Nicole Fréty

Copper diffusion into silicon substrates through TaN and Ta/TaN multilayer barriers

A study of copper (Cu) diffusion into silicon substrates through Ta nitride (TaN) and tantalum (Ta/TaN) layers was investigated based on an experimental approach. TaNx and Ta/TaNx thin films were deposited by radiofrequency sputtering under argon (Ar) and Ar-nitrogen (N) plasma. The influence of the N2 partial pressure on the microstructure and the electrical properties is reported. X-ray diffraction patterns showed that the increase of the N2 partial pressure, from 2 to 10.7%, induces a change in the composition of the δTaN phase, from TaN to TaN1.13, as well as an evolution of the dominant crystallographic orientation. This composition change is related to a drastic increase of the electrical resistivity over a N2 partial pressure of 7.3%. The efficiency of TaN layers and Ta/TaN multilayer diffusion barriers was investigated after annealing at temperatures between 600 and 900 °C in vacuum. Secondary ion mass spectrometry profiles showed that Cu diffuses from the surface layer through the TaN barrier from 600 °C. Cu diffusion mechanisms are modified in the presence of a Ta sublayer.

Numerical simulation of failure prediction for ceramic tools: comparison with forging test results

A fracture prediction criterion for brittle materials has been introduced in the POLLUX finite-element code in order to predict the risk-of-rupture of ceramic tools during a forging operation. POLLUX is a software dedicated to the simulation of forging operations, initially developed by INSA (Lyon). The chosen probabilistic fracture model is based on the weakest-link theory and the statistical theory of Weibull. A surface approach or a volume approach can be retained on the basis of the type of critical flaws in the ceramic. Two different criteria are available in order to characterise the stress state, considering the tensile normal stresses and neglecting the compressive stresses. An identification procedure of the critical flaw type is presented for a particular ceramic material. Statistical parameters of ceramic fracture have been determined experimentally using bending tests performed under environmental conditions close to those of forging. A constitutive equation of the workpiece material has been proposed, issued from torsion tests. In order to validate the model in the case of ceramic tools subjected to multi-axial stress states, a particular configuration has been defined to compare the simulation predictions with the experimental results. A forging test has then been developed, in which a billet of superalloy is formed in a ceramic tool up to its fracture at the temperature of 1423 K. The experimental distribution of tool fracture, according to the strain of the billet, is in good agreement with fracture predictions computed by the simulation.