Recently a cluster ion beam was applied to a projectile for time-of-flight secondary ion mass spectrometry (TOF-SIMS), because the cluster can deposit its energy only near the surface; therefore, it is expected that the cluster ion beam can improve the surface sensitivity and the fragmentation of TOF-SIMS.  A large gas-cluster-ion is expected to be one of the most promising candidates to overcome damage in static SIMS.  For a gas-cluster-ion beam (GCIB), which typically contains several thousand constituent atoms, the mean kinetic energy of each constituent atom should be in the order of several eV, which is the same order as that of covalent bonds.  Therefore, it is expected to achieve specific sputtering of chemical bonds by controlling the energy per constituent atom of the GCIB at an energy resolution of several hundred meV, which means that the SIMS of only the required material can be measured without damaging the substrate material.  In addition, the GCIB has other unique irradiation effects that have been studied in various fields of application, such as high density energy deposition in thin film formation, micro manufacturing, and secondary emission of ions or electrons.  The GCIB simultaneously induces multiple collisions by numerous constituent atoms that have a very low mean kinetic energy of several eV.  It is understood that such a characteristic property of the gas-cluster-ion impact causes unique effects on solid surfaces, which do not occur by irradiation with mono atomic or small cluster ion beams.  However, the mechanisms of the effects in cluster impact are not fully understood.

  We have reported the sputtering and damage produced on highly orientated pyrolytic graphite (HOPG) surface by Ar-GCIB bombardment. It was revealed that the kinetic energy per constituent atom, as well as the size of a cluster, was an essential factor for controlling the formation of damage on the solid surface during cluster impact.  Molecular dynamic (MD) studies have predicted that cluster size is one of the most important parameters in the formation of damage and sputtering.  These studies indicated that the sputtering process could be controlled by precisely adjusting the cluster size, and this has motivated the development of a size-selected-GCIB-TOF-SIMS, which would be a useful tool to perform SIMS with very low damage.  When a cluster size is selected at a size resolution (M/ΔM) of 10, the kinetic energy per constituent atoms can be controlled in the order of 0.1 eV, which seems to be sufficient to achieve breaking of specific chemical bonds, and thus material sputtering; therefore, it is expected that the secondary ions can be measured without fragmentation and damage on the substrate.

  A new cluster time-of-flight secondary ion mass spectrometry (TOF-SIMS) was developed using a size-selected gas cluster ion as a projectile. Since a large gas cluster ion can generate many low-energy-constituent-atoms in a collision with the surface, it causes multiple and ultra low energy sputtering. The mean kinetic energy of constituent atoms is provided by dividing the acceleration energy of the gas-cluster-ion by the number of constituent atoms. Therefore, the sputtering can be controlled to minimize the decomposition of sample molecules and substrate material by precisely adjusting the number of constituent atoms (the cluster size) and/or acceleration energy of the gas-cluster-ion. The cluster size was selected on the basis of the time-of-flight method using two ion-deflectors attached along the ion-beam line. A high resolution of 11.7 was achieved for the cluster size/size width (M/ΔM) of Ar-cluster ions.

K. Moritani, et. al., Appl. Surf. Sci., 255, 948-950, (2008).