Vesseline L. Strashilov

Theoretical and experimental mass-sensitivity analysis of polymer-coated SAW and STW resonators for gas sensing applications

Polymer-coated surface transverse waves (STW) resonators have recently been successfully studied for organic gas sensing applications. The first results indicate increased absolute and even relative sensitivity as compared to similar resonators with surface acoustic waves (SAW). However, the gain in sensitivity is accompanied by the adverse effect of an increased attenuation and the advantage frame is difficult to establish quantitatively. In this paper, a new set of experimental samples with Parylene C-coated quartz substrates are studied. The samples are matched in frequency and wavelength. The results are compared and the obtained features explained using available theoretical algorithms for analyzing layered SAW and Love configurations, and a recently developed STW algorithm. The approximate limits of advantageous applicability of the STW resonator gas sensors are discussed.

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.

The coupling-of-modes approach to the analysis of STW devices. II

For pt. I see ibid., vol. 44, no. 3, p. 652-7 (1997). The method for analyzing surface transverse wave (STW) devices by using a coupling-of-modes (COM) formalism has been completed, covering the STW electromechanical coupling coefficient (ECC). An ECC analytical formula has been derived by fitting numerical results from STW effective permittivity analysis. The ECC exhibits frequency and mass-loading variation. Using this new result, a satisfactory agreement with available experimental frequency characteristics of STW two-port quartz resonators has been achieved, without the necessity of additional experimental information. In its present form, the method is self-consistent and applicable to arbitrary STW layouts.

Efficiency of poly(vinylidene fluoride) thin films for excitation of surface acoustic waves

The applicability of poly(vinylidene fluoride) (PVDF) thin stretched films to the excitation of surface acoustic waves (SAWs) is discussed. Previous results with grating transducers reveal high losses, which are attributed to large attenuation in the polymer. An experimental study was carried out on a LiNbO3 YZ delay line with the input transducer in the form of an interdigital PVDF pair. Untuned insertion losses of 80 dB were obtained at room temperature and frequency of 43 MHz. We demonstrate that these losses are largely caused by the high shear compliance of the film, which leads to degradation of the transducer electromechanical coupling. Employing the piezoelectrically strong thickness-longitudinal mode of the film, a 40 dB loss reduction was achieved. To take full advantage of this mode, a split-electrode grating transducer model with alternating polarity of electrodes is discussed. This is expected to allow efficient SAW excitation at frequencies up to tens of MHz.

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.

Surface transverse waves: properties, devices, and analysis

Surface transverse waves represent a new generation of the surface acoustic wave (SAW) family that offers advantageous properties without further demand for new materials or improved design and technology. The most effective activity in the surface transverse wave (STW) area has been realized during the last decade with high-performance devices achieved and analytical methods developed. The present paper reviews the basic achievements in historical and factual order. A state-of-the-art introduction is combined with discussion on the development tendencies with specific emphasis on sensor technology.

Transmission and reflection coefficients for surface transverse waves propagating under shorted metal strip gratings

Coupled-mode theory is used for the analysis of surface transverse waves (STWs) propagating under metal-strip gratings in the YZ plane of quartz. Analytical expressions are obtained for the reflection and transmission coefficients of waves propagating on rotated Y-cut substrates of weakly piezoelectric trigonal, tetragonal, or hexagonal crystals. A quadratic dependence of the stopband width on the height-to-period ratio of the electrodes is found. This is an agreement with stopband width measurements. The results can be used in the analysis of STW grating structures, particularly the two-port resonator filter.<<ETX>>

Surface transverse waves in polymer-coated grating configurations

The propagation characteristics of surface transverse waves (STW) in polymer-coated surface gratings on rotated Y-cut quartz substrates are studied theoretically. A two-layer Love wave theory is combined with Floquet analysis to derive the wave dispersion and attenuation. The thickness of the polymer layer is found to affect the position and width of the frequency stop band, while the polymer viscosity introduces out-of-band attenuation. Both parameters effectively act in the vicinity of the lower-stop-band edge where STW resonators are mostly operated. The presented algorithm is applicable to the analysis of STW gas sensors and to the improvement of the resonator performance.

COM-theory analysis of STW resonator structures

The dispersion characteristics of surface transverse waves (STWs) on 36/spl deg/ Y cut quartz substrate have been analysed numerically. A closed-form dispersion relation has been introduced in the equations of the coupling-of-modes (COM) theory to account for the variation of trapping with frequency. The transmission and reflection coefficients at the finger edges have been determined from the stopband characteristics. Introducing an electromechanical coupling which varies with the electrode mass loading, quantitative agreement with experimental results on 650 MHz two-port STW resonators has been achieved.

Coupling-of-modes analysis of STW resonators including loss mechanism

Surface transverse wave (STW) resonators exhibit substantial advantages over conventional surface acoustic wave (SAW) resonators. However, their analysis is more involved because of the complicated nature of STW. Many parameters have been studied, but the one that has been difficult to analyze accurately is the quality factor Q, which is of great importance for characterizing the devices. At present, none of the available analytical models is concerned with quantitative loss consideration, and the establishment of reliable design rules is difficult. We present a theoretical study that allows one to conduct coupling-of-modes (COM) STW loss analysis and estimate the resonator Q from material and layout parameters. The COM transmission coefficient /spl chi//sub 11/ is derived by Floquet analysis. Its imaginary part is obtained by numerically fitting available experimental data for the Q-factor of particular resonators. It is a measure of STW propagation loss that adds to the electrode reflection loss.