Microelectronic Interconnects Research Articles

Revealing the complexation of 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid (DPS) with cuprous ion and its contribution to Cu electrodeposition

An accelerator is an organic additive used in Cu electrodeposition, crucial for achieving superfilling during the fabrication of Cu interconnects in microelectronics. 3-N,N-dimethylaminodithiocarbamoyl-1-propanesulfonic acid (DPS) has been proposed and investigated as a substitute for SPS. However, its acceleration mechanism associated with its interaction with Cu ions has not been intensively investigated. In this study, the interaction between DPS and Cu+ was investigated using various spectroscopic analyses, including ultraviolet–visible–near-infrared absorption (UV–VIS–NIR), electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) spectroscopy. UV–VIS–NIR and EPR spectroscopy revealed that DPS forms a complex with Cu+, namely Cu(I)(DPS)2. The kinetic study on the complexation of DPS with Cu+ indicated second-order reaction characteristics. The structure of the complex was suggested using 1H and 13C NMR spectroscopy. Furthermore, electrochemical analyses, including cyclic voltammetry (CV) and linear sweep voltammetry (LSV), were conducted to investigate the electrochemical behaviors of the Cu(I)(DPS)2 complex, demonstrating its acceleration effect on Cu electrodeposition. These results will provide valuable insights for understanding the acceleration mechanism of DPS in Cu electrodeposition.

Microstructure modification and mechanical reinforcement in sub-10 μm scale Cu/Sn–Ag/Cu microbump joints via Co-addition of Zn and Ni in Cu substrates

The trend towards miniaturization of solder joints addresses the need for high-density interconnections in microelectronics. Excessive intermetallic compounds (IMC) and phase transformations cause internal stresses, leading to voiding and cracking in conventional microbumps. Therefore, advanced microbump design must focus on IMC suppression and phase stabilization. This study explores Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps, comparing them to conventional Cu/Sn-3.5Ag/Cu under various reflow times. High-field emission electron probe micro-analysis and transmission electron microscopy reveal improved microstructures. Electron backscatter diffraction confirms the (Cu,Ni)6(Sn,Zn)5 grain size of several hundreds of nanometers. Fracture modes were established based on the correlation between shear testing and fracture surfaces. Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps enhanced shear strength (19.6 %–97.8 %) and failure toughness (10.6 %–132.1 %) compared to Cu/Sn-3.5Ag/Cu. The solid solution strengthened and refined (Cu,Ni)6(Sn,Zn)5 combined with tough bulk (Cu,Ag)5(Sn,Zn)8 in Cu–26Zn–18Ni/Sn-3.5Ag/Cu–26Zn–18Ni microbumps, demonstrate a tough microstructure. These findings provide insights into the design of microbumps with a bonding height in sub-10micron scales.

Corrosion-induced fracture of Cu–Al microelectronics interconnects

We present a mechano-chemical model that couples corrosion, mechanical response, and fracture. The model is used to understand the failure of Cu wires on Al pads in microelectronic packages using a multi-phase field approach. Under high humidity environments, the Cu-rich intermetallic compounds (IMC), Cu9Al4, formed at the interface between Cu and Al, undergo a corrosion degradation process. The IMC expands while undergoing corrosion, inducing stresses that nucleate and propagate cracks along the interface between the Cu-rich IMC and Cu. Furthermore, the volumetric expansion of the IMC may cause damage to the passivation layer and enhance the nucleation of new corrosion pits. We show that the presence of a crack accelerates the corrosion process. The model developed here can be extended to other systems and applications.

BME-Assisted Fabrication of Porous Copper Films with Adjustable Porosity and Ligament Width for Microelectronic Interconnects

Monolithic porous Cu films are synthesized through the use of bicontinuous microemulsion (BME) to serve as a flexible template. The BME system is made up of a series of interconnected linked water and hydrocarbon phases. This one-of-a-kind structure enables precision Cu electrodeposition only in the aqueous phase, avoiding undesirable electrodeposition in the oil phase. We can fine-tune the porosity and ligament width of the resultant copper film by altering the oil phase proportion in the soft template. This Sn-plated porous Cu is capable of both low-temperature reflow and high-temperature service. The flexible template approach shows great potential for creating porous Cu with changeable porosity, allowing for more control over compositions. This approach has potential applications in advanced packaging technologies.

Functionalizing self-assembled monolayers to reduce interface scattering in ruthenium/dielectric for next-generation microelectronic interconnects

As the number of transistor increases in modern devices, the resistivity of metal lines increases with the downscaling of the vias dimension due to the increased in surface scattering of electrons. Herein, we evaluated the electrical performance and the adhesion strength of the Ru/SiO2 substrate upon functionalizing with benzyliminetriethoxysilane (BITES) and aminoproyltriethoxysilane (APTES) self-assembled monolayers (SAMs)s at the Ru/SiO2 interface. The covalent bonding and retention of the SAM film on the SiO2 surface were characterized via angle-resolved X-ray Photoelectron Spectroscopy (AR-XPS) and Time-of-Flight Secondary Ion Mass Spectrometry (Tof-SIMS), respectively. According to the Fuchs–Sondheimer (FS) model and scratch test, the specularity parameter and the adhesion strength of the Ru/BITES/SiO2 simultaneously improved after thermal annealing compared to the non-functionalized Ru/SiO2 substrate. The effect of different SAM’s terminal groups (amine and benzylimine) on the electron scattering behavior was investigated based on the formation of an interface dipole upon chemisorption of organic molecules on the metal film. Our results indicate that SAM with the benzylimine terminal group formed a larger positive dipole at the SAM layer of the Ru/BITES/SiO2 interface compared to the Ru/APTES/SiO2, leading to an increased in specular electron scattering events and, thus an enhanced electrical performance of the Ru/BITES/SiO2.

Soft-template-assisted bottom-up fabrication of tunable porosity monolithic copper film for interconnection in microelectronics

Background: Third-generation devices offer superior attributes, necessitating advanced packaging materials for harsh operating environments. Incorporating 3D porous architectures to increase the reaction area further enhances the reaction rate, making transient liquid phase bonding an ideal interconnection process. Methods: Fabrication of monolithic porous Cu films using a bicontinuous microemulsion (BME) as a dynamic soft template is reported. The BME phase consists of interconnected three-dimensional networks of aqueous and oil phases, separated by surfactants and cosurfactants. This unique structure enables the selective electrodeposition of Cu exclusively within the aqueous region, preventing any unwanted reactions in the oil phase. Tunable porosity in the monolithic Cu film is achieved by adjusting the water to oil phase ratio in the BME. Significant findings: The Sn-plated porous Cu films exhibit low-temperature reflow capability while still maintaining high service temperatures. The utilization of the BME soft template offers a promising approach for fabricating monolithic Cu films with tunable porosity, providing flexibility in controlling the compositions and performance of interconnections. The compatibility of the BME with existing wafer deposition techniques makes it a viable and scalable solution for mass production. The electrodeposition of monolithic Cu arrays on Cu pillars demonstrates its potential applicability in advanced packaging technologies.

Strategy against electromigration-induced stress by passivation thickness design

This study proposes a strategy to determine an appropriate passivation thickness to prevent electromigration (EM) deterioration. Although it is widely recognized that depositing passivation can effectively saturate the interconnect resistance against EM deterioration, the effect of passivation thickness on the lifetime of interconnections against EM has not been thoroughly investigated. In this study, quantitative models were developed to show direct analytical solutions. First, the effects of passivation thickness on the interconnect resistance against EM were examined through experiments using an Al interconnect coated with tetraethyl orthosilicate passivation. Assuming realistic conditions, the behavior of passivation fracturing as a function of passivation thickness was modeled under elastic, elastic-plastic, and elastic-plastic with plastic zone conditions. The models considered the critical tensile stress in the circumferential direction at the inner wall of a long cylindrical tube subject to internal pressure. These three models explain the saturation mechanism of passivation against EM resistance. Based on these findings, an appropriate passivation thickness for the required resistance against passivation fracture due to EM can be determined. This research provides valuable insights for designing reliable interconnects in microelectronics.

Microstructural observation of Bi67In reacting with Cu for microelectronic interconnects

BackgroundLow-temperature solders are considered eco-friendly because of lowered wasted power, warpage, electromigration. In addition, chips are becoming increasingly integrated so as to achieve superior performance. Semiconductor packaging has moved from two-dimensional to three-dimensional integrated-circuits with vertical stacking for increased functionality. Thermal budget impacts electronic device manufacture, specifically in heterogeneous integration. Therefore, reduction of process temperatures is necessary. MethodsBi and In were selected in this study because they are commercial solders with low-melting points. Further, the microstructures and interfacial reactions of Bi–In solders have not been extensively studied. This study entailed a systematic examination of the interfacial reactions occurring between Bi67In and Cu metallization upon reflow and subsequent aging was performed. FindingsIt was observed that In segregates in the middle region of Cu11In9 after reflow. The phase transformations from Cu11In9 to CuIn2 start in the In segregation region. This remarkable phenomenon warrants in-depth research.

High temperature electromigration behavior of cobalt lines observed by in situ transmission electron microscopy

As industrial practices shift away from Cu as the initial back-end-of-line interconnect material due to size limitations, new candidate metals are being tested and characterized. Electromigration resistance is particularly important in ultra-narrow lines and an in situ study provides unique insight into the formation and progression of electromigration damage. This, in turn, helps to inform device design so that electromigration resistant circuits can be produced efficiently. In this work, the authors demonstrate an in situ transmission electron microscopy technique for electromigration analysis of Cu replacement metals in microelectronic interconnects. Using this method, candidate metal lines can be tested at high current densities, ∼5×106 A/cm2, at controllable temperatures over the range of 300–1000 °C. In this work, cobalt lines are tested in the range of the effective valence inversion temperature. The analysis examines void nucleation, growth, and migration as a function of temperature and line geometry. We find that there is a relative insensitivity of failure time to operating temperature, with samples tested between approximately 600 and 900 °C having roughly equivalent failure times. We ascribe this result to a combination of linewidth effects and a decrease in the magnitude of the effective valence approaching the inversion temperature. Failure mechanism is also not affected by temperature in this range, with the primary determining factor being the linewidth and corresponding availability of grain boundaries for diffusive mass transport. We also observe increased lifetimes of devices with uniform temperatures compared to those in which large thermal gradients exist.

Insights into abnormal grain growth in copper thin films for reduced electrical resistivity: A quantitative multi-order-parameter phase-field study under finite element framework

Copper thin films are frequently utilized as microelectronic interconnects. Inducing abnormal grain growth (AGG) may result in reduced electrical resistivity of copper thin films. However, an in-depth understanding of AGG mechanism and quantitative simulation of microstructure evolution during AGG in copper thin films are still missing, prohibiting the regulation and even design of AGG process. In this paper, a multi-order-parameter phase-field (MOP-PF) model coupled with elastic mechanics under a finite element framework was first developed and applied to study the AGG mechanism in copper thin films. It was found that both elastic and grain boundary anisotropies can induce AGG, but elastic anisotropy dominates the evolution of individual grains. Subsequently, a quantitative simulation of microstructure evolution/kinetics during AGG in copper thin films was achieved by inputting the accurate materials parameters from theoretical/experimental data. A further combination with the Mayadas-Shatzkes model led to a quantitative prediction of the evolution of electrical resistivities, from which several feasible strategies were proposed for preparing high-performance copper thin films with reduced electrical resistivities. Furthermore, it is anticipated that the presently developed framework should be generally applicable for quantitative phase-field simulation of grain growth in bulk materials/films.

TSV Filling with Copper Electro-deposition by Using Sodium 3-[[(dimethylamino) thioxomethyl] thio] Propane Sulphonate

Through Silicon Via (TSV) is a promising microelectronic interconnection technology in Three-D integration (3D). Three-component additives are used for non-porous TSV filling. However, various experiments must be carried out to optimize the concentrations of various additives. To avoid this costly process, Two-component additives were developed. In this paper, Sodium 3-[[(dimethylamino) thioxomethyl] thio] propane sulphonate (DPS) in the TSV filling is investigated. Only by DPS in TSV copper sulfate plating, copper ions cannot reach the bottom of the micro-hole, and copper deposits at the TSV orifice. The addition of dimethyl formamide based on the structure of MPS enhances the inhibition of DPS and may show a weak inhibition during the copper plating and filling process of TSV. 0.1 g/L DPS and 1 g/L AESS, with ampere density2 mA/cm2 and a plating time of 12 h, can obtain perfect defect-free TSV filling.

In-situ isothermal aging TEM analysis of a micro Cu/ENIG/Sn solder joint for flexible interconnects

Sn/ENIG has recently been used in flexible interconnects to form a more stable micron-sized metallurgical joint, due to high power capability which causes solder joints to heat up to 200 °C. However, Cu6Sn5 which is critical for a microelectronic interconnection, will go through a phase transition at temperatures between 186 and 189 °C. This research conducted an in-situ TEM study of a micro Cu/ENIG/Sn solder joint under isothermal aging test and proposed a model to illustrate the mechanism of the microstructural evolution. The results showed that part of the Sn solder reacted with Cu diffused from the electrode to form η´-Cu6Sn5 during the ultrasonic bonding process, while the rest of Sn was left and enriched in a region in the solder joint. But the enriched Sn quickly diffused to both sides when the temperature reached 100 °C, reacting with the ENIG coating and Cu to form (NixCu1-x)3Sn4, AuSn4, and Cu6Sn5 IMCs. After entering the heat preservation process, the diffusion of Cu from the electrode to the joint became more intense, resulting in the formation of Cu3Sn. The scallop-type Cu6Sn5 and the seahorse-type Cu3Sn constituted a typical two-layered structure in the solder joint. Most importantly, the transition between η and η’ was captured near the phase transition temperature for Cu6Sn5 during both the heating and cooling process, which was accompanied by a volume shifting, and the transition process was further studied. This research is expected to serve as a reference for the service of micro Cu/ENIG/Sn solder joints in the electronic industry.

Copper-Based Nanomaterials for Fine-Pitch Interconnects in Microelectronics.

ConspectusNanostructured copper-based materials have emerged as a new generation of robust architectures for realizing high-performing and reliable interconnection in modern electronic packaging. As opposed to traditional interconnects, nanostructured materials offer better compliance during the packaging assembly process. Due to the high surface area-to-volume ratio of nanomaterials, they also enable joint formation by sintering through thermal compression at much lower temperatures compared to bulk counterparts. Nanoporous Cu (np-Cu) films have been employed in electronic packaging as materials that facilitate a chip-to-substrate interconnection, realized by a Cu-on-Cu bonding after sintering.In this Account, we discuss the use of self-supported np-Cu films for low-temperature joint formation. The novelty of this work comes from the incorporation of tin (Sn) into the np-Cu structure, thus ensuring lower sintering temperatures with a goal of producing Cu-Sn intermetallic alloy-based joints between two Cu substrates. The incorporation of Sn is done using an all-electrochemical bottom-up approach that involves the conformal coating of fine-structured np-Cu (initially formed by dealloying of Cu-Zn alloys) with a thin layer of Sn.This Account provides insight on existing technologies for using nanostructured films as materials for interconnects as well as the optimization studies for the Sn-coating processes as a new alternative approach. The applicability of the synthesized Cu-Sn nanomaterials for low-temperature joint formation is also discussed. To realize this new approach, the Sn-coating process is administered by a galvanic pulse plating technique, which is optimized to preserve the porosity in the structure with a Cu/Sn atomic ratio that allows for the formation of the Cu6Sn5 intermetallic compound (IMC). Nanomaterials obtained using this approach are subjected to joint formation by sintering at temperatures between 300 and 200 °C under 20 MPa pressure in forming gas atmosphere. Cross-section characterization of the formed joints postsintering reveals densified bonds with minimal porosity that consist predominantly of the Cu3Sn IMC. Furthermore, these joints are less prone to structural inconsistencies compared to existing joints formed using purely np-Cu. The results presented in this Account provide a glimpse into a facile and cost-effective approach for synthesizing nanostructured Cu-Sn films and illustrate their applicability as new interconnect materials.

Comparison of nano-silver solder joints and SAC305 based on ANSYS simulation and life prediction

With the rapid development of microelectronic interconnection technology, the reliability requirements of electronic products are becoming higher and higher. Among them, temperature has the greatest impact on electronic devices. Due to the different coefficients of thermal expansion (CTE) between the many materials of a chip, the inside of a solder joint is subjected to cyclic temperature action. Under the effect of cyclic temperature, the solder joints are subject to internal fracture resulting in the failure of the solder joint. Therefore, it is necessary to find a more reliable material for the solder joint. In this paper, the advantages and disadvantages of pure silver material and low silver material are compared by finite element simulation analysis of nano-silver pure silver material and SAC305 low silver material, and the fatigue life prediction of nano-silveris predicted by a life prediction model. Then, using the Electric Power Research Institute estimation method to estimate the fatigue life of solder joints, the results are compared with those calculated by the finite element method.

Reliability modeling of the fatigue life of lead-free solder joints at different testing temperatures and load levels using the Arrhenius model

Reliability of the microelectronic interconnection materials for electronic packages has a significant impact on the fatigue properties of the electronic assemblies. This is due to the correlation between solder joints reliability and the most frequent failure modes seen in electronic devices. Due to their superior mechanical and fatigue properties, SAC alloys have supplanted Pb-solder alloys as one of the most commonly used solder materials used as interconnection joints on electronic packages. The main aim of this study is to develop a prediction model of the fatigue life of the solder joints as a function of the experimental conditions. Using a customized experimental setup, an accelerated fatigue shear test is applied to examine the fatigue life of the individual SAC305 solder joints at actual setting conditions. OSP surface finish and solder mask defined are used in the studied test vehicle. The fatigue test includes three levels of stress amplitude and four levels of testing temperature. A two-parameter Weibull distribution is used for the reliability analysis for the fatigue life of the solder joints. A stress–strain curve is plotted for each cycle to construct the hysteresis loop at each cyclic load and testing temperature. The acquired hysteresis loop is used to estimate the inelastic work per cycle and plastic strain. The Morrow energy and Coffin Manson models are employed to describe the effects of the fatigue properties on the fatigue life of the solder joints. The Arrhenius model is implemented to illustrate the evolutions in the stress life, Morrow, and Coffin Manson equations at various testing temperatures. The fatigue life of SAC305 solder joints is then predicted using a general reliability model as a function of the stress amplitude and testing temperature.

Synthesis, Structure, and Thermal Properties of Volatile Group 11 Triazenides as Potential Precursors for Vapor Deposition

Group 11 thin films are desirable as interconnects in microelectronics. Although many M-N-bonded Cu precursors have been explored for vapor deposition, there is currently a lack of suitable Ag and Au derivatives. Herein, we present monovalent Cu, Ag, and Au 1,3-di-tert-butyltriazenides that have potential for use in vapor deposition. Their thermal stability and volatility rival that of current state-of-the-art group 11 precursors with bidentate M-N-bonded ligands. Solution-state thermolysis of these triazenides yielded polycrystalline films of elemental Cu, Ag, and Au. The compounds are therefore highly promising as single-source precursors for vapor deposition of coinage metal films.

A Review on Phase Field Modeling for Formation of η-Cu6Sn5 Intermetallic

Formation of intermetallic compounds (IMCs) exhibits remarkable microstructural features and provides opportunities for microstructure control of microelectronic interconnects. Excessive formation of brittle IMCs at the Cu/Sn interface such as η-Cu6Sn5 can deteriorate the reliability and in turn lead to solder joint failure in the Pb-free Sn-based solder joints. Phase field method is a versatile tool for prediction of the mesoscopic structure evolution in solders, which does not require tracking interfaces. The relationships between the microstructures, reliability and wettability were widely investigated, and several formation and growth mechanisms were also proposed for η-Cu6Sn5. In this paper, the current research works are reviewed and the prospective of the application of phase field method in the formation of η-Cu6Sn5 are discussed. Combined phase field simulations hold great promise in modeling the formation kinetics of IMCs with complex microstructural and chemical interactions.

Effect of the reflow process on IMC growth for different devices and complex components

Intermetallic compound (IMC), as an inevitable part between pad and solder, has a severe effect on the strength and reliability of microelectronic interconnection. Here, an investigation was carried out on IMC growth for different devices and complex components. The device-level experiments were conducted with five factors: peak temperature, time duration above solder liquidus temperature, the thickness of solder paste, surface finish types, and package types including ball grid array (BGA) and quad flat package (QFP). Meanwhile, four complex components with the same reflow profile were conducted and compared for component-level experiments. A scanning electron microscopy (SEM) was used to measure the thickness and determine the spatial distribution of the elements through the IMC. The multivariate analysis of the formation and growth of IMC during reflow soldering was studied based on Nernst–Shchukarev’s equation and the results of the experiments. The difference in IMC thickness between BGA and QFP with different factors was discussed and compared separately. The results showed that the peak temperature and time above liquidus played a vital role in the IMC growth and the solder paste thickness and different pad metallization could not be ignored. SEM pictures of the solder and statistical results were revealed that the surface finish type has a marked impact on the formation of the IMC. For printed circuit board (PCB) with numbers of components, the IMC thickness and uniformity of solder joints at corner and center positions showed some regularity differences. Meanwhile, the bump shape (Cu1−x Ni x )6Sn5 IMC was observed for small size BGA with electroless nickel and immersion gold during the reflow process. The results have a significant meaning to optimize its reflow process parameters for complex components, to improve the interconnection reliability in engineering.

Effect of sodium thiazolinyl dithiopropane sulphonate (SH110) addition on electroplating nanotwinned copper films and their filling performance of fine-pitch redistributed layer (RDL)

Nanotwinned copper is a potential microelectronic interconnection material due to its superior strength and conductivity, however, its filling ability is urgently needed to improve before its application in the field of advanced packaging. The effect of additive (sodium thiazolinyl dithiopropane sulphonate, SH110) addition on the surface roughness, microstructure, mechanical properties and filling capacity of nanotwinned copper films was investigated. The surface roughness and grain size were firstly reduced then increased with the increasing concentrations of SH110, reaching the minimum value at 10 ppm. It was noticed that copper films with 10 ppm SH110 also possessed superior tensile strength and elongation, which were measured as 481 MPa and 3.68% on average of 12 μm thick samples by dynamic thermo-mechanical analyzer. Further, their uniformity and flatness of redistributed layers (RDLs) were controlled as 2% and 1.9%, which were significantly improved compared to the samples without SH110 (7.6% and 4.7%). As demonstrated by linear sweep voltammetry analysis and galvanostatic measurement, the SH110 could cooperate well with gelatin and serve as a combination of accelerator and leveler, resulting in the improvement of filling capacity for nanotwinned copper RDLs.

A new type of dual-branch composite structure-based ultrasonic transducer for microelectronic interconnection and packaging

A new type of composite structure-based ultrasonic transducer was proposed and a complete set of design methods was systematically developed. The transducer adopts a cascaded structure of two tapered ultrasonic horns and a piezoelectric resonator which involves two groups of piezoceramic stacks. The performance test experiments show that the newly proposed transducer can provide dual-directional vibration output either in a separate excitation of one stack or in a simultaneous excitation of two stacks, depending on which it is expected to execute different bonding tasks in the meanwhile. Particularly in a simultaneous excitation pattern, the salient features in the energy superposition and power-sharing enable the transducer to cope with a higher power-required occasion without rising the input power of any branch transducer, avoiding the potential risk of performance deterioration caused by the excessive self-heating of the piezoceramic stack due to a long-term operation. The test results also show that the newly proposed transducer possesses a good deal of flexibility, manifesting the feasibility and effectiveness of the design idea, as well as the relevant design method, and giving the transducer a wider application prospect.