Abstract
Aluminum scandium nitride (Al1-xScxN) thin films are commonly applied in piezoelectric MEMS transducers such as RF filters, microphones, PMUTs etc. due to their enhanced piezoelectric properties compared to AlN, low acoustic loss, thermal stability, and CMOS compatibility [1]. In particular, for RF filters, ensuring good c-axis orientation of Al1-xScxN with high Sc content (x=30 at. %) is a key step towards reducing device dimension and thus reaching higher filter resonant frequencies while increasing quality factor and electromechanical coupling coefficient [2]. Current routes for improving structural properties of AlN and AlScN mostly involve application of an AlN seed layer [3] or a gradient seed layer [4]. In this work we present the results of applying a novel hafnium (Hf) seed layer to grow AlN and Al0.7Sc0.3N thin films with strong c-axis orientation on a (111) commercial n-Si substrate.
For this investigation, Hf, AlN and Al0.7Sc0.3N were deposited by pulsed DC magnetron sputtering using a multi-target chamber within an Evatec CLUSTERLINE® 200 II. Hf seed layers with thicknesses between 20 nm and 150 nm were studied and deposited at 450°C. Hf thin film texture and surface morphology were investigated with respect to the sputtering power and the Ar flow. 700-nm AlN and Al0.7Sc0.3N were deposited onto the optimized hafnium layers. The microstructure and texture quality of Hf, AlN and AlScN films were characterized using SEM, EDS AFM, TEM, XRD and optical reflectometry methods.
The analysis of the surface morphology and rocking curve of Hf(0002) shows that low argon flow and power promote the c-axis orientation of the seed layer. The introduction of the seed layer shows significant reduction of ω-FWHM and the abnormal grain density for the overgrown Al(Sc)N films in contrast to the films directly grown on silicon substrate. It confirms that Hf seed provides a good texture template for AlN and AlScN piezoelectric thin films. In particular, by introducing an optimized 20-nm Hf(0002) seed layer rocking curve FWHM of 1° for AlN(0002) and 1.2° for Al0.7Sc0.3N(0002) were achieved.
In future work, we plan to investigate the piezoelectric properties of AlN and Al0.7Sc0.3N with Hf seed of different thicknesses as we have observed that the average grain size of the overgrown piezoelectric layer is proportional to the thickness of the seed layer.
[1] Q. Zhang et al., Materials 14.21 (2021): 6437
[2] M. Park et al., JMEMS 29.4 (2020): 490-498
[3] Su, Jingxiang, et al. Micromachines 13.5 (2022): 783
[4] Beaucejour, Rossiny, et al. JMEMS (2022)
Corresponding author: sarah.risquez@silicon-austria.com
For this investigation, Hf, AlN and Al0.7Sc0.3N were deposited by pulsed DC magnetron sputtering using a multi-target chamber within an Evatec CLUSTERLINE® 200 II. Hf seed layers with thicknesses between 20 nm and 150 nm were studied and deposited at 450°C. Hf thin film texture and surface morphology were investigated with respect to the sputtering power and the Ar flow. 700-nm AlN and Al0.7Sc0.3N were deposited onto the optimized hafnium layers. The microstructure and texture quality of Hf, AlN and AlScN films were characterized using SEM, EDS AFM, TEM, XRD and optical reflectometry methods.
The analysis of the surface morphology and rocking curve of Hf(0002) shows that low argon flow and power promote the c-axis orientation of the seed layer. The introduction of the seed layer shows significant reduction of ω-FWHM and the abnormal grain density for the overgrown Al(Sc)N films in contrast to the films directly grown on silicon substrate. It confirms that Hf seed provides a good texture template for AlN and AlScN piezoelectric thin films. In particular, by introducing an optimized 20-nm Hf(0002) seed layer rocking curve FWHM of 1° for AlN(0002) and 1.2° for Al0.7Sc0.3N(0002) were achieved.
In future work, we plan to investigate the piezoelectric properties of AlN and Al0.7Sc0.3N with Hf seed of different thicknesses as we have observed that the average grain size of the overgrown piezoelectric layer is proportional to the thickness of the seed layer.
[1] Q. Zhang et al., Materials 14.21 (2021): 6437
[2] M. Park et al., JMEMS 29.4 (2020): 490-498
[3] Su, Jingxiang, et al. Micromachines 13.5 (2022): 783
[4] Beaucejour, Rossiny, et al. JMEMS (2022)
Corresponding author: sarah.risquez@silicon-austria.com
| Originalsprache | Englisch |
|---|---|
| Publikationsstatus | Veröffentlicht - 16 Aug. 2022 |