**1. Introduction**

In 2009, Akiyama et al. first reported on the Sc doping of AlN films by the cosputtering process [1]. These AlXSc1-XN films exhibited significantly increased piezoelectric properties depending on the Sc concentration with a maximum at 43 at.% Sc (Al + Sc = 100 at.%). Since then, research and development on AlScN films has increasingly gained attention from research groups as well as industry worldwide. The focus is mostly on MEMS [2,3] and acoustic wave applications [4,5], but energy harvesting applications [6–8] are also gaining traction.

Furthermore, Fichtner et al. demonstrated in 2019 the possibility of ferroelectric switching in AlScN films with scandium concentrations between 27 at.% and 43 at.% [9]. The ferroelectric switching occurs at coercive fields of 2–5 MV/cm, with higher Sc concentrations showing reduced necessary fields for switching but in turn also lower polarization values from above 100 µC/cm<sup>2</sup> to around 80 µC/cm<sup>2</sup> . Further research by different groups worldwide is ongoing [10,11].

One aspect that still proves very challenging for the development of AlScN deposition processes is the increasing probability of the formation of abnormally oriented grains (AOG) at the grain boundaries, especially at higher Sc concentrations [12], tensile film stress [5] and higher film thickness [12,13]. This limits the process window in which films with good piezoelectric properties can be fabricated as well as the possible applications. Therefore,

**Citation:** Barth, S.; Schreiber, T.; Cornelius, S.; Zywitzki, O.; Modes, T.; Bartzsch, H. High Rate Deposition of Piezoelectric AlScN Films by Reactive Magnetron Sputtering from AlSc Alloy Targets on Large Area. *Micromachines* **2022**, *13*, 1561. https://doi.org/10.3390/ mi13101561

Academic Editor: Agne Žukauskait ˙ e˙

Received: 29 July 2022 Accepted: 13 September 2022 Published: 21 September 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

to further extend the material and process range and enable new applications, additional process parameters and adjustment options are necessary. One such process parameter is the pulse mode of powering in reactive pulse magnetron sputter processes. Usually, the configuration can either be unipolar or bipolar pulse mode. This results in different plasma conditions and consecutively film properties. By developing and using a unipolar–bipolar hybrid pulse mode, Barth et al. could freely influence film properties of cosputtered AlScN films from Al and Sc targets in a much wider range, realizing highly oriented films with thicknesses of up to several 10 µm [14]. This paper investigates if such pulse mode variation can be used to deposit highly oriented Al0.7Sc0.3N films from Al0.7Sc0.3 alloy targets with a high deposition rate and good homogeneity on a large area with adjustable film properties in a wider range. This would offer new application opportunities, e.g., in thicker films of several µm for energy harvesting or ultrasonic devices or as stress-optimized thin films in multilayer stacks. parameter is the pulse mode of powering in reactive pulse magnetron sputter processes. Usually, the configuration can either be unipolar or bipolar pulse mode. This results in different plasma conditions and consecutively film properties. By developing and using a unipolar–bipolar hybrid pulse mode, Barth et al. could freely influence film properties of cosputtered AlScN films from Al and Sc targets in a much wider range, realizing highly oriented films with thicknesses of up to several 10 µm [14]. This paper investigates if such pulse mode variation can be used to deposit highly oriented Al0.7Sc0.3N films from Al0.7Sc0.3 alloy targets with a high deposition rate and good homogeneity on a large area with adjustable film properties in a wider range. This would offer new application opportunities, e.g., in thicker films of several µm for energy harvesting or ultrasonic devices or as stressoptimized thin films in multilayer stacks. **2. Materials and Methods** 

(AOG) at the grain boundaries, especially at higher Sc concentrations [12], tensile film stress [5] and higher film thickness [12,13]. This limits the process window in which films with good piezoelectric properties can be fabricated as well as the possible applications. Therefore, to further extend the material and process range and enable new applications, additional process parameters and adjustment options are necessary. One such process

*Micromachines* **2022**, *13*, x 2 of 10

### **2. Materials and Methods** *2.1. Film Deposition*

#### *2.1. Film Deposition* The films were deposited using a reactive pulse magnetron sputter process described

The films were deposited using a reactive pulse magnetron sputter process described in [14]. The double-ring magnetron DRM 400 uses two concentric targets, whose discharges overlap to deposit homogenously on a wide area. Thus, films can be deposited with a very high film growth rate of 200 nm/min with film thickness homogeneity of up to 0.5% on a Ø200 mm area. Film thicknesses can be several tens of microns. By using the pulse unit UBS-C2 developed by FEP and standard DC power supplies, the pulse mode of the pulse magnetron sputtering can be adjusted as unipolar, bipolar or as a unipolar–bipolar hybrid pulse mode. In unipolar pulse mode, a pulsed dc voltage is applied between each of the two targets acting as cathodes and the separate hidden anode. In bipolar pulse mode, the two targets are alternately the cathode and the anode. In hybrid pulse mode, different shares of unipolar (Su) or bipolar (Sb) pulse modes can be applied in a period of 1 ms to influence plasma parameters. The share of unipolar pulse mode is defined as the ratio between the time fraction of unipolar mode t<sup>u</sup> with respect to the total time of one cycle of unipolar–bipolar hybrid pulse mode, i.e., Su = tu/(t<sup>u</sup> + tb). The plasma parameters can be adjusted in a wide range between both pure pulse modes, as shown in the plasma density measurements of an example process of Si sputtering in Figure 1. in [14]. The double-ring magnetron DRM 400 uses two concentric targets, whose discharges overlap to deposit homogenously on a wide area. Thus, films can be deposited with a very high film growth rate of 200 nm/min with film thickness homogeneity of up to 0.5% on a Ø200 mm area. Film thicknesses can be several tens of microns. By using the pulse unit UBS-C2 developed by FEP and standard DC power supplies, the pulse mode of the pulse magnetron sputtering can be adjusted as unipolar, bipolar or as a unipolar– bipolar hybrid pulse mode. In unipolar pulse mode, a pulsed dc voltage is applied between each of the two targets acting as cathodes and the separate hidden anode. In bipolar pulse mode, the two targets are alternately the cathode and the anode. In hybrid pulse mode, different shares of unipolar (Su) or bipolar (Sb) pulse modes can be applied in a period of 1 ms to influence plasma parameters. The share of unipolar pulse mode is defined as the ratio between the time fraction of unipolar mode tu with respect to the total time of one cycle of unipolar–bipolar hybrid pulse mode, i.e., Su = tu/(tu + tb). The plasma parameters can be adjusted in a wide range between both pure pulse modes, as shown in the plasma density measurements of an example process of Si sputtering in Figure 1.

**Figure 1.** Plasma density in dependence of share of unipolar pulse mode in unipolar–bipolar hybrid pulse mode operation (example of Si process, adapted from [14]). **Figure 1.** Plasma density in dependence of share of unipolar pulse mode in unipolar–bipolar hybrid pulse mode operation (example of Si process, adapted from [14]).

In the case of AlScN film depositions, AlSc targets (3N5) with Sc contents of 30 at.% were used. Argon and nitrogen (5N) were used as inert and reactive gas, respectively. The gas flow was between 20 and 60 sccm, and the pressure was between 0.3 Pa and 0.8 Pa. A closed-loop reactive gas control was applied to stabilize the process in the

transition mode [15]. The share of unipolar pulse mode in hybrid pulse mode operation was investigated between 60% and 90% of the period. The chamber base pressure was <sup>2</sup> <sup>×</sup> <sup>10</sup>−<sup>7</sup> mbar. Before coating each sample, a precleaning of the substrate surface was performed using rf bias etching in an Ar atmosphere. There was no additional substrate heating or cooling beyond water cooling of the substrate platform applied.
