Development of high-current ionic liquid ion source toward surface modification
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Development of high-current ionic liquid ion source toward surface modification Mitsuaki Takeuchi, Takuya Hamaguchi, Hiromichi Ryuto, Gikan H Takaoka Photonics and Electronics Science and Engineering Center, Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ABSTRACT Ionic liquid (IL) ion sources with different emitter tip materials and tip numbers were developed and examined on ion beam characteristics with respect to its ILs wettability. As a result of ion current measurements, the most stable emission current was obtained for the graphite emitter tip and the ion current increased with increase of the tip number. The results indicate that the emitter wettability corresponding to the supplying flow rate and the number of emission site play an important role to stabilize and increase the beam current. INTRODUCTION Polyatomic ion beam processes have important characteristics such as equivalently high currents, shallow implantation depths, and low implantation damages. Among them, ionic liquid ion sources (ILIS) have attracted considerable attention because of their unique characteristics such as easy negative ion beam generation and numerous molecular combinations. The ionic liquids (ILs) that are molten salts at room temperature typically exhibit moderate electrical conductivity of 1-100 S/m, high thermal stability and extremely low vapor pressure. Since the typical ILs includes halogens in its anion, reactive etching and surface modification of semiconductor materials have been reported [1]. Moreover, ILIS are studied as the ion source for focused ion beam processing (FIB) [1], ion thrusters [2], and secondary ion mass spectroscopy (SIMS) probes [3]. Several types of ILIS have been reported, such as an externally wetted needle made of tungsten [4,5] and a capillary [6,7]. Figure 1 shows schematic diagrams of three types of ILIS, the externally wetted needle type, the capillary type, and a porous medium type. We have recently proposed an ILIS with a porous medium emitter [8] that has a high current and stable structure in the field of liquid metal ion sources. In this study, we developed an ILIS with different emitter materials and tip numbers, and demonstrated its ion emission characteristics with respect to wettability.
Fig. 1. schematic diagrams of three types of ILIS, (left) the externally wetted needle type, (center) the capillary type, and (right) a porous medium type.
IONIC LIQUID ION EMISSION BY ELECTRIC FIELD Ion emission from conductive liquids is dominated by forces such as electromagnetic force, surface tension, and fluid dynamic force. When the electrostatic force and surface tension balance, a drop of liquid placed on a solid surface applied high electric field is will become deformed and take a unique conical shape known as a Taylor cone. This cone will have a semiangle of 49.3◦ [9]. For the quasi-static condition, the balanced electric field, Eth is defined as Eth = (4γ/ε0r)0.5 where the γ is liquid surface tension, ε0 is the vacuum permittivity, and the r is the radius tip
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