Accurate measurement of porous low-k thin-films by nanoindentation: Densification scaling versus substrate effects
Historically, the integration of insulators with decreasing dielectric constants, k, has been critical in improving devices performance. The most efficient approach to decrease k is by introducing porosity. Since porosity directly impacts the Young’s modulus, the accurate determination of the mechanical properties of porous dielectrics is essential in enabling their implementation into state-of-the-art devices. Currently, nanoindentation is the technique of choice, but as technology relevant thicknesses of dielectrics keep decreasing, substrate effects are unavoidably encountered. These issues have so far been circumvented by measuring bulk materials instead of their thin-film counterparts. This approach is, however, less than ideal as physical properties can be size-dependent. Moreover, pores densification also occurs, potentially leading to overestimated Young’s modulus values. In this paper, we define a novel protocol based on ultra-low load, quasi-static nanoindentation to measure mechanical properties of porous thin-films as a function of porosity (0–60%) and thickness (150–700 nm). We could isolate materials densification from substrate effect, and we demonstrated that the former can be accounted for, while the latter does not happen until indentation depths are remarkably higher than the traditionally accepted indentation limit of 10% of the film thickness. Consequently, extremely accurate mechanical characterization of porous low-k thin-films is now possible.