Plasma-Enhanced Atomic Layer Deposition of Cobalt and Cobalt Nitride: What Controls the Incorporation of Nitrogen?

The present study covers the processes that govern the incorporation of nitrogen in the film during atomic layer deposition (ALD) of cobalt and cobalt nitride prepared from cobaltocene (CoCp2) and NH3 plasma. It is demonstrated that nitrogen incorporation is strongly temperature-dependent; at temperatures of 260 °C and below, the deposited films consist primarily of Co2N, whereas increasing the temperature to 300 °C leads to a mixture of Co3N and Co, and at 350 °C, nominally pure Co is obtained. The sample temperature clearly has a very strong effect on the composition of the deposited film, and in order to understand this temperature dependence, the thermal stability of the CoNx species formed during the interaction between a sample consisting of pure Co and an NH3 plasma is analyzed. X-ray photoemission spectroscopy depth profiling reveals that this plasma treatment converts the top 4 nm of the Co film into CoNx. Temperature-programmed desorption experiments show that this nitride layer starts to decompose at a temperature of approximately 290 °C, which eventually leads to complete removal of all nitrogen from the film. Based on this, a reaction scheme for ALD of Co is proposed; the interaction between the NH3 plasma and the adsorbed CoCp2-derived species leads to the formation of cobalt nitride. At temperatures below 290 °C, the resulting film primarily consists of Co2N, whereas at 300 °C, partial decomposition takes place, resulting in the formation of a mixture of Co3N and pure Co. At 350 °C, decomposition is effectively complete, leading to pure Co with nitrogen contents below 4 at %. Finally, it is argued that the aforementioned mechanism could potentially play a role in plasma-enhanced ALD of other elements that form metastable nitrides such as Ni, Cu, Re, and Ru