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

Abstract

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 pulsedbias 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% as compared to the 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.

Impact of positive ion energy on carbon-surface production of negative ions in deuterium plasmas

“…This change is obviously unfavourable for NI surface production on MCBDD since after a complete scan in bias, the signal is strongly decreased when coming back to -10 V. The ionization probability of the surface exposed at high bias is obviously lower than the ionization probability of the diamond surface exposed at low bias. This conclusion is in line with previous papers showing that defect creation on diamond is unfavourable for NI production 43,32 . Electronic properties of diamond are promoting NI surface production, defect creation is modifying electronic properties and lowering NI yield.…”

Enhancing surface production of negative ions using nitrogen doped diamond in a deuterium plasma

“…The relatively low power coupled to the plasma source results in plasma densities of approximately 10 14 m -3 in the spherical diffusion chamber 54 . The choice of power and pressure was for similarity with previous work 34 . The positive ion composition of the deuterium plasma is measured by the mass spectrometer, described below, to be (84 ± 2) % D 3 + ions, (14 ± 2) % D 2 + ions and (1.1 ± 0.2) % D + ions.…”

Surface production of negative ions from pulse-biased nitrogen doped diamond within a low-pressure deuterium plasma

“…When using a continuous bias in combination with a conductive sample, it is reasonable to expect that V A is equivalent to V S and that the sheath in front of the sample is planar because the sample holder is much larger than the area of the sample from which negative ions are emitted compared to the sheath width. When a pulsed bias is applied to a non-conductive sample 30 , V S becomes time dependent as a result of a build of charge on the sample surface 30 . Therefore equation 1 is rewritten as:…”

“…This implies that, at t = 0 µs, Q = 0 C which means that V S is equivalent to V A as shown in equation refeqn:Vs. This corresponds to the case where a conductive sample is employed 30 .…”

“…Alternative materials to caesium are under investigation, and these include highly oriented pyrolitic graphite (HOPG) 20,[23][24][25] , novel electrides 26 , LaB 6 , MoLa 27 , as well as those that have dielectric properties such as diamond-like-carbon (DLC) and diamond [23][24][25][28][29][30][31][32][33] .…”