A standardized methodology for Al-Cu wire bonded intermetallic compound measurement

Abstract

Copper — Aluminum (Cu-Al) Intermetallic Compound (IMC) is an important physical measurement reference for both wire bondability and product reliability. Most common methodologies for IMC measurement are approaching by vertical direction cross section analysis or chemical solution etching off Cu bonded ball for IMC coverage at the ball bonded area. Moreover, chemical solution etching for IMC measurement is a relatively low cost and time spending method. However, unlike Gold — Aluminum (Au-Al) IMC, Cu-Al IMC color is hard to be told apart from Al and IMC. This study aimed to identify a controlled chemical etching condition to get an idea IMC image within bonded ball area for top view IMC coverage percentage calculation. Also, an IMC coverage percentage calculation method was setup to conduct the relatively consistent results with compared to our traditional method. TSSOP device with 1.0mils Pd (Palladium) coating Cu wire bonded on Al bond pad was selected as the test vehicle. With multiple etching factors experiment by using the same concentration of fuming Nitric acid, the unmolded unit is suggested to etch by 98% weight percent of fuming Nitric acid for 2 minutes in room temperature with proper clean method to make IMC expose ideally. Moreover, for the further IMC coverage percentage calculation, IMC top view image will then be took by OM (Optical Microscope) in 500x resolution. And ImageJ was selected as the software tool for IMC coverage percentage calculation in this study. Analyzed image contained whole bond pad area with Al pad, IMC and Al splash. By converted the photo to 8 bit gray scale image, the threshold intensity between Al bond pad, IMC and Al splash can be calculated systematically. Through histogram analysis, 3 clear stacked distributions will be found within the bond pad area. The coverage area of each composition will be much easy to be defined. The color of Al pad is with the highest intensity, on the contrary, Al splash with the darkest color is with the lowest intensity. Moreover, the color of IMC is at the medium level which is between these 2 intensity regions. In this study IMC coverage percentage was defined as the IMC coverage percentage in the bonded ball area excluded Al splash. Therefore, to get the IMC coverage percentage, the bonded ball area should be defined at first. An area selecting tool in ImageJ will be use to select the bonded ball area. The Al splash excluded bonded ball area will be selected by the software assisting tool with low variation between man to man and good repeatability. However, in the bonded ball area, it is hard to distinguish the color of Al pad and IMC. Therefore, the threshold intensity analysis will be performed on the Al bond pad area to define the Al bond pad threshold intensity by ImageJ. The results will reflect the threshold intensity range of Al bond pad for the IMC percentage calculation. With the range of Al intensity, by using the threshold intensity filtering tool in ImageJ, the Al bond pad area can be captured. By a simple mathematic calculation, ImC coverage percentage will be calculated. The results from this method were compared with our traditional measurement results, the data are matching and the variation is relatively low. Furthermore the man to man variation by this standardized methodology is low and repeatability is also acceptable.