Interface Reliability Research Articles

Interface reliability and lifetime prediction of heavy aluminum wire bonds

In this study a high frequency mechanical fatigue testing procedure for evaluation of interfacial reliability of heavy wire bonds in power semiconductors is presented. A displacement controlled mechanical shear testing set-up working at a variable frequency of a few Hertz up to 10kHz is used to assess the interfacial fatigue resistance of heavy Al wire bond in IGBT devices. In addition, power cyclic tests were conducted on IGBT modules for in-situ measurement of the temperature distribution in the devices and determination of the thermally induced displacements in the wire bond loops. Finite Element Analysis was conducted to calculate the correlation between the thermally and mechanically induced interfacial stresses in the wire bonds. These stress values were converted into equivalent junction temperature swings (ΔTj) in the devices based on which lifetime curves at different testing frequencies were obtained. Comparison of the fatigue life curves obtained at mechanical testing frequencies of up to 200Hz with the power cycling data related to the wire bond lift-off failure revealed a very good conformity in the ranges of 50 to 160K. A lifetime prediction model for Al wire bonds in IGBT modules is suggested by which the loading cycles to failure can be obtained as a function of ΔTj and the mechanical testing frequency. The proposed accelerated shear fatigue testing procedure can be applied for rapid assessment of a variety of interconnects with different geometries and material combinations. Decoupling of the concurrent failure mechanisms and separation of the thermal, mechanical and environmental stress factors allows a more focused and efficient investigation of the interfaces in the devices.

Interfacial reaction and mechanical properties between low melting temperature Sn–58Bi solder and various surface finishes during reflow reactions

Interfacial reactions and joint mechanical reliability of a representative low melting temperature Sn–58 wt%Bi solder with three different surface finishes were evaluated during a reflow process. The surface materials consisted of organic solderability preservative (OSP), electroless nickel–immersion gold (ENIG), and electroless nickel–electroless palladium–immersion gold (ENEPIG). The relationships between the interfacial reaction, surface finish, shear speed, and shear force were elucidated in this study. The interfacial intermetallic compounds (IMCs) on the ENEPIG substrate were sequentially changed during reflowing at 180 °C in the following order: (Pd,Ni)Sn4, (Pd,Ni)Sn4 + Ni3Sn4, and Ni3Sn4. Compared to the Cu6Sn5 IMC on the OSP-finished Cu substrate, the Ni3Sn4 IMCs on the ENIG and ENEPIG substrates were much thinner. The IMCs of each of the different surface finishes grew as the reflow time increased; however, the morphologies of the IMCs were different each other. Layer-type interfacial IMCs created a stable metallurgical interfacial structure during the reflow reaction, resulting in reduced rates of IMC growth and Ni–P consumption. High-speed shear tests were conducted to evaluate the effects of interfacial reactions on the mechanical properties of three Sn–58Bi solder joints. In spite of the different interfacial reactions and IMC formations, there were no large differences among shear forces in the three Sn–58Bi solder joints. Further, the type of surface finish material did not significantly affect the shear force of the Sn–58Bi solder joints under high-speed shear loading.

Non-conductive film with Zn-nanoparticles (Zn-NCF) for 40 μm pitch Cu-pillar/Sn–Ag bump interconnection

Non-conductive film with Zn nano-particles (Zn-NCF) is an effective solution for fine-pitch Cu-pillar/Sn–Ag bump interconnection in terms of manufacturing process and interfacial reliability. In this study, NCFs with Zn nano-particles of different acidity, viscosity, and curing speed were formulated and diffused Zn contents in the Cu pillar/Sn–Ag bumps were measured after 3D TSV chip-stack bonding. Amount of Zn diffusion into the Cu pillar/Sn–Ag bumps increased as the acidity of resin increased, as the viscosity of resin decreased, as the curing speed of resin decreased, and as the bonding temperature increased. Diffusion of Zn nano-particles into the Cu pillar/Sn–Ag bumps are maximized when the resin viscosity became lowered and the solder oxide layer was removed. To analyze the effects of Zn-NCF on IMC reduction, IMC height depending on aging time was measured and corresponding activation energies for IMC growth were calculated. For the evaluation of joint reliabilities, test vehicles were bonded using NCFs with 0wt%, 1wt%, 5wt%, and 10wt% of Zn nano-particles and aged at 150°C up to 500h. NCF with 10wt% Zn nano-particle showed remarkable suppression in Cu6Sn5 and (Cu,Ni)6Sn5 IMC compared to NCFs with 0wt%, 1wt%, and 5wt% of Zn nano-particles. However, in terms of Cu3Sn IMC suppression, which is the most critical goal of this experiment NCFs with 1wt%, 5wt%, and 10wt% showed an equal amount of IMC suppression. As a result, it was successfully demonstrated that the suppression of Cu–Sn IMCs was achieved by the addition of Zn nano-particles in the NCFs resulting an enhanced reliability performance in the Cu/Sn–Ag bumps bonding in 3D TSV interconnection.

Effect of Fe Content on the Interfacial Reliability of SnAgCu/Fe–Ni Solder Joints

Fe–Ni films with compositions of Fe–75Ni, Fe–50Ni, and Fe–30Ni were used as under bump metallization (UBM) to evaluate the interfacial reliability of SnAgCu/Fe–Ni solder joints through ball shear test, high temperature storage, and temperature cycling. The shear strengths for Fe–75Ni, Fe–50Ni, and Fe–30Ni solder joints after reflow were 42.57, 53.94 and 53.98 MPa, respectively, which were all satisfied the requirement of industrialization (>34.3 MPa). High temperature storage was conducted at 150, 175 and 200 °C. It was found that higher Fe content in Fe–Ni layer had the ability to inhibit the mutual diffusion at interface region below 150 °C, and the growth speed of intermetallic compound (IMC) decreased with increasing Fe concentration. When stored at 200 °C, the IMC thickness reached a limit for all three films after 4 days, and some cracks occurred at the interface between IMC and Fe–Ni layer. The activation energies for the growth of FeSn2 on Fe–30Ni, Fe–50Ni, and Fe–75Ni films were calculated as 246, 185, and 81 kJ/mol, respectively. Temperature cycling tests revealed that SnAgCu/Fe–50Ni solder joint had the lowest failure rate (less than 10%), and had the best interfacial reliability among three compositions.

Analytical solution on interfacial reliability of 3-D through-silicon-via (TSV) containing dielectric liner

Interfacial reliability is a challenging issue in through-silicon-via (TSV) technique. To accurately investigate the interfacial reliability of TSV, this paper developed an analytical solution approach, in which the effects of the liner are considered. The validity of the analytical solution is executed by comparison with finite element simulation results. Results show that two approaches have good agreement, with a deviation within 10%, illustrating the validity of the analytical solution developed in this study. Then, using the developed analytical solution, the effects of via diameter, the liner thickness, and the liner materials of TSV on interfacial reliability are investigated with the steady-state energy release rate (ERR). Analytical results show that the steady-state ERR is not only determined by the coefficient of thermal expansion (CTE) mismatch between adjacent materials, but also affected by the products (E×CTE2) of Young’s modulus (E) and CTE2 of the liner. Liner materials with lower E×CTE2 values will lead to lower steady-state ERR. Additionally, the combined effects of copper via diameter and liner thickness on ERR declare that the ERR highly depends on copper via diameter.

Effect of Fe content on the interfacial reliability of SnAgCu/Fe-Ni solder joint

Fe-Ni films with compositions of Fe-75Ni, Fe-50Ni, and Fe-30Ni were used as under bump metallization (UBM) to evaluate the interfacial reliability of SnAgCu/Fe-Ni solder joints through ball shear test, high temperature storage, as well as temperature cycling. The shear strength for Fe-75Ni, Fe-50Ni, and Fe-30Ni solder joints after reflow were 42.57, 53.94, 53.98 MPa respectively, which are all satisfied with the requirement of industrialization (>34.3 MPa ). High temperature storage was conducted at 150°C and 200°C respectively. It was found that higher Fe content in Fe-Ni layer had the ability to inhibit the mutual diffusion at interface region at 150°C, and the growth speed of intermetallic compound (IMC) decreased with the increase of Fe concentration. When stored at 200°C, the thickness of IMC would reach a limitation for all these three films after 4 days, and cracks occurred at the interface between IMC and Fe-Ni layer. Temperature cycling tests revealed that SnAgCu/Fe-50Ni solder joint had the lowest failure rate (less than 10%), which has the best interfacial reliability among three compositions.

Interfacial reliability between hot-melt polyamides resin and textile for wearable electronics application

The interfacial reliability between hot-melt polyamides resin and textile is studied to investigate whether hot-melt polyamides resin is useful as an encapsulation material of wearable electronic devices. Four kinds of hot-melt polyamides resins and six kinds of textile fabrics are used, and the test sample is fabricated by molding polyamides resin on top of textile. To confirm the mechanical reliability between polyamides resin and textile under water washing and dry cleaning condition, the adhesion strength is measured by 90° peel test not only at initial state but also after exposing the sample to moisture and heat. As a result, it can be seen after high temperature and humidity test that peel strength is degraded and fracture mode can be changed from adhesive failure to cohesive failure according to textile fabric. Also, the optimal combination of polyamides resin and textile for better peel strength is obtained.

Cycling effects on surface morphology, nanomechanical and interfacial reliability of LiMn2O4 cathode in thin film lithium ion batteries

In this study, the effects of Li+ intercalation/deintercalation processes on surface morphology, nanomechanical and interfacial properties of sputtered LiMn2O4 cathode film are studied using various techniques, including ex situ field emission scanning electron microscope (FESEM), atomic force microscope (AFM), nanoindentation tests combining with focused ion beam (FIB) sectioning. The results reveal that the spinel LiMn2O4 cathode film shows good cycling performance and morphology stability, accompanying with a slight increase of surface roughness mainly due to the agglomeration of nano-grains caused by phase transformations. The cathode film mechanically fails due to the stress induced by the lattice parameter change upon Li+ intercalation/deintercalation and lattice mismatch related to inhomogeneous phase transformation as well as the Jahn–Teller distortion in the 4V region. The induced stress in the film causes the fragmentation of grains and the initiation/propagation of micro-cracks, leading to the mechanical degradation. The non-uniformity, micro-cracks and nanomechanical degradation have harmful effects on the electrical contact of LiMn2O4 cathode film, resulting in the capacity fading of the lithium ion battery.

Cycling effects on interfacial reliability of TiO2 anode film in thin film lithium-ion microbatteries

This work presents a comprehensive study on the cycling effects on morphological, nanomechanical, and interfacial properties of sputtered TiO2 anode film during discharge/charge cycling. TiO2 film mechanically fails due to the repeated volume change and related generation/relaxation of stress induced by electrochemical phase transformation. The induced stress intensifies the initiation/propagation of cracks, also the interfacial delamination. Both morphology and mechanical property changes have harmful effects on the electrical contact, resulting in the battery aging. This paper also demonstrates that the experiment and analysis method is effective to characterize the interfacial reliability within thin film microbatteries.

Impact of Near-Surface Thermal Stresses on Interfacial Reliability of Through-Silicon Vias for 3-D Interconnects

Continual scaling of on-chip wiring structures has brought significant challenges for materials and processes beyond the 32-nm technology node in microelectronics. Recently, 3-D integration with through-silicon vias (TSVs) has emerged as an effective solution to meet the future interconnect requirement. Among others, thermomechanical reliability is a key concern for the development of TSV structures used in die stacking as 3-D interconnects. This paper examines the effects of thermally induced stresses on the interfacial reliability of TSV structures. First, 3-D distribution of the thermal stress near the TSV and the wafer surface is analyzed. Using a linear superposition method, a semianalytic solution is developed for a simplified structure consisting of a single TSV embedded in a silicon (Si) wafer. The solution is verified for relatively thick wafers by comparing to numerical results from finite element analysis (FEA). The stress analysis suggests interfacial delamination as a potential failure mechanism for the TSV structure. An analytical solution is then obtained for the steady-state energy release rate as the upper bound for the interfacial fracture driving force, while the effect of crack length is evaluated numerically by FEA. With these results, the effects of the TSV dimensions (e.g., via diameter and wafer thickness) on the interfacial reliability are elucidated. Furthermore, the effects of via material properties and dielectric buffer layers are discussed.

Solder Joint Reliability of SnBi Finished TSOPs with Alloy 42 Lead-Frame under Temperature Cycling

Abstract Tin-bismuth (SnBi) is a lead-free alternative to pure tin (Sn) lead-frame finish. SnBi finish is considered by some to be a tin-whisker mitigation strategy. In selecting a SnBi finish, the interfacial strength and reliability of solder interconnects formed with select assembly solders must be considered. To characterize the solder interconnect reliability of SnBi finished parts, sets of test assemblies were created with Sn and SnBi finished thin-small-outline-package (TSOP) parts using SAC305 and SnPb solder. Test assemblies were subjected to temperature cycling and interconnect strength tests. It was found that SnBi finish caused TSOP solder joints to have a shorter fatigue life than Sn finish under temperature cycling testing.

Effect of Hygrothermal Aging on Interfacial Reliability of Silicon/Underfill/FR-4 Assembly

The reliability issues have been converted to the underfill adjacent interfaces since the introduction of the underfill to flip chip package in 1990's. Both thermal cycling and hygrothermal conditioning severely attack the interfaces to de- laminate. The moisture migrating into the underfill decreases the adhesion strength, swells to deform the assembly, and weakens the mechanical and thermal properties of the material. In this study, interfacial reliability of a silicon/underfill/FR-4 assembly exposed at 85degC/85%RH was studied using moire interferometry and micro-digital image speckle correlation (mu-DiSC) techniques. A thermal aging study was simultaneously performed to understand the long-term reliability of the assembly. The results showed that the thermal aging relieved the stresses induced by hygrothermal swelling mismatch between dissimilar materials involved, whereas increased the strains induced by hygrothermal swelling. It indicated the time effect is not negligible when the assembly is subjected to the moisture conditioning, otherwise, the deformation induced by the swelling could be overestimated. The mu-DiSC technique was applied to measure the critical interfacial fracture toughness of the silicon/underfill interface. The results showed that the moisture could significantly reduce the interfacial strength due to the break of hydrogen bonding. By combining the moire and mu-DiSC results, it was concluded that the hygrothermal loading could increase the possibility of interfacial delamination in a flip chip package. Finally, the morphologies of the fractured surface were studied with the aid of scanning electron microscope. Remarkable changes of the failure mode were observed.

Suppression of chemical and electrical instabilities in mesoporous silica films by molecular capping

Mesoporous silica (MPS) thin films are attractive for achieving low relative dielectric permittivity (low k) interlayer isolation in integrated circuit wiring, but are susceptible to instabilities in electrical behavior due to water uptake and copper diffusion. Here, we show that capping MPS films with a trimethyl-group terminated organosilane layer irreversibly suppresses moisture-induced capacitance instabilities, and decreases the relative dielectric permittivity and Cu-induced leakage currents. Analysis of capacitance-voltage and current-voltage characteristics along with infrared spectroscopy shows that the trimethyl organosilanes inhibit hydrogen bonding of water molecules by rendering the dielectric surfaces hydrophobic. These features are promising for tailoring the chemical and interfacial properties and reliability of porous dielectric materials for insulation in device wiring applications.

Relationship Between Interfacial Adhesion and Dielectric Reliability of Cu Alloy Films

AbstractCu alloy has been suggested to enhance reliabilites in future technology. To evaluate the effects of alloying elements on electrical and mechanical reliabilities, Mg and Ru were chosen as alloying elements. Because alloying elements to Cu should not increase resistivity over comparable value to pure Cu, the resistivity of pure Cu, Cu-0.7at%Mg alloy and Cu-2.2at%Ru alloy was compared. Time dependent dielectric breakdown (TDDB) under bias-temperature stress (BTS) tests were accomplished to evaluate the effect of alloying elements on dielectric reliability. The standard four point bending experiments were conducted to measure critical interfacial adhesion energy of conducting metal to SiO2. The relationship of the effect of alloying elements on reliabilities to distribution of alloying elements was analyzed using Auger Electron Spectroscopy (AES). Our results show alloying Mg to Cu can enhance reliability by forming self-passivating layer on its surface and interface while Ru does not.

Improvement of interfacial layer reliability by incorporation of deuterium into HfAlOx formed by D/sub 2/O-ALD

Deuterium was incorporated into the HfAlOx /SiON gate dielectric by the use of heavy water (D/sub 2/O) instead of H/sub 2/O in the atomic layer deposition (ALD) process of HfAlOx. The HfAlOx formed by D/sub 2/O-ALD acts as a deuterium reservoir, and the deuterium atoms are effectively incorporated into the SiON after full CMOS processing. It is clarified that the deuterium incorporation suppresses interfacial trap generation and interfacial SiON breakdown, while charge-trapping in the HfAlOx bulk traps is barely affected. The D/sub 2/O-ALD process is useful for improving the interfacial layer reliability under gate negative stress; therefore it is not only effective for HfAlOx, but also for high-/spl kappa//SiO/sub 2/(SiON) gate stacks with other high-/spl kappa/ materials such as HfO/sub 2/ or HfSiON.

Effects of chlorine on interfacial properties and reliability of SiO2 grown on 6H-SiC

A new process of oxidizing 6H-SiC in dry O2+trichloroethylene (TCE) is used to incorporate chlorine in SiO2. The interface quality and the reliability of 6H-SiC MOS capacitors with gate dielectrics prepared by the process are examined. As compared to the conventional dry O2 oxidation, the O2+TCE oxidation results in lower interface-state density, reduced oxide-charge density and enhanced reliability. This could be attributed to the passivation effects of Cl2 and HCl on the structural defects at/near the SiC/SiO2 interface, and also their gettering effects on ion contamination. Moreover, post-oxidation NO annealing, especially in a wet ambient, can further decrease the interface-state density and the oxide-charge density. Lastly, an increased oxidation rate induced by TCE is observed and should be useful for reducing the normally high thermal budget of oxide growth. All these are very attractive for fabricating SiC MOSFETs with high inversion-channel mobility and high hot-carrier reliability.

Interface Reliability Assessments for Copper/Low-k Products

Multiple new materials are being adopted by the semiconductor industry at a rapid rate for both semiconductor devices and packages. These advances are driving significant investigation into the impact of these materials on device and package reliability. Active investigation is focused on the impact of back-end-of-line (BEOL) processing on Cu/low-k reliability. This paper discusses Cu/low-k BEOL interfacial reliability issues and relates key items from the assembly process and packaging viewpoint that should be managed in order to prevent adverse assembly impact on BEOL interfacial reliability. Reliability failure mechanisms discussed include interface diffusion-controlled events such as the well-known example of Cu electromigration (EM), as well as stress-migration voiding. Interface defectivity impact on dielectric breakdown and leakage is discussed. Lastly, assessments of assembly impact on these Cu/low-k interfacial concerns are highlighted.

Interfacial reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.

Surface Graft Copolymerization Enhanced Lamination of Poly(tetrafluoroethylene) Film to Copper and Epoxy-Based Print Circuit Board (PCB)

Lamination of poly(tetrafluoroethylene) (PTFE) film to a copper foil or to an epoxy-based print circuit board (PCB) Substrate (FR4®) was carried out. Lamination was achieved during surface graft copolymerization of glycidyl methacrylate (GMA) on an Ar plasma pretreated PTFE film at elevated temperature and in the presence of an epoxy resin adhesive. The plasma pretreatment introduces peroxides which are thermally degraded into free radicals to initiate the graft polymerization of GMA on the PTFE surface. The graft copolymerization with concurrent lamination is carried out in the complete absence of a polymerization initiator or system degassing. The modified surfaces and interfaces are characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The adhesion strength between the PTFE film and copper or the FR4® substrate was assessed by the T-peel strength test method. The adhesion strength was affected by plasma pretreatment time, as well as the grafting and curing temperature. The PTFE/GMA-epoxy resin/Cu and PTFE/GMA-epoxy resin/FR4 assemblies exhibit significant higher interfacial adhesion strengths compared to those assemblies in which only epoxy resin or GMA was used. They also exhibit better interfacial adhesion reliability. The PTFE/GMA-epoxy resin/Cu and PTFE/GMA-epoxy resin/FR4® joints delaminated by cohesive failure inside the bulk of PTFE film. The results suggest that the enhanced adhesion between the graft-modified PTFE film and copper or FR4® surfaces is attributable to the formation of covalent bonds between the tethered GMA graft chains on PTFE and the network of epoxy resin.

Surface graft copolymerization enhanced adhesion of an epoxy-based printed circuit board substrate (FR-4) to copper

The lamination of surface modified printed circuit board (PCB) substrate, FR-4(R), from argon plasma pretreatment and UV-induced graft copolymerization with glycidyl methacrylate (GMA), to copper foil was carried out at elevated temperature and in the presence of an epoxy adhesive. The structure and chemical composition of the graft copolymerized surfaces and interfaces of the glass fiber-reinforced and epoxy-based FR-4 substrates were studied by X-ray photoelectron spectroscopy (XPS). The effects of the plasma pretreatment time, the UV illumination time, as well as the curing temperature, on the adhesion strength between the FR-4 substrate and copper were investigated. The assemblies involving GMA graft copolymerized FR-4, or the FR-4-GMA/epoxy resin/Cu assemblies, exhibited a significantly higher interfacial adhesion strength and reliability, in comparison to those assemblies in which only epoxy adhesive alone was used. The enhanced adhesion in the assemblies involving GMA graft copolymerized substrate arises from the fact that the covalently tethered GMA graft chains on the FR-4 surface can become covalently incorporated into the epoxy resin, resulting in the toughening of the epoxy matrix and increased interaction with copper.