The role of plasma in plasma-enhanced atomic layer deposition of crystalline films

The inclusion of plasma in atomic layer deposition processes generally offers the benefit of substantially reduced growth temperatures and greater flexibility in tailoring the gas-phase chemistry to produce specific film characteristics. The benefits plasmas provide, however, come at the cost of a complex array of process variables that often challenge the ability to predict, a priori, the influence of any one input parameter. In this work, the authors attempt to provide some clarity as to how plasmas are formed and controlled and how they can most optimally be employed within the framework of atomic layer deposition. To begin, the authors cover some of the fundamentals of plasma generation along with the production of energetic and reactive species and their transport within the plasma. They then focus on how different plasma generation schemes and geometries, often employed in plasma-enhanced atomic layer deposition (PEALD), differ in their production of energetic and reactive species. They also address the plasma-surface interactions that are critical for film growth and control of crystallinity. Throughout this work, the authors use both current experimental data and a review of previously published works to describe how variations in the approach to plasma generation and the interactions between plasma-produced species and the growth surface influence the plasma reactant step in PEALD processes. The authors highlight two case studies to demonstrate how these relationships can be used to control the phase purity of crystalline titanium dioxide (TiO2) films and grow crystalline growth of semiconducting indium nitride (InN).