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Full Characterization of the Mechanical Properties of 11–50 nm Ultrathin Films: Influence of Network Connectivity on the Poisson’s Ratio
Precise characterization
of the mechanical properties of ultrathin
films is of paramount importance for both a fundamental understanding
of nanoscale materials and for continued scaling and improvement of
nanotechnology. In this work, we use coherent extreme ultraviolet
beams to characterize the full elastic tensor of isotropic ultrathin
films down to 11 nm in thickness. We simultaneously extract the Young’s
modulus and Poisson’s ratio of low-<i>k</i> a-SiC:H
films with varying degrees of hardness and average network connectivity
in a single measurement. Contrary to past assumptions, we find that
the Poisson’s ratio of such films is not constant but rather
can significantly increase from 0.25 to >0.4 for a network connectivity
below a critical value of ∼2.5. Physically, the strong hydrogenation
required to decrease the dielectric constant <i>k</i> results
in bond breaking, lowering the network connectivity, and Young’s
modulus of the material but also decreases the compressibility of
the film. This new understanding of ultrathin films demonstrates that
coherent EUV beams present a new nanometrology capability that can
probe a wide range of novel complex materials not accessible using
traditional approaches