Paper TF2-MoM11
From Chemisorption to Steady-State Growth: Initial Stages of ALD Examined using In Situ X-ray Photoelectron Spectroscopy

Monday, November 9, 2009, 11:40 am, Room B4

One of the least understood aspects of atomic layer deposition (ALD) is the initial stage of growth, which involves the first set of reactions between the thin film precursors and the substrate. As ALD growth is invariably conducted on foreign substrates, the surface must evolve from that representing the starting substrate, to that eventually representing the steady-state growth surface. Here we employ in situ x-ray photoelectron spectroscopy (XPS) to examine the initial stages of growth of TaN_x from the reactions of Ta[N(CH₃)₂]₅ and NH₃ on SiO₂, porous low-κ substrates, and both of these substrates modified with interfacial organic layers (IOLs). In this presentation, first, we will examine the effect of the density of the reactive adsorption sites (-OH groups) on SiO₂ and SiO₂ based porous low-κ substrates. Here we find that the saturation density (coverage) of Ta depends strongly on the initial density of –OH. Moreover, we find that there is a strong correlation between the amount of ligand loss in this first half-cycle, and the density of the reactive adsorption sites. For example, on porous low-κ substrates, reactions involving the loss of a single ligand and formation of –O-Ta[N(CH₃)₂]₄(a) species dominate. In contrast, on surfaces with a high density of –OH (SiO₂), ligand loss is much more significant. Second, another important feature that XPS can probe is the chemical state of the primary thin film constituents. One process observed in the first cycles of growth is the shift in the position of the N(1s) peak, from a binding energy of ca. 398.9 eV to 397.2 eV. The former binding energy corresponds well to that reported for –N(R)₂ species (398.6), while the latter is close to that reported for TaN (397.5) and Ta₃N₅ (396.9). As a final example, we will consider growth of TaN_x on SiO₂ and porous low-κ substrates modified with interfacial organic layers. We have found that adsorption of a branched polymer possessing a high density of –NH₂ species on the porous dielectric increases both the initial uptake of Ta[N(CH₃)₂]₅ in the first half cycle, and the growth rate per cycle. Here an important issue concerns the fate of the IOL—does the ALD thin film quickly and uniformly cover the IOL, or are there reactions between the ALD precursors and the IOL? We find that the fate of the IOL depends strongly on the density and spatial distribution of reactive groups in the organic layer. We will discuss the implications of these observations concerning the use of IOLs as nucleation promoters in ALD.