Cu-Cu direct bonding technology using ultrasonic vibration for flip-chip interconnection

In this study, we evaluated effects of both the oxygen density and aspect ratios of bumps which are the key elements on Cu-Cu bonding using ultrasonic vibration for flip-chip interconnection technology. We assumed that aspect ratios of bumps have conflicting characteristics against transmissibility of ultrasonic energy and bump deformation which is to absorb adjustment error of coplanarity (between the bonding head and the stage) of a bonding machine and variation of bump heights. We used TEG chips with different bump heights and conducted FEM analysis of bumps with different aspect ratios. Deformation amount in both longitudinal and lateral directions was calculated, done under the same loading condition as under the actual bonding operation, and, then, correlation of the calculated result with actual bonding performance, in short bondability was studied. Furthermore, in connection with an issue of oxidation which retards bonding, we investigated into the possible effect from difference of oxygen density by bonding in both air and nitrogen-blow environment. As a result of the overall study, we succeeded in bonding all of the 1,512 bumps in nitrogen-blow environment using 40 µm-high bumps.