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.

Currently, wire bonding is still the dominant interconnection mode in microelectronic packaging, and epoxy molding compound (EMC) is the major encapsulant material. Normally EMC contains chlorine (Cl) and sulfur (S) ions. It is important to understand the control limit of Cl and S in the EMC to ensure good Au wire bond reliability. This paper discussed the influences of Cl and S on the Au wire bond. Different contents of Cl and S were purposely added into the EMC. Accelerated reliability tests were performed to understand the effects of Cl, S and their contents on the Au wire bond reliability. Failure analysis has been conducted to study the failure mechanism. It is found that Cl reacted with IMCs under humid environment. Cl also caused wire bond failure in HTS test without moisture. On the other hand, the results showed that S was not a corrosive ion. It was also not a catalyst to the Au bond corrosion. Whilst, high content of S remain on the bond pad hindered the IMCs formation and caused earlier failure of the wire bond.

In today’s advanced technology world, electronic devices are playing a key role in modern semiconductor products to improve the energy proficiency. These devices are required to be contamination free especially on the bond pad with good adhesion before wire bonding process at the back end. Contamination on the bond pad leads to reliability issues such as pad corrosion, delamination and failure leading to leakage and open fails of electronic devices. Therefore, detection accuracy and sensibility of contamination is important. Auger analysis is the most suitable technique to check bond pad contamination. Auger electron spectroscopy has the capability of analyzing compositional information with excellent spatial resolution. However, charging, noise or artifact is known to be a major concern to the characterization of insulating materials. This paper outlines the strategy that has been utilized to minimize the artifact, noise or charging impact for Auger investigation on a smaller bond pad surrounded by imide passivation layers. The imide passivation layer normally causes the charging effect during Auger analysis, which makes the Auger analysis difficult to be proceed. In addition to that, the charging effect leads to inaccurate analysis. In this paper, we demonstrate a sample preparation method to minimize the charging and artifact of Auger analysis especially for small bond pads.

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.

This paper describes the investigation of donut-shaped probe marker discolorations found on Al bondpads. Based on SEM/EDS, TEM/EELS, and Auger analysis, the corrosion product is a combination of aluminum, fluorine, and oxygen, implying that the discolorations are due to the presence of fluorine. Highly accelerated stress tests simulating one year of storage in air resulted in no new or worsening discolorations in the affected chips. In order to identify the exact cause of the fluorine-induced corrosion, the authors developed an automated inspection system that scans an entire wafer, recording and quantifying image contrast and brightness variations associated with discolorations. Dark field TEM images reveal thickness variations of up to 5 nm in the corrosion film, and EELS line scan data show the corresponding compositional distributions. The findings indicate that fluorine-containing gases used in upstream processes leave residues behind that are driven in to the Al bondpads by probe-tip forces and activated by the electric field generated during CP testing. The knowledge acquired has proven helpful in managing the problem.

At ESREF 2008, the paper “Unusual defects, generated by wafer sawing: Diagnosis, mechanisms and how to distinguish from related failures” had won the Best Paper Award. In the meantime, new experiences were collected, related to new methods as laser sawing and its specific ESD risks and additional failure mechanisms as backside damage, charging of foils, pad corrosion and sawing residue damages. This paper explains in detail these failure sources, including detailed explanations on root causes and physical mechanisms as well as important hints for failure analysts how to distinguish related failure signatures from those, which look similar but are of other origin.

In wafer fabrication (Fab), Fluorine (F) based gases are used for Al bondpad opening process. Thus, even on a regular Al bondpad, there exists a low level of F contamination. However, the F level has to be controlled at a lower level. If the F level is higher than the control/spec limits, it could cause F-induced corrosion and Al-F defects, resulting in pad discoloration and NSOP problems. In our previous studies [1-5], the theories, characteristics, chemical and physical failure mechanisms and the root causes of the F-induced corrosion and Al-F defects on Al bondpads have been studied. In this paper, we further study F-induced corrosion and propose to establish an Auger monitoring system so as to monitor the F contamination level on Al bondpads in wafer fabrication. Auger monitoring frequency, sample preparation, wafer life, Auger analysis points, control/spec limits and OOC/OOS quality control procedures are also discussed.

Contamination by particles is one of the major causes of failures in integrated circuits. In some cases, particles may absorb moisture leading to electrochemical migration, dendrite growth, and electrical leakage and short failures. This work presents two case studies of particle induced corrosion of copper wire bond that resulted in an electrical failure. In the first case, adjacent pin resistive short failures were found to fail due to corrosion and electrochemical migration at wires that were in contact with calcium chloride particles. Analysis showed that the highly hygroscopic calcium chloride particles absorbed moisture and resulted in corrosion and electrochemical migration of the copper wires. For the second case, an electrical open failure after temperature cycle reliability test was found to be due to an organophosphorus particle being in contact with the wire.

Coating of the Cu bond wire with Pd has been a rather widely accepted method in semiconductor packaging to improve the wire bonding reliability. Based on comparison of a Cu bond wire and a Pd-coated Cu bond wire on AlCu pads that had passed HAST, new insight into the mechanism of the reliability improvement is gained. Our analysis showed the dominant Cu-rich intermetallics (IMC) were Cu3Al2 for the Cu wire, and (CuPdx)Al for the Pd-coated wire. The results have verified the Cu-rich IMC being suppressed by the Pd-coating, which has been extensively reported in literature. Binary phase diagrams of Al, Cu, and Pd indicate that the addition of Pd elevates the melting point and bond strength of (CuPdx)Al compared with CuAl that formed with the bare Cu wire. The improvements are expected to decrease the kinetics of phase transformation toward the more Cu-rich IMC. With the suppression of the Cu-rich IMC, the corrosion resistance of the wire bonding is enhanced and the wire bonding reliability improved. We find that Ni behaves thermodynamically quite similar to Pd in the ternary system of Cu wire bonding, and therefore possesses the potential to improve the corrosion resistance.

Recently a phenomenon has been found that shows different corrosion rates over Al bond pad regardless of different densities of Cl, F components on Al bond pads in different products. According to the results of analysis, the products showed different corrosion rates for different etch conditions of the bond pad opening. For the cause analysis, we conducted a cross-sectional profile comparison between two products with Al bond pads. Based on the result of comparison, we discovered that the side wall profile of the Al bond pad is affected by the etch conditions of the bond pad opening. In some severe cases, it was observed that a small void was formed between the side wall and Al bond pad. Under moist conditions, this void provided moisture between Al bond pad and TiN barrier metal that the electricity contacted. Through this study, we could conclude that the moisture in the void between Al bond pad and TiN barrier metal may create a galvanic corrosive condition.

In the authors' previous papers, the failure mechanism and elimination solutions of galvanic corrosion (Al-Cu cell) on microchip Al bondpads in the Al process (0.18un and above) have been studied [1-2]. In this paper, the authors will further study the failure mechanism and root cause of galvanic corrosion (Al-Cu cell) on microchip Al bondpads in the Cu process (0.13um and below) with Ta barrier metal. Based on our results, the root cause of galvanic corrosion (Al-Cu cell) in the Al process is only one way and Al-Cu cell is from Al alloy (Al + 0.5%Cu) on Al bondpads. However, in the Cu process it may be from two ways and Al-Cu cell can be from both Al alloy (Al + 0.5%Cu) on Al bondpads and the Cu metal layer below the barrier metal Ta when Ta has weak points or pinhole. As such, the pinhole defects on Al bondpad caused by galvanic corrosion (Al-Cu cell) in the Cu process might be more serious than that in the Al process. In this paper, TEM is used for root cause identification. Based on the TEM results, galvanic corrosion was due to the weak point/pinhole at the Ta barrier metal layer and Al-Cu diffusion.

Cu wires were bonded to AlSi (1%) pads, subsequently encapsulated and subjected to uHAST (un-biased Highly Accelerated Stress Test, 130 °C and 85% relative humidity). After the test, a pair of bonding interfaces associated with a failing contact resistance and a passing contact resistance were analyzed and compared, with transmission electron microscopy (TEM), electron diffraction, and energy-dispersive spectroscopy (EDS). The data suggested the corrosion rates were higher for the more Cu-rich Cu-Al intermetallics (IMC) in the failing sample. The corrosion was investigated with factors including electromotive force (EMF), self-passivation of Al, thickness and homogeneity of the Al-oxide on the IMC, ratio of the Cu-to-Al surface areas exposed to the electrolyte for an IMC taken into account. The preferential corrosion observed for the Cu-rich IMC is attributed to the high ratios of the surface areas of the cathode and anode that were exposed to the electrolyte, and the passivation oxide of Al with the lower homogeneity. The corrosion of the Cu-Al IMC is just a manifestation of the well-known phenomenon of dealloying. With the understanding of the corrosion mechanisms, prohibiting the formation of Cu-rich IMCs is expected be an approach to improve the corrosion resistance of the wire bonding.

A land-grid array connector, electrically connecting an array of plated contact pads on a ceramic substrate chip carrier to plated contact pads on a printed circuit board (PCB), failed in a year after assembly due to time-delayed fracture of multiple C-shaped spring connectors. The land-grid-array connectors analyzed had arrays of connectors consisting of gold on nickel plated Be-Cu C-shaped springs in compression that made electrical connections between the pads on the ceramic substrates and the PCBs. Metallography, fractography and surface analyses revealed the root cause of the C-spring connector fracture to be plating solutions trapped in deep grain boundary grooves etched into the C-spring connectors during the pre-plating cleaning operation. The stress necessary for the stress corrosion cracking mechanism was provided by the C-spring connectors, in the land-grid array, being compressed between the ceramic substrate and the printed circuit board.

In this study, a comprehensive investigation of the Ag-Al bond degradation mechanism in an electrically failed module using the argon ion milling, scanning electron microscopy (SEM), dual beam focused ion beam-SEM, scanning transmission electron microscopy energy dispersive x-ray spectroscopy, and time-of-flight secondary ion mass spectrometry is reported. It is found that the bond degradation is due to the galvanic corrosion in the Ag-Al bonding area. Specific attention is given to the information of microstructures, elements, and corrosive ions in the degraded bond. In this study, it is believed that the Ag-Al bond degradation is highly related to the packaging designs.

It is well-known that underetch material, contamination, particle, pinholes and corrosion-induced defects on microchip Al bondpads will cause non-stick on pads (NSOP) issues. In this paper, the authors will further study NSOP problem and introduce one more NSOP failure mechanism due to Cu diffusion caused by poor Ta barrier metal. Based on our failure analysis results, the NSOP issue was not due to the assembly process, but due to the wafer fabrication. The failure mechanism might be that the barrier metal Ta was with pinholes, which caused Cu diffused out to the top Al layer, and then formed the “Bump-like” Cu defects and resulted in NSOP on Al bondpads during assembly process.

In wafer fabrication, Fluorine (F) contamination may cause fluorine-induced corrosion and defects on microchip Aluminum (Al) bondpads, resulting in bondpad discoloration or non-stick on pads (NSOP). Auger Electron Spectroscopy (AES) is employed for measurements of the fluorine level on the Al bondpads. From a Process control limit and a specification limit perspective, it is necessary to establish a control limit to enable process monitor reasons. Control limits are typically lower than the specification limits which are related to bondpad quality. The bondpad quality affects the die bondability. This paper proposes a simulation method to determine the specification limit of Fluorine and a Shelf Lifetime Accelerated Test (SLAT) for process monitoring. Wafers with different F levels were selected to perform SLAT with high temperature and high relative humidity tests for a fixed duration to simulate a one year wafer storage condition. The results of these simulation results agree with published values. If the F level on bondpad surfaces was less than 6.0 atomic percent (at%), then no F induced corrosion on the bond pads was observed by AES. Similarly, if the F level on bond pad surfaces was higher than 6.0 atomic per cent (at%) then AES measured F induced corrosion was observed.

This paper presents a quick, reliable, and fully quantitative method of measuring the intermetallic coverage of copper to aluminium bonding at time zero and post reliability stressing. This method is currently used in select manufacturing quality control processes, as well as during product release procedures. By applying this measurement method after various life-tests, it has been possible to collect information on degradation in the copper aluminium system which is currently being used to make a model of the corrosion mechanism in the copper aluminium system.

Lead-free manufacturing regulations, reduction in circuit board feature sizes and the miniaturization of components to improve hardware performance have combined to make data center IT equipment more prone to attack by corrosive contaminants. Manufacturers are under pressure to control contamination in the data center environment and maintaining acceptable limits is now critical to the continued reliable operation of datacom and IT equipment. This paper will discuss ongoing reliability issues with electronic equipment in data centers and will present updates on ongoing contamination concerns, standards activities, and case studies from several different locations illustrating the successful application of contamination assessment, control, and monitoring programs to eliminate electronic equipment failures.

A case study of Fluorine (F)-outgassing is presented in this paper that caused the corrosion of Aluminum bond pad. It will be shown that the source of F-contamination is not the typical residue left behind after the passivation etch with Fluorine-based gas chemistry and the subsequent removal of the etch polymer generated with solvent (chemical) clean. Rather, it is introduced as a result of F-outgas over a period of time from the intermetallic dielectric (IMD) film, fluorosilicate glass (FSG), during the post-fab wafer storage. The methodology used in our failure analysis (FA) lab to identify and characterize this type of failure mode is presented in the paper.