Characterization of pad surface defect by TEM, SEM and EDX

Tiny defects were observed by SEM (scanning electron microscopy) on pad surface. In order to identify the root cause, TEM (transmission electron microscopy) and EDX (energy dispersive X-ray) techniques were used to do failure analysis. The SEM images showed that the tiny surface defects were pits. Higher Cu concentration at pits area was detected compared with normal area based on the EDX data. TEM and EDX analysis showed that the defect could be composed of an Al-Cu (and/or possible pure Cu) core and the halo of aluminum oxide (and/or possible hydroxide). The result can be explained as that the pre-existing Al₂Cu (theta phase) particles and the surrounding aluminum metal might act as the electrodes and favored the occurrence of galvanic corrosion during the backend DI water cleaning process or when the wafer was in a moisture environment. In this paper, the mechanism of galvanic corrosion was further discussed based on the TEM data.