Nucleation Enhancement Mechanism of Ruthenium Metals Thin Films by Atomic Layer Deposition

Noble metal thin films have attracted much attention due to their unique properties and structure, leading to a wide variety of applications in microelectronics, catalysis, energy and photovoltaics. Different parameters determine the metal properties for these applications, where an important one is the deposition process. One of the common deposition methods is atomic layer deposition (ALD), a surface-sensitive thin-film deposition technique. The self-limiting surface reactions during the ALD process promote the layer-by-layer growth mechanism, thus providing significant control over the film thickness and conformality. However, due to an island growth mechanism and nanoparticle formation, it can be difficult to achieve the formation of continuous and pinhole-free layers from noble metal ALD on oxide surfaces, leading to poor-quality films. These nonidealities are mainly due to a lack of nucleation sites for the noble metal ALD on oxide substrates and a poor wettability of these low-surface energy surfaces. In order to maximize the potential of ALD, it is important to fully understand the nucleation and growth mechanism of the ALD process. This work focuses on nucleation enhancement of ruthenium ALD as a model system. Ruthenium metal is a promising candidate in thin film technology because of its low bulk resistivity, high work function, thermal stability even at a higher temperature, low specific electrical resistivity even in its oxidized state, and low solid solubility with strong adhesion to copper. These properties make ruthenium an attractive barrier metal or seed layer for copper electroplating.<br/>We study the nucleation enhancement of ALD ruthenium layers on silicon substrates by using single organometallic monolayers, with and without H₂O exposure, prior to deposition. The nucleation enhancement is demonstrated for ruthenium ALD using three different ALD precursors with different ligands. The results show that pretreatments with organometallic molecules (trimethylaluminum or diethylzinc, with and without H₂O, prior to deposition) reduce the nucleation delay of the ruthenium layer. The surface pretreatment strongly influences the ruthenium nucleation, yielding up to a 2.1-fold increase in surface coverage. The ruthenium ALD nucleation and growth mechanism are studied using scanning electron microscopy (SEM), ellipsometry and X-ray photoelectron spectroscopy (XPS) on the treated and untreated substrates. A theoretical model based on the Avrami nucleation model is developed and fitted to the experimental results, pointing to the possible growth enhancement mechanism, which we propose occurs by increasing the surface diffusion of nanoparticles/adatoms on the pretreated surface. Implications of the nucleation enhancement strategy applied to other metal ALD systems will be discussed.