In integrated circuits chips are connected to the substrate with an electrical bonding. Generally Au bondwires are used on Al chip-pads with a Ni substrate; - denoted as an Al/Au/Ni system. The system can be used up to 150°C. At higher temperatures up to 250°C the system fails due to intermetallic phase growth also known as 'purple pest'. In this talk we present an alternative to the Au/Au/Au system for high temperature assembly, as a gold metallization on the chip side is expensive and not compatible with the CMOS processing. We tested the connection between the AlCu/W pad on the chip and the AuNi pad on the substrat with Al bondwires for different AlCu thicknesses.

To determine the reliability of the bonding the samples are stored at three different temperatures, 250°C, 300°C and 400°C respectively. Resistance and pull force measurements have been made for the characterization at different points in time. For resistance measurements we used a four wire technique. This measurement just takes connection between the aluminium wire and the pad into account. Due to the layout of the chip-pads the transition resistance between the aluminium wire and the AlCu pad and the AlCu/W pad has been measured. Since the testing pull force measurement method is destructive, we prepared a corresponding quantity of bonds. The bonding strength of the bond wires have been evaluated over a period of 1000 hours with different time intervals. In each case 10 resistance tests and 20 pull force measurements for each of the three temperatures have been made.

The data shows that the resistance of the wire connection to the AlCu/W pads remains unchanged for a 250°C constant storage independent of the AlCu thickness of the pads. At a storage temperature of 300°C and 400°C the slope of the resistance depends on the thickness of the AlCu layer. After a 'burn in' process the pull force measurements of the bondwire on AlCu/W pads shows a constant behaviour at all three temperatures independent of the AlCu thickness.

Also the AuNi pads show an increasing resistance of factor three during the 'burn in' process, in the case of 250°C and 300°C the resistance value decreases around some percent over 1000 hours. At 400°C storage temperature the resistance value drops within 500 hours strongly and rises to 1000 hours again. At the beginning also the pull force values reveal a 'burn in' process. After 300 hours the values of the pull force are still above the minimum value in MIL-STD-883G standard at all temperatures. While the values are nearly constant at 250°C, the pull force values indicate a decreasing tendency. In comparison to this the values at 400°C storage get below this minimum value after approximately 700 hours.

We could show by SEM analysis that the connection of the aluminium bondwire and the AuNi pads is affected by the 'purple pest', which depends on the storage temperature. The low-cost AlCuW/Al/AuNi system described here has been successfully tested by now for 1000 hours.

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1. Material characterization for packaging of high temperature integrated circuits