Abstract This paper discusses the basic physics of scanning acoustic microscopy, the counterfeit features it can detect, and how it compares with other screening methods. Unlike traditional optical inspection and IR and X-ray techniques, SAM can identify recycled and remarked chips by exposing ghost markings, fill material differences, delaminations from excessive handling, and popcorn fractures caused by trapped moisture. The paper presents several examples along with detailed images of these telltale signs of semiconductor counterfeiting. It also discusses the potential of developing an automated solution for detecting counterfeits on a large scale.

Abstract This paper assesses the capabilities of scanning acoustic tomography (SAT) for the analysis of bonded silicon wafers. In order to quantitatively evaluate detectability and resolution, the authors acquired images from samples prepared with artificial voids. The samples consisted of two wafers, a cap wafer and a base wafer with dry-etched pits on a silicon-oxide layer. Cap wafers of different thicknesses were used along with transducers of appropriate focal length. The paper describes the experimental setup and test procedures in detail as well as the results.

Abstract Currently gaps in non-destructive 2D and 3D imaging in PFA for advanced packages and MEMS exist due to lack of resolution to resolve sub-micron defects and the lack of contrast to image defects within the low Z materials. These low Z defects in advanced packages include sidewall delamination between Si die and underfill, bulk cracks in the underfill, in organic substrates, Redistribution Layer, RDL; Si die cracks; voids within the underfill and in the epoxy. Similarly, failure modes in MEMS are often within low Z materials, such as Si and polymers. Many of these are a result of mechanical shock resulting in cracks in structures, packaging fractures, die adhesion issues or particles movements into critical locations. Most of these categories of defects cannot be detected non-destructively by existing techniques such as C-SAM or microCT (micro x-ray computed tomography) and XRM (X-ray microscope). We describe a novel lab-based X-ray Phase contrast and Dark-field/Scattering Contrast system with the potential to resolve these types of defects. This novel X-ray microscopy has spatial resolution of 0.5 um in absorption contrast and with the added capability of Talbot interferometry to resolve failure issues which are related to defects within organic and low Z components.

Abstract Failure Analysis labs involved in customer returns always face a greater challenge, demand from customer for a faster turnaround time to identify the root cause of the failure. Unfortunately, root cause identification in failure analysis is often performed incompletely or rushing into destructive techniques, leading to poor understanding of the failure mechanism and root-cause, customer dissatisfaction. Scanning Acoustic Tomography (SAT), also called Scanning Acoustic Microscope (SAM) has been adopted by several Failure Analysis labs because it provides reliable non-destructive imaging of package cracks and delamination. The SAM is a vital tool in the effort to analyze molded packages. This paper provides a review of non-destructive testing method used to evaluate Integrated Circuit (IC) package. The case studies discussed in this paper identifies different types of defects and the capabilities of B-Scan (cross-sectional tomography) method employed for defect detection beyond delamination.

Abstract GHz-SAM using toneburst transducers is a method currently lacking the possibility to measure warped samples easily because large parts of such samples are out of focus due to the limited depth of focus of these types of transducers. This paper shows an approach to use the already established HiSA (High Speed Axis) method to overcome this disadvantage. Therefore warpage of the sample is measured by a white-light interferometer. The detected bow is parametrized and submitted in the control electronics of the HiSA. With this data the HiSA is enabled to keep the distance between sample and transducer precisely constant. This allows eliminating the influence of the warpage on the performance of this method and therefore highly increases the image quality.

Abstract This paper discusses the implementation of GHz-Scanning Acoustic Microscopy (GHz-SAM) into a wafer level scanning tool and its application for the detection of delamination at the interface of hybrid bonded wafers. It is demonstrated that the in-plane resolution of the GHz-SAM technique can be enhanced by thinning the sample. In the current study this thinning step has been performed by the ion beam of a ToF-SIMS tool containing an in-situ AFM, which allows not only chemical analysis of the interface but also a well-controlled local thinning (size, depth and roughness).

Abstract Signal processing and data interpretation in scanning acoustic microscopy is often challenging and based on the subjective decisions of the operator, making the defect classification results prone to human error. The aim of this work was to combine unsupervised and supervised machine learning techniques for feature extraction and image segmentation that allows automated classification and predictive failure analysis on scanning acoustic microscopy (SAM) data. In the first part, conspicuous signal components of the time-domain echo signals and their weighting matrices are extracted using independent component analysis. The applicability was shown by the assisted separation of signal patterns to intact and defective bumps from a dataset of a CPU-device manufactured in flip-chip technology. The high success-rate was verified by physical cross-sectioning and high-resolution imaging. In the second part, the before mentioned signal separation was employed to generate a labeled dataset for training and finetuning of a classification model based on a one-dimensional convolutional neural network. The learning model was sensitive to critical features of the given task without human intervention for classification between intact bumps, defective bumps and background. This approach was evaluated on two individual test samples that contained multiple defects in the solder bumps and has been verified by physical inspection. The verification of the classification model reached an accuracy of more than 97% and was successfully applied to an unknown sample which demonstrates the high potential of machine learning concepts for further developments in assisted failure analysis.

Abstract Scanning Acoustic Microscopy (SAM) is a very important tool in the evaluation of molded plastic electronic components. SAM is used to non-destructively determine the configuration and quality of components using ultrasonic sound waves and consequently is an important test step in the screening, Destructive Physical Analysis (DPA) or Failure Analysis (FA) of plastic components. SAM is performed in a water bath so if internal defects are open to the surface of the device they can fill with water and become invisible to SAM.

Abstract This paper discusses the Failure Analysis methodology used to characterize 3D bonded wafers during the different stages of optimization of the bonding process. A combination of different state-of-the-art techniques were employed to characterize the 3D patterned and unpatterned bonded wafers. These include Confocal Scanning Acoustic Microscopy (CSAM) to determine the existence of voids, Atomic Force Microscopy (AFM) to determine the roughness of the films on the wafers, and the Double Cantilever Beam Test to determine the interfacial strength. Focused Ion Beam (FIB) was used to determine the alignment offset in the patterns. The interface was characterized by Auger Spectroscopy and the precession electron nanobeam diffraction analysis to understand the Cu grain boundary formation.

Abstract This paper demonstrates the application of GHz-SAM for the detection of local non-bonded regions between micron-sized Cu-pads in a wafer-to-wafer hybrid bonded sample. GHz-SAM is currently the only available non-destructive failure analysis technique that can offer this information on wafer level scale, with such high resolution.

Abstract This paper is focused on the de-soldering process on VSON package which was mounted on FR4 substrate board after being subjected to environmental stress. Abnormalities were found at package level during Scanning Acoustic Microscopy (SAM) inspection which is considered to be one of the non-destructive failure analysis processes. Root cause finding involved the investigation of the de-soldering equipment which is suspected to be one of the culprits to contribute to the defect during de-soldering process.

Abstract GHz scanning acoustic microscopy (GHz-SAM) was successfully applied for non-destructive evaluation of the integrity of back end of line (BEOL) stacks located underneath wire-bond pads. The current study investigated two sample types of different IC processes. Realistic bonding defects were artificially induced into samples and the sensitivity of the acoustic GHz-microscope towards defects in BEOL systems was studied. Due to the low penetration depth in the acoustic GHz regime, a specific sample preparation was conducted in order to provide access to the region of interest. However, the preparation stopped several microns above the interfaces of interest, thus avoiding preparation artifacts in the critical region. Cratering related cracks in the bond pads have been imaged clearly by GHz-SAM. The morphology of the visualized defects corresponded well with the results obtained by a chemical cratering test. Moreover, delamination defects at the interface between ball and pad metallization were detected and successfully identified. The current paper demonstrates non-destructive inspection for bond-pad cratering and ball-bond delamination using highly focused acoustic waves in the GHz-band and thus illustrates the analysis of micron-sized defects in BEOL layer structures that are related to wire bonding or test needle imprints.

Abstract Through Silicon Via (TSV) is the most promising technology for vertical interconnection in novel three-dimensional chip architectures. Reliability and quality assessment necessary for process development and manufacturing require appropriate non-destructive testing techniques to detect cracks and delamination defects with sufficient penetration and imaging capabilities. The current paper presents the application of two acoustically based methods operating in the GHz-frequency band for the assessment of the integrity of TSV structures.

Abstract The growing popularity of 2.5D SSIT (Stacked Silicon Interconnect Technology) & 3D package technology in the IC industry had made it more challenging for manufacturers and packaging assembly sites to perform failure analysis and identifying the root causes of failures. There had been some technical papers written on various failure analysis techniques on 2.5D SSIT and 3D IC packages using a variety of equipment for detecting and localizing failures [1, 2]. This paper explains a non-evasive, non-destructive approach of localizing failures on a 2.5D SSIT package by identifying and recognizing certain waveform patterns that the failing devices exhibit in the scanning acoustic microscope A-Scan and in Time domain reflectometry. There are noticeable waveform patterns that an analyst can recognize and used to determine certain types of failure mechanisms that may be present in the device. Please note that it is very important to use the exact same type of package sample when characterizing and comparing waveform patterns as package variability from vendor to vendor and material contents can certainly affect the results.

Abstract This paper discusses the application of two different techniques for failure analysis of Cu through-silicon vias (TSVs), used in 3D stacked-IC technology. The first technique is GHz Scanning Acoustic Microscopy (GHz- SAM), which not only allows detection of defects like voids, cracks and delamination, but also the visualization of Rayleigh waves. GHz-SAM can provide information on voids, delamination and possibly stress near the TSVs. The second is a reflection-based photoelastic technique (SIREX), which is shown to be very sensitive to stress anisotropy in the Si near TSVs and as such also to any defect affecting this stress, such as delamination and large voids.

Abstract This paper describes the detection capability of micro delamination in ICs according to the transducer frequency of SAM (Scanning Acoustic Microscopy) equipment through a case study of non-destructive failure analysis. The analysis of scanning acoustic microscopy is non-destructive, but is difficult to define micro de-lamination or micro crack. In this study, two SAM systems and various transducer frequencies were used to detect micro de-lamination on the lead finger area. In the results of non-destructive analysis by utilizing two systems, one SAM detected the micro delamination but the other SAM did not define micro delamination with C-scan mode. To confirm an accurate delamination phenomenon, we analyze the destructive analysis under cross-sectional inspection and the micro de-lamination was observed. The cause of non-detection for micro delamination is not due to the difference between equipments, but due to the transducer frequency. This paper will be concluded with a discussion on what kind of transducer frequency are selected according to distance from package surface to chip surface, package materials, and micro de-lamination thickness.

Abstract FA cannot consist of simply jumping to conclusions. The FA process is validated through correlation with the initial failure and through interpretation of the obtained results, subjective by definition. This paper illustrates the difficulty of analyzing complex failures caused by multiple factors, including wafer fabrication, assembly, and application conditions. Inter-Layer Dielectric (ILD) delamination was experienced on various ICs from the same 250nm technology. A complete set of techniques (C-SAM, laser and optical microscopy, SEM, FIB cross-sections, TEM, EFTEM, SIMS, Auger, delineation) was used as different pieces of the same puzzle to reveal the multiple factors contributing to the ILD delamination failures. Due to the subtle nature of some of the underlying causes, defining an accurate FA approach with appropriate sample preparation and accurate device traceability was critical to understanding this complex, multivariate issue.

Abstract Contour milling by high precision CNC-milling offers the possibility to delayer precisely into warped and tilted package interfaces e.g. to expose the die backside. The needed data about the warpage of the surface of interest is in this case derived from SAM time of flight- measurements. The combination of these two approaches solves emerging challenges for backside preparation process.

Abstract This paper presents an original approach allowing determining the failure mechanism at the origin of onresistance (RDSon) drift on vertical Power N-MOSFETs, dedicated to automotive application. The studied devices failed after Temperature Cycling (TC) qualification stress. The originality of that paper concerns the necessity to use strategic Failure Analysis (FA) approaches to determine the origin of the defect, without any localization possibilities. In that perspective, an original microprobing sub-mohms resistance electrical characterization of the different conductive interfaces was performed in order to determine the failing layer. Then, physical destructive FA techniques (backside SAM, leadframe peel-off and mechanical cross-section) were combined in order to finely characterize the defect. As a result, problems in die attach layer were highlighted, confirming the electrical probing characterization. At last, root cause of this abnormal die attach will be discuss through review of assembly process parameters. These results allowed implementing corrections and improving product stress resistance.

Abstract Lifted bond balls in Integrated Circuit (IC) have numerous failure mechanisms. A simple external curve can confirm the open, and with package decapsulation, lifted balls can be readily observed. However, the exact cause can be difficult to identify. Most often, a cross section through the balls was performed, but it is far from being able to reveal the reason for lifted bond balls. A comprehensive FA approach is needed. Performing failure analysis through the back side of the die using Scanning Acoustic Microscopy (C-SAM) and Infra Red (IR) inspection helps to observe the conditions of the bond pads. Pulling the die from the mold compound can provide a pristine view of the bond ball-bond pad interface. This allows the detection of contaminants, both organic and inorganic, which cross sections cannot provide.