First-principles study of thermodynamical and mechanical stabilities of thin copper film on tantalum

Abstract

The adhesion, stability, and wetting behavior at interfaces between thin Cu films and clean Ta (110) substrates are investigated by first-principles calculations using density functional theory (DFT) in the local-density approximation. Interfaces between pseudomorphic body-centered-tetragonal thin films of Cu, strained face-centered-cubic thin films of Cu, and a single pseudomorphic monolayer of Cu on body-centered-cubic Ta (110) surfaces are studied. Various high-symmetry interface configurations are considered for each case. The mechanical stability of the interfaces is studied by the ideal work of separation, while the thermodynamic stability is investigated by Gibbs' excess interface energy. All three interfaces are found to be thermodynamically unstable. An energy-weighting scheme extends the use of the DFT calculations to the case of an incoherent misfitting interface. The incoherent monolayer of Cu on Ta is thereby found to be thermodynamically stable. For coverages by more than a monolayer, the Cu atoms are expected to form three-dimensional islands on top of the Cu monolayer. With respect to interface separation, the monolayer is found to be bound more strongly to the Ta substrate than the thin film. Hence, failure is expected to occur not at the Cu/Ta interface but inside the Cu

First-principles simulations of copper diffusion in tantalum and tantalum nitride

“…We find that Cu atoms prefer the bridge sites above Ta atoms along ͓001͔ direction and the interfacial Ta-Cu bond length is 2.66 Å. This result agrees with that of Hashibon et al 23 The bonding charge density and density of states of the interface are presented in Fig. 3: the interfacial cohesion is primarily due to a mixture of covalent/metallic bonding between Cu 4s and Ta 6s states.…”

“…It is clear however from both experimental and theoretical results that Cu forms stable pseudomorphic films on top of Ta, i.e., films which have the same structure as the Ta substrate. 21,23 Since 3D simulations of Cu islands or droplets on Ta based on first-principles are not feasible computationally, we focus on the pseudomorphic thin-film mode in this study. This is a reasonable approximation because the interfacial diffusions are highly localized, taking place near the interface.…”

“…[22][23][24][25][26] It is generally believed that Cu grows on Ta ͑110͒ substrate in a layer-by-layer mode for a small number of monolayers and then switches to a threedimensional ͑3D͒ growth model. The relation between the number of stable Cu monolayers and the instability point is unclear.…”

“…23 Because ͓001͔ and ͓111͔ directions are no longer equivalent, one more type of bridge site has to be considered for the ͑110͒ surface of bcc Ta. There are five in-plane arrangements to be explored: ͑1͒ top sites, ͑2͒ threefold sites where Cu atoms sit at one third of the long diagonal of Ta ͑110͒ unit cell, ͑3͒ half way between ͑1͒ and ͑2͒, ͑4͒ bridge sites along ͓001͔ direction, and ͑5͒ bridge sites along ͓111͔ direction ͓Fig.…”

First-principles study on the stability and electronic structure of Mg/ZrB2 interfaces

“…Although the ideal work of adhesion is lower than the actual mechanical work to separate an interface due to neglect of the interfacial plasticity and diffused degrees of freedom [36], it is still an important and convenient factor to predict the mechanical properties and the chemical bonding strength of an interface [37,38]. W ad is calculated by the …”