High resolution acoustical imaging of high-density-interconnects for 3D-integration

Due to increasing integration density, breakthrough improvements in the resolution achievable by a specific inspection method are of great interest to the 3D Integration community. This requirement is particularly driven by wafer bonding technologies that, in addition to mechanical 3D assembly, were also often used to form buried electrical chip-to chip interconnects with bonding pitch dimensions on the order of 10 μm. Acoustic microscopy is a unique tool for nondestructive inspection of internal structures in opaque materials. However, in acoustics the achievable resolution is strongly dependent on the wavelength of the insonated signal, which varies dramatically with the acoustic wave velocity. In the current study bonded devices with electrical routings in the bonded interface were inspected using acoustic microscopy applying highly focussed ultrasonic transducers with acoustic frequencies ranging from 100 MHz up to 1 GHz. Samples inspected contained high density interconnects (10
⁶
per cm
²
) forming large area arrays (512 × 640) on 10 μm and 15 μm pitch. Depending on the thickness of the top-layer the acoustic frequency and focal length of the transducer was selected for imaging. With a resolution of approx. 1 μm acoustic imaging of the metal-links behind 5 μm of BCB and underfill was performed. Delaminations of the lateral links were detected without opening the mounted polymer layer. The delaminated links were confirmed by FIB cross-sectioning and high-resolution SEM imaging. In addition voids in the underfill material (a low moisture absorption epoxy) have been detected through the top-die. In further experiments acoustic inspection of the interconnects between the two wafers have been performed at 100 MHz, 200 MHz and 400 MHz with focal lengths of 2 mm down to 200 μm. The trade-off between the achievable resolution, acoustic attenuation and the thickness of the top-die has to be taken into account when imaging at a 5 μm scale through opaque polymer materials. It is expected that semi-destructive preparation will be required in practical applications, namely thinning of the top-wafer to an appropriate thickness. However, for performing failure analysis the acoustic inspection of the interconnects and the detection of delaminations at the interfaces between the dielectrics, underfills and metals in the bonded devices will be greatly beneficial for guiding additional destructive imaging and analyses.