Development of Anode Spot Plasma Ion Source for High Current Beam Extraction

Abstract

Anode spot is an additional discharge in front of positively biased electrode exposed to pre-existing (ambient, bulk) plasmas. It is distinguished from the ambient plasma by a double layer, and sustained by the drifted electron accelerated by the double layer potential. The main application of anode spot is used as charged particle source based on its basic characteristics with high plasma density. The main application of anode spot might be a high brightness ion sources for Nano-applications at Seoul National University. We have developed a high-brightness ion source with high power efficiency by generating an anode spot plasma near to extraction aperture, and it has been successfully utilized as ion sources for FIB (Focused Ion Beams) and Nano-MEIS (Nano-Medium Energy Ion Scattering). Recently, it essentially needs the high current large-area ion source in ion implantation field to cover the large area of substrates. We will develop the plasma ion source utilized anode spot (ASPIS, Anode Spot Plasma Ion Source) for high ion beam current extraction to expand its application field. Enlarged the extraction aperture is limited by the size of anode spot that its variation can be a constraint to the ASPIS operating as a high ion current source. It is investigated the correlation between the plasma properties of anode spot including the size variations and the operating parameters of ASPIS. Based on the experimental results, we propose an optimum design parameter and operating range of the ASPIS in order to be used ASPIS as a high current ion source though solving the 0-D particle balance model of anode spot considering ion beam extraction from it. As the diameter of extraction aperture enlarged in range of mm level, it is measured the extracted ion beam currents according to the operating parameters of ASPIS and examined the sustainment conditions of anode spot during the ion beam extraction. The beam currents are also measured with varied the design parameters of ASPIS such diameter of bias electrode and extraction aperture. It is well-known that the ion beam current is proportional to bias current in ASPIS used as high brightness ion source. In contrary, there is the nonlinearity between bias current and ion beam current that the ion beam current is abruptly increased according to the diameter ratio of bias electrode to anode spot. As the diameter of bias electrode increases, the optimum operating pressure for extracted maximum ion beam is gradually decreased and so does beam current. In this manner, the size of the anode spot must be at least larger than the extraction aperture and be about the diameter of the maximum bias electrode to extract high ion beam current with sustainment of anode spot. It is known that the anode spot size is a function of reciprocal of operating pressure, the ionization cross section determined by the double layer potential and mass ratio of electron to ion of operating gas species. However, it is observed that the anode spot size is varied with operating pressure as well as bias voltage. The plasma properties of anode spot as well ambient plasma are analyzed with the consideration on anode spot size variation. As increasing the operating pressure and bias voltage, the bias currents increase with decreasing the double layer potential. In contrary, the size variations of anode spots have different change trends with each operating variables. Because the anode spot controls its size to maintain the ionization rate inside anode spot with respect to the changes on plasma properties of ambient plasma or operating pressure. It suggests the modified formulae related to anode spot size by considered the correlation between the plasma properties including anode spot size and operating parameters of ASPIS. The characteristics of anode spot plasma considered with ion beam extraction is understood by developed the 0-D particle balance of anode spot. The electron is focused on analysis the anode spot properties so far, it is important to consider the generation and loss of ions at anode spot to be used as ion source. The 0-D particle balance model of anode spot is verified that the calculated plasma properties of anode spot with considered the additional ion loss due to beam extraction are compared with the experimental results. When the size of the bias electrode and the extraction aperture were changed, it is found that the maximum ion beam current is extractable at the area ratio of bias electrode to extraction aperture as 2. It is also predictable the size variation of anode spot with respect to bias current that the operating parameters as well as design parameters of ASPIS is suggested with target current. In this study, it is confirmed that the appropriate operation conditions of ASPIS are restricted that the quasi-neutrality conditions of anode spot should be satisfied to sustain the anode spot extracted high current ion beams. In order to apply the anode spot plasma ion source as a large area ion source beyond the high current ion source, this research will be used as a key index to suggest guidelines for ion source design.