Abstract
Wire bonding continues to remain as the dominant chip interconnect technology in the far backend process, regardless of the shrinkage of microchip Al bond pad size and the increase in the number of I/O connections in the modern ICs. The reliability of IC devices is directly affected by the quality of adhesion between wire bond and microchip Al pad. Many factors, such as the wafer fab process residue and corrosion, creep-induced wire breakage and electrostatic damage, may result in poor adhesion. In this paper, we show a p-channel Field-Effect Transistor (pFET) failure caused by a mismatch in the bond pad size and the wire bond diameter as well as electrostatic damage during wire bonding. The failure analysis results, failure mechanism and the design rule of microchip Al bond pad in wafer fabrication are discussed. FA investigations were performed on the high gate leakage (nA to mA level) issue in the packaged pFET. It was found that two major factors contributed to the failure, namely mechanical and electrostatic damage. The mechanical damage was mainly due to incompatible Al pad size and bond wire diameter. More specifically, in the failed device, the bond wire diameter was larger than half size of the bond pad opening, contrary to the general design rules of wire bonding. The failure to adhere to the design rule resulted in the device failure. In addition, the electrostatic damage during wire bonding resulted in defects of poly Si/gate oxide and thus the high gate leakage. In this paper, the FA results, failure mechanism of the high gate leakage and the bond pad design rule will be discussed. Also, it will be demonstrated that to achieve good bonding quality and eliminate mechanical and ESD damage the diameter of the bond wire should be equal to or smaller than half of the bond pad opening.
