Quantitation and Characterization of Copper Plating Bath Additives by Liquid Chromatography with Charged Aerosol Detection and Electrochemical Detection

Apr 28, 2014
Volume 10, Issue 7

Copper plating using an acid bath is a process widely used in the manufacture of a variety of products. Different additives can be used to control different aspects of the copper plating process, and must be accurately quantified. In this article, an approach using a dual liquid chromatography (LC) system with an electrochemical detector (ECD) and a charged aerosol detector (CAD) is described for the quantitation of three additives typically used. The methods are precise and sensitive for the determination of all additives, giving a quantitative measure of suppressor and suppressor degradation. Calibration curves and sample analysis results are reported for all additives. Both analyses can be run using the same sample preparation and on the same system.

Photo Credit: YinYang/Getty Images
Copper plating describes the process of depositing a layer of copper onto the item to be plated using an electric current. It is used in the manufacture of a variety of products ranging from cooking pots to integrated circuits and satellite components. To maintain quality and consistency of plating, the process must be tightly controlled.

The acid copper plating bath method is one of the most common approaches used in copper plating. The main components include copper sulphate and sulphuric acid, in addition to a number of additives. The organic additives influence the quality of the copper deposition and typically include an accelerator (such as bis-(3-sodiumsulphopropyl) disulphide), a suppressor (a polyalkylglycol), and a leveller (either a large-molecular-weight polymer or small molecule containing nitrogen or sulphur). Each modifier has a particular function, determining speed of plating, surface wetting, and surface smoothing. To ensure quality control, the levels of each of these modifiers has to be tracked and controlled.

The most commonly used analytical method for measuring these additives is cyclic voltammetric stripping (CVS), which measures the combined effects of the additives in an iterative process using additive standard additions, which is reported to be very slow (analysis in hours) and not very accurate.1 We have developed a high performance liquid chromatography (HPLC) methodology that has been shown to quantify the majority of additives in copper plating bath samples.

Most modifiers do not possess a chromophore, and within the bath, the levels of accelerator and leveller are present at minute concentrations, therefore limiting the detectors available for quantitation. These HPLC methods can provide selective quantitation of these additives, without the use of sulphuric acid mobile phases,1,2 which cause rapid column deterioration.

A faster method to quantify additives is to perform LC using a dual-gradient HPLC system with two detectors, such as a charged aerosol detector (CAD) and an electrochemical detector (ECD). The ECD uses applied potentials to oxidize analytes, which provides the signal. Any analyte with an oxidizable group can be measured at very low amounts (typically picograms on column). It is effective for measuring low levels of electrochemically active analytes in complex matrices, such as the accelerator and leveller in an acid bath.

The CAD is a non-selective, mass-based detector that can measure nonvolatile analytes, with or without chromophores. The analyte is first converted into aerosol droplets using nebulization. The mobile phase is evaporated, leaving analyte particles that become charged in the mixing chamber. The charge is then measured by a highly sensitive electrometer. Charged aerosol detection is reported to have greater sensitivity and precision than evaporative light scattering (ELS) and refractive index (RI), and it is simpler and less expensive to operate than a mass spectrometer (MS).3 Analyte response is also largely independent of chemical structure, providing clear relationships among different analytes in a sample.

Figure 1: Functioning of electrochemical detection with coulometric cells, (top) a single cell and (bottom) in series.
ECD uses coulometric working electrodes that are more sensitive and selective than traditional amperometric electrodes.4 The selectivity of serially placed coulometric electrodes is presented in Figure 1. The first electrode is typically held at a low potential, the second at a higher potential. As the analytes pass through from one electrode to the other, labile compounds will oxidize at the first electrode, leaving the second (downstream) electrode free to measure more stable compounds without interference. In the example shown in Figure 1, analyte A oxidizes to P on electrode 1 (E1) held at 100 mV applied potential, effectively removing it from further reaction. Analyte B remains unchanged until it encounters electrode two (E2) at 500 mV applied potential. At E2, analyte B oxidizes to Q. This provides a selective means of quantifying different analytes.

Table 1: Gradients.
Both detectors can be run simultaneously, and with this configuration the autosampler can be exchanged between the two systems without interrupting flow to either system.

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