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0.8 µm in the lateral dimension and an axial length of 4-6 µm. The focus of the lens is located 80 µm in front of the transducer’s tip inside the cou- plant (distilled and degassed water is commonly used). On its pathway, the acoustic wave interacts with the elastic material properties and mass density of the propagation materials. Consequently, refraction, reflection, and scattering occur at any material boundaries the wave encounters. Fur- ther, additional interaction phenomena may occur inside the sample where the energy of the incident acoustic wave is partially converted into other wave modes that propagate inside the materials, at the surfaces, or at the interfaces between specific structures in the sample. Regardless, some of

ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 20 NO. 4

Fig. 1 Schematic of the GHz-SAM.

sub-micron voids on 3-µm-thick power metal lines. Additionally, the positive effect of incorporating a 40-nm TiN interlayer in between two AlCu metallization layers was investigated and confirmed. In addition to material composition, properties, and interactions in back end of line (BEOL) systems, void formation—and the critical volume that causes fatal fail- ures—greatlydependsonthemetal structuredesign. These structures need to be adapted based on current density and temperature stability requirements to ensure product reliability. The metal structures can be improved by opti- mizing the width of the metal lines and inserting an addi- tional interlayer material to form a sandwich-like struc- ture. This structure is known to lead to the suppression of large void formation and agglomeration. Samples investigated in this article were manufac- tured employing two different structures of the metal lines. The aluminum(AlCu)metallization had a dimension of 3 × 3 × 800 µm long. Because Ti and TiN are known to have properties that suppress hydrogen-related voiding, [5] TiN was added to the second set of samples. The second set contained a 40-nm thick TiN interlayer material

the incident acoustic waves' energy will return to the transducer, which depending on the mode can occur at different propagation times, as the wave velocities are mode-specific. Those signals are received by the transducer, convert- ed into an electrical signal, and low-noise amplified. To further increase the signal-to-noise ratio (SNR), the signal is preprocessed and then digitized for image formation. A recent extension of the GHz-SAM implemented a high- performance digitizer to capture the unprocessed radio frequency (RF) signals for further offline signal analysis and parametric imaging, particularly in the spectral domain. CASE STUDIES Several case studies have been published by the Fraunhofer IMWS research group to illustrate the success- ful application of GHz-SAM for defect inspection in fields where destructive slice and view analysis was the only alternative for detection and identification of interface defects or crackpropagationpaths. With its relatively large scan field of 2 × 2mmand a possible imaging resolution in the µmregime, superior sensitivity is combinedwith large

area screening and a contrast mecha- nism sensitive to material gradients.

CASE #1: DETECTION OF STRESS INDUCED VOIDS

In the first case study, GHz-SAM was employed to examine the for- mation, growth, and propagation of

Fig. 2 Schematics of samples containing thick AlCu lines with and without a TiN interlayer.

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