Growth mechanism of reactively sputtered aluminum nitride thin films

Aluminum nitride (AlN) thin films grown on the Si (1 0 0) by radio frequency sputtering have been analyzed by X-ray diffractometry, scanning electron microscopy and transmission electron microscopy (TEM). The films of ∼3 μm thick exhibit the [0 0 0 1] preferred growth direction where columnar AlN crystals are grown in a non-epitaxial pattern and aligned almost perpendicular to the SiO2–Si substrate surface. Detailed microstructual analysis from the cross-section TEM of the thin films reveals three areas including the Si substrate (layer (a)), the SiO2 (layer (b)), and the AlN film (layer (c)–(f)). The deposited AlN film appears to consist of four distinct layers characterized by their crystalline phases and grain crystallographic orientations. These include the (c) reaction, (d) transition, (e) alignment, and (f) surface layers. The reaction layer (c) is composed of α-Al2O3 precipitates in an amorphous matrix. Randomly oriented AlN grains start to form in the lower end and become better aligned towards the upper end of the transition layer (d). In layer (e), well-aligned AlN grains have developed to form distinct columnar structure, which continues to grow in size in the surface layer (f). The microstructural observation has enabled us to propose a growth mechanism involving the influence from α-Al2O3.