Mechanical characterization of advanced optic and semiconductor materials via nanomechanical testing

This thesis was concerned with the mechanical characterisation of advanced optic and semiconductor materials using  instrumented nanoindentation and nanoscratch. Nanoindentation was used to measure the elastic modulus and hardness of an optic material, lithium tantalate (LiTaO3) single crystal, and nanoscratch was used to understand its deformation and removal behaviour. Nanoscratch was employed to characterise the mechanical properties and interfacial adhesion of a silicon nitride (SiN)/gallium arsenide (GaAs) film/substrate system for semiconductor applications.LiTaO3 is an advanced optic material and is widely used in nonlinear optics and passive infrared sensors due to its unique optical, piezoelectric and pyro-electric properties. However, LiTaO3 belongs to the category of difficult-to-machine materials and it is difficult to obtain a high quality surface because of the nature of its brittleness. An improved understanding of the deformation characteristics of LiTaO3 is thus crucial to further optimise the current machining process.The elastic modulus and hardness of LiTaO3 obtained from nanoindentation were 251 and 12.6 GPa, respectively. Pop-in events were observed at the applied loads ranged from 305 to 640 μN. The pop-in events were found to be associated with the transition of deformation from elastic to elastoplastic, which were believed to be caused by incipient kink bands. From the atomic force microscopic images of the indents, slip bands were observed when the applied load reached 4 mN. The direction of slipping was along the crystal orientation [1012] . In the nanoscratch tests, it was found that the threshold normal load for transition of removal modes from ductile to brittle was 2.5 mN, corresponding to a scratch depth of 70 nm. When the normal load was greater than this threshold, shear stress induced cracks were generated along the scratch tracks. The removal rate linearly increased with the normal load, but the value reached a plateau at 13×103 nm3/nm after theload was greater than 4 mN.GaAs is widely used in photovoltaic applications, microelectronic devices, mobile phones, satellite communications, microwave point-to-point links and higher frequency radar systems. However, GaAs needs to be protected from oxidation and corrosion during service. SiN has often been chosen to be a passivation layer because of its various desirable properties such as high chemical resistance, high refractive index and low dielectric properties. To ensure the reliability and high performance of the GaAs based devices an appropriate adhesion between the SiN and GaAs is required. As a result, a quantitative method for evaluating the adhesion strength of SiN/GaAs bilayer systems is crucial.The mechanical and adhesion properties of the SiN/GaAs bilayer systems were investigated by use of nanoscratch. Different critical loads (Lc) for film failure which are dominated by delamination and film spallation were found along with other scratch parameters, such as critical friction coefficients (μ), critical scratch normal displacements of the indenter. The adhesion properties were related to the critical scratch condition for film failure by Laugier’s energy approach. The practical adhesion energies per unit area (Wad), which equals to the interfacial fracture energy release rates (Gc), were 2.72, 2.90 and 3.73 J/m2 for the three bilayer systems with SiN thin films deposited under different conditions. It was found that the low compressive residual stress inhibited the crack propagation and increased Wad, while the tensile residual stress was expected to facilitate crack opening and resulted in a lower Wad. However, when the compressive residual stress was excessively high at about 400 MPa, buckling started to take place and Wad decreased.