Cu/Diamond Interfacial Adhesion Improved By Using Amino Group Modified Diamonds to Electroplating of Cu/Diamond Composite
Authors: Naruse Yuto 
Published: December 2020

Introduction

Heat spreaders are used to ensure operation of electronic devices, such as smart phones, pads, LED and screen displays, by dissipating heat generated form integrated circuits. The ideal material working as heat spreaders should have high thermal conductivity. Cu and Cu alloys have been used as heat spreader materials from this point of view, but their heat dissipation performances are approaching their upper limit. Therefore, a heat dissipation material exhibiting even higher thermal conductivity is required for future electronics. Meanwhile, carbon materials are expected as next-generation heat spreader materials due to their extremely high thermal conductivities. Our group has previously reported on electroplating of Cu/carbon composites. In this study, we focused on electroplating of Cu/diamond composites because diamond is an interesting filler material that do not require orientation control due to its isotropic thermal conductivity.
Typically, it has been suggested that the major issue for Cu/diamond composites is the adhesion at Cu/diamond interface because Cu and diamonds show low affinity. Surface modification with a silane coupling agent has been reported to be effective for improving the affinity of the interface, leading to improvement in thermal conductivity by reducing the thermal barrier at the interface [1]. Amino groups (-NH2) are expected to be attracted to the cathode side along with Cu2+ because it is positively charged in acidic solutions and to improve the Cu/diamond interface by forming complex with Cu. In addition, it has been reported that the -NH2 modification of BN improves the thermal conductivity of BN/epoxy resin composites [2]. In this study, we performed electroplating of Cu/diamond composites using -NH2 modified microdiamonds and investigated its effect on adhesion at Cu/diamond interface.
Experimental
The -NH2 modified diamonds were prepared beforehand by the following procedure. As a pretreatment, the microdiamonds (MDs: average diameter of 180 μm) were heated at 1350 ℃ for 2 hours in a vacuum (5 Pa or less), and subsequently heated at 425 ℃ in air. The pretreated MDs were added to 50 cm³ of toluene solution containing 5 wt% of 3-aminopropyltrimethoxysilane (APTMS) and stirred for 6 h at 65 ℃ for silanization. Then, 30 ml of methanol was added to prevent further reaction of unreacted APTMS molecules. -NH2 modified MDs were collected by filtration, sequentially washed with methanol, water and acetone and dried at 80 °C in a vacuum oven over night. Existence of -NH2 on diamond surface was confirmed by using ninhydrin reaction.
Electroplating was performed at room temperature. The bath composition was 1.0 mol/dm3 CuSO4.5H2O aqueous solution adjusted to pH 2.0 using H2SO4. The -NH2 modified MDs were added to the bath. The cathode used was Cu and the anode used was Pt coil, the reference electrode was Ag/AgCl. Galvanostat was used to apply a constant current of 10 mA/cm2. The quantity of electricity was 250 C (theoretical film thickness of 92 μm). The adhesion at the Cu/diamond interface in the composite plating was evaluated by observing the surface and cross section using a scanning electron microscope (SEM).
Results and Discussion
Protrusions were formed on the surface of microdiamond after silanization. Silane oligomer aggregates formed by the self-condensation reaction of APTMS may have bonded to the diamond surface as reported elsewhere [3]. The ninhydrin solution turned from colorless to purple by ninhydrin reaction, implying the presence of -NH2 on modified MDs. From observation of Cu/diamond interface in the composite plating, well adhesion was obtained when using -NH2 modified MDs while an obvious void was observed when using pristine MDs. The result indicated that modification of diamond surface by -NH2 was effective to improve the adhesion at Cu/diamond interface.
Conclusions
MD surface was modified by -NH2 groups through silanization. Surface and cross-sectional observation of Cu/diamond composite platings using -NH2 modified MDs showed well adhesion at Cu/diamond interface. It was found that -NH2 modification of diamond surface was effective to improve the adhesion between Cu and diamond.
Acknowledgement
This work was partially supported by the JST-OPERA Program, Japan [grant number JPMJOP1843] and “Knowledge Hub Aichi”, Priority Research Project and Aichi Prefecture Research, Development Subsidy from Aichi Prefectural Government.

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