The use of plasma-enhanced ALD (PE-ALD) is known to enhance nucleation due to the surface-activating effect of plasma and can be expected to significantly reduce subsurface diffusion, enabling the surface-limited deposition of thin conformal coatings. However, plasma can also have detrimental effects on polymeric substrates, such as polymer degradation or etching.
Our work contributes to a deeper understanding of how oxygen plasma applied during PE-ALD affects film formation and initial growth of ZnO on polymer substrates. In-situ spectroscopic ellipsometry was used as the main technique to monitor the PE-ALD growth of ZnO on selected polymer thin films. To better understand how the chemical structure of the polymer influences plasma-substrate interactions and the ZnO thin film formation, both crosslinked and linear polymers exhibiting varying degrees of reactivity with the ALD precursor were studied.
Our results show that while the plasma efficiently activates the polymer surface to enable rapid ZnO nucleation, it also causes significant substrate etching with etching rates of up to 8 nm/10 s plasma pulse at 100 W plasma power. As a consequence, ZnO film formation on the polymers is a superposition of two competing processes: plasma etching of the polymer substrate and ALD growth of ZnO. During the initial stage of deposition, polymer etching dominates until, at a certain point, ZnO growth takes over and the regime of normal ALD growth behavior is entered. The strength of etching was found to be proportional to the applied plasma power and strongly depends on the type of polymer. Despite the initial etching, the resulting thin films (20-50 nm thick) exhibit sharp interfaces and a quality, in terms of surface roughness, crystallinity and ZnO density, comparable to those of ZnO deposited on silicon.
A closer examination of the first 25 PE-ALD cycles showed that, on the nanometer scale, the simultaneous etching of the polymer and ZnO nucleation leads to a certain degree of intermixing at the interfaces, the extent of which depends on the type of polymer. It was also revealed that, during the first few cycles, no stochiometric ZnO is yet formed but instead Zinc is found to be bonded to hydroxyl groups and presumably oxygen-carbon species from the polymer, forming hybrid bonds. This points to a strong interaction between the polymer substrate and the forming ZnO, which can be expected to result in good film adhesion, a property that is critical in all applications involving mechanical stress and strain.