Fast Simultaneous Thickness Measurements of Gold and Nickel Layers on Copper Substrates

With the recent increase in the price of gold and other precious metals, new ways are being investigated to reduce the consumption of these materials. For example, replacing the uniform plating process with the selective electroplating of gold to tailor-fit the gold layer to the small mating surfaces of the contacts in connectors and integrated circuit packaging hardware will result in large savings of gold. With this new approach, monitoring the thickness of the gold and the nickel layer underneath it at relatively high degrees of accuracy and at high speed becomes an important factor in controlling the deposition process. Today, the most common way to measure the thickness of the gold layer is to use a radioactive beta source which generates high-energy 1851 electrons and then to monitor the backscattered electrons from the gold layer to determine its thickness. Since bright radioactive sources are not available, the system is only adequate for the thickness measurement of materials over 0.5 mm in diameter and with measurement times of 1 min or longer. In addition, the measurement is very sensitive to any change in position of the substrate material relative to the electron detector. X-ray fluorescence has been used by many authors to determine thickness of both the nickel and gold layers.1,2 This is done by monitoring the magnitude of the direct fluorescence from the two layers which are excited by a shorter X-ray wavelength. However, as shown in this paper, the fluorescence intensity produced by the gold saturates at gold thicknesses above 3 jam, which limits the useful range of thickness measurements to less than 3 jum.