Thin and dense YSZ films fabricated at low temperature by ESD and AA-CVD on porous NiO-YSZ for SOEC

International audienc

ESD and ALD Depositions of Ta₂O₅ Thin Films Investigated as Barriers to Copper Diffusion for Advanced Metallization

International audienceDeposits of tantalum oxide thin films on silicon wafer substrates by electrostatic spray deposition (ESD) and atomic layer deposition (ALD) have been studied for their integration as barriers to copper diffusion in interconnect technologies. In the case of ESD, both precursor solution and deposition time have been optimized in order to produce homogeneous films of lowest thickness, T>800°C was studied by transmission electron microscopy. Depending on the thermal treatment, the crystallization occurs with the formation of monoclinic Ta2O5 phase either pure or mixed with a trigonal phase. In the case of ALD, thin-film deposits were optimized as a function of the different parameters of temperature, number of cycles, and tantalum precursor pulse times. As-deposited films were also amorphous but were crystallized in a pure monoclinic Ta2O5 phase after annealing treatment at 980°C. Copper diffusion was tested with high thermal budgets through thin films of T ≥ 700°C. The conformity of such films was examined from deposits realized on patterned substrates

Second order incommensurate phase transition in 25L-Ta2O5

International audienceA new structural state 25L-Ta2O5, obtained from sintering and annealing treatments of a Ta2O5 powder, is identified both by electron diffraction and high resolution imaging on a transmission electron microscope (TEM). According to general rules for the different L-Ta2O5 structures proposed by Grey et al. (J. Solid State Chem. 178 (2005) 3308), a structural model is derived from their crystallographic data on 19L-Ta2O5. This model yields simulated images in agreement with high resolution TEM observations of the structure oriented along its [001] zone axis, but only for a very thin crystal thickness of less than 1.2 nm. Such a limitation is shown to be due to a modulation of the structure along its [001] axis. Actually, from an analysis of a diffuse scattering and of its evolution into satellites reflections as a function of the cooling rate, a second order incommensurate phase transition can be assumed to occur in this compound. The property of single phase samples observed by TEM is also verified by X-ray powder diffraction. In a discussion about studies performed by different authors on incommensurate structures in the system Ta2O5-WO3, it is noticed that TEM results, similar to ours, indicate that phase transitions could be expected in these structures. (C) 2010 Elsevier Inc. All rights reserved

A very promising piezoelectric property of Ta2O5 thin films. I: Monoclinic-trigonal phase transition

International audienceCeramic thin films of tantalum oxide of a new trigonal structure (a=12.713(7) angstrom, alpha = 28.201(0)degrees, space-group R3) were produced by thermal treatments of amorphous deposits on (001)Si wafers, either by electrostatic spray deposition or by injection metal-organic chemical vapor decomposition. This trigonal phase comes from the transformation of a monoclinic phase 11L- or 25L-Ta2O5. The transformation is reversible under oxygen atmosphere and, from results of TEM investigations, occurs mainly via atomic motions along the z unique axis of the monoclinic structure parallel to the polar three-fold axis of the trigonal structure. The non-centrosymmetry and direction of polar axis of the trigonal phase, identified by high resolution TEM imaging, indicate a possibility of very high electric dipole moments linked to a strong piezoelectricity. From results of XPS analyses of both monoclinic and trigonal structures, the binding energies remain similar to those of Ta2O5. As the formation of the trigonal structure gives rise to an important volume expansion, stresses induced in ceramic thin films are likely influencing both properties of birefringence and piezoelectricity which are presented in a 'separated article (part 11). It is mentioned that the formation of trigonal phase does not occur in bulk Ta2O5 samples, for which an incommensurate phase transition has been observed in a previous work

A very promising piezoelectric property of Ta2O5 thin films. II: Birefringence and piezoelectricity

International audienceBirefringent and piezoelectric properties of Ta2O5 ceramic thin films of monoclinic and trigonal structures were analyzed. The birefringence, observed by reflected polarized light microscopy, yields information on thin film microstructures, crystal shapes and sizes and on crystallographic orientations of grains of trigonal structure. Such an information was considered for investigating piezoelectric properties by laser Doppler vibrometry and by piezoresponse force microscopy. The vibration velocity was measured by applying an oscillating electric field between electrodes on both sides of a Ta2O5 film deposited on a Si substrate which was pasted on an isolating mica sheet. In this case, it is shown that the vibration velocity results were not only from a converse piezoelectric effect, proportional to the voltage, but also from the Coulomb force, proportional to the square of the voltage. A huge piezoelectric strain effect, up to 7.6%, is found in the case of Ta2O5 of trigonal structure. From an estimation of the electrical field through the Ta2O5 thin film, this strain likely corresponds to a very high longitudinal coefficient d(33) of several thousand picorneters. Results obtained by piezoresponse force microscopy show that trigonal grains exhibit a polarization at zero field, which is probably due to stress caused expansion in the transition monoclinic-trigonal, presented in a previous article (part I). (C) 2011 Elsevier Inc. All rights reserved