Analysis of bonding failure in CMOS MEMS chips

During post processing of MEMS chips in deep reactive ion etching (DRIE), fluorine containing gases are used to remove the final passivation layers from the top of the bonding aluminum pad area. These gases contaminate the bonding pads and hinder effective wire-bonding onto the pads. This paper is a comparative analysis of bonding pad cleaning of CMOS MEMS chips by three plasma cleaning methods: argon plasma, oxygen plasma using AS201 microwave plasma Asher and oxygen plasma using SS110A DRIE equipment (Tegal). The effectiveness of the chip cleaning method is then analyzed using Energy dispersive X-ray (EDX) spectroscopy to investigate elemental percentage on the pads before and after cleaning. EDX results indicate that Argon plasma cleaning process is the preferable pad cleaning method as it effectively removes the fluorine contaminants from the bonding pads without eroding the top aluminium (metal 3) on the pad. The oxidation level of the aluminium as a result of cleaning is also very low as compared to the other cleaning methods. Finally, a few of the pads cleaned by argon plasma method were successfully bonded using a manual Ball-Wedge wire bonder Model 7700E.