Understanding the role of rf-power on AlN film properties in hollow-cathode plasma-assisted atomic layer deposition

In this study, the authors have carried out real-time process monitoring via in situ ellipsometry to understand the impact of rf-plasma power and plasma exposure time on self-limiting aluminum nitride (AlN) growth character and the corresponding film properties. AlN thin films were grown on Si(100) substrates with plasma-enhanced atomic layer deposition using trimethyl-aluminum (TMA) as a metal precursor and Ar/N-2/H-2 plasma as a coreactant. Saturation experiments have been employed in the range of 25-200 W plasma power and 30-120 s plasma exposure time. In situ multiwavelength ellipsometry identified single chemical adsorption (chemisorption) and plasma-assisted ligand removal events, as well as changes in growth per cycle (GPC) with respect to plasma power. The real-time dynamic in situ monitoring study revealed that GPC and TMA chemisorption thickness gain exhibited plasma power dependent saturation behavior. The amount of chemisorption saturated at similar to 2.3 angstrom for higher rf-power levels, while for 25 and 50 W it went below 1.0 angstrom, which is mainly attributed to incomplete ligand removal. Besides in situ characterization, ex situ measurements to identify optical, structural, and chemical properties were also carried out on 500-cycle AlN films as a function of plasma power. AlN samples displayed a single-phase hexagonal wurtzite crystal structure with (002) preferred orientation for 150 and 200 W, while the dominant orientation shifted toward (100) at 100 W. 50 W and lower rf-power levels resulted in amorphous material with no apparent crystal signature. Furthermore, it was found that when the plasma exposure time was increased from 30 to 120 s for 25 and 50 W, the amount of chemisorption exceeded the thickness gain values recorded for 150-200 W (similar to 2.4 angstrom). However, such a recovery in the chemisorption thickness gain did not restore the crystallinity as the AlN films grown at sub-50 W showed amorphous character independent of plasma exposure time.