Julius Koskela

SAW/LSAW COM parameter extraction from computer experiments with harmonic admittance of a periodic array of electrodes

A novel numerical method for determining the surface acoustic wave and the leaky surface acoustic wave characteristics is proposed. The Greens function method is used to simulate an infinite periodic transducer driven by a periodic voltage. We show that the coupling of modes parameters and the dispersion relation can be extracted from the change in the admittance as the periodicity of the driving voltage is slightly shifted. The method first introduced here leads to significant savings in computing time.

Suppression of the leaky SAW attenuation with heavy mechanical loading

We discuss effects on the propagation of surface acoustic waves (SAW) due to heavy mass loading on Y-cut lithium niobate and lithium tantalate substrates. An abrupt reduction in the leaky-SAW (LSAW) attenuation is observed in the measured admittance of a long resonator test structure on 64/spl deg/-YX-cut lithium niobate for aluminum electrodes of thickness h//spl lambda//sub 0/ beyond 9-10%. This experimental fact is explained theoretically as the slowing down of the leaky wave below the velocity of the slow shear surface-skimming bulk wave (SSBW), such that energy dissipation into bulk-wave emission becomes inhibited. An infinite transducer structure is modeled using the periodic Greens function and the boundary-element method (BEM); the computed theoretical properties well explain for the experimental findings. The model is further employed to quantify the leaky surface-wave attenuation characteristics as functions of the crystal-cut angle and the thickness of the electrodes. The resonance and antiresonance frequencies and the corresponding Q values are investigated to facilitate the selection of crystal cuts and electrode thicknesses. The transformation of the leaky SAW into a SAW-type nonleaky wave is also predicted to occur for gold electrodes, with considerably thinner finger structures.

Acoustic loss mechanisms in leaky SAW resonators on lithium tantalate

We discuss acoustic losses in synchronous leaky surface-acoustic wave resonators on rotated Y-cut lithium tantalate substrates. Laser probe measurements and theoretical methods are employed to estimate the radiation of leaky waves into the busbars of the resonator and the excitation of bulk-acoustic waves. We find that the escaping waves lead to a significant increase in the conductance, typically in the vicinity of the resonance and in the stopband, but that they do not explain the experimentally observed deterioration of the electric response at the antiresonance. At frequencies above the stopband the generation of fast shear bulk-acoustic waves is the dominant loss mechanism.Discusses acoustic losses in synchronous leaky surface acoustic wave (LSAW) resonators on rotated Y-cut lithium tantalate (LiTaO/sub 3/) substrates. Laser probe measurements and theoretical models are employed to identify and characterize the radiation of leaky waves into the busbars of the resonator and the excitation of bulk acoustic waves. Escaping LSAWs lead to a significant increase in the conductance, typically occurring in the vicinity of the resonance and in the stopband, but they do not explain the experimentally observed deterioration of the electrical response at the antiresonance. At frequencies above the stopband, the generation of fast shear bulk acoustic waves is the dominant loss mechanism.

Asymmetric acoustic radiation in leaky SAW resonators on lithium tantalate

We discuss an acoustic loss mechanism in leaky surface-acoustic wave resonators on 36°YX-cut lithium tantalate substrate. Our recent acoustic field scans performed with an optical Michelson interferometer revealed a spatially asymmetric acoustic field atop the busbars of a resonator, giving rise to acoustic beams which escape the resonator area and lead to undesired losses. Here, we link the phenomenon with the inherent crystalline anisotropy of the substrate crystal: the shape of the slowness curves and the asymmetry of the polarization for the leaky surface-acoustic waves propagating at an angle with respect to the crystal X-axis.

Surface transverse waves on langasite

Surface transverse waves are numerically simulated in a periodic transducer on rotated Y-cuts of langasite crystal. The resonance and antiresonance frequencies of the transducer are evaluated as functions of the crystal cut, temperature and the thickness of the aluminum electrodes. For an optimal cut, we demonstrate a vanishing first-order temperature coefficient and find a coupling strength several times higher than that for Rayleigh waves in ST-cut quartz. The transverse waves in langasite differ from those in quartz in that the shear-wave velocity in langasite is lower than that in the aluminum electrodes, resulting in an essentially lower sensitivity of the resonance frequency to the thickness variations of the electrodes.

Analytic model for STW/BGW/LSAW resonators

The propagation and excitation of surface transverse waves (STWs), interacting with bulk-acoustic waves (BAWs), are considered. A parametrized model is proposed for the harmonic admittance of STWs in an infinite periodic grating. The model enables the study of various phenomena, such as unidirectionality, parasitic BAW radiation, and the frequency-dependence of the electromechanical coupling. The analytic formula allows ideal one-port synchronous resonators to be modeled numerically. Results are compared with simulations and experiments.

Mechanism for acoustic leakage in surface- acoustic wave resonators on rotated Y-cut lithium tantalate substrate

We discuss an acoustic loss mechanism in surface-acoustic wave resonators on 36° YX-cut lithium tantalate substrate. Recent acoustic field scans performed with an optical Michelson interferometer reveal a spatially asymmetric acoustic field atop the busbars of a resonator, giving rise to acoustic beams which escape the resonator area and lead to undesired losses. Here, we link the phenomenon with the inherent crystalline anisotropy of the substrate: the shape of the slowness curves and the asymmetry of the polarization for leaky surface-acoustic waves, propagating at an angle with respect to the crystal X-axis.

COM parameter extraction from computer experiments with harmonic admittance of a periodic array of electrodes

A novel numerical method for determining the surface-acoustic wave (SAW) and the leaky surface-acoustic wave (LSAW) characteristics is proposed. The Greens function method is used to simulate the excitation of waves in an electrically driven periodic array of electrodes. For the phase shift between consecutive periods close to /spl pi/, the admittance may also be modeled within the coupling-of-modes (COM) formalism. We show that the dispersion relation and all COM parameters may be determined from the change in admittance as the phase shift is slightly varied, leading to significant savings in the computing time.

Surface acoustic wave impedance element filters for 5 GHz

Surface acoustic wave (SAW) impedance element filter prototypes operating in the 5 GHz range are designed, fabricated, and characterized. The patterning is carried out using direct-writing electron-beam lithography and the lift-off technique. The periodicity p of the resonators in the filters for the 5 GHz regime is on the order of 400 nm. Despite the nonideal finger profile, a low minimum insertion loss of 6.5 dB and a flat passband are measured. Our results suggest that SAW technology itself presents no fundamental physical limitations for its extension into the 5 GHz range.

Surface acoustic wave impedance element ISM duplexer: modeling and optical analysis

Surface acoustic wave (SAW) impedance element antenna duplexers provide compact, high performance, front-end components apt for industrial fabrication. We describe investigations on the design and modeling of a compact ISM antenna duplexer fabricated on a 36/spl deg/ YX-cut LiTaO/sub 3/ substrate based on SAW impedance elements. In particular, we have performed 3-D modeling of the inductive and capacitive electromagnetic couplings caused by the package parasitics for the duplexer. The use of a 1:3 IDT structure for the reduction of the passband width is discussed. The frequency response of the duplexer is predicted with the help of circuit simulation; the modeling is refined by optimization of the model parameters to improve the fit between the measured and simulated responses. We also report scanning optical imaging of the acoustic field within the resonator structures with the help of laser interferometry; this provides insight into the loss mechanisms beyond that attainable in mere electric measurements.