Reliability Results of 0.8 mil Fine Pitch Copper wire bonding on Immersion Gold Plated Pad for Copper Low-k Devices

Abstract

Wire bonding is one of the common interconnection technologies being used in industry. Requirements for numbers of wires increased, bond pad pitch was reduced in order to maintain in small die size, and higher requirement on the reliability of the wire bonding is required to withstand aggressive environments. Many use gold wire as the interconnect wires. Recently, people start to explore more on copper wire. Copper wire gives better reliability and lower cost compare to gold wire. It becomes more and more popular in industry not only due to the advantage in price but also due to its superior in electrical performance. In this paper, the reliability (MSL 3) of the copper wire (0.8 mil) bonded to gold pads on 0.13 um technology Cu/low-k chips will be explored. Ball shear and wire pull results for high temperature storage (at 150degC) for 1000 hours without molding is shown. The experimental test vehicle consists of a 0.31 mm buildup substrate, a 7 mm times 7 mm Cu-low/k mother die with immersion gold plated on Aluminum bond pad and two 3 mm times 5 mm stacked daughter dies. As only the bond pads of 7 mm times 7 mm mother die are immersion gold plated, focus will be put on the reliability of the copper wire on mother die only. As copper wire is stiffer and harder, generally it gives less wire sweep (after molding it < 6% in this case) compare to gold wire. The minimum bond pad pitch between the mother die bond pads of the experimental test vehicle is 53 um with target bonded ball diameter to 40 um. Challenge in fine pitch wire bonding is small intermetallic coverage on ball contact diameter but copper wires on gold pad give nearly 100% coverage which means perfect intermetallic coverage between bonded ball and bond pad. This paper discusses the process of developing immersion gold and also benchmarking the wire bonding process parameters.