Research On Preparation Process And Properties Of Diamond/Al Composites For Thermal Management

With the development of the third generation of wide bandgap semiconductortechnology, thermal management materials with higher performance are in demand.Diamond/aluminum composites are able to meet the needs of thermal management ofthe new generation semiconductor materials and devices. So that they have broadapplication prospects, which have excellent properties, such as high thermalconductivity, low density, isotropic and adjustable linear expansion coefficient.However diamond/Al composites are hard to be machined into complex-shaped parts.So it is necessary to explore the near-net shape forming technology for thesecomposites to accelerate their applications in electronic packaging field. Thisdissertation studies the optimization of electroless copper plating process of diamondparticles, hot pressing-liquid phase sintering method and squeeze infiltration method tofabricate diamond/Al composites as well as their thermo-physical properties. Resultsare as follows:Firstly, a thin (210nm or so) copper coating layer with fine grain size (35nm or so)can be obtained on surface of diamond particles using a potassium borohydride-basedsystem. Raising pH value can significantly enhance the stability of the plating bath.However, when the pH value gets exaggeratively high, the deposition rate drops, and itis bad for the thickening of coating. Potassium ferrocyanide has effect of leveling andgrain refinement on copper coating, while methyl alcohol can inhibit the side reactionand raise the deposition rate. The stability of plating bath and coating quality can beassured with combination of the two.Secondly, the hot pressing-liquid phase sintering process is applied for preparingdiamond/Al composites by using copper plating diamond particles. The relative densityof as-prepared composites is larger than95%; the diamond volume fraction is from50%to63%; the thermal conductivity (TC) reaches as high as413W/m·K, while thecoefficient of thermal expansion(CTE) is only9×10-6K-1. Copper coating on diamondsurface can promote the density and the thermal conductivity of composites, but lowerthe coefficient of thermal expansion. Composites with pure aluminum and AlMg5alloy matrix both have weak interface bonding strength due to the presence of an oxidelayer on the interface, then their TC is lower than that of the substrate and CTE isrelatively large. Composites with AlSi12alloy matrix have strong interfacial bonding strength and high thermal conductivity due to AlSi12matrix selective adhesion ondiamond ｛100｝ faces. The eutectic crystalline silicon phase is prone to precipitate ondiamond ｛100｝ faces whose transition layer can be used as heterogeneous nucleationsubstrate; the liquid phase sintering temperature and time impacts the thermalconductivity of diamond/AlSi12composites significantly; Al4C3content in thecomposites grows with extending the sintering time and temperature.Thirdly, diamond preform is prepared by slip casting with TEOS hydrolysis binderfirstly. Then the diamond/Al composites are fabricated by squeeze infiltration. Theobtained composites exhibit relative density lager than98%, volume fraction from70%to75%, TC as high as256W/m·K, and CTE as low as5.8×10-6K-1, whichbasically meet the requirements of thermal management. By increasing the content ofbinder, the sodium oxide and the calcinations temperature is able to raise the strengthof diamond preform. Interfacial bonding strength of the diamond/Al may be enhancedby extending the impregnation time. The transition layer which consists of C, Al and Oelements locally forms on diamond/Al interface during impregnation process. Itsthickness is about300nm.Fourthly, the theoretical CTE of diamond/Al composites is caculated through Kernermodel and Turner model, which is compared with the experimental data. The interfacethermal conductivity (ITC) and TC of the composites is analyzed withHasselman-Johnson model and differential effective medium (DEM) model,respectively. The results showed that: the CTE of the composites obtained by hotpressing-liquid phase sintering process agrees well with the value caculated by Kernermodel. And the ITC of diamond/AlSi12composites has approximately reachedone-fifth of Kapitza ITC. The TC of diamond/Al composites obtained by squeezeinfiltration has reached two-fifth of DEM theoretically predictive value. Optimizingprocess will be necessary to elevate the TC of the composites in the future work.