Growth of conformal single-walled carbon nanotube films from Mo/Fe/Al2O3 deposited by electron beam evaporation
Introduction
While many prospective applications require control of the position and orientation of individual or groups of CNTs, there are also many applications for films of tangled CNTs as functional device elements such as thin-film transistors [1], chemical sensors [2], flow sensors [3], and electrical contacts [4]. These films may be grown directly on substrates and subsequently processed by lift-off patterning, functionalization, contact metal deposition, CVD deposition of structural layers, and other methods; alternatively the CNTs may be grown in bulk and selectively deposited onto devices by wet chemical methods [5], [6].
Synergy between the catalyst, carbon source, and growth conditions such as temperature and flow rate is vital for obtaining a high-yield of high-quality CNTs by chemical vapor deposition (CVD). The performance of a catalyst in nucleating and continuing the growth of CNTs depends on several factors in addition to its elemental composition, including the size and surface properties of the catalyst particles and the interactions between the catalyst and the support. The supporting layer can significantly promote or hinder CNT growth by the nature of its physical, chemical, and electronic interactions with the catalyst [7], [8]. Especially effective material combinations for CNT growth include Al2O3-supported Mo/Fe with CH4[9], Ni or Co with C2H2[10], and SiO2-supported Mo/Co with CO [11] or alcohol [12].
The performance of a catalyst is also coupled to the deposition process, which can roughly be categorized as a physical method such as magnetron sputtering or e-beam evaporation, or a chemical method for preparing metal clusters in solution and subsequently depositing the clusters on a substrate. For sputtering deposition, Shin et al. demonstrated that the temperature and background pressure of the sputtering process, which affects the grain size and density of Ni thin films, is directly related to the length and diameter of vertically-aligned CNTs grown by thermal CVD of C2H2[13]. In comparison, chemical methods [14], [15], [9] enable direct control of particle composition and particle size, which can give direct control of CNT diameters [16]. However, deposition methods for these solutions, including spin-coating [9], [17], dip-coating [18], and contact printing [19], [20], are less favorable than physical methods for uniformity over large areas, and for coating of microstructures. For example, with contact printing it is difficult to coat oblique features; with dip-coating the solution tends to collect and dry in recessed areas; and with spin-coating, surface tension effects oppose uniform coating of topography and non-uniformity is especially prevalent as solutions wick away from edges and corners of features. It is also straightforward to pattern physically-deposited metal films to micron-scale dimensions using liftoff of standard image-reversal photoresist in acetone, and therefore dictate uniform area-selective growth of CNTs on large substrates.
We present a parametric study of CNT growth from a Mo/Fe/Al2O3 catalyst deposited by e-beam evaporation, and demonstrate uniform and conformal growth of CNT films by atmospheric-pressure thermal CVD on a variety of microstructures. Presence of H2 in addition to CH4 is necessary for CNT growth, and dense films of high-quality SWNTs are grown over a wide range of conditions from a H2/CH4 mixture. Our experiments confirm that Mo promotes SWNT growth from CH4, and indicate that Mo hinders MWNT growth from C2H4, while a Fe/Al2O3 film in C2H4 gives an exceptional yield of vertically-aligned MWNTs. This repeatable and versatile process for enhancing micromachined surfaces with uniform CNT films enable the robust integration of CNTs in micromachined devices.
Section snippets
Catalyst film
A catalyst film of 20 nm Al2O3, 1.5 nm Fe, and 3 nm Mo is deposited by e-beam evaporation in a single pump-down cycle using a Temescal VES-2550, with a FDC-8000 Film Deposition Controller. The substrates are plain (1 0 0) 6″ silicon wafers (p-type, 1–10 Ω cm, Silicon Quest International), which have been cleaned using a standard “pirahna” (3:1 H2SO4:H2O2) solution. After catalyst deposition, no further cleaning or dedicated oxidation is necessary prior to nanotube growth. The Al2O3 is deposited by
Study of growth temperature
To study the effect of temperature, otherwise identical experiments were conducted using 1 × 1 cm samples of the Mo/Fe/Al2O3 film on Si, with 40/360 sccm H2/CH4, at temperatures ranging from 725 to 1025 °C. Fig. 1 shows SEM images of representative samples. No growth occurs at 725 °C, while from 750 to 900 °C a film of densely tangled small-diameter CNTs is grown. At 925 °C, a low density of CNTs protrudes from cracked clusters of large particles on the substrate, and CNTs rarely grow from the
Growth on microstructured silicon substrates
Following the normal CVD procedure discussed previously, high-quality CNT films are also grown directly and conformally on silicon microstructures. Bulk-micromachined structures are fabricated from (1 0 0) silicon wafers by deep-reactive ion etching (DRIE) using a SF6/C4F8 plasma, by reactive ion etching (RIE) using a Cl2 plasma, and by wet etching in aqueous KOH. The Mo/Fe/Al2O3 catalyst film is deposited on the microstructures by the same process as for the flat silicon substrates; the wafer is
Conclusion
We studied growth of high-quality CNT films on bare and microstructured silicon substrates by atmospheric pressure thermal CVD, from Mo/Fe/Al2O3 deposited by e-beam evaporation. Dense SWNT growth occurs in gas mixtures containing at least 5% H2 in CH4, at 750–900 °C. Presence of Mo and Al2O3 in addition to Fe promotes high-yield growth of SWNTs under these conditions. Comparatively, a Fe/Al2O3 film gives vertically-aligned MWNTs in H2/C2H4, but only a very low density of CNTs in H2/CH4. These
Acknowledgements
This project was funded in part by an Ignition Grant from the MIT Deshpande Center for Technological Innovation. A.J. Hart is grateful for the support of a Fannie and John Hertz Foundation Fellowship. Thanks to Yet-Ming Chiang of the MIT Department of Materials Science and Engineering for use of his laboratory space, and the staffs of the MIT Microsystems Technology Laboratories and CMSE Shared Experimental Facilities (NSF DMR-0213282, #CHE-0111370) for assistance with fabrication and
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