Super wide area cross sectioning using broad Ar ion beam on solder bumping

Soldering is a popular jointing method for integrated circuits (IC) and CMOS image sensor packaging methods, such as flip-chip technology [1]. Generally, X-ray tomography is used for defect analysis of devices containing solders. However, with the miniaturization of devices, it has become difficult to observe more complex packaging failure mechanisms using X-ray tomography. Moreover, imaging the defects and internal features of the devices is difficult because of their small size. In addition, component analysis is not possible when using X-ray tomography. In this regard, SEM enables defect analysis by affording high-resolution observation of the cross-section of devices; thus, even the size of defects is smaller than 1 μm can be located. Usually, to understand the details of the interface between the solder and electrode, an electron microscope is used; however, the use of ion beam preparation methods—such as focused ion beam (FIB), which has a high position accuracy, and a broad Ar ion beam—for the preparation of large-area cross-sections is preferred. This is because it is difficult to prepare a fine cross-section owing to the different hardness of the materials. However, the IC is much larger than the process area of both the FIB and broad Ar ion beams. Therefore, a much wider area process is required to analyze several solder bumps simultaneously. Recently, we have been successful in developing a very wide-area Ar ion milling with the shield plate method, which facilitates the preparation of a precise cross-section with a width of approximately 8 mm. Herein, we discuss the principle of this method and demonstrate a wide-area preparation method for a system in package (SiP) with and without liquid nitrogen (LN
2
) cooling.