Long-term strability of various carbon-nanotube-based micro-tip emitters

Abstract

A microfocused x-ray tube using carbon nanotubes (CNTs) as a cold cathode, which has mechanical and chemical stability as well as superior electron emission properties, can overcome several problems that became apparent when thermionic cathode was used [1]. Recently, in order to realize x-ray images with high resolution, direct growth of CNTs on sharp metal tips was attempted to reduce the emission sites [2-3]. In addition, when a m-sized (or below) metallic tip is used as a substrate, adhesion with the substrate is the factor that influences the threshold voltage, maximum emission current, reliability, and lifetime the most. Here, we present the experimental results that regard the electron-emission property as well as the long-term stability of emission current in various CNT-based field emitters having the configuration of CNTs/catalyst/buffer/tip. The emitters are fabricated by varying the diameter of the conical-type tungsten (W) tip, the buffer layer (TiN, Al/Ni/TiN, etc.), and the catalyst material (Ni, Co, etc.). Tip-sharpening is done by the electrochemical etching technique using the KOH solution, which produces a sharp apex of approximately 300 nm ~ 2 m in diameter. The CNTs are grown at 700 oC using an inductively coupled plasma-chemical vapor deposition (ICP-CVD) system. Raman spectroscopy, field emission SEM, and high-resolution TEM are used to analyze the microstructures of all the grown CNTs. The electron-emission measurement and the stability test are carried out using a compactly-designed field emission measurement system, as shown in figure 1. The insets also show the SEM images of the CNTs grown on W-tip. Figure 2 compares some of the long-term (up to 30 hrs) stability results which are monitored from three different CNT-emitters. More detailed dynamic analysis regarding to the stability of emission current and discussion will be presented.