[Show abstract][Hide abstract] ABSTRACT: The thermal compensation at high temperatures for aluminum nitride (AlN) Lamb wave resonators utilizing the lowest symmetric (S0) mode is theoretically and experimentally demonstrated in this work. The turnover temperature can be designed at high temperatures by changing the normalized AlN film thickness (hAlN/λ) and the normalized silicon oxide (SiO2) layer thickness (hSiO2/λ) in the AlN/SiO2 composite layer. The AlN Lamb wave resonators were well temperature-compensated at 214°C and 430°C, respectively, by using different ratios of hAlN/λ to hSiO2/λ. Even though the intrinsic quality factor (Q) degrades and the intrinsic motional impedance (Rm) increases at high temperatures, a Lamb wave resonator shows a Q of 760 at its turnover temperature, 430°C. These results demonstrate that thermally stable AlN Lamb wave resonators have the great potential for harsh environment applications.
[Show abstract][Hide abstract] ABSTRACT: We have developed a wafer-level packaging solution for surface acoustic wave devices using imprinted dry film resist (DFR). The packaging process involves the preparation of an imprinted dry film resist that is aligned and laminated to the device wafer and requires one additional lithography step to define the package outline. Two commercial dry film solutions, SU-8 and TMMF, have been evaluated. Compared with traditional ceramic packages, no detectable RF parasitics are introduced by this packaging process. At the same time, the miniature package dimensions allow for wafer-level probing. The packaging process has the great advantage that the cavity formation does not require any sacrificial layer and no liquids, and therefore prevents contamination or stiction of the packaged device. This non-hermetic packaging process is ideal for passive antenna modules using polymer technology for low-cost SAW identification (ID)-tags or lidding in low-temperature cofired ceramic (LTCC) antenna substrates for high-performance wireless sensors. This technique is also applicable to SAW filters and duplexers for module integration in cellular phones using flip-chip mounting and hermetic overcoating.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 02/2011; 58(2):406-13. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: By adjusting the aluminum nitride (AlN) "overhang" dimension, OH, measured from the center of the outermost electrode to the edge of AlN plate, independently of the interdigital transducer (IDT) pitch, AlN Lamb wave mode resonators (LWRs) with 0.25% relative frequency control were demonstrated. Unlike adjusting electrode number, NE, device length, L, and electrode coverage, η, all of which affect device parameters such as Q, R<sub>m</sub>, C<sub>0</sub>, and k<sub>t</sub><sup>2</sup>, overhang frequency selection technique can linearly shift the center frequency up to 2.5% with no influence on other resonator parameters. Preliminary results of narrowband ladder filters using these finely spaced AlN LWRs as building blocks were fabricated and measured. Fine tuning of bandwidth and center frequency were studied.
Ultrasonics Symposium (IUS), 2010 IEEE, San Diego, CA; 11/2010
[Show abstract][Hide abstract] ABSTRACT: In this study the characterization of an aluminum nitride (A1N) double ended tuning fork (DETF) fabricated on a layer of silicon dioxide (SiO<sub>2</sub>) is presented. The positive temperature coefficients of SiO<sub>2</sub> are used to achieve zero TCF for radio frequency (RF) Lamb wave resonators. This paper shows the possibility to integrate temperature compensated Lamb wave resonators with DETF-based devices on a single chip. The DETF resonates in a quasi-in-plane mode shape with a Q-factor of 578 in air and 3028 in vacuum. Through a laser Doppler velocity (LDV) measurement we also show that, due to the biomorph nature of the structure and the angle of the side walls, the motion of each tine of the DETF is a combination of in-plane bending, out-of-plane bending and torsional motion around the beam main axis.
Ultrasonics Symposium (IUS), 2010 IEEE, San Diego, CA; 11/2010
[Show abstract][Hide abstract] ABSTRACT: In this letter, temperature compensation for aluminum nitride AlN Lamb wave resonators operating at high temperature is presented. By adding a compensating layer of silicon dioxide SiO2 , the turnover temperature can be designed for high temperature operation by varying the normalized AlN film thickness hAlN and the normalized SiO2 film thickness hSiO2. With different designs of hAlN and hSiO2, the Lamb wave resonators were well temperature-compensated at 214 ° C, 430 ° C, and 542 ° C, respectively. The experimental results demonstrate that the thermally compensated AlN Lamb wave resonators are promising for frequency control and sensing applications at high temperature.
[Show abstract][Hide abstract] ABSTRACT: Thermal compensation for aluminum nitride (AlN) Lamb wave resonators operating at high temperature is experimentally demonstrated in this study. By adding a compensating layer of silicon dioxide (SiO<sub>2</sub>), the turnover temperature can be designed for high temperature operation by varying the normalized AlN thickness (h<sub>AlN</sub>/λ) and the normalized SiO2 thickness (h<sub>SiO2</sub>/λ) in the AlN/SiO<sub>2</sub> composite stack. With different designs of h<sub>AlN</sub>/λ and h<sub>SiO2</sub>/λ, the Lamb wave resonators were well temperature-compensated at 214°C, 430°C, and 542°C, respectively. Furthermore, several testing cycles in the full temperature range from 25°C to 700°C were taken to demonstrate the repeatability of the frequency characteristics. This thermal compensation technology is promising for future applications to piezoelectric resonators, filters, and sensors at high temperature.
Frequency Control Symposium (FCS), 2010 IEEE International, Newport Beach, CA; 07/2010
[Show abstract][Hide abstract] ABSTRACT: In this paper, the temperature compensation of AlN Lamb wave resonators using edge-type reflectors is theoretically studied and experimentally demonstrated. By adding a compensating layer of SiO2 with an appropriate thickness, a Lamb wave resonator based on a stack of AlN and SiO2 layers can achieve a zero first-order temperature coefficient of frequency (TCF). Using a composite membrane consisting of 1 microm AlN and 0.83 microm SiO2, a Lamb wave resonator operating at 711 MHz exhibits a first-order TCF of -0.31 ppm/degrees C and a second-order TCF of -22.3 ppb/degrees C(2) at room temperature. The temperature-dependent fractional frequency variation is less than 250 ppm over a wide temperature range from -55 degrees C to 125 degrees C. This temperature-compensated AlN Lamb wave resonator is promising for future applications including thermally stable oscillators, filters, and sensors.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 03/2010; 57(3):524-32. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Thin-film, aluminum nitride (AlN) can be utilized in piezoelectric actuators as an alternative to PZT, enabling high pressure operation without compressive-stress depolarization. To overcome relatively lower piezoelectric coefficients, amplification flexures (e.g. bi-chevron) are used. This paper describes the design, fabrication and testing of a bi-chevron AlN actuator for a high-pressure microvalve. The AlN microvalve can operate at higher pressures (10 MPa) with a fifty-fold increase in stroke/volume ratio compared to current PZT valves. The actuation stroke is compared with theoretical values and finite element method (FEM) results. The error between FEM and empirical data is less than 17 %.
[Show abstract][Hide abstract] ABSTRACT: In this paper, the temperature compensation of aluminum nitride (AlN) Lamb wave resonators using edge-type reflectors is experimentally studied and demonstrated. By adding one compensating silicon dioxide (SiO<sub>2</sub>) layer with an appropriate thickness, the Lamb wave resonator can achieve a zero linear temperature coefficient of frequency (TCF). With the composite membranes including 1 mum AlN and 0.83 mum SiO<sub>2</sub>, a Lamb wave resonator operating at 711 MHz exhibits a secondorder TCF of -21.5 ppb/degC<sup>2</sup>. The temperature-dependent frequency variation is less than 250 parts per million (ppm) over a wide temperature range from -55degC to 125degC. This temperature compensated AlN Lamb wave resonator is promising for future applications to thermally stable oscillators, filters, and sensors.
Frequency Control Symposium, 2009 Joint with the 22nd European Frequency and Time forum. IEEE International, Besançon, France; 05/2009
[Show abstract][Hide abstract] ABSTRACT: We have extended the K-model, a Green's function model previously developed for the precise simulation of surface acoustic wave (SAW) device at radio frequencies (RF), to the computation of Lamb wave resonators (LWR). The effective permittivity for Lamb wave excitation on thin aluminum nitride plates with different electrode configurations is compared. The results of the electromechanical coupling coefficient k<sup>2</sup> computed from the effective permittivity agree closely with the classical approximation based on the open and shorted velocities v<sub>0</sub> and v<sub>m</sub>. Although a backside metallization increases the value of k<sup>2</sup> the frequency spacing of resonance and anti-resonance is reduced due to capacitive feedthrough through the bottom electrode. Finally, we demonstrate the ability of the model to predict the numerous spurious modes that are excited with low-electrode-count interdigital transducers (IDT).
Ultrasonics Symposium, 2008. IUS 2008. IEEE; 12/2008
[Show abstract][Hide abstract] ABSTRACT: In this study we present an analytical model to characterize the thermo elastic damping (TED) for beams in piezoelectric and pyroelectric materials for Q optimization. The model is based on the Euler Bernoulli equation and considers the constitutive equations for piezoelectric and pyroelectric materials. The analytical solution shows that due to the piezoelectric stiffening and the pyroelectric effect the Q-factor is penalized by 15%. The optimization of the Q-factor for piezoelectric beam based resonators is not only related to the beam dimensions, as for isotropic materials, but also to the electrode design.
Ultrasonics Symposium, 2008. IUS 2008. IEEE; 12/2008
[Show abstract][Hide abstract] ABSTRACT: This paper describes an evaluation scheme that prevents phase ambiguity of surface acoustic wave (SAW) delay-line sensors. Although it is well-known that phase evaluation yields accuracies of 150~1500 times higher than time-delay evaluation, the problem of phase ambiguity has prevented phase evaluation of sensors operating over a range larger than 2 pi. This paper addresses this unsolved problem with a complete strategy. Furthermore, the existence of an optimum choice of the relative reflector positions on the sensor is shown. The presented relations enable the design of maximum accuracy SAW delay-line sensors.
IEEE transactions on ultrasonics, ferroelectrics, and frequency control 08/2008; 55(7):1640-52. · 1.80 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this paper we present the temperature compensation of aluminum nitride (AlN) Lamb wave resonators for a future application to XOs and TCXOs for a frequency ranging from 100 MHz to 1000 MHz. The temperature coefficient of frequency (TCF) for the lowest symmetric Lamb wave mode S<sub>0</sub> for AlN plates with h/lambda<0.3 is found to be around -30 ppm/K. A zero TCF resonator is obtained by adding a compensating silicon dioxide layer. The low dispersion of the phase velocity for the S<sub>0</sub>-mode propagating in thin AlN plates reduces not only the fabrication tolerances towards thickness variations of the AlN layer, but also enables resonators operating over a wide frequency range, i.e. from 100 MHz to 1000 MHz, based on two absolute film thicknesses for AlN and SiO<sub>2</sub> achieving near zero TCF over the entire frequency range. The acoustic properties and different layer configurations of zero TCF Lamb wave devices are discussed in detail.
Frequency Control Symposium (FCS), 2008 IEEE International, Honolulu, HI; 06/2008
[Show abstract][Hide abstract] ABSTRACT: This paper describes the development of a MEMS tunable surface acoustic wave (SAW) device that allows an integrated sensor to be interrogated wirelessly in a range of several meters. The component itself requires no power for the wireless communication, as the principle is based on evaluating the reflected signal similar to a radar echo. The structure is similar to a one port reflective SAW delay line device. A MEMS switch is used to modulate the SAW according to the binary sensor output. The encoding solely requires an electrostatic tuning voltage of as low as 3 V for the MEMS switch. We successfully demonstrated that the device assembled on a microstrip antenna wirelessly transmitted ASCII characters over a distance of up to 2 m.
[Show abstract][Hide abstract] ABSTRACT: This paper mainly presents the design and fabrication of a TDMA (time division multiple access) passive wireless pressure sensor using 2.45 GHz surface acoustic wave (SAW) delay lines. The SAW pressure sensor consists of two LiNbO3 substrates. The first layer is a pressure-detective thin substrate, on which an interdigital transducer (IDT) and reflectors are fabricated. The second layer is a support substrate, in which a reference pressure cavity is etched. These two substrates are directly bonded. The pressure measurement was successfully demonstrated in a pressure range of 20˜280 kPa with good repeatability. In addition, the influence of a tire on the wireless interrogation of the sensor was investigated. Automobile tires have little negative influence on wireless communication, at least if they do not rotate.
IEEJ Transactions on Sensors and Micromachines 01/2008; 128(5):230-234.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a process technology to form thin diaphragms with sealed cavities in a lithium niobate (LiNbO3) wafer, which will be used for small and sensitive SAW-based pressure sensors. The process technology uses thermal inversion, wet etching with an electroplated Au mask, etch stop at the thermal inversion layer and Au-Au bonding. By the combination of an etch stop and pinhole-free masking, LiNbO3 thin diaphragms with thicknesses of several tens of mum to 10 mum were fabricated in 128deg Y cut LiNbO3 wafers. The Au-Au bonding of the LiNbO3 wafer gave hermetic sealing. In addition the etch profiles of the LiNbO3 wafer predicted by Wulff-Jaccodine plots and observed in experiments were compared, and good agreement was confirmed.
Ultrasonics Symposium, 2008. IUS 2008. IEEE; 01/2008
[Show abstract][Hide abstract] ABSTRACT: A MEMS magneto-static actuator has been optimized for a minimal phase area while maintaining sufficient actuator force and operational frequency. The geometric design variables, which have been optimized, are: stator width, air gap distance, and number of teeth. The optimized design minimizes the magneto-static actuator phase area with respect to the following constraints: minimum required actuation force, maximum armature mass, and minimum yoke seat length. A genetic algorithm (GA) with a penalty method has been utilized for this optimization. The optimal results have been verified using a sequential quadratic programming (SQP) method. From the optimization, the optimal design variables have been determined: The stator width íµí± íµí± = 4.78 íµí±íµí±, the gap distance íµí± íµí± = 0.02 íµí±íµí±, and number of teeth, íµí± íµí±¡ = 4 which yields a phase area of íµí°* ≅ 42.4 íµí±íµí± 2 . Convergence of GA, confirmed by SQP, strongly suggests that the proposed solution point is indeed the global optimum. INTRODUCTION The performance of small-scale internal combustion engines is greatly impacted by the characteristics of its fuel delivery system . Delivering accurate amounts of fuel is critical to optimal engine performance because the air/fuel mixture determines the stoichiometry of the combustion event, which in turn determines the kinetics of the ignition event as well as heat release rate.
[Show abstract][Hide abstract] ABSTRACT: We have investigated controlling the phase velocity of a surface acoustic wave (SAW) by a microelectromechanical switch fabricated on a high coupling piezoelectric substrate. The principle is based on the interaction of the evanescent surface potential of the SAW with the conductive switch. In theory tuning of the velocity in the range given by v<sub>0</sub> and v<sub>m</sub>, i.e. the velocity for a SAW on a free and metallized substrate, is possible. We have achieved up to 17.6 m/s (0.44 %) velocity tuning on 128degYX LiNbO<sub>3</sub>. A maximum velocity sensitivity of Deltav/v of 15times10<sup>-3</sup>/V and phase sensitivity of 700deg/V was measured. This is five orders of magnitude larger than values obtained for electrical field tuning.