Al coating from ionic liquid and analysis by capillary electrophoresis

The substance class of ionic liquids (IL) has in common that they consist of ions and are liquid at below 100 °C. It does not matter whether the ions are inorganic or organic. It does not matter whether the ions are inorganic or organic. Organic cations (imidazolium, pyrrolidinium, phosphonium, etc., see Fig. 1, left) are often combined with inorganic anions (dicyanamide, thiocyanate, acetate, etc., see Fig. 1, right). This results in a large number of possible compounds.

While ionic liquids are used as lubricants, conductivity additives or process chemicals, e.g. in cellulose processing, they can also be used to make various metals that are otherwise difficult or impossible to access available by electroplating. For example, ionic liquids can be used to deposit titanium, aluminum, silicon, iron and other metals. This paper deals with the deposition of aluminum from ionic liquids. Therefore, only ionic liquids for aluminum deposition are discussed below.

Due to its high conductivity and low weight, aluminum is a particularly suitable raw material for the PCB industry to manufacture innovative new products and open up new fields of application. Various systems are known in the literature for the deposition of aluminum. With regard to aluminum deposition, the tetrachloro-aluminate electrolyte EMIM Cl * AlCl3 (Figure 2) proved to be particularly suitable [1-3].

Fig. 2: Structural formula of the ionic electrolyte for aluminum deposition (R=Et )

Under correct (anhydrous) handling, the electrolyte is extremely stable. Electrolytes stored without water show no deterioration in deposition quality even after years of regular use. However, the electrolyte reacts violently with water and decomposes to form hydrochloric acid. As hydrochloric acid inhibits the process even in small quantities, it is particularly important to ensure reliable analysis of the electrolyte. However, due to the high reactivity even with atmospheric water, it is difficult to establish a suitable measuring method for determining the electrolyte quality. In addition to the analytical challenges of quantifying the main components of the electrolyte system, it is equally important to determine the additive concentration over the service life of the electrolyte.

Chemical analysis

There are many different analytical options for the chemical analysis of IL. Titrations, spectroscopic and chromatographic methods with different equipment are frequently described [4-6]. Which method is ultimately used depends on the chemical composition of the IL, the task to be accomplished and, of course, the equipment of the analytical laboratory.

Capillary electrophoresis (CE), which can be used to measure both cationic and anionic components as well as uncharged organic molecules, is generally suitable as a universal analytical method. For example, CE can be used to separate the metallic components of electroplating baths such as nickel, zinc, manganese and the chromium(III) and chromium(VI) species [7]. Furthermore, anions and organic acids such as chloride, sulphate, lactic acid, EDTA and various phosphates in aggressive electroplating baths can be analyzed in a single measurement [8].

Fig. 3: Schematic diagram of a CE apparatus

So how does a capillary electrophoresis device actually work? Figure 3 shows the schematic structure of a CE device. The sample solutions are analyzed using capillary electrophoresis in a quartz glass capillary filled with buffer solution, which has an internal diameter of between 25 and 100 µm and a total length of 25 cm to 1 m. A high voltage of up to 30 kV is applied to mobilize the sample components in the direction of the detector. The separation is based on the different electrophoretic mobility of the analyte ions in the electric field. The high-voltage supply can be operated with positive or negative polarity, depending on whether positively charged analytes (cations) or negatively charged analytes (anions) are to be determined. The sample application, for which the buffer supply vessel at the capillary inlet is temporarily exchanged for a vessel filled with sample solution, can be realized by applying a pressure (hydrodynamic injection) or an electric field (electrokinetic injection). Only a few nanoliters of the sample or a very small amount of substance in the femtomole range are injected into the capillary.

After the sample injection, the analytes reach the detector, which is installed near the end of the capillary, at different times. For most CE applications, a UV detector is used, which can be used for both direct UV detection and indirect UV detection. This makes it possible to detect all charged sample components in capillary electrophoresis using UV detection. Due to interactions with selected additives added to the buffer solution, uncharged UV-active analytes can also be determined. CE devices based on this principle are available for fully automated analysis. The result of the separation is an x-y representation of the detector signal as a function of the migration time. This representation is called an "electropherogram" in CE.

Chemical analysis of EMIM Cl*1.5 AlCl3 with CE

For the ionic liquid EMIM Cl*1.5 AlCl3 to be considered in the present case, an analytical method should be used with which the three main components EMIM, aluminum and chloride can be quantified. Furthermore, the method should ideally also offer the possibility of detecting potential impurities or decomposition products. Finally, it would also be advantageous if any additives that are added to the IL, for example to improve the layer properties or the deposition rate, could also be analyzed simultaneously.

Fig. 4: CE methods for analyzing the main components of the IL

In order to be able to determine all desired parameters with the CE, three different methods (Figure 4) were developed, which differ in the composition of the buffer system, the coating and the dimensions of the capillary and in the detection. A fourth method is also available for analyzing certain additives, although this was not relevant for this report.

Aluminum and the cationic EMIM present in the buffer system CE-KI01 can be quantified simultaneously with indirect detection. The same method can be used to determine potential cationic impurities in the IL, such as sodium, calcium, magnesium, 1-MIM and amines.

The main anionic component chloride can be quantified using the CE-AI01 buffer system, which can also be used to detect other anionic components. Finally, it is also possible to determine the EMIM content using the CE-KD01 buffer system with direct UV detection. This system is particularly suitable for the very sensitive detection of any impurities originating from the synthesis as well as decomposition products.

Fig. 5: Chemical analysis of the main components of IL

Quantification of the main components

Before injection in the CE device, a defined dilution step is required to obtain the concentration of the main components in the linear concentration range of the detector. Each step of the sample preparation requires rapid handling, preferably in the absence of air, to prevent the release of hydrogen chloride gas, as this would falsify the analytical result for the chloride concentration. Figure 5 shows the resulting electropherograms for the main component analysis. Using this method, the theoretically expected content in the ionic liquid was confirmed by analyzing the main components. There was good agreement between the theoretical and experimental results (Table 1).

Table 1: Measured and theoretical content of the sample.

Furthermore, the sample proved to be very clean: no cationic or anionic impurities could be detected. The peaks for sulphate and carbonate shown in the anion system are additions to the IL, which are used to test the efficiency of the separation.

Analysis of impurities and additives

The IL samples were analyzed for potential known and unknown UV-absorbing impurities using another CE method that uses direct UV detection. All samples tested proved to be very pure. Only very small amounts of 1-methylimidazole were detected as an impurity in all ILs. The content was approximately 0.02 % in each case. In order to check whether further impurities could be detected in principle, test measurements (Figure 6) were carried out after quantitative addition of the most important potential impurities or decomposition products to the IL.

Fig. 6: Purity control of the IL

It can be seen that the separation of the most important impurities from the main component is ensured. For this purpose, 0.1 % of each impurity was added to the IL and the limits of quantification could be calculated for each impurity based on the resulting signal-to-noise ratio.

A very good sensitivity was achieved for all components, which far exceeds the purity requirements for the IL (Table 2).

Table 2: Limits of quantification for potential impurities

The separation of nicotinamide (NA) from the main peak and the impurities was also tested as a proxy for the addition of additives. For this purpose, 12 mM NA was added. Figure 6b shows a problem-free separation of the nicotinamide from all other components. This method therefore makes it possible to monitor the availability and consumption of additives as part of process control. The aim of further investigations is to extend the method to other additives.

Monitoring the reactants

The same method can also be used to monitor the purity of the starting materials. For example, the starting product EMIM Cl can be screened for impurities, which can be helpful as an input control before use in the synthesis of the IL. The example in Figure 7 shows the electropherograms of two batches of the starting product EMIM Cl.

Fig. 7: Purity control of the reactant EMIM Cl

Both batches have a low concentration of 1-methylimidazole, the quantification of which resulted in a content of 0.08 %. This impurity is transferred to the IL during synthesis and can also be detected there. However, experience has shown that this impurity does not interfere with the application of the aluminum coating. In one of the reactant batches examined, however, a previously unidentified impurity was found after the main peak. A comparison of the spectral data (Fig. 8) shows that this has a similar UV spectrum with a slightly shifted maximum compared to the main EMIM peak and is therefore presumably chemically related.

Fig. 8: Comparison of the spectral data

This impurity is also transferred directly into the IL during synthesis and could influence the coating process in a previously unknown way. It will therefore be the subject of further investigations. For the unknown impurity, an evaluation was carried out using the area percentage method with reference to the main peak. A content of 0.44 fl% was found in the batch in question.

The comparison of the purities of IL and reactant allows important conclusions to be drawn as to whether the detected impurity is a decomposition product or an original contamination. For example, the results can be used as a selection aid when choosing suppliers and when establishing additional purification steps prior to use in IL synthesis.

Outlook for use

The main focus of developments to date has been on the printed circuit board industry [9, 10]. The BMBF-funded project "Aioli - Al deposition from ionic liquids as a substitute for typical metallization in information and communication technology (AioLi)" dealt with the fundamental issues of aluminium deposition on printed circuit boards. As part of the project, for example, 18 µm thin-wire bonds were deposited on a structured Al layer. Further work in the follow-up project "Alma" was dedicated to the special challenges of the PCB industry, e.g. via deposition. Various via diameters were coated (diameter variation: 200 µm, 400 µm and 800 µm).

In addition to the specific issues of the PCB industry, various pilot projects have shown that other motifs, such as screws (illustration on p. 1129 left) or similar motifs, can also be coated with aluminum.

It is also possible to vary the substrate materials. Various metals and alloys as well as conductive polymers were tested. Of particular note here is the successful deposition on a Lego brick (illustration on p. 1129, right).

In addition to the analytical monitoring of the electrolyte, the possible recycling of the electrolyte was investigated in the "Alma" research project. In initial experiments, it was possible to recycle an electrolyte contaminated by water, from which it was no longer possible to separate it, for separation. IoLiTec is currently focusing its research on further developments in this area and aims to contribute to the realization of the "Rent an ionic liquid" vision for a climate and environmentally friendly circular economy.

Acknowledgements

We would like to thank our partners from the research institutes (Silvia Braun: Fraunhofer ENAS in Chemnitz and Sascha Loebel: TU Chemnitz) and our industrial partners (Mike Becker: NB Technologies GmbH in Bremen and Karl-Thomas Süß: Jenaer Leiterplatten GmbH in Jena) for the exciting (in the truest sense of the word) and constructive cooperation within the framework of the ZIM cooperation project "Alma" (FKZ: KK5090401FF0 (ICA). Funding was therefore provided by the Federal Ministry of Economics and Climate Protection (BMWi).

INFO

List of abbreviations:

AlAluminum

CECapillary electrophoresis

EDTAEthylenediaminetetraacetic acid

EMIM1-Ethyl-3-methylimidazole

EMIM Cl*AlCl31-Ethyl-3-methylimidazo-

lium tetrachloroaluminate

1-EIM1-Ethylimidazole

ILIonic liquid

Imimidazole

1-MIM 1-Methylimidazole

4-MIM 4-Methylimidazole

NANicotinamide

VUcontamination

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