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/N₂/H₂ plasma as metal precursor and coreactant, respectively. Growth saturation experiments within 100–250 °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 °C, followed by a stronger surge at 250 °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 ∼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 °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.