Effect of Metal Additives on Flatness of Aluminum Plated Films Using Organic Solvent
Authors: Koiwa Ichiro , Ogata Ryo 
Published: December 2020

1. Introduction
Zinc and zinc alloy films formation is the most common for corrosion resistance treatment. However, the zinc will be exhausted after 10 or 15 years later. Aluminum and aluminum alloy film formation is the one of the most promising candidates for new corrosion resistance treatment. An electrodeposition method is one of the most common method for corrosion resistance treatment. Unfortunately, a deposition potential of aluminum ion is less novel than that of hydrogen evaluation in aqueous solution. Many researchers have investigated organic or ionic solvents for aluminum electrodeposition. In this study, a dimethyl sulfone has been used as organic solvent because of its low volatility. Flatness is important for multi-function such as reflection. In this study, two types of additives are investigated to improve flatness, one is ammonium chlorides and the other is niobium chloride.
2. Experimental
In this study, dimethyl sulfone, (CH3)2SO2, is used as a solvent because of its nonvolatile characteristic. Aluminum chloride, AlCl3, is used as an aluminum source. Copper plates are used as a substrate. First, the ammonium chlorides have been investigated to make film surface flatter, ammonium chloride (NH4Cl), methyl ammonium chloride, [(CH3)NH3]Cl, di-methyl, ammonium chloride, [(CH3)2NH2]Cl, and tri-methyl ammonium chloride, [(CH3)3NH]Cl. These additives are added 0.001 mol in the bath whose contained 30g of (CH3)2SO2 and 12.75g of AlCl3. Second, the niobium chloride, NbCl5, has been investigated by changing addition amount from 0.05 mol% to 0.4 mol%. An aluminum wire is used as reference electrode. The prepared baths have been heated at 150oC for 30 minutes to dehydrate.
3. Results and Discussion
3.1. Ammonium Chloride Additives
Table 1 shows effect of ammonium chloride additives on surface roughness of aluminum films. As shown in Table 1. Leveling effect has been clearly observed. Addition of ammonium chlorides decrease the roughness of plated aluminum films. The surface roughness of plated aluminum film without additives is much larger than that of plated with ammonium chloride additives. Especially fro tri-methyl ammonium chloride, [(CH3)3NH]Cl, the surface roughness is the smallest than those of other plated films at both deposition voltage, 4.0 and 6.0V.
3.2. Niobium Chloride Additives
Figure 1 shows effect of niobium chloride addition and deposition voltage on reflection. As a reference, the copper substrate has been measured. The aluminum film plated without niobium chloride addition shows no reflection. The other films which are plated with niobium chloride additive shows reflection. Figure 2 shows roughness of the films which plated with or without niobium chloride additive. Three types of roughness have been measured. The two types of roughness are measured along a line but different direction, longitudinal direction and lateral direction, Ra (longitudinal direction) which is shown in green and Ra (lateral direction) which is shown in blue. One type of roughness has been measured in area, Sa, which is shown in red. The plated aluminum film shows larger roughness values than those of copper substrate. Therefore, the plated aluminum films increase all three roughness values. On the other hand, all plated aluminum films with niobium chloride additive show lower all roughness values than those of copper substrate. The aluminum film with niobium chloride additive plated -2.7 V shows the lowest roughness values, Ra (longitudinal direction), Ra (lateral direction) and Sa.
4. Conclusion
In this study, two types of additives have been investigated to improve surface flatness of plated aluminum films. Especially for Especially for tri-methyl ammonium chloride, [(CH3)3NH]Cl, the surface roughness is the smallest than other plated films at both deposition voltage, 4.0 and 6.0 V. The films plated with niobium chloride additive show flatter surface than that of only copper surface. The aluminum film with niobium chloride additive plated -2.7 V shows the lowest roughness values, Ra (longitudinal direction), Ra (lateral direction) and Sa. Both ammonium chlorides and niobium chloride additive are effective as a lever, improving surface flatness additive.
Table 1 Effect of ammonium chloride additives on surface roughness of Al films.

Additives

Deposition Potential(V)

Surface Roughness(μm)

Without additive

-4.0

5.805

-6.0

0.741

With NH4Cl

-4.0

0.130

-6.0

0.034

With [(CH3)NH3]Cl

-4.0

0.052

-6.0

0.049

With [(CH3)2NH2]Cl

-4.0

0.834

-6.0

0.224

With [(CH3)3NH]Cl

-4.0

0.040

-6.0

0.051

Figure 1