INSTITUTE OF PHYSICS PUBLISHING and INTERNATIONAL ATOMIC ENERGY AGENCY NUCLEAR FUSION Nucl. Fusion 42 (2002) L27–L30 PII: S0029-5515(02)54403-3 LETTER Chemical sputtering of hydrocarbon films by low-energy Ar + ion and H atom impact C. Hopf, A. von Keudell and W. Jacob Centre for Interdisciplinary Plasma Science, Max-Planck-Institut f ¨ ur Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching, Germany E-mail: von Keudell@ipp.mpg.de Received 8 August 2002, accepted for publication 4 October 2002 Published 31 October 2002 Online at stacks.iop.org/NF/42/L27 Abstract Erosion of hydrocarbon films is investigated in a particle-beam experiment employing sources for argon ions and hydrogen atoms. Sputtering by argon ions sets in above a threshold of 58 eV and reaches a yield of 0.5 at an ion energy of 800eV. Sputtering by argon ions with an additional flux of thermal hydrogen atoms towards the surface occurs above a threshold of 1.3 eV and reaches a yield of about three at an ion energy of 800 eV and a hydrogen atom to argon ion flux ratio of 400. A pronounced dependence of the yield on this flux density ratio is observed. It is proposed that the shift of the threshold energy as well as the change in the absolute yields is due to the process of chemical sputtering: within a collision cascade of the incident ions, broken bonds are instantaneously passivated by the abundant flux of atomic hydrogen. This leads to the formation of hydrocarbon molecules within the ion penetration range, which diffuse to the surface and desorb. This has important implications for the lifetime of plasma wall components in the divertor region of next step nuclear fusion devices. PACS numbers: 52.40.Hf, 61.80.Jh, 81.65.Cf 1. Introduction In nuclear fusion research most of the current devices have plasma facing components made of carbon materials. Atomic hydrogen as well as energetic species from the fusion plasma cause erosion of the carbon tiles such as the divertor surfaces and therefore limit the lifetime of the first wall [1, 2]. If the plasma is operated in the detached regime, the particle flux onto the divertor surface consists mainly of low-energy ions. Despite this low ion energy, however, a significant erosion of the divertor surfaces is still observed. A quantitative understanding of this process is therefore urgently desired. The temperature activated chemical erosion of C : H films by thermal hydrogen atoms was intensively investigated in experiments employing hydrogen atom beams [3]. A reaction scheme was developed, which is able to predict the temperature and hydrogen flux dependence of chemical erosion with high accuracy [4]. Physical sputtering of C : H films by hydrogen ions was also comprehensively studied in various experiments employing hydrogen ion beams [5, 6]. Incident ions penetrate into the solid and cause a collision cascade of recoils by displacing bonded target atoms. If the projectile itself or a recoil transfers sufficient kinetic energy to a surface atom or group to overcome its surface binding energy, physical sputtering occurs. Measured sputtering yields are in agreement with predictions by TRIM.SP computer simulations [7]. It was observed, however, that C : H film erosion by hydrogen ions occurs even at low substrate temperatures and low ion energies, where on the one hand no chemical erosion is expected and on the other hand the ion energies are below the threshold for physical sputtering [6]. It was postulated that this low-energy, low-temperature film erosion occurs similar to physical sputtering, but with a very low threshold energy of only 1–2 eV since weakly bound C x H y surface groups are sputtered [6]. In this article, the underlying mechanism for sputtering at low ion energies is investigated. 2. Experiment Film erosion is studied in a particle-beam experiment employing two sources for low-energy ions and for hydrogen atoms. The two beams interact with a so-called ‘hard’ 0029-5515/02/120027+04$30.00 © 2002 IAEA, Vienna Printed in the UK L27