Non-contact wafer testing

Probing ICs while they are still on the wafer normally requires that contact be made between the automatic test equipment (ATE) and IC. This contact is usually made with some form of mechanical probe. A set of mechanical probes will often be arranged together on a probe card, which is attached to the wafer prober. The wafer is lifted by the wafer prober until metal pads on one or more ICs on the wafer make physical contact with the probes. A certain amount of over-travel is required after the first probe makes contact with the wafer, for two reasons:

• to guarantee that all probes have made contact (to account for non-planarity of the wafer)
• to break through the thin oxidized layer (if the metal pad is Aluminum) on the pad

There are numerous types of mechanical probes available commercially: their shape can be in the form of a cantilever, spring, or membrane, and they can be bent into shape, stamped, or made by microelectromechanical systems processing.

Using mechanical probes has certain drawbacks:

• mechanical probing can damage the circuits under the probe pad on the IC^[1]
• repeated probing can damage the probe pad on the IC, making further probing of that IC impossible
• the probe card may be damaged from repeated contact, or become contaminated with debris created by contact with the wafer^[2]
• the probe will act as a circuit and affect the results of the test. For this reason, the tests performed at wafer sort cannot always be identical and as extensive as those performed at the final device test after packaging is complete^[3]
• since the probe pads are typically on the perimeter of the IC, the IC can soon become pad-limited. Shrinking pad sizes makes design and manufacturing of smaller and more accurate probes a challenge