The growth on silicon substrates of arrayed bundles of multiwalled carbon nanotubes (CNTs) by metal catalyzed chemical vapor deposition of carbon from ethylene has been characterized and optimized. We find that, while CNTs can grow on bare Si substrates, the growth is substantially more reproducible if a thin (similar to 3 nm) barrier layer of aluminum oxide is used between the Si surface and iron catalyst. Optimum Fe thickness and growth temperature are 3.0 nm and 650 degrees C, respectively. We find that the CNT length increases linearly with time at a rate of 3-4 mu m/min for up to 2 h of CNT growth, after which the growth ceases. The length of the resulting CNT can thus be controlled up to a maximum length of similar to 500 mu m. Such control over CNT bundle length will be crucial in the incorporation of these bundle arrays into high-intensity electron field emission devices. (c) 2006 American Vacuum Society.