Fast solid-phase bonding based on indium film-modified copper crystal structure | Scientific Reports

Scientific Reports volume 15, Article number: 17847 (2025) Cite this article

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The special shape of Cu/In layer and ultrasonic vibration are used to realize fast bonding at room temperature, thus solving the problems of high thermal stress and signal delay caused by high temperature in the traditional reflow soldering process. The indium film-modified copper crystal microlayer substrates are used as bonding couples, and ultrasonic vibration and pressure are applied to the bonding contact area to realize the rapid solid-phase bonding of two copper substrates. The microstructure, intermetallic compounds and average shear strength at the bonding interface are analyzed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction (XRD) and bond strength tester. Under ultrasonic vibration and small pressure, the micro-cone structures of Cu/In layers are inserted into each other to form a stable physical barrier structure. The atoms of the thin indium layer at the bonding interface transform into the high-quality phase Cu2In by rapid diffusion driven by ultrasonic energy. When the thickness of the indium layer at the bonding interface is 250 nm, the bonding pressure is 7 MPa, and the bonding time is 1 s, the relatively optimal bonding quality is obtained, and the holes at the bonding interface disappear. The results of heat treatment experiments show that this solid-phase bonding technique can obtain good bond strength without additional heat treatment. The special morphology of the Cu/In layer and ultrasonic vibration allow the bonding to be completed quickly at room temperature. The bonding quality is good and small bond sizes can be obtained.

Low-temperature bonding technology is a hot research topic in 3D packaging. Low-temperature bonding technology avoids the adverse effects of high-temperature manufacturing processes on temperature-sensitive devices which is especially important for some temperature-sensitive electronic components1,2,3,4,5. In copper-copper bonding, various metal nanostructures which have special composition of nano-sized particles and internal spaces are used as intermediate fillers6,7,8,9,10. Jiang et al.11 used a unique Cu-Sn nanocomposite sandwich with Cu nanowire arrays embedded in Sn. Due to this unique sandwich structure, transient liquid-phase bonding of Cu/Cu was realized at 250 °C. The intermetallic compounds at the joints were fine equiaxed Cu-Sn grains with an average size of 1.6 μm, but there were typical defects at the bonding interface, including various types of voids and cracks. Fang et al.12 used micron-sized Ag pastes to achieve fast pressureless and low-temperature bonding of Cu in air. The sintering performance of the micron-sized Ag paste was improved by a surface activation process. The Cu joint structure has excellent interfacial properties with a bond strength of 25 MPa, which is superior to conventional Sn-Pb solder. Li et al.13 sintered ultra-small 5 nm Cu nanoparticle aggregates to obtain 25.36 MPa high-strength Cu-Cu bonded joints at 250 °C and a low bonding pressure of 1.08 MPa. However, the preparation of nanoparticles is complicated. Jhan et al.14 used electrodeposition to prepare Cu nanocrystal structures with an average grain size of 78 nm. The Cu-Cu direct bonding was achieved by joining two Cu nanocrystals in a formic acid atmosphere. 71 MPa was achieved after optimizing the bonding parameters. After optimizing the bonding parameters, a high bond strength of 71 MPa can be obtained. This is attributed to the fact that the nanocrystalline texture of the Cu surface provides a high surface area and fast diffusion paths (grain boundaries) for the atoms to diffuse into each other. Efficient grain growth was generated at the bonding interface, resulting in a well-bonded joint. Li et al.15 used highly (110)-oriented vertical nanotwinned Cu layers to obtain direct Cu-Cu bonding at low temperature and pressure. The two Cu films were bonded at 200–250 °C and 2 MPa for 1 h. A few voids were observed at the interface. Higher bonding pressures and temperatures are needed to promote creep and grain boundary migration at the interface to improve the bonding quality. However, too high pressure and temperature are not desirable in practical applications. A low-temperature bonding method based on Ni/Cu micro-nano cone-arrays has been reported in recent years16,17,18,19, in which the bonding temperature is as low as 160 °C, and effective bonding is realized below the melting point temperature of the solder. However, the presence of holes at the interface affects the material properties and this bonding technique requires long bonding times (up to several minutes) to achieve full diffusion. This will create additional thermal stresses at the bonding interface20,21, so it is necessary to achieve rapid formation of intermetallic compounds at the bonding interface.

Ultrasonic-assisted bonding technology refers to the bonding of metals on both sides under the action of ultrasonic waves and pressure22,23,24,25. This bonding technique can realize low-temperature bonding between chips in a very short bonding time with low bonding pressure, which reduces the damage to the chip by hot pressure and improves the reliability of the package compared with the traditional hot-pressure bonding26,27,28,29,30. Various substrate and interlayer materials such as Ni, Al, and Cu have been used in ultrasonic bonding31,32,33,34,35,36. Ji et al.37 achieved direct bonding between alumina and ceramics under ultrasound using pure aluminum as an intermediate layer. The bonding time was 90 s. Alumina nanoparticles were obtained at the bonding interface with high fracture strength, but high temperature heat treatment was required after bonding. Wang et al.38 investigated an ultrasonic bonding technique between Ni microcone films and lead-free solder. A cavity-free bonding interface was achieved at a bonding pressure of 7 MPa and a bonding time of 1 s. However, the size of the solder ball is too large to affect the bonding performance. The large size of the solder balls affects the application of the process, especially in high-density and small-size interconnects. Heat dissipation appears to be important in chip 3D stacked packages, but Ni has poor electrical conductivity. Li et al.39 realized a high melting point bonded interface by bonding tin as a filler layer under ultrasonication and 0.6 MPa pressure for 4 s at room temperature in an atmospheric environment. Under the ultrasonic friction energy, Sn melted instantaneously and reacted with the Cu substrate. The different intermetallic compositions at the bonding interface were realized by controlling the bonding time and the thickness of the intermediate Sn layer. With the depletion of Sn, Cu3Sn and Cu6Sn5 intermetallic compounds were formed at the bonding interface, while different types of holes were formed at the interface, and these interfacial voids adversely affected the interconnections. A direct ultrasonic bonding method for Cu pillar chips was proposed by Roshanghias et al.40,41. The ultrasonic energy significantly reduces the bonding pressure, temperature and time, but formic acid vapor must be introduced to protect the bonding region from oxidation.

An ultrasound-assisted rapid solid-phase bonding technique is proposed in the paper. This technique is based on Cu/In micro-nanolayers. The bonding interface is flanked by indium film-modified copper micron needle cone structures. This makes the bonding preparation process simpler. Defect-free bonding at the interface is realized under ultrasonic vibration. Due to the special morphology of the Cu/In layer and the assistance of ultrasonic vibration, the bonding is completed quickly at room temperature. Compared with the previous bonding technologies that require harsh conditions such as ultra-high vacuum, high temperature, and high surface flatness, the bonding technology proposed in this paper has less stringent conditions, and can obtain smaller bonding dimensions with high efficiency and low energy consumption, which is in line with the development trend of green packaging.

Copper micro-nano-needle cone structures are obtained by chemical plating method. The size of the needle cone morphology can be controlled by the crystalline modifier model42. As shown in Fig. 1(a), the height of the copper micro-nano-needle layers is 1–3 μm, and the diameter of the root of the needles is 500 nm–2 μm. The tips of the copper needles are relatively sharp and have a typical cone shape. The copper cones grow in different directions. Indium nanolayers are plated onto copper micro- nano structures using a commercial solution where the thickness of the indium nanolayers was controlled by the plating time. An image of an indium nanolayer (plating time of 10 s) plated onto a copper micro-nanoarray structure is shown in Fig. 1(b). The copper-indium micro-nano layers maintain a relatively sharp tip structure. The cross-sectional morphology of the Cu-In micro-nanolayer is shown in Fig. 1(c). It is found that the surface of the Cu-In micro-nanolayer still maintains the needle cone structure. The distribution of Cu and In atoms at the cross-section is shown in Fig. 1(d) and Fig. 1(e), respectively. The In layer is found to be uniformly and continuously covered above the Cu needle cone structure. The average of three measurements of different areas is taken. The thickness of the indium layer on the Cu/In micro-nanoarray structure is about 250 nm. The surface Cu/In micro-nano layer structure is schematically shown in Fig. 2.

Morphology of micro nano layer: (a) SEM of copper needle array structure (b) SEM of Cu-In micro-nano needle (c) The cross-sectional morphology of the Cu-In micro-nano needle (d) The distribution of Cu atoms at the cross-section (e) The distribution of In atoms at the cross-section.

Structure of Cu/In micro-nano layer on surface.

A schematic diagram of ultrasonic vibration-assisted rapid solid-phase bonding is shown in Fig. 3. Solid-phase bonding was performed on a customized ultrasonic bonding instrument (RHESCA, PTR- 1102), and the ultrasonic vibration was set to 20 kHz. The procedure is as follows: First of all, two identical copper substrates modified with indium film (model C194) were stacked on the bottom plate of the ultrasonic bonding instrument. It can be seen that the top and bottom sides of the bonded joint are the same. The ultrasonic energy was loaded on the top of the bonded joint. The loading speed of the loader was set at 2.0 mm/min. The bonding pressure was set at 5–7 MPa, and the bonding time was set at 1–1.5 s. The whole bonding process was completed at room temperature without vacuum environment or inert gas protection. After the bonding was completed, the shear strength was measured using the push-ball destructive shear mode of the ultrasonic bondmeter. The positioning distance was 30 μm and the measurement speed was set to 0.3 mm/s. For each bonding condition, 10 bonded samples were collected for testing the shear strength. The shear strength was averaged to minimize experimental error. Scanning electron microscope (SU8220, Hitachi, Japan), and transmission electron microscope (FEI Tecnai G2 F20S) were used to analyze the interfacial morphology after bonding. The elemental information was analyzed by energy dispersive spectroscopy (EDS, accelerating voltage of 200 keV) under the electron microscope.

Schematic diagram of ultrasonic vibration-assisted rapid solid-phase bonding.

A copper-indium micro-nano cone structure electroplated with indium for 10 s was chosen to study the effects of pressure and bonding time on the bonding quality. Copper metal is susceptible to oxidation during the encapsulation process, which can be avoided by coating indium nanolayers on the copper cone array structure. Indium has good cold soldering properties, and it can also be used as a buffer layer to minimize the generation of holes at the bonding interface. The quality of bonding can be analyzed by observing the microscopic morphology of the plated surface and bonding interface.

The cross-sectional image of the bonding interface is shown in Fig. 4 to investigate the morphology of the copper microcone structure and indium interlayer at the bonding cross-section. As shown in Fig. 4(a), at a bonding pressure of 5 MPa and a bonding time of 1 s, some of the copper-indium pin-cone structures are inserted into each other, and some of the pin-tip structures are fractured. The bonding interfaces rub against each other under the effect of transverse ultrasonic vibration. Indium has good cold soldering properties. Under the friction condition, the indium nanolayer at the bonding interface is attached to the copper pin array structure. Some fractured copper cone tips are dispersed on the internal indium interface. Since the copper-indium pin-cone structures have not yet been fully inserted, distinct holes are formed between the copper-indium microcones at the bonding interface. As shown in Fig. 4(b), at a bonding pressure of 6 MPa and a bonding time of 1.5 s, no obvious concave holes were found at the bonding interface. Due to the long bonding time, the transverse friction resulted in linear crack holes at the bond interface region. The formation mechanism of these holes was different from that of the holes caused by hot press bonding. It is concluded that ultrasonic vibration has an important effect on the interface morphology. The shrinkage and filling of the pores between the concave regions of the copper-indium microneedle cones are accelerated. Linear holes are formed at the top of the copper-indium tip, which are caused by the lateral movement between the indium and copper layers.

The presence of holes seriously affects the bonding quality, especially the linear holes are easy to extend and expand and prone to crack formation. In order to obtain satisfactory bonding quality, the bonding pressure was adjusted to 7 MPa and the bonding time was set to 1 s, as shown in Fig. 4(c). The copper micro-nano cone-array structures are well embedded in each other due to the different copper cone orientations. A stable physical separation layer is formed at the bonding interface, and basically no holes or cracks are observed at the bonding interface. The results show that the deformation of the copper micro-nano cone-array structures is multi-directional. This is due to the transverse vibration of the ultrasound, the direction of force on the tip is constantly changing, and the copper cone structure remains essentially conical. With the increase of pressure, the needle tip partially deforms into a bow-shaped structure, as shown in Fig. 5. As a result, this important bonding parameter (i.e., bonding pressure) was set to 7 MPa in the experiment.

The atomic number fractions of copper and indium at the bonding interface were obtained by EDS point scanning and line scanning, and the composition of the bonding interface was analyzed. Figure 4(d) shows the line scan analysis results at the scan line in Fig. 4(c). According to the average atomic ratio, it can be seen that Cu atoms exist inside the In phase. It means that Cu diffuses in the bonding interface and Cu exists in the whole bonding interface, so the substances in the interface are all intermetallic compounds. From the results of the line scan (Fig. 4(d)), it is found that the average quantitative atomic ratios (both expressed as atomic number fractions) in the flat part of the curve on the left are 67.8% for Cu and 32.2% for In, which is more in line with the atomic number ratios of Cu2In. The results of the point-scan analysis of Point 1 are shown in Fig. 4(e). The atomic number fraction of Cu is 67.14% and that of In is 32.86%, which is still in line with the atomic number ratio of Cu2In (2:1). As the interface layer is very thin, the heat generated by ultrasonic waves accelerates the diffusion of Cu, which can be completely transformed into Cu2In under certain bonding conditions. Therefore, no phase transformation will occur in the subsequent aging treatment, and no Kirkendall cavities will be formed43. It can be seen that intermetallic compounds of moderate thickness in the interface have no adverse effect on the bonding strength of the interface, on the contrary, it improves the bonding strength. The Cu atom distribution at the bonding interface is shown in Fig. 4(f). The results of In atom distribution at the bonding interface is shown in Fig. 4(g).

Morphology of Cu-In-Cu bonding interface under different bonding conditions: (a) 5 MPa, 1 s (b) 6 MPa, 1.5 s (c) 7 MPa, 1 s (d) EDS line scanning analysis (e) EDS point scanning analysis (f) Cu atomic distribution at the bonding interface (g) In atomic distribution at the bonding interface.

Morphology of the bowed structure.

The results of transmission electron microscopy observation and analysis of the specimens under bonding pressure of 5 MPa and bonding time of 1 s are shown in Fig. 6. By studying the bonding mechanism of this bonding technique, it is found that the insertion between the copper-indium micro-nanolayers is not tight under this bonding condition. There are obvious holes. A low magnification image of the bonding interface is shown in Fig. 6(a). Several copper cones with indium plated layers are shown, and it is clear that copper micro-nano cone-array structures are plated on the copper plate. Deformation of the copper-indium micro-nano cone alloy is observed, transforming from a cone to a trapezoidal shape, but no solid-state bonding is achieved. Cracks in the upper portion of the copper-indium micro-nano cones indicate that the voids will increase further, leading to failure at the bonded joints. As shown in Fig. 6(b), one copper cone is inserted into the other side of the copper cone, and the indium-plated layers on the cones are interconnected and squeezed together, with the copper-indium boundaries marked with lines in Fig. 5(b). The copper needle cones are deformed by ultrasound. The indium layers on the lower side of the interface, which are distributed inside the copper needles, are tightly bonded after the extrusion and no voids are created.

Two indium layer regions, B and C, are selected in the Cu cone for further study (shown in Fig. 6(b)). The high resolution images of the B and C regions are shown in Fig. 6(c)-(d). Under the dual effects of mechanical insertion and ultrasonic friction, indium diffuses rapidly from indium to indium and indium to copper. Lattice fringes of Cu2In can be recognized at the bonding interface by measuring the crystal spacing, as shown in Fig. 6(c). Due to the frictional extrusion of the indium layer, the indium atoms form various alignments as shown in Fig. 6(d). Cu2In and Cu coexist in the region near the copper cone, and their transitions are separated by several nanometers. Some amorphous regions can be seen in Fig. 6(d). These amorphous regions imply the presence of atomic level bonding, which ensures the strength of the bonding interface.

The insertion is incomplete because the pressure is too low. Lattice fringes of Cu7In3 can be recognized on the copper cone region. Indium is not fully converted to Cu2In during the 1 s ultrasonic bonding time. As shown in Fig. 6(c)-(d), the selected electron diffraction patterns are labeled to the [011] band axis for indium and [001] band axis for copper. It shows that the bonded copper pin cone still has the original structure.

TEM morphology of copper-indium bonding interface: (a) low power image (b) close-up image of indium filled copper needle valley (c) high resolution contact images of copper-indium in B regions (d) high resolution contact images of copper-indium in C regions.

High-quality interconnect samples were obtained by ultrasonic bonding under the optimal process parameters of 10 s indium plating time, 7 MPa bonding pressure, and 1 s bonding time. The effect of heat treatment on the average shear strength of the copper-indium-copper bonding interface was analyzed by aging the bonded samples.

The samples obtained under the bonding interconnect parameters of 7 MPa pressure and 1 s bonding duration were placed in an oven at 160 °C for aging heat treatment for 60 min and 360 min, respectively.

The XRD spectra of the bonded interfaces obtained after heat treatment of the samples at 160 °C for different times are shown in Fig. 7. When the heat treatment time is 60 min, the diffraction peaks of Cu and Cu2In are strong. This is due to the thin indium layer with a thickness of about 250 nm for 10 s of indium plating. The diffusion of copper atoms is faster under the combined effect of vertical pressure and lateral ultrasonic energy. The indium layer reacts completely with the copper to form Cu2In.

The intermetallic compound Cu2In do not undergo a phase transition after a short heat treatment time of 60 min. There is no excess indium at the bonding interface to further react with copper atoms, and the interface composition is stable. Cu2In is a high-quality phase that improves the mechanical properties of Cu/In/Cu joints44. The intensity of the Cu7In3 diffraction peak is stronger when the heat treatment time is 360 min. After a long time of heat treatment, the interfacial reaction of copper and indium continues to take place, and Cu2In is transformed into Cu7In3. It can be hypothesized that the intermetallic compounds at the bonding interface will all be converted to Cu7In3 after a long time of heat treatment. Previous studies have shown that Cu7In3 has poorer shear properties than Cu2In45. Moreover, the melting points of Cu7In3 and Cu2In are almost the same, so it is not necessary to obtain the Cu7In3 phase by a longtime heat treatment. On the contrary, relatively good mechanical properties can be obtained by heat treating the joints consisting of Cu2In for a short period of time. This can also be verified from the results of the shear test experiments as well as the fracture surface results that follow.

XRD pattern of bonding interface at 160 °C for different heat treatment time.

The heat treatment temperature was set to 160 °C, and shear tests were conducted on the bonded interfaces after heat treatment for different times (all bonding times were 1 s), as shown in Fig. 8. From Fig. 8, it can be found that the shear strength of the interface (bonding pressure of 6 MPa), which has a low average shear strength, shows a significant increase after heat treatment. With the increase of heat treatment time, the increase of shear strength becomes slower after more than 120 min. When the heat treatment time reaches 600 min, the average shear strength is about 22 MPa, which is still far from the shear strength achieved by reflow soldering (40 MPa). Before heat treatment, the average shear strength of the bonding interface with high shear strength (bonding pressure of 7 MPa) is up to 33 MPa, which is close to the shear strength achieved by reflow soldering. After heat treatment, the shear strength of the interfaces increases, but more slowly. After the heat treatment time exceeds 300 min, the shear strength tends to decrease. This shows that the heat treatment process do not significantly increase the shear strength. Instead, the shear strength decreases after a long time of heat treatment, and it can be assumed that this ultrasonically-assisted bonding technique does not require additional heat treatment to obtain a more desirable shear strength.

Relationship between shear strength of bonding interface and heat treatment time.

The fracture surface after shear experiment is shown in Fig. 9. After heat treatment at 160 °C for 60 min, the specimens have tough nest fracture surfaces, as shown in Fig. 9(a). It shows that the bonded interface has good toughness and exhibits excellent plasticity. The Cu-In-Cu interface is partially intact, and the fracture site is mainly a Cu2In section. This is due to the low average shear strength of the Cu-In-Cu interconnect bonding interface, although the heat treatment time is short, but the diffusion of atoms at the bonding cross-section is faster. The bonding interface is fully embedded. The intermetallic compound Cu2In formed has excellent plasticity and toughness. With the extension of the heat treatment time, the atoms further diffuse.

Cu2In intermetallic compound has been transformed to form Cu7In3 intermetallic compound. Cu7In3 is more brittle and less plastic, thus affecting the quality of the interconnect bond. As shown in Fig. 9(b), after heat treatment at 160 °C for 300 min, the shear surface is relatively flat and uniform, with quasi-dissolution fracture dominating. With the extension and expansion of cracks, secondary disintegration and localized tearing occurred on the main disintegration surface. There are a large number of along-crystal fractures in Fig. 9(b). The fracture surfaces mainly occurred in the Cu/In bonding layer. The indium-containing fracture Cu7In3 is found at the fracture location.

The results of the heat treatment experiments show that the ultrasound-assisted transient solid-phase bonding of copper-indium micro-nanolayers is highly reliable. Relatively optimal bonding quality can be obtained at a bonding pressure of 7 MPa and a bonding time of 1 s. There is no need to increase the heat treatment temperature to increase the interconnection strength of the bond, and the high reliability can be maintained in the future use.

Cross section of bonding interface under different parameters: (a) 160 °C, 60 min (b) 160 °C, 300 min.

A solid phase bonding technique was investigated. Indium nanolayers were electroplated on copper micro-cone structures. Indium has good cold soldering properties.

Under ultrasound assistance, the copper-indium micro-nanolayers were instantaneously bonded at room temperature under suitable pressure (7 MPa) and suitable bonding time (1 s) to obtain void-free joints at room temperature with good bonding quality.

Based on the special morphology of the Cu micro-cone arrays, the indium layer fills the holes between the fractured Cu micro-cones under ultrasonic energy. At the bonding interface, Cu2In intermetallic compounds were generated based on the rapid diffusion of copper. Cu2In is a high-quality phase with good mechanical properties, which is favorable for improving the bond strength in use.

Appropriately increasing the bonding pressure facilitates the close puncturing of the copper micro-cones with each other and creates a physical shield.

Ultrasonic vibration, mechanical insertion, and rapid diffusion are the main mechanisms involved in the bonding process.

By means of secondary copper-indium micro-nanolayers, successful bonded joints can be obtained at room temperature without the need for a vacuum environment or protection by inert gases and without additional heat treatment.

Data is provided within the manuscript.

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This study was funded by Specialized Focus Areas for General Colleges and Universities in Guangdong Province(2024ZDZX3098), the Special Funds for Intelligent Manufacturing Modern Industrial College (No. 0220119).

School of Intelligent Manufacturing and Materials Engineering, Gannan University of science and technology, No. 156, Kejia Rd., Zhanggong District, Ganzhou City, 341000, Jiangxi Province, People’s Republic of China

Xiao Jin, Luo Jia, Zhou Qi-xing & Zhang hao

School of Advanced Manufacturing, Guangdong Songshan Polytechnic College, Shaoguan, 512100, People’s Republic of China

Jun-hui Liu

School of Artificial Intelligence and Electrical Engineering, Guangzhou College of Applied Science and Technology, Guangzhou, 511300, People’s Republic of China

Huang Xi-feng

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Xiao Jin: Funding acquisition, Formal analysis, Conceptualization.Luo Jia: Software.Zhou Qi-xing: Software.Zhang hao: Formal analysisLiu Jun-hui: Investigation.Huang Xi-feng: Investigation.

Correspondence to Xiao Jin.

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Jin, X., Jia, L., Qi-xing, Z. et al. Fast solid-phase bonding based on indium film-modified copper crystal structure. Sci Rep 15, 17847 (2025). https://doi.org/10.1038/s41598-025-02798-y

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Received: 19 November 2024

Accepted: 15 May 2025

Published: 22 May 2025

DOI: https://doi.org/10.1038/s41598-025-02798-y

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