Environmentally friendly electrodeposition: using citrate based electrolytes to deposit nickel and nickel-iron
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Date
28/06/2016Author
Perry, Richard
Metadata
Abstract
The production of magnetic materials is of great interest for use in the micro-fabrication industry.
In particular, Permalloy (Ni80Fe20) is used in the production of micro-electromechanical systems
(MEMS) due to its favourable magnetic properties (high relative permeability, low coercivity and
high magnetic saturation). This leads to applications in devices such as inductors, transformers
and micro-actuators. The electrodeposition of NiFe is also of fundamental electrochemical interest,
as there is anomalous thermodynamic behaviour, with the less noble (iron) metal depositing
preferentially to the more noble (nickel) metal.
To enable consistent alloy deposition nickel and nickel-iron baths are currently almost exclusively
based on boric acid. Boric acid has an important role in the deposition of NiFe films but its
role(s) in the electro-deposition mechanism is (are) not wholly understood. Recently (2011) boric
acid has been identified as a “substance of very high concern” based on the criteria established
by EU chemical regulation, REACH. In anticipation of increased regulation an alternative was
sought to provide a benign alternative to boric acid in the NiFe plating bath suitable for use in
micro-fabrication.
Initial work was performed to benchmark the performance of existing boric acid based electro-deposition
baths. Cyclic voltammetry was performed, which demonstrated the deposition of nickel
and nickel-iron from boric acid baths. Coulombic efficiencies up to 93 % were measured for the
deposition of nickel using the electrochemical quartz crystal microbalance (EQCM) on platinum
electrodes. For nickel-iron deposition control of the film composition was demonstrated on copper
electrodes through varying the iron (II) concentration, current density and temperature. A citrate
bath for the deposition of nickel-iron was then developed and characterised. Cyclic voltammetry was
performed in these citrate baths demonstrating the deposition of nickel and nickel-iron. Optimal
conditions for depositing Ni80Fe20 were demonstrated to be an elevated temperature (60 °C) with
a current density of 20 mA cm-2 and a pH of 3.
Using the EQCM the efficiency for nickel deposition was measured to be > 80 %. The effects of
sodium saccharin and sodium dodecyl sulfate as additives were investigated; these were shown to
influence morphology but not the coulombic efficiency. Decreasing the pH was shown to lower the
efficiency of nickel deposition from the citrate bath.
Comparisons of key properties were made between NiFe films deposited from a boric acid bath
and the citrate bath developed in this work. Test structures were used to compare the strain in
the films; no significant difference was found. For 2.2 μm thick Ni80Fe20 films the sheet resistance
was measured using Greek cross structures as 0.078 ± 0.004 Ω/square for films deposited from
the boric acid bath and 0.090 ± 0.006 Ω/square from the citrate bath. The magnetic saturation,
Ms, was measured as 895 ± 66 emu cm-3 for deposits from the boric acid bath and 923 ± 111
emu cm-3 from the citrate bath. These again show no significant difference in these values within
experimental error. Coercivities for these films were measured to be between 20 and 120 A m-1.
In combination, this work demonstrates the development and characterisation of a new citrate
based electrodeposition bath for nickel and nickel-iron. Similar chemical, electrical, mechanical
and magnetic properties were found from films deposited from both baths, thus demonstrating the
suitability of the citrate bath for the deposition of nickel-iron films in microfabrication.