Wafer-to-wafer bonding is widely used to support the production of both integrated circuits and MEMS devices. Bonding wafers together is accomplished in a variety of ways including anodic, thermal compression, and adhesive bonding. The bond can be either permanent or temporary. Permanent wafer bonding is used to combine two materials together that remain together for the life of the device. Temporary bonding is primarily used to support a device wafer during certain processing steps, and then the support wafer is removed once those steps are completed. Both types of bonds must be uniform and free of voids or defects. Examples of bonding defects include particulates, cracks, or delaminations. In permanent bonding, void-free bonds are considered critical for long device life and good yield. For temporary bonding, voids may result in yield loss on a die scale as well as overall adhesive failure across the wafer stack. A common process using temporary bonding is thinning of the device substrate. During this process, a point can be reached where the thinned silicon will not support itself across a void and will crack and/or separate. Also, voids in bonded pairs may contain residual gases, which can expand when the outer surface is exposed to vacuum and/or elevated temperature, thus destroying the thinned wafer. With these and many other issues associated with voids or defects, their detection is critical to the success of all bonding solutions. Scanning acoustic microscopy has been shown to be very effective in detecting voids in bonded silicon wafers and even in packaged die. This technique is very useful, however, it is expensive and time consuming. Infrared microscopy has also been shown to detect defects in the bonding material. This technology uses a focal plane array that is sensitive in the shortwave infrared radiation spectrum where silicon is semitransparent. The limitations with this technology are resolution and differentiation between different types of defects. In this paper, an alternative methodology is reported that utilizes a very sensitive near-infrared radiation (NIR) camera technology from FLIR Systems, Inc. (FLIR), coupled with colorizing software to enhance the output. Included in the report is an analysis of the performance and limitations of the technique by a comparison to scanning acoustic microscopy and optical profilometry.
Keywords: silicon wafer bonding, bond inspection, through-silicon imaging, infrared microscopy, color enhancement