Interfacial fracture and moisture effects in the silicon dioxide/titanium nitride/aluminum multilayer system

One of the key factors affecting the long-term reliability of thin film structures in microelectronics devices is the fracture resistance of their interfaces. Of the many different interfaces formed during device fabrication, the SiO2/TiN interface is amongst the most common. In order to measure the fracture energy of this interface, we have been utilizing a superlayer technique, in which a narrow strip of a material, having a large intrinsic tensile stress, is deposited and lithographically defined onto the SiO₂/TiN/A1 thin film structure. The elastic strain energy in the micro-strip provides an additional driving force for debonding the weakest interface in the structure. Based on shear-lag theory, the stress states and stored strain energy in the superlayered system have been studied in detail. The results enable us to quantitatively establish the relation between the interfacial fracture and the length of the strip that remains intact after debonding from both ends. The interface fracture energy is found to be directly related to the shear-lag zone size, as well as strip width, intrinsic stress, thickness, and other elastic/plastic properties of each layer.; The SiO₂/TiN interface was found to be prone to sub-critical decohesion by crack extension along the interface when exposed to moisture. As with conventional sub-critical crack growth in bulk silica, the rate of decohesion is dependent on both the relative humidity and the strain energy release rate. The superlayer microstrip technique has been used to measurement the intrinsic fracture energy while investigating the sub-critical crack growth behavior along the SiO₂/TiN interface, as well as measuring the threshold interface fracture resistance under different humidities. Another major observation of this work is that the decohesion velocity of narrow strips is highly sensitive to the exposure time in the humid environment, suggesting that the moisture diffuses into the interface ahead of the propagating decohesion crack, causing a form of hydrolytic weakening of the interface. In seeking to establish the origin of this behavior, measurements of moisture diffusion along the SiO₂/TiN interface are accomplished by combining Secondary Ion Mass Spectroscopy (SIMS) Imaging with the diffusion of isotope. Analyses of images formed using 2D and 18O revealed that both diffuse along the interface with a diffusivity of (6 ± 2) × 10–13 cm2/sec, a value about 10 4 times faster than the bulk diffusion in the SiO₂ dielectric layer. The temperature dependence of both interfacial diffusion and bulk diffusion in the SiO₂ dielectric layer have been studied over a range of 8°C to 90°C and the activation energies are obtained as 0.21 eV and 0.74 eV, respectively. Interface roughness shows no significant effects on the indiffusion of moisture. This is the first reported direct measurement of moisture diffusion at a low temperature range either in silicate glass or along the interface between silicate glass and another material. Finally, the dynamic imaging SIMS technique has also been extended to the measurement of moisture diffusion in low dielectric materials. The value for PAE was 0.5–2.5 × 10–7 cm2/s.