Invited Paper TF+MI+NS-ThA1
Coating and Infilling 3D Geometries by Low-T CVD : HfB₂ throughout 0.5 mm Thick CNT Forests

Thursday, November 2, 2017, 2:20 pm, Room 21

We describe the use of extremely conformal chemical vapor deposition to coat carbon nanotube forests throughout their depth with a film of HfB₂, a mechanically hard, electrically conductive and highly refractory alloy. CVD uses the precursor Hf(BH₄)₄ at partial pressures ≤ 15 Torr at substrate temperatures ≤ 200°C. The CNT forests are up to 500 µm thick and can be shaped by pre-depositing patterns of seed metal on the underlying substrate prior to their synthesis.

Coating CNT forests affords a new class of composite foams with adjustable mechanical properties : the HfB₂ coating both stiffens individual tubes and “welds” them together wherever they touch, whereas in the uncoated forest the tubes can slide relative to one another. We fabricate cylindrical pillar structures with HfB₂ thicknesses from 3-50 nm. As measured by nanoindentation using a flat punch, the Young’s modulus varies over three orders of magnitude as E ~ ρ^1.7, where the mass density ρ is dominated by the HfB₂ coating. The maximum stiffness and strength are 56 and 1.9 GPa, respectively.

A major question is how CVD can achieve a nearly uniform coating in such a deep structure. The challenge is inherent in the diffusion-reaction kinetics : the transport of reactants occurs by molecular diffusion, which is slow in narrow cross-sections; while at the same time the reaction (consumption) rate must be high enough to be useful. In combination, these factors imply that the partial pressure (flux) of reactants must diminish with depth; if the film growth rate varies monotonically with flux, then the coating thickness must decline with depth in the structure.

The solution is to (i) employ a precursor that exhibits a hard saturation in the growth rate vs. partial pressure, (ii) use a high partial pressure above the sample, and (iii) grow at low temperature to limit the reaction rate. Under these conditions, even though the partial pressure of precursor drops considerably from the top to the bottom of the sample, the growth rate remains almost constant. To achieve the high precursor pressure, we employ a static (unpumped) reaction tube apparatus. We previously derived a master relationship [1] that predicts the precursor pressure (p) necessary to achieve a desired step coverage (SC) and growth rate (GR) in a given aspect ratio (AR) given knowledge of the atomic density (r), growth rate coefficient (K) and molecular diffusivity (D₀): p = (GR AR) * ((c r kT) / (2 D K) * (1 - SC))^0.5

We will discuss extension of this method to other thin film systems and substrate geometries.

1. A. Yanguas-Gil, Y. Yang, N. Kumar, and J. R. Abelson, JVST A 27, 1235 (2009) [DOI: 10.1116/1.3207745]