Effects of Bath Composition and Current Density on the Electrodeposition Behavior of Fe–Ni Invar Alloy

Fe–Ni invar alloy (Fe 64–Ni 36 wt%) has a very low coefficient of thermal expansion (CTE) than any other metals. For this reason, it has been used as fine metal mask (FMM) in RGB patterning of OLED manufacture process. However, as the resolution of OLED display is getting higher, the thickness of FMM is getting thinner and then the conventional extruded invar sheet cannot be used directly. The electrodeposition of invar can be the alternative for fabrication of thin FMM. In this study, the Fe–Ni alloy were electrodeposited varying bath compositions and current density. Also, the effects of ferric ion (Fe3+), produced during electrodeposition on inert anode, on the behavior of deposition were investigated. Finally, the Fe–Ni alloy with 36–40 wt% Ni were obtained at 50 mA/cm2 in 0.30 M Fe2+ bath. The back side and front side composition of the deposits were analyzed to evaluate the composition uniformity of the Fe–Ni alloys. Generally, the Fe content of back side of the deposits were higher than that of front side.


Introduction
Fe-Ni alloy with Fe 64-36 wt% Ni, commercially known as invar, has a low thermal expansion coefficient (CTE) than any other metals [1].Based on its dimensional stability, Fe-Ni invar has been used as the materials of fine metal mask(FMM), a key component in RGB patterning of organic light-emitting diode (OLED) display, to avoid distortion of patterns during evaporation process at high temperature [2][3][4].
Nowadays, commercialized FMM has been produced by wet etching process of invar film with over 20 μm in thickness made by rolling process.However, it is impossible to get higher resolution than QHD with extruded invar thickness [5,6], therefore, several studies about alternative technology for higher resolution have been conducted.Electrodeposition is considered as one of the methods to make thinner FMM of 15 μm or less, whose resolution is higher than QHD by relatively simple equipment and process, compared to other technologies [7].
However, the properties of electrodeposited Fe-Ni film, such as composition, microstructure, surface morphology, are significantly influenced by various process conditions (bath composition, current density, temperature, pH, additive and etc.) [8,9,11].For this reason, it is important to understand the effects of each variable on the deposits in order to apply electrodeposition to mass production of high resolution FMM.In this study, the electrodeposition of Fe-Ni alloy was conducted at various bath composition of ferrous ion (Fe 2+ ) and current density.Also, ferric ion (Fe 3+ ) was added to bath to observe the effect of ferric ion on Fe-Ni electrodeposition behavior.Finally, Fe-Ni films close to invar composition were obtained and the uniformity of deposited was evaluated.

Experimental Details
In this study, Fe-Ni alloy electrodeposition was conducted in sulfate-chloride bath using dimensionally stabilized anode (DSA) as anode, Ti plate with exposed area of 2 cm × 2 cm as cathode, which cleaned with 10 wt% NaOH solution and 10 wt% H 2 SO 4 solution in ultrasonic cleaner before electrodeposition.The electrolyte consisted of 0.95 M nickel(II) sulfate, 0.17 M nickel(II) chloride, 0.25-0.4M iron(II) sulfate, 0.5 M boric acid, 0.05 M malonic acid and 2 g/L sodium saccharin.Iron(III) sulfate was added for investigation of Fe 3+ effects.The pH of bath was adjusted to 2.3 with 20 wt% NaOH solution and 10 wt% H 2 SO 4 solution.The temperature of bath was maintained at 50 ℃ in water jacket and bath was stirred with 300 rpm using 28 mm magnetic bar.The schematic diagram of the electrodeposition cell is shown in Fig. 1.The current density was varied from 10 to 200 mA/cm 2 , and deposition time was adjusted with each current density to maintain total charge of 192 C. The Fe-Ni deposits was cleaned with distilled water and dried after electrodeposition.
The compositions of the Fe-Ni deposits were analyzed using energy dispersive X-ray spectroscopy (EDS).Field emission scanning electron microscope (FESEM) was used for analysis of surface morphology and thickness for calculation of current efficiency.Potentiodynamic polarization was conducted at three electrode system, consisting of Ag/ AgCl electrode as reference electrode, Ti plate as working electrode, and DSA as counter electrode.The scan rate was 5 mV/s and operating voltage range was from 0.05 V (vs open circuit voltage, OCV) to − 1.8 V (vs Ag/AgCl).Residual stress of deposits were analyzed using deposit stress analyzer model 683, and the condition of temperature and total charge for the electrodeposition of strip were room temperature (RT) and 45 C, respectively.

Results and Discussion
Figure 2 shows the composition of the deposits from bath with 0.25, 0.30, 0.35, 0.40 M iron(II) sulfate (FeSO 4 ).For all bath conditions, the Fe content of deposits were higher than Fe content of the baths.It means that Fe-Ni alloy was electrodeposited anomalously, as active Fe preferentially deposited than Ni [10,11].The deposits became Fe-richer as the concentration of Fe 2+ increased.In 0.35 M Fe 2+ bath, the Fe-Ni film with 35.4 wt% Ni, similar to invar composition, was obtained.The surface morphologies of the deposits are shown in Fig. 3.The morphology of deposits at low Fe 2+ concentration showed the bright surface, however the surface morphology was changed to rougher and lost the  [12].
To investigate the effect of Fe 3+ in bath, Fe-Ni electrodeposition was conducted in the modified bath containing 0, 0.05, 0.10 M iron(III) sulfate, which the sum of Fe 2+ and Fe 3+ was kept at 0.30 M. As shown in Fig. 5, the Fe content of deposits from the bath with higher Fe 3+ content was lower.In addition, the current efficiency was reduced from 56.5 to 51.8% as the concentration of Fe 3+ increased (Fig. 6).
It means that Fe 3+ produced on the DSA suppress Fe reduction on the cathode due to consumption of charge for Fe 2+ / Fe 3+ redox reactions.
Figure 7 shows the composition of deposit from 0.35 M Fe 2+ bath depending on current density.The composition change in the high current density region over 50 mA/cm 2 was negligible compared to that in the low current density region from 10 to 50 mA/cm 2 .It is due to the current density reached the diffusion controlled region as shown in the cathodic polarization plot (Fig. 8), where charge transfer rate was so fast that total reaction rate is determined by diffusion of reactants [13,14].Figures 9 and 10 show the residual stress and current efficiency at various current density respectively.All deposits showed the tensile stress and it was increased with current density.The current efficiency  was decreased from 73% at 10 mA/cm 2 to 44% at 100 mA/ cm 2 .As the current density became higher, the rate of charge transfer for reduction of proton in bath get faster whereas the rate of reduction of metal ions became diffusion-controlled.As the results, more hydrogen gases were evolved on deposit, leading to low efficiency and high tensile stress [15,16].
The composition range of electrodeposited Fe-Ni, which is showing minimum CTE, was 36-41 wt%, on the other hand the pyrometallurgically produced one was about 36 wt% Ni [17].For this reason, the Fe-Ni alloy was electrodeposited at 50 mA/cm 2 in 0.30 M Fe 2+ bath for 5, 10, 15, 30 min.Figure 11 shows thickness of deposits(δ) and current efficiency depending on deposition duration.For all conditions, the thickness of deposits were thinner than theoretical thickness(δ exp ) and the difference between them became larger, leading to decrease in current efficiency.The results of composition analysis are shown in Fig. 12.The Ni contents of the front side of deposits were 36-40 wt% Ni regardless of deposition time, however, the back side of deposits contain more Fe than front side at all conditions.The Fe-Ni alloy was electrodeposited anomalously, and the commonly reported mechanism of anomalous codeposition was shown below [18][19][20][21].In these equation, M is iron group metal (Fe, Ni, Co) and the adsorption ability of each metal monohydroxide ion (MOH + ) is strong in order of Fe, Co, Ni [22].Thus Fe(OH) + , preferentially adsorbed on the cathode, inhibited adsorption of Ni(OH) + and caused higher Fe content of the back side of deposits compared to that of the front side.

Conclusion
The effects of bath composition and current density on Fe-Ni electrodeposition behavior and properties of deposits were investigated.The Fe-Ni alloy films with 36-40 wt% Ni were deposited at optimized conditions.The anomalous codeposition of Fe-Ni, the preferential deposition of more active Fe, was shown in all conditions and it was profound at higher Fe 2+ concentration and higher current density.However, the decrease in current efficiency was observed at the same time, due to Fe 2+ /Fe 3+ redox reaction and hydrogen evolution.For efficient and stable electrodeposition, Fe 3+ in the bath should be controlled as low as possible and the excessively high current density should be avoided.In addition there was the difference in composition between the front and back of deposits in all deposition time.Therefore, for the application of electrodeposition to fabrication of FMM, the uniformity of deposits' properties should be secured by additional process.

Fig. 1
Fig. 1 Schematic image of the Fe-Ni electrodeposition cell

Fig. 4
Fig. 4 Current efficiency at various concentration of Fe 2+

Fig. 12
Fig. 12 Composition of front and back side of deposits vs. deposition time