Surface roughness and adhesion analysis study on ultrasonic gold ball onto aluminium bond pad

Abstract

The advancement of technology in the micro and nano electronic niche has changed the aspects of interconnection of electrical connectivity. Surface adhesion of electronic device fabrication has propelled tribology element on layers and bonding of this devices as an important element. Surface roughness has been an integral part of macro electromechanical systems, in the advent of microelectronics surface roughness has been again a focus research area. In this work, a study on surface roughness attribute in gold ball adhesion on aluminium bond pad was investigated. Using design of experiment approach (DOE), bond force as a critical to function parameter was chosen in this work to study surface roughness influence. A range of different surface roughness of aluminium bond pad on pieces of wafer was created using different plasma settings. Using three different values of bond force, multiple gold ball bonding were performed on the series of different surface roughness on aluminium bond pad for three trials. A modified box-plot was plotted for average ball shear value against surface roughness range. All three trials depicted similar polynomial plot trend with a leading negative coefficient. Using simple differentiation coupled with the standard deviation value from the trials, an optimized range for surface roughness was obtained from the equations created. The optimized ranges of surface roughness deduced from this experiment were from 2.10 nm till 6.38 nm. A final footprint experiment was done to understand and correlate to the ball shear experiments. The footprint experiment shows a better understanding of adhesion without shear value. Footprint adhesion scales were tabled and ranked, using this footprint adhesion plot against surface roughness was plotted. The plot depicted similar polynomial trend as the previous experiments. The highest footprint adhesion was also within the range obtained previously. The adhesion of footprint more than 80% gold ball remnant on the aluminium bond pad were within the optimized surface roughness range of 2.10 nm till 6.38 nm. It can be finally concluded that surface roughness at micro level becomes an important critical attribute to ensure the wire bond quality. The experimental approach has also given new means of obtaining the suitable optimized surface roughness range for a quality and robust wire bond adhesion onto bond pads. The work has successfully established surface roughness importance in the wire bonding process.