Nucleation period, surface roughness, and oscillations in mass gain per cycle during W atomic layer deposition on Al 2 O 3

Nucleation phenomena are critical for the fabrication of W / Al 2 O 3 nanolaminates using atomic layer deposition (ALD) techniques. The nucleation and growth of W ALD on hydroxylated Al 2 O 3 ALD surfaces and Al 2 O 3 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 Al 2 O 3 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 Al 2 O 3 surface and displayed "substrate-inhibited growth." The W ALD nucleation period was shorter for higher Si 2 H 6 and WF 6 reactant exposures. The most rapid nucleation of W ALD on the Al 2 O 3 surface occurred with much larger Si 2 H 6 and WF 6 exposures on the initial ALD cycle with the WF 6 exposure prior to the Si 2 H 6 exposure. By analyzing the individual Si 2 H 6 and WF 6 mass gain per cycle (MGPC), three main regions were identified in the W ALD nucleation and growth: initial deposition on Al 2 O 3 , 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 Si 2 H 6 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 Al 2 O 3 surfaces with recent simulations of ALD nucleation helps to establish the relationship between the nucleation period and surface roughness with island growth during nucleation.