APPLIED PHYSICS LETTERS 94, 043116 共2009兲
Electron-shading effect on the horizontal aligned growth of carbon nanotubes Yang Chai,a兲 Zhiyong Xiao, and Philip C. H. Chan Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, People’s Republic of China
共Received 2 November 2008; accepted 6 January 2009; published online 29 January 2009兲 Based on the well-accepted electron-shading theory during plasma processing, we designed microstructures to control the local built-in electric-field on the substrate surface. The distortion magnitude of the electric-field is largest near the sidewalls of the microstructures, creating a horizontal electric-field in this region. We showed that the horizontally aligned carbon nanotubes 共CNTs兲 were grown by making use of this built-in electric-field during the plasma-enhanced chemical vapor deposition process, with a tactical choice of geometries and materials of the microstructures on the substrate. This technique opens up a way to selectively and controllably grow horizontally aligned CNTs on the substrate surface. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3077155兴 Carbon nanotubes 共CNTs兲 exhibit unique structural and electronic properties, which make them promising for many applications in electronics, such as field effect transistor,1,2 diode,3 integrated circuit interconnect,4,5 thermal interface material,6 and field emitter. The orientation of the CNT is particularly important for the practical application because of its one-dimensional feature. Chemical vapor deposition 共CVD兲 method has been considered one of the methods for controllably growing the CNTs.7 Vertically aligned CNTs have been successfully demonstrated by either thermal CVD or plasma-enhanced CVD 共PECVD兲 method.3–7 Many electronic applications of the CNT, such as field effect transistor and interconnect line, also require the horizontally aligned CNTs. Therefore, fabricating the horizontally aligned CNTs in a controllable way is a critical issue for the practical application of the CNT. PECVD is widely used for manufacturing very large scale integrated circuits. The introduction of the PECVD method in the CNT growth process provides additional control mechanism.7 There is a self-generated electric-field between the plasma and the substrate in the plasma environment.6–8 When the substrate surface is uniformly flat, the built-in electric-field is vertically directed from the plasma to the substrate surface. Utilizing this vertical built-in electric-field, people have fabricated the vertically aligned CNT film perpendicular to the substrate surface by PECVD method.6–8 Several research groups have also reported the inclined CNTs relative to the substrate surface by tilting the angle of the sample holder.9,10 In this study, we demonstrated that the horizontally aligned CNTs can be grown by making use of the built-in electric-field induced by plasma during the PECVD growth process. This was achieved with a tactical choice of materials and geometries of the microstructures on the substrate. We intentionally designed and fabricated the microstructures to distort the direction of the built-in electric-field. The local electric-field near the sidewalls of the microstructures can be modified to be horizontally directed. This horizontal built-in electric-field guides the as-grown CNTs to be well-aligned a兲
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on the substrate surface. This technique can selectively and controllably grow CNT for electronic applications on a large scale. The process flow for fabricating the test structure is schematically described in Figs. 1共a兲–1共d兲. We started with p-type doped silicon substrate. 共1兲 A 5000 Å SiO2 layer was grown on the silicon substrate. 共2兲 The oxide patterns were formed by photolithography and reactive ion etching. 共3兲 A 5 nm Fe was then deposited by e-beam evaporation techniques as the catalyst for the CNT growth. Excessive catalyst was removed in an acetone lift-off leaving catalyst only in the photolithography patterned area. 共4兲 We then loaded the samples into an ASTEX microwave PECVD system to grow the CNTs. At the beginning of the plasma processing, the floating substrate exposed to the plasma is immediately bombarded by electrons, ions, and neutral molecules. Since the electron is the lightest and hottest particle, the average velocity of the electrons is much higher than that of either the ions or the neutrals 共over 10 000 times兲.11 The floating substrate accu-
FIG. 1. 共Color online兲 关共a兲–共d兲兴 Process flow for fabricating the test structure with patterned SiO2 on the Si substrate. Schematic drawing of 共e兲 the electron distribution in our designed structure; 共f兲 the distorted electric-field near the sidewall of the SiO2 island. We only illustrate the distribution on the sidewall with catalyst here.
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mulates net negative charges very rapidly and starts to repel the further electron flux. Due to the repulsing potential, the neighborhood of the substrate has fewer electrons than the rest of the plasma 共the so-called “sheath” region兲. The substrate couples with the plasma via the sheath.11 At the steady state, the potential of the substrate surface is tracked to the plasma according to the well-known sheath potential Vsh 共Refs. 8兲 Vsh = 共kTe兲 / 共2e兲ln共M / 2.3m兲, where Te is the electron temperature and m and M are the masses of the electron and ion in the plasma, respectively. Electrons that overcome the sheath potential arrive on the substrate with an isotropic angular distribution because of the random thermal movement, while ions are accelerated in the presheath to the Bohm velocity. Ion velocity in the direction perpendicular to the substrate is larger than the thermal velocity of the ions in the horizontal direction. This initial directional difference is greatly amplified in our designed structure as the particles traverse across the sheath.12 We schematically illustrate the surface charging of our designed SiO2 / Si structure during the plasma processing in Fig. 1共e兲. Due to the isotropic thermal movement, the electrons accumulate not only on the bottom Si surface but also on the sidewall of the SiO2 islands. The ion flux is directed to the bottom Si surface because of the sheath potential acceleration. According to the Gaussian’s law, the nonuniform charge distribution on the microstructure distorts the direction of the electric-field. The interaction between the charge distribution and the electric-field continues until the charging of the electrons and ions reaches a steady state. The electricfield lines near the SiO2 island are mostly ended on the sidewalls charged with electrons, as schematically shown in Fig. 1共f兲. The ions are deflected by the local electric-field near the sidewall of the island to produce off-axis ion trajectories. This so-called electron-shading effect was widely discussed in the previous plasma etching literatures.12–14 We fabricated the horizontally aligned CNTs using an ASTEX microwave PECVD system with the microwave frequency of 2.45 GHz and the output power of 300 W.6 The pressure of the PECVD reaction chamber was pumped to 0.01 Torr by a mechanical pump, and the substrate temperature was raised to 850 ° C by a rf heater before the CNT growth process. We used methane as carbon source gas. The working pressure was kept at 8.5 Torr. We started the CNT growth by igniting the plasma. The growth process was kept for 2 min. We then turned off the plasma and the heater and pumped out the residual reaction gas until the reaction chamber was cooled down to room temperature. Figure 2共a兲 shows the as-grown CNTs using the designed structure in Fig. 1. We observed that the horizontally aligned CNTs were grown from the catalyst in the direction of the electric-field. The electric-field can guide the alignment of the CNTs either during the fabrication process or after the growth.15–17 The polarizability along the CNT axis ␣储 is much higher than that perpendicular to the CNT axis ␣⬜, resulting in an infinite dipole moment along the CNT axis P = ␣E in electric-field E. Under the torque on the dipole movement = 兩P ⫻ E兩 = ␣储E2 sin cos , the CNT growth is guided in the direction of the built-in electric-field, where is angle between the CNT and the electric-field.15 The thickness of the SiO2 island is 5000 Å. The widths of the island and catalyst stripe are 5 and 3 m, respectively. The density of the horizontally aligned CNTs grown from a 5 nm thick
Appl. Phys. Lett. 94, 043116 共2009兲
FIG. 2. 共Color online兲 SEM images of the horizontally aligned CNTs originated from 共a兲 a SiO2 island, 共b兲 a SiO2 trench, and 共c兲 a circle pattern. 共d兲 SEM images of the horizontally aligned CNTs originated from a poly-Si trench and island 共inset兲. 共e兲 SEM image of the as-grown CNTs without the designed microstructure, showing vertically aligned feature. 共f兲 SEM image of the as-grown CNTs from the designed structure with catalyst on the SiO2 side. Inset: schematic drawing of the electric-field distribution of the designed structure with the SiO2 trench.
catalyst is around 12– 16/ m. By controlling the dimensions of the microstructure and catalyst, we are able to obtain the horizontally aligned CNTs with different geometrical features. The horizontally aligned CNTs can be grown in the SiO2 trench, as shown in Fig. 2共b兲. We also designed a circular structure. Figure 2共c兲 shows that the as-grown CNTs from the circular structure distribute along the radial direction. The CNTs are horizontally aligned along the normal direction of the circle. This result further validates the electron-shading mechanism in this experiment. Since the applications in field effect transistor or interconnect line require the CNTs to be grown on the SiO2 surface, we also prepared the structure with poly-Si pattern on the SiO2 film using the same masks. Using this structure, we also obtained the horizontally aligned CNTs on the SiO2 surface, as shown in Fig. 2共d兲. Different from the external electric-field method,15 which has low throughput, our method utilize the built-in electric-field created during plasma processing. It produces the horizontally aligned CNTs on a wafer-scale. The length of the CNTs in our experiment is around 6 m even we prolonged the growth time to 5 min. On the same chip, we also designed the catalyst pattern without the island or trench microstructure. Under the same growth conditions, we obtained the vertically aligned CNT film with the thickness of around 17 m, as shown in Fig. 2共e兲. The disparity of the CNT length is related to the electric-field distribution. As shown in Fig. 1共f兲, the electric-field nearest to the sidewall is horizontal, while it is inclined or vertical far away from the sidewall. The horizontal electric-field exists locally near the sidewall of the island or trench. Therefore, once reaching a certain critical length, the electric-field distribution will not favor the growth of the horizontally aligned CNTs.
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We also conducted a control experiment with the identical SiO2 pattern but different catalyst locations. The catalyst stripe was deposited on the top side of the SiO2 trench edge, as schematically shown in inset of Fig. 2共f兲. According to the electron-shading effect, the electric-field in this region is vertically directed, thus not favoring the horizontal CNT growth. Although the local electric-field in this region is vertically directed, the 0.5 m width catalyst is too thin to produce dense CNTs. As the result, the as-grown CNTs also have no apparently vertical alignment, as shown in Fig. 2共f兲. In summary, we have shown that the localized surface charging distorts the direction of the built-in electric-field in plasma. In order to utilize the electron-shading effect, we designed the microstructures and successfully fabricated horizontally aligned CNTs on the substrate surface. Our approach leads to selective and controllable CNT growth for the application requiring the horizontally aligned feature of the CNTs. The authors are supported by the Croucher Foundation under Grant No. CAS-CF05/06.EG01 and RGC CERG under Grant Nos. 611305 and 611307. 1
Appl. Phys. Lett. 94, 043116 共2009兲
Chai, Xiao, and Chan
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