Autonomous Synthesis of Sputtered Nitride Thin Films with Controlled Cation Composition

Autonomous experimentation has emerged as an efficient approach to accelerate the pace of materials discovery. Although instruments for autonomous synthesis have become popular in molecule and polymer science, solution processing of hybrid materials and nanoparticles, examples of autonomous tools for physical vapor deposition (PVD) are scarce yet important for the semiconductor industry.<br/>Although some of the few existing reports of autonomous PVD focused on the optimization of material properties such as resistivity and crystallinity, precise control of cation and anion composition in inorganic thin films is of paramount importance. For example, it has been theoretically predicted and experimentally demonstrated that short-range ordering tunes the optical absorption edge in the long-range disordered alloy (ZnSnN)_1-x(ZnO)_2x at a very specific composition of x = 0.25. Also, the resistivity and bandgap of ternary nitrides and their alloys depend mostly on cation composition.<br/>In this presentation, we will discuss the design and implementation of a closed-loop autonomous workflow for sputter deposition of thin films with controlled composition, leveraging a highly automated sputtering reactor custom-controlled by Python, optical emission spectroscopy (OES), and Bayesian optimization. By fabricating Zn_xTi_1-xN_y thin films with simultaneous monitoring of optical emission lines from the co-sputtering of elemental targets, we will show that cation composition, spanning a wide range, can be expressed as a linear function of emission lines.<br/>Informed by OES, a control algorithm with Bayesian optimization as its decision-making agent can effectively optimize the power on each sputtering source, achieving deviations from the targeted cation composition within relative 3.5 %, even for 15 nm-thick films, which demonstrates that the methods described in this work can reliably synthesize thin films with specific composition and minimal human interference.<br/>Moreover, we will show that our approach can enhance reproducibility in the synthesized films by assuring that the desired composition will be achieved regardless of the N₂/Ar mixture and total power applied on elemental targets. Finally, we will also discuss how substrate effects and chamber pressure affect the calibration of the current approach and suggest future improvements of its accuracy.<br/><br/>This presentation will be based on the following preprint: https://arxiv.org/abs/2305.11122