Abnormal Grain Growth in AlScN Thin Films Induced by Complexion Formation at Crystallite Interfaces

Abstract

Sputter deposited Al (1-x) Sc x N thin films with a Sc content from x ¼ 0 to 43 at% are investigated by electron microscopy in order to study and explain the formation and growth of abnormally oriented grains (AOG). It is found that the latter did not nucleate at the interface with the substrate, but at high energy grain boundaries, at which systematically higher Sc concentrations are detected. The AOGs are thus formed during the growth of c-textured grains. They grow faster than those, and finally protrude from the c-textured film surface, having at their end a pyramidal shape with three facets of a hexagonal wurtzite crystal: one (0001) and two (11 20) facets. Process conditions favoring less compact grain boundaries, and lower surface diffusion across grain boundaries are thought to promote nucleation of AOGs. Finally, a 4-step growth mechanism explaining the nucleation from a Sc-rich complexion and proliferation of AOGs with increasing film thickness is proposed.

Material Parameter Extraction for Complex AlScN Thin Film Using Dual Mode Resonators in Combination with Advanced Microstructural Analysis and Finite Element Modeling

“…In our case, however, the instability does not occur at the interface, but in later growth stages (with a few exceptions), as explained in ref. . Interestingly, when making the same deposition on a wafer that is fully covered with a Pt layer, such abnormally oriented grains are not observed with this composition.…”

“…In ref. it is proposed that Sc concentration fluctuations at grain boundaries may lead to an epitaxial nucleation of a ScN complexion in the rocksalt phase. This one switches back to the wurtzite phase with growing grain size, because the growing grain must adapt to the average Sc concentration.…”

“…We show that this can be achieved with a combination of finite element modeling and transmission electron microscope (TEM) nanodiffraction mapping. The latter point is of scientific interest for the extraction of physical properties of films with a composition in the transition region between wurtzite and rocksalt structure, where it is probably not possible to achieve phase pure films with no abnormally oriented grains …”

“…However, the growth of AlScN films is more delicate as compared to AlN films. The main problem is the occurrence of abnormally oriented grains, which are due to secondary nucleation during film growth caused by some irregularities impeding good surface diffusion of adatoms . A further challenge in AlScN thin film microacoustic devices is the required knowledge of all relevant materials constants in order to design novel devices as a function of composition, particularly for novel types of resonators with more complex mode shapes of the lamb type.…”

Clandestine nanoelectromechanical tags for identification and authentication

“…Furthermore, although purposeful layout variations can help enhance the entropy of the tags, they add additional steps to the implementation of the labels and increase their cost. In this work, the Sc 0.3 Al 0.7 N film used for implementation of the NEMS tags offers large intrinsic uncertainties due to the purely randomized formation of cubic conical clusters within the hexagonal grains 16,17 . This inherent characteristic corresponds to the tendency of the film morphology transition from hexagonal aluminum nitride (AlN) to cubic scandium nitride at high concentrations of scandium doping.…”