*Article* **Vertical and Lateral Etch Survey of Ferroelectric AlN/Al1**−**xScxN in Aqueous KOH Solutions**

**Zichen Tang <sup>1</sup> , Giovanni Esteves <sup>2</sup> , Jeffrey Zheng <sup>3</sup> and Roy H. Olsson III 1,\***


**Abstract:** Due to their favorable electromechanical properties, such as high sound velocity, low dielectric permittivity and high electromechanical coupling, Aluminum Nitride (AlN) and Aluminum Scandium Nitride (Al1−xScxN) thin films have achieved widespread application in radio frequency (RF) acoustic devices. The resistance to etching at high scandium alloying, however, has inhibited the realization of devices able to exploit the highest electromechanical coupling coefficients. In this work, we investigated the vertical and lateral etch rates of sputtered AlN and Al1−xScxN with Sc concentration x ranging from 0 to 0.42 in aqueous potassium hydroxide (KOH). Etch rates and the sidewall angles were reported at different temperatures and KOH concentrations. We found that the trends of the etch rate were unanimous: while the vertical etch rate decreases with increasing Sc alloying, the lateral etch rate exhibits a V-shaped transition with a minimum etch rate at x = 0.125. By performing an etch on an 800 nm thick Al0.875Sc0.125N film with 10 wt% KOH at 65 ◦C for 20 min, a vertical sidewall was formed by exploiting the ratio of the 1011 planes and 1100 planes etch rates. This method does not require preliminary processing and is potentially beneficial for the fabrication of lamb wave resonators (LWRs) or other microelectromechanical systems (MEMS) structures, laser mirrors and Ultraviolet Light-Emitting Diodes (UV-LEDs). It was demonstrated that the sidewall angle tracks the trajectory that follows the 1212 of the hexagonal crystal structure when different *c*/*a* ratios were considered for elevated Sc alloying levels, which may be used as a convenient tool for structure/composition analysis.

**Keywords:** aluminum scandium nitride (AlScN); aluminum nitride (AlN); wet etch; potassium hydroxide (KOH); ferroelectric
