X-ray photoelectron spectroscopy and Auger electron spectroscopy investigation on the oxidation resistance of plasma-treated copper leadframes

Abstract

X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) have been employed in the investigation of the surface chemistry and oxidation-resistant property of the C-, N-, and Cu-containing films, formed by the plasma treatment of pristine copper. The copper oxidation process has been elucidated using realistic microelectronic packaging-assembly conditions, results of which are in agreement with established models. From the chemical shift in binding energy and change in full-width-half-maxima of the XPS core-level peaks, the heated copper has been observed to first form the monovalent cuprous oxide (Cu2O) followed by nucleation and growth of the divalent cupric oxide (CuO) above the Cu2O. The copper hydroxide [Cu(OH)2] has been observed to form on the Cu2O, CuO, and copper metal surfaces. This knowledge of copper oxidation has been used as a yardstick for the study of film decomposition and subsequent oxidation of exposed copper. Variation of the plasma treatment time produced two films of varied thickness [CuCxNy(A):15 nm and CuCxNy(B):75 nm], with the longer time producing the thicker films. AES results also suggest that the two films have the analogous film stoichiometry of 1:1:0.16 (Cu:C:N). Further XPS and AES analyses reveal the thickness dependence of the oxidation-resistant property, with the thicker film having the better oxidation resistance. A difference in film thickness of ∼5 times delayed the film decomposition temperature by 50 °C. AES depth profiles on the heated (250, 300, and 350 °C) films show the gradual decay of the film thickness with heating temperatures. Oxide growth predominates with the complete decomposition of the films. Present results suggest neither the presence of entrapped N2 gas in the copper oxide structure nor an oxide structure with the C and N decorations at the copper oxide grain boundaries. One possible film decomposition mechanism may be through the breaking of the Cu–N bonds under suitable heating conditions, thereby allowing the formation of the Cu–O bonds as the oxidation proceeds.