Growth characteristics, optical properties, and crystallinity of thermal and plasma-enhanced ALD AlN films

Using real-time in-situ spectroscopic ellipsometry and ex-situ atomic force microscopy and X-ray diffraction, we have investigated the growth characteristics, especially ocusing on the initial growth (nucleation) regime, optical properties and crystalline structure of thin films of aluminum nitride (AlN) grown on Si(111) by atomic layer deposition (ALD) in both thermal and plasma-enhanced modes. For the thermal ALD, trimethylaluminum (TMA) and ammonia (NH3) were used as precursors. The self-limiting growth was observed in a very narrow temperature range (i.e. 330−350 °C). The plasma-enhanced ALD (PEALD) was performed using TMA and either H2/N2 or NH3 plasma in the same temperature range. Highly uniform and conformal AlN films were obtained (Fig. 1). We demonstrate that, for the thermal ALD, the nucleation stage is governed by the formation and laterally-preferential growth of 3D islands, which consequently leads to a fully covered surface at a film thickness of about 2 nm, followed by the linear ALD growth. In contrast, the linear growth starts immediately in PEALD and leads to a significant lower surfaces roughness (Fig. 2). The study on optical properties of the AlN films showed a strong dependence of the refractive index on the ALD modes used (Fig. 3) and film thickness. In the presentation, we will discuss in detail (1) the growth characteristics, including the nucleation regime, closure and linear growth, (2) the evolution of optical constants during the growth; and (3) the crystalline structures of the AlN films grown in different ALD modes.