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The interface of Au/Si(100) eutectic bonding was investigated by infrared (IR) microscope and related to the bond strength. A strong relationship between the IR images and the bond strengths was found. Bond strength test showed that a strong bond has many square black spots in the IR images, whereas a poor bond has fewer or no square black spots. In order to study the nature of the relationship, the dissolution behavior of the bare Si(100) surface after bonding was investigated. During the Au/Si(100) eutectic reaction, the dissolution of the bare Si(100) surface primarily occurs by the formation of the craters which result in many square black spots in the IR images. The formation of the craters is ascribed to the anisotropic nature of Au/Si reaction that results in three-dimensional dissolution behavior on the bare Si(100) side. In order to further test the anisotropy hypothesis, Au/Si(111) bonding was also studied. Under the same bonding conditions, triangular black spots were observed in the IR images and triangular pits were found on the bare Si(111) surface. The analysis suggests that the craters on the bare Si(100) surface, in other words the square black spots in the IR images, are the indication of Au/Si(100) eutectic reaction. More craters mean a reaction between Au and Si(100), which occurs uniformly at the Au/Si(100) bonding interface compared to the case of fewer craters. No crater indicates that there is no eutectic reaction in the region. Therefore, the IR microscope may be used to evaluate and compare the different bond strengths qualitatively.