January - February 2003 The International Reference for Chip-Scale Electronics, Flip-Chip Technology, Optoelectronic Interconnection and Wafer-Level Packaging Email the editor

By Otto P. Weeden III, Keithley Instruments, Inc., Tempe, Arizona

Effective parametric testing requires tight control of contact resistance along the signal path, which is affected by the test head, cabling, probe card, probe needle, and bond pad on the wafer.

Parametric testers are widely used for semiconductor product and process development, and for production monitoring.

Test signal integrity depends on a high-quality wafer probe contact. A major detriment to signal quality is contact resistance (CRes), which has become more important as signal voltages drop, contact pressures decrease, and new device technologies draw higher currents. High CRes tends to reduce probe test yields, because it usually requires re-probing to check results.

Contact Force

Contact resistance depends heavily on bond pad and probe materials, the condition of the probe, and the amount of scrub by the probe tip on the bond pad (a function of probe overdrive).

Contact forces that are too high damage bond pads. If these forces are too low, the tip may not scrub through the pad oxide layer, and will produce high CRes and unreliable results. Tip force increases linearly with additional Z motion (vertical travel).

Smaller diameter needles with longer extensions are typically employed to reduce tip force. For most pad materials, tungsten-rhenium (97%-3%) probe tips are often a good choice. They possess a tight lattice structure that produces a smooth surface, which is unlikely to attract contaminants, is easy to clean and manifests a stable CRes.

Overdrive

The term "overdrive" refers to the amount of Z travel after initial probe touchdown. The standard is 1-3 mils (25-76µm). Proper overdrive ensures that all probes contact the bond pads and provide scrubbing action.

Overdrive also depends on tip shape, probe force, tip diameter, planarity of the probes and bond pad material. Too little overdrive results in poor connection and high CRes. Excessive overdrive may result in device damage.

Probe tips before and after cleaning are shown. Contact resistance is strongly influenced by tip dimensions, material, shape and condition, among other variables.

Planarity

As probes are repeatedly flexed, fatigue affects stiffness, resulting in planarization changes. Poor planarization causes excessive overdrive, premature probe wear, overstressed and damaged probe needles and probes that scrub off the pad.

Planarity is the vertical distance between the highest and lowest probe tip and is specified as a total indicator reading (TIR). In most cases, maximum planarity should not exceed 1.0 mil (25µm) TIR. Planarization can be assessed by examining scrub marks and probe tip wear.

Probe Tip Wear and Contamination

Probe tip wear and the resulting contamination cause an increase in CRes. The best way to enhance long-term performance of probe tips is to incorporate periodic cleaning in the test protocol. This gain must be weighed against its cost: reduced test throughput while the prober is out of service, and reduced probe life (because cleaning removes some of the tip material).