###### 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 (Al\({}_{\mathbf{1-x}}\)Sc\({}_{\mathbf{x}}\)N) 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 Al\({}_{\mathbf{1-x}}\)Sc\({}_{\mathbf{x}}\)N 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 Al\({}_{\mathbf{0.875}}\)Sc\({}_{\mathbf{0.125}}\)N film with 10 wt% KOH at 65 \({}^{\circ}\)C for 20 min, a vertical sidewall was formed by exploiting the ratio of the \(\left\{10\overline{1}\right\}\) planes and \(\left\{1\overline{1}00\right\}\) 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 \(\left\{\overline{1}2\overline{1}\overline{2}\right\}\) 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. aluminum scandium nitride (AlScN); aluminum nitride (AlN); wet etch; potassium hydroxide (KOH); ferroelectric +
Footnote †: journal: Accepted: 27 June 2022  
Therefore, AlScN has become a competitive candidate for different filter designs targeting 5G NR mmWave (K\({}_{\text{a}}\) band, 26 GHz) [8; 9; 10] or higher. As a result, extensive research has been conducted on its material properties, growth, characterization and device fabrication. Etching is a key step in the fabrication of AlScN devices. Like other III-V nitride alloys, existing etching techniques on AlN/Al\({}_{1-x}\)Sc\({}_{x}\)N can be grouped into two categories: dry etching and wet etching. For dry etching, ion-milling that uses Ar exclusively and Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE), which utilizes BCl\({}_{3}\)/Cl\({}_{2}\)/Ar mixtures, are common methods [11]. The latter is the more routinely used technique for dry etching polycrystalline AlN, and etch rates can reach up to 420 nm/min [12] for an ICP power of 800 W. Nevertheless, this etch rate drops dramatically with increasing scandium concentration. For an Al\({}_{0.85}\)Sc\({}_{0.15}\)N film, the etch rate declined to 64% of that of AlN [13]; for Al\({}_{0.73}\)Sc\({}_{0.27}\)N, 42% under the same etch condition [14]; and for Al\({}_{0.64}\)Sc\({}_{0.36}\)N, 10% of that for AlN [15]. As for single-crystalline Al\({}_{1-x}\)Sc\({}_{x}\)N, the reduction in etch rate occurred much faster: at x = 0.02, the etch rate already reduced to 15% of AlN, and at x = 0.15, it was 12.7% [16]. Not only has the existence of scandium retarded the etch rate, but its non-volatile etching by-products also re-deposit during the etch process, resulting in a roughened and tapered side wall less than 76\({}^{\circ}\) if Ion Beam Etching (IBE) is not used [17; 18]. The poor selectivity requires very thick, hard masks during processing and makes it challenging to stop the AlScN etch on underlying metal electrode materials. Both the slow etch rate and low selectivity can limit the maximum AlScN film thickness realizable in a MEMS process.