Electrodeposited metal films are often in a state of stress and this has been the subject of extensive experimental investigation and theoretical analysis1-4. We have been measuring stress development in situ during electrodeposition and correlating the results with in-situ AFM imaging during electrodeposition under similar conditions. In this paper we present results on the early stages of copper deposition from acidic CuSO4 electrolytes with and without chloride as an additive.
To examine the effect of growth rate, sequential galvanostatic depositions were carried out (after an initial potentiostatic deposition of a thin layer of copper) over a relatively large range of growth rates (from 0.22 nm s-1 to 6.9 nm s-1) in additive-free electrolyte. 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 Chason’s kinetic model3 gave a good fit of our experimental stress measurements in the region of constant grain size (~970 nm). The parameters from this fit were then used to model the steady state stress as a function growth rate and grain size. The values predicted by the model are plotted against the values from the experimental measurements in Figure 1. It can be seen that there is good agreement between model and experiment.
The effect of added chloride in the electrolyte was also investigated. Even at low concentrations (< 1 ppm), the presence of chloride 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. However, in chloride-containing electrolyte, the stress became even more tensile on interruption of deposition.
Our experimental results on the change from one type of behaviour to the other as the chloride concentration was increased will be described and possible mechanisms will be discussed.
References
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Figure 1
Relation between Changes in Photosynthetic Rate and Changes in Canopy Level Chlorophyll Fluorescence Generated by Light Excitation of Different Led Colours in Various Background Light