Comparative study on delineation of Cu-Al intermetallic using chemical treatment and ion milling

Intermetallic (IMC) is an alloy formed between two or more metals. Understanding IMC growth and formation is important in determining the bond integrity and the conductivity between metals. While Au-Al IMC is easily detected, Cu-Al IMC is hardly seen due to a slower interdiffusion rate between Cu and Al. This Cu-Al IMC can create a big challenge in analyzing suspected wire bond to bond pad interface failures, since defects may be difficult to find, and it makes delineating the different phases of IMC layers difficult. These limitations may lead to inaccurate findings and may result in invalid analysis conclusions. Chemical treatment has been proposed to be a good approach for IMC delineation. We evaluated the solution of hydrochloric acid (HCI) and isopropyl alcohol (IPA) with five different mixing ratios (4:1, 3:1, 2:1, 1:1, and 0:1 by volume) to delineate the Cu-Al IMC. Another methodology, focused ion beam (FIB) ion milling, is a surface preparation technique which uses an ion beam to strip away an extensive two-dimensional surface area for microscopic analysis. A comparative study was performed between chemical treatment and ion milling on the success of delineation of the Cu-Al IMC. Chemical treatment is a lower cost method which provides IMC delineation at a wider area (entire bonding interface), while ion milling is a higher cost method which does at a smaller area (selected bonding region). Both techniques are found to be very useful in IMC analysis for failure mechanism identification and root cause determination.