Inspection of Multilayered Electronic Devices via Scanning Acoustic Microscopy Using Synthetic Aperture Focusing Technique

Scanning acoustic microscopy (SAM) is a standard tool for the inspection of electronic devices. Modern devices can contain many interfaces, complex structures and various sub-devices in different height levels. Especially for safety relevant devices a full inspection of all these devices is needed. Current test strategies are compromising between the inspection time and the probabiltiy of failure detection. To improve the detection capabilities of SAM on modern electronic devices, Synthetic Aperture Focusing Technique (SAFT) is investigated in this paper. Several overlapping measurements of the same reconstruction point in a volume are combined to yield a large synthetic aperture with a narrow synthetic beam, yielding depth independent high lateral resolution and dynamic focusing. SAFT is commonly used in medical ultrasound or in non-destructive testing, e.g., for detecting crack tips, but has hardly been applied to SAM yet. Scan data is recorded with a scanning acoustic microscope for reference samples (polycarbonate block with drilled holes) utilizing a focused transducers with center frequency of 15 MHz. The focus is placed on the sample surface to generate the virtual point source and by this a wide opening angle in the sample. Reconstruction is performed in frequency domain as well as in time domain and different directivity patterns are examined. The comparison of the results on the reference sample shows the strengths and weaknesses of the different reconstruction methods. For real electronic samples the choice of the method strongly depends on the sample structure and the defect to be detected.