Improving Geometric Uniformity in Dynamic Chemical Vapor Deposition of Carbon Nanotube Forests.

Geometric nonuniformities are often observed in the catalytic chemical vapor deposition (CVD) of vertically aligned carbon nanotubes (VACNTs), known as CNT forests. These nonuniformities typically occur in the form of sloped heights and empty regions within the catalyst-covered substrate. To realize the true potential of carbon nanotube forests in unidirectional mass and energy transport applications, it is critical to develop robust manufacturing processes to produce geometrically uniform CNT forests on large-scale substrates in a repeatable manner. Our custom-designed reactor with an IR heating multizone furnace with rapid thermal processing capability allows the programming of dynamic recipes with the catalyst formation temperature decoupled from the CNT nucleation and growth temperature. In this work, we present an approach for tuning the geometric uniformity of CNT forests based on the combined effects of substrate holder design and dynamic recipes during CVD. We propose a mechanism that explains the observed geometric nonuniformities based on both the temperature profile across the catalyst chip, which depends on the substrate holder design, and the temperature range for CNT growth, which depends on the catalyst formation temperature. We performed a comparative study of the properties of alumina layers after annealing with two different substrate holder designs. We found that the actual temperature experienced by the sample depends on the substrate holder, which supports our proposed mechanism. Our work provides insight into the growth of CNT forests with large-scale spatial uniformity and high structural quality.