In situ AFM possibilities under metals electrodeposition in magnetic field.

• Download (539.58 Kb)

R.G. Fedorov^a, A.V. Khlynov^b
a - Research Scientist, Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Moscow, Russia,
b - Development Engineer, NT-MDT Company, Zelenograd, Russia
www.ntmdt.com

The present work demonstrates the possibilities of the in situ atomic force microscopy for investigation of surface morphology and crystallographic structure of metals electrodeposited in external homogeneous magnetic field (MF) at the examples of Cu and Co deposition onto a Au(111) electrode in the OPD range from sulfate electrolyte.

Introduction

Due to the high interest in thin metallic layers with defined physical properties and microstructure, new methods for preparation and control are required. One of the techniques is superimposition of external magnetic fields during the deposition. It offers many possibilities to influence the electrodeposition process and thus to control the microstructure of the thin film interface.

Experimental

              

Fig. 1. NTEGRA Aura microscope in the MFM configuration with a longitudinal magnetic field generator: 1 – measuring head; 2 – magnetic field generator; 3 – mount.

    The in situ AFM investigations were performed with a NTEGRA Aura microscope in the MFM configuration (production of the NT-MDT Co., Zelenograd, Russia, www.ntmdt.com) (Fig.1). Electrodeposition was performed potentiostatically using bipotentiostat furnished with the NTEGRA microscope.
The measurements under the influence of magnetic field were carried out in a modified electrochemical in situ AFM cell (you can find the standard electrochemical cell description at the www.ntmdt.com). The magnetic field was generated by longitudinal magnetic field generator with magnetic circuits with flat poles. The induction of the magnetic field in parallel to the Au surface was about 0.1 T.
    The working electrode was an Au film with the thickness of 200-300 nm sprayed on glass. To obtain atomic flat Au(111) terraces, the substrate was flame annealed immediately before each measurement. In the case of electrodeposition of Co, the potentials were measured vs. an Ag/AgCl microelectrode and in the case of Cu electrodeposition, a Cu wire was used as a reference electrode. A flame-annealed Pt wire was used as a counter electrode. Electrodeposition was performed from 0.05 mM CuSO₄ and 0.05 M CoSO₄ electrolytes each of them containing 1 mM H₂SO₄ which was prepared on the basis of Milli-Q water and reagents of high-purity grade. Measurements were performed by CSG01 probes.

Results

Cu electrodeposition

The electrodeposition was carried out in an applied magnetic field (B = 0.1 T) (b) as well as in the absence of the field (B = 0) (a) in a double-pulse regime with NTEGRA Aura in the MFM configuration: at a high initiation overpotential (ηinit = -60 mV) and at a low growth overpotential (ηover = -20 mV). Derived AFM images were edited using mathematical apparatus and filters supported in the Nova software (Fig. 2). 

Fig. 2. AFM images of copper films electrodeposited on Au(111) without MF (a) as well as in MF (B = 0.1 T) (b) were obtained by CSG01 probes.

Co electrodeposition

    The effect of a magnetic field on the electrodeposition of Co was also examined in a double-pulse mode with NTEGRA Aura in the MFM configuration: an initiation impulse (E = -1000 mV) of rather short duration (τ = 150 s) and growth pulse (E = -750 mV) of long duration (τ = 2000 s) (Fig. 2).

Fig. 3. AFM images of Co films electrodeposited on Au(111) without MF (a) as well as in MF (B = 0.1 T) (b) were obtained by CSG01 probes.

Discussion

    The effect of the surface morphology and crystal structure significant variation in the case of the electrodeposited metal films in MF is generally explained by the magnetohydrodynamic (MHD) convection induced by the Lorenz force. The Lorenz force depends on the charge and movement rate of ions in electrolyte, on magnetic induction as well as on relative localization of the force lines of MF and electrode surface. The largest effect of this force and, consequently, the largest MHD effect are achieved with MF oriented in parallel to the electrode surface (i.e., when the external magnetic field is oriented perpendicularly to the direction of the ion flux). The MHD convection enhances the mass transport of ionic species thus changing the electrode interfacial conditions, such as the surface pH and ionic adsorption, and results in an increase in the deposition current.
In addition to the Lorenz force, a significant influence on electrochemical processes is produced by the magnetic field gradient and paramagnetic force. Due to a small cell size and an extremely low scan region, it can be assumed that the magnetic field lines are oriented in parallel to the surface, and the field gradient is negligible. The paramagnetic force depends on magnetic susceptibility of metal ions, on MF induction and is independent of its direction.
    Cu ions possess a comparatively low magnetic susceptibility and the MHD effect is the key factor that influences electrodeposition in MF enhancing the mass transfer rate of ions and reducing the diffusion layer thickness. The surface concentration of Cu²⁺ ions is still maintained at a sufficiently high value due to MHD convection, the deposition rate is increased. Fig.1 demonstrates that both in conventional conditions (B=0) and under superposition of magnetic field, the deposit growth occurs preferentially on the grain boundaries of Au(111) surface where the maximum number of structural defects, steps and kinks, is present. In the case the magnetic field superposition, the Cu crystallites are greater in size and in faceting.
    Apart from MHD convection, the paramagnetic force can produce an additional effect on the electrochemical processes of nucleation and growth of Co deposit in MF due to a sufficiently high magnetic susceptibility of the metal ions. The morphology of the deposit obtained without MF (fig.3а, B=0) represents coalescent crystallites with the size of 0.2-0.5 mm and clear faceting. The angles between lateral facets are predominantly 60° or 120°, which correlates well with the crystallographic structure of different metallic deposits on a substrate with a (111) orientation.
    In the case of superimposition of an external magnetic field, the surface profile of the Co deposit is smoother. A great number of crystallites have no definite faceting. Such morphologic modification of the Co deposit surface can be explained by an increase in the nucleation rate in MF. In (1), a dependence between the MF induction value and galvanostatic deposition potential of Co is studied. At an increase in the field induction, the deposition occurred at more positive potentials. This is explained by MHD convection resulting in a decrease in diffusion limitations.
    The presented results were obtained under potentiostatic conditions. When the values of the deposition potentials are similar, the nucleation rate is higher in the presence of superimposed MF than in the case of the absence of MF owing to a decrease in diffusion limitations. The high amount of crystallites in MF can be explained by the lower size of diffusion zones near the crystallites due to an increase in the mass transfer rate under the MHD convection conditions.

References

Matsushima H., Ispas A.,Bund A., Plieth W., Fukunaka Y.Magnetic field effects on microstructural variation of electrodeposited cobalt film // Journal of solid state electrochemistry. 11. 2007. p. 737 – 743.