IES990008A2 - Magnetic device for electrochemistry - Google Patents

Abstract

A permanent magnet device is described which generates a magnetic field in the vicinity of the electrode of an electrochemical cell. The device may be used to increase the rate of electrodeposition and improve the morphology and electrical characteristics of the electrodeposit. By varying the magnitude and direction of the magnetic field, or by rotating the magnetic field created by the device it is possible to further control the quality of the electrodeposit.

Description

APPLICATION to..

Magnetic device for electrochemistry Abstract: A permanent magnet device is described which generates a magnetic field in the vicinity of the electrode of an electrochemical cell. The device may be used to increase the rate of electrodeposition and improve the morphology and electrical characteristics of the electrodeposit. By varying the magnitude and direction of the magneticfield, or by rotating the magnetic field created by the device it is possible to further control the quality of the electrodeposit.

Background of the invention Electrodeposition and electroplating of metal layers from ion-containing solutions are important industrial processes. Electroplating is used to produce decorative, protective, functional or hard coatings on various metallic substrates. It may also be beneficial to coat electrodes themselves with active layers in order to modify their characteristics. Another process is electropolymerization, where a polymeric deposit is produced electrochemicallyfrom solution. In all these processes, it is desirable to control the quality of the electrodeposit, including properties such as thickness, deposition rate, surface morphology, porosity, composition, mechanical properties, adherence and electrical conductivity. This is traditionally done by chemical and physical means such as use of chemical additives and choosing the composition and concentration of the electrolyte, the pH of the solution and the working voltage. Mechanical motion of the electode and stirring of the solution are sometimes used. Ultrasonic excitation in another such technique.

For example, in order to make interconnects in the semiconductor industry' it may be advantageous to use use electrodeposition instead of a physical deposition method for the metal layers, typically copper. In order to achieve smooth, uniform layers it is necessary to operate at a low plating rate and it is customary to include additives in the electrolytesolution which are known in the art.

There have been some reports in the electochemical literature that a steady magneticfield can influence the deposition rate of metals such as copper, silver and zinc. Examples are F. Z, Fahidy, Electrochimica Acta, vol 18, p.607 (1973); E. Chassaing and R Wiart, Electrochimica Acta, vo| 29, p.649 (1984); J. P. Chopart et al. Electrochimica Acta, vol 36, p.459 (1991);V. Noninski, Electrochimica Acta, vol 42. p.251 (1997); G Hinds et al, Journal of Applied Physics vol. 83, p.6447 (1998).

The present invention permits the control of deposition rates and other process parameters of electrodeposits by using permanent magnets to generate a suitable magne IE 990008 Description of the invention.

The invention consists of an electrochemical cell associated with an a permanent magnetaassembly which createsa magneticfield in the vicinity of an electrode, usually the cathode of the electrochemical cell. The permanent magnets may be immersed in the electrolyte and the magnets may be coated painted or cased in an inert material to avoid direct contact between the magnetic material and the electrolyte (Figure 1). Alternatively, the magnets may be mounted outside the cell (Figure 2). In that case segments of soft magnetic material may optionally be mounted inside the cell to concentrate and direct the magneticflux in the vicinity of the electrode (Figure3).

The permanent magnetb can be made of hexagonal ferrite, neodymium-ironboron, samarium-cobalt, samarium-iron-nitrogen, alnico or another permanent magnet material.The magnetic circuit is typically designed to deliver a magnetic field typically parallel or perpendicularto the electrode surface where electrodeposition is taking place. In the case of non-planar electrodes the field pattern may be chosen to fit the form of electrode. The direction of the field required may depend on the orientation of the electrode relative to the Earth’s surface. The principles of magnetic circuit design are known in the art; a reference is Advances in Permanent Magnetism by R. J. Parker, John Wiley, New York 1990.

In magnitude of the magnetic field required to exert an effective benficial influence on may be estimated form the dimensionless ratio R = Bj/gp, where B is the magnetic flux density in teslas, j is the current density at the electrode in amperes per square meter, g is the acceleration due to gravity amd p is the desnsity of the electrolye in kilograms per cubic meter. The magneticfield will be most effective when this ratio is in the range 0.1 - 10. A typical value is 1. Hence, for example, for a current density of 10,000 A/m2, a density of 1000 kg/m3and g ~ 10 m/s2 a field of 1 tesla will be effective.

By generatingthe magneticfield with a permanent magnetflux source, there are the advantages that no power supply with a continuous supply of energy is needed, and no cooling of the magnet is required. The flux source may produce a fixed or variable magneticfield, and the direction of the field may be varied during the electrodeposition process. In particular, it may be beneficial to rotate the field during electrodeposition. Variation of the field direction and magnitude may be achieved by moving the magnets.

Some examples of the implementation of the device are now disclosed. 1) Eleetrodepositionof copper was carried out from a 0.2 M solution of CuSOq at pH 1, using copperelectrodeswith a current density of approximately 20,000 A'm2. The cell was incorporated in a permanent magnet variable flux source incorporating two IE 990008 nested counter-rotating magnetic cylinders. The plateau current was recorded as a function of magnetic flux density from 0 - 1 T, with the results shown in Table 1.

Table I Magnetic flux density (T) 0 0.1 0.2 0.4 0.6 0.8 1.0 plateau current (mA) 44 59 76 84 96 104 111 2) The quality of the coppper electrodeposits obtained with and without the magnetic field was examined. Without the field, the plated surface of the cathode was irregular with areas of rapid dendritic growth. When a uniform field of 0.6 T was applied during electrodeposition in an electrochemical cell placed in a cylindrical segmented permanent magnet, the surface was smoother, and the dendrites were eliminated. The mean surface roughness was decreased from 2.2 microns to less than 1 micron. 3) The inf uenceof the magnetief eld is related to the the pH of the solution. The effect increases markedly at low pH. In a uniform field of 0.6 T applied during electrodeposition in an electrochemical cell placed in a cylindrical segmented permanent magnet, the variation of current with pH of the electrolyte solution is shown in Table2.

Table 2. pH of electrolyte solution Plateau current (mA) 3.0 2.5 2.0 1.5 1.0 0.8 0.6 110 129 140 4) By rotating the permanent magnet with respect to the electrochemical cell during electrodeposition at a frequency of approximately 1 Hertz, a further improvement in the deposition rate and the quality of the electrodeposit was observed. The rotation of the field is very convenient to achieve with the permanent magnet electrochemical cell, but would be very awkward in an electromagnet system.

These examples are intended to be illustrative. The permanent-magnet electrochemical cell of the invention can be applied in other ways, including electropolymerization of pyrrole for example, coating electodes with electoactive IE 990008 polymers and oxides, and generally when control of mass transport or redox reactions near the electrode is required.

Claims (5)

Claims.

1) An electrochemical cell for electrodeposition associated with a structure of permanent magnets and optionally soft magnetic matrial which produces a magnetic field at an electrode of the electrochemical cell, the magnetic flux density of the field being in the range 0.01 tesla to 2 tesla

2. ) An electrochemical cell for electrodeposition associated with a structure of permanent magnets and optionally soft magnetic matrial which produces a magnetic field at an electrode of the electrochemicalcell, the magnetic flux density of the field being varied by movement of the permanent magnets during an electrochemical process in the range from zero to a maximum not exceeding? teslaand optionallyin sign .

3. ) An electrochemical cell for electrodeposition associated with a structure of permanent magnets and optionally soft magnetic matrial which produces a magnetic field at an electrode of the electrochemical cell, the direction of the magnetic field being varied or continuously rotated during the course of electrolysis by movement of permanent magnets of the device.

4. ) An electrochemical cell for electrodeposition of copper or other good elecrical conductors which incorporates a structure of permanent magnets and optionally soft magnetic matrial which produces a magnetic field at the cathode of the electrochemical cell, the magnetic flux density of the field being in the range 0.01 tesla to 2 tesla and preferably in the range 0.2 to 1 teslaand optionally variable in magnitude and direction includingthe possibility of rotation during the course of electrodeposition.

5. ) An electrochemical process which is conducted in a magnetic field in the range 0.1 to 1 tesla in an electolytic solution of pH less than 1.5 with a current density in the range 100 to 100000 amperesper square meterwhere the direction of the magnetic field produced by a structure of permanent magnets is continuously rotated during the process.