In-Situ Measurements of Stress during Electrodeposition of Copper Nanofilms: Surface and Grain-Boundary Migration of Atoms and the Effect of Chloride Ions
Authors: Murphy Joseph A , Lenihan Catherine , Lynch Robert Patrick , Buckley D. Noel 
Published: February 2020

Thin metal films deposited on a substrate are usually in a state of stress and this has been the subject of extensive experimental investigation and theoretical analysis1-4. Various studies of the stress generated during the electrodeposition and aging of copper films have been reported.

In this paper we report results of in situ stress monitoring during deposition of copper nanofilms from acidic CuSO4 electrolytes with and without chloride as an additive. At low growth rates, steady state stress was compressive. As growth rate was increased, the stress became less compressive and eventually became tensile. Crossover from compressive to tensile stress occurred at a growth rate of ~1 nm s-1. Both in-situ AFM imaging during deposition and ex-situ SEM imaging were used to characterise the evolution of grain size during the sequence of galvanostatic depositions used in the stress measurements. In general, grain size increased with continued deposition but eventually reached a constant value.
Simulations based on a kinetic model3 gave an excellent fit of our experimental stress measurements in the region of constant grain size (~870 nm). In the region of variable grain size, a fit of the model to the experimental data gave a grain size that is in good agreement with the average (520 nm) obtained from SEM and AFM.
The presence of chloride in the electrolyte considerably reduced the tensile stress. Chloride-free and chloride-containing electrolytes also showed very different behaviours after interruption of electrodeposition. In chloride-free electrolyte, the tensile steady-state stress observed during deposition changed to compressive stress on interruption of the deposition: typical results are shown in Fig. 1. However, in chloride-containing electrolyte, the stress became even more tensile on interruption of deposition.
The observed behaviour on current interruption offers strong evidence to support Chason’s model3 of stress development in thin films. According to that model, stress originates primarily from effects at grain boundaries. As grain boundaries form, tensile stress tends to develop because of a so-called “zipping” effect whereby the surfaces of adjacent grains near the film surface tend to attract once the separating distance becomes small. When the attraction between adjacent grains becomes strong enough, they “zip” together, closing the gap between them and creating tensile stress as the material stretches slightly to fill the gap. Similar effects occur near the film surface as existing grains continue to grow: when the atoms at the edges of terraces on two adjacent grains get sufficiently close to each other they bond to each other even if that bond causes stress in the layer. This tensile stress can be relaxed or made compressive by the insertion of atoms into the grain boundary by diffusion from the surface. The rate at which this happens is driven by the difference in chemical potential (∆µ) between atoms on the surface and those in the top of the grain boundary (i.e. near the surface). Our experimental results will be discussed in detail in terms of this model.

References

1. O. E. Kongstein, U. Bertoci and G. R. Stafford, J. Electrochem. Soc., 152, C116 (2005).
2. S. Ahmed, T. T. Ahmed, M. O'Grady, S. Nakahara and D. N. Buckley, J. Applied Physics, 103, (2008).
3. E. Chason, Thin Solid Films, 526, 1-14 (2012).
4. Eric Chason, Alison Engwall, Fei Pei, Manon Lafouresse, Ugo Bertocci, Gery Stafford, Joseph A. Murphy, Catherine Lenihan, and D. Noel Buckley,J. Electrochem. Soc., 160, D3285-D3289 (2013)

Figure 1