S. Lehtonen

Short Reflectors Operating at the Fundamental and Second Harmonics on 128 ◦ LiNbO 3

In this work, we study numerically the operation of surface acoustic wave (SAW) reflectors comprising a small number of electrodes on the 128 degree YX-cut lithium niobate (LiNbO3) substrate. The electrodes have a finite thickness, and they are either open circuited or grounded. The center-to-center distance between adjacent electrodes d corresponds roughly either to half of the characteristic wavelength d proportional to lambda0/2 or to d proportional to lambda0, for the reflectors operating at the fundamental and second harmonic modes, respectively. We use software based on the finite-element and boundary-element methods (FEM/BEM) for numerical experiments with a tailored test structure having 3 interdigital transducers (IDTs), simulating experimental conditions with an incident wave and reflected and transmitted SAWs. Using the fast Fourier transform (FFT) and time-gating techniques, calculation of the Y-parameters in a wide frequency range with rather a small step allows us to determine the reflection coefficients, and to estimate the energy loss due to bulk-wave scattering. The detailed dependences of the attenuation and reflectivity on the metallization ratio and the electrode thickness are given for the classic 128 degree-cut of LiNbO3.

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.

SPUDT filters for the 2.45 GHz ISM band

Filters based on using single-phase, unidirectional transducers (SPUDT) consisting of /spl lambda//4 and wider electrodes are presented. The design variants exploit the significant difference between the reflectivity of short-circuited /spl lambda//4 electrodes and that of floating wide electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The surface acoustic wave (SAW) devices operating at 2.45 GHz have critical dimensions of about 0.3-0.4 /spl mu/m, accessible to standard optical lithography. When matched, the fabricated SPUDT filters exhibit minimum insertion losses of 5.5-7.9 dB together with 3 dB passbands of 89-102 MHz. The majority of the insertion loss can be attributed to the attenuation on free surface and inside the grating, and to the resistivity of the electrodes.

Minimum-loss short reflectors on 128/spl deg/ LiNbO/sub 3/

We consider the interaction of surface acoustic waves (SAWs) with short electrode gratings encompassing only few electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/). The qualifications of the reflectors are evaluated by comparing the part of incident SAW energy scattered by the structure into the bulk to the energy reflected back as a SAW.

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.

Unidirectional SAW transducer for GHz frequencies

A SPUDT structure employing /spl lambda//4 and wider electrodes is introduced. The design and its variants exploit the significant difference between the reflectivity of short-circuited /spl lambda//4 electrodes and that of floating wide electrodes on 128/spl deg/ LiNbO/sub 3/. The SAW devices operating at 2.45 GHz have critical dimensions of about 0.4 /spl mu/m, accessible for standard optical lithography. When matched, the fabricated SPUDT filters exhibit minimum insertion losses of 5.5-7.9 dB together with 3 dB pass-bands of 95-102 MHz.

Unidirectional SAW transducer for gigahertz frequencies

A single-phase unidirectional transducer (SPUDT) structure using /spl lambda//4 and wider electrodes is introduced. The considerable difference between the reflectivity of short-circuited /spl lambda//4 electrodes and that of floating /spl lambda//2-wide electrodes on 128/spl deg/ lithium niobate (LiNbO/sub 3/) is exploited. The surface acoustic wave (SAW) device operating at 2.45 GHz has critical dimensions of about 0.4 /spl mu/m, accessible for standard optical lithography.

Extraction of the SAW attenuation parameter in periodic reflecting gratings

In this paper, the extraction of the coupling-of-modes (COM) model attenuation parameter gamma in a finite grating is considered. We use test structures comprising identical transmitting and receiving transducers and a grating centered in the acoustic channel along the propagation direction of the surface acoustic wave (SAW). The extraction procedure proposed is based on studying the magnitude of the ratio of the reflection and transmission coefficients of the grating, R/T, obtained through time gating from the S parameter measurements of the test devices. In particular, we found that the level of the notches of R/T directly depends on the attenuation of SAW in the grating. A simple closed-form expression for the attenuation normalized to the grating length, gammalambda0, depending on the characteristics of |R/T|, is given. The proposed method is applied to the measurement data for selected grating topologies to yield estimates of the attenuation

Second harmonic reflectors

In this work, we study theoretically the operation of surface-acoustic wave (SAW) reflectors on the second harmonic frequency. There is a renewed interest in second harmonic reflectors for high-frequency devices and for sensor and tag applications. Here, we demonstrate that the reflectivity of the 2nd harmonic reflectors can be very high, e.g., on 128/spl deg/-LiNbO/sub 3/. We have simulated performances of a few typical SAW devices with such reflectors. Two times larger period of the structure makes it easy for production even in the frequency range from 2 to 3 GHz.

Second-harmonic reflectors on 128/spl deg/ LiNbO/sub 3/

In this work, we study theoretically the operation of long surface acoustic wave reflectors, comprising a large number of electrodes, at the fundamental and second harmonic frequencies on the 128/spl deg/ LiNbO/sub 3/ substrate for various electrode thicknesses and metallization ratios. Numerical simulations utilizing tailored test structures and time gating indicate that the reflectivity of the second-harmonic reflectors can be very high for certain geometries. Furthermore, our simulations suggest that inside the stopband the total losses for the second harmonic are of the same order as those for operation at the fundamental harmonic.