Study on bond pad damage issue in bare Cu wire bonding on SMOS8MV wafer technology

Because of lower cost, better thermal/electrical properties and reliability performances, Cu wire was replacing the traditional Au wire gradually these days in wire bonding IC connections. While Cu was harder than Au, Cu wire bonding would need higher bonding force and more ultrasonic power; which might introduce mechanical damages to bond pad and form cracking or cratering. In this paper the bond pad damage issue was found in a SOIC 32ld device with SMOS8MV wafer technology; and FIB cross section confirmed it terminated in IMD2 (Inter Metal Dielectric) layer just under the pad top metal layer. Optimizations of wire bonding parameters, including EFO (Electric Flame Off) and ball bond parameters, failed to eliminate this pad damage issue; while the experiments found the variations in a same cratering check result and the differences between 2 different cratering check results were so obvious that the improper cratering check method should be suspected as one root cause instead of unoptimized wire bonding parameters. Total 5 cratering check methods were tried on samples built with same bonding parameters and one method of two-step etching under room temperature got the least damaged pad ratio. This method cleared up cratering check noises except on the issued pad with the fixed location. Cross sections found an additional IMD2 layer between metal2 and pad top metal in the wafer structure of the issued pad, compared with that of normal pads; which might weaken the structure and introduce bond pad damages during wire bonding or cratering check. And these two metal layers, metal2 and pad top metal, were stacked together directly to clear up this bond pad damage issue of the issued pad finally. This bond pad issue was solved after tries on wire bonding parameters, cratering check method and wafer structure; and a further research would be continued to unmask the mechanism.