Molecular Dynamics Simulation of Cluster Ion Impact on Solid Surface

Abstract

In this thesis, the impact process of cluster ions on solid surfaces was studied using molecular dynamics (MD) simulation. Cluster is an aggregated material which consists of a few to thousand atoms. The impact process of cluster ion on solid surface is of great interest because the effect of impact by cluster ion cannot be explained by the summation of individual monomer ions, and it is termed as 'nonlinear effect.' In order to understand the nonlinear effect by cluster, the dynamics of collisional process between cluster and solid surface should be examined. MD simulation is one method of computer simulation to solve numerically the Newton's equation of motion for each atom in the system using difference equation technique, so MD can make it possible to trace the time evolution of coordinates and velocity for every atom with high resolution. The basic theory of molecular dynamics and the acceleration method are described in chapter 2. For this study, the original MD program was developed, which can accelerate the calculation speed of collisional process of high-energy atoms with a solid surface by applying different timestep to each atom depending on its velocity. Due to this acceleration technique and recent progress in computers, it can be possible to simulate the system with a large number of atoms, or more than hundred simulations in order to obtain statistics. In the following chapters, MD simulation is used to examine the impacts of various types of clusters on a number of well defined substrates. Chapter 3 describes the typical impact process of cluster on solid surface examined using large argon cluster and silicon substrate. The differences between cluster and monomer impact are shown in penetration range, damage formation and sputtering. The energy dependence of penetration depth of cluster was examined and it was fond that the penetration depth is proportional to the cube root of the incident energy. This is due to a large number of collisions between cluster atoms and surface atoms, which cause isotropic propagation of incident energy. Through this multiple-collision process, a crater-shaped damage is formed on the surface. In chapter 4, the impacts of carbon cluster onto carbon substrate are examined both by MD simulation and experimentally. Carbon is a suitable material to generate well-defined small size clusters such as C7 , C19 and C60 in experiments. It was found that the penetration depth of carbon clusters with several keV/atom is similar to that of monomer ion, but a larger number of displacements are formed with the cluster size of larger than 10. From this study, the boundary size of cluster size where a cluster shows the nonlinear effect is discussed. Shallow junction formation by boron cluster implantation into silicon substrate is discussed in chapter 5. Decaborane (B10H14) is a stable material of boron cluster and each boron atom can be irradiated with 1/10 energy of the total acceleration energy. This implies that the low-energy implantation can be obtained easily. The penetration depth by B10H14 is shown to be the same as that by boron monomer ions with same acceleration energy per atom. Furthermore, it is found that B10H14 implantation can form larger number of displacements in the near surface region, with lower atomic dose than monomer ions. The high-yield damage formation on the surface suggested to suppress the transient enhanced diffusion (TED). These properties of B10H14 implantation are considered as advantageous for small LSI fabrication. In chapter 6, the impact of uorine cluster and neon cluster onto silicon substrate are compared, in order to examine the sputtering effect by reactive cluster ion. Fluorine cluster shows higher sputtering yield than both uorine atom and neon cluster at low energy region. It is suggested that the high-density atomic and energy deposition by cluster ion impact prompt the formation of volatile silicon- uoride materials and desorption of uoride materials. As the incident energy increases, uorine and neon cluster shows similar sputtering yield because the physical sputtering effect through atomic collisions is major effect in high energy region. From these results, characteristics of cluster ion impact depending on incident energy, cluster size and cluster and substrate species are discussed.