Thin Ti film is commonly used as an adhesion-promoting layer for different metallization schemes, Cu/Ti/SiO2, Au/Ti/GaAs to name a few [1]. This research reports results on a comparison study for Ti/Si vs. Ti/GaAs adhesion measured by means of the superlayer indentation test [1, 2].
The thermodynamic work of adhesion for any thin film will depend on the condition of the substrate surface at the time of deposition. Most often sputter clean cycle is used prior to film deposition. Unlike monoatomic substrate such as Si, compound substrate surface compositions can be significantly altered by sputter cleaning. We propose a mechanism by which such a process can lead to significantly reduced adhesion for the Ti/GaAs interface.
A 1 μm thick compressively stressed superlayer of TiW was deposited on top of Ti films of varying thicknesses, sputtered under various conditions on Si and GaAs wafers. Blister size along with materials elastic properties, residual and indentation stresses provide a basis for calculating thin film practical work of adhesion [2].
While Ti films on Si wafers provided consistent adhesion values, Ti film adhesion on GaAs scattered as high as 100 J/m2. In order to understand the discrepancy several blisters of TiW/Ti films have been removed from substrates with an adhesive tape. Fracture surfaces were characterized with SEM. Opposite to Ti films on Si, where cracks initiated in Si and then spread to the interface, Ti/GaAs fracture started at the interface, kicked into the GaAs substrate and then back again to the interface, leaving chips and pieces of GaAs on the debonded Ti surface. Opposite to Si, there were distinct radial cracks present in the GaAs substrate. GaAs fracture toughness is three times less than that of Si, which makes it more susceptible to radial cracking. There is a competition between the interfacial adhesion of Ti to GaAs and the fracture toughness of GaAs itself, which has caused large scatter in adhesion measurements.