Surface modification of aluminum on a silicon chip by citric acid treatment

Aluminum is widely used as a conductive material in the semiconductor and electronics fields. However, the inevitable formation of an oxide layer on its surface during atmospheric heat treatment accompanying the fabrication process increases surface resistivity. This necessitates a surface treatment to mitigate the negative effects of the oxide layer. In this study, a silicon chip containing aluminum wires was immersed in a 0.1 wt% citric acid solution as a simple surface treatment technique. In situ electrical current measurements from arrays of aluminum wires located on the silicon chip were performed during the treatment. X-ray photoelectron spectroscopy and infrared spectroscopy were conducted before and after the treatment. The results revealed a significant removal of the contamination/oxide layer formed on the chip surface, and the outermost stacked layer on the aluminum surface turned hydrophilic (ca. 0.3 Å/min at 50 °C), resulting in a considerable decrease in the electric resistance around the surface. As a demonstration, a sensor made of Al and Au arrays was placed on the silicon chip, and the citric acid treatment helped enhance its response to humid air by more than 1000, with the steady current response realized in a treatment time of 2–3 min. This promising surface treatment method is expected to help enhance the removal of oxide layers formed on metal-based components.