Optimization of external and internal conditions for high thermal conductive Cu-diamond composites produced by electroplating

Diamond & Related Materials 98 (2019) 107478

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Optimization of external and internal conditions for high thermal conductive Cu-diamond composites produced by electroplating Yongpeng Wu, Liyan Lai, Yan Wang, Hong Wang, Guifu Ding

T

⁎

National Key Laboratory of Science and Technology on Micro/Nano Fabrication, School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Copper matrix composites Bimodal diamond Electroplating Current density Forced convection Temperature

Aiming to realize the potential application of Cu-diamond composites with high thermal conductivity (TC) in microelectronic industry, we recently utilized electroplating technology to synthesize void-free Cu-diamond composites (TC above 600 W/m K). In this work, the detailed external conditions (current density, temperature and forced convection) and internal condition (diamond content) for improving TC of the composites were investigated. It was found that the nucleation energy along crystal orientation varied with the current density, and copper nodules tended to appear at high current density. Microstructures of the composites were severely affected by the temperature, since the additives were deactivated at high temperature. Additionally, the adsorption of additives changed oppositely at strong forced convection, leading to the decrease of electroplating efficiency. Bulk Cu-diamond composites were synthesized with different plating time to verify the electroplating condition for void-free microstructure and compactly combined diamond/copper interface. Using the optimal conditions, the composites with bimodal diamond particles were produced by electroplating for the first time, reaching a higher diamond volume fraction than that with single particles and a high TC of 651 W/m K. The improved TC mechanism of bimodal diamond composites was explained by double EMT model. This work revealed the detailed effect of current density, temperature and forced convection on void-free copper matrix composites, which were helpful for improving the diamond content and the TC value of the bimodal diamond composites.

1. Introduction In recent years, micro-electronics industry has developed rapidly in high integration and heat management becomes significant important [1,2]. For lowering the operating temperature, high thermal conductivity (TC) and low coefficient of thermal expansion (CTE) materials are urgently needed. Metal matrix composites possess the suitable properties (higher TC than 300 W/m K, adjustable CTE of 4–9 ppm/K) by adding carbon reinforcement, which meet the demand mentioned above [3–5]. Diamond is one of the most valuable reinforcement materials for its high TC (~2200 W/m K) and low CTE (2.3 ppm/K) [6–9]. Diamond reinforcing copper matrix composites (Cu-diamond) are widely studied in heat management [9–11]. Currently, Cu-diamond composites are synthesized by infiltration [11–14], hot-pressing sintering [15], high pressure sintering [8,16–18], spark plasma sintering [9,19–21] and powder metallurgy [7,22–24]. Nevertheless, these high temperature sintering methods need rigorous conditions (~1000 K and 60 MPa). It is not only energy-intensive, but

⁎

also incompatible with the microelectronic process. Therefore, it is emergent to find an alternative way for producing high TC Cu-diamond composites under moderate conditions. Recently, we have successfully synthesized void-free Cu-diamond composites by electroplating and achieved the high TC (above 600 W/ m K) [25]. This low-temperature technique overcomes the constraints in high temperature sintering methods, such as high temperature, thermal mismatch and non-wetting. Moreover, composite materials produced by electroplating are compatible with microelectronic devices, which is good for high integration. The TC of Cu-diamond composites is effected by diamond/copper interface, which is caused by the external electroplating conditions and internal condition of diamond content. In this study, we focus on the electroplating conditions (external conditions) to optimize the synthetic process for void-free Cu-diamond composites, which is beneficial for producing high-quality bimodal composite materials with higher diamond content and TC values than before. Electroplating current density, temperature and forced convection are investigated. It is found

Corresponding author. E-mail address: [email protected] (G. Ding).

https://doi.org/10.1016/j.diamond.2019.107478 Received 25 May 2019; Received in revised form 17 July 2019; Accepted 17 July 2019 Available online 19 July 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.

Diamond & Related Materials 98 (2019) 107478

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where the subscript t and p referred to the test sample and random powder pattern, respectively. n was the number of calculated peaks. LFA 467 HyperFlash Laser flash analyzer was used to measure the TC k of composite coatings (Eq. (2)). The density ρ of composite coatings was determined using Archimedes method. The specific capacity c of composite materials was obtained by Eq. (5), according to the mixture rule. The diamond content (Vdia) of composite materials was examined by Eq. (3) and derived Eq. (4). The thermal diffusivity α was obtained directly from LFA 467. The sample disks with a size of diameter 12.7 mm and thickness 1 mm were used.

that copper nodules tend to appear at the current density higher than 40 mA·cm−2. Moreover, at the temperature above 40 °C, the additive system (DVF-B and DVF-C) is deactivated and the microstructure of Cudiamond composites is degenerated. Additionally, the adsorption of DVF-C is enhanced by strong forced convection, leading to the decrease of electroplating efficiency. A bulk Cu-diamond composite sample is produced with different plating time, accompanied with the SEM examination of void-free microstructure. Bimodal diamond particles reinforcing composites are prepared using the optimal conditions, and the diamond content (internal condition) is higher than that of single particles. This work studies the electroplating conditions, making the effects of current density, temperature and forced convection on voidfree Cu-diamond composites clear and explicit. This work proposes a method to obtain high diamond content Cu-diamond composites using bimodal diamond particles.

k = αρc

(2)

ρ = ρdia × Vdia + ρCu × (1 − Vdia)

(3)

Vdia =

2. Experimental

ρ − ρCu ρdia − ρCu

ρ × c = cdia × Vdia × ρdia + cCu × (1 − Vdia) × ρCu

(4) (5)

2.1. Materials and preparation 3. Results and discussions

The high thermal conductive diamond particles (Element Six Corp., PDA999, 1700 W/m K) were cubic-octahedral shape. Four kinds of diamond particles used in this study were 140/170 (mean size of 100 μm), 230/270 (mean size of 60 μm), 325/400 (mean size of 40 μm) and 400/500 (mean size of 30 μm). To produce composite materials with single diamond particles, the detailed preparation process, apparatus and composition of electrolyte have been reported in our previous report [25,26]. In this study, electroplating parameters were discussed, including current density, temperature and forced convection. Table 1 showed the electroplating parameters used in this study. The 140/170 diamond (mean size of 100 μm) was used for optimization of various electroplating parameters. To produce composite materials reinforced with bimodal diamond particles, an alternate electroplating process was implemented using the optimal conditions (20 mA/cm2, 25 °C, 150 rpm). Firstly, the composites with 100 μm diamond particles were synthesized in the first electrolyte bath (with 100 μm diamond particles) for 3 h. Secondly, the unfinished composites were moved to another electrolyte bath for synthesizing with 60 μm, 40 μm or 30 μm diamond particles for 5 h, respectively. Then, the two procedures were repeated alternately for several times to obtained the bulk composites. The plating bath was refreshed for every 24 h electroplating time.

3.1. Effect of current density The effect of current density on the synthesis of Cu-diamond composites is studied at the conditions of temperature 25 °C, magnetic agitation 150 rpm, plating time 240 min. When the current density is lower than 30 mA·cm−2, the flat copper matrix with uniformly distributed diamond particles can be seen (Fig. 1(a–d) and Fig. 2(a–d)). Previous reports have shown that optimal concentration ratio of additive system (DVF-B and DVF-C, known as accelerator and inhibitor) can promote voids full-filling and level copper matrix, which is resulted from the different coefficients of diffusion and adsorption [29–32]. In this study, the specific additive system (DVF-B and DVF-C) is utilized for eliminating the voids/gaps in Cu-diamond composites and leveling copper nodules. With the suitable current density (below 30 mA·cm−2), DVF-B works in the small intervals between diamond particles,

2.2. Characterization Scanning electron microscopy (SEM, Ultra55, ZEISS) with the voltage of 10 kV was used to characterize the microstructure of composite materials. X-ray diffraction (XRD, D8 Advance, Bruker-axs) with Cu Kα radiation was used to analyze the phase structure. The preferred XRD orientation index ηhkl was calculated using Eq. (1), which showed the value > 1 [27,28].

ηhkl =

Ihkl, t / Ihkl, p 1 n

n

∑1 Ihkl, t / Ihkl, p

(1)

Table 1 The electroplating parameters. Parameters

Values

Current density Temperature Magnetic agitation Plating time Diamond content

5–60 mA/cm2 5–60 °C 0–600 rpm 4–70 h 10 g/L

Fig. 1. SEM images of Cu-diamond synthesized by different current density: (a) 5 mA·cm−2; (b) 10 mA·cm−2; (c) 20 mA·cm−2; (d) 30 mA·cm−2; (e) 40 mA·cm−2; (f) 60 mA·cm−2. 2

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Fig. 2. Enlarged SEM images of Cu-diamond synthesized by different current density: (a) 5 mA·cm−2; (b) 10 mA·cm−2; (c) 20 mA·cm−2; (d) 30 mA·cm−2; (e) 40 mA·cm−2; (f) 60 mA·cm−2.

especially the micro voids/gaps, promoting the copper deposition; while DVF-C inhibits copper deposition in the large intervals and on the copper heaves/nodules, as shown in Fig. 3. Thus, in this cooperative situation, copper matrix with the flat surface is obtained and copper matrix is compactly combined with diamond particles (Fig. 2(a–d)). Increasing the current density to 40 mA·cm−2, copper nodules appear on the surface. Such unbalanced electrodeposition is explained with crystal nucleation and growth. The nucleation rate is expressed as Eq. (6) [33]:

T ω = B exp ⎛⎜− 2 ⎞⎟ ⎝ η ⎠

(6)

where T is the temperature, B is the constant for copper matrix, ω is the nucleation rate and η is the polarized potential. With the low current density (30 mA·cm−2), the cathodic polarization potential η is low, leading to limited nucleation. At the low nucleation rate, copper grains tend to grow up. However, with the help of DVF-C, the over-growth of copper grains on copper heaves/nodules is restrained, leading to fine copper grains (Fig. 1(a–d) and Fig. 2(a–d)). At high current density (over 40 mA·cm−2), however, cathodic polarization potential is enhanced and nucleation is promoted. The excess

Fig. 4. (a) XRD patterns of composites synthesized with different current density; (b) enlarged XRD patterns from 42.5° to 75°; (c) the crystallographic orientation indexes of (111), (200) and (220).

Fig. 3. The schematic diagram of the distribution of additives among the diamond particles. 3

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copper grains are electrodeposited on high cathodic polarization potential spots and aggregated to form copper heaves/nodules. In this situation, the DVF-C is incapable to restrain the excess copper deposition on the heave/nodules. Copper heaves form on the surface and develop to large nodules as the deposition proceeds (Fig. 1(e) and (f)). Once the copper nodules appear, large voids/gaps occur when combining diamond particles (Fig. 2(e) and (f)), resulting in the badly combined interface and the decrease of thermal diffusion coefficient. Fig. 4(a) and (b) shows the XRD analysis of composites synthesized with different current density. Diamond shows a very high intensity of (111) peak, which is the sign of preferred crystallographic orientation [28,34]. No graphite peak appears at 26.5°, indicating that the composites contain no graphite phase [35], which is due to room temperature synthesis. As for copper matrix, it can be calculated that the (111) orientation index decreases from 1.76 to 1.18, while the (200) orientation index increases from 0.85 to 1.09 and the (220) orientation index increases from 1.39 to 1.78, as shown in Fig. 4(c). The energy needed for nucleation along crystal orientation (Whkl) is regarded as Eq. (7) [36–38]:

Whkl =

Bhkl zFη / Na − Ahkl

(7)

where Ahkl and Bhkl depend on the direction [hkl]. Na is the Avogadro constant, F is the Faraday constant, z is the number of electrons, η is the over-voltage of cathodic polarization. According to Eq. (7), at low overvoltage, W111 < W100 < W110, while at high over-voltage, W110 < W100 < W111 [36–38]. Metal would deposit on the crystallographic direction with the lowest nucleation energy. With the low current density (below 10 mA·cm−2), W111 is the smallest nucleation energy and copper matrix shows [111] preferred orientation. With the increase of current density (over 10 mA·cm−2), W110 decreases and the preferred orientation of [220] increases (Fig. 4(c)).

Fig. 5. SEM images of composites synthesized at different electroplating temperature: (a) 5 °C; (b) 15 °C; (c) 25 °C; (d) 40 °C; (e) 60 °C; (f) 25 °C, using the electrolyte preheated at 60 °C.

3.2. Effect of temperature The Effect of electroplating temperature on the synthesis of Cudiamond composites is investigated at the conditions of current density 20 mA/cm2, magnetic agitation 150 rpm, plating time 240 min. Increasing the temperature from 5 to 25 °C, copper matrix shows the flat surface with fine copper grains and diamond particles are distributed uniformly (Fig. 5(a–c)). No voids/gaps or pores appear, leading to the compactly combined interfaces (Fig. 6(a–c)). However, as the electroplating temperature increases to 40 °C, copper matrix is coarse with large copper grains. Increasing the temperature to 60 °C, the number of diamond particles reduces obviously and copper matrix shows coarse surface. The efficiency of the electroplating decreases largely (Fig. 6(e)). Additionally, copper nodules begin to appear on copper matrix (Fig. 5(e)). As known to all, the nucleation energy barrier is proportional to 1/η, and the balance between nucleation and growth is controlled by overpotential η [39]. Moreover, the over-potential decreases with the increase of electroplating temperature [40]. Therefore, at high electroplating temperature, the over-potential decreases and nucleation energy barrier enhances. Thus, copper grains tend to grow up and large grains appear at high electroplating temperature, as shown in Fig. 5 and Fig. 6. To investigate the active additives at high temperature, another composite sample is electroplated at 25 °C using the same electrolyte pretreated at 60 °C (heating in an oven at 60 °C). Fig. 5(f) and Fig. 6(f) show the morphology of copper matrix, which is nearly the same as Fig. 5(e) and Fig. 6(e), indicating that the additives have lost their activity at such a high temperature. Thus, the electroplating efficiency decreases as the temperature increases. Copper nodules appear since the inhibition of DVF-C is lack at high temperature. Similar to Fig. 4, diamond shows preferred crystallographic orientation along [111] direction without the noticeable graphite peak at

Fig. 6. Enlarged SEM images of composites synthesized at different electroplating temperature: (a) 5 °C; (b) 15 °C; (c) 25 °C; (d) 40 °C; (e) 60 °C; (f) 25 °C, using the electrolyte preheated at 60 °C.

26.5° (Fig. 7(a) and (b)). As for copper matrix, it can be calculated that the (111) orientation index decreases from 1.32 to 0.12 and (200) orientation index decreases from 0.82 to 0.14, while the (220) orientation index increases from 1.85 to 3.73, as shown in Fig. 7(c). The over-potential decreases with the increase of electroplating temperature [40]. Thus, at high temperature (> 40 °C), the over-potential decreases, W111 < W100 < W110. When the temperature is lower than 30 °C, the 4

Diamond & Related Materials 98 (2019) 107478

Y. Wu, et al.

Fig. 8. SEM images of composites synthesized with the different flow rate: (a) 0 rpm; (b) 80 rpm; (c) 150 rpm; (d) 300 rpm; (e) 450 rpm; (f) 600 rpm.

consumed and become insufficient, leading to the formation of micro cracks (Fig. 9(a)). As the stirring speed increases from 80 to 300 rpm, cupric ions and additives can be replenished into the intervals and voids/gaps timely. Thus, cracks-free Cu-diamond composites can be obtained (Fig. 8(b–d) and Fig. 9(b–d)). Increasing the stirring speed from 450 to 600 rpm, diamond particles are still distributed uniformly. However, the electroplating efficiency reduces (Fig. 8(e–f) and Fig. 9(e–f)). With such intensively stirring, copper ions are difficult to reach the deep intervals, resulting in the low concentration of cupric

Fig. 7. (a) XRD patterns of composites synthesized at different temperature; (b) enlarged XRD patterns from 42.5° to 75°; (c) the crystallographic orientation indexes of (111), (200) and (220).

orientation indices of (111), (200) and (220) seem unchanged. However, the sharp changes of orientation indices appear at the temperature above 40 °C, resulting from the deactivation of additives. Such changes of orientation indices agree well with the microstructure of the composites (Fig. 5 and Fig. 6). 3.3. Effect of forced convection Copper electro-crystallization is strongly affected by the concentrations of cupric ions and additives, which are affected by forced convection. When no forced convection is added in the electrolyte, cupric ions and additives transfer to the bottom of intervals and voids/ gaps between diamond particles by concentration gradient. Since the diamond particles heap up on the substrate by gravity, the concentration gradient is incapable to transfer cupric ions and additives to the reactive surface. As the electrodeposition proceeds, the reactants are

Fig. 9. Enlarged SEM images of composites synthesized with the different flow rate: (a) 0 rpm; (b) 80 rpm; (c) 150 rpm; (d) 300 rpm; (e) 450 rpm; (f) 600 rpm. 5

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Fig. 12. The schematic diagram of ideal distribution with (a) single diamond particles; (b) bimodal diamond particles. Fig. 10. The schematic diagram of the distribution of copper ions by forced convection and gradient diffusion.

3.5. Bimodal diamond particles reinforced composites Fig. 12 shows the schematic diagram of ideal distribution of single and bimodal diamond particles in the composites. Since diamond particles can hardly get into the space formed by themselves (Fig. 12(a)), another small diamond particles are used to form the bimodal diamond composites (Fig. 12(b)). To investigate the actual distribution, three kinds of composites reinforced with 100 μm and 60 μm, 100 μm and 40 μm, 100 μm and 30 μm bimodal diamond particles are synthesized (Fig. 13). The small diamond particles (60 μm, 40 μm, 30 μm) are mainly distributed among the large diamond particles (Fig. 13(a)). The copper matrix is flat without nodules and voids. All the diamond/copper interface is compactly combined, suggesting the void-free composites (Fig. 13(b)). The above bimodal diamond composites are cut by UV-laser, to reveal the internal distribution and microstructure (Fig. 14). After cutting, the composites are subjected to surface cleaning using the mixed solution of hydrogen peroxide and ammonium hydroxide. Similarly, the small diamond particles (60 μm, 40 μm, 30 μm) are mainly distributed among the large diamond particles. No internal voids can be found and the diamond/copper interface shows compactly combined. It can be concluded that the proposed bimodal diamond composites are successfully synthesized. The diamond content of the bimodal diamond composites is measured according to the Eq. (4), as shown in Table 2. Compared with our previous report [25], the diamond content of bimodal diamond composites is higher than that with single diamond particles (42.0 vol%), which is due to the dense packing distribution of bimodal diamond particles shown in Fig. 12(b). However, the improved space of diamond content is still large, which may be resulted from the pressure-free condition of electrodeposition and compression-free distribution of diamond particles. Additionally, the bimodal diamond (100 μm + 40 μm) composite sample has the highest diamond content. Too large diamond particles cannot fully get into the space formed by the main diamond particles, while too small diamond particles cannot obtain the high diamond content. Table 3 shows the TC values of Cu-diamond composites reinforced with single diamond (100 μm) and bimodal diamond (100 μm + 40 μm). The bimodal diamond particles improve the diamond content up to 60.1%, and the TC of the composite sample increases to 651 W/m K, which is larger than that with single diamond particles (615 W/m K) [25]. Since there are two sizes of diamond particles in the composites, the theoretical TC model is complex and single diamond H-J model and DEM model are not applicable. Although the quantitative measurement of each bimodal diamond content is unavailable for electrodeposition, the double EMT model (DEMT) is useful for understanding the detailed TC mechanism of bimodal diamond composites, as shown in Eq. (8) [42].

ions and the low rate of electrodeposition (Fig. 10). Moreover, the strong convection enhances the adsorption of DVF-C selectively by increasing the flux of chloride ions, but has no effect on the adsorption of DVF-B [41]. The high concentration of DVF-C may be another reason for reducing the copper deposition rate.

3.4. Bulk Cu-diamond composites synthesized with different time In order to verify the above electroplating conditions, bulk Cu-diamond composites are synthesized for 70 h. The composites produced with different plating time are examined by SEM, as shown in Fig. 11(a–c). Utilizing the optimal synthesis parameters (20 mA/cm2, 25 °C, 150 rpm), void-free composites are produced as the electroplating proceeds. Diamond particles are distributed uniformly and combined compactly with copper matrix during the whole synthesis process. To examine the inner microstructure, the bulk Cu-diamond composite sample (70 h) is cut by UV-laser, revealing the sliced profile image, as shown in Fig. 11(d). No voids can be seen inside the bulk composites, suggesting that void-free composite sample with the compactly combined diamond/copper interface is successfully synthesized.

Fig. 11. SEM images of composites synthesized with different electroplating time: (a) 120 min; (b) 240 min; (c) 70 h; (d) the sliced Cu-diamond composites synthesized with 70 h. 6

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Fig. 13. SEM images of composites reinforced with bimodal diamond particles: (a) 100 μm and 60 μm; (b) 100 μm and 40 μm; (c) 100 μm and 30 μm. (d–f) are the corresponding enlarged SEM images of (a–c).

Fig. 14. SEM images of the sliced composites reinforced with bimodal diamond particles: (a) 100 μm and 60 μm; (b) 100 μm and 40 μm; (c) 100 μm and 30 μm. (d–f) are the corresponding enlarged SEM images of (a–c).

and

Table 2 The density and diamond content of bimodal diamond composites. Bimodal composites

100 μm + 60 μm

100 μm + 40 μm

100 μm + 30 μm

Density Diamond content

6.241 g/cm3 50.0 vol%

5.960 g/cm3 60.1 vol%

6.046 g/cm3 53.6 vol%

kp

kc =

kmeff

2 ⎛ eff − ⎝ km ⎛1 − ⎝

kp hr 1

kp

+

eff

km

kp

− 1⎞ V1 + ⎠ kp hr 1

⎞ V1 + ⎠

kp

+

eff

km

2kp

+

eff

km

hr 1

+2

(8)

where

( − (1 −

2 kmeff

= km

kp

kp

km

hr 2

kp

km

+

) )V

− 1 Veff + kp hr 2

eff

+

kp km kp km

2kp

+ +

hr 2 2kp hr 2

+2 +2

V2 1 − V1

(10)

r1 and r2 are the radiuses of large and small diamond particles, respectively. V1 and V2 are the diamond content of large and small diamond particles, respectively. V_eff refers to the effective matrix reinforced with small diamond particles, and km_eff refers to the TC of the effective matrix. The parameters are shown in Table 4. Such numerical results explain the TC mechanism of bimodal diamond composites. With the addition of small diamond particles, the TC of the effective matrix is improved to 462 W/m K (according to Eq. (9)), which is higher than copper matrix (400 W/m K). Thus, the TC of bimodal diamond composites is improved by such high TC matrix (according to Eq. (8)), as shown in Fig. 15.

+2

hr 1 2kp

Veff =

(9)

Table 3 The TC of Cu-diamond with single diamond and bimodal diamond.

Previous report [25] In this study

Diamond

r (μm)

Vp (%)

ρ (g/cm3)

c (J/kg K)

α (mm2/s)

k (W/m K)

Single Bimodal

100 100 and 60 100 and 40 100 and 30

42.0 50.0 60.1 53.6

6.63 6.24 5.96 6.04

415.50 420.26 425.63 423.02

223.2 240.1 256.6 239.7

615 630 651 612

7

Diamond & Related Materials 98 (2019) 107478

Y. Wu, et al.

Table 4 Theoretical parameters for DEMT model calculation with V1 = 40% and V2 = 20%. kp (W/m K)

km (W/m K)

h (W/m2 K)

r1 (μm)

r2 (μm)

V1 (%)

V2 (%)

V_eff (%)

km_eff (W/m K)

kc (W/m K)

1700

400

4.76 × 107

50

20

40

20

33.3

462

635

[8] [9]

[10] [11]

[12]

[13]

Fig. 15. The theoretical TC values of km_eff and kc calculated with different V2, according to DEMT model.

[14]

[15]

4. Conclusion

[16]

Void-free Cu-diamond composites with high TC were successfully synthesized by electroplating. The detailed electroplating conditions had been investigated. The current density needed to be lower than 40 mA·cm−2 for restraining the formation of copper nodules and gaps. At the high temperature over 40 °C, large copper grains formed on the surface of copper matrix and the electroplating efficiency decreased, which was due to the deactivation of additives. Forced convection from 80 to 300 rpm was required not only for the transfer of cupric ions and additives, but also for the high efficiency of electroplating. To verify the electroplating condition for void-free microstructure, a bulk Cu-diamond composite sample was produced with different plating time. Under the optimal conditions, bimodal diamond particles reinforcing composites were produced and the diamond content was higher (60.1%) than that of single diamond particles (42.0%). Thus, the TC of the bimodal diamond composites was improved to 651 W/m K. The DEMT model was used to explain the improved TC mechanism of bimodal composites. This work showed the detailed electroplating conditions for synthesizing void-free Cu-diamond composites and proposed a bimodal electroplating method, which was helpful for improving high TC composites.

[17] [18]

[19]

[20]

[21]

[22]

[23]

[24]

Acknowledgments

[25]

This work was supported by the National Defense Science and Technology Innovation Special Zone project (1816321TS00107401).

[26]

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[27]

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