'Enhancement of the crystallization process of TbxFe1-x thin films upon the formation of alpha-Tb phase'

TbxFe1−x thin films deposited by sputtering on Mo were investigated structurally and magnetically. The microstructure consists of TbFe2 nanoparticles embedded in an amorphous matrix, and the Tb content can be correlated with an increase in the volume of these nanoparticles. Similar microstructure and behavior were found when TbFe2 was deposited on glass and on a Pt buffer layer. Nevertheless, thermal treatments promote a different effect, depending on the mechanical stiffness of the buffer layer. The layers deposited on Mo, a rigid material, show crystalline TbFe2 together with α-Tb phase upon thermal treatment. In contrast, TbFe2 does not crystallize properly on Pt, a material with a lower stiffness than Mo. Intermediate results were observed on the film deposited on glass. Experimental results show the impact of the buffer stiffness on the crystallization process. Moreover, the formation of α-Tb appears to be fundamental to crystallized TbFe2 on layers deposited on rigid buffers

Growth of AlN oriented films on insulating substrates.

This work describes the structural and piezoelectric assessment of aluminum nitride (AlN) thin films deposited by pulsed-DC reactive sputtering on insulating substrates. We investigate the effect of different insulating seed layers on AlN properties (crystallinity, residual stress and piezoelectric activity). The seed layers investigated, silicon nitride (Si3N4), silicon dioxide (SiO2), amorphous tantalum oxide (Ta2O5), and amorphous or nano-crystalline titanium oxide (TiO2) are deposited on glass plates to a thickness lower than 100 nm. Before AlN films deposition, their surface is pre-treated with a soft ionic cleaning, either with argon or nitrogen ions. Only AlN films grown of TiO2 seed layers exhibit a significant piezoelectric activity to be used in acoustic device applications. Pure c-axis oriented films, with FWHM of rocking curve of 6º, stress below 500 MPa, and electromechanical coupling factors measured in SAW devices of 1.25% are obtained. The best AlN films are achieved on amorphous TiO2 seed layers deposited at high target power and low sputtering pressure. On the other hand, AlN films deposited on Si3N4, SiO2 and TaOx exhibit a mixed orientation, high stress and very low piezoelectric activity, which invalidate their use in acoustic devices.

Influence of Crystal Quality on the Excitation and Propagation of Surface and Bulk Acoustic Waves in Polycrystalline AlN Films

We investigate the excitation and propagation of acoustic waves in polycrystalline aluminum nitride films along the directions parallel and normal to the c-axis. Longitudinal and transverse propagations are assessed through the frequency response of surface acoustic wave and bulk acoustic wave devices fabricated on films of different crystal qualities. The crystalline properties significantly affect the electromechanical coupling factors and acoustic properties of the piezoelectric layers. The presence of misoriented grains produces an overall decrease of the piezoelectric activity, degrading more severely the excitation and propagation of waves traveling transversally to the c-axis. It is suggested that the presence of such crystalline defects in c-axis-oriented films reduces the mechanical coherence between grains and hinders the transverse deformation of the film when the electric field is applied parallel to the surface.

Effect of substrate-target distance and sputtering pressure in the synthesis of AlN thin films

In this work, we analyze the influence of the processing pressure and the substrate–target distance on the synthesis by reactive sputtering of c-axis oriented polycrystalline aluminum nitride thin films deposited on Si(100) wafers. The crystalline quality of AlN has been characterized by high-resolution X-ray diffraction (HR-XRD). The films exhibited a very high degree of c-axis orientation especially when a low process pressure was used. After growth, residual stress measurements obtained indirectly from radius of curvature measurements of the wafer prior and after deposition are also provided. Two different techniques are used to determine the curvature—an optically levered laser beam and a method based on X-ray diffraction. There is a transition from compressive to tensile stress at a processing pressure around 2 mTorr. The transition occurs at different pressures for thin films of different thickness. The degree of c-axis orientation was not affected by the target–substrate distance as it was varied in between 30 and 70 mm.