Effect of in-situ free air ball laser heating on bonding strength and grain structure for copper wire bond

Abstract

As the need for integrated circuits increase year after year, researchers around the globe are constantly experimenting for innovative approaches to use copper wire as a medium of interconnectivity. Copper is being used as an alternative to replace gold to produce interconnections in microelectronics. However, the use of copper wire is known to pose threats to low-k devices with thin structure. The impact of hard copper free air ball onto bonding surfaces is undesirable as it can damage the bond pad and the silicon die. In-situ heating of free air ball is expected to reduce its hardness and mitigate columnar grains. This research paper investigates the effects of in-situ free air ball laser heating on bonding strength and grain structure for 99.999% purity (5N) copper wire bonding. Wire bonding was performed on bond pads consisting of NiPdAu metallization and was wire bonded under 16 different temperature conditions. Results of this study showed significantly higher as-bonded ball shear strengths and reduced columnar grains when in-situ laser heating is applied. The results of this study can be helpful for semiconductor plastic packaging with copper wire bonding on thin die structures by further improving copper wire bonding quality without the need to use excessive bonding temperature, higher bonding force and excessive ultrasonic energy.