Microbump Structure Research Articles

A Cu/Ni/Cu/Sn1.8Ag microbump structure for void-free interface under long time high-temperature storage

Microbumps play a crucial role in interlayer interconnections for advanced packaging. Among the prevalent microbump structures are Cu/Sn and Cu/Ni/Sn configurations. However, as the bump size continues to shrink, excessive Kirkendall voids tend to form in Cu/Sn microbumps, leading to potential reliability issues. While Cu/Ni/Sn structures can suppress Kirkendall void formation, Ni3Sn4 intermetallic compound (IMC) exhibits inferior physical properties compared to Cu3Sn and Cu6Sn5. In this study, a novel Cu/Ni/Cu/Sn1.8Ag (CNCT) Sn-rich microbump structure with controlled Cu thickness is proposed to suppress Kirkendall void formation while promoting Cu6Sn5 and Cu3Sn as interfacial IMCs. A Ni barrier layer is inserted to partition the Cu pillar and impede Cu atom replenishment from the bottom side. The remaining Cu pillar at the top is designed to be 3μm thick, which is intended to be fully consumed by 8μm Sn1.8Ag solder while simultaneously serving as redundant protection for the bottom Cu pillar against unexpected Sn spillage. The effects of high temperature storage (HTS) at 150°C on IMC evolution in CNCT microbump bonding were investigated for 1500 hours. No significant voids were observed at the bonding interface during HTS. Interfacial positions recorded from 0 to 500 hours show that the Cu3Sn IMC grows unidirectionally toward the Cu side but not toward the Sn side, indicating a greater diffusion flux of Sn atoms compared to Cu atoms. The diffusion coefficients of Cu in Cu3Sn (DCu,ε), Cu6Sn5 (DCu,η), and Sn in Cu3Sn (DSn,ε), Cu6Sn5 (DSn,η) at 150°C were calculated as 1.36×10−18 m2/s, 5.01×10−19 m2/s, 9.53×10−19 m2/s and 1.10×10−18 m2/s, respectively. The dramatic decrease of DCu,η suggests that the diffusion capacity of Cu atoms into Sn is significantly suppressed. The notable transformation of Cu3Sn to Cu6Sn5 occurred after 500 hours. By 1500 hours, Cu3Sn was greatly consumed, and the bonding interface tended to form a stable Cu/Ni/Cu6Sn5/Ni/Cu structure. Diffusion models were established and discussed to elucidate the mechanisms governing both stages.

Laser-based fabrication of superwetting titanium alloy with enhanced corrosion and erosion-corrosion resistance

Titanium (Ti) alloy is vulnerable to erosion and corrosion effect of sea water in the application of marine equipment and prone to degradation and failure. In order to further enhance the erosion-corrosion resistance of Ti alloy, a laser-heat hybrid treatment is applied to fabricate superwetting Ti alloy surface. The surface morphology, chemical composition and surface wettability of Ti alloy before and after laser processing are characterized. The surface behaviors of the superwetting Ti alloy surface during corrosion and erosion-corrosion interaction are evaluated by electrochemical station and two-phase flow erosion equipment. The experimental results indicate that the periodic micro-bump structure induced by laser texturing and the low surface energy after heat treatment contribute to the surface wettability transformation into superhydrophobicity with the contact angle of 156.2°. Compared with the untreated Ti alloy surface, the laser-heat-treated superhydrophobic surface exhibits a smaller corrosion current density and a larger potential in NaCl solution, showing better corrosion resistance. The surface erosion-corrosion resistance has also been increased by 18% due to the decrease of solid-liquid contact area caused by air cavity of superhydrophobic surface. This can provide a solid laser-based fabrication method of Ti alloy for long-term and stable application in marine environment.

Three-Dimensional Integrated Fan-Out Wafer-Level Package Micro-Bump Electromigration Study.

To meet the demands for miniaturization and multi-functional and high-performance electronics applications, the semiconductor industry has shifted its packaging approach to multi-chip vertical stacking. Among the advanced packaging technologies for high-density interconnects, the most persistent factor affecting their reliability is the electromigration (EM) problem on the micro-bump. The operating temperature and the operating current density are the main factors affecting the EM phenomenon. Therefore, when a micro-bump structure is in the electrothermal environment, the EM failure mechanism of the high-density integrated packaging structure must be studied. To investigate the relationship between loading conditions and EM failure time in micro-bump structures, this study established an equivalent model of the vertical stacking structure of fan-out wafer-level packages. Then, the electrothermal interaction theory was used to carry out numerical simulations in an electrothermal environment. Finally, the MTTF equation was invoked, with Sn63Pb37 as the bump material, and the relationship between the operating environment and EM lifetime was investigated. The results showed that the current aggregation was the location where the bump structure was most susceptible to EM failure. The accelerating effect of the temperature on the EM failure time was more obvious at a current density of 3.5 A/cm2, which was 27.51% shorter than 4.5 A/cm2 at the same temperature difference. When the current density exceeded 4.5 A/cm2, the change in the failure time was not obvious, and the maximum critical value of the micro-bump failure was 4 A/cm2~4.5 A/cm2.

Effect of liquid crystal polymer micro-bump structure prepared by mesh method on the frictional corrosion performance of micro-arc oxide coated pure aluminum disks

In this study, micro-arc oxidation (MAO) technology was used to coat the surface of pure aluminum disks. Then the liquid crystal polymer (LCP) was used in the mesh method to form a micro-bump structure on the MAO coating. Compared with a single MAO coating, the micro-bump composite coating has a low coefficient of friction, high corrosion resistance, and excellent resistance to frictional corrosion. The microstructure and composition of both coatings were characterized by metallurgical microscopy, SEM, EDS, XRD, and XPS. And the protection mechanism, as well as the physical model of the LCP micro-bump coating, are presented and discussed. In summary, the preparation of micro-bump structures on MAO surfaces using the mesh method provides a feasible improvement idea for the shortcomings of MAO technology.

Focused Ion Beam Preparation of Low Melting Point Metals: Lessons Learned From Indium.

Indium (In) and other low melting point metals are used as interconnects in a variety of hybridized circuits and a full understanding of the metallurgy of these interconnects is important to the reliability and performance of the devices. This paper shows that room temperature focused ion beam (FIB) preparation of cross-sections, using Ga+ or Xe+ can result in artifacts that obscure the true In microbump structure. The use of modified milling strategies to minimize the increased local sample temperature are shown to produce cross-sections that are representative of the In bump microstructure in some sample configurations. Furthermore, cooling of the sample to cryogenic temperatures is shown to reliably eliminate artifacts in FIB prepared cross-sections of In bumps allowing the true bump microstructure to be observed.

Experimental and numerical studies on micro-bumps without melting of gold films with different thicknesses induced by ultrafast laser

The micro-bumps of metal films induced by ultrafast laser have potential applications in various fields, suchas biotechnology and energy, etc. The mechanism of micro-bump deformation of the gold film irradiated by single ultrafast laser pulse is controversial. In this study, the experimental research of single ultrafast laser pulse irradiating gold films with different thicknesses from 20 nm to 200 nm is carried out, with the laser fluence varying from 1 to 50 J/cm 2. The micro-bumps without melting are observed by the Scanning Electron Microscope (SEM) and different separation thresholds can be obtained. The separation threshold of gold films ranges from 5.1 to 43.5 J/cm 2 and it increases with the thickness of gold film increasing. The heights of micro-bumps are measured by Atomic Force Microscope (AFM). One-dimension (1D) Molecular dynamic (MD) simulations are carried out to analyze the temperature and stress evolutions. Large-scale two-dimension (2D) MD simulations are furthermore performed to obtain the micro-bump structure. MD simulations showing a good agreement with experiments, and the essential mechanism responsible for micro-bump without melting is clearly analyzed. It is also analyzed that the heights of micro-bumps are related to the laser fluences and radii of spot.

One-Step Fabrication Method of GaN Films for Internal Quantum Efficiency Enhancement and Their Ultrafast Mechanism Investigation.

The third-generation semiconductors are the cornerstone of the power semiconductor leap forward and have attracted much attention because of their excellent properties and wide applications. Meanwhile, femtosecond laser processing as a convenient method further improves the performance of the related devices and expands the application prospect. In this work, an approximate 3 times improvement of the internal quantum efficiency (IQE) and a 5.5 times enhancement of the photoluminescence (PL) intensity were achieved in the GaN film prepared using a one-step femtosecond laser fabrication method. Three types of final micro/nanostructures were found with different femtosecond laser fluences, which could be attributed to the decomposition, melting, bubble nucleation, and phase explosion of GaN. The mechanisms of the microbump structure formation and enhancement of IQE were studied experimentally by the time-resolved reflection pump-probe technique, X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Simulations for the laser-GaN interaction have also been performed to ascertain the micro/nanostructure formation principle. These results promote the potential applications of femtosecond lasers on GaN and other wide band gap semiconductors, such as UV-light-emitting diodes (LEDs), photodetectors, and random lasers for use in sensing and full-field imaging.

Effect of Electromigration-Induced Joule Heating on the Reliability of Sn-Ag Microbump with Different UBM Structures

The effect of current stress-induced Joule heating on two different under-bump metallization (UBM) structures in Sn-Ag microbumps was investigated with current stressing at 150°C and a current density of 5 &!thinsp;× 104 A/cm2. Both Ni UBM and Cu UBM configuration microbump structures underwent extensive electromigration (EM) testing, with results revealing a longer lifetime with the Cu UBM configuration than the Ni UBM structure. The observed EM failure mechanism in the Ni UBM configuration was identified as a void formation within the bump interconnected Al trace and not due to damage accumulation inside the microbump structure. The intermetallic compound developed inside the microbump was formed and maintained its stability throughout the current stressing period. To identify the main driving force of damage accumulation in the Al trace, the current density and temperature distributions in the Sn-Ag microbumps were analyzed numerically via the finite element method. The simulation results showed higher Joule heating with the Ni UBM than the Cu UBM microbump configuration, along with the bump geometrical contribution of add-on higher Joule heating in the Ni UBM microbump structure.

Ni Barrier Symmetry Effect on Electromigration Failure Mechanism of Cu/Sn–Ag Microbump

Ni barrier symmetry effect on the electromigration (EM) failure mechanism of Cu/Sn–Ag microbump were systematically investigated by studying the intermetallic compound (IMC) growth characteristics at 150 °C with a current density of 1.5 × 105 A/cm2. In the symmetric Ni barrier structure, Cu diffusion to Sn–Ag solder was restricted by the Ni barrier at both interfaces and the Ni3Sn4 phase formed by the inter-diffusion between Ni and Sn atoms just after bonding, which was gradually transformed to (Ni,Cu)3Sn4 phase and later to (Cu,Ni)6Sn5 during current stressing with relatively slow resistance increase with time. By the way, in the asymmetric structure, extensive Cu6Sn5 phase grew by the inter-diffusion between Cu and Sn atoms due to there is no Ni barrier at the upper interface, which was rapidly transformed into only Cu6Sn5 and Cu3Sn IMCs during electron downward flow, with relatively fast resistance increase with time. Therefore, the symmetric Ni barrier structure is very effective in restricting extensive IMC reactions during EM of Cu-solder microbump structure.

A Pilot Study On The Novel Non-Invasive Nerve-Holder With Negative-Pressure Suctions.

In performing neurosurgical operation on peripheral nervous system, the most important first step is to robustly hold the target nerve, since the nerve-holding stability and reliability significantly affect the result of surgical operation. However it is not straightforward to robustly hold peripheral nerve during the surgical operation, because the peripheral nerve is too flexible and slippery. In this study, we design a novel peripheral nerve-holder that can be used for the neurosurgical operation. Considering the anatomical characteristics of the peripheral nerve that small bundles of nerve fibers (i.e., fascicles) are structured inside the outermost layer of the nerve bundle (i.e., epineurium), we aim to develop a non-clamping and non-invasive type nerve-holder to protect the nerve fibers. For the aim, the negative-pressure suction method is applied to the proposed holder. And, in order to hold the nerve more robustly, micro-bump structure is fabricated on the suction surface contacting with the nerve. This paper introduces the concept, working principle, characteristics, and in-vitro experimental results on feasibility evaluation of the proposed holder.

Numerical study of microbump failure in 3D microelectronic structures

Microbump failure in 3D microelectronic chip stacks is studied numerically using the finite element method. The microbump structure consists of a solder joint sandwiched between copper pads connected to through-silicon vias. The model system is subject to prescribed shear deformation, with possible superimposed tension or compression. A ductile damage model for solder is implemented to investigate failure propensity and cracking pattern as affected by the loading mode and underfill material. Failure of the solder is found to be sensitive to the loading mode, with a superimposed tension or compression on shear easily changing the crack path and tending to reduce the solder ductility.

3-D Wafer-Level Packaging Die Stacking Using Spin-on-Dielectric Polymer Liner Through-Silicon Vias

In this paper, we report on the processing and the electrical characterization of a 3-D-wafer level packaging through-silicon-via (TSV) flow, using a polymer-isolated, Cu-filled TSV, realized on thinned wafers bonded to temporary carriers. A Cu/Sn micro-bump structure is integrated in the TSV process flow and used for realizing a two-die stack. Before TSV processing, the Si wafers are bonded to temporary carriers and thinned down to 50 μm. The actual TSV and micro-bump process uses 3 masks, two Si-deep-reactive ion etching steps and a polymer liner as a dielectric. The dimensions of the TSV structure are: 35 μm OTSV, 5 μm thick polymer liner, 25-μm-O Cu TSV, 50 μm deep TSV, and a 60 μm TSV pitch.

飞秒激光辐射下微突起结构形成的超快热弹性模拟

Finite element method and ultrafast thermoelasticity model are combined to simulate the microbump formation irradiated by a femtosecond laser. It has been shown that the effect of microbump formation is related to the characteristic of incident femtosecond laser and the thermoelasticity properties of the film. The numerical results exhibit good agreements with the experimental results in both the shape and height of the conical microbump structure, which verify the effectiveness of the ultrafast thermoelasticity model in experiments. It should be helpful for selecting appropriate materials for nanotexturing of thin films by ultrafast lasers.