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

Abstract

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 (CoCp 2 ) and NH 3 plasma. It is demonstrated that nitrogen incorporation is strongly temperature-dependent; at temperatures of 260 °C and below, the deposited films consist primarily of Co 2 N, whereas increasing the temperature to 300 °C leads to a mixture of Co 3 N 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 CoN x species formed during the interaction between a sample consisting of pure Co and an NH 3 plasma is analyzed. X-ray photoemission spectroscopy depth profiling reveals that this plasma treatment converts the top 4 nm of the Co film into CoN x . 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 NH 3 plasma and the adsorbed CoCp 2 -derived species leads to the formation of cobalt nitride. At temperatures below 290 °C, the resulting film primarily consists of Co 2 N, whereas at 300 °C, partial decomposition takes place, resulting in the formation of a mixture of Co 3 N 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.

Self-limiting nitrogen/hydrogen plasma radical chemistry in plasma-enhanced atomic layer deposition of cobalt

“…These trench N species on Co(100) surface will be a potential source of N impurities in deposited Co thin films. 30,45…”

“…Finally, if the temperature is up to 623 K, the deposited thin film is nominally pure Co, which is due to the decomposition of cobalt nitride. 30 Detailed studies are needed to unravel the formation of surface cobalt nitride and removal of these N species, which is out of the scope of this work.…”

“…These trench N species on Co (100) surface will be a potential source of N impurities in deposited Co thin films. 30,45 We applied Bader charge analysis to the Co(001) and Co (100) surfaces in each step of NH x removal. The results are summarized in Table 2.…”

“…The formation of CoN x was reported during the deposition of Co thin films using CoCp 2 and NH 3 plasma at temperatures below 300 °C. 30 For deposition temperatures above 350 °C, decomposition of CoN x was effectively complete, resulting in pure Co thin films. 30 The detailed mechanism of plasma assisted metal deposition has not yet been established and requires deeper investigations.…”

“…30 For deposition temperatures above 350 °C, decomposition of CoN x was effectively complete, resulting in pure Co thin films. 30 The detailed mechanism of plasma assisted metal deposition has not yet been established and requires deeper investigations. A plausible proposal is that highly reactive radicals from the hydrogen/nitrogen-plasma source must be present for successful PE-ALD of metals.…”