Stefan Mertin

Properties of AlScN thin films for hybrid BAW/SAW resonator fabrication

New hybrid bulk-surface acoustic wave resonators using scandium doped aluminium nitride thin films were simulated, using material constants from piezoelectric measurement techniques for thin films. Afterwards they were fabricated and an electromechanical coupling of 4.3% was achieved.

Enhanced piezoelectric properties of c-axis textured aluminium scandium nitride thin films with high scandium content: Influence of intrinsic stress and sputtering parameters

Aluminium scandium nitride (ASN) exhibits a largely enhanced piezoelectric response as compared to aluminium nitride (AlN), which makes it an upcoming piezoelectric material for next generation RF filters, sensors, actuators and energy harvesting devices. In this work, process-microstructure-property relationships of such ASN films containing up to 40 at% Sc were investigated. Hereby, the influence of the process parameters on the film structure, the intrinsic stress and the piezoelectric response was carefully investigated.

Ex-situ AlN seed layer for (0001)-textured Al 0.85 Sc 0.15 N thin films grown on SiO 2 substrates for shear mode resonators

Partial substitution of Al by Sc in AlN wurtzite films leads to a strong enhancement of the piezoelectric properties as long as the wurtzite phase is maintained. This is very promising for improving piezoelectric MEMS devices and enlarging their application range. Nucleation of (0001)-AlScN works particularly well on Pt (111) thin films. However, in some applications, the growth on insulating substrates may be required. Even though AlN can be grown well oriented on smooth amorphous surfaces of SiO2, it is not the case for AlScN. AlN seed layers are an evident solution when there is no vacuum break between the AlN and AlScN growth. However, when a vacuum break is unavoidable, an oxide layer is formed on AlN, which makes the “regrowth” difficult. In this contribution we show that a mild RF etch can solve the problem.

Ex-situ AlN seed layer for (0001)-textured Al 0.84 Sc 0.16 N thin films grown on SiO 2 substrates

It is more difficult to nucleate AlN-ScN alloy thin films (AlScN) in pure (0001)-texture than it is with pure AlN thin films. AlN thus can serve as seed layer for AlScN. Equipment limitations may lead to the problem of a vacuum break between AlN and AlScN deposition, as it leads to oxidation of the AlN surface. This issue was studied with high resolution TEM and electron diffraction. The formed oxide layer disturbed a lot the epitaxial growth, leading to additional grain orientations. A mild RF etching step introduced before AlScN deposition was able to remove the oxide layer, and allowed for growing Al0.84Sc0.16N in local epitaxy on AlN, as shown by Hyper map EDX images. The resulting AlScN films show a pure (0001) texture. Double beam laser interferometry and finite element modeling were used to determine d33, f of both layers together as 6.6 pm/V, and of 6.85 pm/V for AlScN alone when using the standard value of 3.9 pm/V for pure AlN. At the same time, the relative dielectric constant of Al0.84Sc0.16N was determined as 14.1.

Sputter deposition technology for Al (1−x) Sc x N films with high Sc concentration

Aluminium scandium nitride (Al1−xScxN) with its strongly enhanced piezoelectric response is the upcoming piezoelectric material of choice in next generation RF filters, sensors, actuators and energy harvesting devices. This paper will concentrate on the deposition technology for Al1−xScxN films with high Sc content. Films with Sc concentrations close to 43 at% have been grown on 200-mm substrates using a cluster type sputter deposition tool. The piezoelectric response will be discussed and correlated with the deposition parameters and film structural properties. The steps required to deliver a high-volume production solution for high Sc concentration will be described.