Near-Field Synthetic Aperture Focusing Technique to enhance the inspection capability of multi-layer HBM stacks in Scanning Acoustic Microscopy

Summary:

This contribution explores the improved inspection of High Bandwidth Memory (HBM) stacks with Scanning Acoustic Microscopy (SAM). Due to the complexity of the multi-layer structure advanced signal processing methods are applied. To overcome the limitations of detection capabilities vs. inspection time, Synthetic Aperture Focusing Technique (SAFT) is used. In contrast to previous trails applying SAFT on SAM data, this contribution introduces the Near Field SAFT where the reconstruction is done also for layers between the transducer and its focus, thus in the near field of the transducer. This allowes measurements with common working distances and provides higher frequencies and better resolution. Systematic evaluations on different measurement setups and transducers with various center frequencies and focal lengths are performed to find an optimal measurement setup.