Positive sheath behaviour in low pressure Argon plasma

We measure positive ions by means of a Hiden mass spectrometer (EQP 300) in a helicon discharge. Our helicon ‘PHISIS’ (plasma helicon to irradiate surfaces in situ) is used at low injected RF power, in a capacitive regime. A copper sample is centred in the expanding chamber, 40 mm away from the mass spectrometer nozzle. We measure ion energy distribution function (IEDF) for different sample positive bias. We show the absence of ions having energies corresponding to the full acceleration inside the positive sheath. We attribute this effect to secondary electron emission induced by electrons extracted from the plasma to the sample surface, which changes the potential profile and neutralise low energy ions created close to the sample surface (radiative recombination).