, No. 55 Research Institute, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 4 May 2011 Received in revised form 26 September 2011 Accepted 26 September 2011 Available online 20 October 2011 Keywords: Gas sensor Surface acoustic wave Piezoelectric film Sensitivity optimization
a b s t r a c t Acoustic transmissions in surface acoustic wave (SAW) gas sensors based on multilayered structures consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate are studied with the transfer matrix method, in which the shifts of the central frequencies of SAW sensors induced by the variations of surface conductivities and sorbed masses are evaluated. According to the results, the sensitivities of SAW gas sensors using different piezoelectric films are optimized by simultaneously considering the variations of surface conductivities and masses caused by sorbates on the surfaces of the sensors. It is found that the frequency shift induced by the variation of surface conductivity is closely related with the initial conductivity and relative dielectric constant of the sensitive layer, while the frequency shift induced by the sorbed mass is almost independent on them. Furthermore, the operating frequencies, SAW modes and characteristics of sensitive layers are optimized in order to achieve high sensitivities according to the responses of the SAW gas sensors to different types of the detected gases. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Surface acoustic wave (SAW) chemical sensors have been widely studied and applied in the detection of toxic, inflammable and explosive gases [1–7] in virtue of high sensitivity, small size, and potential for wireless sensing [8–10]. In the development of SAW sensors, the multilayered structures consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate become the focus of research, in which piezoelectric films take the place of the bulk ones to excite the SAW. In this type of SAW sensors, when silicon wafers are used as the substrates [1], the sensors have the potential to be integrated with electric circuits for micro-electro-mechanical systems (MEMS), which can realize the miniaturization, portability and long distance wireless data communication [10–12]. In the researches of SAW chemical sensors, it has been known that in addition to changing the sorbed masses, the matters sorbed by the sensitive layers can change the conductivities of the sensitive layers [13,14], which also influence the SAW velocities or center frequencies of the sensors. Then, similar to the mass sensitivity Sm [15], the conductivity sensitivity Sc can be defined to
Abbreviations: SAW, surface acoustic wave; MEMS, micro-electro-mechanical system; ECC(s), electromechanical coupling coefficient(s). ∗ Corresponding author. Tel.: +86 25 83597484; fax: +86 25 83593690. E-mail address: tx [email protected] (L. Fan). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.09.077
be the relative variation of the central frequency of the SAW sensor caused by the variation of the conductivity. According to the related theory [15–17] and numerous experiments, Sm is always negative, i.e., the center frequency of the SAW sensor is always decreased by the sorbed mass. However, the conductivity of the sensitive layer can be increased or decreased by the sorbates, which decreases or increases the center frequency of the SAW sensor, respectively. If the center frequency is increased by the variation of the conductivity, an “abnormal” response of the sensor can occur [18–21]. Especially, for the SAW gas sensor, the influence of sorbed mass induced by the gas is weak and then the influence of the variation of the surface conductivity becomes important [22,23]. Therefore, both Sc and Sm must be considered simultaneously in the performance optimizations of SAW gas sensors. In this paper, the transfer matrix method [24–27] is used to evaluate and optimize both Sc and Sm of the SAW sensor based on the multilayered structure consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate. The influences of the initial conductivities, relative dielectric constants and thicknesses of the sensitive layers, the properties of piezoelectric films, modes of SAW waves and operating frequencies on the sensitivities of SAW sensors are evaluated. Based on the results, the optimized sensitivities of the SAW gas sensors can be obtained by using proper SAW modes, sensitive layers, piezoelectric films and operating frequencies according to different detected gases.
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
115
Fig. 1. Multilayered structure and dispersion curve of sensors: (a), (c) Rayleigh wave; (b), (d) Lamb wave.
2. Theory The sorption of gaseous matters by the sensitive layers can be classified into two types: adsorption and absorption [28]. Adsorption is sorption onto a surface and absorption can be seen as dissolving into the absorbent material, the latter often happens when a sensitive layer sorbs the detected gas [28,29], which changes the conductivity and increases the mass of the layer. Fig. 1 shows the models of SAW sensors based on multilayered structures, in which piezoelectric films are deposited on nonpiezoelectric substrates and covered by sensitive layers, x1 is the propagation direction of the SAW and x2 is the normal direction. Fig. 1(a) shows the model of the sensor based on the Rayleigh wave, in which the substrate is semi-infinite and the Rayleigh wave is dispersive due to the multilayer. Fig. 1(b) shows the model based on the Lamb wave, in which the multilayered structure forms a thin plate. The corresponding dispersion curves of the Rayleigh wave and the A0 and S0 modes of the Lamb wave are shown in Fig. 1(c) and (d) respectively. The acoustic and electromagnetic waves in the rth layer can be expressed as [24–27]:
As the conductivity of the sensitive layer (r = 1) is considered, the electromagnetic wave equation in the sensitive layer must contain the dissipation item as [18]: [1] [1] [1] ¨ [1] [1] eikl u¨ k,il − εik − J˙ k,k = 0 ,ik [1]
[1]
(3)
[1]
[1]
in which Jk,k = ik ,ik is the conduction current and ik is the conductivity of the material. For the simple harmonic wave, Eq. (3) can be rewritten as:
[1] [1] eikl uk,il
−
[1] εik
[1]
−
ik
[1]
jω
,ik = 0
(4)
Comparing Eq. (4) with Eq. (2), it shows that Eq. (4) can be obtained by replacing εik of Eq. (2) with an equivalent dielectric constant εik − ik /jω. The solutions of equation set (1) and (2) or (1) and (4) for the SAW wave (Rayleigh and Lamb waves) can be expressed in the same forms as: uj =
8
˛jq Bq exp j k1 x1 + k2q x2 − ωt
(5)
q=1 [r]
[r]
[r]
[r]
[r]
cijkl uk,il + ekij ,ik = [r] u¨ j
(1) =
[r] [r] eikl uk,il
[r] [r] − εik ,ik
=0
[r]
and εik are the elastic, piezoelectric and dielectric matrices of the rth material, respectively, i, j, k, l = 1, 2, 3 indicate the directions of [r] x1 , x2 and x3 . [r] is the density, uj and [r] are the particle displacement and electric potential, respectively.
˛4q Bq exp j k1 x1 + k2q x2 − ωt
(6)
q=1
(2)
where the superscript r = 1, 2, 3 indicates the sensitive layer, piezo[r] [r] electric film and non-piezoelectric substrate, respectively. cijkl , ekij
8
in which ˛jq and ˛4q are the amplitudes of the partial wave solutions and Bq is the undetermined coefficients; k1 and k2q = bq k1 are the wave numbers in the x1 and x2 directions, respectively; ω and t are the angle frequency and time, respectively. The stress and electric displacement of the normal direction x2 can be expressed as: [r]
[r]
[r]
[r]
[r]
T2j = c2jkl uk,l + ek2j ,k
(7)
116
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 3. Influence of 0 and ε of sensitive layer on Sc of sensor with ZnO piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
Defining = D˙ + J, then the normal component of the vector must be continuous at surfaces and interfaces of the multilayered structure. For the simple harmonic wave, Eq. (10) becomes:
Fig. 2. Frequency responses of Sc in sensors with different piezoelectric films: (a) ZnO; (b) AlN; (c) PZT.
[r]
[r]
[r]
[r]
[r]
D2 = e2kl uk,l − ε2k ,k
(8)
Substituting Eqs. (5) and (6) into Eqs. (7) and (8) results in the equation set of the stress and electric displacement. The mechanical boundary conditions are independent on the electric loss, while the electric boundary conditions are dependent on it. When the material is electrically lossless and no charge exists at the surfaces and interfaces, the Gaussian equation is: Di,i = 0
(9)
Therefore, the normal component of the electric displacement D is continuous at the surfaces and interfaces. While when the conduction current is considered, the Gaussian equation becomes: D˙ i,i + Ji,i = 0
(10)
Di,i −
Ji,i jω
=0
(11)
Therefore, similar to Eq. (4), the new electric boundary condition can also be obtained by replacing εik in the boundary condition of the lossless material with the equivalent dielectric constant εik − ik /jω. Then the dispersion curves of the Rayleigh and Lamb waves can be obtained from the coefficient matrix of the equation set of the boundary conditions with or without the electrical loss, and the influence of the conductivity on the SAW velocity or the central frequency of the sensor can be evaluated. Generally, the sensitive layer is non-piezoelectric, however, due to the existence of the piezoelectric film, the acoustic and electric fields in the multilayered material are coupled with each other and the fields are connected at the interfaces of the multilayer material, then the variation of the conductivity of the sensitive layer changes the acoustic velocity of the SAW in the multilayered structure, which can be expressed by the piezoelectric stiffened elastic constant: CKL + jCKL = CKL +
eKj Kj [Ki eiL ]
Ki εij − ij /jω Kj
(12)
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 4. Influence of 0 and ε of sensitive layer on Sc of sensor with AlN piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
where CKL is the equivalent elastic matrix without the piezoelecT , tric effect, eKj is the equivalent piezoelectric matrix and eiL = eKj the superscript T indicates the transposition; CKL is the real part of is the the equivalent piezoelectric stiffened elastic matrix and CKL imaginary part indicating the electric loss caused by the conduction current. They are all expressed with the contracted subscript for convenience. Ki = [k1 , k2 , 0] and Kj = KiT . Then the influence of the conductivity on the acoustic velocity is related to the real part of the stiffened part of the equivalent elastic constant:
=− dCKL
2 eKj Kj [Ki eiL ] εij ij
ε2ij ω + ij2 /ω
2 dij
(13)
Eq. (13) shows that a positive shift of the conductivity dij results , and vice versa. in a negative shift of equivalent elastic constant dCKL Therefore, if the sorbate increases the conductivity of the sensitive layer, the equivalent elastic constant is decreased, which decreases the SAW velocity or the central frequency of the sensor. On the other hand, if the sorbate decreases the conductivity of the sensitive layer, the SAW velocity and central frequency of the sensor increase. In this case, when the frequency shift induced by the conductivity variation is larger than that coming from the sorbed mass, an “abnormal” response can occur, in which the central frequency of the sensor increases after sorbing gases [19–21].
117
Fig. 5. Influence of 0 and ε of sensitive layer on Sc of sensor with PZT piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
Eq. (13) shows that several factors dominate the influences of the surface conductivity on the SAW velocity, i.e., the Sc of the sensor. First, an optimized angle frequency ω brings the maximum , then the maximum S can be obtained at the optimized fredCKL c quency. Meanwhile, the frequency response of the Sc is similar to that of the electromechanical coupling coefficient (ECC) because they are both related with the coupling between the acoustic field and electromagnetic field [18]. Second, the piezoelectric constant eKj influences the Sc , which is mainly related with the piezoelectric film in the sensor. Finally, the equivalent conductivity ij and relative dielectric constant εij also have the influences on the Sc , therefore, the multilayered materials with the proper conductivities and relative dielectric constants must be selected in the SAW sensors. The numerical simulations are based on the multilayered structures shown in Fig. 1, in which a silicon wafer and a WO3 layer are taken as the substrate and sensitive layer respectively, and three types of piezoelectric films of ZnO, AlN and PZT are used to excite the SAW, the materials are usually used in SAW gas sensors. It is assumed that the thickness of the piezoelectric film is 1 m, the silicon substrate is semi-infinite for the Rayleigh wave in Fig. 1(a) and 3 m for the Lamb wave in Fig. 1(b), the thickness of the sensitive layer is 0.2 m except that the influences of different thicknesses of the sensitive layers are evaluated.
118
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 6. Maximum Sc of sensor with ZnO film and different 0 and hs of sensitive layer: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
3. Conductivity sensitivity Sc 3.1. Frequency response of Sc In order to optimize the Sc of the SAW sensor, the relative frequency shift induced by the variation of the conductivity of sensitive layer is obtained with the transfer matrix method. According to practical experiments of gas sensors, the initial conductivities 0 of sensitive layers before the sorption of any gases vary in a wide range of about 0.01–100 S/m3 [29,30]. Meanwhile, the relative variations of the conductivities induced by the sorbed gases also vary from several per mille [31] to several percents [13]. Therefore, in the simulations, the initial conductivities of the sensitive layers are chosen in a wide range of 0.1–60 S/m3 . Furthermore, in order to compare the results, the variations of the conductivities induced by the sorbed gases are assumed to be 10 mS/m3 , then the relative variations of the conductivities are in the range of about 0.01–10%. Fig. 2 shows the relative frequency shifts in the sensors with different piezoelectric films and SAW modes. As shown in Fig. 2(a) and (b), the frequency responses of the frequency shifts in the sensors with ZnO and AlN piezoelectric films are similar, in which the optimized frequencies for the Sc of the Rayleigh wave and the A0 mode of Lamb wave are in the low frequency ranges and the optimized frequency for that of the S0 mode of Lamb wave is higher. However, in the sensor with PZT piezoelectric film, as shown in Fig. 2(c), high Sc of the Rayleigh wave and A0 mode can be obtained at frequencies higher than the optimized frequency for the S0 mode.
Fig. 7. Maximum Sc of sensor with AlN film and different 0 and hs of sensitive layer: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
with the sheet conductivity of M = Vε0 1 + εp , where ε0 is the dielectric constant of air and εp is the relative dielectric constant of the bulk piezoelectric material. Similarly, in the multilayered structure, a large relative dielectric constant of the piezoelectric film also increases the optimized initial conductivity of the sensitive layer for high Sc . PZT is a type of piezoelectric material with high relative dielectric constant ε, which is 50–100 times larger than that of ZnO or AlN, so the initial conductivities of sensitive layers are much larger than those with the ZnO or AlN films for optimizing Sc of the sensors. Furthermore, it can be seen that the Sc keeps constant while ε of the sensitive layer varies in a large range, but Sc decrease when ε exceeds a value εc [as indicated in Fig. 3(a)]. As shown in Figs. 3 and 4, for the ZnO and AlN films, the Sc of the three wave modes decrease when ε increases to the order of the relative dielectric constants (about 10) of ZnO or AlN. While as shown in Fig. 5, with the PZT film, the Sc of the three wave modes decrease when ε increase to about 500, which is also the same order as the relative dielectric constants of PZT. 3.3. Optimization of thickness of sensitive layer Another important factor determining the Sc is the thickness of the sensitive layer hs , which is evaluated in Figs. 6–8 according
3.2. Influence of initial conductivity and relative dielectric constant of sensitive layer Figs. 3–5 show the influences of the initial conductivity 0 and relative dielectric constant ε of the sensitive layer on the Sc . As shown in the three figures, Sc decreases to zero if 0 becomes too small or too large, which means that regardless of the piezoelectric films and the wave modes, the initial conductivity of the sensitive layer must be set in a narrow “window” [as indicated in Fig. 3(a)] for obtaining high Sc . Furthermore, the conductivity window for high Sc is dependent on the piezoelectric film. For ZnO or AlN film, the high Sc can be obtained with the initial conductivity in the range of about 0.5–5.0 S/m3 , while for the PZT piezoelectric film, the conductivity window shifts to about 10–100 S/m3 . In a homogenous bulk piezoelectric material [32,33], the maximum SAW velocity shift induced by the variation of surface conductivity is obtained
Fig. 8. Maximum Sc of sensor with PZT film and different 0 and hs of sensitive layers: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 9. Influence of 0 and ε of sensitive layer on Sm of sensor with ZnO piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
to different initial conductivities of the sensitive layer. It can be seen that with smaller 0 , larger hs of the sensitive layers are required for obtaining high Sc . As shown in Figs. 6 and 7, similar results can be obtained in the sensors with ZnO or AlN piezoelectric films: by optimizing the thicknesses of the sensitive layers, high Sc of Rayleigh wave and S0 mode of the Lamb wave are achieved with 0 in the range of 0.5–3.0 S/m3 , and besides, high Sc of the A0 mode of the Lamb wave can also be obtained with a smaller 0 = 0.1 S/m3 . For the PZT film, to achieve high Sc , a sensitive layer with large initial conductivity 0 must be selected, as shown in Fig. 8, high Sc is obtained with the 0 larger than 10.0 S/m3 . The results are also attributed to the high relative dielectric constant of PZT, which makes the optimized sheet conductivity of the sensitive layer increase greatly, and so larger 0 is required for optimizing Sc . 4. Mass sensitivity Sm As described above, the sorption of gas into the sensitive layer increases the mass. Then the frequency shift (decrease) induced by the sorbed gas can also be evaluated by the transfer matrix method. Based on experiments, the detection limits of SAW gas sensors can be less than 100 pg/cm3 , and typically the mass sensitivities of 10–100 Hz/(ng cm2 ) can be achieved [9], so in the simulation, a sorbed mass of 20 ng/cm2 is assumed.
119
Fig. 10. Influence of 0 and ε of sensitive layer on Sm of sensor with AlN piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave.
4.1. Influence of initial conductivity and relative dielectric constant of sensitive layer Figs. 9–11 shows the relative frequency shifts induced by the sorbed masses in the SAW sensors with three types of piezoelectric films of ZnO, AlN and PZT, in which the influences of the initial conductivity 0 and relative dielectric constant ε of the sensitive layer on the Sm are evaluated. Different from the Sc , the Sm is almost independent on 0 and ε, in which the largest variation is only a decrease of 15% in the Sm of the S0 mode of the Lamb wave in the sensor using PZT as the piezoelectric film. It is reasonable that the electrical parameters 0 and ε have more obvious influences on the Sc than those on the Sm . 4.2. Optimization of operating frequency and thickness of sensitive layer The frequency responses of the Sm in the sensors with different thicknesses of sensitive layers hs are shown in Figs. 12–14, in which 0 and ε of the sensitive layers are set as 1 S/m3 and 10, respectively, since they hardly influence the Sm . It can be seen that in the three types of sensors with different piezoelectric films, similar frequency responses of the Sm are obtained. For the Rayleigh wave and A0 mode of the Lamb wave, high Sm can be obtained by increasing the operating frequency, while for the S0 mode of the
120
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
Fig. 11. Influence of 0 and ε of sensitive layer on Sm of sensor with PZT piezoelectric film at operating frequency 200 MHz: (a) Rayleigh wave; (b) A0 mode and (c) S0 mode of Lamb wave. Fig. 12. Frequency response of Sm of sensor with ZnO film and different hs of sensitive layer: (a) Rayleigh mode; (b) A0 mode and (c) S0 mode of Lamb wave.
Lamb wave, high Sm is obtained at one optimized frequency. On the other hand, as shown in Figs. 12–14, the influences of hs on the Sm of the Rayleigh wave and A0 mode of the Lamb wave are slight, especially in the sensor using ZnO as the piezoelectric film, the Sm of the Rayleigh wave and A0 mode of the Lamb wave are almost independent on hs . While for the S0 mode of the Lamb wave, the Sm at the optimized frequency is rapidly increased by decreasing the thickness of sensitive layer. Furthermore, in the sensor with the AlN piezoelectric film, a peak of the Sm occurs at a lower frequency when the thickness of the sensitive layer increases to 0.3 m, as shown in Fig. 13(c). 5. Optimization of performance Both the Sc and Sm must be considered simultaneously in the optimizations of the performances of the SAW gas sensor. Additionally, high ECC must also be assured for obtaining high signal-to-noise ratios in the applications of SAW sensors. According to the responses of the sensor to different sorbed gases, two conditions are considered. First, when the sorbed gas extracts and consumes electrons from the sensitive layer, which decreases the conductivity of the sensitive layer and increases the central frequency of the sensor, the frequency shifts induced by both conductivity variation and sorbed mass counteract each other and decrease the sensitivity of the
sensor. Fortunately, the frequency shift induced by the variation of the conductivity reduces to zero when the initial conductivity of the sensitive layer is outside the range of the conductivity window, and meanwhile, the initial conductivity hardly influences the Sm . Therefore, the influence of the conductivity variation on the sensitivity of the sensor can be easily eliminated by choosing a sensitive layer with an initial conductivity outside the conductivity window. On the other hand, if the sorbed gas produces electrons into the sensitive layer, which increases the surface conductivity, both the variation of conductivity and sorbed mass decreases the central frequency of the sensor. In this case, it is favorable if high Sc and Sm can be obtained simultaneously. Therefore, the initial conductivity of the sensitive layer must be set in the conductivity window and a small relative dielectric constant of the sensitive layer must be selected simultaneously. Furthermore, in the performance optimizations of the SAW sensors, high ECCs are also required. Since the Sc and the ECC are closely related with each other, they can be optimized simultaneously [18], then the optimized operating frequencies of the sensors can be chosen according to the frequency responses of the Sc in Fig. 2 and those of the Sm in Figs. 12–14.
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
121
Fig. 13. Frequency response of Sm of sensor with AlN film and different hs of sensitive layer: (a) Rayleigh mode; (b) A0 mode and (c) S0 mode of Lamb wave.
Fig. 14. Frequency response of Sm of sensor with PZT film and different hs of sensitive layer: (a) Rayleigh mode; (b) A0 mode and (c) S0 mode of Lamb wave.
As shown in Figs. 12–14, high Sm of the Rayleigh wave and A0 mode of the Lamb wave are obtained with high operating frequencies. Comparing them with Fig. 2(a) and (b), in the sensors with ZnO or AlN films, the optimized frequencies for high Sc of the Rayleigh wave and A0 mode of the Lamb wave are in the range of low frequency. In this case, Sc and Sm cannot be optimized simultaneously with the same operating frequency. Then, by considering both sensitivities and ECCs, in the sensors with ZnO or AlN films and based on the Rayleigh wave or A0 mode of the Lamb wave, the operating frequencies are chosen to be the optimized frequencies for high Sc and ECC, but at some expense of Sm . On the other hand, as shown in Fig. 2(c), in the sensor with the PZT piezoelectric film, the high Sc of the Rayleigh wave and A0 mode of the Lamb wave can be obtained with high frequencies, which is basically in agreement with the optimized frequencies for the Sm . Furthermore, as shown in Figs. 12–14, in three types of sensors, the Sm for the S0 mode of the Lamb wave reaches the maximum in the frequency range of 500–700 MHz, with which a high Sc can also be obtained, as shown in Fig. 2. Therefore, using the S0 mode, the Sc , Sm and ECC can be optimized simultaneously in the frequency range of 500–700 MHz, which is favorable to the SAW gas sensors.
6. Conclusion 1. The transfer matrix method is used to theoretically simulate the sensitivity of the SAW gas sensors fabricated by the multilayered structures consisting of sensitive layer/piezoelectric film/non-piezoelectric substrate by simultaneously considering the Sc and Sm . Based on the results, the sensitivities of the sensors can be optimized by proper selection of the operating frequencies, structural parameters and properties of the multilayered materials in the sensors. 2. It is found that the central frequencies of the sensors are generally decreased by sorbates, but sometimes they are increased by the sorbates, i.e., the “abnormal response” phenomena, which is attributed to the decrease of the conductivity by the sorbates, and in this case, the frequency shifts induced by the variation of conductivity and the sorbed mass counteract each other. If the frequency shift induced by the variation of conductivity dominates over that induced by the sorbed mass, the sensitivity of the sensor is mainly determined only by Sc , which can be optimized according to the results obtained above. Otherwise, the frequency shift induced by the variation of conductivity has adverse effects on the sensitivity, fortunately, the influence can
122
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123
be eliminated by setting the initial conductivity of the sensitive layer out of the “conductivity window”. However, it is favorable if a proper sensitive layer with the conductivity increased by the sorbate can be achieved, in this case, the frequency shifts induced by both effects superpose with each other, then Sc , Sm and ECC can be optimized simultaneously. 3. At last, although the influences of the Sc have been proved by “abnormal response” in the experiments [19–21], it is not easy to evaluate the contributions of Sc and Sm of the sensors separately due to the coaction of both effects. In order to achieve a check of the simulations, two experimental projects are suggested. First, when the initial conductivity of the sensitive layer is set out of the “conductivity window” or the relative dielectric constant is large, the influence of the Sc can be eliminated. In this case, the Sm can be independently evaluated by selecting the structural parameters and operating frequencies of the sensors. However, when the conductivity sensitivity is evaluated, the influence of the sorbed mass cannot be separated from the response of the sensor. It was found that in the gas sensor detecting a light gas, such as the hydrogen, the influence of the surface conductivity becomes more obvious [22], then in the type of sensors, by neglecting the influence of the sorbed mass, the conductivity sensitivity can be approximately evaluated.
Acknowledgements This work is supported by the National Basic Research Program of China No. 2012CB921504, National Natural Science Foundation of China, Nos. 10904067, 11174142, Ph.D. Programs Foundation of Ministry of Education of China, No. 20090091120050 and the PAPD of Jiangsu Higher Education Institutions.
References [1] C. Caliendo, P. Verardi, E. Verona, A. D’Amico, C.D. Natale, G. Saggio, M. Serafini, R. Paolesse, S.E. Huq, Advances in SAW-based gas sensor, Smart Mater. Struct. 6 (1997) 689–699. [2] W.P. Jakubik, M.W. Urbanczyk, S. Kochowski, J. Bodzenta, Palladium and phthalocyanine bilayer films for hydrogen detection in a surface acoustic wave sensor system, Sens. Actuators B 96 (2003) 321–328. [3] W.P. Jakubik, M.W. Urbanczyk, SAW hydrogen sensor with a bilayer structure based on interaction speed, Sens. Actuators B 106 (2005) 602–608. [4] S.J. Ippolito, S. Kandasamy, K. Kalantar-Zadeh, W. Wlodarski, Layered SAW hydrogen sensor with modified tungsten trioxide selective layer, Sens. Actuators B 108 (2005) 553–557. [5] A.C. Fechete, W. Wlodarski, K. Kalantar-Zadeh, A.S. Holland, J. Antoszewski, S. Kaciulis, L. Pandolfi, SAW-based gas sensors with rf sputtered InOx and PECVD SiNx films: Response to H2 and O3 gases, Sens. Actuators B 118 (2006) 362–367. [6] H.C. Hao, K.T. Tang, P.H. Ku, J.S. Chao, C.H. Li, C.M. Yang, D.J. Yao, Development of a portable electronic nose based on chemical surface acoustic wave array with multiplexed oscillator and readout electronics, Sens. Actuators B 146 (2010) 545–553. [7] W. Wang, S.T. He, S.Z. Li, M.H. Liu, Y. Pan, Advances in SXFA-coated SAW chemical sensors for organophosphorous compound detection, Sensors 11 (2011) 1526–1541. [8] D.S. Ballantine, S.J. Martin, H. Wohltjen, R.M. White, E.T. Zellers, Acoustic Wave Sensors Theory, Design, and Physico-chemical Applications, Academic Press, New York, 1997, pp. 3–7. [9] M.J. Vellekoop, Acoustic wave sensors and their technology, Ultrasonics 36 (1998) 7–14. [10] A. Pohl, A review of wireless SAW sensors, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 47 (2000) 317–332. [11] E.T. Zellers, R.M. White, S.W. Wenzel, Computer modelling of polymer-coated ZnO/Si surface-acoustic-wave and lamb-wave chemical sensors, Sens. Actuators A 14 (1988) 35–45. [12] W. Wang, K. Lee, T. Kim, I. Park, S. Yang, A novel wireless, passive CO2 sensor incorporating a surface acoustic wave reflective delay line, Smart Mater. Struct. 16 (2007) 1382–1389. [13] W. Lim, J.S. Wright, B.P. Gila, J.L. Johnson, A. Ural, T. Anderson, F. Ren, S.J. Pearton, Room temperature hydrogen detection using Pd-coated GaN nanowires, Appl. Phys. Lett. 93 (2008) 072109. [14] N. Al-Hardan, M.J. Abdullah, A. Abdul Aziz, The gas response enhancement from ZnO film for H2 gas detection, Appl. Surf. Sci. 255 (2009) 7794–7797.
[15] S.W. Wenzel, R.M. White, Analytic comparison of the sensitivities of bulkwave, surface-wave and flexural plate-wave ultrasonic gravimetric sensors, Appl. Phys. Lett. 54 (1989) 1976–1978. [16] J. Enderlein, S. Makarov, E. Chilla, H.J. Fröhlich, Mass sensitivity of temperaturestabilized surface acoustic wave delay line on GaAs, Sens. Actuators B 24-25 (1995) 65–68. [17] C. Zimmermann, D. Rebiere, C. Dejous, J. Pistre, E. Chastaing, R. Planade, A love-wave gas sensor coated with functionalized polysiloxane for sensing organophosphorus compounds, Sens. Actuators B 76 (2001) 86–94. [18] L. Fan, F.M. Zhou, C. Wang, H.D. Gao, S.Y. Zhang, H. Zhang, X.B. Wang, Influence of surface conductivity on sensitivity of acoustic wave gas sensors based on multilayered structures, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 58 (2011) 451–460. [19] R. Arsat, X.F. Yu, Y.X. Li, W. Wlodarski, K. Kalantar-zadeh, Hydrogen gas sensor based on highly ordered polyaniline nanofibers, Sens. Actuators B 137 (2009) 529–532. [20] M.Z. Atashbar, A.Z. Sadek, W. Wlodarski, S. Sriram, M. Bhaskaran, C.J. Cheng, R.B. Kaner, K. Kalantar-zadeh, Layered SAW gas sensor based on CSA synthesized polyaniline nanofiber on AlN on 64 degrees YX LiNbO3 for H2 sensing, Sens. Actuators B 138 (2009) 85–89. [21] S.J. Ippolito, S. Kandasamy, K. Kalantar-zadeh, W. Wlodarski, K. Galatsis, G. Kiriakidis, N. Katsarakis, M. Suchea, Highly sensitive layered ZnO/LiNbO3 SAW device with InOx selective layer for NO2 and H2 gas sensing, Sens. Actuators B 111-112 (2005) 207–212. [22] A.Z. Sadek, C.O. Baker, D.A. Powell, W. Wlodarski, R.B. Kaner, K. Kalantar-zadeh, Polyaniline nanofiber based surface acoustic wave gas sensors—effect of nanofiber diameter on H2 response, IEEE Sens. J. 7 (2007) 213–218. [23] R. Arsat, M. Breedon, M. Shafiei, P.G. Spizziri, S. Gilje, R.B. Kaner, K. Kalantarzadeh, W. Wlodarski, Graphene-like nano-sheets for surface acoustic wave gas sensor applications, Chem. Phys. Lett. 467 (2009) 244–267. [24] S.G. Joshi, Y. Jin, Propagation of ultrasonic Lamb waves in piezoelectric plates, J. Appl. Phys. 70 (1991) 4113–4120. [25] J.T. Stewart, Y. Yong, Exact analysis of the propagation of acoustic waves in multilayered anisotropic piezoelectric plate, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 41 (1994) 375–390. [26] H.D. Gao, S.Y. Zhang, X. Qi, K. Wasa, H.D. Wu, Influence of material parameters on acoustic wave propagation modes in ZnO/Si bi-layered structures, IEEE Trans. Ultrason., Ferroelec., Freq. Contr. 52 (2005) 2361–2369. [27] L. Fan, S.Y. Zhang, K. Zheng, W. Lin, H.D. Gao, Calculation of electromechanical coupling coefficient of Lamb waves in multilayered plates, Ultrasonics 44 (2006) e849–e852. [28] D.S. Ballantine, S.J. Martin, H. Wohltjen, R.M. White, E.T. Zellers, Acoustic Wave Sensors Theory, Design, and Physico-chemical Applications, Academic Press, New York, 1997, pp. 272–292. [29] H. Nakagawa, N. Yamamoto, S. Okazaki, T. Chinzei, S. Asakur, A roomtemperature operated hydrogen leak sensor, Sens. Actuators B 93 (2003) 468–474. [30] S.T. Shishiyanu, T.S. Shishiyanu, O.I. Lupan, Sensing characteristics of tin-doped ZnO thin films as NO2 gas sensor, Sens. Actuators B 107 (2005) 379–386. [31] S. Virji, R.B. Kaner, B.H. Weiller, Hydrogen sensors based on conductivity changes in polyaniline nanofibers, J. Phys. Chem. B 110 (2006) 22266–22270. [32] A. Wixforth, J.P. Kotthaus, Quantum Oscillations in the surface-acoustic-wave attenuation caused by a two-dimensional electron system, Phys. Rev. Lett. 56 (1986) 2104–2106. [33] J. Pedros, F. Calle, R. Cuerdo, J. Grajal, Z. Bougrioua, Voltage tunable surface acoustic wave phase shifter on AlGaN/GaN, Appl. Phys. Lett. 96 (2010) 123505.
Biographies Li Fan received his B.S. and Ph.D. degrees in acoustics from Institute of Acoustics, Nanjing University, Nanjing, P.R. China, in 2002 and 2007, respectively. Since then he has been with the Lab of Modern Acoustics, Institute of Acoustics of Nanjing University, where he focuses on the research of thermoacoustic refrigeration and SAW sensors. Now he is an associate professor of the School of Physics, Nanjing University. As the author and co-author, he has published more than 20 papers. Huan Ge is now an undergraduate student of School of Physics, Nanjing University, and will start to pursue his master degree in Acoustics from Institute of Acoustics, School of Physics, Nanjing University, in September, 2011. Shu-yi Zhang graduated from the Department of Physics, Nanjing University, Nanjing, China, and then from the graduate school of the same University with a speciality in acoustics. Since then, she has been with the Department of Physics and Institute of Acoustics, Nanjing University. Since 1986, she has been appointed as a Professor of the Department of Electronic Science and Engineering, Nanjing University (1986), and then as a Member of Chinese Academy of Sciences (1991). Now she is a Professor of School of Physics, Nanjing University (2010). She has also been a Visiting Associate Professor of Wayne State University, USA, (1985), a Visiting Professor of ESPCI, Paris, France, (1988), and also The University of Tokyo, Japan, (1991). During 1992–2001, she has been the Director of the Institute of Acoustics, Nanjing University, in the period as a Chairperson, she organized 9th Int. Conf. on Photoacoustic and Photothermal Phenomena, and 7th and 8th Int. Workshops on Acoustics, in
L. Fan et al. / Sensors and Actuators B 161 (2012) 114–123 Nanjing, and then edited 3 Proceedings of the Conferences (Workshops). Her research fields include molecular acoustics, surface acoustic waves, acousto-optic interaction, photoacoustics and ultrasonic physics, devices and NDE, etc. As the author and co-author, she has published more than 300 papers and 4 chapters included in 4 monographs on photoacoustics and acoustics published in USA and Europe. Hui Zhang received his B.E. degree from Dalian Jiaotong University, China in 2000, M.E. degree from Southeast University, Nanjing, China in 2003, and Ph.D. degree in acoustics from the Institute of Acoustics, Nanjing University (NJU), China, in
123
2006. Since then, he has worked in the Lab of Modern Acoustics, at the Institute of Acoustics of Nanjing University. Now he is an associate professor of the School of Physics of NJU. His research interests are in numerical modeling of bulk acoustic wave resonators, acoustic actuators, and acoustic sensors. Jian Zhu was born in 1962 and received her Ph.D. degree from the School of Instrument Science and Engineering in Southeast University, Nanjing, China. Now she is the director of the second center in the Electronics Technology Group Corporation of China, No.55 Research Institute, Nanjing, P.R. China.