Real-time in situ ellipsometric monitoring of aluminum nitride film growth via hollow-cathode plasma-assisted atomic layer deposition

The authors report on the real-time monitoring of self-limiting aluminum nitride growth process by using multiwavelength in situ ellipsometry. Aluminum nitride (AlN) thin films were grown on Si(100) substrates via hollow-cathode plasma-assisted atomic layer deposition (HCPA-ALD) using trimethylaluminum (TMA) and Ar/N-2/H-2 plasma as metal precursor and coreactant, respectively. Growth saturation experiments within 100-250 degrees C temperature range were carried out without interruption as extended single runs featuring 10-cycle subruns for each parameter change. The sensitivity of the multiwavelength ellipsometry provided sufficient resolution to observe not only the minuscule changes in the growth-per-cycle (GPC) parameter, but also the single chemical adsorption (chemisorption) and plasma-assisted ligand removal events. GPC values showed a slight increasing slope within 100-200 degrees C, followed by a stronger surge at 250 degrees C, signaling the onset of thermal decomposition. The real-time dynamic in situ monitoring revealed mainly the following insights into the HCPA-ALD process of AlN: (i) film growth rate and TMA chemisorption amount exhibited plasma power dependent saturation behavior, which was also correlated with the substrate temperature; (ii) time-dependent refractive index evolution indicated a nonconstant relationship: a faster increase within the first similar to 100 cycles followed by a slower increase as the AlN film gets thicker; and (iii) a considerable improvement in crystallinity was observed when the substrate temperature exceeded 200 degrees C. Besides in situ optical characterization, ex situ optical, structural, and chemical characterization studies were also carried out on 500-cycle grown AlN films as a function of substrate temperature. All AlN samples displayed a single-phase wurtzite polycrystalline character with no detectable carbon and relatively low (<5%) oxygen content within the bulk of the films. Moreover, regardless of the deposition temperature, HCPA-ALD grown AlN films exhibited highly stoichiometric elemental composition. Published by the AVS.