SAW based passive sensor with passive signal conditioning using MEMS A/D converter

Transcription

1 Available online at Sensors and Actuators A 141 (2008) SAW based passive sensor with passive signal conditioning using MEMS A/D converter Jae-Geun Oh, Bumkyoo Choi, Seung-Yop Lee Department of Mechanical Engineering, Sogang University, Sinsu-Dong, Mapo-Gu, Seoul, Republic of Korea Received 8 March 2006; received in revised form 14 September 2007; accepted 14 September 2007 Available online 29 September 2007 Abstract A monolithically packaged surface acoustic wave (SAW) radio transponder and pressure sensor are developed for the application to a tire pressure monitoring system (TPMS). The device contains the wireless transponder, which converts analog signal into digital one without any auxiliary electronic circuits and transmits the converted data wirelessly. No power sources are needed for wireless transponder and pressure sensor. The touch-mode pressure sensor converts externally applied pressure into capacitance, and the SAW radio transponder radiates sensor values with pulse train to the interrogation (measurement) unit. The realization of the mechanical A/D conversion is possible since the SAW radio transponder is connected to the touch-mode capacitive pressure sensor. The SAW radio transponder and touch-mode sensor are fabricated using a surface micromachining and a bulk micromachining technologies, respectively. The performance of the integrated, passive and wireless pressure sensor meets the design specifications such as linearity, sensitivity and noise figure. Finally, experimental results for the radio transponder and the sensor without power source are presented Elsevier B.V. All rights reserved. Keywords: Wireless sensor; SAW radio transponder; Capacitive sensor; Touch mode; Micromachining technology 1. Introduction SAW radio transponders have been studied by many researchers as wireless sensors [1,2]. The major application fields are RFID tag and the component of communication systems. Recently, wireless sensors have been considered as one of the emerging technologies because of a lot of application such as the environmental monitoring and the structural health monitoring systems. In Europe, the wireless sensors are developed for temperature sensors [3], torque sensors [4] and pressure sensors, which are embedded in to measure the tire pressure [5].In general, tire pressure is not easy to measure because of the difficulty in wiring power-supply and electronic circuits. Therefore, some alternatives are considered. One is inductively coupled passive/wireless pressure sensors using MEMS technology. This technology is based on the wireless power transmission such as RF tag and SAW radio transponder. However, the pressure sensor has some limitations such as a small detecting range about 4 cm Corresponding author. Tel.: ; fax: address: bkchoi@sogang.ac.kr (B. Choi). long and unstable outputs, and thus it is affected by environment conditions and temperature. The second alternative is a wireless power transmission. Wireless power transmission is studied for the application to an intra-ocular pressure sensor, which is powered wirelessly. However, the transmission range of delivered wireless power is small for practical use. Furthermore, it is difficult to deliver RF power to a distant place more than 10 m from sensor location without violating regulations on wireless communication. Therefore, SAW radio transponder might be thought to solve above limitations because it receives incident radio signals and it can reflect them into the air. The reflected signals launching on the surface of SAW radio transponder contain some information about internal status of SAW transponder such as the impedance and the loaded mass on the substrate. The varying impedance affects the reflected signal (wave) from SAW transponder, and the reader unit, which is called as the interrogator in this study, can read out this reflected signal. The varying impedance is implemented with the variable capacitance and thus the variable capacitive sensor should be developed for the external load impedance connecting to SAW radio transponder. Various types of SAW pressure sensor have been developed over 10 years. Most of the SAW pressure sensors use varying /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.sna

2 632 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) impedance, which has a high Q-factor to improve sensitivity. Due to the resonance circuitry like nature of the impedance loaded inter digital transducers (IDT), the applicability of a capacitive pressure sensor to the load for the splitfinger IDT depends on the Q-factor of the impedance. A low Q-factor reduces both the achievable magnitude and the phase modulation dramatically. Silicon based micromachined capacitive pressure sensors showed a very low full scale range of 1 2%. In this study, a conventional micromachining process was investigated using a borosilica glass for low cost and easy implementation, and a new design concept for measuring pressure using embedded MEMS A/D converter is proposed. The MEMS A/D converter developed is a fully passive A/D converter connecting SAW transponder and micromachined touch-mode pressure sensor. The conventional capacitive pressure sensor measures capacitance between two electrodes. Therefore, the key parameter in the micromachined capacitive pressure sensor is the gap between two electrodes. However, the gap is not an important parameter in touch-mode pressure sensor. The touch-mode pressure sensor operates at the instants of contacting two electrodes. The lower electrodes cannot move because the lower electrode is fixed the on substrate. Only the upper electrode can move. When two electrodes are touched, the touched area is increased as the external pressure is increased [6,7]. One of key parameters affecting the performance of the pressure sensor is the touching zone. This study uses the touch-mode pressure sensor and the piezoelectric characteristics of the SAW transponders for passive operation of the wireless sensor system. 2. Device design To use the SAW device as a wireless transponder, some characteristics of SAW device such as intrinsic and given parameter in frequency domain should be analyzed using analytical methods. Among various methods, P-matrix is the most significant and well-known formalism to calculate SAW device s characteristics such as the efficiency of propagation, the phase and amplitude of reflected wave in frequency domain. The P-matrix is calculated from the s-parameter, which is measured by the specially designed equipment like network analyzer. The distribution of group velocity of the elastic wave traveling on the piezoelectric substrate, a direction of acoustic axis and a direction of vibration should be completely analyzed for the optimal design of SAW radio transponder and for using piezoelectric materials such as LiNbO 3, quartz and LiTaO 3. The velocity of surface acoustic wave can be calculated with the use of elastic coefficient, piezoelectric constant, dielectric constant and density of the piezoelectric substrate such as LiNbO 3 and LiTaO 3 of which lattice structure composed in single crystal. Thus, a shape of inter digital transducers (IDT) finger can be designed on the basis of the characteristics of piezoelectric substrate, which is mentioned previously. In this study, the dimension and shape of the SAW IDT and reflector are designed as depicted in Fig. 1. In Fig. 1, the substrate of SAW transponder assumed to be YZ-LiNbO 3 and the propagation velocity, v, is equal to 3488 m/s. If an operating frequency band is industrial science and medical (ISM) band of 315 MHz, a width of one IDT (a), spacing between fingers (b) and an acoustic wavelength can be described as following equations: λ = v p f = m (1) a = b = λ = m (2) 4 From the above equations and Fig. 1, a length of wireless transponder (d) and a length of finger (h) are 0.55 mm and 1.66 mm, respectively. A length of finger is designed as times long as one acoustic wavelength to compress the effect of bulk acoustic wave (BAW) which is generated simultaneously with SAW from IDTs on SAW substrate [1]. The P-matrix is an essential idea to predict the efficiency of reflected wave, generated at the reflector IDT on SAW transponder and to evaluate the electro-mechanical coupling. An electro-mechanically coupled equivalent model of an IDT on piezoelectric substrate and the P-matrix are shown below [8] (Fig. 2(a)). Fig. 1. The schematic illustration of SAW IDT for passive/wireless sensor system.

3 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) Fig. 2. An electro-mechanically coupled models of SAW IDT: (a) equivalent model, (b) s-parameter model. The P-matrix had been made by Tobolka in 1979 and this idea was improved by Hartmann and Abbott [9,10]. P-matrix is used for analyzing SAW devices in RF (radio frequency) band and represents the efficiency of SAW device when the high frequency signal is transmitted on the SAW device. However, this concept does not show a direct conversion of physical phenomena. In other words, P-matrix cannot be directly measured. Thus the measurable parameters like ABCD-matrix should be introduced. ABCD-matrix is measurable parameter using special equipment called network analyzer. The conversion of ABCD-matrix to P-matrix is possible mathematically and P-matrix and ABCDmatrix is defined in Eqs. (3) and (4), respectively. b 1 b 2 i 3 ( b1 b 2 p 11 p 12 p 13 = p 21 p 22 p 23 p 31 p 32 p 33 ) ( s11 s 12 = s 21 s 22 )( a1 a 2 ) a 1 a 2 u 3 (3) (4) where a and b are wave amplitudes in s-parameter, and i 3 and u 3 are the current and the voltage at port 3, respectively. The upper left 2 2-submatrix describes the scattering of wave entering into the structure from the outside: p 11 and p 22 are the reflection coefficients, while p 12 = p 21 is the transmission coefficient. The remaining components of the P-matrix are relevant only for transducers. The components p 13 and p 23 describe the excitation efficiency of the IDT. The components p 31 and p 32 measure the current generated in the IDT by the arriving waves. The 33-component of the P-matrix (p 33 ), the admittance, describes the acoustic and electrostatic currents due to the drive voltage V. The above matrices in Eqs. (3) and (4) are strongly related. Measurable parameters are the s-parameter in the ABCD-matrix by using of network analyzer. This study aims to develop the passive and wireless transponder, which is designed to connect the reflector s IDT with the external load by the wire bonding. The amplitude of reflected wave is deeply affected by external load impedance. Fig. 3 shows the schematics of the SAW transponder with external load. As shown in Fig. 3(a), when the reflector IDT on SAW transponder and the external load are connected, the reflection Fig. 3. (a) Schematic illustration of SAW traveling wave in piezoelectric substrate and (b) the schematic of traveling wave launched at the SAW IDT.

4 634 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) coefficient, p 11, for describing the amplitude with phase, can be expressed as in Eq. (5). P 11 (Z load ) = P sc 11 + P 2 13 P 33 + (1/Z load ) P 13 = L sin(ql/2) (δα + κ α)sin(ql/2) jα qcos(ql/2) ql/2 q cos(ql) + jδ sin(ql) (6) P 33 = 4α2 (δ + κ) (δ + κ)(1 cos(ql)) jq sin(ql) q 3 q cos(ql) + jδ sin(ql) ± j 4α2 + jωcl (7) δ κ where χ, δ and q =± δ 2 χ 2 are the reflectivity, the detuning parameter and the wave number which is expressed as a function of frequency at eigen mode, respectively. In addition, L, N, α and C are the total width of IDT, the number of fingers, the transduction coefficient and the capacitance per length, respectively. The total width of IDT is expressed more detailed as L = Np where p is λ/2. P11 sc is the reflection coefficient, assumed that P11 sc = 0 in this study (below 1 GHz) since the acoustic reflection of an short circuited IDT is near 0. The electrical load consists of serial circuits of sensor impedance (resistive, capacitive or inductive) with the inductance of the bonding and connecting wires. The smaller the inductance is, the further the shortcut-point moves towards the origin. Because the amplitude of reflected wave is determined by P 11, the amplitude of it is thought to be controlled by the load impedance, Z load, except for the intrinsic parameters such as P 13,P 33,P11 sc. In other words, the conventional sensor with varying impedance can be read-out wirelessly when connected with SAW transponder by the external reader system. As mentioned above, the impedance variation affects the variation of reflected wave and the sensitivity in wireless sensor to be developed through this study. Therefore, the pressure sensor is to be designed as the external impedance has large deviation in capacitance or inductance since the deviation of external impedance affects sensitivity of wireless sensor. The (5) sensitivity and detection range of the wireless sensor, planned to be developed using SAW transponder with a touch-mode capacitive pressure sensor, is affected strongly by the operational frequency and the reflection coefficient. When surface acoustic wave propagates on piezoelectric substrate, the propagation loss is 6 db/ s at 2.45 GHz. The propagation loss at 433 MHz is a negligible 0.25 db/ s, whereas that at 900 MHz is approximately 1 db/ s. The precise analysis about SAW transponder and external load is conducted for extraction of design parameters because the optimal design of wireless pressure sensor depends on many design parameters such as the shape of IDT, the external impedance, the frequency range and intrinsic characteristics of piezo-substrate such as propagation velocity, thermal expansion coefficient and piezoelectric constant, etc. The MEMS A/D converter is the second step for developing passively operated pressure sensor system. The embedded MEMS A/D converter is easily implemented using conventional micromachining process. The schematic illustration is shown in Fig. 4. When applied pressure increased, the measured signal at the reader unit is like pulse train. As an external pressure increases, the number of pulse decreases. The reader unit calculates the applied pressure by counting the number of pulses. Various sensing mechanisms by micromachining technology are the piezo type sensing using piezoelectric effect, capacitive sensing, and resonant sensing, etc. Among the abovementioned sensing methods, the capacitive type is broadly used as a pressure sensor to use for wireless telemetry in the recent years. There are some merits for the capacitive sensor such as simple structure and temperature independency. However, the measured value in capacitive sensor should be calibrated since it shows non-linear characteristic and the calibration is not easy. The reason for using the parallel plate capacitor in MEMS field for pressure sensors is that the fabrication is relatively easier than other types of sensors. Moreover, that is the controllability about full scale deviation in capacitance is possible by the geometry (dimension) of sensor s structure. The goal of this study is to develop the touch-mode capacitive pressure sensor, changing its touched area due to applied pressure, which is shown in Fig. 5. The touch-mode capacitive pressure sensor is well-matched with Fig. 4. A schematic illustration of embedded MEMS A/D converter with SAW wireless transponder.

5 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) Fig. 5. Structure of a capacitive pressure sensor: (a) normal mode, (b) touch mode. Fig. 6. A brief flow diagram of interrogation system and SAW sensor. the SAW transponder since its touched area is changed linearly by applied pressure and the capacitance deviation is larger than normal mode capacitive sensor. The normal mode capacitive sensor, shown in Fig. 5(a), is assumed to be a small deformation. But, the touch-mode one is assumed to be a large deformation. The large deformation cannot be solved easily because of the moving boundary conditions and diaphragm stretching after contacting to lower electrode. The governing equations are shown in Eqs. (8) and (9). Dieter et al. found an approximated solution in the region of small deformation. Wang et al. accomplished the FEA simulation and measurement of square membrane in the region of large deformation. The simulation has been done using ABAQUS with the square membrane elements [6]. 4 F x F x 2 y F y 4 (( w 2 ) = E x y 2 w x 2 ) w y (8) 4 w x w x 2 y w 4 y = h D ( P h + F 2 y 2 2 w x F y 2 2 w x 2 2 F 2 x y 2 ) w x y where D represents the flexural rigidity(d = Eh 2 /12(1 ν 2 )), P the applied pressure, w the deflection in (x, y), E Young s modulus, v Poisson s ratio, and F the stress function. Therefore, in this study, the analysis in touch mode will be done with the FEA about the governing equation in large deformation, given in Eqs. (8) and (9). Eq. (10) can be used as a capacitance in touch mode since it contains parameters such as gap between two electrodes (d), area of electrode (A) and insulator that is evaporated on lower electrode C = ε o ε a ε i dx dy ε a t m + ε i (d w(x, y)) (9) (10) Fig. 7. The detailed schematic diagram of interrogator unit and the photographic image of experimental setup.

6 636 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) where C is the total capacitance between electrodes, ε o the dielectric constant of free space, ε a the dielectric constant of air(=1.0), ε i the dielectric constant of isolation layer, t m the thickness of isolation layer, d the initial gap between two electrodes, and w(x, y) deflection. In the case of Fig. 5(b), in which the linear solution is no more valid, the FEA simulation should be conducted. Choi et al. [11] achieved a study about the effect by stretching in diaphragm that does not exert large effect on small deformation in case the deformation of the upper electrode enters the touch mode. The non-linear characteristics of the touch-mode pressure sensor had been analyzed. Moreover, the sudden change of capacitance on entering the touch mode had been analyzed, too. ANSYS was used for calculating the touched area due to the applied pressure. The interrogator (reader) unit for sending pulse to the SAW transponder, receiving reflected signal, and launching at SAW transponder, is shown below. The interrogation units are similar to those used in radar applications. We can construct interrogation units based on pulse radars, pulse compression radar, and frequency modulated continuous wave (FMCW) radar architectures. The design goal of the interrogation unit of the pulse type shown in Fig. 6 is to achieve the best performance values, the operation over a broad frequency range, and a high degree of modularity. A heterodyne receiver architecture with a 65 MHz intermediate frequency (IF) stage is realized. The detailed illustration of reader unit is shown in Fig. 7. The receiving level at the interrogator decreases, in accordance with the radar equation, with the fourth power of the distance between the SAW sensor element and the interrogator. The maximum read-out distance γ is given by the transmitted power P 0 of the interrogator, the respecting gain of the interrogator and sensor antenna, G I and G e, the insertion loss of the sensor element D, the noise energy kt 0 of the receiving antenna (in equilibrium with environment at T 0 ), the system bandwidth B, the system noise figure F, the electromagnetic wavelength λ of the operation frequency, and the needed signal/noise ratio S/N to be r = 1 4π 4 P 0 G 2 I G2 e λ4 (11) kt 0 BF(S/N)D Fig. 8. The simulation results of Si square membrane; applied pressure (31 psi), thickness of membrane (5 m), gap between two electrodes (20 m), length of membrane (1200 m). In Europe only two frequency bands, which are compatible with SAW technology, are allowed for unlicensed low power devices (LPDs). The allowed equivalent isotropic radiation power P 0 (EIRP) for LPDs is 25 mw. The sensor element Table 1 Material property of Si membrane Yield strength (10 10 dyne/cm 2 ) Knoop hardness (kg/mm 2 ) Young s modulus (10 12 dyne/cm 2 ) Density (g/cm 3 ) Thermal conductivity (W/cm C) Thermal expansion (10 6 / C) Diamond SiC TiC Al 2 O Si 3 N Iron SiO Si Steel (max. strength) W

7 Table 2 Brief summary of experimental conditions and results J.-G. Oh et al. / Sensors and Actuators A 141 (2008) Parameter Value Unit Remark Transmitting power 0 20 dbm Antenna gain of interrogator 0 dbi Omni-direction Antenna gain at SAW 0 dbi Omni-direction Band width 10 MHz Receiving noise level dbm K Loss due to mismatch 7 db Measured SAW attenuation 50 db Measured value Free space path loss db d =40cm Detecting range 0.4 m Maximum value Center frequency 315 MHz Signal to noise ratio (SNR) 15 db Required level Receiving power ant Critical level of receiving power dbm System margin 3.09 dbm Voltage at receiv. ant. 1.1 mv Measured value Delivered transmitting power to the SAW 4.45 dbm Calculated value Theoretically calculated distance for detection m RF power of 13 dbm Fig. 9. (a) A monolithically packaged SAW transponder and (b) the batch fabricated touch-mode pressure sensor.

8 638 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) Fig. 10. (a) The captured signals at the recoing antenna of interrogator/measuring system aparting from the SAW sensor: 50 W terminator is connected to two ports of the reflector (signal and ground) and (b) no load is applied (opened). has an insertion loss D in the order of db, depending on the used frequency range. The main loss mechanisms are the distribution of the energy to the individual reflectors and attenuation of SAW on its propagation path. The typical antenna gain at the SAW element is in the order of 0 dbi and at the interrogator of 0 dbi. The typical requested S/N ratio is 10 db. Under the presumptions from Eq. (11) for a pulsed single interrogation cycle, the maximum distance r of 40 cm is obtained in the 434 MHz band. 3. Simulation and experiments This study represents the passive operation of wireless SAW sensor system and the method for improving sensor performance using touch-mode pressure sensor. The MEMS embedded A/D converter is possible only with the change of the lower electrode s pattern. To extract design parameter, the commercial finite element method (FEM) package, called ANSYS, is used to simulate the behavior of touched upper electrode. As shown in Fig. 8, when the touch mode is processed, the upper electrode is touched down the lower electrode. From the simulation results, 1200 m length of square membrane is touched down to the lower electrode by the touched length of 320 m, when an external pressure is up to 31 psi. In this simulation, the material properties are shown in Table 1. In addition, the gap between two electrodes is 20 m. From the simulation result, it had been shown that the touched area was increased due to the change of applied external pressure. If the lower electrode is split into several strips, the total number of echoed pulse is changed, which is shown in Fig. 4, due to the change of applied pressure. Therefore, the analog value of pressure, which is the area of touched area, can be converted as digital pulse train. To prove the feasibility of the embedded MEMS A/D converter, we made the experiments for the characteristics of SAW transponder. At first, we had made SAW reflective delay line, which reflects three echoed pulses with the time interval of 1 s. The first echo is the reference and wake signal. The second signal contains the external load condition, and the third signal is the operation ending signal. When the external load impedance of 50 terminator is connected to the second reflective delay IDT, the amplitude of echoed signal is larger than non-external loaded case (Fig. 10). The ceramic packaged SAW transponder and the touch-mode pressure sensor is shown in Fig. 9. Table 2 shows the calculated and measured value of SAW passive/wireless transponder. From the above results, the 50 impedance matched to the external load will strengthen the echoed signal. Moreover, two touched electrodes, connected individually to each IDT splitfingers, will show weak signal of echoed signal as shown in Fig. 10(b). However, non-touched IDT splitfingers, connected to two electrodes in touch-mode pressure sensor, will show strong signal power of echoed signal as in Fig. 10(a). The arrayed lower electrodes as shown in Fig. 4 will show the digital pulse train, contain external load information, and this pulse makes it easy to calculate pressure value at the reader s signal processing unit. Unlike previously developed SAW passive pressure sensor, the result of this study proves the feasibility of new idea of the embedded MEMS A/D converter with low cost reader circuitry. In this study, the maximum detecting range is 40 cm long. However, more sophisticated coherent detector such as I/Q demodulator will improve detecting range up to 1 m. 4. Conclusions This paper presents a passively operated remote sensor system using MEMS A/D converter. A SAW transponder is used to develop passive operation. Furthermore, in order to overcome low Q-factor shown in Si based sensors, we developed a new touch-mode sensor using the embedded A/D converter. The basic experimental makes us to predict the performance of wireless sensor. The experimental results showed that the maximum distance for detection is about 40 cm, which is the maximum value of theoretical radar equation when the RF power of 13 dbm and antenna gain of 0 dbi are used. Moreover, the feasibility of the MEMS A/D converter is experimentally proven. This study can be applied to develop a wireless sensor which does not use any electrical power source. The main usage will be

9 J.-G. Oh et al. / Sensors and Actuators A 141 (2008) ubiquitous sensor node for structural health monitoring system or in vivo bio-sensors. Acknowledgement This work was supported by Korea Research Foundation Grant (KRF D20063). References [1] H. Subramanian, et al., Design and fabrication of wireless remotely readable MEMS based microaccelerometers, Smart Mater. Struct. (1997) [2] F. Schmidt, G. Scholl, Wireless SAW identification and sensor systems, Int. J. High Speed Electron. Syst. 10 (4) (2000) [3] A. Pohl, G. Ostermayer, L. Reindl, F. Seifert, Spread spectrum techniques for wirelessly interrogable passive SAW sensors, in: Proceedings of the 1996 IEEE Symposium Spread Spectrum Techniques and Applications, 1996, pp [4] G. Scholl, F. Schmidt, T. Ostertag, L. Reindl, H. Scherr, U. Wolff, Wireless passive SAW sensor systems for industrial and domestic applications, in: Proceedings of the 1998 IEEE International Frequency Control Symposium, 1998, pp [5] G. Schemitta, et al., Wireless pressure and temperature measurement using SAW hybrid sensor, in: IEEE Ultrasonic Symposium, 2000, pp [6] Q. Wang, W.H. Ko, Modeling of touch mode capacitive sensors and diaphragms, Sensors Actuators A-75 (1999) [7] W.H. Ko, Q. Wang, Touch mode capacitive pressure sensors, Sensors Actuators A-75 (1999) [8] R.B. Brown, et al., Practical implementation of coupling-of-modes theory for SAW device modeling, in: IEEE Ultrasonic Symposium Proceedings, 1989, pp [9] G. Tobolka, Mixed matrix representation of SAW transducers, in: IEEE Transactions on Sonics and Ultrasonics, 1979, pp [10] C.S. Hartmann, et al., A generalized impulse response model for SAW transducers including effects of electrode reflections, in: IEEE Ultrasonic Symposium Proceedings, 1988, pp [11] B. Choi, G. Lee, Modelling of stretching effects on the contacted diaphragm of pressure transducers by LIGA process, Sensors Actuators A-69 (1998) Biographies Jae-Geun Oh received his BS in electronic engineering and his MS in mechanical engineering from Sogang University, Seoul, Korea in 1995 and 1999, respectively, and PhD in mechanical engineering from Sogang University, Seoul, Korea in From 1996 to 1998, he was a research engineer at R&D Center in Kia Motors, where he worked on design of electronic engine control unit for engine management system. From 2005 to 2006, he joined MDT R&D Center for developing microsensors. He was in charge of the design and fabrication of the passive radio sensor system, where he worked on design of passive radio sensor system and developing of TPMS (Tire Pressure Monitoring System) sensor. He is currently a research professor of mechanical engineering at Sogang University, Seoul, Korea. His research interests include microelectromechanical systems (MEMS), microfabrication technologies, RF MEMS and the ubiquitous sensor issues. Bumkyoo Choi received his BS in mechanical engineering and his MS in mechanical design engineering from Seoul National University, Seoul, Korea in 1981 and 1983, respectively, and PhD in engineering mechanics from the University of Wisconsin, Madison, in From 1984 to 1986 he was a research engineer to the CAD/CAM Laboratory at KAIST (Korea Advanced Institute of Science and Technology), where he worked on the structure analysis and design for machine elements. From 1992 to 1994, he was a technical staff member of CXrL (Center for X-ray Lithography) in the University of Wisconsin where he developed a computer code for thermal modeling of X-ray mask membrane during synchrotron radiation. From 1994 to 1997 he joined Samsung Electronics for developing microsensors and microactuators. He was in charge of the design and fabrication of the micromirror for display system. He is currently a professor in the Dept. of Mechanical Engineering of Sogang Univ., Seoul, Korea. His research interest includes microelectromechanical systems (MEMS), micromachining and microfabrication technologies, and modeling issues. Seung-Yop Lee was born in Seoul, Korea on April 20, He received the BS degree in mechanical engineering from Seoul National University, Seoul, Korea in 1989 and the MS/PhD degrees in mechanical engineering from the University of California, Berkeley, in 1990 and 1995, respectively. He is currently a professor of the Department of Mechanical Engineering, Sogang University, Seoul, Korea. His research interests include information storage devices and biomimetic sensors and actuators with biomedical applications.