Plasma-enhanced atomic layer deposition: Correlating O2 plasma parameters and species to blister formation and conformal film growth

Plasma-enhanced atomic layer deposition has gained a lot of attraction over the past few years. A myriad of processes have been reported, several reviews have been written on this topic, and there is a lot of interest for industrial applications. Still, when developing new processes, often heuristic approaches are used, choosing plasma parameters that worked for earlier processes. This can result in suboptimal plasma process conditions. In order to rationally decide which parameters to use, we systematically studied an inductively coupled RF oxygen plasma source (13.56 MHz) for powers up to 300 W, a pressure range between 10−4 and 10−2 mbar, and a flow range between 10 and 400 sccm. We discerned between chemically active “radical” species (atomic O and excited, metastable O2) and ionic particles ( O2+, O+, O2−, and O−), which can have an additional physical effect to the film. Optical emission spectroscopy (OES) was used to study the generation of O2+ and atomic O in the plasma source region. It is shown that the concentration of plasma species increases in a linear way with the plasma power and that the atom-to-ion fraction increases with both the power and the gas flow. To study the effect of plasma species in the remote region, near the sample position, an electrostatic quadrupole analyzer was used to gauge fluxes of O2+, O+, O2−, and O−. Even a moderate increase in pressure can drastically reduce the ion flux toward the substrate. The formation of bubbles or blisters in films can be linked to ion-induced compressive stress, and, hence, it can be mitigated by an increase in the gas pressure. Finally, Al2O3 was deposited in lateral high-aspect ratio structures to investigate the effect of plasma power and gas pressure on the partial pressure of radical species. Simulated profiles were fitted to experimental deposition profiles to estimate trends in the radical partial pressure, and a linear relationship between radical partial pressure and the power was found. This correlated with the density of atomic O species as observed in the OES measurements in the plasma source region. The methods presented in this work are also applicable to characterize other reactor geometries, plasma sources, and gas mixtures.