VUV/HCOOH pre-treatment for Al-to-Al ultrasonic bonding

Abstract This study aims to develop surface modification pretreatment for ultrasonic bonding to optimize aluminum-to-aluminum metal direct bonding for multilayer packaging applications. Vacuum ultraviolet (VUV) irradiations in air or formic acid atmosphere were adopted. Experimental results indicate that VUV pre-treatment could alter the functional group bonding on the aluminum surface, especially the increase in the content of Al–OH bonds. The atmosphere of formic acid vapor gave rise to the formation of formate ligands via monodentate coordination mode. An appropriate amount of Al–OH and formate ligands enhance the strength of aluminum/aluminum joints. However, excessive organic acid treatment resulted in incomplete reaction residues, leading to a deterioration in bonding.