Synthesis and characterization of ion-crafted nano/micro field ion emitters

Abstract

At the present time, interest is focused on the production of cold electron emitters and vacuum microelectronic devices [1]. The methods for manufacturing the above systems by the mechanical assembly of individual needles, directional growth of whisker crystals from gaseous media, photolithography followed by chemical etching, and directional solidification of eutectic oxide metal alloys are known [2]. An interesting alternative method for fabricating such large area field emitter arrays is the nuclear track etch technique [3– 7]. In this technique, the hollow etched tracks are filled with a suitable cathode material and then a replica technique is employed. The advantage of multiple emission structures manufactured by track etch technique is that they offer a wide range of cathode materials with a broad range of the diameter of emission electrodes. In this technique, the field emitter pins are statistically distributed and quite higher areal density can be achieved. Because of the conical shape of the emitter pins generated using track etch technique, they are better emitters than those generated by microlithographic techniques. Further, the main advantage of track etching technique over conventional techniques is that several tedious processing steps involved are eliminated. A field emitter is a metallic needle-electrode held at negative voltage with respect to a flat counter electrode. Under good vacuum conditions, the needle cusp emits a strongly bundled electron beam of high intensity. The brightness of field emitters can be several orders of magnitude higher than that of thermal cathodes. Therefore field emitters are beginning to replace directly heated tungsten filaments in many applications, for example, in scanning electron microscopy. The most important parameter of the field emitter pin is its smallest radius of curvature at the outermost tip: the maximum field strength is inversely proportional to the tip radius. Large area field emitter arrays are promising devices for many applications ranging from high quantum yield photocathodes to energy saving radar tubes. Their working depends critically on keeping tip radii as close as possible within the given specifications. Copper points arranged on copper backing are the most easily made structures and show promise for future application as explosive electron emitters. The high electrical and heat conduction qualities of the point material ensures a small loss of substance from their surface and consequently a long lifetime. Another trend in the production of emitters is the use of triode field emitting electron systems. The advantages of such cathodes is a small electrode gap and consequently low values of supply and control voltage, which can directly be controlled at the output of the microelectronic devices. The low absolute energy of the secondary ions minimizes erosion of the cathode and extends its lifetime. Cathodes of this type form the basis for display panels, which may replace cathode ray tubes. The Template Synthesis technique entails synthesis of desired materials (metals, semiconductors, and metal-semiconductor junctions) of very low dimensions [3–11]. The underlying principle of this technique is well known. It is an electrochemical process in which metallic ions in a supporting solution are reduced to the metallic state at the cathode, which, if closely covered by a nuclear track filter (NTF) membrane, would lead to the formation of growth of plated film as an embodiment of microor nanostructure. The etched pores of the membrane used would act as templates. The generated structures can either be heterogeneous or homogeneous depending upon the pore size and geometry, with complete control over the aspect ratio. As is evident, the Template Synthesis is a membrane based technology. One of the types of membrane used here is known as a track etch membrane or nuclear track filter (NTF). NTFs have emerged as a spin-off from solid state nuclear track detectors—solid state materials capable of storing tracks of energetic, heavily ionizing ions which can subsequently be chemically amplified as see-through pores or channels of well defined geometry and spatial density. NTFs have been put to numerous filtration applications besides their use in the synthesis of nano/microstructures and devices. Martin [11], Chakarvarti and Vetter [7] have produced extensive reviews of the Template Synthesis technique along with morphological revelations of the structural ensembles so generated.