Paper TF-FrM1
Low-temperature CVD with Growth Inhibitors to Afford Uniform, nm-thick Films in Structures with Aspect Ratio ~ 200

Friday, November 4, 2011, 8:20 am, Room 110

In order to grow uniform, smooth and pinhole-free films of nm thickness, the nucleation step must occur with high areal density on the substrate and the nuclei must have a narrow size distribution. If the film must be deposited inside of a deep structure of very high aspect ratio, then the growth process must be strictly governed by the surface reaction rate of the precursor species rather than by the rate at which precursor is transported to the growth surface. Here, we present a method of control in chemical vapor deposition that simultaneously meets both of the above criteria.

We previously reported that the steady-state film growth rate is reduced when a molecular species, called the inhibitor, is added to the CVD process. The inhibitor interacts transiently with the growth surface but does not decompose and incorporate its elements into the film, i.e., the inhibitor is not consumed. We also reported that the inhibitor greatly improves the film nucleation step, e.g. for the growth of HfB₂, a high performance diffusion barrier, on SiO₂ substrates. In the absence of the inhibitor the nucleation density is low and the size distribution is broad, such that some islands attain a height of > 10 nm before the film even coalesces. In the presence of the inhibitor, nuclei attain a height of ~ 1 nm, but then grow only very slowly. Additional nuclei continue to form and fill in the bare substrate, such that coalescence occurs at a thickness of ~ 2 nm with a rms surface roughness of ≤ 0.5 nm.

The use of a growth inhibitor allows the nucleation regime of film growth to afford excellent nm-thick coatings in extremely high aspect ratio features. Note that when the film growth rate is low, so also is the consumption of precursor, such that a partial pressure of precursor species persists to the bottom of the feature. The inhibitor further assures that the nucleation process will be uniform. In features of aspect ratio > 200, we show that a film is deposited on all surfaces with a rms roughness of 0.6 nm. We also discuss the mechanisms that can afford the observed results. We suggest that the ability to reduce (homogenize) the size distribution and increase the areal density of nuclei will greatly extend the useful range of CVD precursor-substrate combinations which can afford nm-thick coatings in very high aspect ratio features.