Fig 5 - uploaded by Hongqiang Zhang
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Particle structure in SNF after ultrasonic dispersion (a) SEM image exhibiting the agglomeration of particles; (b) Schematic of the "frame" and "filler" configuration.
Source publication
In this work, an organic-free silver nanostructured film (SNF) was applied to bond SiC chips and ceramic substrates, which was fabricated by ultrafast pulsed laser deposition (PLD). Unlike the commercial nano silver paste which contains 10%–30% organics, the bonding temperature of SNF could be reduced to only 180 °C, with the average shear strength...
Contexts in source publication
Context 1
... microstructure of deposited SNF is shown in Fig. 4a, which contains micron and nano-sized particles (Fig. 3b). According to the composition and phase analysis, the film was pure silver structure without oxidation ( Fig. 4c-d). The magnified view of deposited particles is shown in Fig. 5a, and particles were agglomerated. The structure of these particles could be described by Fig. 5b. Large-sized particles (average diameter >400 nm) were connected with necks, and this structure was designated as "frame". Meanwhile, small-sized particles (average diameter <100 nm) formed branched clusters surrounding the "frame" ...
Context 2
... is shown in Fig. 4a, which contains micron and nano-sized particles (Fig. 3b). According to the composition and phase analysis, the film was pure silver structure without oxidation ( Fig. 4c-d). The magnified view of deposited particles is shown in Fig. 5a, and particles were agglomerated. The structure of these particles could be described by Fig. 5b. Large-sized particles (average diameter >400 nm) were connected with necks, and this structure was designated as "frame". Meanwhile, small-sized particles (average diameter <100 nm) formed branched clusters surrounding the "frame" particles. This structure was designated as "filler". The formations of "frame" and "filler" particles ...
Citations
... This indicated the weak strength of those joints under thermal cycling, as shown in Figure 6b,d. In addition, the shear strength of the joints was compared with that of the sintered joints reported in recent studies [60][61][62][63][64][65][66][67][68][69]. Clearly, the joints showed a high shear strength and good reliability, showing their potential for packaging applications. ...
The effects of the sintering duration and powder fraction (Ag-coated Cu/SnAgCu) on the microstructure and reliability of transient liquid phase sintered (TLPS) joints are investigated. The results show that two main intermetallic compounds (IMCs, Cu6Sn5 and Cu3Sn) formed in the joints. The Cu6Sn5 ratio generally decreased with increasing sintering time, Cu powder fraction, and thermal treatment. The void ratio of the high-Cu-fraction joints decreased and increased with increasing sintering and thermal stressing durations, respectively, whereas the low-Cu-fraction counterparts were stable. We also found that the shear strength increased with increasing thermal treatment time, which resulted from the transformation of Cu6Sn5 and Cu3Sn. Such findings could provide valuable information for optimizing the TLPS process and assuring the high reliability of electronic devices.
... As soon as the glass content increased to 3 wt.% as shown in Figure 8b, the silver particles grew significantly, reigniting the driving force for densification, whereas pores and imperfections were significantly reduced [33]. The reinitialization of the driving force would have stimulated the grain growth and the formation of a more compact and dense structure of the silver film. ...
Low-temperature lead-free silver pastes deserve thorough investigation for sustainable development and application of MgTiO3 ceramics in electronic devices. In this study, a series of Bi2O3-B2O3-ZnO-SiO2-Al2O3-CaO glasses with suitable softening temperatures were prepared via melt quenching using a type of micrometer silver powder formed by silver nanoparticle aggregates. The composite pastes containing silver powder, Bi2O3 glass powder and an organic vehicle were then screen-printed. The effects of glass powder concentration and sintering temperature on the microstructure of the surface interface were also investigated. The results showed that the silver paste for microwave dielectric ceramic filters (MgTiO3) possessed good electrical conductivity (2.28 mΩ/□) and high adhesion (43.46 N/mm2) after medium temperature (670 °C) sintering. Thus, this glass powder has great application potential in non-toxic lead-free silver pastes for metallization of MgTiO3 substrates.
... The film consisted of various sizes of 167 nanoparticles, as presented in the magnified SEM image from 168 the cross section in Fig. 2(b). These hybrid-sized nanoparticles 169 had a cooperative effect to increase the stacking density 170 effectively [31]. Pre-sintered necks were also observed in The integrity of the bump array after the lift-off process was 188 studied as shown in Fig. 3. Two different PRs with topcut and 189 undercut wall respectively were used as masks. ...
All-Cu interconnects with fine pitch scalability and excellent electrical performance are highly considered as the next interconnection node for the coming era of chiplet integration. However, current all-Cu interconnection through Cu-Cu direct bonding relies much on expensive chemical mechanical polishing (CMP) process to reduce surface roughness to several nanometers. Herein, Cu nanoparticle film prepared by pulsed laser deposition (PLD) was successfully patterned into micro bumps of 120 μm pitch and sintered at 160~250 °C to form all-Cu interconnects without using CMP pretreatment. The fabricated bumps had a low Young’s modulus of only 1GPa and could produce vertical collapse of several microns in bonding process, providing compliance with surfaces having high roughness and poor coplanarity. The bonded interconnects also exhibited excellent mechanical quality with shear strength >20 MPa at 160 °C, 15 MPa and >90 MPa at 250 °C, 20 MPa, which was superior to most reported Cu-Cu bonding using patterned nanomaterials. Patterning mechanism in PLD process involving incident characteristic of Cu nanoparticle flux, evolution of bump morphology was investigated and bump morphology’s influence on bonding properties was analyzed in detail. The strategy illustrated here could lead to develop a low-temperature, low-pressure assembly technique with less reliance on CMP for all-Cu interconnection.
... Sintered Ag has become a state-of-the-art die-attach material for wide-bandgap (WBG) semiconductors due to its excellent electrical and thermal conductivity, high service temperature, and long-term reliability. 1,2 To date, various sintered Ag materials have been developed, such as nano/micro Ag paste, 3,4 nano-Ag film 5,6 and nanoporous Ag. 7,8 The bonding temperature of most nano-Ag materials is above 200°C, but there is still urgent demand to further decrease the process temperature in multi-step packaging and large-area bonding. For example, the GaN radio frequency (RF) microsystem requires multi-step packaging, and the bonding temperature of each step needs to be lower than the previous one. ...
Achieving high strength and reliable bonding below 150°C using Ag nanoparticle pastes is still a challenge. This work developed an organic-free nano-Ag multilayer film consisting of a compact layer and a loose layer using pulsed laser deposition (PLD). A high shear strength of 71.2 MPa was achieved with bonding at 150°C, well above the reported values. A value of 18.6 MPa was achieved even with bonding at 50°C, meeting the MIL-STD-883 K standard requirement. The shear strength of sintered joints was strongly dependent on the diffusion behavior and microstructure evolution of loose layers. The sub-10-nanometer grains and high quantity of lattice disorders in the Ag nanoparticles induced a high diffusion driving force, ensuring high-strength bonding inside the bondline. In addition, pre-bonding and nano-bump effects of the deposited compact layer enhanced the interfacial bonding between bondline and metalized surfaces. This work provides a promising method for robust die attachment below 150°C.
... It can be further confirmed that the supersaturated Ag-Cu nanoalloy film was fabricated successfully according to Bragg's law, as only Ag peaks shifted to a higher degree for the Ag-Cu nanoalloy film based on the XRD results (Fig.1c). These nanoparticle film has high surface energy as well as stacking faults and dislocations due to the high cooling rate during PLD process, which provides a driving force for the sintering bonding process [29]. Thus the die attach process between SiC power chip and Ag coated DBC substrate can be carried out successfully at 250 °C (Fig.1d). ...
Nano-Ag sintering can realize superior thermal, electrical, and mechanical properties for power electronic packaging, while it suffers from unstable microstructure and electrochemical migration. In this work, a supersaturated Ag2.8wt%Cu nanoalloy film has been developed using pulsed laser deposition, which can be sintered at 250 °C in air for electronic packaging without sacrificing bondability. It was surprisingly found that both thermal stability and electrochemical migration resistance were significantly improved when alloying the Ag with 2.8wt% Cu. The in situ formed Cu2O/CuO from Ag-Cu nanoalloy existed on the pore surface and along the grain boundaries of Ag/Ag-Cu nanoalloy, which stabilized the microstructure of bondline from 400 h to 1400 h at 300 °C. The synergistic effect of Ag-Cu nanoalloy and Cu oxides realized an obvious protective effect in the process of anodic dissolution, exhibiting a short-circuit time 2.4 times of the pure Ag electrode. The die attach process is compatible with most commercial equipment and Si/SiC dies with improved performance, enabling the supersaturated Ag-Cu nanoalloy to be a promising material for high reliability power electronic packaging.
... As an innovative organic-free metal micro-nano particles film preparation approach, pulsed laser deposition (PLD) technology has been developed in our previous research to applied in die-attachment [36,37]. In this work, a novel rapid and low temperature sintering method with ultra-high bonding strength has been developed using Cu nanoparticle film (CNF). ...
... through a scanning galvanometer in a self-controlled vacuum system. The deposition mechanism is the same as that in Ref. 37. Spattering nanoparticles were deposited on the surface of samples (Cu die and Si die) in Ar atmosphere, whose deposition pressures were ranged from 500 Pa to 2000 Pa. ...
... The microstructure of deposited CNF is shown in Fig. 2a, with a thickness of 100 µm, which contains submicron and nano-sized particles ( Fig. 2b), forming a "frame-filler" structure. According to the quantity and volume proportion distribution in Fig. 3, particles with diameters larger than 120 nm act as "frame", and the smaller particles are designated as the "filler", agglomerating together and surrounding the "frame" particles, which is similar to the structure of silver nanoparticle film in Ref. 37. The composition of particles with multiple sizes is more favorable to the sintered microstructure than single size particles [26]. ...
It is of great interest but very difficult to achieve both rapid (few minutes) and low temperature (< 200 ℃) sintering bonding using Cu nanoparticles, which is mainly due to the diffusion dominated process and complex organics in the nanoparticle paste. In present work, a copper nanoparticle film (CNF) fabricated by pulsed laser deposition (PLD) has been developed. Under the reduction of ethylene glycol (EG) and protection of inert atmosphere, it can be sintering bonded at even 140 ℃, having adequate shear strength higher than that of Pb-5Sn. A remarkable high strength of 65 MPa for Cu-DBC joints and 50 MPa for Si-DBC joints were achieved when sintered at 250 ℃ for only 1 min. The rapid and low temperature sintering of CNF is mainly attributed to the efficient oxide film removal by EG at low-temperature, since it is an organic free dry film without the complex organics which decompose or evaporate at high temperature. The relationships among joint strength, porosity of microstructure and interfacial connection ratio have also been discussed.
... By means of the back scattered electron (BSE) mode of scanning electron microscopy (SEM), the grains can be made visible due to their different contrast values [22]. In this study, the mechanism of grain growth and twin formation in the bulk sintered Cu nanoparticles was investigated using a quasi-in-situ method combining SEM observation and ion beam etching. ...
... Fig. 10 shifting can be also found during Cu nanoparticle sintering as shown in Fig. 6 (a), Fig. 7 (c-d) and Fig. 8 (c). There are also some tiny voids along the initial interface due to incomplete sintering as shown in Fig. 10 (b1), the residual organics could also induce voids [22,40]. Some new voids exposed as shown in Fig. 10 (b3) after the sample was slightly etched to make the grain structure visible. ...
Sintered Cu interconnection for power electronics has attracted considerable interest recently. Investigation of grain growth during Cu nanoparticle sintering provides insight into the strengthening mechanism of the sintered structure. Currently, the literature on Cu nanoparticle sintering mechanism is limited and mainly focuses on the transmission electron microscopy (TEM) observation of a limited number of nanoparticles with an ultrathin region. This study employs a quasi-in-situ method to investigate the mechanism of grain growth and twin formation in the bulk sintered Cu nanoparticle structure. The grains were found to continuously grow accompanied by orientation unification which is attributed to grain boundary (GB) migration or dislocation motion at elevated temperature. Two mechanisms governing twin formation have been observed and detailed. The grain and pore size and porosity in the sintered Cu structure under different sintering conditions were measured and correlated to the joint strength. Porosity was found to be the dominant factor affecting joint strength rather than grain or pore size. The original bonding interface between the sintered structure and the Cu substrate has high porosity. However, subsequent GB movement during heating causes this interface to shift into the sintered structure, eliminating the high porosity interface. Meanwhile, a model predicting porosity evolution was used to identify the dominant diffusion mechanism. This study constitutes the first detailed experimental observation of nanoscale grain growth, twin formation and GB movement at the interface and in the bulk sintered Cu structure. This novel mechanism of GB shifting could provide new guidance for strengthening the sintered Cu interconnections for power electronics.
... As an innovative organic-free metal micro-nano particles film preparation approach, pulsed laser deposition (PLD) technology has been developed in our previous research to applied in die-attachment [36,37]. In this work, a novel rapid and low temperature sintering method with ultra-high bonding strength has been developed using Cu nanoparticle film (CNF). ...
... through a scanning galvanometer in a self-controlled vacuum system. The deposition mechanism is the same as that in Ref. 37. Spattering nanoparticles were deposited on the surface of samples (Cu die and Si die) in Ar atmosphere, whose deposition pressures were ranged from 500 Pa to 2000 Pa. ...
... The microstructure of deposited CNF is shown in Fig. 2a, with a thickness of 100 µm, which contains submicron and nano-sized particles ( Fig. 2b), forming a "frame-filler" structure. According to the quantity and volume proportion distribution in Fig. 3, particles with diameters larger than 120 nm act as "frame", and the smaller particles are designated as the "filler", agglomerating together and surrounding the "frame" particles, which is similar to the structure of silver nanoparticle film in Ref. 37. The composition of particles with multiple sizes is more favorable to the sintered microstructure than single size particles [26]. ...
... Some researchers have investigated the mechanical properties of sintered nanoCu paste under high temperatures [8][9][10][11]. Fan et al. [10]performed high-temperature nanoindentation tests on sintered nano Cu particles at temperatures ranging from 140 to 200℃, revealing that the high temperature decreased the creep resistance of the sintered structure. ...
... Since traditional bonding materials such as solder alloys and conductive adhesive could not work satisfactorily at such high temperature, sintered Ag has been widely researched as die-attach material for semiconductor industry due to its superior electrical/thermal conductivity and high melting temperature [4][5][6]. Previous researches indicated that organic-free sintered Ag is suitable for high-temperature operation, as it showed strong shear strength and good performance under high temperature storage (HTS) test [7,8]. ...
Thermal stress caused by the coefficients of thermal expansion (CTE) mismatch of materials is the main failure mechanism in power electronic packaging. High junction temperature (Tj) would induce larger thermal stress and accelerate failing process. In this paper, two packaging strategies to reduce thermal stress of power module for high-temperature operation were presented based on organic-free sintered Ag bonding and wireless packaging structuring: reduce elastic modulus of sintered Ag layer by changing sintering parameters; and reduce the CTE mismatch by changing the ribbon material. Finite-element analysis (FEA) simulations confirmed that these strategies can effectively reduce the maximum and average thermal stress in power electronics packaging. Furthermore, by combining these two strategies, high-temperature power cycling test indicated that the wireless packaging with Cu30Mo70 ribbon and 5 MPa sintering pressure exhibited excellent durability and reliability in the temperature swing at the junction (ΔTj∆Tj) over 150 K.
