An Improved Model of Electrical Contact Resistance of Pad-Probe Interaction during Wafer Test

Semiconductor wafer test requires a stable electrical contact resistance between each individual I/O pad and the probe needle. Due to the strong sensitivity of thin layers of IC's to mechanical stress, low-force probe cards are mostly used. Those probes face the challenge of a randomly increased and instable electrical contact resistance during continuing insertions, whose root cause is not fully understood yet. Therefore, we firstly validated the contact behavior of a single probe on different pad metal alloys and layer thicknesses with respect to Holm's theory. We could confirm the force and material dependency of the contact resistance for Au and Al pad metallization's but we did observe a very strong pad metal thickness influence. Further on we evaluated the effective contact area related to probe tip diameter and surface topography, which has a significant effect on the probability for an increased and instable electrical contact resistance. As proven in high-resolution 3D microscopy strong variances regarding probe tip shape and roughness within a probe card were observed. Based on the experimental findings we could develop an improved model how the roughness and pad material built-up is related to the alpha-spot formation and thus the contact resistance stability.