The iButton temperature logger is a robust and independent system that measures temperature and records the temperature data in a non-volatile memory section. Although iButtons are a relatively mature product, failures due to Battery Depletion and Battery-On-Reset (BOR) occur. Despite the implementation of major corrective actions to improve product reliability and robustness, customer failures due to Depleted Battery and BOR have not been fully eliminated. One recent quality issue involved a high failure rate on one iButton variant at the customer’s quality control before calibration, which prompted a thorough failure analysis to nail down the real cause of failure. This paper presents methodical failure analysis (FA) steps and processes that led to the identification of failure mechanisms. As a result, the real issue was detected leading to a more accurate corrective action and device reliability.

By using fluorocarbon gases for aluminum (Al) pad open plasma etch, the pad inevitably has a thin surface remnant layer of Al-oxyfluoride (AlOF) by-product. This layer is chemically stable and does not directly cause issues in chip testing or wire bonding. This is true until open Al pads were exposed to a humid environment causing pad corrosion over time. The F-assisted corrosion created so-called black mushroom (BM) defects on the Al pads according to the defects appearance, resulting in the non-stick pads for wire bonding. Experimental tests were carried out to induce the Al pad corrosion via placing random fab-out wafers in a cassette pod hosting about 90% RH over a period up to a week. Optical imaging revealed BMs nucleated, primarily at Al grain boundaries. BMs were found all to be composed of O, F, and Al. In the cross section, BMs were shown to have separations of F-rich region next to Al and O-rich region towards the surface. In addition, BMs were composed of small crystallites and were porous. The former indicates an ionic bonding involving in O, F, and Al. The latter indicates the corrosion generated gaseous byproduct. A moisture (H 2 O) involved cyclic chemical reaction incorporating these analyses has been formulated. Factors to prevent BM formation were discussed.

Copper ball bonding is the most widely used interconnection method in microelectronic packages. It has enabled many modern technologies, but the bond can fail due to corrosion. This paper concerns quantitative analyses of corrosion products of passing and failing copper ball bonds, and correlation with the corrosion thermodynamics. The role each element in the aluminum-copper intermetallic compound plays during crevice corrosion is described, and relative abundances of the oxidized elements are estimated. New insights regarding mechanisms of the highest vulnerability to corrosion attack in the thin film-stack across the bond are presented. Limited data indicate the same corrosion mechanisms for Au ball bonds.

Microscopic imaging and characterization of semiconductor devices and material properties often begin with a sample preparation step. A variety of sample preparation methods such as mechanical lapping and broad ion beam (BIB) milling have been widely used in physical failure analysis (FPA) workflows, allowing internal defects to be analyzed with high-resolution scanning electron microscopy (SEM). However, these traditional methods become less effective for more complicated semiconductor devices, because the cross-sectioning accuracy and reliability do not satisfy the need to inspect nanometer scale structures. Recent trends on multi-chip stacking and heterogenous integration exacerbate the ineffectiveness. Additionally, the surface prepared by these methods are not sufficient for high-resolution imaging, often resulting in distorted sample information. In this work, we report a novel correlative workflow to improve the cross-sectioning accuracy and generate distortion-free surface for SEM analysis. Several semiconductor samples were imaged with 3D X-ray microscopy (XRM) in a non-destructive manner, yielding volumetric data for users to visualize and navigate at submicron accuracy in three dimensions. With the XRM data to serve as 3D maps of true package structures, the possibility to miss or destroy the fault regions is largely eliminated in PFA workflows. In addition to the correlative workflow, we will also demonstrate a proprietary micromachining process which is capable of preparing deformation-free surfaces for SEM analysis.

Failure to apply the proper systematic analysis procedure can result in loss of valuable evidence required to understand the root cause of package failures. For example, in the case of marginal current leakage fail, decapsulation from package front-side may result in loss of the electrical failure signal so that root cause of the leakage failure cannot be understood. In such case, a systematic backside fault isolation method can improve the success rate of isolating the defect. These electrical failures are often due to zero solder bond line thickness (BLT), or filler particle compression on the die, which are key assembly defects encountered in clip style surface mount packages (SMX). In this paper, the first case study is to determine the failure mechanism of an electrical short. A silicon micro-crack propagating through the junction at the dimple clip center, which is due to the ultra-thin solder BLT close to zero micron is found to be the root cause of failure. The second case presents the failure mechanism for a low leakage fail. The pointed tip of a silica filler particle compressed on the die surface leads to excessive leakage.

A typical workflow for advanced package failure analysis usually focuses around two key sequential steps: defect localization and defect characterization. Defect localization can be achieved using a number of complementary techniques, but electro optical terahertz pulse reflectometry (EOTPR) has emerged as a powerful solution. This paper shows how the EOTPR approach can be extended to provide solutions for the growing complexity of advanced packages. First, it demonstrates how localization of defects can be performed in traces without an external connection, through the use of an innovative cross-sectional probing with EOTPR. Then, the paper shows that EOTPR simulation can be used to extract the interface resistance, granting an alternative way of quantitative defect characterization using EOTPR without the destructive physical analysis. These novel approaches showed the great potential of EOTPR in failure analysis and reliability analysis of advanced packaging.

Advanced packages such as 2.5D will continue to grow in demand as performance increases are needed in various applications. Failure analysis must adapt to the changes in the interfaces, materials and structures being developed and now utilized. Traditional techniques and tools used for selectively removing materials to isolate and analyze defects need to evolve alongside these packages. A CF4-free Microwave Induced Plasma (MIP) process is used to remove underfill (UF) with minimal alteration of other materials on the samples, a process which has become more difficult on 2.5D modules. UF is removed using this MIP process to allow subsequent analysis on interposer interconnects and ìbumps in crosssection. SEM inspection, Electron Beam Absorbed Current (EBAC), and FIB are techniques used post cross-sectional UF removal of these 2.5D structures. The benefits of the specific MIP process through case studies are presented. Specifically, the use of an automatic cleaning step and a CF4-free downstream O2 plasma allow easy removal of UF without damaging other structures of interest with little tool recipe development.

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.

Fan Out - Panel Level Packaging (FO-PLP) has redistribution layers (RDLs) which connect IC to a substrate. And each layer in the RDLs is connected through copper micro-vias. Viarelated defects including via separation are very critical because they can escape from electrical test and be found in the field. So many cleaning methods have been developed to keep the target pad surface free of oxides or organic contamination before forming vias. In this paper, we present a via separation case caused by alkaline cleaning introduced before seed metal deposition for electroplating of copper. We investigated the cause by analyzing the microstructure and chemical composition using a focused ion beam (FIB) and a transmission electron microscope (TEM) equipped with an energy dispersive spectrometer (EDS). Via separation, interestingly occurred at the interface between the seed Ti and the seed Cu not the interface between the seed Ti and the target pad..Cu surface which is known to be weak. We suggest a mechanism that structural imperfections at the outer rim of via bottom and galvanic couple of titanium and copper are involved in the separation of vias. Since two dissimilar metals of Ti and Cu are in direct contact, galvanic corrosion can occur in the presence of alkaline solution and discontinuities in the seed Ti layer. We found that galvanic corrosion in the studied system can be further complicated by the existence of copper oxide and titanium oxide as well as Cu and Ti.