Adhesion study of low-k/Si system using 4-point bending and nanoscratch test

Chemical vapour deposited (CVD) low-k films using tri-methyl-silane (3MS) and tetra-methyl cyclo-tetra-siloxanes (TMCTS) precursors were studied. A 4-point bend test (4PBT) was performed to assess the adhesion property of the low-k films to Si substrates and the results were compared with that of simpler method, nanoscratch test (NST), as a quality control tool despite its drawbacks. Adhesion energy, G_c, of the low-k/Si interface as measured by 4PBT and critical scratch load, P_c, as obtained by NST display a linear relationship with hardness and modulus of the low-k film. The lowering of G _c as the hardness of the film decreases can be explained by the effects of the C introduction into the SiO networks found in these films. Lower carbon content for higher hardness films is thought to cause them to be more "silica-like", and thus, exhibit better adhesion with the Si substrate. Two failure modes were observed for specimens under 4PBT. On one hand, films with low hardness (<5 GPa) exhibit low G_c (<10 J/m²) with an adhesive separation of low-k from the Si substrate. On the other hand, films of high hardness (>5 GPa) display interfacial energies in excess of 10 J/m² with delamination of epoxy from the Si substrate, thus, indicating excellent adhesion between the low-k films and Si substrate. For the low hardness films, good correlation exists between P _c and G_c. However, the two data points of the high hardness films that gave the two highest P_c and G_c values do not lie on the correlation line drawn for the low hardness film data points due to different factors governing the failure in both tests and a change in the 4PBT failure mechanism.