Metal Interconnects Research Articles

Experimental and Computational Studies on Octyl Hydroxamic Acid as an Environmentally Friendly Inhibitor of Cobalt Chemical Mechanical Polishing.

Octyl hydroxamic acid (OHA) was investigated as an inhibitor in H2O2-based alkaline silica dispersions for the polishing of cobalt (Co) films for interconnect applications. A combination of experiments and density functional theory (DFT) was used to investigate the inhibition effect and the mechanism of OHA on the Co surface. On the basis of the experiments, it can be proven that OHA has an inhibition effect on Co, which came from the inhibition of the cathodic reaction. The X-ray photoelectron spectroscopy (XPS) experiments show that the adsorption of OHA weakened the oxidation of the Co surface and protected the Co surface from corrosion. On the basis of the calculations, it can be proven that the OHAketone (ion) is most likely to react with the Co surface, and it can adsorb on the Co surface by Co-O bonds. This study provides important microscopic insights for understanding the corrosion protection of Co interconnect metals and helps to explain the corrosion inhibition mechanism of the organic-metal interface during the CMP process.

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.

Co-W Barrier Layers for Metallization of Copper Interconnects: Thermal Performance Analysis.

The back-end-of-line (BEOL) copper interconnect structure has been subjected to downscaling for the last two decades, while the materials used for conforming and assuring its physical integrity during processing have faced significant obstacles as the single-digit nanometer process node is implemented. In particular, the diffusion barrier layer system comprised of Ta/TaN has faced major constraints when it comes to the electrical performance of the smaller Cu lines, and thus alternative formulations have been investigated in recent years, such as Ru-Ta or Co-W alloys. In this work, we assess how PVD (physical vapor deposition) deposited equimolar Co-W films perform when exposed to different vacuum annealing temperatures and how these films compare with the Ta adhesion layer used for Cu seeding in terms of dewetting resistance. The stacks were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDX) mapping. The Cu film at the surface of the Cu/Co-W system exhibited grain growth starting at 300 °C, with the formation of abnormally large Cu grains starting at 450 °C. Sheet resistance reached a minimum value of 7.07 × 10−6 Ω/sq for the Cu/Co-W stack and 6.03 × 10−6 Ω/sq for the Cu/Ta stack, both for the samples annealed at 450 °C.

Change in the Microstructure of Ferritic Stainless Steel with Surface Roughness and the Number of Thermal Cycles

One of the candidates for metallic interconnects of solid oxide fuel cells is ferritic stainless steel, Crofer 22 APU. Ferritic stainless steel Crofer 22 APU specimens with different surface roughness were prepared by grinding with SiC powder papers of various grits and then thermally cycled in air. Variation in the microstructure of the samples having different roughness with thermal cycling was investigated. Polished Crofer 22 APU specimens after three and five thermal cycles had relatively flat oxide layers with thicknesses of about 13.8 and 17.9 μm, respectively. Micrographs of a trench made by milling with FIB (focused ion beam) for a Crofer 22 APU specimen ground with grit 80 SiC powder paper after 8 thermal cycles (total oxygen exposure time of 200 h at 1073 K), captured by ESB (energy selective back-scattering) and SE2 (type II secondary electrons), showed that the surface of the sample was very coarse and its oxide layer was undulated. In the oxide layer, the phase of the sublayer was Cr2O3, and that of the top layer was (Cr, Mn)3O4 spinel. The surface of the sample ground with grit 80 SiC powder paper was very rough after 60 thermal cycles (total oxygen exposure time of 1500 h at 1073 K). The polished Crofer 22 APU is a better applicant to an interconnect of SOFC than those with rougher surfaces.

In situ triggering metallicity in 3D graphene via constructing wrinkle configuration

Freestanding 3D graphene with a controllable electronic state is quite desirable for high-power all-carbon electronics. Although field-effect doping and charge doping have been demonstrated to manipulate the electronic properties of graphene derivatives, using these strategies to achieve nanoscale control remains a challenge. Herein, we developed a strategy for in situ triggering metallicity in semiconductor 3D graphene films (GFs) by building 1D wrinkles. Moreover, the controllable engineering of wrinkles represents a feasible way to realize the manipulation of 1D metallic states with characteristic width ranging from nanoscale to mesoscale. The global metallicity is achieved in GFs by constructing wrinkle percolation networks. The idea of in situ triggering metallicity in semiconducting graphene demonstrates potential application in high-power nanoelectronics. A striking case is that in situ constructing 1D metallic wrinkle provides a promising candidate as the metallic wire interconnects in all-carbon electronics.

Investigation of a Metallic Interconnect Extracted from an SOFC Stack after 40,000 h of Operation

An in-depth investigation was performed on a metallic interconnect extracted from an SOFC stack operated for 40,000 h. The characterization was performed on the surface and the cross-section, paying attention to the evolution of the materials due to the interaction with the dual atmosphere of the stack under operating parameters. The interaction between materials (i.e., metal substrate, coatings and atmospheres) and stack components (i.e., current collectors and MIC) generated several modifications that affected the surface and, in some cases, the bulk of the interconnect. The careful metallographic preparation allowed for the performance of an intensive microscopical characterization of the cross-sections all along the interconnect profile, from the inlet to the outlet of the fuel stream. The formation of thermal grown oxides on both sides and their evolution were studied and described. The interconnect, after 40,000 h, was still suitable for operation, but the few bulk changes due to the diffusion of Ni and the TGO that formed at the fuel side suggest the introduction of fuel side coatings to increase the life expectations of the whole stack.

Performance degradation in proton-conducting ceramic fuel cell and electrolyzer stacks

Proton-conducting ceramics are emerging as enabling materials for efficient electrochemical electricity generation, energy storage, and fuels synthesis. In this work, we present longer-term degradation results for protonic-ceramic fuel cells and electrolyzers based on a BaCe0.4Zr0.4Y0.1Yb0.1O3−δ (BCZYYb) electrolyte. The cells are packaged within unit-cell stacks, including metallic interconnects, current collectors, sealing glasses and gaskets. Durability is found to be superior in protonic-ceramic electrolyzers in comparison to fuel cells. Operating conditions have a large impact on degradation rates; better stability is found at fuel-cell operating temperatures above 600 °C, and electrolyzer steam feeds below 20 %. We find that both fuel-cell and electrolyzer degradation is greatly reduced via the introduction of a gadolinium-doped ceria interlayer between the electrolyte and the air–steam electrode. Fuel-cell degradation falls to 1.2% khr−1 under methane fuel at 600 °C; electrolyzer degradation is reduced to 1% khr−1 at 550 °C and 50% steam. Further analyses of electrochemical impedance spectroscopy and distribution of relaxation times provide insight to root processes and degradation phenomena in protonic electroceramics.

Liquid Crystal Elastomer with Integrated Soft Thermoelectrics for Shape Memory Actuation and Energy Harvesting.

Liquid crystal elastomers (LCEs) have attracted tremendous interest as actuators for soft robotics due to their mechanical and shape memory properties. However, LCE actuators typically respond to thermal stimulation through active Joule heating and passive cooling, which make them difficult to control. In this work, LCEs are combined with soft, stretchable thermoelectrics to create transducers capable of electrically controlled actuation, active cooling, and thermal-to-electrical energy conversion. The thermoelectric layers are composed of semiconductors embedded within a 3D printed elastomer matrix and wired together with eutectic gallium-indium (EGaIn) liquid metal interconnects. This layer is covered on both sides with LCE, which alternately heats and cools to achieve cyclical bending actuation in response to voltage-controlled Peltier activation. Moreover, the thermoelectric layer can harvest energy from thermal gradients between the two LCE layers through the Seebeck effect, allowing for regenerative energy harvesting. As demonstrations, first, closed-loop control of the transducer is performed to rapidly track a changing actuator position. Second, a soft robotic walker that is capable of walking toward a heat source and harvesting energy is introduced. Lastly, phototropic-inspired autonomous deflection of the limbs toward a heat source is shown, demonstrating an additional method to increase energy recuperation efficiency for soft systems.

Thermal-Aware EM Reliability for Advanced Metal Interconnects of Complementary FET

Electromigration (EM) of metal interconnects is the continuous reliability concern for high-density integration beyond 5-nm technology, especially for the buried power rails in vertically stacked device structure. In this work, EM reliability of alternative metal interconnects based on complementary FET (CFET) is investigated by kinetic Monte Carlo (KMC) method. The microscopic evolutions of void formation in power rails can be well reproduced by considering the coupled effects of thermal and stress distribution during the EM simulation. The influences of grain boundary in different metal materials on EM lifetime are analyzed to provide the optimal strategy of process integration. Thermal-aware EM reliability is further predicted, indicating the importance of self-heating on EM evaluation of power rails in CFET.

Evaluation of the thermal stability of TiW/Cu heterojunctions using a combined SXPS and HAXPES approach

Power semiconductor device architectures require the inclusion of a diffusion barrier to suppress or at best prevent the interdiffusion between the copper metallization interconnects and the surrounding silicon substructure. The binary pseudo-alloy of titanium–tungsten (TiW), with >70 at. % W, is a well-established copper diffusion barrier but is prone to degradation via the out-diffusion of titanium when exposed to high temperatures (≥400 °C). Here, the thermal stability of physical vapor deposited TiW/Cu bilayer thin films in Si/SiO2(50 nm)/TiW(300 nm)/Cu(25 nm) stacks were characterized in response to annealing at 400 °C for 0.5 h and 5 h, using a combination of soft and hard x-ray photoelectron spectroscopy and transmission electron microscopy. Results show that annealing promoted the segregation of titanium out of the TiW and interdiffusion into the copper metallization. Titanium was shown to be driven toward the free copper surface, accumulating there and forming a titanium oxide overlayer upon exposure to air. Annealing for longer timescales promoted a greater out-diffusion of titanium and a thicker oxide layer to grow on the copper surface. However, interface measurements suggest that the diffusion is not significant enough to compromise the barrier integrity, and the TiW/Cu interface remains stable even after 5 h of annealing.

Patterning challenges for direct metal etch of ruthenium and molybdenum at 32 nm metal pitch and below

Ruthenium and molybdenum are candidate materials to replace Cu as the back-end-of-line interconnect metal for the tightest pitch features for future technology nodes. Due to their better figure of merit ρ0 × λ (ρ0 bulk resistivity, λ electron mean free path), it is expected that the resistance of <10 nm wide Ru and Mo metal lines can be significantly reduced compared to Cu. An important advantage for Ru and Mo is that both materials, in contrast to Cu, can be patterned by means of so-called direct metal etch, through reactive ion etching or atomic layer etching and can potentially be implemented without barrier. An integration scheme with direct metal etch instead of damascene patterning could simplify the overall patterning flow and eventually opens the possibility for exploring new integration concepts and patterning approaches. However, the learning on direct metal etch of Ru and Mo in the literature is scarce, especially at the relevant dimensions of today's interconnects. In this work, we will focus on the major patterning challenges we have encountered during the development of direct metal etch processes for Ru at 18 nm pitch and Mo gratings at 32 nm pitch. We have observed that the direct metal etch of Ru at these small dimensions is impacted by the growth of an oxidized layer on the sidewalls of the hard mask, which originates from the sputtering of the hard mask in combination with the O2-based Ru etch chemistry. This results in a narrowing of the trenches to be patterned and can easily lead to an etch stop in the smallest features. We will discuss several mitigation mechanisms to remove this oxidized layer, as well as to avoid the formation of such a layer. For patterning Mo with a Cl2/O2-based chemistry, the major patterning challenges we encountered are the insufficient sidewall passivation and the oxidation of the patterned Mo lines. The sidewall passivation issue has been overcome with an in situ thin SiO2-like deposition after partial Mo etch, while a possible mitigation mechanism for the Mo oxidation could be the in situ encapsulation immediately after Mo patterning.

Electrical Reliability of Flexible Silicon Package Integrated on Polymer Substrate During Repeated Bending Deformations

Abstract Although the reliability of flexible electronics during bending deformation is of great interest nowadays, the mechanical reliability that has mainly been investigated is of single electronic components or simple devices, such as metal interconnect, transparent conductive electrode, or thin-film devices, rather than that of the real package sample having complex structure and various materials. This study systematically investigated the mechanical reliability of flexible Si package sample consisting of Si die, polymer bump, and polymer substrate, by using individual resistance monitoring of the metal line, bump array, and total interconnect. For the bending test, the sample consisting of only Si die and polymer substrate shows abrupt electrical resistance increase below a bending radius of 3 mm, due to cracking of the Si die. For the bending fatigue test, the electrical resistance increases after 2000 cycles in 5 mm bending radius, due to fatigue failure of the metal line and bump array. Both the maximum bendability and fatigue lifetime can be significantly improved by covering with the molding layer. Finite element method simulation is conducted to analyze the mechanical stress distribution of the flexible package with and without molding layer during bending deformation. This study based on experimental results and simulation analysis can provide helpful guidelines for the design of highly reliable flexible packages.

Ag metal interconnect wires formed by pseudoplastic nanoparticles fluid imprinting lithography with microwave assistant sintering

Nanoimprint technology has the advantages of low cost, high precision, high fidelity and high yield. The metal nanoparticle fluid is non-Newtonian fluid, which is used as the imprint transfer medium to realize high fidelity of pattern because of its shear thinning effect. In order to functionalize the metal nanoparticles microstructure, the subsequent sintering step is required to form a metal interconnect wire. Metal interconnect wire with fewer grain boundaries and fewer holes have excellent mechanical and electronic properties. In this paper, the pseudoplastic metal nanoparticle fluid was formed by Ag nanoparticle and precursor solution, and then the thermal diffusion process was completed by microwave sintering after interconnects were embossed. The influence of microwave and thermal atmosphere on the microstructure and performance of Ag Interconnect wires was analyzed and discussed, and the Ag Interconnect wires performance was determined under the influence of time and temperature parameters. In our experiments, the interconnects after microwave sintering can achieve 39% of the conductivity of bulk silver. The microwave sintering module might be integrated as the heat treatment module of the metal micro/nano pattern directly imprint lithography.

Evaluation of inkjet-printed spinel coatings on standard and surface nitrided ferritic stainless steels for interconnect application in solid oxide fuel cell devices

Inkjet printing technology was employed for the application of protective layer coatings in SOFC metallic interconnects. Aqueous-based spinel coatings were inkjet-printed on standard and surface nitrided K41 ferritic stainless-steel substrates. Inkjet-printed substrates were exposed to high-temperature oxidation and Area Specific Resistance (ASR) tests for 1000 h at 700 °C in air with 3% volume humidity, simulating SOFC cathode environment. Performance of inkjet printed coatings and effect of nitriding stainless-steel substrates were evaluated based on chromium migration/retention and Area Specific Resistance. Sol-gel infiltration was introduced to develop a scaffold layer over the porous microstructure. With the ASR reduced to a level ∼60 mΩ cm2 and chromium concentration in the getter (cathode) material below 1 atomic%, close to the detection threshold, the protective layers produced via inkjet printing present a promising solution for SOFC interconnector applications.

Fracture toughness of reactive bonded Co–Mn and Cu–Mn contact layers after long-term aging

Creating a tough bond for the electrical contact between metallic interconnects and ceramic solid oxide cells (SOC) in a stack is challenging due to restrictions on the assembly temperature. The reactive oxidation bonding in the formation of Co2MnO4 (CoMn) and Cu1.3Mn1.7O4 (CuMn) spinel oxides from metallic precursors could provide a potential solution for achieving tough and well-conducting contact layers. These contact layers are deposited from metallic precursors onto CoCe-coated AISI441 substrates to achieve high toughness even after aging for 3000 h at typical operating temperatures for SOCs. The interface fracture energy of CoMn and CuMn contact layers was measured for as-sintered and aged samples by using a modified four-point bending test. After the fracture test, X-ray diffraction, electron microscopy, and energy-dispersive X-ray spectroscopy were used to determine phase evolution and possible reactions at the contact layer/interconnect interface. The results show that the interface fracture energy of sintered CoMn contact layer (6.1 J/m2) decreased to 2.9 J/m2 after aging at 850 ○C for 3000 h while the fracture energy for CuMn increased from 6.4 J/m2 to 19.7 J/m2.

A Mechanistic Model for Plastic Metal Line Ratcheting Induced BEOL Cracks in Molded Packages

Metal line ratcheting and passivation cracking in back-end-of-line (BEOL) structures are significant reliability concerns for molded packages that are in widespread use at the present time. When metal lines plastically deform due to ratcheting, the passivation overcoat accumulates stress at the corner upon temperature cycling and is eventually susceptible to fracture. Since packaging materials’ interaction with the die is the cause of the failure, the problem is inherently multiscale in nature requiring the analysis model to span from package dimension to BEOL length scale. In this article, the mechanistic cause for stress accumulation is elucidated. Furthermore, a global–local modeling strategy is applied to model the passivation crack initiation and growth. A global model with coarse mesh was built of the package. The local region around the interconnect metal line in the die was modeled using boundary conditions extracted from the global model. A novel load decomposition technique is developed to identify the loading mode that best correlates with the experimentally observed fracture. It is shown that shear is the dominant loading mode inducing the observed die cracks. Furthermore, it is demonstrated that the thermal expansion mismatch between the mold compound and the lead frame with the silicon die induces the shear load on the BEOL structure. Due to the fact that mold compound is applied at a temperature that is higher than that seen during thermal cycling, the direction of the induced shear load is constant regardless of whether the package is heated or cooled. As the metal line plastically yields during every temperature cycle, the plastic deformation ratchets or accumulates in the same direction over the course of the thermal cycling test. The yielding of metal line results in stiffness reduction, leading to steady accumulation of stress in the passivation corner, causing it to fracture eventually.

Volatile and Nonvolatile Memory Devices for Neuromorphic and Processing-in-memory Applications

The motivation for driving semiconductor devices can be found in the development of advanced computers which can contribute to the betterment in our daily lives. The contribution has been largely made by semiconductor logic devices traveling the pavements identified as technology nodes for device shrinkage that enables high-speed and low-power operations. Lighter and faster processors are the everlasting goals in electronics and computer science, and have been concerned with logic technologies. However, the vast amount of data that should be dealt are consistently requiring an innovative way out of the conventional serial data communication and processing. Data need to be processed in a shorter time but the irreducibilities in logic switching time, data propagation time in metallic interconnection accompanying RC delay, and the time amount spent in the serial communication between logic and memory units should be quenched. It is quite hard to control the former two factors which are largely determined by physical limits and fabrication technology ones in recent days but the latter still has room for reduction by novel devices and architectures specifically designed for maximizing the parallelism in data processing and communication. The semiconductor memories let aside the advancements in processor technologies now is being moved to the center of renovation toward the future computers in the ultimate architecture. In this review, the roles and requirements of semiconductor memories for memory-oriented processors are investigated in the highlights of applications in the neuromorphic system and processing-in-memory (PIM) architectures.

Solid state infiltration of porous steel with aluminium by the forcefill process

Abstract We report first results on producing a metal – metal composite by forcing solid aluminium to fill a porous steel preform. This process is akin to forcefill used for filling of vias when producing metallic interconnects in microelectronics. Using scanning electron microscopy with energy-dispersive X-ray spectroscopy, it was shown that for nominal pore size as small as 100 μm, the pores could be filled with aluminium throughout the entire preform used, thus demonstrating the viability of the process. An interesting aspect of this solid state ‘infiltration’ is that the tortuous random paths taken by the plastically flowing Al involve numerous kinks. Some of them may induce large localised shear deformation similar to that produced by equal channel angular pressing, which is used to fabricate bulk ultra-fine-grained materials. It is, therefore, conjectured that towards the exit end of the channels the infiltrating material may develop an ultra-fine grain structure and exhibit enhanced strength. It is suggested that the process described can be used to produce metal– metal (or ceramic – metal) composites with unusual mechanical properties. Thin fibres of the material that has percolated through the porous preform can be expected to have a sub-micron grain structure and to possess improved mechanical properties.

High power density & energy density Li-ion battery with aluminum foam enhanced electrode: Fabrication and simulation

Compared with aluminum foil (Al foil), the three-dimensional (3D) aluminum foam (Al foam) has the characteristics of 3D interconnected metal wires and through-hole structure with high specific surface area, which exhibits great potential for application in high-power energy storage devices. In this work, 3D Al foam-based and Al foil-based cathode sheets were fabricated and assembled into pouch cells. In-situ XRD was employed to analyze the electrochemical reaction behavior of the two types of porous electrodes. The 3D Al foam-based cathode demonstrated faster electron transfer and lithium-ion diffusion as compared to the Al foil-based electrode, which is helpful for a uniform electrochemical reaction and decreases the electrochemical polarization. Electrochemical simulations and porous electrode models were built to understand these phenomena. The Al foam-based LiFePO4 batteries exhibit much better power and energy performance than Al foil-based LiFePO4 battery. The power density of the Al foam pouch cells is 7.0–7.7 kW/L when the energy density is 230–367 Wh/L, which is the highest power and energy density among reported Al foam-based devices. The new findings open up opportunities for the development of high-power and high-energy-density commercial batteries.

Fabrication of glass ceramic sealants with ceramic fiber filler for solid oxide fuel cells

In this study, ceramic fibers are used as a filler material for glass ceramic sealant in solid oxide fuel cells to improve the thermal cycle behavior. Beside the bare glass ceramic sealant for comparison, multilayered sealants with different ceramic fiber contents are fabricated to investigate the effect of ceramic fiber quantity also. The mechanical performances of the samples are measured via tensile tests by placing them between two metallic interconnector plates after the glass formation process as well as after 1, 5 and 10 thermal cycles. The results show that the mechanical strength in general tends to decrease with increasing the ceramic filler content, which can be attributed to poor adhesion due to reduced glass ceramic composition. On the other hand, thermal cycle behavior of the samples with ceramic fibers is found to be improved at some extend. This may be due to the behavior of ceramic filler network and relatively slow crystallization with increasing the amount of the filler as proven by microstructural observations. Especially for the sample including 4 ceramic fiber interlayers each having 0.030 g ceramic fibers, the mechanical strength shows an increasing trend with the number of thermal cycles.