Room-temperature direct bonding of CMP-Cu film for bumpless interconnection

As the trend of microelectronic systems moves toward higher performance and speed, ultra-high density interconnection technology must be developed. To satisfy this requirement, a new concept called bumpless interconnection for next generation of packaging is proposed, which might bridge to global interconnection on chip. This technology will be most suitable and inevitable for ultra-high density interconnection when pad and pitch sizes are reduced to a few micrometers. Also the combination of a thin chip and a flexible substrate will be required for such interconnection since pads in the size of micrometers can not cope with the thermal stress in the bonding process. The surface activated bonding (SAB) method enables direct bonding at room-temperature. Thereby the SAB method is considered to be a most appropriate method for bumpless interconnection. Another requirement for bumpless interconnection is the bonding between Cu thin films because Cu is the most promising conductive material. Since SAB method requires no heat, large initial contact area must be maintained to obtain enough interconnection, For the purpose, the surface of Cu thin film must be highly flattened, for example, by Cu process. In this paper, a few fundamental experiments and preliminary results of investigations on the feasibility of CMP-Cu direct bonding at room temperature for bumpless interconnection are presented.