Pulsed DC bias for the study of negative-ion production on surfaces of insulating materials in low pressure hydrogen plasmas

In this work negative-ion production on the surface of a sample negatively DC biased in a hydrogen plasma is studied. The negative ions created under the positive ion bombardment are accelerated towards the plasma, self-extracted and detected according to their energy and mass, by a mass spectrometer placed in front of the sample. The use of a pulsed bias allows applying a quasi-DC bias on insulating material during a short period of time and offers the possibility to extend the measurement method to nonconductive samples. The pulsed-bias tests were performed first with Highly Oriented Pyrolitic Graphite (HOPG), a conductive material, to demonstrate the feasibility of the method. By changing the pulsed-bias frequency it was possible to obtain HOPG material with different hydrogen surface coverages and hence different surface states leading to an increase of negative-ion production by up to 30-50 continuous bias case. To establish a protocol for insulating materials, charge accumulation on the surface during the bias pulse and influence of the bias duration and frequency were explored using microcrystalline diamond (MCD) thin layers. By using a pulse short enough (10 μs) at 1 kHz frequency, it has been possible to measure negative-ions on MCD sample at a quasi-constant surface bias of 130 V, with only 1 V variation during the measurement. Negative-ion surface production on MCD has been studied in pulsed mode with surface temperature from room temperature to 800°C. It is shown that pulsing the bias and increasing the temperature allows limiting defect creation on MCD which is favorable for negative-ion production. Consequently, at 400°C the yield on MCD in pulsed mode is one order of magnitude higher than the yield on HOPG in continuous mode at room temperature.