Nano-Processing with Gas Cluster Ion Beams

Abstract

In this thesis, fundamental properties of gas cluster ion beams and their non-linear irradiation effects are studied. Applications in the fabrication of nano-structures (nano-processes) are demonstrated, as based on knowledge of the interactions between clusters and solid surfaces. In chapter 2, a cluster source which provides an intense neutral cluster beam by supersonic expansion from a Laval nozzle is described, and the high current cluster ion beam equipment is explained. By optimization of both ionization and transport conditions of the cluster beam, a high cluster ion current density of a few μA/cm2 was achieved. The detailed cluster size distribution following a supersonic expansion and the characteristics of the cluster beams are discussed in chapter 3 based on data obtained with a high resolution time of flight mass spectrometer. The formation of inert, reactive and complex gas clusters was verified, and their average cluster size was 2000atoms/cluster. With increasing cluster size, the ionization and collision cross-section increased, however, the kinetic energy of the impact was compensated by the cohesive energy of a large cluster. In chapter 4, interactions of cluster and target atoms in an energetic cluster ion impact are discussed. Most of the kinetic energy of cluster ions was deposited with high density on the surface regions of the targets, and subsequently, multiple collisions between targets and clusters occurred. This dense energy deposition resulted in intrinsic non-linear sputtering effects, such as high yield sputtering and crater formation, which could not be explained by the summation of the irradiation effects induced by the same number of monomer ions. The lateral sputtering effect, which is explained in that many sputtered atoms with cluster ions are emitted in the horizontal direction on the surface plane, was clarified experimentally for the first time, and this was verified by STM observations of single traces of cluster ion impacts. In chapter 5, an enhancement of the sputtering effects with reactive cluster ion beams and their applications are discussed. Since the impact area of the target by a cluster ion occurred under high temperature and high pressure conditions, chemical reactions on the target surface were enhanced. In the case of reactive cluster ion irradiation, dissociation of reactive molecules and clusters occurred simultaneously, and subsequently, enhancement of the etching rate was observed as a consequence of the production of volatile materials. Reactive cluster ion etching could be applied for Si fine pattern etching, and it provided solutions for charging up, isotropic etching, microloading and radiation damage problems. In chapter 6, the surface smoothing effect and mechanisms with cluster ions are discussed. The cluster ion exhibited marked surface smoothing effects and it was made clear from both experimental and simulation results that the lateral sputtering effect was significant for surface smoothing. Very smooth surfaces of CVD diamond films and SiC single crystal substrates were obtained using the gas cluster ion beam processing; these materials are difficult to etch using conventional processes. From these results, it can be summarized that gas cluster ion beam processing is effective in the fabrication of nano-structures and applications in the industrial field are expected.