Combinatorial Synthesis of Highly-Oriented Ferroelectric AlScN Thin Films using Metal-Ion Synchronized High-Power Impulse Magnetron Sputtering.

Piezoelectric micro-electro-mechanical systems (MEMS) are one of the building blocks of modern electronics and are used in many applications such as RF filters, resonators, and sensors. AlScN in wurtzite structure is one of the most promising materials for such applications due to its high-temperature stability and linear frequency response compatibility with CMOS processing.[1] Since the demonstration of ferroelectric switching in AlScN the material has gained a renewed interest for a number of alternative applications.[2]<br/>The functional properties of AlScN, such as the piezoelectric response or the switching behavior are generally improved by high Sc contents. However, under those conditions the heterostructural nature of the alloy system and low Sc miscibility lead to a high degree of structural frustration. Hence, the synthesis of Sc-rich AlScN with high crystallinity and uniform texture remains an important challenge, especially at moderate substrate temperatures.<br/>In this work, we present the utilization of metal-ion synchronized high-power impulse magnetron sputtering (MIS-HiPIMS) along with a combinatorial screening approach to deposit highly oriented AlScN films. A substrate bias potential is used to accelerate the ions onto the growing film. By synchronizing this potential to the metal-rich part of each HiPIMS-pulse we can tailor the ion kinetic energy of the film-forming species, while simultaneously avoiding process-gas incorporation and point-defect formation.[3]<br/>Combinatorial libraries of AlScN are deposited with different biasing conditions as well as substrate temperatures leading to a set of roughly 300 distinct synthesis conditions. The combinatorial libraries are then fully characterized with respect to their phase constitution, structure and composition using state-of-the-art techniques. Residual stress, piezoelectric coefficients and ferroelectric switching behavior are studied for selected samples.<br/>We find, that applying synchronized substrate-bias potentials significantly improves the crystallinity as well as the texture of the AlScN thin films, especially at low deposition temperatures. Depending on the kinetic energy of the constituent metal-ions (i.e. Al and Sc) the residual stress in the films can be varied over a large range from 1.5 GPa (tensile) to -3.0 GPa (compressive). This is strongly correlated with a change in Sc solubility from 42 at.% to 22 at.%, respectively. In addition, we demonstrate that accelerating the ions with potentials as low as -30 V promotes growth along the substrate normal and virtually eliminates misoriented grains.<br/>The synthesis approach demonstrated here can be transferred to many different piezoelectric and ferroelectric thin film materials. Furthermore, the ability to precisely tailor the residual stress and the potential to deposit highly-textured thin films on structured as well as temperature-sensitive substrates might open up exciting applications, <i>e.g.</i> for flexible electronics, in the future.<br/><br/>[1] M. Akiyama <i>et al.</i> <i>Advanced Materials</i> <b>2009, </b>21 (5), 593-596.<br/>[2] S. Fichtner <i>et al.</i> <i>Journal of Applied Physics</i> <b>2019</b>, 125, 114103.<br/>[3] J. Patidar <i>et al.</i> <i>Surface and Coating Technology</i>, <b>2023</b> in press. (arXiv:2301.11183)