In this study, we investigate the oxidation behavior of copper at temperatures below 300 °C and its mechanism. A methodology to slow down the oxidation rate is then proposed based on the observed mechanism. The oxides formed after oxidation at low temperatures have fine crystal sizes; the rate constants reach 2×10 -15 m 2 /s and 6×10 -14 m 2 /s at 200 °C and 300 °C, respectively. A passivation treatment at 600 °C in nitrogen produces a thin oxide layer composed of relatively large Cu 2 O crystals. The presence of such a layer slows down the oxidation rate constants by an order of magnitude. This study demonstrates that the oxidation of copper at low temperatures is controlled by the grain boundary diffusion. Increasing the crystal size in the surface oxide reduces the oxidation rate significantly.

The criteria for the formation of a CuAlO 2 reaction phase at the Cu/Al 2 O 3 interface are explored. Oxygen solutes up to 2 wt% were introduced into the copper first. The bonding was carried out at 1075°C. The reaction phase was observed only when the oxygen solute in copper before bonding was higher than 1.3 wt%. The CuAlO 2 phase is polycrystalline and covers only part of the interface. The CuAlO 2 grains interact with the crack, which improves the interface strength. As CuAlO 2 is not continuous at the interface, the thermal conductivity of the Al 2 O 3 /Cu/Al 2 O 3 laminate is affected little.

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Direct bonding copper technique employs a eutectic liquid from Cu-O system to bond copper to alumina. Many semispherical voids are found at the Al 2 O 3 -Cu interface after bonding. Both the amount and the size of voids increase with the increase of oxygen content in copper. The void formation is resulted from the reduction of Cu 2 O precipitates during bonding. Based on this observation, an approach to prevent the formation of voids is proposed. The approach introduces a sacrificial coating onto Al 2 O 3 substrate before bonding. The coating consumes the oxygen from the reduction, and the amount of voids is then reduced significantly.

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The thermal cycling reliability of ceramic/metal laminates is critical for their applications in microelectronic modules. A eutectic bonding process is used to prepare copper/sapphire bilayers in this study. Due to elastic and thermal mismatches between alumina and copper, the Cu/sapphire bilayer cannot pass a thermal cycling test. The thermal cycling reliability can be improved through the use of a metallic nickel interlayer. During the bonding process, the nickel interlayer was oxidized first, reacted with alumina to form a NiAl2O4 spinel phase. The thermal diffusivity of the bilayer with spinel interphase remains high after the temperature cycling test.

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