P1E-5 Understanding Ultrasound-Induced Aluminum Oxide Breakage During Wirebonding

A few models have earlier been proposed to explain Al oxide breakage during US wirebonding as presented by L. Levine, but no widely accepted theory exists. We propose a model to describe the AlOx breakage mechanism during tangential US excitation at constant pressure in a wirebonder. The model is based on theoretical estimations and experimental measurements. We measure with a laser Doppler vibrometer the relative wire-base displacement and propose that stick-slip and micro-slip behavior is prevalent during ultrasonic bonding. Displacement was measured at rim of the rectangular 14 mum thick and 80 mum width Al wire and at the silicon microchip base. A rectangular shaped wire was used to have probe light good reflection. We combined displacement measurements, detailed SEM analysis of contact interfaces and FEM of the bond structure during the bonding process. We also made a synthesis of the current bonding process knowledge. As a synthesis we propose a two-step model, including early stage scrubbing and later microweld expansion. Validation of the proposed model is discussed. This work and the obtained results are steps towards a fundamental quantitative US bonding theory that is necessary to develop reliable bonding technologies towards finer-pitch and more reliable interconnections.