Acoustic inspection of high-density-interconnects for 3D-integration

The increasing demand on the complexity of microelectronic components will soon require architectures that build in the third dimension. A number of current projects and world-wide research in this field focuses on technologies that aim at combining individual devices on wafer level into complex systems that have a variety of features. However, the interfaces available for interconnecting the individual components are limited and thus a three-dimensional approach will provide the ultimate solution. An architecture that enables the above mentioned features will largely challenge conventional inspection techniques for quality control and failure analysis. Of particular interest for 3D-integrated devices are methods that operate non-or semi-destructively. Acoustic microscopy remains a powerful tool for investigating opaque samples and their internal structure. However, due to the continued miniaturization and the demand of higher resolution acoustic microscopy has to go to smaller wavelengths. In the current study bonded devices with electrical routings in the bonded interface were inspected using acoustic microscopy applying highly focussed ultrasonic transducers operating at 1 GHz. Inspected samples contained high density interconnects (10(6) per cm(2)) forming large area arrays on 10 mu m and 15 mu m pitch. Acoustic inspection was performed through the top die of the device after thinning to 10 mu m thickness. The acoustic inspection was also performed through a polymer mechanical key after removal of the top die. Defects detected acoustically were confirmed by FIB-cross sectioning and high-resolution SEM imaging. Defects found were delaminations in the electrical cross-links and voids in the underfill material when inspecting samples with and without a thinned top-die. The trade-off between the achievable resolution, acoustic attenuation and the thickness of the top-die has to be taken into account when imaging through opaque materials. It is expected that semi-destructive preparation will be required in practical applications. However, for performing failure analysis the acoustic inspection and the detection of delaminations at the interfaces will be greatly beneficial for guiding additional destructive analyses.