Molecular dynamics simulations of ballistic He penetration into W fuzz

\u3cp\u3eResults are presented of large-scale Molecular Dynamics simulations of low-energy He bombardment of W nanorods, or so-called 'fuzz' structures. The goal of these simulations is to see if ballistic He penetration through W fuzz offers a more realistic scenario for how He moves through fuzz layers than He diffusion through fuzz nanorods. Instead of trying to grow a fuzz layer starting from a flat piece of bulk W, a new approach of creating a fully formed fuzz structure 0.43 μm thick out of ellipsoidal pieces of W is employed. Lack of detailed experimental knowledge of the 3D structure of fuzz is dealt with by simulating He bombardment on five different structures of 15 vol% W and determining the variation in He penetration for each case. The results show that by far the most important factor determining He penetration is the amount of open channels through which He ions can travel unimpeded. For a more or less even W density distribution He penetration into fuzz falls off exponentially with distance and can thus be described by a 'half depth'. In a 15 vol% fuzz structure, the half depth can reach 0.18 μm. In the far sparser fuzz structures that were recently reported, the half depth might be 1 μm or more. This means that ballistic He penetration offers a more likely scenario than He diffusion through nanorods for how He moves through fuzz and may provide an adequate explanation for how He penetrates through the thickest fuzz layers reported so far. Furthermore, the exponential decrease in penetration with depth would follow a logarithmic dependence on fluence which is compatible with experiments. A comparison of these results and molecular dynamics calculations carried out in the recoil interaction approximation shows that results for W fuzz are qualitatively very different from conventional stopping power calculations on W with a similarly low but homogeneous density distribution.\u3c/p\u3

Chemical–mechanical relationship of amorphous/porous low-dielectric film materials

We have performed a series of atomic simulations, from which the chemical–mechanical relationship of the amorphous/porous silica based low-dielectric (low-k) material (SiOC:H) is obtained. The mechanical stiffness of the low-k material is a critical issue for the reliability performance of IC backend structures. Due to the amorphous nature of the low-k material, a molecular structure model is required, and we present an algorithm to generate such models. In order to understand the variation in the mechanical stiffness and density resulting from modifications to the chemical configuration, sensitivity analyses have been performed using the molecular dynamics (MD) method. Moreover, a fitting equation, based on homogenization theory, is used to represent the MD simulation results in terms of the mean characteristics of the chemical configuration. The trends indicated by the simulation results exhibit good agreement with experimental results. In addition, the simulation result shows the Young’s modulus of the SiOC:H is dominated by the concentration of basicbuilding blocks Q and T, whereas the density is influenced by all the basic building blocks