**1. Introduction**

Acoustic filters are key components in the evolution of radio frequency communication systems. Quartz crystals, lithium tantalate (LiTiO3) and lithium niobate (LiNbO3) surface acoustic wave (SAW) filters made the first two generations (Global System for Mobile communication, or GSM and Code-Division Multiple Access, or CDMA) of mobile networks possible [1,2], followed by AlN Bulk Acoustic Wave (BAW) resonators for the 3G (WCDMA) and 4G-LTE networks [3]. As the road map to 5G and beyond unfolded, filter requirements have increased and call for filters that exhibit lower insertion loss, higher temperature stability, steeper skirts and wider bandwidth. To achieve these metrics, a new material with enhanced piezoelectric coefficients is needed, since it can lead to more efficient coupling that directly transfers to increased filter bandwidth. In 2001, Takeuchi identified, by first principal calculation, that wurtzite AlScN had the potential [4] to achieve higher piezoelectric coefficients than AlN. Akiyama et al. later showed this through measurement of co-sputtered AlScN films [5–7] that demonstrated a peak d<sup>33</sup> of 27.6 pC/N [6].

**Citation:** Tang, Z.; Esteves, G.; Zheng, J.; Olsson, R.H., III. Vertical and Lateral Etch Survey of Ferroelectric AlN/Al1−xScxN in Aqueous KOH Solutions. *Micromachines* **2022**, *13*, 1066. https://doi.org/10.3390/ mi13071066

Academic Editor: Agne Žukauskait ˙ e˙

Received: 28 May 2022 Accepted: 27 June 2022 Published: 2 July 2022

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Therefore, AlScN has become a competitive candidate for different filter designs targeting 5G NR mmWave (K<sup>a</sup> band, 26 GHz) [8–10] 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/Al1−xScxN 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 BCl3/Cl2/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 Al0.85Sc0.15N film, the etch rate declined to 64% of that of AlN [13]; for Al0.73Sc0.27N, 42% under the same etch condition [14]; and for Al0.64Sc0.36N, 10% of that for AlN [15]. As for single-crystalline Al1−xScxN, 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◦ 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. Table 1 summarizes selected studies on the dry etch rates of AlN/Al1−xScxN:


**Table 1.** Published dry etch rates of AlN/Al1−xScxN in ICP-RIE.

Compared to dry etching, wet etching can be rather advantageous owing to its less expensive tooling. Common etchants include tetramethylammonium hydroxide (TMAH) [19–21], phosphoric acid (H3PO4) [22,23] or phosphoric acid-based solution [21] (PWS, containing 80% H3PO4, 16% H2O and 4% HNO3), potassium hydroxide (KOH) [22,24,25] and AZ400K Developer (contain 10 wt% of KOH) [26]. Among them, we found KOH to be rather attractive due to its availability and non-toxicity with a relatively high etch rate and high etch selectivity to silicon nitride (SiNx). The current research focusing on AlN etching in KOH is abundant, yet very few details were disclosed regarding the etching of Al1−xScxN, let alone a complete survey of its etch rate as a function of Sc alloying. For the data available, the etch rate varies substantially based on the temperature, crystallinity and etchant concentration. Table 2 summarizes previous studies on the etch rates of AlN/Al1−xScxN in aqueous KOH.


**Table 2.** Published vertical etch rates of AlN/Al1−xScxN in aqueous KOH solutions.

Note: For comparison, the etch rates were converted to 45 ◦C with activation energy *E<sup>a</sup>* given by the author, or assuming *E<sup>a</sup>* = 15.85 kcal/mol if data were unavailable.

The lack of variable control and the limited data make it difficult to draw solid conclusions on the factors affecting the etch rate, though from the AlN/Al1−xScxN dry etch results and the KOH wet etch of its III–V alloy kin Al1−xInxN [27]/Al1−xGaxN [29], one would assume that the etch rate decreases with the increasing Sc concentration. Moreover, even though prior etching studies reported the anisotropic nature of the etch, with preferential etching of the c-plane 0001 in N-polar AlN/Al1−xScxN [28,29] while exposing the 1011 planes [31] from whose boundary the sidewall that follows is the 1212 of the hexagonal crystal structure, lateral etch rates have not been reported. To date, there have been but a few papers [32,33] discussing KOH etching of AlN/Al1−xScxN in detail, and only AlN and Al0.80Sc0.20N were studied. Therefore, we report a thorough survey on the vertical and lateral etch rates of sputtered AlN/Al1−xScxN in aqueous KOH solutions vs. scandium concentrations, where the KOH concentrations and solution temperatures were strictly regulated. We found that the vertical etch rate declines steadily with increasing scandium concentration, whereas the lateral etch rate experiences a V-shaped transition with a minimum value of 0.043 ± 0.002 nm/s at x = 0.125. By etching the Al0.875Sc0.125N film in 10 wt% KOH at 45 ◦C for 10 min, a nearly 90◦ sidewall was produced by exposing the 1100 planes. This technique is capable of generating a vertical sidewall without pre-treatment and could be beneficial for the fabrication of numerous kinds of microelectromechanical systems (MEMS) device such as lamb wave resonators (LWRs), laser mirrors and Ultraviolet Light-Emitting Diodes (UV-LEDs).

The findings are presented in four sections: (1) Introduction, containing the problem selected, literature review, novelty and section description; (2) Experiment, containing the deposition methods, film characterization, etch mask patterning, wet etching process steps and data interpretation methodology; (3) Results and Analysis, containing the illustration of results and detailed analysis; (4) Conclusion, summarizes the paper and the novelty of the research.
