Direct Bonding of an Electroformed Cu Substrate and Si Chip at Room Temperature under Atmospheric Conditions

A Cu-based heat spreader with a smooth Au bonding film was fabricated by electroforming to provide efficient cooling via direct bonding with the semiconductor. The Cu substrates were electroformed using Cu/Au and Cu/Ta/Au seed layers deposited on smooth SiO2/Si substrates; subsequently, they were mechanically stripped at the Au/SiO2 interface. The Cu substrates were directly bonded with Au-metallized Si chips in air after surface activation with argon plasma. When the Cu substrate was electroformed using the Cu/Au seed layer, Cu diffused through the Au film and CuO was generated on the stripped Au surface. Thus, the Cu substrate was insufficiently bonded to the Si chip as CuO increased the surface roughness and prevented intimate contact between the Au surfaces. Meanwhile, the Cu substrate electroformed using the Cu/Ta/Au seed layer had a smooth Au surface without Cu impurities because the Ta layer blocked the diffusion of Cu. Thus, it formed a strong bond with the Si chip, equivalent to the bulk strength of Si. It is expected that a Cu substrate electroformed on a Cu/Ta/Au seed layer has potential as a heat spreader that can directly bond with semiconductors at room temperature under atmospheric conditions. (c) 2019 The Electrochemical Society.