Atomic Layer Deposition is a well-established method to deposit thin, conformal films. Several techniques are routinely used to specify the ALD window and to ensure that a proper ALD process is obtained. There are however “side” processes that can lead to film growth/deposition without a classical (i.e. ligand exchange) mechanism, which makes it difficult to control deposition, particularly when selective deposition is sought. It is therefore important to use techniques that can identify such side processes. departure from standard ALD. In this talk, we show that in-situ IR spectroscopy, by detecting ligand exchange and substrate reactions, is a critical tool to explore such anomalies, which cannot be uncovered by conventional ex situ XPS, ellipsometry, QCM methods. We illustrate this point with Cp-based metal precursors with water as a co-reactant. For instance, alkylamido-Cp precursors are attractive because they are readily oxidized (react strongly with water), yet stable, and produce environmentally-mild products. We have explored by in-situ IR spectroscopy the deposition of TiO2 films using (EtCp)Ti(NMe2)3 with water on OH-terminated SiO2 surfaces. This precursor initially reacts with surface OH at ≥150 °C via loss of its NMe2 groups. However, once chemisorbed, it appears to be unreactive with water; i.e., there is no observable ligand exchange. Yet, TiO2 deposition does occur and is shown to originate from gas phase reactions with trace water in the reactor, as evidenced by its dependence on the vapor pressure of water in the reactor chamber. This striking lack of reactivity between chemisorbed (EtCp)Ti(NMe2)3 and water is attributed to the electronic and steric effects of the EtCp group. Sc(MeCp)2(Me2pz) is another Cp-based precursor considered to deposit Scandium oxide (Sc2O3) thin films. This precursor also reacts with hydroxyl-terminated silicon oxide at 150 °C. However in this case, and in contrast to (EtCp)Ti(NMe2)3, there is a strong reduction of the SiO2 underlying surface, leading to the formation of Sc2O3, independent of gas phase reactions. Such processes have mostly been ignored in the ALD community for lack of positive identification. For instance, most papers dealing with Cp-based precursors have concluded that water is a good co-reactant for oxide deposition. This conclusion needs to be re-examined by monitoring ligand exchange to determine if alternative mechanisms are in play, particularly when considering selective deposition.

Challenges for selective atomic layer deposition

255th National Meeting (2018)

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