Interface structure regulation and thermal expansion coefficient of tungsten-coated diamond/copper composites

Diamond/copper composite is considered to be the most potential new generation of electronic packaging materials due to its excellent properties such as high thermal conductivity and low thermal expansion coefficient in theory, which is expected to solve the difficult problems of heat dissipation and thermal stress accumulation of electronic components. However, the diamond and copper (Cu) matrix are not wetted, and the thermal expansion coefficient of the prepared composite is large, which is difficult to meet the requirements of electronic packaging. Therefore, an interface layer was deposited on the diamond surface between diamond and Cu matrix by magnetron sputtering method to improve the wettability between diamond and Cu matrix. The thermal expansion coefficient of the composite is optimized by adjusting the interface structure (phase), and the influence mechanism of the interface structure on the thermal expansion coefficient of the composite is expounded.

Firstly, tungsten (W) coating with thickness of 300 nm was deposited on the diamond surface by magnetron sputtering. Secondly, the tungsten coated diamond particles are subjected to vacuum heat treatment at 900°C ~ 1200°C held for 1 h ~ 4.5h with the condition of the rise and fall rate of 5°C/min. The use of high temperature conditions to promote the diffusion of C and W atoms and chemical reaction, to achieve the regulation of the coating phase composition. The graphitization degree of diamond surface after heat treatment was analyzed by Raman spectroscopy, and the phase composition of tungsten coated diamond particles was quantitatively analyzed by X-ray diffractometer. Thirdly, the heat treated tungsten coated diamond particles and Cu are prepared into bulk composites by pressure infiltration method. The principle is to use capillary action and external force to impregnate Cu between diamond particles. The infiltration temperature is 1200°C, the holding time is 15 min, and the infiltration pressure is 4 MPa. The interface of the composites was obtained by electrochemical polishing (current density 1 A/cm, voltage 26 V, electrolyte dilute nitric acid) or picosecond laser cutting technology, and the interface microstructure as well as fracture structure of the composites were observed by field emission scanning electron microscopy. Finally, the thermal expansion coefficient of the composites was measured by thermal analyzer under the air conditions in the temperature range of 50℃~ 500℃ and at the heating rate of 5℃/min.

(1) The effect of heat treatment parameters on the phase transformation of tungsten-coatings on diamond surface. At 900°C, the coatings on diamond surface are composed of a large amount of metal W phase and a small amount of WC phase, in which the content of WC phase is 34.6 wt.%. When the heat treatment temperature rises to 1000°C, there are more WC phase in the coatings, the relative content increasing to 41.2 wt. %. While a small amount of WC phase in the coatings was also observed and the relative content is 19.7 wt.%. When the heat treatment temperature continued to rise to 1100°C, the metal W phase disappeared, and the coatings on the surface of the diamond particles was composed of WC and WC phases, corresponding relative contents of 77.5 wt.% and 22.5wt.%, respectively. The heat treatment temperature is further increased to 1200°C, the WC phase disappears, and the coatings is composed of the WC phase and a small amount of graphite phase. The graphite phase has been further confirmed by Raman spectroscopy. After heat treatment at 1200°C, D, 2D and G peaks were detected on the diamond surface, indicating the formation of graphite carbon of sp2 type on the diamond surface. (2) The influence of phase evolution of tungsten coatings on the interface structure of diamond/copper composite. When the coatings on diamond surface is composed of a lot of metal W phase, there is an obvious gap at the interface of diamond/copper composite, and the interface structure can be described as Diamond/gap/Cu. When the coatings on diamond surface is composed of WC and WC, the interface of diamond/copper composite possesses good interfacial bonding, and the interface structure is Diamond/carbide/Cu. When the coatings on diamond surface is composed of WC and graphite, and the interface of diamond/copper composite has also a good interfacial bonding, and the interface structure can be described as Diamond/graphite/WC/Cu. (3) The change of thermal expansion coefficient of diamond/copper composites with different interface structures. With the gradual transformation of the W phase into the corresponding carbide phase (WC, WC) in the coatings, the coefficient of thermal expansion exhibits a fluctuating trend with initial decrease followed by an increase, consistent with variations predicted by Turner and Kerner models. In cases where the interface structure of the composites is Diamond/WC/WC/Cu, its optimized coefficient of thermal expansion can be as low as 6.35×10 K (50℃). Conclusions: (1) The phase composition of tungsten coating can be fine-regulated by changing the process of the heat treatment, but the temperature is best not to more than 1200°C. Otherwise graphite layer will be formed on the diamond surface. (2) The phase evolution of tungsten coatings significantly affects the interface bonding state of diamond/copper composites, and the diamond/Cu composites whose interface structure is composed of carbide (WC, WC) phases have good interface bonding. (3) The bonding states of diamond/copper composites with different interface structures have a significant influence on its thermal expansion coefficient. With the gradual transformation of the W phase into the corresponding carbide phase (WC, WC) in the coatings, the coefficient of thermal expansion exhibits a fluctuating trend with initial decrease followed by an increase, consistent with variations predicted by Turner and Kerner models. The composite with interface structure of Diamond/WC/WC/Cu has a thermal expansion coefficient as low as 6.35×10 K (50℃). Good interface bonding and the low thermal expansion coefficient and high elastic modulus of the interface layer play a critical role in enhancing load transfer efficiency and reducing the coefficient of thermal expansion for the composite. This work provides a theoretical basis for optimizing the coefficient of thermal expansion of diamond/copper composite and the selection of the interface carbide layer.