| AVS 63rd International Symposium & Exhibition | |
| Advanced Ion Microscopy Focus Topic | Thursday Sessions |
| Session HI+MI+NS-ThA |
| Session: | Ion Beam Based Imaging and Nanofabrication |
| Presenter: | Marek Schmidt, Japan Advanced Institute of Science and Technology, Japan |
| Authors: | M.E. Schmidt, Japan Advanced Institute of Science and Technology, Japan Y. Oshima, Japan Advanced Institute of Science and Technology, Japan L.T. Anh, Japan Advanced Institute of Science and Technology, Japan X. Zhang, Japan Advanced Institute of Science and Technology, Japan T. Kanzaki, Japan Advanced Institute of Science and Technology, Japan M. Akabori, Japan Advanced Institute of Science and Technology, Japan A. Yasaka, Hitachi High-Tech Science Corporation, Japan M. Muruganathan, Japan Advanced Institute of Science and Technology, Japan T. Shimoda, Japan Advanced Institute of Science and Technology, Japan H. Mizuta, Japan Advanced Institute of Science and Technology, Japan |
| Correspondent: | Click to Email |
A larger number of gas molecules (among them helium, nitrogen and neon) can be ionized by the gas field ion source (GFIS) and used as projectiles in focused ion beam (FIB) systems. Among them, the nitrogen stands out as it forms a very strong covalent bond. It is not yet fully understood how this N2 molecule behaves during field ionization and sample interaction, i.e. if and when the bond is broken. Previously, it has been shown that cross section studies are very useful in analyzing beam/sample interaction [1]. Here, we report scanning transmission electron microscopy (STEM) analysis of cross sections extracted from silicon bombarded with ionized N2 molecules. The extracted implantation depths for ion energies of 25 and 16 keV are compared with theoretical values and suggest that the bond is broken during sample interaction. We use first principle molecular dynamics simulation to support this finding, in particular that the covalent bond is broken within the first few atomic layers of the impinged silicon target.
All nitrogen ion implantation was carried out in the GFIS-FIB nanofabrication system [2] located at the Japan Advanced Institute of Science and Technology. Line implantation was carried out on cleaned silicon. Following the cross section preparation STEM observation was conducted. For the 25 keV beam, an implantation depth of ~75 nm is observed, while this decreases to ~35 nm for the case of 16 keV. These values match the theoretically predicted values for the case that two nitrogen atoms are ionized with a single charge (N2+), and split upon impact. The splitting is also predicted by the molecular dynamics simulation we conducted.
These results help to give a clearer picture of the nitrogen ionization in a GFIS and the resulting beam. After ionization of the N2 molecule through electron tunnelling into the atomically sharp emission tip, the ion is accelerated to the energy E = E0and focused onto the sample. Upon interaction with the sample surface, the covalent bond is momentarily split. Consequently, each of the nitrogen atoms has only half of the energy E = E0/2. The ion charge is dissipated in the substrate by transfer of an electron.
The help of M. Uno with the usage of the GFIS-FIB is acknowledged. The authors thank M. Ito for the help with TEM cross section preparation. This work is supported by the Center Of Innovation (COI) program of the Japan Science Technology Agency.
[1] R. H. Livengood et. al, Nucl. Instrum. Methods Phys. Res. Sect. Accel. Spectrometers Detect. Assoc. Equip., vol. 645, no. 1, pp. 136–140, Jul. 2011.
[2] F. Aramaki et. al, in Proc. SPIE 8441, Yokohama, Japan, 2012, vol. 8441, p. 84410D–84410D–6.