Detecting Trace Copper Contamination in Electroplating Solutions (Trace Cu Detection in Acidic Silver/Tin Solutions)

The Problem:

Contamination is a major concern for the semiconductor industry. With the advent of tin and tin/silver solders for lead-free/RoHS compliant electro-deposition, the risk of trace copper contamination (<20 ppm) due to leaching of copper into the acidic bath solutions requires real-time monitoring to ensure proper deposition.

The Solution:

By incorporating a real-time copper ion monitor into electrodeposition equipment, a technician can quickly identify when the plating solution needs to be replaced before viable product yields begin to drop.

Experimental Setup:

A proof of concept study was conducted with tin and silver/tin salt solutions dissolved into sulfuric acid to generate a plating solution. Subsequently, the plating solutions were spiked with serial additions of aqueous copper sulfate and measured with a Guided Wave M508plus UV-VIS spectrometer coupled to a 100 mm Teflon™ Axial Flow Cell.

Results:

As shown in Figure 1, the M508plus is able to discern the difference between 0 and 4 ppm Cu with a linear response up to 70 ppm Cu in Sn. The addition of Ag to the Sn Sulfuric Acid solution does not interfere with the optical determination of Cu ion concentration.

Figure 1: Partial Least Squares Regression of Cu (ppm concentration) in an Sn acidic plating solution.

To test the viability of a photometer, such as the ClearView® db, for the real-time monitoring of trace Cu contamination of electroplating solutions, three visible wavelengths were selected for Multiple Linear Regression (MLR) on the spectral data for the 0-20 ppm Cu in Sn samples. As shown in Figure 2, a calibration was achieved which estimates the average error for a photometer would be +/-0.8 ppm over the calibration range.

Figure 2: Multiple Linear Regression of 0-20 ppm Cu contamination of a Sn acidic plating solution.

Conclusion:

Real time detection of trace (part per million) contamination of electroplating solutions can be achieved with both Guided Wave analyzers; the M508plus full spectrum UV-Vis spectrometer or the ClearView db photometer. Understanding when to replace the plating solution, due to copper contamination can be achieved at a reduced cost with the Guided Wave ClearView db photometer and a 100 mm Teflon™ Axial Flow Cell. Either analyzer can be installed alongside existing plating systems, or manufacturers of plating workstations can purchase an OEM version of the analyzer, to monitor the buildup of copper ions in Sn and Sn/Ag sulfuric acid plating solutions. The proof of concept study presented in this application note illustrates that Guided Wave process analyzers can detect as little as 4 ppm copper in the Sn and Ag plating solution.