Evaluation of directly bonded silicon wafer interface by the magic mirror method

The magic mirror method was used to evaluate directly bonded Si wafers. An unbonded region at the silicon-to-silicon interface was detected as a bubble in an optical image by this method. The bubbles at the interface generate a locally convex distortion at the surface and this convex surface is visualized as a dark region using the magic mirror method. A comparison of the resulting effect was made with transmission X-ray topography and ultrasonic flaw detection, and good correspondence was found among the three methods, especially between the magic mirror method and transmission X-ray topography. We applied this method to the observation of changes of interface bubbles before and after thermal annealing.