Fabrication of vacuum tube arrays with a sub-micron dimension using anodic aluminum oxide nano-templates

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Abstract

Vacuum tube arrays with a sub-micrometer dimension were fabricated by using anodic aluminum oxide (AAO) nano-templates. Ni nanowires deposited electrochemically inside the pores of the AAO nano-templates were used as the field emitters. The pores were sealed by means of angled evaporation of titanium. The field emission measurement was carried out in atmospheric environment outside of a vacuum chamber. The field emission characteristics show low turn-on voltages of 11.0–14.0 V. This phenomenon is attributed to the fact that the distances between the tips of Ni nanowires and the anodes are much smaller than those of conventional designs. Curvature in the Fowler–Nordheim plots at low applied voltage region is due to the variation of the enhanced field at each nanowire tip by the distribution in the length of Ni nanowires.

Introduction

Vacuum devices based on field emission have attracted considerable attention in recent years because they have several advantages over solid-state devices. They are robust at ambient temperature as well as in radiation environments. They have no power dissipation during electron transport because of the ballistic nature of transport in a vacuum [1]. Consequently, vacuum devices can generate higher power at high frequencies. These characteristics guarantee many applications, including active elements for integrated-circuits, flat-panel displays, electron guns, and microwave power tubes [2], [3], [4].

Most of the field-emission-based vacuum devices are fabricated by the Spindt process. However, it requires sophisticated processing skills and expensive equipment such as selective etching and electron beam lithography. High voltage is required for the operation of these devices because of the inter-electrode distance of several hundred micrometers. In addition, the Spindt process is difficult to apply to a large area. To overcome these difficulties, we have used anodic aluminum oxide (AAO) technology, which is capable of controlling the dimensions of the structure such as pore diameter, pore length, and pore density with a few nanometer resolution without using electron beam lithography [5], [6], [7]. AAO technology can also be easily applied to a large area [8].

In this study, we have fabricated field emitter arrays (FEAs) with integrated anodes by using AAO nano-template. Due to the integrated structure, these vacuum tubes can be operated as stand-alone devices, outside of a vacuum chamber. This work is the basis to devise a novel fabrication process for triode vacuum tube arrays with sub-micrometer dimensions.

Section snippets

Experimental

First, a cleaned and electropolished aluminum sheet of high purity (99.999%) was anodized in an oxalic acid solution. After chemically etching the anodized layer in a mixture of chromic acid and phosphoric acid, the second anodization was performed under the same conditions. Details of the fabrication process have been reported elsewhere [9]. Ni nanowires were electrodeposited in the pores of the AAO nano-template. In order to facilitate the growth of Ni nanowires, the voltage was dropped

Results and discussion

Fig. 2 shows the progressive change of the surface morphology of a sample. The virgin specimen exhibits surface roughness in μm scale before electropolishing (Fig. 2(a)), but these large surface irregularities are completely removed after electropolishing (Fig. 2(b)). Fig. 2(c) shows the AAO template after 2-step anodization. Pores of an identical dimension are hexagonally closed-packed after the 2-step anodization. The average inter-pore distance is 110 nm, and the average pore diameter is 70

Conclusion

We have fabricated vacuum tube arrays with a sub-micrometer dimension by using AAO nano-templates. Current–voltage characteristics show low turn-on voltages of 11.0–14.0 V. This phenomenon is attributed to the fact that the distances between the tips of Ni nanowires and the anodes are much smaller than those of conventional designs. Curvature in the FN plots at low applied voltage region is due to the variation of enhanced field at each nanowire tip by the distribution in the length of Ni

Acknowledgements

This work was supported by the National R&D Project for Nano Science and Technology and by the Brain Korea 21 Project in 2003.

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    Citation Excerpt :

    Preparing anodic aluminum oxide (AAO) templates has been studied for a considerably long time and many papers reported the details (Wang and Han, 2003; Kim and Cho, 2006; Pellin et al., 2005) and (Li et al., 2004). Similar work has been reported by Sun and Xu (2005), Lee et al. (2005), Chen et al. (2005), Hwang et al. (2005), and Kong (2005), as well as Wang et al. (2005). In the work performed by Zhao et al. (2005a), a two-step anodic oxidization method was used to obtain nanopores with uniform sizes and thin barrier layers.

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