Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films.

Asma Chargui,Nicolas Martin,Alexis Mosset,Pascal Vairac,Valérie Potin,Sébastien Euphrasie,Raya El Beainou

doi:10.3390/nano10010081

Abstract

Tungsten films were prepared by DC magnetron sputtering using glancing angle deposition with a constant deposition angle α = 80°. A first series of films was obtained at a constant pressure of 4.0 × 10−3 mbar with the films’ thickness increasing from 50 to 1000 nm. A second series was produced with a constant thickness of 400 nm, whereas the pressure was gradually changed from 2.5 × 10−3 to 15 × 10−3 mbar. The A15 β phase exhibiting a poor crystallinity was favored at high pressure and for the thinner films, whereas the bcc α phase prevailed at low pressure and for the thicker ones. The tilt angle of the columnar microstructure and fanning of their cross-section were tuned as a function of the pressure and film thickness. Electrical resistivity and surface elastic wave velocity exhibited the highest anisotropic behaviors for the thickest films and the lowest pressure. These asymmetric electrical and elastic properties were directly connected to the anisotropic structural characteristics of tungsten films. They became particularly significant for thicknesses higher than 450 nm and when sputtered particles were mainly ballistic (low pressures). Electronic transport properties, as well as elastic wave propagation, are discussed considering the porous architecture changes vs. film thickness and pressure.

Highlights

Among the solid-state physical properties, understanding the propagation of electrons and elastic waves in materials remains a challenging task
It is admitted that the crystal symmetries mainly affect the direction-dependent properties of materials due to an intrinsic anisotropy at the atomic scale coming from spatial differences between bonds in the crystal structure
We report on tilted columnar tungsten thin films sputter-deposited by magnetron sputtering using the GLAD technique with a constant deposition angle of α = 80◦

Summary

Introduction

Among the solid-state physical properties, understanding the propagation of electrons and elastic waves in materials remains a challenging task. As a result of the loss of this intrinsic anisotropy, structuring at the micro- and nanoscale appears as an attractive strategy to produce directional behaviors in surfaces and thin solid films [4,5,6,7] These spatially organized surfaces and films may exhibit asymmetric characteristics in electronic and ionic transports, electromagnetic wave propagations, magnetic properties, or even mechanical and tribological performances [8,9,10]. A higher pressure produced more resistive films with an increase in the elastic wave velocity, where anisotropic behaviors were reduced The evolution of these physical properties as a function of the film’s thickness and argon sputtering pressure is discussed, taking into account the evolution of the crystallographic structure and the films’ morphology at the micro and nanoscale

Materials and Methods

Morphology and Structure

Second Series

Elastic Wave Propagation