Brillouin scattering by surface acoustic modes for elastic characterization of ZnO films

Surface Brillouin scattering study of tantalum nitride (TaN) thin films

Elastic properties of TaN films on (001)Si are investigated by surface Brillouin scattering (SBS). The velocity dispersion is obtained from Brillouin frequency shifts and surface acoustic wave phase velocities derived from measured SBS spectra. The observed Rayleigh surface acoustic and Sezawa waves indicate a soft on hard configuration. The elastic stiffnesses from the best fit of the calculated and measured spectra were C₁₁=288 and C₄₄=80GPa. Using these elastic constants in the surface elastodynamic Green's function yielded theoretical Brillouin spectra in agreement with measured spectra. Employing a least squares fitting procedure to the elastic constants gives uncertainties in C₁₁ and C₄₄ as ±3.7 and ±2.5GPa, respectively.

THIN-FILMS FOR SAW DEVICES

Thin-films play a significant role in surface acoustic wave (SAW) technology. Thin metal films are essential for the generation and detection of SAWs on piezoelectric subtrates, can provide for directional control, influence the velocity and propagation loss, and can be configured to produce a wide variety of time and frequency signal processing functions for electronic systems. Dielectric films have been used to selectively modify the propagation properties of surface waves, protect the surfce, and improve the coupling efficiency and temperature characteristics of SAW devices. Piezoelectric films have extended SAW device developments to amorphous and nonpiezoelectric crystals for enhanced frequency and loss performance, lower cost, and microelectronic integration. Semiconductor, optical, and magnetic films have shown promise for active acoustoelectronic, acoustooptic, and magnetoacoustic devices. Chemically sensitive films are the basis for SAW sensor developments. Active research continues today in improving the properties and utilization of thin films on SAW substrates through advances in processing and characterization.

Elastic Properties of FeNi/Cu and FeNi/Nb Metallic Superlattices Investigated by Brillouin Light Scattering

The Brillouin light scattering technique has been exploited in order to investigate the elastic properties of periodic superlattices consisting of alternating layers of Fe20Ni80 (permalloy) and either Cu or Nb. These multilayers, with total thicknesses ranging between 0.2 and 0.7 mm and with periods of typically 3-5 nm, present a polycrystalline structure with (110) texture for the Nb layers and (111) texture for both the FeNi and Cu layers. Measurement of the frequency position of the Brillouin peaks corresponding to the Rayleigh and Sezawa acoustic modes allowed the effective elastic constants of these structures to be determined. The values obtained are compared with those calculated from the elastic constants of the bulk materials, taking into account the polycrystalline nature of the superlattices and the crystallographic orientation of the layers.

Estimating materials parameters in thin-film BAW resonators using measured dispersion curves

The dispersion curves of Lamb-wave modes propagating along a multilayer structure are important for the operation of thin-film bulk acoustic wave (BAW) devices. For instance, the behavior of the side resonances that may contaminate the electrical response of a thin-film BAW resonator depends on the dispersion relation of the layer stack. Because the dispersion behavior depends on the materials parameters (and thicknesses) of the layers in the structure, measurement of the dispersion curves provides a tool for determining the materials parameters of thin films. We have determined the dispersion curves for a multilayer structure through measuring the mechanical displacement profiles over the top electrode of a thin-film BAW resonator at several frequencies using a homodyne Michelson laser interferometer. The layer thicknesses are obtained using scanning electron microscope (SEM) measurements. In the numerical computation of the dispersion curves, the piezoelectricity and full anisotropy of the materials are taken into account. The materials parameters of the piezoelectric layer are determined through fitting the measured and computed dispersion curves.

The effect of an SiO(2) buffer layer on the SAW properties of ZnO/SiO(2)/GaAs structure

The effect of an SiO(2) buffer layer on the surface acoustic wave (SAW) properties of ZnO/SiO(2)/GaAs structure is examined. Both theoretical and experimental results show that the coupling coefficient is increased appreciably by providing an SiO(2 ) film between the ZnO film and the GaAs substrate. Adding an SiO (2) film is also beneficial to the promotion of quality of ZnO thin film. The results could be useful for the further development of monolithic SAW devices.