Comparison of structure and mechanical properties of SiO2-like films deposited in O-2/HMDSO pulsed and continuous plasmas
Résumé
In many applications (ophthalmic lenses, car headlights, etc.), silicon oxide films are used as protective coatings in order to improve the mechanical properties of polymers. The Plasma Enhanced Chemical Vapor Deposition (PECVD), which allows deposition of dense films at temperature near the ambient, is particularly relevant for this. In this work, SiO2-like films have been deposited in a helicon reactor operated in inductive mode at low pressure with pulsed O2/HMDSO plasmas. The influence of the plasma-off time on the structure and mechanical properties of deposited films are studied and compared to those of the film deposited with a continuously applied power. Ellipsometric measurements and infrared spectra show that all the films are SiO2-like films, transparent from 1.5 to 5 eVand possess a refractive index (n=1.46 at 1.95 eV) close to the thermal oxide one. Good mechanical properties are observed from the depth sensing indentations by Fischerscope tester. The plastic hardness is found to decrease from 15.5 to 13.5 GPa when the plasma-off time increases, whereas the elastic modulus is the same (78 GPa). In continuous mode, the hardness value obtained is higher than the fused silica one (10 GPa) which is explained by a high compressive stress of 185 MPa, as determined from Newton's ring method. Plasma pulsing allows to reduce the compressive stress to 40 MPa. On the AFM micrographs, one can observe a buckling around the indentation print due to this compressive stress in films. The interfacial fracture toughness has been estimated from the radius of the delaminated area and linked to previous stress measurements. Moreover, all the films present a good resistance to delamination