Effect of chemical etching solution on Cu/Al IMC result

To ensure a reliable Cu wire bond on Al bond pad, Freescale has learnt that good IMC plays a very important role. However, unlike Au/Al IMC measurement method which has been well established, Cu/Al IMC measurement method is still relatively new in semiconductor industry. This study was aimed to establish an effective chemical solution to etch off Cu bonded ball to check IMC coverage at the entire ball-bond-to-bond-pad interface, with consistent and reliable result. In this paper, effect of chemical etching solution on Cu/Al IMC result was studied by using different combination of Nitric Acid molar concentration. The key factors were found to be etching temperature and different combination of Nitric Acid molar concentration. The chemical etching method was carefully designed to etch away Cu wire while only retaining the Cu/Al IMC on Al bond pad. The test vehicle used was 668TePBGA-II. Samples were baked at 175C for 48hrs. Cu ball bond was then removed with different combination of Nitric Acid molar concentration. After etching, IMC was inspected under 500x microscope magnification. Images were taken and software was used to accurately calculate IMC % with reference to effective bonded ball area. SEM/EDX was used to confirm the presence of Cu/Al IMC on the surface of Al bond pad. EDX was used to analyze the phase diagram of Cu-Al system to identify the possible IMCs formed between Cu and Al. Through this study, Cu/Al IMC phases were found to be Cu₉Al₄ (69.2 atomic % Cu), Cu₃Al₂ (60.0 atomic % Cu), Cu₄Al₃ (57.1 atomic % Cu), CuAl (50.0 atomic % Cu) and CuAl₂ (33.3 atomic % Cu). The result showed that Nitric Acid concentration from 9 to 11 mol/L yielded the most reliable and repeatable IMC result. Apart from that, other advantages of the final recipe included low price and reasonable short etching time.