Thin-film Interconnects Research Articles

Ultrathin damage-tolerant flexible metal interconnects reinforced by in-situ graphene synthesis

Conductive patterned metal films bonded to compliant elastomeric substrates form meshes which enable flexible electronic interconnects for various applications. However, while bottom-up deposition of thin films by sputtering or growth is well-developed for rigid electronics, maintaining good electrical conductivity in sub-micron thin metal films upon large deformations or cyclic loading remains a significant challenge. Here, we propose a strategy to improve the electromechanical performance of nanometer-thin palladium films by in-situ synthesis of a conformal graphene coating using chemical vapor deposition (CVD). The uniform graphene coverage improves the thin film’s damage tolerance, electro-mechanical fatigue, and fracture toughness owing to the high stiffness of graphene and the conformal CVD-grown graphene-metal interface. Graphene-coated Pd thin film interconnects exhibit stable increase in electrical resistance even when strained beyond 60% and longer fatigue life up to a strain range of 20%. The effect of graphene is more significant for thinner films of < 300 nm, particularly at high strains. The experimental observations are well described by the thin film electro-fragmentation model and the Coffin-Manson relationship. These findings demonstrate the potential of CVD-grown graphene nanocomposite materials in improving the damage tolerance and electromechanical robustness of flexible electronics. The proposed approach offers opportunities for the development of reliable and high-performance ultra-conformable flexible electronic devices.

BENDING FATIGUE BEHAVIOR OF AG NANOWIRE/CU THIN-FILM HYBRID INTERCONNECTS FOR WEARABLE ELECTRONICS

Enhancing the mechanical reliability of metal interconnects is important for achieving highly reliable flexible/wearable electronic devices. In this study, Ag nanowire and Cu thin-film hybrid interconnects were explored as a novel concept to enhance mechanical reliability under bending fatigue. Bending fatigue tests were conducted on the Cu thin films and Cu/Ag nanowire/polyimide (CAP) interconnects. The increase in resistance was larger for the Cu thin films than for the CAP. The single-component Cu electrodes showed multiple crack initiation and propagation due to bending strain, which degraded the electrical conductivity. In CAP, however, no long-range cracks were observed, even after 300,000 cycles of bending, although a wavy structure was observed, probably due to the delamination of the Ag nanowires under repeated bending. Our study confirms that flexible Ag nanowire and metal thin-film hybrids can enhance the mechanical reliability of metal thin-film interconnects under bending fatigue.

Study of Electromigration-Induced Void Nucleation Problem Dominated by Bulk, Grain Boundary, and Interfacial Diffusion Based on an Improved Energy Approach

In this present article, an improved electromigration-induced void nucleation model is developed based on our previous work. The Gibbs free energy function of the electromigration-induced void nucleation process is reconstructed to well describe the electromigration-induced void nucleation process. The mass diffusion energy term in the Gibbs free energy function is obtained by the electromigration damage entropy model framework proposed by Basaran et al. The bulk diffusion, grain boundary diffusion, and interface diffusion effect during electromigration can be all considered. Thus, the applicability of the newly developed model is expanded and can be widely used not matter in the thin-film interconnects but also in the RDL and TSV interconnect structures in advanced electronic packaging. The electromigration-induced void initiation time and the critical void nucleation radius can be obtained analytically based on the present model. Several experiments conducted on these new structures are illustrated. It is shown that the theoretical results of the electromigration-induced void nucleation time can well match the experiments.

TiB2-modified PDC electrical solder joints for electrical interconnection in extreme environments

The use of electrical solder joints for die attachment and interconnection in harsh environments remains a significant challenge. The interconnections between traditional metal solder joints and ceramic/semiconductor devices often fail during thermal cycling owing to mismatched coefficients of thermal expansion. In this study, TiB 2 -modified polymer-derived ceramic (PDC) electrical solder joints were developed for electrical interconnections. Owing to the close contact between the PDC paste and the metal/ceramic/semiconductor, as well as the TiB 2 percolation path formed inside the solder joint, the solder joint exhibited extremely low resistance and reliable electrical connection. Thermal cycling tests showed that the PDC solder joints had negative resistance temperature coefficients, excellent repeatability, and oxidation resistance up to 800 °C. After being oxidized at 800 °C for 24 h, the resistance at the solder joint only changed by 1.4 Ω. This solder joint can not only be used for metal devices but also for ceramic/semiconductor device electrical interconnections in harsh environments. • A TiB 2 -modified PDC solder joint was developed for metal/ceramic/semiconductor device interconnections in harsh environments. • The solder joint exhibited excellent thermal stability and high-temperature oxidation resistance. • The feasibility of solder joints for thin-film sensor interconnections was verified.

Phase field simulation of the instability and splitting processes of elliptical inclusions in interconnects due to anisotropic interface diffusion under electric and stress fields

Electromigration and stress migration induced failure of thin-film metal interconnects is one of the most challenging material reliability issues for microelectronic circuits toward ultra-large-scale integrated circuits. Based on the theory of anisotropic interface diffusion, a modified Cahn–Hilliard phase field model is established to elucidate the instability and splitting processes of elliptical inclusions under the multi-physics field. The reliability of the model is verified by comparing numerical and theoretical solutions for the evolution of circular inclusions under electric and stress fields, respectively. The numerical results elaborate on the role of the conductivity ratio, the elastic modulus ratio, the aspect ratio, the electric field, the stress field, the linewidth, and the anisotropic interface diffusion mobility on morphological evolution using an adaptive mesh finite element method. The numerical results show that the larger the electric and stress fields, the greater the aspect ratio larger than 1 or smaller than 1, and the more easily the elliptical inclusions split into several small inclusions or get destabilized. The smaller the linewidth, the easier it is for the inclusions to migrate toward the edge of the line, severely reducing the conductivity of the line. Under anisotropic interface diffusion, lower misorientations favor a steady-state, whereas higher values render the inclusion unstable, splitting, or bifurcating into more small ones. Moreover, the splitting time of the elliptical inclusion decreases with an increase in the electric field, the stress field, and the misorientations, then increases, and subsequently decreases with an increase in the aspect ratio.

Improvement of the Electromigration Resistance of Al Thin-Film Interconnection by Controlling the Crystallinity of Grain Boundaries

The degradation process of the crystallinity of grain boundaries in Al thin-film interconnections was visualized by applying EBSD analysis under high current density. The grain boundary diffusion was accelerated by electromigration only along random grain boundaries with low crystallinity. The crystallinity (IQ value obtained from EBSD analysis) of grain boundaries varied drastically as strong functions of manufacturing process such as deposition method, temperature of thin-film deposition, gas pressure during the deposition, substrate material, and so on. The effective activation energy of self-diffusion of component element along random grain boundaries was found to be dominated by the crystallinity of random grain boundaries. The measured activation energy obtained from the low-crystallinity interconnections was much lower than that obtained from the thermodynamically stable bulk material, and therefore, the diffusion constant due to electromigration was drastically increased, resulting in the drastic decrease of the lifetime of the interconnections. The lifetime of the interconnections was, therefore, improved by increasing the crystallinity of grain boundaries.

Effect of conductivity on the electromigration-induced morphological evolution of islands with high symmetries of surface diffusional anisotropy

We report on the electromigration-induced morphological evolution of islands (vacancies, precipitates, and homoepitaxial adatom clusters) using a phase-field method with high symmetries of surface diffusional anisotropy. The analysis emphasizes on islands migrating in the {100} and {111} planes of the face-centered-cubic crystal, which resembles fourfold and sixfold symmetries, respectively. The numerical results intend to elaborate on the role of conductivity contrast between the island and the matrix and the misorientation of the fast diffusion direction with respect to the applied electric field on the morphological evolution. Based on numerical results, a morphological diagram is constructed in the plane of misorientation angle and conductivity contrast delineating a rich variety of morphologies, which includes steady-state, time-periodic, zigzag oscillations, and island breakup. While the shape of the island is primarily dictated by the conductivity contrast, the migration modes depend on the misorientation. The various migration modes are further distinguished based on the shape of the island such as a faceted wedge or seahorse morphology, an oscillatory characteristic such as standing wave or traveling wave time-periodic oscillations, and different breakup features. The steady-state kinetics obtained from the fourfold and sixfold symmetries are critically compared with the twofold symmetry, isotropic analytical, and numerical findings. Our result suggests that the steady-state velocity decreases with the symmetry fold of the island. Furthermore, the influence of variation in conductivity contrast and misorientation on kinetics in the time-periodic oscillations are discussed. Finally, the numerically obtained stable facets are compared with the analytically derived orientations. The observed results have direct repercussions in terms of the fabrication of nanopatterns and the performance of thin-film interconnects.

Crystallinity-Induced Acceleration of Intergranular Cracking in Thin-Film Interconnections under High Current Density

Crystallinity-Induced Acceleration of Intergranular Cracking in Thin-Film Interconnections under High Current Density

Crystallinity-Induced Acceleration of Intergranular Cracking in Thin-Film Interconnections under High Current Density

Crystallinity-Induced Acceleration of Intergranular Cracking in Thin-Film Interconnections under High Current Density

Role of conductivity on the electromigration-induced morphological evolution of inclusions in {110}-oriented single crystal metallic thin films

The electromigration-induced microstructural evolution of inclusions such as voids, precipitates, and homoepitaxial islands is of technological importance to the reliability, the performance of the thin film interconnects, and surface nanoengineering. In the present article, we report the results on the migration of cylindrical inclusion in the {110}-oriented single crystal of face-centered-cubic metals under the action of electromigration. To this end, we employ a phase-field model based on the Cahn-Hilliard equation with anisotropy in adatom mobility. Emphasis is laid on the role of conductivity contrast between the inclusion and the matrix, and the misorientation of the fast diffusion directions with respect to the applied electric field. Numerical simulations indicate that lower misorientations favor a steady state, while higher values render the inclusion unstable, initiating a complex cycle of splitting and coalescence. At intermediate misorientations, the inclusion undergoes a time-periodic oscillation, the amplitude and the frequency of which is strongly dependent on the values of conductivity. Furthermore, higher conductivity of the matrix relative to the inclusion promotes a transverse elongation, while the similar conductivities lead to slitlike features along the direction of the electric field. Finally, a morphological map is constructed by delineating the dependence of various migration modes on conductivity contrast and misorientation. Results presented here have important implications on void dynamics in interconnects and the fabrication of nanostructures of desired features and dimensions.

Paper-like Foldable Nanowave Circuit with Ultralarge Curvature and Ultrahigh Stability.

Highly foldable conducting interconnects are fundamental elements for multipurpose flexible electronic circuits, including wearable electronics and biomedical devices. Traditional metalized thin-film interconnects demonstrate stable electronic performances in rigid devices but low deformation tolerance in flexibility. Recently, several remarkable research studies on flexible electronics have been carried out, as interconnect structures of serpentine, wavy, and nanowire networks. However, all of the reported flexible interconnects possess either mechanical instability or fabrication difficulty, which restrict their practical applications. Here, we report a new flexible circuit system, which consists of nanowave structure metal interconnects with highly foldable and large-scale manufactured features. This kind of nanowave interconnects presents both stable and prominent electrical performances under mechanical deformation (down to 0.2 mm bending radius with interconnecting resistance variation less than 10%). Further, a highly flexible paper-like wireless accelerometer based on the nanowave interconnects is fabricated and characterized under several extreme strain situations. Our approach affords a comprehensive direction for constitutional realization of new flexible designs and implements the assembly of next-generation foldable electronic equipment.

Effect of Thermomigration–Electromigration Coupling on Mass Transport in Cu Thin Films

Self-induced temperature gradients produced due to passage of electric current through thin film interconnects with bends can be very large, making thermomigration an important mass transport mechanism, in addition to electromigration. Here, we study effects of thermomigration–electromigration coupling on mass transport in Cu films deposited on SiO2/Si substrate, as per the Blech configuration, with a W or Ta interlayer. We observed a slowly growing depletion zone at the anode in addition to a rapidly expanding depleted zone at the cathode. Moreover, we also observed that the extent of the depletion zone at the cathode varied non-monotonically with the inverse of the length of the sample. These seemingly “anomalous” observations are attributed to the coupling between thermomigration and electromigration, where thermomigration becomes dominant as the current density is increased, the sample length is decreased and the affinity between interlayer and the Cu film is weakened. The findings in this work are augmented by finite element modeling of thermomigration–electromigration coupling in Cu film. An overview of impact of these findings on fabrication of thin film device-level interconnects is also presented.

Crystallinity dependence of reliability of electroplated gold thin film interconnections

Crystallinity dependence of reliability of electroplated gold thin film interconnections

Inverse Blech Length Phenomenon in Thin-Film Stripes

In electronics, current-induced mass depletion at the cathode is often seen to decrease with decreasing sample length; this is called the $B\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}h$ $l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h$ $e\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}f\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}t$. However, in the presence of a self-induced temperature gradient in copper film with a geometry that allows current crowding, the authors observe strong coupling between thermomigration and electromigration, which $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}s$ mass depletion with decreasing sample length. This $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}e$ $B\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}h$ $l\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}g\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}h$ phenomenon warrants a revision of design practices currently used to avoid current-induced damage in multilevel microelectronic devices.

Characterization of the quality of grain boundaries in aluminum thin-film interconnection used for semiconductor devices

Characterization of the quality of grain boundaries in aluminum thin-film interconnection used for semiconductor devices

Effect of texture and grain size on the residual stress of nanocrystalline thin films

Residual stresses develop in thin film interconnects mainly as a result of deposition conditions and multiple thermal loading cycles during the manufacturing flow. Understanding the relation between the distribution of residual stress and the interconnect microstructure is of key importance to manage the nucleation and growth of defects that can lead to failure under reliability testing and use conditions. Dislocation dynamics simulations are performed in nanocrystalline copper subjected to cyclic loading to quantify the distribution of residual stresses as a function of grain misorientation and grain size distribution. The outcomes of this work help to evaluate the effect of microstructure in thin films failure by identifying potential voiding sites. Furthermore, the simulations show how dislocation structures are influenced by texture and grain size distribution that affect the residual stress. For example, when dislocation loops reach the opposite grain boundary during loading, these dislocations remain locked during unloading.

Effect of the Crystallinity on the Electromigration Resistance of Electroplated Copper Thin-Film Interconnections

Dominant factors of electromigration (EM) resistance of electroplated copper thin-film interconnections were investigated from the viewpoint of temperature and crystallinity of the interconnection. The EM test under the constant current density of 7 mA/cm2 was performed to observe the degradation such as accumulation of copper atoms and voids. Formation of voids and the accumulation occurred along grain boundaries during the EM test, and finally the interconnection was fractured at the not cathode side but at the center part of the interconnection. From the monitoring of temperature of the interconnection by using thermography during the EM test, this abnormal fracture was caused by large Joule heating of itself under high current density. In order to investigate the effect of grain boundaries on the degradation by EM, the crystallinity of grain boundaries in the interconnection was evaluated by using image quality (IQ) value obtained from electron backscatter diffraction (EBSD) analysis. The crystallinity of grain boundaries before the EM test had wide distribution, and the grain boundaries damaged under the EM loading mainly were random grain boundaries with low crystallinity. Thus, high density of Joule heating and high-speed diffusion of copper atoms along low crystallinity grain boundaries accelerated the EM degradation of the interconnection. The change of Joule heating density and activation energy for the EM damage were evaluated by using the interconnection annealed at 400 °C for 3 h. The annealing of the interconnection increased not only average grain size but also crystallinity of grains and grain boundaries drastically. The average IQ value of the interconnection was increased from 4100 to 6200 by the annealing. The improvement of the crystallinity decreased the maximum temperature of the interconnection during the EM test and increased the activation energy from 0.72 eV to 1.07 eV. The estimated lifetime of interconnections is increased about 100 times by these changes. Since the atomic diffusion is accelerated by not only the current density but also temperature and low crystallinity grain boundaries, the lifetime of the interconnections under EM loading is a strong function of their crystallinity. Therefore, it is necessary to evaluate and control the crystallinity of interconnections quantitatively using IQ value to assure their long-term reliability.

Mechanical Testing for Stretchable Electronics

Stretchable electronics have been a subject of increased research over the past decade (Lacour, S., et al., 2006, “Mechanisms of Reversible Stretchability of Thin Metal Films on Elastomeric Substrates,” Appl. Phys. Lett., 88(20), p. 204103; Lacour, S., et al., 2004, “Design and Performance of Thin Metal Film Interconnects for Skin-Like Electronic Circuits,” IEEE Electron Device Lett., 25(4), pp. 179–181; and Maghribi, M., et al., 2005, “Stretchable Micro-Electrode Array,” International IEEE-EMBS Conference on Microtechnologies in Medicine and Biology, pp. 80–83.). Although stretchable electronic devices are a relatively new area for the semiconductor/electronics industries, recent market research indicates that the market could be worth more than $900 million by 2023 (PR Newswire, 2015, “Stretchable Electronics Market Worth $911.37 Million by 2023,” PR Newswire, Albuquerque, NM.). This paper investigates mechanical testing methods designed to test the stretching capabilities of potential products across the electronics industry to help quantify and understand the mechanical integrity, response, and the reliability of these devices. Typically, the devices consist of stiff modules connected by stretchable traces (Loher, T., et al., 2006, “Stretchable Electronic Systems,” Electronics Packaging Technology Conference (EPTC '06), pp. 271–276.). They require electrical and mechanical connectivity between the modules to function. In some cases, these devices will be subject to biaxial and/or cyclic mechanical strain, especially for wearable applications. The ability to replicate these mechanical strains and understand their effect on the function of the devices is critical to meet performance, process, and reliability requirements. In this paper, methods for simulating biaxial and out-of-plane strains similar to what may occur in a wearable device on the human body are proposed. Electrical and/or optical monitoring (among other methods) can be used to determine cycles to failure depending on expected failure modes. Failure modes can include trace damage in stretchable regions, trace damage in functional component regions, or bulk stretchable material damage, among others. Three different methods of applying mechanical strain are described, including a stretchable air bladder method, membrane test method, and lateral expansion method.

Merging of Thin‐ and Thick‐Film Fabrication Technologies: Toward Soft Stretchable “Island–Bridge” Devices

Soft stretchable thick–thin-film devices are prepared by merging different fabrication technologies with different functional materials. The new “island–bridge” devices rely on screen-printed thick-film island electrodes, connected by thin-film serpentine interconnects. Such a hybrid fabrication process enables to realize high-performance stretchable materials toward a new class of soft electronics with new capabilities that cannot be achieved via individual fabrication routes.

Dominant Factors of Electromigration Resistance of Electroplated copper Thin-film Interconnections

Dominant Factors of Electromigration Resistance of Electroplated copper Thin-film Interconnections