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Wireless passive SAW sensors using single-electrode-type IDT structures as programmable reflectors
Sensors and Actuators A 122 (2005) 203–208
Wireless passive SAW sensors using single-electrode-type IDT structures as programmable reflectors Qiuyun Fu ∗ , Helmut Stab, Wolf-Joachim Fischer Department of Electrical and Information Technology, Semiconductor and Microsystems Technology Laboratory, Dresden University of Technology, Noethnitzer Str. 64, 01062 Dresden, Germany Received 7 September 2004; received in revised form 22 April 2005; accepted 2 May 2005 Available online 17 June 2005
Abstract Sensor information can be read out wirelessly by an antenna if the surface acoustic wave (SAW) delay line is connected to an external classical sensor. This is a passive device also called impedance-loaded SAW sensor. In the last decade, many researchers have made great efforts researching such devices, in which the double-electrode-type interdigital transducer (IDT) has been used as programmable reflector because its short-circuit reflection coefficient is very small. In this paper, the single-electrode-type IDT structure is proposed to be used as programmable reflector. The metallic periodic structure deposited on 128◦ YX LiNbO3 shows an extraordinary characteristic: for a certain ratio of the film thickness to the periodicity, the reflection of the short-circuited single-electrode-type IDT almost disappears. This characteristic enables great amplitude modulation when the impedance-loaded SAW sensor employs the single-electrode-type IDT as programmable reflector. The fundamental operating frequency of SAW devices using the single-electrode-type IDT structure is twice that of SAW devices using the doubleelectrode-type IDT structure with the same critical dimension. This means higher operating frequencies for such devices can be reached with the achievable linewidth in current manufacturing technologies. © 2005 Elsevier B.V. All rights reserved. Keywords: Passive; Wireless passive SAW sensors; Programmable reflector; Single-electrode-type IDT
1. Introduction Sensor information can be read out wirelessly by an antenna if the surface acoustic wave (SAW) delay line is connected to an external classical sensor. This is a passive device also called impedance-loaded SAW sensor [1,2], as shown in Fig. 1. The SAW delay line consists of three IDTs. The first connected to the antenna is a transmitting/receiving IDT. The second is used as a reference. The third connected to an external sensor is used as a programmable reflector. The operating principle is briefly described here: the transmitted pulse from an interrogation unit is received by the antenna and inputs to the transmitting/receiving IDT. Then SAW is excited on the piezoelectric substrate and propagates to the reference and the programmable reflector where it is ∗
Corresponding author. Tel.: +49 351 46336337; fax: +49 351 46337021. E-mail address: [email protected] (Q. Fu).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.05.006
partially reflected. The transmitting/receiving IDT converts the reflected SAW into electrical signals responding to the interrogation unit through the antenna. Since the reflection coefficient of the programmable reflector changes with the impedance variation of the external sensor, the sensor information is included in the echo of the programmable reflector. Finally, the echoes are transferred to a PC or other devices for post processing. P11t is defined as the total reflection coefficient of the programmable reflector with the external load, given by P11t = P11sc +
2 4P13
P33 +
1 Zsm
(1)
where P13 and P33 are the P-matrix elements of the reflector [3]. P11sc is the reflection coefficient of the reflector when short-circuited. Zsm is the impedance of the external sensor and matching circuits.
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the single-electrode-type IDT structure is twice that of SAW devices using the double-electrode-type IDT structure. 2. Stopband analysis of metallic gratings on 128◦ YX LiNbO3
Fig. 1. Impedance-loaded SAW sensor. Response 1 is the echo of the reference. Response 2 is the echo of the programmable reflector.
In the last decade, the double-electrode-type IDT structure (Fig. 2(b)) has been proposed to be used as programmable reflector [1,6] because P11sc of the double-electrode-type IDT structure is very small. In this paper, the single-electrodetype IDT structure (Fig. 2(a)) is proposed to be used as a programmable reflector. The metallic periodic structure deposited on 128◦ YX LiNbO3 shows an extraordinary characteristic: for a certain ratio of the film thickness to the periodicity, the reflection of the short-circuited single-electrodetype IDT almost disappears. This characteristic is similar to that of the double-electrode-type IDT structure. Thus, it is possible to use the single-electrode-type IDT structure on 128◦ YX LiNbO3 as programmable reflector. A relatively large linewidth is preferable for manufacturing purpose for high frequency devices. As shown in Fig. 2, the double-electrode-type IDT structure employs λ/8 electrode width, while the single-electrode-type IDT structure employs λ/4 electrode width. When their operating frequencies are the same, the critical linewidth of SAW devices using
Fig. 2. Uniform IDTs. λ is the periodicity of the IDT structure and w is the aperture: (a) single-electrode-type IDT structure and (b) double-electrodetype IDT structure.
In order to verify the possibility to use the single-electrodetype IDT as programmable reflector, the relationship between the stopband and the reflection coefficient of a programmable reflector with a load is analyzed here. Eq. (1) shows that the total reflection P11t depends on P11sc , P13 , P33 and Zsm . P33 is the admittance of the reflector when no incident wave comes, which can be matched by an external circuit. If we want to use the IDT as a programmable reflector and get maximal amplitude modulation dependant only on the external load, we should get minimal P11sc and relatively larger |P13 |. The relation of P-matrix elements to coupling-of-modes (COM) parameters [4] shows that |P11sc | of an singleelectrode-type IDT is proportional to the stopband width of the corresponding short-circuited grating (see Fig. 3(b)). Thus, if the stopband width of the corresponding shortcircuited grating is 0, we can get minimal P11sc of the singleelectrode-type IDT structure with the value of 0. In addition, from the determination of COM parameters [4], it is seen that |P13 | is proportional to the stopband width of the corresponding open-circuited grating (see Fig. 3(a)). Therefore, if the stopband of the corresponding open-circuited grating is relatively wide, a relatively large |P13 | can be obtained.
Fig. 3. Metallic gratings: p is the periodicity of gratings. (a) The opencircuited grating. (b) The short-circuited grating.
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Fig. 4. Dispersion relation of SAW on 128◦ YX LiNbO3. SC denotes shortcircuited grating. OC denotes open-circuited grating [4].
The stopband characteristics of Aluminium gratings on 128◦ YX LiNbO3 calculated by the software FEMSDA [4] are illustrated in Fig. 4, where h is the thickness of the metal film. VB = 4025 m/s is the velocity of the slow-shear surface skimming bulk wave (SSBW) and used to normalize the frequency. Fig. 4 shows that, with the increase of h/p, the stopband width of the open-circuited grating increases and that of the short-circuited grating decreases. The width of the stopband indicates the intensity of the reflection. This phenomenon comes from the following reason [5]: the reflection effect r is the sum of two factors rp and rm . The rp comes from the piezoelectric perturbation and rm results from the mechanical perturbation. For the open-circuited grating, |r| = |rp | + |rm |. For the short-circuited grating, |r| = |rp | − |rm |. Therefore, there exists such a point where the stopband of the shortcircuited grating is 0, whereas the stopband of open-circuited grating is wide, as shown in Fig. 5. This point comes up at h/p ≈ 0.06. This means, the single-electrode-type IDT structure deposited on 128◦ YX LiNbO3 at h/λ ≈ 0.03 is able to be used as programmable reflector.
3. Design and simulation of the SAW chip 3.1. Transmitting/receiving IDT The admittance of the transmitting/receiving IDT is a very important factor. Generally, wireless passive SAW sensors will work in a 50 -matching environment. If the transmitting/receiving IDT can be self-matched to 50 , the loss in
Fig. 5. Dispersion relationship calculated by FEMSDA. The region inside the dashed circle in (a) is enlarged in (b).
the desired frequency range will be small. When designing real devices, we can let the real part of the admittance be close to 0.02 −1 and symmetric to the center frequency, then use a series inductors or shunt inductors to tune out the imaginary part of the admittance (the static capacitance). Diffraction and attenuation are problems that also should be considered, because the distances between the transmitting/receiving IDT and the reflectors are relatively big. To avoid diffraction, a relatively larger aperture is necessary. In addition, the propagation losses on the substrate should be small. 128◦ YX LiNbO3 satisfies this requirement. The design parameters based on the estimation using the conventional method are adjusted as follows: λ = 4.58 m, Np = 20, w = 50λ and p = 2.29 m, where Np is the number of finger pairs of the IDT. Fig. 6 gives the simulation results of the transmitting/receiving IDT by using the COM model. The thickness of the aluminium film h has a great effect on the conductance of the IDT G. When h is 0.131 m, G is symmetric and close to the expected value 0.02 −1 . This is just the point h/λ ≈ 0.03. When h is smaller than this value, then the peak
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Fig. 6. Effect of thickness of the aluminium film on IDT conductance.
of G moves toward the low frequency direction. When h is larger than this value the peak of G moves toward the high frequency direction. 3.2. Programmable reflector As indicated in Section 2, the single-electrode-type IDT structure can be employed as programmable reflector when the aluminium film is deposited on 128◦ YX LiNbO3 and h/λ ≈ 0.03. Therefore, the programmable reflector can use the same parameters as that of the transmitting/receiving IDT. P11t of this type IDT with external loads is calculated, as shown in Fig. 7. The P11t depends on the external load and shows a very large amplitude modulation, which meets the requirement for a programmable reflector. 4. Application to sensors—results of simulation and experiment 4.1. Capacitive sensor In this application, a series inductor L is used as a matching circuit. A variable capacitor C can be taken as the impedance of an external capacitive sensor. It is shown in Fig. 8. Because the resonance point can easily be changed with the series inductor, the SAW chip can be connected to conventional capacitive sensors in different capacitance ranges, as shown in Fig. 9. S11 is the amplitude of the response pulse of the entire devices in the time domain. Because bond wires behave as resistors and inductors connected serially [59], the situation L < 2nH in Fig. 9 is very unlikely to occur. It is necessary to include the inductance and resistance of bond wires in the simulation. In Fig. 10, Lw and Rw are the inductance and resistance of the bond wires, respectively. The simulation and measurement results are shown in Fig. 11. In this occasion, the sensitive capacitance range for a capacitive sensor is from 2.6 to 6 pF.
Fig. 7. Dependency of P11t on external loads: (a) dependency of P11t on R when L = 0 and C = 0 and (b) dependency of P11t on L and C when R = 0.
Fig. 8. SAW reflector with a variable capacitor.
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Fig. 12. SAW reflector with a variable resistor. Fig. 9. Dependency of S11 on L and C.
4.2. Resistive sensor Conventional resistive sensors connected to this SAW chip can also be read out wirelessly by an antenna. Because of the parasitic effects of bond wires, a capacitor C should be used to tune with the inductance of bond wires to get good amplitude modulation, as shown in Fig. 12. The inductance and resistance of bond wires Lw , Rw , and the tuning capacitor C compose a matching circuit. A variable resistor R can be taken as the impedance of an external resistive sensor. The simulation and measurement results are shown in Fig. 13. Without the tuning capacitor, the S11 changes little due to the inductance of bond wires. The simulation is shown in Fig. 14. This shows the tuning matching circuit is necessary for getting a large amplitude modulation. Fig. 10. SAW reflector with a variable capacitor including bond wires.
Fig. 11. Dependency of S11 on C (Lw = 14 nH, Rw = 4 ).
Fig. 13. Dependency of S11 on R (Rw = 3 , Lw = 11 nH, C = 2.78 pF).
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Acknowledgements This work is mainly funded by the German Research Council (DFG – Graduiertenkolleg “Sensorik”). The authors are thankful to the research group of Prof. Ken-ya Hashimoto for their free software FEMSDA.
References
Fig. 14. Dependency of S11 on R (Rw = 3 , Lw = 11 nH, C = 0 pF).
For the above two applications, the effective amplitude modulation range can be more than 10 dB. If shorter bond wires or flip chip technology are used for packaging it, Lw can be smaller. As a result, greater amplitude modulation and larger sensitive ranges for sensors can be obtained.
[1] L. Reindl, W. Ruile, Programmable reflectors for SAW-ID-tags, in: Proceedings of the IEEE Symposium on Ultrasonics, 1993, pp. 125–130. [2] F. Schmidt, G. Scholl, Wireless SAW identification and sensor systems, Int. J. High Speed Electron. Syst. 10 (4) (2000) 1143–1191. [3] G. Tobolka, Mixed matrix representation of SAW transducers, IEEE Trans. Sonics Ultrason. SU-26 (6) (1979) 426–428. [4] Ken-ya Hashimoto, Surface Acoustic Wave Devices in Telecommunications Modeling and Simulation, Springer, 2000, pp. 218–219. [5] Y. Suzuki, et al., Some studies on SAW resonators and multiple-mode filters, in: Proceedings of the IEEE Symposium on Ultrasonics, 1976, pp. 297–302. [6] L. Reindl, et al., Hybrid SAW-device for a European train control system, in: Proceedings of the IEEE Symposium on Ultrasonics, 1994, pp. 175–179.
Biographies 5. Conclusion It was shown that single-electrode-type IDT deposited on the substrate 128◦ YX LiNbO3 at certain ratio of the metal thickness to the periodicity of the IDT structure can be used as programmable reflector. The achievable amplitude modulation is sufficient for sensor applications. The advantage of the single-electrode-type IDT approach to the conventional double-electrode-type IDT approach can be outlined as follows. • Reducing the manufacturing cost: In our sample, the work frequency is 845 MHz, the minimal linewidth is 1.145 m. If the double-electrode-type IDT is used as programmable reflector for 845 MHz, the minimal linewidth will be about 0.5725 m and the manufacturing cost will increase. • Increasing the operating frequency: The current achievable linewidth for manufacturing SAW is about 0.3 m. Therefore, the limit of operating frequencies for the double-electrode-type IDT is lower than 2 GHz, but for single-electrode-type IDT the operating frequency can reach 3 GHz. Wireless passive SAW sensor by using the singleelectrode-type IDT structure as programmable reflector was also introduced in this paper. Its applications for external capacitive and resistive sensors were presented. In these sensors, the tasks of the transponder and the front-end sensor unit are separated, which makes measurements easier and protects the transponder from the operating environment.
Qiuyun Fu was born in Hubei, China, in 1972. She received the Bachelor and the Master degrees of electronic engineering at Huazhong University of Science and Technology, China, in 1994 and 2000, respectively. She is currently working towards the PhD in electronic engineering at Dresden University of Technology, Germany. From July 1994 to September 1997, she worked at Accelink Technologies Co. Ltd., China. From July 2000 to February 2002, she worked at ASIC center of FiberHome Telecommunication Technologies Co. Ltd., China. Her current research interests include simulation and design of RF SAW devices, Si-ASIC hybrid devices. Helmut Stab received his diploma degree in physics in 1968 and his PhD degree in physics in 1976 from the Humboldt University of Berlin, Germany. From 1975 to 1993 he had worked in the field of surface acoustic waves at the Institute of Electron Physics of the Academy of Sciences in Berlin. He has investigated SAW excitation and propagation phenomena in layered piezoelectric crystals. He was engaged in the development of surface acoustic wave filters, convolvers, and sensors. From 1993 to 1997 he was with the Dresden University of Applied Sciences and since 1997 he has been working at the Semiconductor and Microsystems Technology Laboratory of the Dresden University of Technology in the field of chemical and physical SAW sensors and systems. He is currently working in the field of passive remote requestable SAW identification and sensor systems. Wolf-Joachim Fischer received the MS and PhD degrees in electrical engineering from the Dresden University of Technology (TUD), in 1973 and 1976, respectively. From 1973 to 1991 he was with ZMD Inc., where he was engaged in the design of integrated circuits. In 1991 he joined in the Fraunhofer-Institute for Microelectronic Circuits and Systems, where he leads a department of IC design. Since 1994 he is a professor for microsystem technology in the Department of Electrical Engineering of the TUD. His main research and teaching interests are in the fields of intelligent microsystems and IC design for wireless transponders and chipcards.
Wireless passive SAW sensors using single-electrode-type IDT structures as programmable reflectors Qiuyun Fu ∗ , Helmut Stab, Wolf-Joachim Fischer Department of Electrical and Information Technology, Semiconductor and Microsystems Technology Laboratory, Dresden University of Technology, Noethnitzer Str. 64, 01062 Dresden, Germany Received 7 September 2004; received in revised form 22 April 2005; accepted 2 May 2005 Available online 17 June 2005
Abstract Sensor information can be read out wirelessly by an antenna if the surface acoustic wave (SAW) delay line is connected to an external classical sensor. This is a passive device also called impedance-loaded SAW sensor. In the last decade, many researchers have made great efforts researching such devices, in which the double-electrode-type interdigital transducer (IDT) has been used as programmable reflector because its short-circuit reflection coefficient is very small. In this paper, the single-electrode-type IDT structure is proposed to be used as programmable reflector. The metallic periodic structure deposited on 128◦ YX LiNbO3 shows an extraordinary characteristic: for a certain ratio of the film thickness to the periodicity, the reflection of the short-circuited single-electrode-type IDT almost disappears. This characteristic enables great amplitude modulation when the impedance-loaded SAW sensor employs the single-electrode-type IDT as programmable reflector. The fundamental operating frequency of SAW devices using the single-electrode-type IDT structure is twice that of SAW devices using the doubleelectrode-type IDT structure with the same critical dimension. This means higher operating frequencies for such devices can be reached with the achievable linewidth in current manufacturing technologies. © 2005 Elsevier B.V. All rights reserved. Keywords: Passive; Wireless passive SAW sensors; Programmable reflector; Single-electrode-type IDT
1. Introduction Sensor information can be read out wirelessly by an antenna if the surface acoustic wave (SAW) delay line is connected to an external classical sensor. This is a passive device also called impedance-loaded SAW sensor [1,2], as shown in Fig. 1. The SAW delay line consists of three IDTs. The first connected to the antenna is a transmitting/receiving IDT. The second is used as a reference. The third connected to an external sensor is used as a programmable reflector. The operating principle is briefly described here: the transmitted pulse from an interrogation unit is received by the antenna and inputs to the transmitting/receiving IDT. Then SAW is excited on the piezoelectric substrate and propagates to the reference and the programmable reflector where it is ∗
Corresponding author. Tel.: +49 351 46336337; fax: +49 351 46337021. E-mail address: [email protected] (Q. Fu).
0924-4247/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2005.05.006
partially reflected. The transmitting/receiving IDT converts the reflected SAW into electrical signals responding to the interrogation unit through the antenna. Since the reflection coefficient of the programmable reflector changes with the impedance variation of the external sensor, the sensor information is included in the echo of the programmable reflector. Finally, the echoes are transferred to a PC or other devices for post processing. P11t is defined as the total reflection coefficient of the programmable reflector with the external load, given by P11t = P11sc +
2 4P13
P33 +
1 Zsm
(1)
where P13 and P33 are the P-matrix elements of the reflector [3]. P11sc is the reflection coefficient of the reflector when short-circuited. Zsm is the impedance of the external sensor and matching circuits.
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the single-electrode-type IDT structure is twice that of SAW devices using the double-electrode-type IDT structure. 2. Stopband analysis of metallic gratings on 128◦ YX LiNbO3
Fig. 1. Impedance-loaded SAW sensor. Response 1 is the echo of the reference. Response 2 is the echo of the programmable reflector.
In the last decade, the double-electrode-type IDT structure (Fig. 2(b)) has been proposed to be used as programmable reflector [1,6] because P11sc of the double-electrode-type IDT structure is very small. In this paper, the single-electrodetype IDT structure (Fig. 2(a)) is proposed to be used as a programmable reflector. The metallic periodic structure deposited on 128◦ YX LiNbO3 shows an extraordinary characteristic: for a certain ratio of the film thickness to the periodicity, the reflection of the short-circuited single-electrodetype IDT almost disappears. This characteristic is similar to that of the double-electrode-type IDT structure. Thus, it is possible to use the single-electrode-type IDT structure on 128◦ YX LiNbO3 as programmable reflector. A relatively large linewidth is preferable for manufacturing purpose for high frequency devices. As shown in Fig. 2, the double-electrode-type IDT structure employs λ/8 electrode width, while the single-electrode-type IDT structure employs λ/4 electrode width. When their operating frequencies are the same, the critical linewidth of SAW devices using
Fig. 2. Uniform IDTs. λ is the periodicity of the IDT structure and w is the aperture: (a) single-electrode-type IDT structure and (b) double-electrodetype IDT structure.
In order to verify the possibility to use the single-electrodetype IDT as programmable reflector, the relationship between the stopband and the reflection coefficient of a programmable reflector with a load is analyzed here. Eq. (1) shows that the total reflection P11t depends on P11sc , P13 , P33 and Zsm . P33 is the admittance of the reflector when no incident wave comes, which can be matched by an external circuit. If we want to use the IDT as a programmable reflector and get maximal amplitude modulation dependant only on the external load, we should get minimal P11sc and relatively larger |P13 |. The relation of P-matrix elements to coupling-of-modes (COM) parameters [4] shows that |P11sc | of an singleelectrode-type IDT is proportional to the stopband width of the corresponding short-circuited grating (see Fig. 3(b)). Thus, if the stopband width of the corresponding shortcircuited grating is 0, we can get minimal P11sc of the singleelectrode-type IDT structure with the value of 0. In addition, from the determination of COM parameters [4], it is seen that |P13 | is proportional to the stopband width of the corresponding open-circuited grating (see Fig. 3(a)). Therefore, if the stopband of the corresponding open-circuited grating is relatively wide, a relatively large |P13 | can be obtained.
Fig. 3. Metallic gratings: p is the periodicity of gratings. (a) The opencircuited grating. (b) The short-circuited grating.
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Fig. 4. Dispersion relation of SAW on 128◦ YX LiNbO3. SC denotes shortcircuited grating. OC denotes open-circuited grating [4].
The stopband characteristics of Aluminium gratings on 128◦ YX LiNbO3 calculated by the software FEMSDA [4] are illustrated in Fig. 4, where h is the thickness of the metal film. VB = 4025 m/s is the velocity of the slow-shear surface skimming bulk wave (SSBW) and used to normalize the frequency. Fig. 4 shows that, with the increase of h/p, the stopband width of the open-circuited grating increases and that of the short-circuited grating decreases. The width of the stopband indicates the intensity of the reflection. This phenomenon comes from the following reason [5]: the reflection effect r is the sum of two factors rp and rm . The rp comes from the piezoelectric perturbation and rm results from the mechanical perturbation. For the open-circuited grating, |r| = |rp | + |rm |. For the short-circuited grating, |r| = |rp | − |rm |. Therefore, there exists such a point where the stopband of the shortcircuited grating is 0, whereas the stopband of open-circuited grating is wide, as shown in Fig. 5. This point comes up at h/p ≈ 0.06. This means, the single-electrode-type IDT structure deposited on 128◦ YX LiNbO3 at h/λ ≈ 0.03 is able to be used as programmable reflector.
3. Design and simulation of the SAW chip 3.1. Transmitting/receiving IDT The admittance of the transmitting/receiving IDT is a very important factor. Generally, wireless passive SAW sensors will work in a 50 -matching environment. If the transmitting/receiving IDT can be self-matched to 50 , the loss in
Fig. 5. Dispersion relationship calculated by FEMSDA. The region inside the dashed circle in (a) is enlarged in (b).
the desired frequency range will be small. When designing real devices, we can let the real part of the admittance be close to 0.02 −1 and symmetric to the center frequency, then use a series inductors or shunt inductors to tune out the imaginary part of the admittance (the static capacitance). Diffraction and attenuation are problems that also should be considered, because the distances between the transmitting/receiving IDT and the reflectors are relatively big. To avoid diffraction, a relatively larger aperture is necessary. In addition, the propagation losses on the substrate should be small. 128◦ YX LiNbO3 satisfies this requirement. The design parameters based on the estimation using the conventional method are adjusted as follows: λ = 4.58 m, Np = 20, w = 50λ and p = 2.29 m, where Np is the number of finger pairs of the IDT. Fig. 6 gives the simulation results of the transmitting/receiving IDT by using the COM model. The thickness of the aluminium film h has a great effect on the conductance of the IDT G. When h is 0.131 m, G is symmetric and close to the expected value 0.02 −1 . This is just the point h/λ ≈ 0.03. When h is smaller than this value, then the peak
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Fig. 6. Effect of thickness of the aluminium film on IDT conductance.
of G moves toward the low frequency direction. When h is larger than this value the peak of G moves toward the high frequency direction. 3.2. Programmable reflector As indicated in Section 2, the single-electrode-type IDT structure can be employed as programmable reflector when the aluminium film is deposited on 128◦ YX LiNbO3 and h/λ ≈ 0.03. Therefore, the programmable reflector can use the same parameters as that of the transmitting/receiving IDT. P11t of this type IDT with external loads is calculated, as shown in Fig. 7. The P11t depends on the external load and shows a very large amplitude modulation, which meets the requirement for a programmable reflector. 4. Application to sensors—results of simulation and experiment 4.1. Capacitive sensor In this application, a series inductor L is used as a matching circuit. A variable capacitor C can be taken as the impedance of an external capacitive sensor. It is shown in Fig. 8. Because the resonance point can easily be changed with the series inductor, the SAW chip can be connected to conventional capacitive sensors in different capacitance ranges, as shown in Fig. 9. S11 is the amplitude of the response pulse of the entire devices in the time domain. Because bond wires behave as resistors and inductors connected serially [59], the situation L < 2nH in Fig. 9 is very unlikely to occur. It is necessary to include the inductance and resistance of bond wires in the simulation. In Fig. 10, Lw and Rw are the inductance and resistance of the bond wires, respectively. The simulation and measurement results are shown in Fig. 11. In this occasion, the sensitive capacitance range for a capacitive sensor is from 2.6 to 6 pF.
Fig. 7. Dependency of P11t on external loads: (a) dependency of P11t on R when L = 0 and C = 0 and (b) dependency of P11t on L and C when R = 0.
Fig. 8. SAW reflector with a variable capacitor.
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Fig. 12. SAW reflector with a variable resistor. Fig. 9. Dependency of S11 on L and C.
4.2. Resistive sensor Conventional resistive sensors connected to this SAW chip can also be read out wirelessly by an antenna. Because of the parasitic effects of bond wires, a capacitor C should be used to tune with the inductance of bond wires to get good amplitude modulation, as shown in Fig. 12. The inductance and resistance of bond wires Lw , Rw , and the tuning capacitor C compose a matching circuit. A variable resistor R can be taken as the impedance of an external resistive sensor. The simulation and measurement results are shown in Fig. 13. Without the tuning capacitor, the S11 changes little due to the inductance of bond wires. The simulation is shown in Fig. 14. This shows the tuning matching circuit is necessary for getting a large amplitude modulation. Fig. 10. SAW reflector with a variable capacitor including bond wires.
Fig. 11. Dependency of S11 on C (Lw = 14 nH, Rw = 4 ).
Fig. 13. Dependency of S11 on R (Rw = 3 , Lw = 11 nH, C = 2.78 pF).
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Acknowledgements This work is mainly funded by the German Research Council (DFG – Graduiertenkolleg “Sensorik”). The authors are thankful to the research group of Prof. Ken-ya Hashimoto for their free software FEMSDA.
References
Fig. 14. Dependency of S11 on R (Rw = 3 , Lw = 11 nH, C = 0 pF).
For the above two applications, the effective amplitude modulation range can be more than 10 dB. If shorter bond wires or flip chip technology are used for packaging it, Lw can be smaller. As a result, greater amplitude modulation and larger sensitive ranges for sensors can be obtained.
[1] L. Reindl, W. Ruile, Programmable reflectors for SAW-ID-tags, in: Proceedings of the IEEE Symposium on Ultrasonics, 1993, pp. 125–130. [2] F. Schmidt, G. Scholl, Wireless SAW identification and sensor systems, Int. J. High Speed Electron. Syst. 10 (4) (2000) 1143–1191. [3] G. Tobolka, Mixed matrix representation of SAW transducers, IEEE Trans. Sonics Ultrason. SU-26 (6) (1979) 426–428. [4] Ken-ya Hashimoto, Surface Acoustic Wave Devices in Telecommunications Modeling and Simulation, Springer, 2000, pp. 218–219. [5] Y. Suzuki, et al., Some studies on SAW resonators and multiple-mode filters, in: Proceedings of the IEEE Symposium on Ultrasonics, 1976, pp. 297–302. [6] L. Reindl, et al., Hybrid SAW-device for a European train control system, in: Proceedings of the IEEE Symposium on Ultrasonics, 1994, pp. 175–179.
Biographies 5. Conclusion It was shown that single-electrode-type IDT deposited on the substrate 128◦ YX LiNbO3 at certain ratio of the metal thickness to the periodicity of the IDT structure can be used as programmable reflector. The achievable amplitude modulation is sufficient for sensor applications. The advantage of the single-electrode-type IDT approach to the conventional double-electrode-type IDT approach can be outlined as follows. • Reducing the manufacturing cost: In our sample, the work frequency is 845 MHz, the minimal linewidth is 1.145 m. If the double-electrode-type IDT is used as programmable reflector for 845 MHz, the minimal linewidth will be about 0.5725 m and the manufacturing cost will increase. • Increasing the operating frequency: The current achievable linewidth for manufacturing SAW is about 0.3 m. Therefore, the limit of operating frequencies for the double-electrode-type IDT is lower than 2 GHz, but for single-electrode-type IDT the operating frequency can reach 3 GHz. Wireless passive SAW sensor by using the singleelectrode-type IDT structure as programmable reflector was also introduced in this paper. Its applications for external capacitive and resistive sensors were presented. In these sensors, the tasks of the transponder and the front-end sensor unit are separated, which makes measurements easier and protects the transponder from the operating environment.
Qiuyun Fu was born in Hubei, China, in 1972. She received the Bachelor and the Master degrees of electronic engineering at Huazhong University of Science and Technology, China, in 1994 and 2000, respectively. She is currently working towards the PhD in electronic engineering at Dresden University of Technology, Germany. From July 1994 to September 1997, she worked at Accelink Technologies Co. Ltd., China. From July 2000 to February 2002, she worked at ASIC center of FiberHome Telecommunication Technologies Co. Ltd., China. Her current research interests include simulation and design of RF SAW devices, Si-ASIC hybrid devices. Helmut Stab received his diploma degree in physics in 1968 and his PhD degree in physics in 1976 from the Humboldt University of Berlin, Germany. From 1975 to 1993 he had worked in the field of surface acoustic waves at the Institute of Electron Physics of the Academy of Sciences in Berlin. He has investigated SAW excitation and propagation phenomena in layered piezoelectric crystals. He was engaged in the development of surface acoustic wave filters, convolvers, and sensors. From 1993 to 1997 he was with the Dresden University of Applied Sciences and since 1997 he has been working at the Semiconductor and Microsystems Technology Laboratory of the Dresden University of Technology in the field of chemical and physical SAW sensors and systems. He is currently working in the field of passive remote requestable SAW identification and sensor systems. Wolf-Joachim Fischer received the MS and PhD degrees in electrical engineering from the Dresden University of Technology (TUD), in 1973 and 1976, respectively. From 1973 to 1991 he was with ZMD Inc., where he was engaged in the design of integrated circuits. In 1991 he joined in the Fraunhofer-Institute for Microelectronic Circuits and Systems, where he leads a department of IC design. Since 1994 he is a professor for microsystem technology in the Department of Electrical Engineering of the TUD. His main research and teaching interests are in the fields of intelligent microsystems and IC design for wireless transponders and chipcards.