Development of gas cluster ion beam irradiation system with an orthogonal acceleration TOF instrument K. Ichiki, a * J. Tamura, b T. Seki, a,e T. Aoki c,e and J. Matsuo d,e Surface damage induced on biomolecules with gas cluster ion beam (GCIB) irradiation is significantly lower than with atomic or small cluster ion beams, and for this reason, surface analysis techniques such as secondary ion mass spectrometry (SIMS) have become one of the most important applications of GCIB, particularly for microscale chemical imaging of biomolecular species. Because of the low duty-cycle in time-of-flight (TOF)-SIMS, only less than 0.1 % of the incident ion beam is used for analysis, meaning that analysis with high spatial resolution can practically be extremely lengthy. The duty cycle can be significantly improved with the orthogonal acceleration (oa) TOF method because with this method secondary ion mass spectra can be measured at high mass resolution without requiring a pulsed primary ion beam. In this study, we developed a gas cluster ion irradiation system mounted on an oa-TOF instrument and investigated the sputtering yield and secondary ion yield of arginine. Copyright © 2012 John Wiley & Sons, Ltd. Keywords: Ar cluster; arginine; secondary ions; sputtering yield; oa-TOF Introduction A gas cluster is an aggregate of more than several hundred gaseous atoms, in which the very weakly bound constituent atoms strike the target at the same time, generating multiple collisions with the surface atoms. The penetration depth of a gas cluster ion is much shallower than that of a monoatomic ion of the same total energy, because the individual constituents of gas cluster ions have a very low energy; for example, each Ar atom in a 10-keV Ar 1000 cluster has an energy of only 10 eV. Gas cluster impact induces various unique irradiation effects in the near-surface region, such as shallow implantation, thin film formation, surface smoothing and high-rate etching. [1–3] In the last decade, the use of gas cluster ion beam (GCIB) has expanded into various industrial applications, and recently etching and analysis of organic materials have emerged to be among its most important applications. [4,5] An ion with an energy of a few keV/atom penetrates more than 10 nm into the organic surface and significantly affects the surface with phenomena such as chain scission, cross-linking and carboniza- tion. [6,7] Therefore, irradiation with monoatomic ions are suitable for surface modification of physical and chemical properties, but not for damage-less sputtering or soft ionization of organic materials. [8,9] On the other hand, under GCIB irradiation, high secondary ion yields, high etching yields and constant etching rates with little or no damage to the underlying structure have been reported for various organic and polymeric materials such as leucine, arginine, poly(bisphenol A carbonate), polystyrene and poly(methyl methacrylate). [10–12] Therefore, the mapping of chemical information as a function of depth (called depth profiling) and the localization of biomolecular species (called 3D secondary ion mass spectrometry (SIMS) imaging) could be improved by using GCIB. Organic SIMS with cluster ion beam has been investigated, and the effective useful molecular ion yields for organic targets were estimated to be in the order of 10 5 to 10 4 secondary ions per incident ion. [13] A spatial resolution of a few mm is required for 3D SIMS imaging of small organic samples such as cells. In a usual time-of-flight (TOF)-SIMS system, the duty cycle, which is defined as the ratio of secondary ion ON time to total cycle time, is in the order of 0.1%, because primary ion beam has to be pulsed for TOF analysis. It therefore takes about 100 h per image to complete a secondary ion map of 100  100 pixels by using a GCIB of DC current 100 mA/cm 2 . The orthogonal acceleration (oa)-TOF mass spectrom- eter simultaneously samples all masses without requiring a pulsed primary ion beam, which considerably improves the duty cycle, with typical values exceeding 10%. [14] In this study, we developed a compact GCIB equipment combined with a commercial oa-TOF system and measured the molecular sputtering yield and secondary ion spectrum of an arginine sample with an argon cluster ion beam. Experimental Figure 1 shows the schematic diagram of our oa-TOF system. We connected a compact GCIB equipment to the oa-TOF system (AccuTOF, Jeol Ltd, Akishima, Japan). In this system, neutral Ar * Correspondence to: K. Ichiki, Department of Nuclear Engineering, Kyoto University, Sakyo, Kyoto, 606–8501, Japan. E-mail: ichiki.kazuya@nucleng.kyoto-u.ac.jp a Department of Nuclear Engineering, Kyoto University, Sakyo, Kyoto, 606-8501, Japan b JEOL Ltd, Tokyo, Japan c Department of Electronic Science and Engineering, Kyoto University, Nishikyo, Kyoto, 615-8510, Japan d Quantum Science and Engineering Center, Kyoto University, Uji, Kyoto, 611-0011, Japan e CREST, Japan Science and Technology Agency (JST), Chiyoda, Tokyo 102-0075, Japan Surf. Interface Anal. 2013, 45, 522–524 Copyright © 2012 John Wiley & Sons, Ltd. SIMS proceedings paper Received: 28 October 2011 Revised: 1 June 2012 Accepted: 4 June 2012 Published online in Wiley Online Library: 5 July 2012 (wileyonlinelibrary.com) DOI 10.1002/sia.5092 522