Mechanical testing and microstructural characterization of TiN thin films

摘要

Mechanical properties of titanium nitride (TiN_x) thin films have been investigated using the bulge test and the depth sensing nanoindentation measurements. The bulge test was performed on the square free standing membranes made by means of standard micromachining of silicon wafers, while the nanoindentation was conducted on the films adhered to their supporting substrate. Thin layeres of titanium nitride (t=300 - 1000 nm) were deposited in a r.f. magnetron sputtering system on the Si(100) wafers containing a layer of low stress LPCVD silicon nitride (SiN_y). The bulge test was first conducted on the silicon nitride film to determine its proper residual stress and Young's modulus. Then, the composite membrane made of TiN_x together with underlying silicon nitride was bulged and the related load-displacement variation was measured. Finally, using a simple rule of mixture formula the elastic mechanical properties of TiNx coatings were calculated. Both the Young's modulus and residual stress showed increasing values with negative bias voltage and nitrogen to titanium ratio, but the substrate temeprature between 50-570 deg C was found less significant as compared to the other parameters. Nanoindentation data extracted from dynamically loading-unloading of TiN films converged to the bulge test measurements for compact coatings, but diverged from the bulge test data for porous coatings. Scanning electron microscopy observation of the cross sectioned specimens showed that TiN films first grow by formation of the nanocrystallites of size mostly between 10 - 15 nm. These nanocrystallies give rise to the columnar morphology beyond a thickness of 50-100 nm. The columns change their aspect with deposition parameters, but remain nearly perpendicular to the film surface. Relationship between microstructural evolution of columns and mechanical properties of coatings are discussed in terms of deposition parameters.