Study on the mechanical properties of ultra-low dielectric film by tensile test

Abstract

The ultra-low dielectric constant (ULK) materials is implemented in the copper/low-k interconnection structures in industry in order to minimize the RC delay and crosstalk noise. The mechanical property of ULK dielectrics is weak compared to traditional dielectrics. So it poses a significant risk to the device reliability, not only in the fabrication process, but also in packaging processes and reliability tests. When using numerical simulations to assess the structural integrity associated with low-k integration, accurate mechanical property of ULK materials is also an important parameter that needed to be measured. In this paper, we present a method of measuring the mechanical properties of ULK materials. This method is suitable for a variety of materials which can be deposited or grown on a silicon substrate. The fabricating processes mainly include lithography, temporary bonding, grinding, Si etching and debonding. In addition, we designed two kinds of test structures. One is with pre-crack, and the other is without pre-crack. Each structure has two kinds of geometry sizes. The tensile test was done by using the in-situ tensiometer apparatus with uniaxial stretching. The stress-strain diagrams of the specimens show an elastic modulus $\mathbf{E=1.2}$ GPa and tensile strength is about 13.5 Mpa for the structure without pre-crack, which is lower than that of bulk low-k material. The samples with two kinds of geometry sizes have similar modulus values which can be converted by the measured results from the diagram. While other specimens show a critical energy release rate ${\mathrm{G}_{\mathrm{c}}=0.25\mathrm{J}/\mathrm{m}^{2}}$ for the structure with pre-crack, which is also lower than that of bulk material. The results illustrate that the ability to resist fracture of low-k thin films is weak. It provides a possible method to measure the mechanical properties of ULK thin films.