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9 May 2005 NDT of wafer direct bonding by non-confocal transmission phase sensitive acoustic microscopy
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Acoustic Micro Imaging (AMI) has long been established as a method of NDT of the wafer-to-wafer bonding quality in directly bonded wafers. In conventional imaging systems a C-Mode Scanning Acoustic Microscope operating in reflection is utilized. In this paper a non-confocally adjusted Phase Sensitive Acoustic Microscope (PSAM) operating in transmission at a frequency of 85 MHz is employed for imaging. This mode of operation results in a time-dependent point spread function, which together with full-transient data acquisition allows for its optimization in terms of resolution in the post-processing stage. Furthermore, the information contained in the images produced by varying time-dependent PSF is used for identification of bonding defects in directly bonded wafers. Both completely disbanded and weak bond regions in the wafer-to-wafer interface are identified. These latter areas are present, e.g. at the rim of the entirely disbanded regions as an intermediate interface condition between fully bonded and completely disbanded states. Mode conversion of the ultrasound waves at the solid-solid and solid-liquid wafer boundaries has been exploited to excite shear waves that are sensitive to weak bonds. A short burst transducer excitation and time-selective post-processing of the acquired data is employed to prevent overlap with the direct transmission signal or its echo sequences and in this way making visible the amplitude variation induced by the interface bond degradation.
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Evgeny Twerdowski, Reinhold Wannemacher, Axel Schindler, Nasser Razek, and Wolfgang Grill "NDT of wafer direct bonding by non-confocal transmission phase sensitive acoustic microscopy", Proc. SPIE 5768, Health Monitoring and Smart Nondestructive Evaluation of Structural and Biological Systems IV, (9 May 2005);

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