G. Stan, C.V. Ciobanu, S.W. King
National Institute of Standards and Technology,
Keywords: nanoscale mechanical properties, atomic force microscopy
Summary:The continuous advances in semiconductor device fabrication demand various characterization techniques capable of proving quantitative measurements at the nanoscale. A prominent scanning probe-based technique for nanoscale elastic property measurements is contact resonance atomic force microscopy (CR-AFM). Mostly operated in the elastic modulus range from few GPa to hundreds of GPa, CR-AFM was used to test different materials and structures at the nanoscale and considered for discerning the mechanical response of subsurface inhomogeneities and buried domains. It remains, however, to directly prove the extent of its quantitative capabilities both in terms of elastic modulus and depth sensitivity. In this work, we revise few CR-AFM developments with direct applicability to low-k dielectric materials and structures that are relevant to the fabrication of semiconductor devices. In particular, we develop a quantitative methodology to test the elastic modulus and depth sensitivity of CR-AFM against a set of low-k dielectric bilayer films with the top layer of various thicknesses. We have analyzed the measurements with a semi-analytical model and three-dimensional finite element analysis. Both analyses confirmed the expected elastic moduli of the layered structures and provided a robust quantitative estimation of the subsurface depth and material sensitivities of CR-AFM. We also developed a correlative measurement-model analysis to assess the convoluted contributions of the structural morphology and mechanical properties to the contact stiffness used by AFM-based subsurface imaging. The results explain the inherent difficulties associated with solving concurrently the material contrast and location of subsurface heterogeneities in nanomechanical subsurface imaging.