Lilia Arapan

A micromachined thermally compensated thin film Lamb wave resonator for frequency control and sensing applications

Micromachined thin film plate acoustic wave resonators (FPARs) utilizing the lowest order symmetric Lamb wave (S0) propagating in highly textured 2 µm thick aluminium nitride (AlN) membranes have been successfully demonstrated (Yantchev and Katardjiev 2007 IEEE Trans. Ultrason. Ferroelectr. Freq. Control 54 87–95). The proposed devices have a SAW-based design and exhibit Q factors of up to 3000 at a frequency around 900 MHz as well as design flexibility with respect to the required motional resistance. However, a notable drawback of the proposed devices is the non-zero temperature coefficient of frequency (TCF) which lies in the range −20 ppm K−1 to −25 ppm K−1. Thus, despite the promising features demonstrated, further device optimization is required. In this work temperature compensation of thin AlN film Lamb wave resonators is studied and experimentally demonstrated. Temperature compensation while retaining at the same time the device electromechanical coupling is experimentally demonstrated. The zero TCF Lamb wave resonators are fabricated onto composite AlN/SiO2 membranes. Q factors of around 1400 have been measured at a frequency of around 755 MHz. Finally, the impact of technological issues on the device performance is discussed in view of improving the device performance.

Highly Mass-Sensitive Thin Film Plate Acoustic Resonators (FPAR)

The mass sensitivity of thin aluminum nitride (AlN) film S0 Lamb wave resonators is theoretically and experimentally studied. Theoretical predictions based on modal and finite elements method analysis are experimentally verified. Here, two-port 888 MHz synchronous FPARs are micromachined and subsequently coated with hexamethyl-disiloxane(HMDSO)-plasma-polymerized thin films of various thicknesses. Systematic data on frequency shift and insertion loss versus film thickness are presented. FPARs demonstrate high mass-loading sensitivity as well as good tolerance towards the HMDSO viscous losses. Initial measurements in gas phase environment are further presented.

Thin-film zero-group-velocity Lamb wave resonator

A concept for the development of thin film micro-acoustic resonators is demonstrated. The basic principles for the design and fabrication of zero-group-velocity Lamb acoustic wave resonators on c-textured thin aluminum nitride films are presented. The experimental results demonstrate that the zero-group-velocity waves can be employed in high frequency resonators with small form factors.

Sensitivity Features of Thin Film Plate Acoustic Wave Resonators

Thin film plate acoustic resonators devices operating in the lowest order symmetric Lamb wave mode (S0) in c-oriented aluminum nitride (AlN) membranes on Si were fabricated and tested for their sensitivities to pressure and mass as well as for their ability to work in liquid environment.

IC-compatible power oscillators using Thin Film Plate Acoustic Resonators (FPAR)

In this study, two-port 880MHz FPAR devices operating on the lowest order fast symmetric Lamb wave mode (S0) in c-oriented AlN membranes on Si, were fabricated and subsequently tested for their power handling capabilities in a feedback-loop power oscillator circuit. The S0 Lamb waves were excited and detected by a classical two-port resonator structure, as in Rayleigh SAW (RSAW) resonators. Incident power levels of up to 24 dBm (250 mW) for the FPARs were provided by a high-power sustaining amplifier in the loop. No measurable performance degradation was observed. The results from this study indicate that IC-compatible S0 FPAR devices can dissipate orders of magnitude higher RF-power levels than their RSAW counterparts on quartz and are well suited for integrated microwave power oscillators with thermal noise floor (TNF) levels below −175 dBc/Hz.

Temperature compensation of thin AlN film resonators utilizing the lowest order symmetric lamb mode

Micromachined thin film plate acoustic wave resonators (FPAR) utilizing the lowest order symmetric Lamb wave (S0) propagating in highly textured 2 mum thick Aluminum Nitride (AlN) membranes have been successfully demonstrated. However, a notable drawback of the proposed devices is their non-zero temperature coefficient of frequency (TCF) which lies in the range -20 ppm/K to -25 ppm/K. In this work temperature compensation of thin AlN film Lamb wave resonators is studied and demonstrated. Temperature compensation, while retaining at the same time the device electromechanical coupling, is experimentally demonstrated. The zero TCF Lamb wave resonators are fabricated onto composite AlN/SiO2 membranes. Q factors of around 1400 have been measured at a frequency of around 755 MHz.

Coupled Mode approach to the analysis of thin film S0 lamb wave resonators

In this work the applicability of the Couplings-of-Modes (COM) approach to the analysis of one- and two- port thin AlN film plate acoustic resonators (FPAR), utilizing the S0 Lamb wave, is discussed. Analysis based on the Fluquet-Bloch theorem as well as COM parameter extraction from a micromachined FPAR test structure are simultaneously used to verify the applicability of the COM approach. Subsequently, COM based design of 2-port FPARs for use in feed back loop oscillators and cascaded filters is demonstrated. The proposed theoretical approach can easily be adapted to the case of S0 Lamb wave resonators utilizing reflections from suspended membrane edges.

Acoustic Transducers as Passive Cooperative Targets for Wireless Sensing of the Sub-Surface World: Challenges of Probing with Ground Penetrating RADAR

Passive wireless transducers are used as sensors, probed by a RADAR system. A simple way to separate the returning signal from the clutter is to delay the response, so that the clutter decays before the echoes are received. This can be achieved by introducing a fixed delay in the sensor design. Acoustic wave transducers are ideally suited as cooperative targets for passive, wireless sensing. The incoming electromagnetic pulse is converted into an acoustic wave, propagated on the sensor substrate surface, and reflected as an electromagnetic echo. According to a known law, the acoustic wave propagation velocity depends on the physical quantity under investigation, which is then measured as an echo delay. Both conversions between electromagnetic and acoustic waves are based on the piezoelectric property of the substrate of which the sensor is made. Investigating underground sensing, we address the problems of using GPR (Ground-Penetrating RADAR) for probing cooperative targets. The GPR is a good candidate for this application because it provides an electromagnetic source and receiver, as well as echo recording tools. Instead of designing dedicated electronics, we choose a commercially available, reliable and rugged instrument. The measurement range depends on parameters like antenna radiation pattern, radio spectrum matching between GPR and the target, antenna-sensor impedance matching and the transfer function of the target. We demonstrate measurements at depths ranging from centimeters to circa 1 m in a sandbox. In our application, clutter rejection requires delays between the emitted pulse and echoes to be longer than in the regular use of the GPR for geophysical measurements. This delay, and the accuracy needed for sensing, challenge the GPR internal time base. In the GPR units we used, the drift turns out to be incompatible with the targeted application. The available documentation of other models and brands suggests that this is a rather general limitation. We solved the problem by replacing the analog ramp generator defining the time base with a fully digital solution, whose time accuracy and stability relies on a quartz oscillator. The resulting stability is acceptable for sub-surface cooperative sensor measurement.

Polymer coated thin film plate acoustic resonators (FPAR) for gas sensing applications

Mass sensitivity of thin aluminum nitride (AlN) film S0 plate wave resonators is theoretically and experimentally studied. Here, two-port 888MHz synchronous thin film plate acoustic resonators (FPAR) are micromachined and subsequently coated with plasma-polymerized hexamethyldisiloxane (pp-HMDSO) thin films of various thicknesses. Systematic data on frequency shift and insertion loss versus film thickness are presented in a comparative manner. Measurements in gas phase environment are further presented in a comparative manner.