Fundamental Research On Oxidation Of AlN Substrates And Direct Bonding Copper Process

With the rapid development of microelectronic technology,the integration level and the chip power density increases steadily,requiring higher and higher heat dissipation ability of the electronic devices and systems.Direct bonding copper?DBC?substrates have widely used in multi-chip power semiconductor module electronic circuits,replacing the traditional PCB ones.Aluminum nitride?AlN?is regarded as one kind of the best ceramic substrate materials in DBC due to its attractive properties,i.e.,high thermal conductivity,dielectric strength and mechanical property and the controllable coefficient of thermal expansion,matching with that of the chips.In this thesis,the polycrystalline AlN ceramic substrates doped with 2 wt%Y₂O₃were oxidized at 1000-1300?in air.Microstructures,growth kinetics and formation mechanism of the oxide layers on the AlN substrates were investigated.The relationships of bending strength and thermal conductivity of the oxidized AlN substrates with the oxide layer thicknesses were also studied.Comparative studies on the Cu²⁺-activated then oxidized and directly oxidized AlN-DBC substrates were finally concentrated on the interface structure,peeling strength and temperature cycling reliability of the AlN-DBC substrates.The oxide layer on the AlN substrate is composed of?-Al₂O₃ nanocrystallines,containing the longitudinal and transverse cracks and interconnected micropores.The nano Al₂O₃ tubular arrays form by oxidation of the AlN grain boundaries and grow into the AlN grains in the direction perpendicular to AlN grain boundaries.Hence,the“core-shell”structures of the non-oxidized AlN core wrapped by the continuous Al₂O₃shell layer are formed.The micropores in the“core-shell”structures connect with the continuous big tubes along the prior AlN grain boundaries,acting as the paths of O₂/N₂input/output during the oxidation process.A linear relationship between the oxide layer thickness and the oxidation time is followed,indicating a reaction-controlled kinetic mechanism of the oxidation of the AlN substrates.The activation energy of the oxidation process is calculated to be 260.5 kJ/mol.With the increase of the oxide layer thickness,the bending strength and thermal conductivity of the oxidized AlN ceramic substrates decrease monotonously,while the coefficient of thermal expansion of the oxidized AlN ceramic substrates increase.Based on the Kim model and the series model,the mathematical models of the bending strength and thermal conductivity of the oxidized AlN substrates changed with the oxide layer thickness were built,respectively.Compared with the previous models,these newly-built ones in this work are closer to the experimental data,in other words,they have higher accuracies.During oxidizing the Cu²⁺activated AlN substrates,CuAl₂O₄ rods and spherical particles generate,resulting in the increase of the surface area and surface roughness of the AlN substrates.The interfacial transition layer of the Cu²⁺activated then oxidized AlN-DBC substrates consists of the Cu oxide layer,Cu-Al-O intermediate layer and Al₂O₃ layer from the Cu foils to the AlN substrates.The peeling strength and temperature cycling reliability of the Cu²⁺activated then oxidized AlN-DBC substrates are apparently higher than the directly oxidized ones.The Cu²⁺activated then oxidized AlN-DBC substrates are mainly peeled off via the AlN substrates,meanwhile,the directly oxidized AlN-DBC substrates through the transition layer.Thus,the formers have higher interfacial bonding strength and reliability.