Surface Acoustic Wave Propagation Loss Research Articles

Comparative performance of piezoceramic and crystal SAW filters

Bulk elastic, piezoelectric, and dielectric constants of four lead zirconate titanate piezoceramics, Pz24, Pz26, Pz27, Pz28, and a modified lead titanate, PTS, are measured and used to theoretically compute the effective permittivity curve of each material from which the surface acoustic wave (SAW) properties are deduced. In parallel, experimental measurements of the SAW properties are carried out by using a curve fitting algorithm on the real and imaginary parts of the electrical input impedance of an unapodised single electrode SAW transducer. The SAW propagation losses are also measured using a SAW delay line. For these ceramics, the effects of a hot isostatic pressing (HIP) post sintering process on the performances of the device are also studied. All these results are discussed and show that ceramic materials, particularly PTS, have potential for SAW applications.

Study of mechanical and dielectric losses contribution to the surface-acoustic-wave attenuation in piezoelectric ceramics

The surface-acoustic-wave (SAW) propagation losses of two ceramics, a lead zirconate titanate composition and a modified lead titanate composition, are studied. Contrary to crystals, where only mechanical losses contribute, dielectric losses and scattering at the grain boundaries have to be taken into account. A first-order perturbation theory is extended to include dielectric losses to calculate the surface attenuation. Based on the measurement of the mechanical and the dielectric losses of the two ceramics as a function of frequency, the frequency behavior of the SAW propagation losses of each material is theoretically calculated and values are compared to those obtained experimentally at 25 MHz. Both results show that the modified lead titanate ceramic has low SAW propagation losses, 0.22 dB/mm at 25 MHz, while the lead zirconate titanate has propagation losses of 2 dB/mm at 25 MHz. Using results published in literature, the effect of the scattering contribution is evaluated. Finally, other SAW properties of ceramics, i.e., the free-surface-wave velocity, the surface coupling coefficient, and the surface permittivity are measured and reported. All these results are discussed and the suitability of both materials for SAW applications is examined.

Surface Acoustic Wave Properties on Proton-Exchanged 128°-Rotated Y-Cut LiNbO3

The characteristics of surface acoustic waves on proton-exchanged 128°-rotated Y-cut LiNbO3 were investigated at a frequency of 100 MHz. The changes of the phase velocity and the electromechanical coupling coefficient as a function of the proton-exchange depth were measured. From these experiments, the elastic constants and the piezoelectric constants of the proton-exchanged layer were determined. The effects of annealing on the phase velocity and the surface acoustic wave propagation loss were also measured.

SAW propagation loss mechanism in piezoelectric ceramics

The surface acoustic wave (SAW) propagation loss mechanism of piezoelectric ceramics is examined using lead titanate (PbTiO3) ceramics modified with additives of Nd2O3, MnO2, and In2O3, which have zero temperature coefficients of SAW delay time. The propagation loss L in the frequency range 30 to 300 MHz consists of two parts. One is the Rayleigh scattering loss LS caused by grain, and the other is the true dissipative loss LF caused by internal friction. Thus, total loss results in L(dB/cm) = LS+LF = 3.2×103D3f4+6.8×10−4f2 (where D and f denote average grain size in cm and frequency in MHz, respectively). It is also revealed experimentally that the internal friction causing the latter loss has a close correlation with the dielectric loss through piezoelectric combination. Therefore, materials with fine grain (<0.5 μm) and low dielectric loss (<10−3) must be developed to reduce SAW propagation losses which prevent the piezoelectric ceramics from being used in higher frequency (≳100 MHz) SAW applications. Based on these requirements, fine grained (∼0.5 μm) specimens are fabricated. The propagation losses obtained from these specimens, 9 dB/cm at 100 MHz, are the lowest values ever reported for piezoelectric ceramics. This result suggests that the present ceramics have high potential as materials for high-frequency SAW devices.

Determination of capture cross section and surface-states concentration profile using the surface-acoustic-wave convolver

Energy distribution of surface states and majority-carriers capture cross section is determined using the surface-acoustic-wave (SAW) convolver. The semiconductor is placed a small distance above the SAW delay line, with a uniform air gap between the two media. A fast rise time dc pulse is applied across the semiconductor-delay line structure, and the resulting change in the SAW propagation loss is observed. The transient response of the SAW propagation loss represents the emission or trapping of majority carriers from surface states. From this transient response, the capture cross section and the concentration profile of the surface states are determined. The results obtained agree with the already known distribution of fast surface states; it is constant at the middle of the gap and increases towards the conduction band, whereas the capture cross section is constant in the middle of the gap and decreases toward the conduction band. This new SAW technique is simple, sensitive, and requires no contact to the semiconductor surface.

Optical image scanning by electronic control of surface acoustic wave attenuation in CdS

A new technique of reading optical images in a CdS surface acoustic wave delay line is presented. The principle is based on controlling surface-acoustic wave propagation loss by applied gate bias.