Surface Thermolytic Behavior of Nickel Amidinate and Its Implication on the Atomic Layer Deposition of Nickel Compounds

Atomic layer deposition (ALD) is a highly important technology to fabricate nickel and nickel compound thin films. The quality of the ALD films relies much on the surface chemistry reactions involved in the ALD process. Aiming to achieve high-quality ALD films, a careful surface chemistry study is carried out in this work to investigate the surface thermolysis behavior of an amidinate-type nickel precursor, bis­(N,N′-di-tert-butylacetamidinato)­nickel­(II) (Ni­(amd)₂). Using the in situ technique of X-ray photoelectron spectroscopy, this work reveals a number of implications which are important for the engineering of the ALD processes. The Ni­(amd)₂ precursor is shown to be reactive to the SiO_x surface even at room temperature, which suggests a good suitability for low-temperature ALD. The surface amidinate moiety is found to decompose at 250 °C, which suggests the limitation of Ni­(amd)₂ for high-temperature ALD. On the other hand, the byproduct of the surface reaction, amidine, can be adsorbed on the surface at low temperature, which might be trapped in the deposited film, inducing carbon and nitrogen impurities. Therefore, if the goal is to achieve a low impurity, a relatively higher deposition temperature (e.g., at 150–200 °C) would be favored. To further demonstrate the important value of the above implications from the thermolysis study, an example of the ALD of NiO is investigated experimentally. Indeed, the NiO films can be successfully deposited at an ever-low temperature of 90 °C, and increasing the deposition temperature to 200 °C can eliminate the accumulation of carbon and nitrogen impurities at the film–substrate interface, whereas the deposition at 250 °C leads to drastic increase in impurities, likely owing to the thermal decomposition of the surface amidinate moiety. All these experimental findings are well consistent with the implications from the surface thermolysis study, which highlights the important value of this surface thermolysis approach for rational ALD process engineering. Given the similarity and broad use of the amidinate-type metal precursors, the results reported herein should also be of significant value for many other ALD processes.