Paper TF-TuM6
Atmospheric Pressure ALD in Porous Substrates: The Effect of Pressure on Step Coverage

Tuesday, November 8, 2016, 9:40 am, Room 105A

ALD is renowned for its ability to deposit thin films into high aspect ratio structures with step coverages realized that are unparalleled by other gas-phase deposition techniques. This is one of the reasons ALD has become a key deposition technique in microelectronics fabrication. In the past years, Spatial ALD concepts have evolved for high throughput, large-area and roll-to-roll ALD applications in e.g. photovoltaics and flexible electronics. A new challenge for Spatial ALD is coating inside porous and 3D substrates, e.g. for applications in energy storage, catalysis and membranes. In many cases, Spatial ALD is performed at atmospheric pressure.

There are several studies where the relation between precursor dose and step coverage has been investigated. The most famous one is the kinetic model derived by Gordon et al. [1] that gives an analytical estimation of the precursor dose required to conformally coat a pore as a function of its aspect ratio. The pressure dependence of conformal coating in pores is in the transport of precursor molecules by diffusion along the length of the pore, given by the diffusion coefficient. Unfortunately, the diffusion coefficient is not a variable in the Gordon model.

We have derived an alternative kinetic model that makes use of similar assumptions as used in the Gordon where the pressure dependence of the diffusion coefficient was included. Three regimes can be identified for diffusion inside pores: pressure dependent Fickian diffusion for large diameter pores, pressure independent Knudsen diffusion for small pore diameters and a transition regime between the two. Combining the pressure dependence of the diffusion coefficient, the kinetic model and experimental data we can calculate the required precursor dose required to conformally coat a pore as a function of pore diameter and reactor pressure. A similar analysis can be performed on the required purge time to empty a pore of reactants.

The main results we will show are that 1) for pores smaller than ~1 µm diameter, there is no difference in atmospheric and low pressure ALD with respect the required precursor dose, 2) for larger pores there is a pressure dependence of the diffusion coefficient and 3) in terms of deposition rate it is beneficial to use higher reactor pressures to allow high precursor partial pressures leading to high diffusion- and reaction rates. Based on these experimental and modeling results we will finally give an outlook to the feasibility of large-area or roll-to-roll atmospheric pressure Spatial ALD of conformal coatings in high aspect ratio substrates.