2.1.1. Growth

Based on the crystallinity of AlN/Al1−xScxN, several methods can be used for film growth. Single-crystal AlN can be grown via physical vapor transport (PVT) on substrates up to 60 mm in diameter [34–36]. Single-crystal Al1−xScxN can be grown by molecular beam epitaxy (MBE) [9] on 100 mm wafers [37] with scandium concentration x from 0.06 to 0.36 [38]. The growth of Sc alloyed AlN via metal–organic chemical vapor deposition

(MOCVD) had suffered from the lack of Sc precursors [39]; however, research in this field is catching up quickly, and the ability to grow 36% Sc alloyed Al1−xScxN films on 100 mm wafers has been demonstrated [40,41]. High-quality polycrystalline Al1−xScxN films can be deposited with physical vapor deposition (PVD) methods such as magnetron sputtering. This method features a high deposition rate, low growth temperature, and the capability of up-scaling in substrate size [42–46], and has been adopted by a variety of tool manufacturers for industrial mass production [47].

Al1−xScxN depositions were performed in an Evatec CLUSTERLINE® 200 II Physical Vapor Deposition System (Evatec AG, Trübbach, Switzerland) at a substrate temperature of 350 ◦C with 150 kHz pulsed DC bias with an 88% duty cycle. No RF substrate bias was applied and the AlScN materials were deposited directly onto silicon substrates. No precleans were applied to the substrates. Two 100 mm metal targets were used, Al (99.999%) and Sc (99.99%), with a target-to-substrate distance of 88.5 mm. Before deposition, the chamber was pumped to a base pressure lower than 1.0 <sup>×</sup> <sup>10</sup>−<sup>7</sup> mbar. A 15 nm AlN film was deposited first onto a 100 mm Si (100) wafer as the seed layer by sputtering Al in a pure nitrogen environment with a target power of 1000 W and N<sup>2</sup> flow of 20 sccm. Subsequently, a 35 nm linearly graded Al1−xScxN layer was deposited by gradually increasing the Sc target power while maintaining the Al target power constant. Finally, a 750 nm bulk Al1−xScxN layer was deposited by fixing both the Al and Sc target powers. The 20 sccm N<sup>2</sup> flow was maintained during the process and no Ar was used throughout. The chamber pressure remained close to 8.0 <sup>×</sup> <sup>10</sup>−<sup>4</sup> mbar during the deposition. A total of 15 films were deposited with Sc concentration ranging from 0% to 42%. Table 3 summarizes the correlation between Sc target power and Sc concentration based on our previous research [48].

**Table 3.** Scandium concentration vs. Sc target power.


### 2.1.2. Surface Metrology

The adoption of the AlN seed layer, the Al1−xScxN gradient layer, and pure nitrogen sputtering environment greatly reduced the occurrence of abnormally oriented grains (AOGs). AOGs are a series of wurtzite Al1−xScxN crystals that do not have their c-axis perpendicular to the substrate [49]. They could erupt from the crystalline interface if grown under unfavorable conditions, especially for films with higher Sc concentration [50]. If not suppressed, they may occupy the entire film surface [49,51,52], severely degrading device performance [53,54] and locally slowing the etch rate. To examine the film quality, atomic force microscopy (AFM) scans were conducted using a Bruker Icon AFM, and most of the films measured showed a roughness of <2 nm. Figure 1a shows the surface of an Al0.64Sc0.36N film within a 20 <sup>×</sup> <sup>20</sup> <sup>µ</sup>m<sup>2</sup> field. The root mean square (RMS) surface roughness R<sup>q</sup> is 0.840 nm and R<sup>a</sup> is 0.641 nm. The film quality is also supported by the rocking curve measurements, which were performed by a Rigaku Smart Lab X-RAY Diffractometer (XRD) with a high resolution Parallel Beam (PB) Ge (220) × 2 monochromator (Rigaku Corporation, Tokyo, Japan). The Omega scan data are centered at 18.13 ◦ with a full width at half maximum (FWHM) of 1.80 ◦ for a 500 nm film deposited on Si (100) using the same process, indicating that the film is highly *c*-axis textured. All films had a similar quality based on the AFM and XRD measurement.

**Figure 1.** (**a**) AFM of the Al0.64Sc0.36N Film. (**b**) Rocking curve measurement (Omega Scan) of an Al0.64Sc0.36N film. **Figure 1.** (**a**) AFM of the Al0.64Sc0.36N Film. (**b**) Rocking curve measurement (Omega Scan) of an Al0.64Sc0.36N film.

#### *2.2. Film Patterning 2.2. Film Patterning*

To selectively etch AlN/Al1−xScxN, the film must be patterned so that only locations of interest would be exposed in the etchant. Silicon nitride (SiNx) was chosen as our hard mask, as it has an etch rate of 0.67 nm/min in 30% KOH at 80 °C [55], which is negligible compared to the etch rates of Al1−xScxN. A 200 nm SiN<sup>x</sup> film was deposited on top of the Al1−xScxN film in an Oxford Plasma Lab 100 Plasma-enhanced Chemical Vapor Deposition (PECVD) machine (Oxford Instruments Plasma Technology, Bristol, United Kingdom), and on the backside of the wafer as well to protect the Si substrate during the etch. Afterwards, photoresist MICROPOSIT® S1813 was spin-coated and exposed in a Karl Süss MA6 Mask Aligner (SÜ SS MicroTec SE, Garching, Germany) via contact lithography. The exposed film was developed in TMAH-0.26N developer, then transferred to an Oxford 80 plus Reactive-ion Etching (RIE) etcher (Oxford Instruments Plasma Technology, Bristol, United Kingdom). A 30 s O<sup>2</sup> descum was performed first to remove the remaining resist, followed by 3 min of SiN<sup>x</sup> etch with CHF3/O<sup>2</sup> mixture. Finally, the resist was stripped in MICROPOSIT® remover 1165 with ultrasonic bath at 60 °C and plasma cleaned. Figure 2 illustrates the fabrication process. To selectively etch AlN/Al1−xScxN, the film must be patterned so that only locations of interest would be exposed in the etchant. Silicon nitride (SiNx) was chosen as our hard mask, as it has an etch rate of 0.67 nm/min in 30% KOH at 80 ◦C [55], which is negligible compared to the etch rates of Al1−xScxN. A 200 nm SiN<sup>x</sup> film was deposited on top of the Al1−xScxN film in an Oxford Plasma Lab 100 Plasma-enhanced Chemical Vapor Deposition (PECVD) machine (Oxford Instruments Plasma Technology, Bristol, UK), and on the backside of the wafer as well to protect the Si substrate during the etch. Afterwards, photoresist MICROPOSIT®S1813 was spin-coated and exposed in a Karl Süss MA6 Mask Aligner (SÜSS MicroTec SE, Garching, Germany) via contact lithography. The exposed film was developed in TMAH-0.26N developer, then transferred to an Oxford 80 plus Reactive-ion Etching (RIE) etcher (Oxford Instruments Plasma Technology, Bristol, UK). A 30 s O<sup>2</sup> descum was performed first to remove the remaining resist, followed by 3 min of SiN<sup>x</sup> etch with CHF3/O<sup>2</sup> mixture. Finally, the resist was stripped in MICROPOSIT® remover 1165 with ultrasonic bath at 60 ◦C and plasma cleaned. Figure 2 illustrates the fabrication process.
