Secondary Ion Mass Spectrometry with Gas Cluster Ion Beams
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SECONDARY ION MASS SPECTROMETRY WITH GAS CLUSTER ION BEAMS Noriaki Toyoda1, Jiro Matsuo2, Takaaki Aoki2, Shunichi Chiba2, Isao Yamada3, David B. Fenner4 and Richard Torti4 1 Material Processing Center, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139. 2 Ion Beam Engineering Experimental Lab., Kyoto Univ. Sakyo, Kyoto, 606-8501, JAPAN. 3 Laboratory of Advanced Science and Technology for Industry, Himeji Institute of Technology, Hyogo, 678-1205, JAPAN. 4 Epion Corporation, 37 Manning Road, Billerica, MA 01821. ABSTRACT Secondary Ion Mass Spectrometry (SIMS) with Gas Cluster Ion Beams (GCIB) was studied with experiments and molecular dynamics (MD) simulations to achieve a high-resolution depth profiling. For this purpose, it is important to prevent both ion-mixing and the surface roughening due to energetic ions. As the Ar cluster ion beam shows surface smoothing effects and high secondary-ion yield in the low-energy regime, the cluster ion beam would be suitable for the primary ion beam of SIMS. From MD simulations of Ar cluster ion impact on a Si substrate, the ion-mixing is heavier than for Ar monomer ions at the same energy per atom, because the energy density at the impact point by clusters is extremely high. However, the sputtering yields with Ar cluster ions are one or two orders of magnitude higher than that with Ar monomer ions at the same energy per atom. Comparing at the ion energy where the ion-mixing depths are the same by both cluster and monomer ion impacts, cluster ions show almost ten times higher sputtering yield than Ar monomer ions. Preliminary experiment was done with a conventional SIMS detector and a mass resolution of several nm was achieved with Ar cluster ions as a primary ion beam.
INTRODUCTION With the shrinking size of electronic, magnetic or photonic devices, the control of the thickness of films or the depth profile of dopants become extremely important. Secondary Ion Mass Spectrometry (SIMS) is a powerful technique to obtain depth profiles of the element in the samples. However, degradation of the depth resolution due to the ion-mixing and surface roughening with high energy ions cannot be overlooked when a depth resolution below 1nm is required [1]. Reducing the ion energy is the most effective way to avoid ion-mixing, however the secondary ion yield decreases at the same time, which results in the reduction of the signal-to-noise ratio. In addition, surface roughening with energetic ions causes degradation of the depth resolution. Therefore, an ion beam, which has high secondary ion yields in the ultra low energy regime and has surface smoothing effects, is desired for the primary ion beam of high depth resolution SIMS. We have developed the Gas Cluster Ion Beam (GCIB) process to replace the conventional ion beam in the low-energy region, such as a GCIB assisted thin film deposition [2], surface smoothing [3,4] and ultra shallow ion implantations [5]. Clusters are huge aggregates with several to thousands of atoms bound by Van der Waals force. As each
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