Search

Despite the rapid progression of silicon carbide (SiC) power devices, the thermal characteristic evaluation during power cycling at high temperature (>200 °C) is an issue. In this article, a fast and miniaturized evaluation system with online thermal characteristic measurement function was introduced by an n-doped 4H SiC thermal engineering group (TEG) chip. Online thermal resistance measurement of a power module structure by Ag sinter joining with micron/submicron Ag particles paste in low temperature, low pressure, and cooling system by a thermal interface material bonding was performed. High-temperature reliability was systemically investigated by power cycling tests by switching on / off the power source which is connected to the SiC-TEG chip by Au wires. The total thermal resistance of the power module from the SiC-TEG chip to the cooling system increased from 0.5 to 0.53 K/W with the enhanced power source, and remained almost same after 20 000 power cycling at a swing temperature ΔTj of 150 °C. Furthermore, the SiC-TEG power module structure with the die attached with Pb and Pb-free solders, alongwith the same power source as sinter Ag paste was also measured. The Ag sinter joint possesses the lowest thermal resistance and highest high temperature reliability during power cycling compared with Pb and Pb-free die-attach materials.

Ag sinter joining technology is emerging as a die attach material for next-generation power modules in high-temperature applications. Thermal shock test has revealed that the fracture characteristics and reliability of sintered Ag joint were influenced by thermo-mechanical stress. This was study conducted to understand the microstructure, vertical crack formation, and fracture behavior of sintered Ag joints which were designed with different metallization layers on a direct bonded aluminum (DBA) substrate, at different thermo-mechanical stresses during thermal shock tests. Two kinds of metallization layers were designed as Ti/Ag and Ni/Ti/Ag layers. A finite element model (FEM) simulation confirmed that the Ni layer prohibited Al hillock-like deformation and generates different thermo-mechanical stresses during the thermal shock test from − 50 °C to 250 °C. Depending on the degradation of the interfaces for both of the Ag-sintered joints, the sintered Ag grain necking thickness and microstructure characteristics including the Ag grain structures, which have a dominant influence on the bonding strength in terms of long-term reliability, are considerably different from the results by an electron back scatter diffraction (EBSD) analysis. This paper proposes a novel metallization technology that can induce joint fracture with complete recrystallization of sintered Ag joints by effectively suppressing interfacial degradation. The mechanism of this technology was systematically analyzed through experiments and FEM simulations.

Reactive bonding using nanoscale multilayers based on the high specific surface/interface energy is a promising low-temperature and low-pressure wafer-bonding process. Herein, Si wafers were bonded using nanoscale Ag/Cu multilayers in N2 or the ambient atmosphere. A flawless joint composed of Ag-rich and Cu-rich face-centered cubic (fcc) phases was achieved in N2 with 22.2 MPa shear strength. However, a peculiar “fcc-(Ag)+voids/Cu2O/fcc-(Ag)+voids” three-layer sandwich structure with 11.1 MPa shear strength was formed due to significant out segregation of Cu toward the bonding-interface in the ambient atmosphere. The bonding mechanism and the role of oxygen were unveiled based on CALPHAD (CALculation-of-PHAse-Diagram) thermodynamic modeling.

This study was carried out to develop a DBA (direct bonded aluminum) substrate with Ni/Ti/Ag metallization to achieve highly functional thermal shock stability of Ag sinter joining in GaN (Gallium Nitride) power modules. GaN /DBA die-attached module structures by Ag sinter joining was performed during harsh thermal shock cycling tests within a temperature range of − 50/250 °C. In the case of DBA without a Ni metallization layer (Ti/Ag), severe degradation occurred at the interface between the sintered Ag and Al due to significant plastic deformation of the Al layer. The shear strength decreased from an initial value of 33.1 MPa to 22.3 MPa after 500 cycles. With EBSD investigation, it was determined that the Al layer underwent sub-grain rotation recrystallization during thermal shock cycles. This led to a non-uniform grain orientation distribution at center and corner locations. On the other hand, Ni/Ti/Ag metallization showed that it can prevent severe Al deformation due to the superior rigidity achieved by Ni metallization. The die-shear strength maintained almost the same value as its initial value, even after 500 cycles. In addition, a numerical simulation analysis determined that the Ag sinter joining structure on the DBA substrate with Ni/Ti/Ag metallization had high functionality in stress relaxation. This study provided a novel approach to design thermal shock stability Ag sinter joining for next-generation power modules in high-temperature applications.

Ag sinter joining provides superior mechanical and thermal/electrical properties and is considered to become a leading next-generation wide band-gap (WBG) die-attach material. However, the microstructural evolution and mechanical characteristics of Ag sinter joining when subjected to high temperature have never been directly compared to those same characteristics of solder materials. In this study, we have evaluated the high-temperature and long-term reliability of a GaN/DBA die-attached module by pressure-less Ag sinter joining and Pb–5Sn solder in a harsh thermal aging test. Both the Ag sinter joining and Pb–5Sn solder were subjected to a thermal aging test of up to 1000 h at 250 °C. Initial shear strength of the Ag sinter joint exceeded 42 MPa, and increased stably up to 1000 h without any defects such as interface oxidation, diffusion, or mechanical deformation. The increase in shear strength of the Ag sinter joints was the result of necking growth of the sintered Ag during thermal aging. On the other hand, the shear strength of the Pb–5Sn joints exhibited substantially decreased shear strength (by 60%) after aging 250 h. NixSnx intermetallic compounds (IMC) were also formed and serious interface degradation occurred during the aging process. These microstructure changes and mechanical characteristics have an important influence on mechanical reliability and, with that in mind, the tendency of fracture mechanism was investigated in detail by SEM–EDX. This study systematically examines the fracture mechanism on the microstructure of a DBA substrate and on high-temperature packaging during thermal aging tests for WBG semiconductor device applications.

The application of Co-W-P plating technology in high-temperature package structure is advantageous from a point of structural reliability because Co-W-P metallization is known to deliver strong bonding to both high-temperature-compatible Ag-sintered joining and high-temperature-compatible encapsulation resins. However, Co-W-P, unlike a noble metal, has a potential risk of surface oxidation in the module fabrication process. This surface oxidation can result in a decrease in resin adhesion. In this paper, the effects of W content (7 wt%, 11 wt%, 21 wt%) in Co-W-P metallization on both the oxidation resistance and the resin adhesion were studied. The resin adhesion on the annealed Co-W-P metallization with a high W content (21 wt%) was found to be sufficiently strong even after 250 °C anneal for 1 h. This resin adhesion strength was not present in other Co-W-P metallization tests. SEM–EDS analysis revealed that the oxidization of the Co-W-P-metallized surface during the anneal process proceeded more slowly in the case of the Co-W-P metallization with a doping 21 wt% W. XPS analysis revealed that Co(OH)2, necessary for a chemical reaction with the resin, exists mainly on the Co-W-P-metallized surface in the case of doping 21 wt% W, even after 250 °C anneal. XRD analysis revealed its structure to be a characteristic Co-W solid solution, unlike the structures found in other Co-W-P metallization. The findings in this study are significant for the promotion of Co-W-P metallization in the module fabrication process, as well as to the fundamental understanding of oxidation resistance and adhesion behavior on Co-W-P metallization.

In this work, two methods were proposed to improve the bonding quality of Ag sinter joining on Au surface finished substrate (Ag Au joint). The first method was preheating treatment to Au surface finished substrate before sintering. The second method proposed was increasing the initial thickness of the Au layer on the substrate. The bonding strength of the sintered specimen improved from 13.8 MPa to 25.4 MPa by a 250 °C substrate preheating treatment. Also, bonding strength increased from 13.3 MPa to 24.4 MPa as the initial thickness of Au layer was increased from 0.1 to 0.8 μm. SEM, EDS and XRD characterizations were conducted to analyze improvement of bonding strength, diffusion behavior between Au and Ag, and structure of Au layer. The results indicated that the enhancement of bonding strength was attributed to the increase of Au grain-size because larger Au grains can alleviate grain boundary diffusion that can diminish bonding strength. As a result of this finding, an empirical equation that indicates the correlation between bonding strength and Au grain-size was also proposed.

The use of a Co-W-P metallization substrate in SiC power modules is expected to improve high temperature reliability because Co-W-P metallization has been found to induce strong bonding strength to both sintered Ag joints and encapsulation resins. To progress in the development of this technology, the successful applicability of Co-W-P metallization into the module fabrication process is of critical importance. In this paper, the effects of annealing a Co-W-P metallization substrate in the die attach process onto its resin adhesion was studied with representative annealing conditions for Ag sintering. Initially, mild annealing at 200 °C for 1 h was applied and showed strong resin adhesion greater than 15 MPa at 225 °C and an ideal cohesion fracture mode of resin, the same as that found in that of the fresh Co-W-P case. However, more severe annealing resulted in lower resin adhesion. For example, annealing at 280 °C for 1 h resulted in a poor resin adhesion below 15 MPa, as well as a delamination fracture mode between the resin and the Co-W-P metallization. This mechanism was investigated with the use of SEM–EDS and XPS analysis. It was observed that annealing at 200 °C induced a slight oxidization of Co, but Co(OH)2 for the chemical reaction to resin, still remained on the Co-W-P surface. On the other hand, annealing at 280 °C formed an alternative main component (CoO). The shortage of Co(OH)2 on the top surface created by severe oxidization was found to induce poor resin adhesion. The results from this research are significant to future designs and applications of a module fabrication process using Co-W-P metallization substrates, as well as to the fundamental understanding of adhesion behavior on Co-W-P metallization.

One of the applications of wide band gap semiconductors is high temperature operation. That application requires high temperature compatible (i) joining materials such as sinter Ag, (ii) encapsulation resins such as imide type primers or molding compounds, and (iii) metallization for those materials. Ag metallization, the best candidate metallization for sinter Ag materials, has difficulty in bonding to encapsulation resins. Conversely, Ni/Au-flash metallization enables strong resin adhesion but also demonstrates poor reliability for sintered Ag joints. There is no single metallization compatible to both sintered Ag and encapsulation resin for high temperature application. This paper reports on a single metallization, electroless plated CoW metallization, which has demonstrated the capability to achieve both (i) high-temperature reliability (250 °C for 500 h) for sintered Ag joints and (ii) high-temperature adhesion (at 225 °C) for encapsulation resins. Such results have not been achieved with either Ag or Au metallization. The shear strength of sintered Ag joints on CoW metallization exceeded 40 MPa. TEM observation revealed excellent bonding between the sintered Ag and the metal Co of the CoW metallization. Furthermore, CoW metallization also showed strong resin adhesion (about 21 MPa) at 225 °C. XPS analysis identified metal Co for bonding to sinter Ag and, Co(OH)2 and WOx for bonding to resin on the top surface of CoW metallization layer. The foregoing results indicate that CoW may well represent a new metallization process for the fabrication of high reliability and high-temperature compatible SiC power modules.

In this study, the influence of annealing at 250 °C for 500 h on Ohmic contact of backside electrode of Silicon Carbide Schottky-Barrier-Diode (SiC-SBD) was investigated. Considering the prevalence status of the SiC-SBD as power device applications, the experiment was implemented on a commercial based SiC-SBD device. The annealing has brought remarkable transition in the structure and the impurity profile at the surface and the Ag/Ni interface of the device as a result of oxidation. For these evaluations, dual-beam Time-of-flight Secondary Ion Mass Spectrometry is regarded as an available analytical technique on the high sensitivity and imaging ability. In the deeper region of the backside electrode, an amorphous Ni–Ti alloy layer was found between the Ni/Ti interface of the annealed sample, which is also considered the result of diffusion due to the annealing. On the other hands, some nuggets of glassy-carbon and Ti cubic structures were observed in the Ni(Si)–C layer of both the non-annealed and the annealed samples. These structures are deduced to be formed during the fabrication process of the backside electrode and to be stable against the annealing. For these evaluations, Transmission Electron Microscopy (TEM) and Scanning TEM (STEM)—Energy Dispersion X-ray analysis were optimized with the high lateral resolution.

Sinter joining with Ag particle paste has emerged as a promising lead-free die-attachment solution for power devices owing to its low-temperature and pressureless processing characteristics. Using this process, the achieved performance is superior to that of traditional solders. In this work, a robust, direct Cu bonding was realized using a low-cost, hybrid Ag paste. The bonding strength of the bare Cu joint structure achieves approximately 30 MPa when bonded at 250 °C without assisted pressure in air. To understand this excellent result, the bonding microstructure was investigated via SEM, EDS, XPS and TEM. This research revealed that the direct Cu bonding is closely related with the Cu2O nanoparticles formed on the bare Cu surface under the effect of solvent and self-generate Ag nanoparticles during the sintering. These two nanoparticles can form an intact bond, which allows a preferable bonding strength at pressureless, low-temperature and atmospheric sintering condition. Also, a possible mechanism has been proposed to interpret the formation of this successful direct Cu bonding.

Silicon carbide (SiC) power modules with Ag sinter-bonding die attach were designed on the basis of thermal stress analysis for reliable high-temperature operations. Both the finite-element analysis (FEA) simulations and preliminary experiments confirmed that inserting the direct-bonded-copper (DBC) substrates can effectively reduce the maximum thermal stress in the module. A prototype SiC power module using sintered Ag die attach with a DBC substrate was designed and fabricated. The modules exhibited excellent durability in power cycling between 65 °C and 250 °C up to 20 000 cycles. FEA calculations of cumulative thermal strain and stress distributions adequately predicted the initial cracking position in the specimens after prolonged power cycles, observed by scanning electron microscopy.

GaN wurtzite crystal is commonly regarded as eminently brittle. However, our research demonstrates that nanodeconfined GaN compressed along the M direction begins to exhibit room-temperature plasticity, yielding a dislocation-free structure despite the occurrence of considerable, irreversible deformation. Our interest in M-oriented, strained GaN nanoobjects was sparked by the results of first-principles bandgap calculations, whereas subsequent nanomechanical tests and ultrahigh-voltage (1250 kV) transmission electron microscopy observations confirmed the authenticity of the phenomenon. Moreover, identical experiments along the C direction produced only a quasi-brittle response. Precisely how this happens is demonstrated by molecular dynamics simulations of the deformation of the C- and M-oriented GaN frustum, which mirror our nanopillar crystals.

N. Wakasugi, C. Chen, K. Hirao, S. Nagao and K. Suganuma, "Thermal evaluation of metalized ceramic substrates for use in next-generation power modules toward international standardization," 2020 IEEE 8th Electronics System-Integration Technology Conference (ESTC), Tønsberg, Vestfold, Norway, 2020, pp. 1-4, doi: 10.1109/ESTC48849.2020.9229827., Wide bandgap (WBG) semiconductors such as SiC and GaN have opened their market as the next generation of high power modules. As advanced electronic power modules must deal with increasing power density, power modules with an insulating ceramic substrate are subjected to extremely high temperature due to high current and voltage. Currently, the junction temperature of Si power devices is kept low enough, around 150 , to maintain the module durability. Nevertheless, the temperature will be soon beyond 200 for WBG devices. Thus thermal dissipation performance has one of the key technologies in designing modules. Directly bonded copper (DBC) or aluminum (DBA) substrates have been widely used as two of the typical ceramic insulating substrates for high power modules. If the extensive heat from a junction is kept inside a module, the module will be easily damage due to the increasing temperature resulting in severe thermal stress inside. Designing a proper module structure requires not only each material property but also complex component shape/structure/layout including each interface heat transfer. An accurate measurement method of precise thermal properties for such a complex metallized ceramic substrates is currently still missing. To meet the urgent requests for the next generation of high power modules, a new and simple measurement method of the thermal properties has been proposed for complex metallized ceramic substrates. First we have designed a micro heater SiC chip as an accurate and controllable heat source instead of a SiC active die. The heater chip was die-attached on metallized ceramic substrate mimicking real module packaging structure. The thermal resistance of the metallized ceramic substrate was evaluated. Due to the high power of the chip, i.e. 1 kW/cm2, the obtained thermal resistance has an excellent accuracy within a few percent error, when adequate cooling system is used. The thermal resistance includes those of die-attach material and thermal interface materials (TIM). The developed method thus enables precise comparison in thermal properties of high power modules, which was proposed as an ISO standard.

A novel die-attach material, Cu particle paste with self-reduction and self-protection characteristics, was designed by simply adding ascorbic acid (AA) into Cu paste. The self-reduction characteristic is due to the addition of AA into the paste, as it reduces the Cu oxide layer on the metal Cu, even at room temperature. The self-protection characteristic is due to the decomposition of AA preventing further oxidation during sintering process. These characteristics are beneficial for the sintering of Cu particles and for enhancing the bondability. The increase to bonding strength is the direct result of the concentration of AA in the Cu paste. The bonding strength of Cu joints sintered from Cu paste with 1.3 wt% AA addition was as high as 24.8 MPa, thus making it attractive as a die-attach material. On the other hand, the bonding strength of those sintered from Cu paste without AA addition was only 9.7 MPa. These results strongly suggest that the self-reduction and self-protection characteristics resulting from AA addition are necessary and crucial for sintered joints fabricated from readily oxidized Cu particle paste. This novel Cu paste is an efficient, safe, and environmentally-friendly die attachment material, especially well-suited for applications in high power semiconductor devices. Keywords: Cu particle pastes, Self-reduction, Self-protection, Ascorbic acid, Sintered Cu joints, Power electronics

Homogeneous columnar Cu film with fully embedded nanotwins was successfully fabricated on Ti/Cu seed layer on silicon wafer. The nanotwins with thickness of tens of nanometers are generally parallel to the silicon surface, showing a strong (111) preferred orientation. The acid concentration was found to be important in influencing the formation of nanoscale twins. By adjusting the acid concentration, the nanotwins can be induced from the top columnar grain to middle columnar grain and reach the bottom equiaxed grain, and a microstructural transformation model was given. A theory focusing on the cathode overpotential was proposed to reveal the effect of acid concentration on the growth mechanism of nanoscale twins. An appropriate adsorption proportion of hydrogen on cathode (acid concentration 17 ml L−1) could increase the overpotential which supplies adequate nucleation energy for nanoscale twins formation.

Next generation power semiconductors, e.g. SiC and GaN, are emerging for the further minimization and high current/voltage of power devices with high reliability covering wider operating environments than those based on Si. To implement high reliability operation, the key technology is the control of the temperature distribution in the module, and thermal stress caused by the heat generated by power loss. In the present study, we have developed SiC micro-heater chip with temperature probe to evaluate thermal characteristics of an assembled system of Ag sinter die-attach on metalized ceramic substrate (Cu/Si3N4/Cu) during the repetitive power cycling. The test specimens were fixed on a water cooling system, and steady-state heat resistance of the system was measured during the power cycling. For comparison, Pb-Sn, Sn-Cu-Ni-P, Sn-Ag-Sb-Cu solders were used as die-attach material bonded on the same metalized ceramic substrates. The maximum applied power exceeds 200 W with cycles of 2 seconds of heating and 5 seconds of cooling, and the test cycles was over 5000 cycles. The power cycle number dependence on the temperature swing and thermal resistance characteristics would be discussed, in connected with the power cycle testing for real power devices.

Improving the reliability of solder joints for die attachment in power modules is one of the most important issues in creating environmentally friendly vehicles such as hybrid electric vehicles. Power modules must have highly reliable solder joints that must be thermostable at temperatures over 175 °C in the future. In die attachment, soldering surfaces are often finished with Ni plating, so for Cu/Ni plating/Sn–Cu solder joints it is necessary to suppress both Ni diffusion into the solder as well as growth of the (Ni,Cu)3Sn4 intermetallic compound (IMC). Ni diffusion in Ni plating can be suppressed by the presence of a continuous (Cu,Ni)6Sn5 IMC layer at the Ni plating/solder interface. To form this IMC, we investigated the interfacial reactions and growth behavior of IMC layers in the presence of composite Sn–0.7Cu solder with added Cu balls. Adding 2.5 mass% of Cu balls prompted the formation of a continuous (Cu,Ni)6Sn5 IMC layer at both the electroless Ni–P and the electrolytic Ni plating, and this IMC layer worked well as a Ni diffusion barrier during a high-temperature storage test at 200 °C for 1000 h.

In the present study, we introduce a heater dummy chip equipped with temperature sensor, with extremely high capability of high heat-density using SiC wafer. As often used in high voltage/temperature SiC devices exhibit their advantage of high thermal stability and heat conductivity. Our testing heater chip (thermal test engineering group, TEG) consist of insulated SiC and Pt thin-film heater and temperature sensor realizes extreme heat density of 250 W/mm2 with reliability above 300°C of high temperature. The TEG chip was the n-doped 4H SiC chip, which is used generally for SiC devices, coated with an alumina insulation film and molded the Pt heater and sensor by lithography technique. Using the TEG chip, we demonstrate simple but accurate heat resistant measurements of several types of metal bonded ceramic substrates. The combination of SiC TEG chip and heat resistance measurement system is also applied to power cycle testing, avoiding numerous problems arising from using actual working device chips. Therefore, this study systematically was carried out the thermal measurement technology of SiC with repetitive power cycle by TEG chip as new concept heat system. Through this progress, the technologies developed in the study may have further applications in evaluation and testing of various packaging materials targeted for high temperature/wattage power modules.

Electrochemical techniques were employed to study the electrochemical corrosion behavior of Sn-9Zn-xTi (x = 0, 0.05, 0.1, 0.2 wt.%) lead-free solders in neutral 0.5M NaCl solution, aiming to figure out the effect of Ti content on the corrosion properties of Sn-9Zn, providing information for the composition design of Sn-Zn-based lead-free solders from the perspective of corrosion. EIS results reveal that Ti addition was involved in the corrosion product layer and changed electrochemical interface behavior from charge transfer control process to diffusion control process. The trace amount of Ti addition (0.05 wt.%) can refine the microstructure and improve the corrosion resistance of Sn-9Zn solder, evidenced by much lower corrosion current density (icorr) and much higher total resistance (Rt). Excess Ti addition (over 0.1 wt.%) led to the formation of Ti-containing IMCs, which were confirmed as Sn3Ti2 and Sn5Ti6, deteriorating the corrosion resistance of Sn-9Zn-xTi solders. The main corrosion products were confirmed as Sn3O(OH)2Cl2 mixed with small amount of chlorine/oxide Sn compounds.

Micron Ag paste had a more affordable price, feasible large-scale synthesis, and longer storage life compared to nano Ag paste, thus it attracts much industrial interest for die attachment of high-power devices. However, the previous studies of high-temperature reliability were mainly focused on nano Ag joints, the research about reliability of micron Ag joints, especially low-temperature and pressureless, was very limited. Therefore, we evaluated high-temperature stability of low-temperature and pressureless micron Ag joint, involving in the changes of mechanical behaviors, evolution of microstructure and interfacial reliability. The average joint strength of micron Ag joints was independent of aging time and kept approximately 35 MPa after aging for 1000 h. The fracture of the micron joint was dominated by the ductile deformation of Ag grains during the fracture process. On the other hand, the microstructure of porous structure evolved greatly during aging process. Ag grains were oriented randomly before and after aging process, but the Ag grains increased slightly from 827.2 nm initially to 1178.4 nm after 1000 h aging. Meanwhile, the pores size in porous structure increased, the number decreased significantly, and the porosity also decreased slightly. Moreover, the barrier layers at interfaces of micron Ag joint remained stable and reliable during aging at 250 °C. The results would promote the large-scale application of the commercially available micron Ag paste in high-power devices.

A bonding technology is introduced by using surface polished porous Ag in die-attachment structure. Bonding strength did not change much as the chip size varied from 3 × 3 mm2 to 15 × 15 mm2. This confirms that the technology was not influenced by the chip size, and thus can be used in large area bonding. Bonding mechanism based on stress-induced migration was discussed with the three dimensional finite element analyses. Transmission electron microscopy (TEM) observation further confirmed that single crystal hillocks and Ag particles formed at the bonding interface, bridging the interface together.

Submicron Cu particles mixed with Cu complex are used successfully to fabricate highly conductive Cu films for printed electronics. This study investigates the effects of Cu particle size on microstructure, conductivity and on the long-term stability of printed Cu films. In particular, the oxidation behaviors of printed Cu films at high temperatures are studied from the evolutions in microstructure and chemical composition. The submicron Cu particles are sintered efficiently due to the help of in-situ formed Cu nanoparticles from the decomposition of the Cu complex. Low resistivity of 5.8 μΩ cm is easily achieved. At temperatures of 140 °C and 180 °C, the printed Cu films prepared from 800 nm Cu particles are more stable than that from those 350 nm particles, which can be attributed to larger Cu particles possessing higher oxidation resistance. At 220 °C, the result becomes opposite because the loose structure with many large voids in the printed Cu films from large particles provides sufficient space for oxygen and accelerate the break of pathways between adjacent particles by the formation of Cu oxides layers. This indicates the long-term stability of printed Cu films is attributed to not only the intrinsic oxidation of Cu to Cu2O but also the degradation of microstructures. At all events, the printed Cu films from submicron Cu particles with Cu complex exhibit excellent oxidation resistance and are superior to those from Cu nanoparticles. This presents significant potential and favorable prospects for the fabrication of highly reliable and cost-effective printed electronics.

Topographical patterns are widely applied in many manufacturing areas due to the unique role in modifying performance related to physical, chemical and biological fundamentals. The patterns are usually realized by buckling or wrinkling, self-assembly or epitaxy, and lithography techniques. However, the combination of satisfactory controllability, ridge robustness, cost and dimensional precision is still difficult to achieve by any of the strategies above. A novel, simple and low-cost nanopatterning technique named "photodegradation copying method" with high technological flexibility has been initially proposed in this study. As a perfect example, a nanoridge-patterned surface has been successfully realized on a polymeric film thanks to the selective photodegradation of polymer and the shielding effect of silver nanowire (AgNW) networks. Roughness, wettability and transmittance of the polymeric film became simply and effectively controllable by adjusting the photodegradation time or the size and distribution of AgNWs. In addition, the ridge-patterned film could also be employed as a substrate in transfer printing for more flexible devices. Various topographical nanopatterns are expected to be simply realized by the photocopying method, just replacing nanowires with other masks like nanodisks, nanocubes, nanotriangles, and so on. This promising photocopying technique is believed to play an important role in the development of topographical nanopatterns, and enable more intriguing applications simply, flexibly and inexpensively.

An interatomic potential for SiC based on the angular-dependent potential (ADP) model is developed based on reference data obtained by first-principles calculations. The reference data include not only the energy, stress and interatomic forces of equilibrium and strained crystals but also the stress–strain relationship obtained in ideal strength analyses, with the aim to make the potential suitable for simulations of fracture. The constructed potential successfully reproduces the critical stress of the 3C and 4H structures under various loading conditions. Another distinct feature of the developed potential is the inclusion of point charges in the pairwise term, which enables the description of an environment with excess electric charge. Molecular dynamics simulations using the potential demonstrate fracture of thin films, where ductile and brittle fracture behaviors are found depending on crystal orientations and local stress conditions, and dislocation motion under shear stress, where excess electric charge is seen to influence the mobility of C-core and Si-core partial dislocations.

This work introduces the possibility of using Ag sinter-paste as a novel high-temperature and high-current wire bonding solution. We investigated the electromigration (EM) behavior and lifetime of the sintered Ag wiring under high current density and high temperature required for the design of power electronic devices. The sinter Ag wiring fabricated on the two Cu substrates were tested under current densities of 2.7 × 104 A/cm2 at temperature of 250 °C. The microstructure evolution of sintered wiring was characterized after EM test. The resistance of sintered wiring did not change even after EM test for 300 hours, which confirms that the Ag-paste sinter wire bonding is rather stable than aluminum wire bonding under high temperature and high current density. No degradation was observed in microstructure of sintered wiring after EM test. Thus, it is expected that Ag paste sinter wire bonding is one of potential alternative interconnection technology for power electronic devices.

We introduce a Ag-Ag direct bonding process with the aid of ultrasonic bonding, which uses silver “Nano-volcanic eruption of Ag” caused by stress migration at 250 °C. Various preheating temperatures before the ultrasonic pulse and various sputtering methods such as direct current (DC) and radio frequency (RF) have also been evaluated. A novel bonding process which combines ultrasonic bonding and stress migration bonding (SMB) method is established. The bonding achieved a die-shear strength of more than 50 MPa and a nearly-perfect voidless bonding interface. High temperature storage (HTS) test at 250 °C was used to evaluate the thermal-resist ability of bonded SMB joints. The results indicate that the bonded joints show no obvious changes in the interfacial morphology and the die-shear stress after HTS test for 1000 h is still higher than 30 MPa. The thermally-stable SMB joints can be applied to advanced devices such as thin-wafer multi-chip integrations and next-generation power devices.

Metal paste sintering die-attach is recently attracting much attention as an alternative to Pb containing high temperature solders, particularly required for power device packaging with post-Si wide band-gap semiconductors. For high voltage and high power devices, which are used in electric vehicles, railway trains, or power grid systems, SiC MSOFET/SBD devices are emerging replacing Si IGBT devices. These SiC devices have two prominent advantages to traditional Si based devices: fast switching and high maximum junction temperature TJ. The excellent characteristics serve for miniaturization of the device module; the former allows to use smaller capacitor and reactors because of the high frequency, and the latter excludes cooling system without affecting the device life time. However, the thermal reliability should be critically tested before used in industrial applications. We have hence conducted comprehensive reliability tests using several types of metal sintering die-attach including Ag and Cu. High temperature storage tests at 250°C certify that the device structure is truly thermostable, and thermal cycling between −50°C and 250°C indicates that the thermomechanical stress caused by device package design is the key for high reliability of power devices. Power cycling demonstrates the usefulness for effective acceleration tests to estimate the device life time. Our results conclude that present metal paste die-attach is ready for use in the product instead of high temperature solders.

Electrochemical behavior of Zn- x Sn ( x = 40 wt%,30 wt%,20 wt%) high-temperature lead-free alloys was investigated in aerated 0.5 M NaCl solution by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques. OCP results show that the corrosion state of the alloys working surface reached a relatively dynamic equilibrium after the alloys were immersed in 0.5 M NaCl solution for 100 h. EIS analysis reveals that the total corrosion resistance R t of alloys depended on the amount of Sn, and Zn-40Sn possessed the highest value 2228.2 Ω cm 2 . No passivation behavior was observed during corrosion process by potentiodynamic polarization measurement. The dominant corrosion products were confirmed as Zn 5 (OH) 8 Cl 2 ·H 2 O, ZnO and Zn(OH) 2 . Moreover, the surface and cross-section morphology of corrosion product suggested that the corrosion layer was more homogeneous and denser for higher Sn-content, Zn-40Sn obtained the best protective layer from corrosion product. The results recommend that Zn- x Sn high-temperature solder alloy with higher Sn content appears to be more attractive in terms of superior corrosion properties. The exact corrosion process was also discussed in detail.

Thermostable die-attach method alternative to soldering with Pb content is urgently required for utilizing wide-bandgap power semiconductors like SiC or GaN with a high maximum junction temperature TJ. Ag nano-or micro particle paste sintering attracts wide attention for this purpose, but Cu paste has also being developed mainly because of the cost pressure. Recently Cu sub-micron particle paste dispersed polyethylene Glycol solvent has demonstrated low temperature sintering at 250°C in N2 gas, resulting in high strength of die-attach. In the present study, the developed Cu paste die-attach has accepted to SiC Schottky barrier diode in TO-247 standard packaging with highly thermostable mold materials. The produced sample devices are subjected to high temperature storage tests at 250°C and thermal cycling tests between −50°C and 250°C. The device including the novel die-attach structure exhibited excellent thermal stability. Furthermore, power cycling tests has been conducted, and the results appear prospective for long life time and high reliability with high TJ exceeding 250°C. The presented study thus conclude that Cu submicron paste sintering is a promising candidate of power device die-attach usable for heavy industrial applications.

A one-step polyol method was employed to synthesize bimodal Cu particles with average diameters around 200 nm and 1000 nm, respectively. The bimodal Cu particles were mixed with a reductive solvent of polyethylene glycol (PEG) to form a paste. The Cu paste was used as die bonding material to prepare Cu joints under N2 or vacuum sintering atmosphere. The results showed that the strength of the Cu joints in N2 atmosphere was always higher than that in vacuum. The shear strength of a Cu joint processed at 350°C under only 0.4 MPa bonding pressure in N2 was above 40 MPa, which was far higher than that obtained using single-sized nano-Cu particle paste. It is related to the dense packing of the bimodal Cu particles and slow decomposition behavior of the reductive PEG solvent. The reductive PEG solvent in the Cu paste, which effectively removed oxides on the surface of the Cu particles, was necessary for easy-oxidized Cu pastes. These results suggested that Cu pastes with suitable particle sizes, reducing solvent and sintering atmosphere could be a proper candidate for low-temperature and low-pressure bonding process.

Sintering Ag particles for connection of power devices is a hot topic due to the superior thermal and electrical properties of Ag to conventional solder paste. However, there are still some dilemmas for the sintering Ag particles such as high cost of nano Ag particles, requirements of pressure and surface metallization. In this work, we realized a robust die attachment on bare DBA substrate with sintering micron Ag flake particles. The bonding strength of die attachment can reach about 35 MPa under pressureless, atmospheric, and 200 °C sintering condition. By analysis the structure of sintered Ag particles, we found the micron flakes can be sintered into a uniform porous structure as low as 200 °C. The sintered die attachment structure shows a imitate attachment to the bare DBA substrate according to TEM observations of bonding interface. The high-temperature storage results indicate this die attachment has excellent thermal reliability that no significant degradation occurred even after 1000 h aging at 250 °C.

A Cu particles paste was developed as a candidate of die attach materials for next generation power devices. The evaluation including thermal shock and power cycling test were performed on SiC-DBC die-attach structure to test the reliability of sintered Cu joints. The thermal shock reliability of SiC die-attached on a DBC substrate was carried out from -50 °C to 250 °C in the ambient atmosphere. SiC MOSFETs bonded by the Cu paste were evaluated by power cycle test from 25 °C to 200 °C. In both test condition, the sintered Cu joints showed good stability. The shear strength increased with the thermal shock cycles increased, which can be attributed to Cu oxidation during test. The power cycle test also showed no obvious deterioration occurred.

Sinter Ag joining as an attractive die attach material received more and more attention. Usually, to obtained an excellent strength bonding, sinter Ag joining need contact with the Ag, Au metallization layer on both chips and substrates. The electro- and electroless-plated Ni(P) technology was well developed and widely used. This work focuses on the bonding quality and high temperature reliability of sintering micron-sized Ag particles on an electro-less Ni-P plated substrate. High die shear strength of sinter Ag joining structure was obtained, which over 40 MPa with a sintering temperature 300Υ in air and pressure-less conditions. In addition, the high temperature aging test also was investigated at the aging temperature 250Υ for 500 h. The die shear strength keeps 30 MPa after 500h aging. The bonding mechanism of sinter Ag joining on the electro-less Ni-P plated structure was discussed by SEM, EDS analysis.

The structural and mechanical properties of ZnO nanowires (NWs) have been systematically investigated by using molecular dynamic simulations based on the empirical Buckingham potential. Under tensile loading in direction, ZnO NWs undergo four-stage deformation: elastic stretching of initial Wurtzite structure, Wurtzite to body-centered tetragonal (BCT) phase transformation, stretching of the resulting BCT structure and eventually brittle fracture. The entire deformation process is significantly size dependent. As the NW size decreases, the Young's modulus dramatically increases. The critical stress for both phase transformation and fracture decreases while the critical strain increases with increasing the NW size; both converge to constant values when the size is sufficiently large. The strain energy density for the initiation of phase transformation appears independent of the size, which implies that the size-dependent phase transformation is dominated by the size effect of the Young's modulus. © 2017 The Author(s). Published by Elsevier B.V. This is an open access article under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/

Molybdenum oxide (MoOx) films show great potential for use in electrical devices because they display a diversity of valence states of the Mo cation. A simple and mild sol–gel route was developed to synthesize MoOx hole transport layers (HTLs) for organic solar cells to replace the traditional evaporation process for MoOx films, which is difficult to achieve with low-cost mass production. The oxygen vacancies and valency of the Mo in the MoOx HTLs can be well controlled by introducing and adjusting the amount of hydrogen peroxide (H2O2) in the precursors. This method gives MoOx HTLs with a high conductivity, transparency and well-aligned band structure, with the highest occupied molecular orbital in the active layer. The molar ratio of Mo to citric acid in the precursor solution and the annealing temperature of the process have important roles and were optimized as Mo:citric = 0.025:0.075 mol L−1 and 200 °C, respectively, for the fabrication of the most efficient solar cells. The performance of the organic solar cells containing the solution-processed MoOx HTLs was comparable with that of reference devices using evaporated MoOx HTLs or poly(3,4-ethylenedioxythiohene):poly(styrenesulfonate) HTLs. HTLs obtained via this route show promise for universal application in a variety of solar cells and provide valuable insights for the production of fast, large-scale, low-cost and renewable sources of energy.

Submicron Cu particle ink was developed to successfully achieve highly reliable and highly conductive Cu patterns on low-cost, transparent, and flexible substrates by an optimized two-step sintering process involving low temperature heat-welding and subsequent flash light sinter-reinforcement. The Cu ink contains a special additive of the Cu–amino complex made from copper(II) formate and 2-amino-2-methyl-1-propanol solvent. Low temperature heat-welding promotes the decomposition of the Cu–amino complex into fresh metallic Cu particles, which as nano-welders can in situ weld those big submicron Cu particles. The subsequent flash light sintering further reinforces the connection between big Cu particles with the assistance of these active nano-welders and strengthens the adhesion between sintered Cu patterns and polymer substrates due to the local soft-effect. The achieved resistivities of sintered Cu patterns on polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polyimide (PI) substrates are 26.5 μΩ cm, 15.9 μΩ cm and 7.2 μΩ cm at a low welding temperature of 140 °C for 10 min and subsequent flash light energies of 1080 mJ cm−2, 1273 mJ cm−2 and 2073 mJ cm−2, respectively, at which the same electrical properties cannot be obtained from either pure nano-Cu or submicron Cu particle ink as reported in previous research studies. Moreover, bending fatigue and oxidation-resistance tests indicate that the sintered Cu patterns have superior mechanical and environmental stability. Finally, flexible and foldable LED circuits and flexible dipole antennas were successfully fabricated to demonstrate the applicability of the sintered Cu patterns for printed electronic devices. It should be noted that this method opens a new way for making highly reliable and highly conductive Cu patterns on low-cost, transparent, and flexible substrates with big Cu particles instead of nanoparticles under a suitable sintering process, which may largely decrease the cost and enhance the application of Cu inks for flexible electronic devices.

The mechanical properties of sintered Ag paste with microporous structure have been investigated by tensile and shear tests, focusing on the temperature-dependent plastic deformation at various temperatures from 25°C to 300°C, corresponding to the target operating temperature range of emerging wide-bandgap semiconductor devices. Specimens were prepared by sintering hybrid Ag paste consisting of microflake and submicron spherical Ag particles, simulating a typical bonding process for power semiconductor die attach. Mechanical tests revealed that the unique microstructure caused a brittle-to-ductile transition at temperature of around 160°C, remarkably lower than that of bulk Ag. The obtained Young’s modulus and shear modulus values indicate obvious softening with increasing temperature, together with a remarkable decrease in Poisson’s ratio. These plastic behaviors at elevated temperature can be explained based on Coble creep in the microporous network structure. Fracture surfaces after tensile and shear tests indicated unique features on scanning electron microscopy, reflecting the variation in the ductile behavior with the test temperature. Furthermore, these temperature-dependent mechanical parameters were employed in three-dimensional finite-element analysis of the thermomechanical stress distribution in wide-bandgap semiconductor module structures including Ag paste die attach of different sizes. Detailed thermal stress analysis enabled precise evaluation of the packaging design for wide-bandgap semiconductor modules for use in high-temperature applications.

Solder joint reliability in power modules is one of the most important issues for hybrid, electric, and fuel cell vehicles; these modules must have highly reliable solder joints, i.e., they must be highly thermostable at temperatures over 175 °C in the future. The soldering surfaces in power modules are often finished with electroless Ni–P plating. Thus, for Cu/Ni–P plating/Sn–0.7Cu joints, it is necessary to suppress Ni diffusion into the solder. Ni diffusion can be suppressed in the presence of a continuous Cu6Sn5 intermetallic compound (IMC) layer at a Ni–P plating/solder interface. To form this IMC, we investigated the composite Sn–0.7Cu solder added with Cu balls. It was confirmed that the addition of 2.5 wt% Cu balls formed a continuous (Cu, Ni)6Sn5 IMC layer between the solder and the Ni–P plating. It is concluded that the IMC layer works well as a Ni diffusion barrier in multiple reflow tests, of which the peak temperature was 330 °C, and in a high-temperature storage test at 200 °C for 1000 h.