Measurement and analysis of contact resistance in wafer probe testing

When testing IC chips using a wafer probe card, maintaining a low and stable contact resistance is essential. However, the electrical contact between the probe and the bonding pad of the IC chip becomes unstable following repeated probing operations since particles from the chip surface gradually accumulate on the probe tip. The contamination caused by these particles causes the contact resistance to increase. Accordingly, this study develops an experimental procedure for investigating the effect of the particle contamination on the magnitude and stability of the contact resistance. Initially, an experiment is performed to establish the contact resistance between a clean tungsten probe and various specimen surfaces, i.e. aluminum, gold and copper, at various levels of overdrive. Subsequently, an experiment is conducted to investigate the accumulation of surface particles on the probe tip following multiple contacts of the probe with the wafer surface. The extent of particle contamination following 10,000, 30,000 and 50,000 contacts, respectively, is examined using a scanning electron microscope (SEM). The contact resistance of the contaminated probes is then measured at various levels of overdrive. The experimental results are then integrated to establish a suitable tradeoff between the contact resistance, the overdrive displacement, and the number of contacts.The results from the contact resistance experiment conducted using a clean tungsten probe indicate that the surface specimens with a lower resistively generate a lower contact resistance. For example, the contact resistance between the tungsten probe and the copper foil is approximately 100 mΩ, and becomes stable at an overdrive of 45 μm. However, the contact resistance increases with an increasing number of contacts. In general, the probe should be removed for cleaning following 30,000 contacts to ensure that a contact resistance of less than 1 Ω is maintained.