In this work, metallic nanocones were examined with regard to their suitability for use as field emitters in vacuum electron sources. The focus was on the optimization of the manufacturing process, the modeling of the emission current for different emitter configurations as well as the experimental investigation of the manufactured electron sources.
The manufacturing of the metallic nanocones is based on a process for the electrodeposition of metal in ion track-etched polymer membranes. For this purpose, polycarbonate films, which were previously bombarded with heavy ions and thus show a higher etch rate along the ion tracks, were asymmetrically etched to create conical pores with a random spatial distribution. After an in-depth consideration of the
process parameters’ influence, a new etching system was developed. This system enables the production of sufficiently large polymer membranes that can be used for the integration of nanocones on usual 100mm-wafer. Since the polycarbonate membranes use to have a low hydrophilicity and thus a low
wettability by the electrolyte in the following electrodeposition, an surface activation of the membranes by an oxygen plasma was added to the process. With a micro-electroplating system and the prepared membranes gold was deposited in the pores, applying nanocones to gilded glass wafers in this way. The
polycarbonate membranes used result in cones with a height of 24µm and a base diameter of about 3µm. The tip diameter of the cones is in the submicron or nanometer range.
To optimize the vacuum electron source, both the field emitter on the cathode and the grid required for electron extraction have been modeled. Since in the literature only inadequate models of randomly distributed emitters were available up to now, an electrostatic model based on simulations with finite
element methods (FEM) was developed. This model enables the prediction of the total emission current of a large-area field emitter array and, thus, the derivation of an optimal emitter density depending on the emitter geometry and the applied electric field. For the selected cone geometry an optimal emitter density in the range of 1 to 2 · 10^4 Emitter/cm^2 resulted. In addition, the electrostatic fields generated by
the extraction grid were also examined by means of FEM simulations with regard to the field fluctuations, as these can significantly influence the current emission of the cones. From this, a rough empirical formula has been derived to predict how to choose the grid geometry in dependence of the emitters’ height for the field inhomogenities to be neglectable.
Furthermore, extensive field emission measurements were carried out on cathodes with metallic nanocones. Cathodes with cones of similar geometry, but with three different nominal emitter densities were investigated. With a characteristic field enhancement factor of approximately 1160, a cathode with an emitter density of 4 · 10^5 Emitter/cm^2 exhibited supreme field emission properties. Current stability studies were performed at room temperature and in an ultra-high vacuum of 10^−9 mbar. A maximum cathode emission current of 142 µA at an extraction voltage of 339 V was achieved. Such currents were not obtained using cathodes with a lower emitter density of 6 · 10^4 Emitter/cm^2 or a higher emitter density of 1 · 10^6 Emitter/cm^2 . However, with emission currents above 100 µA a decrease of the current was found, which can be explained with the degradation of the emitter tips by the sputtering of ionized residual gas particles. Nevertheless, a stable emission current of (31,0 ± 1,3) µA was measured over a period of 50 h.
A discrepancy between the optimal emitter density of approximately 1 · 10^4 Emitter/cm^2 predicted by the
modeling of the emitters and the experimental investigations can be explained by a high number of defects, such as cones that have not grown completely, and the non-equal emitter tip radii of the cathodes. Based on these measurements, miniaturized vacuum electron sources were made, which were adapted to the installation space on a Leybold IE514 ionization manometer. For this purpose, cathodes with a nominal emitter density of 1 · 10^5 Emitter/cm^2 were produced after an improvement in the manufacturing process with regard to the homogeneity of the cone growth. Via a screw connection, these were frictionally mounted together with a spacer made of a structured mica plate and a tungsten fabric serving as an
extraction grid. Subsequent field emission measurements showed that the tungsten fabric is unsuitable despite its high optical transparency. In order to achieve the required emission currents, grids with smaller grid geometries in the lower µm-range are necessary. It also became clear that it is necessary to position the extraction grid as close as possible to the field emitters while taking into account the field fluctuations
generated by the grid. Revised vacuum electron sources were mounted and measured with grids consisting of a microtechnically manufactured, metallized silicon nitride membrane. This showed clear improvements in the field emission properties. However, due to short circuits between the cathode and grid, no further current stability measurements could be carried out. In the final discussion further optimization potential of the vacuum electron sources is shown. Overall, the present work proves the general suitability of metallic nanocones for use as field emitters in vacuum electron sources. It forms the basis for the further production of vacuum electron sources, which can meet the requirements for the use in an ionization manometer with regard to the emission current’s amount and stability to be achieved. | English |