Growth of the large area horizontally-aligned carbon nanotubes by ECR-CVD
Introduction
In the past decade, carbon nanotubes (CNTs) have attracted intensive interest in nanotechnology due to their remarkable properties and applications, such as nanoelectronics [1], [2], [3], [4], [5], [6], [7], [8], field emitters [9], [10], [11], [12], [13], hydrogen storage [14], [15], scanning probe [16] and supercapacitors [17]. The CNTs have been synthesized by many methods, including arc discharge [18], laser ablation [19], microwave plasma chemical vapor deposition [20], thermal chemical vapor deposition (CVD) [21], etc. Most of these methods are designed to grow the vertically-oriented CNTs with respect to the substrate surface [22], [23]. However, for microelectronic applications, it often demands to make horizontal interconnection between components. In these applications, horizontally-oriented CNTs have started to attract attention in both academic and industrial communities [24], [25], [26], [27], [28]. Other applications of the horizontally-oriented CNTs are to grow a single nanotube for study of single nanotube properties [28]. The purposes of this study were to investigate the main parameters that control the orientations of the CNTs or each single nanotube, and then to examine their properties and structures. In consideration of the advantages of high dissociation percentage of the precursor gases, high uniformity of plasma energy distribution, and possible large area film deposition, the electron cyclotron resonance chemical vapor deposition (ECR-CVD) method is adopted for this study.
Section snippets
Experimental
The horizontally-aligned CNTs were synthesized on 100 mm Si wafers by ECR-CVD method with CH4/H2 as the source gases. The Co catalyst for CNTs growth was applied on Si wafer by physical vapor deposition method, and followed by hydrogen plasma etching treatment for 15 min to become well-distributed nano-sized catalysts. To produce the horizontally-aligned CNTs, a Ti wafer about specimen size and ∼2 mm in thickness was placed 1–2 mm on top of the specimen to guide the source gases to flow
Effect of the guided flow under negative bias on the growth direction of CNTs
The SEM morphologies of the CNTs are shown in Fig. 1a and b for Sample A, Fig. 1c and d for Sample B, Fig. 1e and f for Sample C. Fig. 1b, d and f are at higher magnifications than the corresponding Fig. 1a, c and e. Where the Samples A and B are grown under the guided flow (inserting Ti wafer on the top of specimen) for 5 and 15 min deposition times, respectively. In contrast, the Sample C is grown under the un-guided flow (no Ti wafer above the specimen) for 15 min deposition time. In
Conclusions
Large-area well-aligned CNTs with vertical and horizontal orientations could be obtained by manipulating the gas flow direction and the electric field on the deposition substrate. This novel process permits to synthesize horizontally-aligned CNTs, which is a very promising way to apply CNTs in microelectronic devices in the future. The possible growth mechanism of horizontal CNTs is proposed. The field emission properties can be manipulated by changing the orientation of CNTs.
Acknowledgements
The authors would like to thank the supports of the National Science Council (Contract No.: NSC90-2216-E-009-034, -035 and -040) and the Ministry of Education of Taiwan (Contract No.: 89-E-FA06-1-4).
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