The connection between sputter cleaning and adhesion of thin solid films

Abstract

The aim of this work was to study the adhesion of thin films to given substrates. Experiments were carried out on carbon, aluminum, chromium and tungsten films deposited on steel and polished TA6V titanium alloy substrates. Numerous parameters have an influence on the adhesion. These include stresses in the film, contamination, chemical bonding between the film and the substrate, the physical properties of the substrate and the roughness of the substrate. Sputter cleaning appeared to be one of the most effective methods approches in the PVD (physical vapor deposition) process to improve the adhesion of thin films. Argon was used as the sputtering gas. The sputtering time was varied from 0 to 70 min. The adhesion of thin films was characterized by mechanical tests. The film cracking induced by these mechanical tests was observed by optical microscopy. We noticed that there was a clear correlation between sputter cleaning time and adhesion of films. The level of oxygen in the interface was studied with SIMS (secondary ion mass spectrometry) and NRA (nuclear reaction analysis); there was no linear reduction of the amount of oxygen as a function of sputter cleaning time. According to the SIMS results, there was no clear dependence between the amount of oxygen at the interface and adhesion. Measurement of oxygen content at the interface was quite a complicated problem and dynamic-SIMS was found to be a valuable method to perform assessment of this parameter when the thickness of the films exceeds 100 nm. Other phenomena connected to sputter cleaning were physical changes in the surface of the substrate, such as the removal of contaminated top layers, the decrease in substrate grain size and changes of texture. The major disadvantage of the sputter cleaning method was the surface contamination. The main contamination sources were residual water vapor and backscattered atoms introduced by the sputtering process.