Effect Of Chromium Interlayer On Interfacial Thermal Conductance Across Copper/diamond Interface

As the demand for electronic components with smaller sizes,multiple functions,and higher efficiency increases continuously,heat transfer has become one of the principal challenges facing future microelectronic technology.The application of advanced thermal management materials in electronic packaging has become an effective way to solving the problem of heat dissipation in electronic devices and to improving the performance of electronic systems.Diamond particles reinforced copper matrix(Cu/diamond)composites act as a new generation of thermal management materials for electronic packaging owing to their high thermal conductivity(?),relatively low density,and adjustable coefficient of thermal expansion(CTE)matched with semiconductor materials.As Cu and diamond are difficult to react chemically and Cu does not wet diamond under the common circumstance,weak interfacial bonding is incurred.The ? of Cu/diamond composites is affected by the interfacial bonding state,and large interfacial thermal resistance is derived.As a result,the superior thermal conductivity of diamond cannot be brought into play adequately.Interfacial thermal conductance(h)plays a decisive role in attaining high thermal conductivity in Cu/diamond composites.Metal matrix alloying or diamond surface metallization with carbide-forming elements can improve the interfacial bonding and increase the h and further the ? of Cu/diamond composites.Chromium is an important carbideforming element.Currently,the understanding of the effect of Cr modification is only limited to the change of thermal conductivity of Cu/diamond composites.The effect mechanisms of Cr modification on the interfacial structure and interfacial thermal conductance are still not clear.It is a technical challenge to experimentally survey the interfacial structure and interfacial thermal conductance.In this thesis,Cu/Cr bilayer films are deposited onto a single-crystalline diamond substrate by an unbalanced direct current magnetron sputtering(DCMS)deposition system to form a Cu/Cr/diamond sandwich structure,and the Cr interlayer is further converted to carbide after heat treatment.Then,a time-domain thermoreflectance(TDTR)technique is used to experimentally measure the h between Cu and diamond.By depositing metal films on single-crystalline diamond substrate to simulate the real interfacial structure in Cu/diamond composites,it is thus possible to experimentally measure the h of Cu/diamond.The Cu/Cr/diamond structure was prepared by magnetron sputtering,and the effect of the Cr interlayer thickness on the h between Cu and diamond was clarified.The thesis investigates the heat transport behavior between a 200 nm Cu layer and a single-crystalline diamond substrate inserted by a Cr interlayer having a series of thicknesses from 5 nm up to 150 nm.The TDTR measurements are compared with theoretical predictions to detect the effect of the Cr interlayer thickness on the h between Cu and diamond.The results show that the introduction of Cr interlayer improves the h between Cu and diamond owing to the Cr interlayer with acoustic properties in between Cu and diamond.The h value is 57 MW m-2 K-1 for the Cu/diamond sample without Cr interlayer and dramatically increases to 270 MW m-2 K-1 when the thickness of Cr interlayer is 5 nm.The h value increases with decreasing Cr interlayer thickness because of the decrease in thermal resistance of Cr interlayer.The high h values were observed for the Cr interlayer thicknesses below 21 nm since the phononic transport channel dominates the thermal conduction in the ultrathin Cr.The combination of a diffusion mismatch model(DMM)with a diffusive two-temperature model(TTM)considers both the phononic and electronic thermal transports across the Cu/diamond interface and predicts the h between Cu and diamond more accurately when the Cr interlayer thickness is less than 21 nm.The interfacial structure evolution of the Cu/Cr/diamond sample during heat treatment was investigated by combining magnetron sputtering with heat treatment.The research mainly involves the surface morphology,surface roughness,element chemical state and depth profile information of Cu,Cr,and C elements,and interfacial microstructure for the Cu/Cr/diamond samples subjected to heat treatment.The results show that the heat treatment can promote the reaction between Cr atoms in the interlayer and C atoms in the diamond substrate,and the in-situ growth of Cr3C2 between the Cu layer and the diamond substrate is realized.After different heat treatments(673-873 K,0.5-2 h),the surface roughness of Cu/Cr/diamond samples increases obviously.The effect of heat treatment temperature on the surface roughness of the samples is greater than that of heat treatment time.The Cu grains on the top Cu layer agglomerate and grow during heat treatment.A part of Cr atoms in the Cr interlayer diffuses through the Cu layer and reaches the exterior surface of the Cu layer.Simultaneously,some C atoms in the diamond substrate diffuse through the Cr interlayer and the Cu layer to reach the exterior surface of the Cu layer.The Cr atoms and C atoms form Cr3C2.Finally,a mixed layer of Cr and Cr3C2 with a thickness of 29 nm is formed on the exterior surface of the Cu layer.A 50 nm-thick Cr film was deposited on diamond substrate by magnetron sputtering at 773 K,and the conversion of Cr into Cr3O2 was regulated by changing the length of holding time.The influence of phase composition of the interfacial layer on the h between Cu and diamond was investigated.The results show that the h value is 96 MW m-2K-1 with a metallic Cr interlayer,increases to 168 MW m-2 K1 with a mixed Cr and Cr3C2 interlayer,and decreases to 86 MW m-2K-1 with a completely converted Cr3C2 interlayer.The partial conversion of metallic Cr interlayer into Cr3C2 improves the interfacial bonding and further reduces the acoustic impedance mismatch between Cu and diamond;however,the full conversion of Cr into Cr3C2 reduces the h value due to the lower thermal conductivity of Cr3C2 than Cr.To summarize,this thesis successfully prepares Cu/Cr3C2/diamond samples by high temperature magnetron sputtering and elucidates the formation mechanism of chromium carbide between Cu and diamond.The effects of thickness and phase structure of the interlayer on the h between Cu and diamond are detected.The prediction model of h for metal/nonmetal interface is modified to correlate the experimental data.Based on the above studies,the relationship between the preparing parameters,interfacial microstructure,and heat conducting properties is established.This study points out the direction for improving the heat conduction of metal/nonmetal interface and provides references for interface modification to attain high thermal conductivity in Cu/diamond composites.The results can also be applied to the structural optimization and thermal design of electronic packaging applications.