Novel Approach in Mechanical Characterization of Thin Films for Nano Electronics
Mechanical characterization of nano-thin layers with high lateral resolution is as important for nanoelectronics as challenging. We have investigated mechanical properties of porous organosilicate glasses (OSG) with ultralow values of dielectric permittivity (dielectric constant k < 2.4). We have used the atomic force acoustic microscopy (AFAM) method to determine the indentation modulus for two sets of the OSG thin films. The first set has consisted of three samples of nominal porosity varying from 27% to 40% with thickness of 650 nm. The values of the indentation modulus obtained for these samples have varied from 4 GPa to 7 GPa depending on the pore concentration and been in excellent agreement with those determined by nanoindentation. Having confirmed the reliability of the AFAM method, we have determined the indentation modulus of OSG films with the nominal porosity of 30% and film thickness decreasing from 350 nm to 46 nm, a level challenging for standard nanoindentation methods. Interestingly, we have observed an increase of the indentation modulus with the decreasing film thickness. To ensure that our results were free of the substrate influence, we have used tips of different radii and applied them within different ranges of the static load. We have also analyzed the ratio of the sample deformation as well as the tip radius to the film thickness. The results of our analysis have been compared for all the measurement series and showed, which ones have been free of the substrate influence. The stiffening observed for the porous films could be explained by evolution of the pore topology as a function of the film thickness.
Our results prove the capabilities of the AFAM method in studies of mechanical properties of thin films, especially those made of novel functional material used in nanoelectronics and lacking a bulk equivalent.