Novel Ultrasonic Flip Chip Bonding Approach Utilizing Electroplated Aluminium Pillars for Advanced Packaging

Ultrasonic Flip-Chip bonding (US-FCB) is a potential integration method for narrow pitch ranges (less than 100 μ m) [1], enabling the integration flexibility of individual chips into a package. A mono-metallic interconnect is more reliable and involves fewer fabrication process steps compared to solder reflow. The US-FCB was primarily investigated for metals such as gold (Au) and Copper (Cu) due to their material reliability, superior conductivity and technology availability [2]. However, both Au and Cu present integration challenges that often require additional metallization layers and elevated bonding temperatures [3], [4]. A new alternative approach is the use of aluminium (AI), offering mono-metallic interconnection at temperatures below 50°C [5], [6]. The enabling technology for Al US-FCB is the electrochemical deposition (ECD) of Al from ionic liquids at the wafer level [6], [7]. Utilizing the same pillar fabrication method as for Cu or Au, the same benefits in geometric flexibility and thickness variation can be achieved. In comparison to Cu or Au, the use of Al as an interconnection material in US-FCB reduces the need for additional metallization layers on microelectronic contact pads.