Scanning Acoustic Microscopy Beyond Conventional Applications
Authors: Brand Sebastian , Petzold Matthias 
Published: February 2020

The ultimate aim in the development of concepts, methods and architectures in microelectronic technologies is the continuous reduction of lateral dimensions by increasing integration density, functionality and processing performance of electronic and microelectronic components. Recent trends therefore employ all three spatial dimensions for further increasing integration density and functionality. Novel challenges and additional requirements on the physical and electrical behavior arising from these integration concepts result in the need for specifically adapted compositions and materials. Also existing testing methods are likely compromised due to challenges of either their resolution capabilities or the underlying contrast mechanisms. As novel architectural concepts require the development of appropriate processing technologies the robustness, reliability and performance of the individual processing steps are of superior importance with respect to their global applicability and thus require comprehensive evaluation prior to introduction into industrial manufacturing.

In process development, quality assessment, failure analysis and others inspection methods operating non-destructively are of major importance. The application of multiple tests employing a variety of contrast mechanisms is extremely valuable for assessment, defect localization and evaluation. Scanning acoustic microscopy (SAM) employ elastic waves that interact with a specimens mechanical properties. With the elastic waves capability of penetrating optically opaque materials acoustic microscopy enables the assessment of internal structures and features and allows the depth specific assignment. The contrast mechanism is based on the wave interaction with gradients in either elastic properties or mass density and can be evaluated both quantitatively and qualitatively. The properties of the elastic waves employed by the acoustic microscope can be adjusted and specifically adapted to the requirements of the inspection task. This provides a high degree of freedom for acoustically based analyses and makes SAM a valuable and powerful tool for non-destructive evaluation and assessment. Limitations and challenges of current conventional acoustic microscopy for the inspection of novel 3D-integration concepts mainly have their origin in the lateral resolution required for imaging feature sizes in the low micrometer range. However, in some cases phenomena are required that cannot be exploited easily and may require an adapted set-up for acoustic insonation. In a broad range of applications additional and specifically adapted signal analysis can be of high values for the detection and identification of defects and deviations.    

The current paper focusses on scanning acoustic microscopy and its applications and challenges as non- and semi-destructive testing method for 3D integration but also for microelectronic and wafer bonding technologies. The development and modification of an acoustic GHz-microscope and its application for inspection of 3D-relevant technologies is described. With lateral imaging resolutions of 1 µm and below TSVs of 10 µm diameter have been inspected for filling-voids and rim-delaminations. Due to the large numerical apertures employed in the GHz-SAM wave modes like Rayleigh- or shear modes can be excited in addition to the pure compressional wave component potentially providing access to new contrast phenomena. For assisting result interpretation however, modelling and numerical simulation of the complex wave propagation became necessary and will be described. The high sensitivity to surface and near-surface structures is illustrated by the detection and localization of sub-surface µ-cracks describing the value of acoustic GHz-microscopy for non-destructive crack detection in near-surface regions of highly integrated microelectronic components.

In additionally to GHz-SAM conventional scanning acoustic microscopy supplemented by signal analysis approaches operating in time- and spectral domain and their value for defect localization and evaluation will be presented.