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RESEARCH PRODUCT

Time-of-flight telescope for heavy-ion RBS

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Abstract This paper describes a time-of-flight (TOF) spectrometer for Heavy-Ion Rutherford Backscattering Spectrometry (HI-RBS) recently installed at IMEC for thin film analysis. The TOF telescope allows the use of ion beams heavier than He, with advantages in terms of depth and mass resolution and sensitivity compared to conventional RBS based on planar Si detectors. The start timing-signal is produced by the secondary electrons emitted from a thin C foil when traversed by a backscattered ion; the electrons are deflected in an electrostatic mirror towards a Micro-channel plate (MCP) assembly which provides a fast timing response. The stop signal is obtained directly from a second MCP assembly perpendicular to the ion trajectory. The setup was tested with a variety of incident beams. An energy resolution as good as 10, 14, 17 and 25 keV was achieved for 4 He, 12 C, 16 O and 35 Cl beams respectively. The associated depth resolution improves up to a factor of three, compared to conventional RBS. The different contributions to the energy resolution in the near-surface region are theoretically evaluated. The main limiting factors in the low and high energy range are the tandem effect and the telescope time resolution (550 ps), respectively. In order to reduce the tandem effect, a new design of the first timing gate, with the C foil at ground potential, is currently under development.