Bumpless interconnect of 6-μm pitch Cu electrodes at room temperature

Bumpless interconnect of 6-mum-pitch Cu electrodes was realized at room temperature with the surface activated bonding (SAB) method. In this study, we propose a novel bumpless structure, where the electrodes and a surrounding Cu frame are fabricated with the same height to increase bond strength and demonstrate the feasibility of a sealing interconnection between Cu surfaces. The Cu damascene process, assisted by the reactive ion beam etching (RIE) process, was used to fabricate the Cu structures. 1,048,748 electrodes were fabricated in a square of about 6-mm² area; 923,521 connections were placed inside the 10-mum-wide Cu frame. These 923,521 electrodes were arranged into a spiral chain to enable the detection of the positions with insufficient interconnection by electrical resistance measurements. Using the SAB conditions optimized in the previous studies, we found that 744,769 electrodes among them were successfully interconnected. The failure in the electrical interconnection reappeared in some lines near the frame of all samples. The optical beam induced resistance change (OBIRCH) analysis, in which particularly high resistance is presented as a bright contrast, showed that these insufficient connections might be due to sample preparation error rather than a bond defect, because high resistance was observed only at some specific electrodes amid the line in this area. In other well-bonded area, the mean contact resistance was as low as 0.08 Omega; it is considered that a sealing effect was achieved at the frame structure because there was little increase in the contact resistance in high temperature storage test performed at 150degC for 1000 h, in ambient air.