Copper ion diffusion in porous and nonporous SiO2-based dielectrics using bias thermal stress and thermal stress tests

The success of future gigascale integrated circuit chip technology depends critically upon the introduction of low dielectric constant (low-k) materials as interlayer dielectrics, and their integration with copper, the low resistivity interconnect metal of choice. Currently the interest is focused on porous silica-based films due to their ultra low-k value and high compatibility to the current silica technology.Nanoporous silica-based films were deposited by surfactant templated self-assembly spin-on deposition (SOD). Other low-density silica-based films were deposited by plasma enhanced chemical vapor deposition (PECVD). A dense thermal SiO2 was used as a reference. The diffusion of Cu into the different dielectrics studied was evaluated by characterizing metal-insulator-semiconductor (MIS) devices subjected to thermal stress (TS) and bias thermal stresses (BTS). The diffusion of Cu ions induced by TS in the temperature range of 300 up to 500 °C can be described by an Arrhenius-like dependence. The lowest activation energy value (0.71 ± 0.04 eV) was obtained for the porous SOD film, which can be attributed to the fast Cu ions surface diffusion through the interconnected pore structure of the film. The activation energies increased with increasing film density, and the highest value (1.02 ± 0.12 eV) was obtained for thermal SiO2, which can be attributed to the slow Cu ions bulk diffusion through the dense dielectric. For the PECVD SiO2 film an intermediate activation energy value of 0.84 ± 0.06 eV was obtained. These activation energy values are comparable to the values reported in the literature for similar types of dielectric films. When subjecting these MIS capacitors to BTS up to 2 MV/cm at 300 °C, a general Cu-related phenomenon of no inversion layer was found. This phenomenon was not reported previously in the literature for low-k dielectrics, and has only recently been reported by our group for thermal SiO2. This failure mechanism was related to the degradation of the dielectric by Cu ions, which was accelerated in the case of the interconnected pore structure of the porous SOD dielectrics.