Review: Copper Fine Particle/Nanoparticle-Based Sintering Joining Material

Abstract

Wide-bandgap power semiconductors have garnered attention for their potential use in electronic power control. For joining materials of these power semiconductors, sintering of metal nanoparticles as the bonding material has been focused. Silver nanoparticles have been extensively researched so far as these bonding materials; however, studies have shifted the focus to copper nanoparticles from the perspectives of cost and ion migration resistance. Nevertheless, copper nanoparticles are known for their low oxidation resistance, leading to a decline in sintering performance. Therefore, research has been conducted to tailor copper nanoparticle pastes to prevent their oxidation. Additionally, various techniques during the sintering process have been considered to enhance the sinter bonding. This paper introduces the paste design attempts and examples of the sintering process by using copper nanoparticle paste bonding materials.

 

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 88 (2024) 270–280.

Fig. 5 Examples of devices utilizing power semiconductors. (Illustrations are sourced from “Irasutoya”)

1. Metal Nanoparticles and Melting Point Depression

Fine particles and nanoparticles have been intensively researched and widely applied in various industrial fields. Within this domain, nanoparticles are defined by IUPAC (International Union of Pure and Applied Chemistry) and ISO (ISO TC229) as “particles with a diameter of less than 100 nm” [1]. The size reduction limit achievable through mechanical grinding is at the submicron level (0.1 µm = 100 nm), and considering the comparison with cell sizes, this 100 nm threshold is regarded as one of the size limits, thus termed nanoparticles [2]. For particles with diameters larger than those of nanoparticles, there is no specific definition. However, particles with diameters of approximately 100 nm–1 µm are often referred to as submicron particles and particles with diameters of approximately 1 µm–1000 µm as microparticles. In this paper, to distinguish them from nanoparticles, we will collectively refer to both submicron and microparticles as fine particles.

When particle size becomes smaller than the wavelength of light (400 nm–800 nm), the mean free path of molecules in air (70 nm at atmospheric pressure), or even smaller than the thickness of magnetic domain walls in magnetic materials (a few nanometers to tens of nanometers), distinctive properties begin to emerge that differ from those of bulk materials. This manifestation of properties not observed in bulk materials, caused by particle miniaturization, is referred to as the size effect (or the quantum effect). This phenomenon occurs because the specific surface area drastically increases as particles become smaller, leading to an increase in the proportion of atoms present on the particle surface. Surface atoms exhibit weaker interactions and are less stable, resulting in an increase in surface energy. Because of this increased surface energy, nanoparticles exhibit unique properties.

One such famous phenomenon of nanoparticles is melting point depression. Baffet et al. [3] reported the relationship between the particle size and melting point of gold nanoparticles. Figure 1 depicts the relationship between gold nanoparticle size and melting point. Additionally, Lai et al. [4] studied the relationship between the particle size and melting point of tin nanoparticles. Figure 2 illustrates this relationship. It has been shown that as the particle size decreases, the melting point decreases. Particularly, a significant decrease in melting point is observed when the particle size falls below 10 nm (= 100 Å). Our study [5] also demonstrated that the reduction in particle size of tin nanoparticles affects the conductivity of the tin film generated upon dispersion liquid application (Fig. 3). Melting point depression occurs due to the increase in surface energy, resulting in a decrease in latent heat. Therefore, for particles in regions where significant melting point depression does not occur, low-temperature sintering may still be possible for similar reasons.

Fig. 1

Size dependence of the melting points of gold nanoparticles. Dots are experimental data and solid line presents theoretical model. (Reproduced from Ref. [3]. Copyright 1976. American Physics Society)

Fig. 2

Size dependence of the melting points of tin nanoparticles. Dots are experimental data and solid line represents theoretical model. (Reproduced from Ref. [4]. Copyright 1996. American Physics Society)

Fig. 3

Surface SEM images of tin nanoparticle-coated films. (a) Average particle size of 38 nm, (b) average particle size of 88 nm. In (a), it can be observed that smaller particles have fused to form necking. (Reproduced from Ref. [5]. Copyright 2016. The Royal Society of Chemistry)

2. Synthetic Methods for Metal Fine Particles/Nanoparticles

There are two main procedures for producing metal nanoparticles: physical methods, known as top-down methods, and chemical methods, including bottom-up methods and aggregation methods (Fig. 4) [6–8].

Fig. 4

Schematic diagram of methods for synthesizing metal nanoparticles. Physical method (top-down approach) and chemical method (bottom-up approach). (Reproduced with permission from Ref. [8]. Copyright 1998. The Royal Society of Chemistry)

Physical methods, or top-down methods, involve fabricating metal nanoparticles by processing bulk metal materials. Examples of physical methods include pulverization techniques such as ball milling [9]. While these methods offer the advantage of easily producing metal nanoparticles, controlling the shape and producing extremely small nanoparticles can be challenging. Since the shape and size of metal nanoparticles significantly influence their properties, these methods are not suitable for producing particles with uniform properties.

Chemical methods, bottom-up methods, or aggregation methods is a technique used to produce nanoparticles from atoms, molecules, and clusters. The term “aggregation method” is also used to refer to the process of gathering atoms and molecules into particles. The bottom-up approach can be further categorized into several types, including solid-phase synthesis [10–12], liquid-phase synthesis [8, 13–15], gas-phase synthesis [16–18], biosynthesis [19], and the liquid plasma method [20–22]. Among these, the chemical reduction method in liquid [13, 15, 23], which is a representative example classified under liquid-phase synthesis methods, involves reducing metal ions with a reducing agent in a solvent containing capping agents or protective agents to produce metal nanoparticles. This method is one of the simplest ways for producing metal nanoparticles. Laser ablation [10, 24], ion sputtering [17], or magnetron sputtering [11, 12] is a technique for synthesizing nanoparticles directly from bulk metals. The liquid plasma method also includes techniques that directly synthesize nanoparticles from bulk metals using plasma [20]. Although the raw materials used in these methods are bulk metals, they are generally classified as bottom-up approaches because nanoparticles are formed through the aggregation of metal atoms and clusters. While these bottom-up methods are more complex than top-down approaches, they offer advantages in terms of control over particle size distribution and morphology, as well as the ability to synthesize large quantities.

3. The Necessity of Bonding Materials in Next-Generation Power Semiconductors

3.1 Power semiconductors

Power semiconductors are semiconductor devices for the power conversion, characterized by their ability to handle high voltages and large currents. They are used in a wide range of applications, from small-scale power devices to large-scale power supply systems. Power semiconductors serve as the core of electronic equipment and transportation systems, including rectification (converting AC to DC), frequency conversion (changing frequency of AC), voltage regulation (adjusting DC voltage), and inversion (converting DC to AC). Such power conversion requires rapid switching of high voltages and large currents, enabled by the electrical properties of semiconductors. Compared to conventional power conversion methods that rely on resistors, power conversion using power semiconductors results in less energy loss due to heat, making them more energy efficient. These power semiconductors are found in electronic devices for power conversion, such as power systems, railway vehicles, and home appliances (Fig. 5). Familiar household applications include inverters used in air conditioners and solar power systems. Unlike conventional semiconductors that perform the tasks such as computing and memory storage, power semiconductors must function reliably under extreme conditions, such as high temperatures and high voltages. As improvements in the power conversion efficiency of power semiconductors lead to further reductions in power dissipation, continuous efforts are being made to advance the technology.

Fig. 5

Examples of devices utilizing power semiconductors. (Illustrations are sourced from “Irasutoya”)

3.2 Next-generation power semiconductors

At present, Si wafers are used in most power semiconductors. Mass production of large Si wafers, which can exceed 300 mm in diameter, is now well underway, making them affordable and easily accessible. However, the use of Si wafers for high-power and high-voltage applications, as well as further advances in integration and reduced on-state voltage, is approaching its limits due to the physical properties of the Si material itself and is not expected to develop significantly in the future. Consequently, wide-bandgap power semiconductors using wafers made from materials such as silicon carbide (SiC) and gallium nitride (GaN) have attracted attention [25–29]. SiC has several crystalline polymorphs, each with different physical properties. Table 1 shows the material properties of Si, 4H-SiC (suitable for power devices), and GaN [30]. Compared to Si, 4H-SiC and GaN have approximately three times the bandgap, more than six times the breakdown field, and allow wide control of p-type and n-type doping. In addition, 4H-SiC has a higher thermal conductivity than Si. These properties allow operation at higher voltages and temperatures than is possible with Si wafers. The superior material properties of these wafers enable power semiconductors to achieve lower power dissipation and higher frequencies, resulting in a significant improvement in power semiconductor performance.

Table 1 Material properties of various semiconductors [30].

3.3 Joining materials for next-generation power semiconductors

Next-generation power semiconductors utilizing wide-bandgap materials can withstand higher voltages and operate at higher temperatures. However, the increased operating temperatures have raised concerns about the reliability of the metal joints in power semiconductors. Traditionally, Sn-based lead-free solder, which is inexpensive and easy to use, has been widely used as a bonding material. Because next-generation power semiconductors are expected to operate at temperatures above 250°C, Sn-based Pb-free solder, which has a liquidus temperature of around 200°C, cannot be used as a bonding material for next-generation power semiconductors. Therefore, there is a growing need for new, reliable bonding materials that can withstand the higher operating temperatures of power semiconductors. One such bonding material attracting attention is low-temperature sintering materials using fine particles/nanoparticles of metals with high melting point, which are currently being actively researched.

4. Joining Using Metal Fine Particle/Nanoparticle Pastes

4.1 Joining using metal fine particles/nanoparticles

As a bonding material with high reliability at high temperatures, it is naturally expected that metals with melting points significantly higher than the operating temperature will be used. However, it is difficult to join materials using metals with high melting points in the bulk state. Furthermore, when conventional solders are replaced with high-melting-point alternatives, many of them contain harmful lead, making them undesirable for use in electronic components from an environmental viewpoint. Therefore, a promising approach is to use fine particles/nanoparticles, taking advantage of their low-temperature sintering properties to form bonds that can withstand high temperatures. This method, known as low-temperature sinter bonding, is gaining attention.

The specific procedure is as follows: A paste is prepared by dispersing metal fine particles/nanoparticles at high concentrations in an organic solvent. The paste is applied to a substrate, and a chip is placed on top. Upon heating, the solvent in the paste evaporates and is removed, allowing the remaining metal nanoparticles to sinter at low temperatures, thus bonding the chip to the substrate.

The advantage of metal nanoparticles is that they can be sintered at low temperatures to form bulk metal with reliable bonding up to high temperatures. Metal nanoparticles can sinter at significantly lower temperatures than the melting point of the bulk metal, and once sintered, they lose their quantum size effects, allowing for the formation of stable, high-temperature-resistant bonds through irreversible reactions.

4.2 Silver nanoparticle pastes

Silver nanoparticles have been widely used as bonding materials in power semiconductors [31–35]. Silver has a sufficiently high melting point (961.8°C), excellent electrical conductivity (1.59 µΩ·cm), and high thermal conductivity (429 W/(m·K)). The high melting point enhances the reliability of the bond, while the excellent electrical conductivity makes it suitable for high-current applications. Additionally, the high thermal conductivity helps to improve the heat dissipation performance of power semiconductors. Moreover, silver, being a noble metal, exhibits excellent oxidation resistance, reducing concerns about oxidation-induced degradation of mechanical properties or conductivity. Due to these properties, silver has been selected and researched as a bonding material for metal nanoparticles. Recently, the dynamics of bonding have begun to be investigated. In particular, detailed observations using transmission electron microscopy (TEM) have reported the successful sintering of a bilayer of lattice-disordered silver nanoparticles (Fig. 6) [35]. In-situ TEM observations of nanoparticle sintering during heating have also been investigated [36]. However, the high cost of silver (110.35 JPY/g as of January 16, 2024) and its low resistance to ion migration [37] have been highlighted as concerns.

Fig. 6

Sintering dynamics of the cooperative bilayer of lattice-disordered nanoparticles (CBLDN). (a) Comparison of neck growth speed between the CBLDN (solid line with circles) and nanopastes (dashed line with triangles) (lower x-axis). The solid line with squares shows neck growth ratio varying with the reciprocal of the sintering temperature (upper x-axis) for CBLDN. (b) High-resolution TEM (HRTEM) image showing the CBLDN nanoparticles with high-density stacking faults. The inset shows the corresponding FFT pattern. (c) TEM image showing a polycrystal CBLDN nanoparticle with many nanograins. (d) HRTEM image showing CBLDN nanoparticles with sub-5 nm grains and orientation difference. (e) HRTEM image of a representative region containing four nanograins (marked with A to D) in the CBLDN. Insets show the FFT patterns of the four grains, the misorientation at the A–B grain boundary, and the edge dislocation in grain D. (f) Molecular dynamics simulation on sintering 5 nm nanoparticles at 900 K for 2 ns. Case I: lattice-ordered nanoparticle pair. Case II–IV: nanoparticle pairs with various configurations of lattice disorders. Each case shows a slice of the nanoparticle and the final state after sintering. The atom colors in the slice represent different lattice types. Green: face-centered cubic (fcc); red: hexagonal close-packed (hcp); blue: body-centered cubic (bcc); white: other lattice types. (Reproduced with permission from Ref. [35]. Copyright 2019. American Chemical Society) (online color)

4.3 Advantages and challenges of copper nanoparticle paste

Copper is a metal with a high melting point (1084.6°C), possessing electrical conductivity (1.68 µΩ·cm) and thermal conductivity (398 W/(m·K)) second only to silver. Additionally, copper is tolerant to ion migration as compared to silver, and its cost is approximately one-hundredth that of silver (1.26 JPY/g as of January 16, 2024), making it highly advantageous for industrial applications. However, copper is easily oxidized by oxygen, resulting in the formation of Cu₂O or CuO on the surface of nanoparticles. These oxides hinder the sintering of nanoparticles and lead to reduced mechanical properties, thermal conductivity, and electrical conductivity after sintering. Figure 7 shows the thermodynamic phase diagram of copper, Cu₂O, and CuO as a function of temperature and oxygen partial pressure, indicating that oxidation readily occurs even at low temperatures [38]. In addition, the presence of water molecules accelerates copper oxidation. As the particle size in the paste decreases, the surface energy increases, making the particles more reactive and thus more susceptible to oxidation, which can even lead to combustion in some cases. Due to these issues, bonding materials using copper nanoparticles have been considered challenging.

Fig. 7

Thermodynamic phase diagram of copper (Cu-Cu₂O-CuO) as a function of temperature and oxygen partial pressure. (Reproduced from Ref. [38])

In response to these challenges, recent studies have explored the use of capping agents and protective molecules to prevent oxidation [39], as well as the sintering behavior of copper nanoparticles under inert gas atmospheres. These efforts have led to the development of methods for low-temperature sintering of copper nanoparticles while preventing oxidation [40, 41]. As a result, the problem of oxidation in copper nanoparticle pastes is gradually being overcome. Additionally, alternative sintering techniques that do not require concerns about oxidation during the process, such as laser sintering [42, 43] and intense pulsed light (IPL) sintering [44], have also been investigated.

5. Example of Copper Fine Particle/Nanoparticle Paste Bonding Material

Various efforts have been made to design copper fine particle/nanoparticle pastes that exhibit high shear strength while reducing sintering costs, improving oxidation resistance and enabling low-temperature and short-duration bonding processes. Some examples are presented below.

5.1 Copper nanoparticles protected and capped with organic molecules

Copper nanoparticles have a large specific surface area, resulting in a high surface energy, which makes them susceptible to oxidation. Therefore, methods have been explored to coat the surface of copper nanoparticles with organic materials that inhibit contact with oxygen and water molecules to prevent oxidation. Polymers and surfactants are commonly used as surface coating molecules. In particular, copper nanoparticles coated with polyvinylpyrrolidone (PVP) [45–47], other functional polymers [39, 48–50], cetyltrimethylammonium bromide (CTAB) [47, 51, 52], and carboxylic acids [40, 41] have been reported.

Jianfeng et al. [45] synthesized PVP-capped copper nanoparticles. The copper nanoparticles were dropped onto a copper substrate, and a copper wire was placed on top. The specimens were pressed at 5 MPa and sintered in air at different temperatures. As a result, the shear strength increased in proportion to the sintering temperature, and at temperatures above 220°C, the copper wire itself fractured rather than the joint, indicating a shear strength of over 13.5 MPa.

Tang et al. [47] investigated the role of PVP and CTAB in the formation mechanism of copper nanoparticles. PVP is a polymer with substituent groups capable of coordinating to metal ions, allowing it to interact with the Cu²⁺ precursor ions and lower the reduction potential, thus stabilizing the reduction process. Furthermore, the steric effect of PVP inhibits agglomeration among the copper nanoparticles. On the other hand, CTAB is a cation with a long-chain alkyl group and increases the absolute value of zeta potential by adsorption on the surface of copper nanoparticles. This results in the formation of an electrical double-layer structure around the surface of the copper nanoparticles, which prevents the particles from agglomerating through both steric and electrostatic effects. These effects result in the formation of copper nanoparticles with excellent atmospheric stability and dispersibility.

While PVP and CTAB are effective as stabilizers, they are difficult to decompose and require prolonged high-temperature treatment for complete removal during sintering. At low temperature and short duration sintering, organic residues were present on the particle surface, inhibiting nanoparticle sintering. Therefore, our research group [40, 41] proposed copper nanoparticles protected by medium-chain carboxylic acids as a target material suitable for low-temperature sintering. The synthesized nanoparticles were relatively stable, and their hydrophobic surfaces allowed easy dispersion in organic solvents. Even after dispersion using a bead mill, low-temperature sintering below 200°C was possible, and the sintered films showed excellent electrical conductivity.

5.2 Copper nanoparticle pastes with reducing agent

The surface oxide of copper nanoparticles inhibits particle sintering at low temperatures. This is a major challenge preventing the widespread use of copper fine particles/nanoparticles. One proposed solution to this problem is to suppress the formation of surface oxides on copper nanoparticles by adding a reducing agent to the paste.

Gao and Suganuma [53] focused on ascorbic acid as a reducing agent. They prepared a copper nanoparticle paste by dispersing nanoparticles in ethylene glycol with the addition of 10 mass% ascorbic acid, which was confirmed to inhibit the formation of Cu₂O on the nanoparticle surface. Using this ascorbic acid-added paste, sintering was carried out at 300°C for 30 min under a N₂ flow with an applied pressure of 0.4 MPa, achieving a shear strength of 24.8 MPa, a significant improvement in strength compared to the paste without ascorbic acid.

5.3 Surface-treated copper nanoparticle pastes

Another method to reduce the effects of the surface oxide on copper nanoparticles during sintering is surface treatment. Pretreatment of copper nanoparticles with hydrochloric acid or formic acid to remove the surface oxide layer has been considered.

Liu et al. [54] performed surface treatment on PVP-capped copper nanoparticles by immersing them in a mixed solution of ethanol and formic acid, converting the surface oxide layer to copper formate. The copper paste was prepared by dispersing the surface-treated copper nanoparticles in ethylene glycol and sintered at 260°C for 5 min under 10 MPa pressure in a 5% H₂/N₂ atmosphere, achieving a shear strength of 43.4 MPa (Fig. 8). The shear strength of the untreated copper nanoparticle paste was 23.9 MPa, indicating that the low-temperature reduction of the copper formate layer formed by the surface treatment exposed the metallic copper surface, promoting sintering and increasing the shear strength.

Fig. 8

Schematic illustration of the formation of highly conductive and high-strength copper-copper joints using copper formate treatment. (Reproduced with permission from Ref. [54]. Copyright 2016 American Chemical Society)

5.4 Copper pastes with bimodal particles

As shown in Fig. 1 and Fig. 2, the size effect of metal nanoparticles becomes prominent when the particle diameter is less than 10 nm. Therefore, it can be seen that the size of the copper nanoparticles should be less than 10 nm in diameter in order to obtain the full effect of lowering the sintering temperature. However, copper nanoparticles smaller than 10 nm are more susceptible to oxidation due to increased surface energy and tend to aggregate, making them difficult to handle, especially in air. In addition, the increase in specific surface area requires larger amounts of organic capping and protective agents to disperse and stabilize the particles. Large amounts of protectants are difficult to thermally decompose and evaporate during sintering, and there is concern that residues will reduce electrical conductivity, thermal conductivity, and shear strength. Thus, it is challenging to use pastes composed solely of copper nanoparticles smaller than 10 nm as bonding materials.

One solution to these problems is to use particles with a broad particle size distribution. For example, it has been reported that films prepared from smaller tin nanoparticles having melting points near room temperature and larger tin nanoparticles exhibit good conductivity [5]. Similarly, by preparing a bimodal nanoparticle paste, where relatively larger particles are mixed with nanoparticles, the oxidation resistance of the paste can be relatively improved while maintaining the high sintering performance of the nanoparticles.

Huang et al. [55] prepared the paste using copper particles with a bimodal size distribution of 10 nm and 160 nm. By performing two-step sintering of this paste at 200°C for 5 min and then at 280°C for 10 min without applying pressure, a shear strength of 65.24 MPa was successfully achieved.

Lai et al. [56] used commercially available copper particles with a broad size distribution (200 nm to 1.8 µm) to prepare a copper particle paste. Sintering at 220°C under 2 MPa pressure resulted in a shear strength of 42.7 MPa, and sintering without applying pressure at 260°C resulted in a shear strength of 27.1 MPa.

Namgoong et al. [57] performed surface treatment on copper fine particles with diameters of 340 nm and 2 µm by stirring them for 15 min in a solution containing 0.3 mol of oxalic acid in 100 mL of ethanol. The resulting bimodal copper fine particles were dispersed in a solvent mixture of glycerin and a reducing liquid polymer to prepare the paste. The paste was sintered in air at 300°C under 2 MPa pressure for 1 min, resulting in a joint with a shear strength of 20.8 MPa (Fig. 9).

Fig. 9

Estimated compression sinter-bonding mechanism at 300°C of bimodal-sized copper oxalated-coated Cu particles in the effective reducing formulation. (Reproduced from Ref. [57].)

5.5 Copper formate complex pastes

Challenges in sinter bonding using copper nanoparticles include the high sintering temperatures required, the often long bonding times, and the tendency to form voids during sintering. Copper nanoparticles with a particle size of less than 10 nm may be used for bimodal pastes in some cases, but they are significantly more susceptible to oxidation, and their synthesis and handling, as well as the preparation of the paste with such nanoparticles, is not easy. For this reason, copper particles with relatively large diameters are often used, but higher temperatures and longer times are required for the sintering of these particles. Additionally, during the sintering process, the nanoparticles connect by necking each other, but it is difficult to achieve complete sintering of the nanoparticles to form a dense joint, which inevitably leads to the formation of voids in the joint. To address these issues, copper(II) formate complexes have been proposed as a paste material. Copper(II) formate decomposes around 220°C, producing metallic copper and gas as byproducts (eq. (1)) [58–60].

  

\begin{equation} \text{Cu$^{\text{II}}$(HCOO)$_{2}$}\to \text{Cu$^{0}$} + \text{2CO$_{2}$} + \text{H$_{2}$} \end{equation} (1)

The metallic copper generated through this decomposition would be extremely fine nanoparticles, which sinter quickly with neighboring copper nanoparticles to form a dense joint with minimal voids. In addition, since the byproducts are gaseous, there is no concern of residue affecting the joint.

Choi et al. [59] prepared a copper(II) formate paste using α-terpineol as a solvent. Sintering this paste in air under 20 MPa pressure at 250°C for 1 min resulted in a joint with a shear strength of 23 MPa. Under 13 MPa pressure at 225°C for 3 min, a high shear strength of 71 MPa was obtained.

There have also been reports of using copper formate directly as a bonding material without the use of solvents. Lee et al. [60] prepared compressed pellets of copper formate powder and placed them between copper substrates for sintering in air. Sintering under 10 MPa pressure at 250°C for 30 s resulted in a shear strength of 21.9 MPa. Extending the sintering time to 300 s produced a dense joint interface with minimal oxidation (Fig. 10).

Fig. 10

Schematic representation of formation of preform comprising copper formate particles and achievement of highspeed sinter bonding. (Reproduced from Ref. [60])

5.6 Pastes using particles with core–shell structure

It has been mentioned that copper particles/nanoparticles have issues with oxidation resistance. Another solution to suppress the surface oxidation of copper particles is to employ a core–shell structure, in which the surface of the copper particles is coated with a different oxidation-resistant metal. From the perspective of thermal conductivity, electrical conductivity, and oxidation resistance, nanoparticles with a copper core–silver shell structure are the most investigated. By using such core–shell structured particles, it is possible to increase the oxidation resistance of the particles, promote sintering, and improve mechanical properties.

Liu et al. [61] synthesized copper–silver core–shell nanoparticles with a particle size of 86.15 nm and a silver shell thickness of 6.8 nm. Compared to particles without a silver shell, the core–shell nanoparticles showed no oxide peaks in the X-ray diffraction pattern and exhibited a higher oxidation onset temperature according to thermogravimetric analysis. When the copper–silver core–shell nanoparticles were dispersed in butanol to prepare a paste and sintered under argon at 300°C for 30 min under 5 MPa of pressure, a shear strength of 19.7 MPa was obtained. This was higher than that of the copper nanoparticle paste without the silver shell, demonstrating the superiority of the silver shell in oxidation resistance during sinter bonding.

Kim and Lee [62] also reported rapid sintering using copper–silver core–shell fine particle pellets. They synthesized copper–silver core–shell fine particles with a diameter of 351 nm, applied a load of 2 tons for 2 min to form the fine particle pellets. Using these pellets, a shear strength of 20 MPa was obtained under short time sintering conditions of 30 s at 350°C under 5 MPa pressure. Scanning electron microscope (SEM) observations of the sintered bodies suggested that the silver had rearranged and filled the gaps between the sintered copper fine particles (Fig. 11).

Fig. 11

Schematic diagram indicating the mechanism and sequence of sinter bonding at 350°C using a Cu Core – Ag shell preform. (Reproduced from Ref. [62].) (online color)

Similarly, Lee et al. [63] reported the synthesis of copper–silver core–shell microparticles with various particle sizes and the preparation of a paste with terpineol. SEM observations revealed that the numerous tiny silver nodules are formed on the surface of the particles after heating. In thermogravimetric analysis to evaluate the oxidation onset temperature, weight gain was observed at temperatures above 200–250°C, depending on the particle size, indicating the formation of Cu₂O. Using core–shell particles with a diameter of 200 nm, it was reported that sufficient shear strength comparable to high-temperature solder could be obtained with a short sintering time of 10 min at 225°C under 10 MPa of pressure.

6. Sintering Conditions for Copper Fine Particle/Nanoparticle Pastes

Efforts have been made not only to improve the structure of the copper particles and the preparation of the paste, but also to improve the sintering performance of the paste by controlling various conditions during sintering.

6.1 Applying pressure during sintering

Applying pressure during the sintering of copper fine particle/nanoparticle pastes can promote the fusion between the particles and form a dense sintered structure. The equation for the sintering densification rate based on the Mackenzie-Shuttleworth model [64] is as follows:

  

\begin{equation} \frac{\text{d}\rho}{\text{d}t} = \frac{3}{2}\left(\frac{\gamma}{r} + P_{\text{applied}}\right)(1 - \rho)\left[1 - \alpha \left(\frac{1}{\rho} - 1\right)^{\frac{1}{3}}\ln\frac{1}{1 - \rho}\right]\frac{1}{\eta} \end{equation} (2)

where dρ/dt is the densification rate, P_applied is the applied pressure during sintering, γ is the surface energy, r is the particle radius, α is a constant, ρ is the density, and η is the viscosity. From eq. (2), it can be seen that as applied pressure during sintering increases, the densification rate also increases.

Hu et al. [65] investigated the role of applied pressure in the sintering of copper nanoparticles using molecular dynamics simulations. The results showed that the application of pressure induced plastic deformation between the copper nanoparticles, enhancing the atomic diffusion at both surface and volume. Furthermore, when the applied pressure was sufficiently high, defects formed in the necking regions, changing the microstructure, increasing the dislocation density, and accelerating mass transport at the necking sites. These effects demonstrated that the application of pressure enhanced sintering between copper nanoparticles. It was also shown that preheating at low temperatures is effective.

Yamakawa et al. [66] conducted sintering experiments under three pressure conditions (0, 5, and 15 MPa) during the sintering of copper nanoparticle pastes. As a result, the shear strength improved with increasing applied pressure. Additionally, the increase in shear strength with temperature tended to be greater with higher applied pressures. Wu et al. [67] sintered copper nanoparticle films fabricated by pulsed laser deposition under four pressure conditions (2, 5, 10, and 20 MPa) after reduction in ethylene glycol at 140°C. In this case as well, the results showed that the shear strength increased with the applied pressure. The increase in applied pressure led to a decrease in the porosity of the sintered area and an improvement in the interfacial connection ratio between the copper nanoparticle film and the direct bonding copper (DBC) substrate.

6.2 Sintering under reductive atmosphere

Copper fine particles/nanoparticles are easily oxidized by oxygen and water in the atmosphere, which hinders sintering. Moreover, heating promotes this oxidation reaction, and oxidation becomes more pronounced at temperatures above 200°C, where particle sintering typically occurs. Even in a nitrogen atmosphere, oxidation may progress due to trace amounts of impurities such as oxygen. Therefore, methods for sintering in a reducing atmosphere to prevent oxidation and to promote sintering have been investigated.

Gao and Nishikawa et al. [68] oxidized copper fine particles with an average particle size of 4.2 µm at 280°C, and then grinded and mixed with terpineol to prepare a paste. When the resulting paste was sintered at 300°C for 30 min in a formic acid reducing atmosphere, the shear strength increased with the use of copper particles with longer pre-oxidation time, and high shear strength of up to 46.5 MPa was obtained. It was reported that this high strength was due to copper oxide fine particles and nanoparticles generated during the oxidation process filling the gaps between the copper fine particles, and upon reduction in the formic acid atmosphere, a dense sintered structure was formed.

Li and Suga et al. [69] prepared a paste by mixing copper microparticles with a particle size of 500 nm and ethylene glycol. This paste was sintered under three different atmospheric conditions (nitrogen, formic acid, and activated formic acid) at 5 MPa and 250°C for 30 min. As a result, shear strengths of 5 MPa, 30 MPa, and 53 MPa were obtained under nitrogen, formic acid, and activated formic acid atmospheres, respectively, clearly demonstrating that the higher the reducing ability in each atmosphere, the higher the shear strength of the sintered bonding.

Tokura et al. in our research group [41] also reported that when copper nanoparticles protected by carboxylic acid were sintered in a nitrogen atmosphere containing hydrogen, necking growth was greater compared to sintering in a nitrogen atmosphere without hydrogen.

6.3 Applying ultrasonic vibration during sintering

The application of ultrasonic vibration is also a sintering method that contributes to achieving high shear strength. Ji et al. [70] used a paste prepared by dispersing copper core–silver shell nanoparticles protected with PVP in a mixed solvent of ethanol/water and applied ultrasonic vibration during sintering. As a result, a high shear strength of 54.27 MPa was achieved even at the low sintering temperature of 180°C. In contrast, when sintering was performed under 5 MPa pressure without ultrasonic vibration, the shear strength was only 3.91 MPa, demonstrating that the application of ultrasonic vibration significantly promotes atomic diffusion during sintering, leading to a dense bond and significantly improved shear strength.

7. Conclusions

This review paper introduced research on copper fine particles and nanoparticles used as bonding materials for power semiconductors. In the implementation process of wide bandgap power semiconductors, there is a demand for new bonding materials capable of withstanding high operating temperatures. Among these, bonding materials using copper particles, which are cost-effective and highly resistant to ion migration, have attracted attention. To overcome the challenge of low oxidation resistance of copper particles, several approaches have been explored, including organic capping, the addition of reducing agents to pastes, surface treatment of copper particles, bimodal copper particles, copper formate complex pastes, and core–shell structured copper particles. Additionally, methods to promote sintering such as applied pressure, reducing atmospheres, and the application of ultrasonic vibration in the sintering process have also been investigated. These studies have demonstrated the achievement of shear strengths over 20 MPa, comparable to that of Sn-based lead-free solder, through the sintering of copper particle pastes. Future challenges include reducing the interparticle void at lower temperatures, lower pressures, and shorter sintering times, as well as ensuring the long-term reliability of copper nanoparticle bonds under temperature cycling. Further research is expected in the future to address these challenges.

Acknowledgements

This work was partially supported by JSPS KAKENHI Grant Numbers JP23K17855, JP24K01174, and JKA Foundation and its promotion funds from KEIRIN RACE (2023-M354). The authors thank H. Tsukamoto (Hokkaido University) for their research assistance.

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