Acoustic GHz-microscopy and its potential applications in 3D-integration technologies

3D-integration is one of the most challenging approaches addressed by researchers in the field of microelectronics in the recent years. With the intension on integrating different components in three dimensions in one device performance and functionality will increase dramatically by reducing the devices footprint. A major challenge in 3D-integration concepts is the electrical interconnection of the stacked individual components. These interconnecting technologies employ micro bumping, temporary wafer bonding, wafer thinning and through silicon vias (TSVs). The increasing complexity and the miniaturization result in new requirements on testing, diagnostics, failure analysis and metrology techniques, methods and tools. Scanning acoustic microscopy (SAM) is a powerful tool for non-destructively inspecting internal structures and features. It employs elastic waves that can be focused and used for imaging and quantitative analyses. However, at conventionally used frequencies (5 MHz - 250 MHz) imaging resolution compromises the application on devices and technologies required in 3D-integration approaches. The current paper reports on the use of acoustic GHz-microscopy for the inspection, defect localization and its ability for identification of abnormalities in through silicon vias. Investigated were artificial and real defects in the TSV-fillings (voids) and the condition of the TSV-walls (rim-delaminations). Acoustic frequencies used in the current work ranged from 400 MHz up to 1.2 GHz allowing for imaging resolutions in the 1 μm - regime. However, highly focused acoustical lenses as employed here require large numerical apertures which inevitably result in a very complex wave propagation and acoustic field inside a solid sample. To improve the understanding and interpretation acoustic intensity fields have been simulated numerically. Results obtained by acoustic GHz-microscopy have been evaluated complimentarily by FIB-cross-sectioning and SEM imaging which gave a valuable insight into the abilities for acoustic TSV-inspection by GHz-SAM.