Measurements of RF plasma sheath thickness in magnetic field up to 500 mT using a floating spiral probe

The thickness of the radio frequency (RF) plasma sheath developing in front of a capacitively coupled electrode has been measured using a floating probe. The probe presented in this contribution is based on the floating loop probe first developed by Godyak et al. To study the sheath potential in a smaller vacuum vessel, the probe has been designed in a spiral shape to maximize its sheath capacitance while occupying a reduced volume. During the time oscillation of the sheath, the probe's position influences the time variation of the probe's floating potential, unveiling the sheath thickness. Due to the strong direct current (DC) bias developing on the electrode and the significant RF frequency compared to the plasma ion frequency, the ions responded to the time-averaged sheath electric field. The measured time-average sheath thickness was comparable to the existing DC models with and without a magnetic field for pressures ranging from 2 to 10 Pa. Within this range of pressure, the sheath thickness decreased from 13 to 5 mm, due to the density increase. In a magnetic field, the sheath potential drop decreased and the plasma density increased, leading to a reduction of the sheath size. With the spiral floating probe, no magnetic pre-sheaths were observed up to 500 mT as predicted by Ahedo criterion.