In this paper, we investigate a novel fast and reliable method to check the bonding quality of silicon wafers. It is based on illuminating the wafers with a high frequency waves (110 - 170 GHz) using quasi-optical technique. The reflected energy is used to evaluate the bonding strength. The reported experimental study is compared with the Infrared images.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.