Thermal-cycling-induced surface roughening and structural change of a metal layer bonded to silicon nitride by active metal brazing

Abstract

We applied thermal cycles with a range of −40 to 250 °C to metalized silicon nitride substrates, to both sides of which copper layers had been bonded using the active metal brazing method, and systematically evaluated the effects of the thermal cycles on the roughening of the surface of the metal layer, and on the changes in the structure of the copper layer. The roughening of the metal surface increased with the number of thermal cycles. The period of the long-periodic component of the roughened surface (surface waviness) was 100–300 μm. This value is almost identical to the diameter of the copper grains. The results of electron back-scatter diffraction pattern analysis revealed strain spanning the entire copper crystal, with severe crystal strain appearing near the high-angle grain boundaries in the copper layers subjected to the above thermal cycles, due to repeated cyclic thermal stress arising from a difference in the coefficients of thermal expansion between the metallic and ceramic layers. In a sample subjected to 1000 thermal cycles, the strain within a grain or in the vicinity of a grain boundary becomes more severe as the distance from the ceramic join decreases, while the strain tends to be smaller at the surface of the metal layer. This appears to be due to the copper grains near the surface being subjected to stress, giving rise to out-of-plane displacement, such that the strains tend to be released.