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ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4 4 EDFAAO (2018) 4:4-12

1537-0755/$19.00 ©ASM International ®

HIGH RESOLUTION ACOUSTIC GHz MICROSCOPY Sebastian Brand, Michael Kögel, and Frank Altmann

Fraunhofer Institute for Microstructure of Materials and Systems, IMWS, Center for Applied Microstructure Diagnostics CAM Halle, Germany sebastian.brand@imws.fraunhofer.de

INTRODUCTION Nondestructive acoustic inspection techniques are widely used methods for defect localization. With their special ability to penetrate optically opaque materials, these techniques enable analysis and imaging through housing or encapsulation materials to sense irregulari- ties, which accompany potential defects and thus provide information on their 3D position inside the device. With this ability, acoustics-based techniques are also of high value for the quality screening often performed in the industrial manufacturing environment, especially for high-reliability applications, e.g., the automotivemarket, where a low failure rate is mandatory. However, the indirect access through intermediate materials goes along with a reduction in the techniques’ resolution capabilities, and artifacts can occur that are caused by interactions of the sensing rays on their propagation path through the sample. The result is a compromise between the nondestructive nature of the technique and its achievable lateral and axial resolution. Therefore, both the precision and sensitivity of a defect localization method need careful consideration, as they are highly relevant for guiding the subsequent physical preparation required to provide close access for imaging and analysis techniques that allow for superior lateral resolution. Inspection for delamination, cracks, and voids by scanning acoustic microscopy (SAM) is highly efficient because the contrast mechanism is based on the acoustic waves’ interaction with the material’s mechani- cal properties. This interaction also leads to phenomena such as scattering, reflection, and refraction. Therefore, voids, cracks, and delamination, which can be considered gaseous inclusions in the package, manifest as large gra- dients in the material’s mechanical properties. SAM can detect such material gradients with superior sensitivity. Ongoing advances in microelectronics technologies, such as 3D integration and system inpackage (SiP), enable significant improvements in performance and integration

density, resulting in increasingly complex systems while reducing a device’s spatial requirements. However, new failuremechanisms and failuremodes are inevitably con- nected to these novel technologies, challenging existing methods and tools for nondestructive defect detection and localization. Nonetheless, these tools and techniques are necessary both during process development and in high volume industrial manufacturing. Conventional acoustic microscopy that operates in the frequency band below 400 MHz is highly sensitive to the aforementioned defects, but lacks lateral resolution and sensitivity when the failure sites get too small. With the focus on high resolution failure analysis and metrology applications, a novel SAM device with the spectral range extending up to 2 GHz was developed in the Fraunhofer IMWS lab in collaboration with industry partners. This substantial increase in frequency leads to a significant decrease inwavelength, which combinedwith theapplicationof highly focusedacousticobjective lenses, would theoretically allow for lateral resolution capabili- ties of below1µm. [3] However, this increased lateral resolu- tion comes at the expense of the achievable penetration depth, which renders the technique highly sensitive to surface and subsurface areas. The method is therefore ideally suited for thin structures in the thickness range below 5 µm, which are highly relevant to current micro- electronic technologies. GHz-SAM Figure 1 shows a schematic of the gigahertz-scanning acoustic microscope (GHz-SAM). A signal, which is gen- erated either by an arbitrary waveform generator or a gated oscillator, is power amplified and directed to the acoustic transducer. There, the signal is converted into an elastic (mechanical) wave and submitted through a droplet of coupling medium (to reduce reflectivity) into the sample. The transducer contains a large numerical aperture acoustic lens that focuses the mechanical GHz waves into an extremely narrow spot of approximately

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