Copper Pillars Research Articles

Analysis of warpage and reliability of very thin profile fine pitch ball grid array

With the evolution of advanced integrated circuit (IC) packaging technology, the use of experiments to identify package performance and life expectation will take a significant amount of time and cost to finish the job. To reduce the cost of research and testing, predictive analyses of reliability and performance using simulation tools have become a feasible approach for the IC assembly industry. Therefore, this study utilized Moldex3D molding simulation software to analyze very thin profile fine pitch ball grid array (VFBGA) packages and established a numerical analysis procedure from the molding and curing process, the post-mold cure (PMC) process, to a thermal cycling test (TCT) to predict the amount of package warpage during processing and reliability after TCT.The results showed that the warpage trends of both experiments and simulations during the same temperature ramping process were similar. In the thermal cycling analysis, potential failure locations were found to be at the copper pillars and redistribution layer (RDL), where the maximum Von Mises stress occurred at the lowest temperature (−65 °C). The fatigue life model, Coffin–Manson model, was used to calculate the potential fatigue life at the two locations, resulting in 1689 cycles (copper pillars) and 9706 cycles (RDL L1).

Reliability Simulation Analysis of TSV Structure in Silicon Interposer under Temperature Cycling.

As semiconductor integration scales expand and chip sizes shrink, Through Silicon Via (TSV) technology advances towards smaller diameters and higher aspect ratios, posing significant challenges in thermo-mechanical reliability, particularly within interposer substrates where mismatched coefficients of thermal expansion exacerbate issues. This study conducts a thermo-mechanical analysis of TSV structures within multi-layered complex interposers, and analyzes the thermal stress behavior and reliability under variable temperature conditions (-55 °C to 85 °C), taking into account the typical electroplating defects within the copper pillars in TSVs. Initially, an overall model is established to determine the critical TSV locations. Sub-model analysis is then employed to investigate the stress and deformation of the most critical TSV, enabling the calculation of the temperature cycle life accordingly. Results indicate that the most critical TSV resides centrally within the model, exhibiting the highest equivalent stress. During the temperature cycling process, the maximum deformation experiences approximately periodic variations, while the maximum equivalent stress undergoes continuous accumulation and gradually diminishes. Its peak occurs at the contact interface corner between the TSV and Redistribution Layer (RDL). The estimated life of the critical point is 3.1708 × 105 cycles. Furthermore, it is observed that electroplating defect b alleviates thermal stress within TSVs during temperature cycling. This study provides insights into TSV thermal behavior and reliability, which are crucial for optimizing the design and manufacturing processes of advanced semiconductor packaging.

Development and Characterizations of Fine Pitch Flip-Chip Interconnection Using Silver Sintering

Flip-chip interconnects made of silver are promising candidates to overcome the intrinsic limits of solderbased interconnects and match the demand for increased current densities of high-performance microprocessors. Dipbased interconnects have been demonstrated to be a promising approach to form electrical interconnects by sintering paste between copper pillars and pads. However, the quality of the process is limited by residual porosity and poor performances of the sintered joint formed between the pillar and the pad during sintering if a pressure > 50 MPa is not applied in order to decrease the final porosity. In this study, development has been focused on varying key dipping process parameters allowing a pressureless sintering process. Dip-transfer process was optimized on test vehicle and has shown electrical continuity over 700 interconnections with diameter down to 50 μm. We demonstrate high reliability of the process with microstructural observations, tomography X and thermal cycle up to 200 cycles without breakdown.

Performance of a foam copper-based chromium potassium alum salt pill in an adiabatic demagnetization refrigerator

Hydrated paramagnetic salt pills are important components in an adiabatic demagnetization refrigerator (ADR) for temperatures below 500 mK. To improve the heat transfer performance between salt crystals and external thermal links, thermal buses are commonly employed inside the salt pill. Common forms of thermal buses are either thin copper pillars made from electric discharge machining (EDM) or wire bundles, both of which generally involve considerable machining or labor. This paper introduces a novel design and fabrication of a simple foam copper-based paramagnetic salt pill. The foam copper has 30 pores per inch (PPI), a porosity of 98 %, 0.005 m2/g specific surface area, and an estimated pore diameter of 0.82 mm. The complete design, assembling process, and growth of the foam copper-based chromium potassium alum (CPA) salt pill (40 mm in length and 23 mm in diameter) are introduced. A single-stage ADR is built based on this salt pill to test its performance, while another salt pill with the wire thermal bus has also been tested for comparison. A typical cool-down from 4.3 K @ 4 T with a demagnetization rate of 0.0025 T/s led to a no-load temperature of 176.5 mK, slightly better than that using a wire thermal bus. In terms of cooling power per gram of salt, the cooling capacities at 600 mK of both salt pills have also been measured and both have shown similar performance. This preliminary study has shown the potential of the foam copper-based salt pill though more research is required to optimize its performance.

Photo-Imageable Dielectrics Enabling Structured MEMS and 2.5D / 3D Bonding Systems

DuPont has developed a new Photo-Imageable Dielectric (PID) to complement its existing permanent bonding materials. The CYCLOTENEâ?¢ family of dielectric materials are well known for use as a permanent dielectric and have seen significant use in commercial applications including BSI CMOS image sensors, integrated passive devices and as an optical bonding adhesive. For new applications, there is a desire to have a thermal stable material that can be patterned to eliminate the material from the metal joining region or from the light path. We will demonstrate in this paper that the solvent developed PID as well as our existing aqueous developable PID are both capable of wafer to wafer bonding after photo imaging and development. In addition to bonding using a commercial wafer to wafer bonding tool, we have shown that wafer to wafer bonding is achievable using a two-stage vacuum laminator. Optimization of the curing process will be discussed to prevent any separation or reflow of the PID as well as achieving the maximum adhesion energy after full cure. Finally, we will show the use of the negative tone PID as a solder collar or polymer dam to enable high density flip chip bonding using 15?m Sn/Ag capped copper pillars with a 1:1.5 pitch to a and 18?m polymer openings with a 1:1.08 pitch in 10mm x 10mm dies as well as outline the process we will use to extend this approach to Cu-Cu hybrid bonding.

Multifunctional Composite Material with Efficient Microwave Absorption and Ultra-High Thermal Conductivity

The increasing demands for electronic devices to achieve high miniaturization, functional integration, and wide bandwidth will exacerbate the heat generation and electromagnetic interference, which hinders the further development of electronic devices. Therefore, both the issues of microwave absorption and heat dissipation of materials need to be addressed simultaneously. Herein, a multifunctional composite material is proposed by periodic arrangement of copper pillars in a matrix, based on the wave-absorbing material. As a result, the equivalent thermal conductivity of the composite structure is nearly 35 times higher than the wave-absorbing matrix, with the area filling proportion of the thermal conductivity material being 3.14%. Meanwhile, the reflectivity of the composite structure merely changes from −15.05 dB to −13.70 dB. It is proved that the designed composite structure possesses both high thermal conduction and strong microwave absorption. The measured results accord well with the simulation results, which demonstrates that the thermal conductivity of the composite structure can reach more than 10W⋅m−1⋅K−1 without significant deterioration of the absorption performance.

Synthesis and Atomic Transport of CoSn3 NanoIMC by In Situ TEM.

In order to optimize the interfacial properties by adding Co to the bumps of copper pillars and to overcome the strong tendency of Co to oxidize, an intermetallic compound (IMC) "capsule" was developed for the purpose of transporting elements through the intermetallic compound. In this study, we present a comprehensive analysis of the transformation process of CoSn2 nanoparticles into CoSn3 at the nanoscale using in situ heating transmission electron microscopy (TEM). The experimental results reveal that CoSn2 nanoparticle growth occurs through polymerization, whereas CoSn3 nanoparticle formation relies on the reaction between CoSn2 and Sn. During the initial stages of the reaction, Co dissolves and diffuses into Sn, leading to the nucleation and growth of CoSn2 in Sn via Ostwald ripening. As the input energy increases, vacancies in CoSn2 drive a reaction at the Sn/CoSn2 interface, resulting in the generation of CoSn3. In this process, Sn nanoparticles enter the CoSn2 structure through the "Anti Structure Bridge (ASB) mechanism" to fill vacancies. Following the codeposition process, CoSn3 nanoparticles were successfully plated within the Sn layer of the Cu-pillar bumps. Upon reflow heating, the CoSn3 nanoparticles exhibited a preference for precipitating the vacant sites within the Sn layer. This process facilitated the release of Co atoms from CoSn2, enabling their diffusion throughout the entire Sn layer.

Pre-applied underfill Technique for Fine-pitch Cu Pillar 3D Die Stacking to Enable 2.5/3D Advanced Packaging

Increased performance and smaller form factor have driven demand for next generation architectures for integrated circuit 3D (IC) chip packaging. Advanced chip packaging technologies, such as 2.5D and 3D, offer greater chip compatibility and lower power consumption. Given these advantages, the adoption of advanced packaging is inevitable. One key driver is the copper pillar interconnect technology which offers several benefits such as improved electromigration resistance, improved electrical and thermal conductivity, simplified under bump metallization (UBM), and higher input/output (I/O) density. The fine pitches that copper pillars are capable of helps the technology to supersede solder bump technology, which reaches its lowest pitch around 40 microns. Finer pitches allow for a higher I/O count, which enhances performance. One of the major innovations has been 2.5/3D integration, a technique of vertically stacking multiple dies. However, traditional encapsulation techniques become challenging because traditional capillary underfill methods become restrictive as enlarged surface area is required in order to provide underfill protection on multiple surfaces. Here, we discuss a novel approach of vertically stacking dies with an integrated underfill process that minimizes the required area for underfilling in fine-pitch IC chip packages by creating a technique to apply underfill during thermo-compression bonding. This process offers a reliable method for vertical stacking of multiple chips and eliminates post-processing need for underfill application. Additionally, the process provides precise temperature control to both the substrate and die by using a mechanism of dual heat application. This is used in combination with a controlled variable application of compressive force on the package. Through fine-tuning of the force and temperature applied to the IC components, the bonding process can be tailored to simultaneously provide the ideal underfill curing conditions while forming solder joints. In this work, we used dies with 80µm pitch Cu pillar flip chip copper pillars with tin/silver caps bonded to Electroless Nickel and Immersion Gold (ENIG) pads in conjunction with commercially available pre-applied underfill material. The interconnect strength was determined though destructive shear testing and cross-sectional analysis to evaluate for the bonding quality and strength. A scanning electron microscope was used to visually examine joint properties, and X-ray analysis was conducted to review for underfill voiding. Finally, completed assemblies were thermal-cycled to evaluate for the degradation of joint strength and voiding of underfill under induced stress.

In Situ Study of Twin Boundary Stability in Nanotwinned Copper Pillars under Different Strain Rates

The nanoscopic deformation of ⟨111⟩ nanotwinned copper nanopillars under strain rates between 10-5/s and 5 × 10-4/s was studied by using in situ transmission electron microscopy. The correlation among dislocation activity, twin boundary instability due to incoherent twin boundary migration and corresponding mechanical responses was investigated. Dislocations piled up in the nanotwinned copper, giving rise to significant hardening at relatively high strain rates of 3-5 × 10-4/s. Lower strain rates resulted in detwinning and reduced hardening, while corresponding deformation mechanisms are proposed based on experimental results. At low/ultralow strain rates below 6 × 10-5/s, dislocation activity almost ceased operating, but the migration of twin boundaries via the 1/4 ⟨101¯ ⟩ kink-like motion of atoms is suggested as the detwinning mechanism. At medium strain rates of 1-2 × 10-4/s, detwinning was decelerated likely due to the interfered kink-like motion of atoms by activated partial dislocations, while dislocation climb may alternatively dominate detwinning. These results indicate that, even for the same nanoscale twin boundary spacing, different nanomechanical deformation mechanisms can operate at different strain rates.

Laser-Assisted Micro-Solder Bumping for Copper and Nickel-Gold Pad Finish.

Flip-chip bonding is a key packaging technology to achieve the smallest form factor possible. Using copper as a direct under-bump metal and performing bonding under little force and at a low temperature eliminates the processing step for the deposition of a suitable wetting metal and offers an economical solution for electronic chip packaging. In this paper, various samples with copper and nickel–gold surface finishes are used to apply an in-house solder bumping, flip-chip bonding and reflow process to exhibit the bump-bond feasibility. Native oxides are reduced using process gases, and copper surface protection and solder wetting are achieved using copper formate. Lead-free 40 µm solder balls were bumped on 80 µm copper pads and 120 µm copper pillars to demonstrate a full intermetallic Cu–Cu bond as a base study for stacking applications. Using a low-force bonding technique, various chips with different dimensions were bonded at 0.5–16 MPa, followed by a reflow step at a maximum temperature of 270 °C. Then, 30 µm solder balls are utilized to bump the samples with NiAu and Cu bond pads at 50 µm pitch. A mean shear strength of 44 MPa was obtained for the 30 µm Cu samples. To the best of our knowledge, 30 µm solder bumping directly on the copper pads by producing copper formate is a novel research contribution.

Effects of Commercial No-Clean Flux on Reliability of Fine Pitch Flip-Chip Package With Solder Bumps and Copper Pillars

In this work, two commercial no clean fluxes, namely, extremely low residue no-clean flux (NCF) (NC-1) and ordinary NCF (NC-2), were used for the preparation of a fine pitch flip-chip package with a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.6\,\times \,7.6$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> Si die on a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$17\,\times \,17$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> organic substrate. One water-soluble (WS) flux was also used for comparison. Moisture sensitivity of the package was evaluated using JESD22-A113D (30 °C/60% relative humidity (RH), 192 h, <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3\times $ </tex-math></inline-formula> reflow at 260 °C). Thermal cycling tests were performed following JESD22-A104D (−65 °C to 150 °C). The high-temperature storage life (HTSL) of the packages was investigated using JESD22-A103C (175 °C). Underfill delamination or voids in the package were investigated by confocal scanning acoustic microscopy (C-SAM). Failure characteristics of the packages were also studied using scanning electron microscopy (SEM). After moisture sensitivity level (MSL) test, vehicles prepared by NC-1 and NC-2 showed failure. SEM images revealed severe delamination between underfill and solder mask in the presence of amine/amide and carboxylic acid-based NCF residue. Solder bump crack were found at < 500 cycles for samples prepared with NC-1 which contains tertiary amine and long carbon chain (C8:C10:C12) carboxylic acid-based. However, failure occurred at < 1500 cycles for NC-2 containing secondary amide and shorter carbon chain length (C3) carboxylic acid. Samples prepared using WS did not show any failure for up to 2000 cycles. Thus, NCF-treated assemblies performed poorly in moisture sensitivity and temperature cycling tests.

Investigation of Cu-Sn-Cu transient liquid phase bonding for microsystems packaging

ABSTRACT Fabrication, characterization, and thermomechanical reliability of electroplated Cu-Sn-Cu-based transient liquid phase (TLP) bonding required in microsystems packaging are reported. Die-level bonding was achieved using 8-μm-thick electroplated copper pillars covered with 2-μm tin. A mechanical test specimen having rectangular ring-shaped structures was fabricated to measure the strength of Cu-Sn joints. An electrical test vehicle having 250 copper pillars connected in a daisy chain series was designed. The effect of bonding force, temperature, and durations on the shear strength and electrical resistance of the Cu-Sn-Cu joints were investigated. The mechanism behind the improved bonding at higher force, temperature, and duration is reported. Samples bonded at a lower bonding force (5 kg) failed, while those bonded at a higher force showed good strength. A complete transformation of Sn into stable Cu3Sn was essential to achieve good bonding. Due to the presence of Cu3Sn IMC, the bonding strength of samples bonded at higher temperatures and longer duration was higher. Thermal shock and humidity tests were conducted to check the long-term reliability of Cu–Sn–Cu bonded joints. The percentage change in the electrical resistance of a complete daisy chain remained <5% even after 1000 thermal shock cycles.

Near‐field radio frequency identification leaky‐wave antenna based on artificial transmission line

The artificial transmission line (ATL) has a good characteristic of wavelength shortening, which can effectively cut down the size of microwave devices. In this article, the near-field radio frequency identification antenna based on an ATL was proposed. It mainly includes the radiating unit structure on the upper layer and the feeding structure on the lower layer. The two layers are connected by copper pillars for feeding. The height of the copper pillars is 16 mm (h0), which can obtain the maximum bandwidth. The antenna unit based on ATL consists of three curved microstrip lines, two interdigital microstrip lines, and four open microstrip lines. The interdigital structure on the upper layer of the antenna unit mainly affects the resonant frequency of the antenna unit, and the asymmetric interdigital microstrip lines increase the impedance matching bandwidth of the antenna unit. Compared with previous antennas, the proposed antenna features a low profile, and a good miniaturization based on an ATL by bending inductance and interdigital capacitance. The leaky-wave characteristic makes the energy radiate at a certain angle on the surface of the antenna. The leaky-wave characteristic and circular polarization characteristic of the antenna achieve a uniform field distribution. The simulated and measured bandwidth cover from 0.78 to 1.1 GHz for |S11| less than −10 dB. More importantly, the antenna has a large recognition area. The measured results demonstrate that the proposed reader antenna has a 100% read rate in 400 × 350 × 400 mm3 area in the metal shelf.

The Effect of Grain Orientation of β-Sn on Copper Pillar Solder Joints during Electromigration

The grain orientation of Sn-based solder joints on copper pillars under the combined action of electron wind force and temperature gradient greatly affects their electromigration damage. The copper pillars with Sn-1.8Ag lead-free solder on the top was subjected to a current density of 1.5 × 104 A/cm2 at 125 °C to study the electromigration behaviors. The grain orientation was characterized by scanning electron microscopy (SEM) equipped with electron backscattered diffraction (EBSD) detector. Metal dissolution and voids formation in the cathode as well as massive intermetallic compounds(IMC) accumulation in the anode were observed after electromigration. Closer examination of solder joints revealed that the Sn grain whose c-axis perpendicular to electric current may have retarded Cu diffusion to anode and IMC accumulation. In addition, the newly formed Cu6Sn5 exhibited preferred orientation related to the electric current direction.

Fine-pitch Copper Pillar Flip Chips in High Reliability Applications

Abstract To keep up with the demands for smaller antennas with increased performance and decreased costs, most next generation architectures mandate higher IC (integrated circuit) chip integration. Compared to conventional packaging configurations, advanced chip packaging technologies, such as 2.5D and 3D, offer greater chip compatibility and lower power consumption. Given these advantages, the adoption of advanced packaging is inevitable. Within advanced packaging, the copper pillar interconnect is a key enabling technology, and the next logical step. This technology offers several benefits, including improved electromigration resistance, improved electrical and thermal conductivity, simplified underbump metallization (UBM), and higher I/O (input/output) density. The fine pitches that copper pillars allow helps the technology to supersede solder bump technology, which reaches its lowest pitch around 40 microns. Finer pitches allow for a higher I/O count, which increases performance. In this work, assembly of ultra-thin MMIC (monolithic microwave integrated circuit) GaN (Gallium Nitride) fine-pitch copper pillar flip chip assemblies on high density interposers was successfully demonstrated. Using 150μm pitch copper pillar flip chip, the assembly processes for both organic PCB (printed circuit board) and silicon interposers were evaluated, with both an ENIG (Electroless Nickel Immersion Gold) and eutectic tin-lead solder pad finish evaluated. For the 2D/2.5D/3D assembly process development, a standard in-house pick and place tool was used, followed by mass solder reflow, finished with an underfill for reliability test. The interconnect robustness was determined by die pull strengths, a flux stamping investigation, and cross-sections. Complete reliability and qualification test data on GaN copper pillar flip chip 2D assembly was completed, including 700 temperature cycles and UHAST (unbiased highly accelerated temperature/humidity stress test).

An Investigation into Thermomigration Failure of Flip Chip Solder Joint Interconnects used in High Reliability Applications

Abstract The experimental study described herein investigates thermomigration of both eutectic Sn/Pb and lead free Sn/Ag/Cu solder joints, with the additional focus on the development of an Arrhenius failure model of time-to- failure as a function of temperature. Flip chip test vehicles were assembled with daisy chain die, with and without underfill, attached to ceramic and organic package substrates, using standard reflow processes. Three different under bump metallurgy technologies were investigated: sputtered thin film Al/NiV/Cu, thick electroplated nickel, and thick electroplated copper (i.e., copper pillars). The test vehicles were subjected to unbiased high temperature storage (HTS) testing with temperatures ranging from 125°C to 200°C. Failure was determined from continuity testing at regular intervals until an open circuit or high resistivity was detected. The results demonstrate the robustness of Al/NiV/Cu UBM at temperatures less than 130°C, and of the thick electroplated UBM at temperatures of 150°C. Based on the HTS test results, and corresponding analyses, a thermomigration failure model was generated for eutectic Sn/Pb and lead free Sn/Ag/Cu solder alloys using an Arrhenius-like fit to the failure data.

The Pivotal Role of Uniformity of Electrolytic Deposition Processes to Improve the Reliability of Advanced Packaging

Abstract Heterogeneous integration is considered as the key technology to create large, complex System in Package (SiP) assemblies of separately manufactured, smaller components. Proper control of the uniformity of each process step constitutes one of the main challenges during integration of the different components into a higher-level assembly. In this context, processes that create thick layers by electrochemical deposition are especially susceptible to variations across the substrate. Such processes include copper pillar and bump as well as tin-silver applications. Insufficient coplanarity of electrolytic copper would result in significant reliability issues or evolution of stress in the package. Upcoming hybrid bump designs with features of different dimensions pose additional challenges to the electrolytic copper and tin-silver processes. Purposeful adjustment of differences between the heights of pillars of different diameters may be required after the copper process step in order to obtain the best uniformity for the complete stack with tin-silver on top. In addition to coplanarity, the electrolytic process should allow modification shape of the individual pillar or bump. In this context, a versatile copper electrodeposition process will be discussed that allows adjustment to a broad variety of uniformity parameters and combinations thereof. In combination with suitable tin-silver deposition processes, this process is expected to significantly improve the reliability of copper pillars and bumps for advanced packaging applications.

Effect of the functional group of polyethylene glycol on the characteristics of copper pillars obtained by electroplating

Five polyethylene glycol (PEG)-based compounds with varying terminal functional groups were investigated as suppressors to investigate the effect of different terminal functional groups on the ability of PEG-based suppressors to inhibit Cu pillar electroplating. The inhibition ability of the suppressors increased with increasing steric hindrance of the aromatic functional group and length of the alkyl group. PEG-based compounds bearing terminal functional groups with large steric hindrances significantly impeded the diffusion of electroplating accelerator, thus enhancing the inhibitory effect. When the molecular weight of PEG was less than 1000, possessing a terminal functional group had a greater effect on the inhibitory ability than increasing the molecular weight. The wetting ability of the electrolyte increased as the inhibitory ability of the suppressor increased. The significant steric effect of PEG p-(1,1,3,3-tetramethyl butyl)-phenyl ether was confirmed, which contributed to the convex shape of the Cu pillars and the growth of small-sized Cu grains. Conversely, the interaction between PEG and Cu electrode surface was weak. Thus, flat-top Cu pillars were obtained when PEG was used as the suppressor.

Crack identification and evaluation in BEoL stacks of two different samples utilizing acoustic emission testing and nano X-ray computed tomography

A novel approach is presented to evaluate the mechanical stability of back end of line (BEoL) stacks. A SRAM test vehicle manufactured in 28 nm technology with copper pillar bumps is used. In a second step, the validity of the approach is also shown for a second sample manufactured in 22 nm technology and the experimental results are compared. To inflict damage to the BEoL stack of the respective semiconductor device, external forces are applied to the copper pillars (Cu-pillars) by shear loading using a tribo-indenter system. An acoustic emission (AE) detection sensor is attached to the sample to measure acoustic waves during the shear experiment as an indication for damage. The damage progression in the BEoL stack is extremely fast, hence details can't be resolved anymore with the piezo sensors of the tribo-indenter. However, due to the much higher temporal resolution, AE measurements give a more precise insight into the damage process. The resulting damages are imaged using nano X-ray computed tomography (nXCT) as well as scanning electron microscopy (SEM). The results provide a better understanding of the origin and the propagation of damages in the BEoL stack.

Compressive properties of porous Cu reinforced by inserting copper pillars or tubes

Open-cell metal foams suffer from severe deterioration in mechanical properties due to their enriched three-dimensional interconnected holes. In this paper, reinforcing copper pillar(s) or tube(s) are embedded into the foam matrix to form “composite” structure to enhance the open-cell copper foams. To do this, a simple positioning device is designed for preparation of the green porous copper aligned with directional through hole(s) based on a tapping method. Then the reinforcing pillar(s) or tube(s) are inserted into the hole(s) and sintered together. By this means, the mechanical properties of the copper foams are significantly improved. The energy absorption capacity of the composite foams has also been improved because of a higher and wider yield platform compared with the unreinforced copper foams.