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In this work negative-ion production on the surface of a sample negatively DCbiased in a hydrogen plasma is studied. The negative ions created under thepositive ion bombardment are accelerated towards the plasma, self-extracted anddetected according to their energy and mass, by a mass spectrometer placed infront of the sample. The use of a pulsed bias allows applying a quasi-DC biason insulating material during a short period of time and offers the possibilityto extend the measurement method to nonconductive samples. The pulsed-biastests were performed first with Highly Oriented Pyrolitic Graphite (HOPG), aconductive material, to demonstrate the feasibility of the method. By changingthe pulsed-bias frequency it was possible to obtain HOPG material withdifferent hydrogen surface coverages and hence different surface states leadingto an increase of negative-ion production by up to 30-50% as compared to thecontinuous bias case. To establish a protocol for insulating materials, chargeaccumulation on the surface during the bias pulse and influence of the biasduration and frequency were explored using microcrystalline diamond (MCD) thinlayers. By using a pulse short enough (10 $\mu$s) at 1 kHz frequency, it hasbeen possible to measure negative-ions on MCD sample at a quasi-constantsurface bias of 130 V, with only 1 V variation during the measurement.Negative-ion surface production on MCD has been studied in pulsed mode withsurface temperature from room temperature to 800{\textdegree}C. It is shownthat pulsing the bias and increasing the temperature allows limiting defectcreation on MCD which is favorable for negative-ion production. Consequently,at 400{\textdegree}C the yield on MCD in pulsed mode is one order of magnitudehigher than the yield on HOPG in continuous mode at room temperature.

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