Nucleation period, surface roughness, and oscillations in mass gain per cycle during W atomic layer deposition on Al2O3

Nucleation phenomena are critical for the fabrication of W/Al2O3 nanolaminates using atomic layer deposition (ALD) techniques. The nucleation and growth of W ALD on hydroxylated Al2O3 ALD surfaces and Al2O3 ALD on fluorinated W ALD surfaces was studied using in situ quartz crystal microbalance (QCM) and ex situ atomic force microscope (AFM) techniques. The QCM investigations revealed that Al2O3 ALD readily nucleated on the fluorinated W surface and displayed “substrate-enhanced growth.” In contrast, W ALD required 4–10 ALD cycles to nucleate on the hydroxylated Al2O3 surface and displayed “substrate-inhibited growth.” The W ALD nucleation period was shorter for higher Si2H6 and WF6 reactant exposures. The most rapid nucleation of W ALD on the Al2O3 surface occurred with much larger Si2H6 and WF6 exposures on the initial ALD cycle with the WF6 exposure prior to the Si2H6 exposure. By analyzing the individual Si2H6 and WF6 mass gain per cycle (MGPC), three main regions were identified in the W ALD nucleation and growth: initial deposition on Al2O3, W island growth and coalescence, and steady state growth. The root mean square (rms) roughness of the resulting W ALD film was dependent on the Si2H6 exposures and the number of ALD cycles required to nucleate the W ALD. A linear dependence was observed between the rms roughness and the number of ALD cycles required to reach one-half the maximum W MGPC. The W ALD also displayed very periodic oscillations in the W MGPC that were consistent with island nucleation and growth. Four local minima and three local maxima were observed in the W MGPC versus the number of ALD cycles. Comparing the results for W ALD on Al2O3 surfaces with recent simulations of ALD nucleation helps to establish the relationship between the nucleation period and surface roughness with island growth during nucleation.