Probe Card Simplifies Test and Replacement

Constant price pressure on memory devices drives the need for lower test costs. Many companies increase test throughput by testing more devices simultaneously. In the past, test probe cards have allowed parallel testing of additional die — from 32 devices under test (DUTs) to 64 to 128 — reducing the number of test platforms. Thus, being able to perform a one- instead of four-touchdown (TD) test on a 300 mm wafer can result in throughput gains.

Conservatively, it takes 10 minutes to test a flash die. This is done once before the wafer is changed, which requires an additional two minutes. It takes another three minutes to change the wafer boat; thus, only ~80% of a tester's time is spent testing the wafer. Other requirements, such as setup time and scheduled and unscheduled maintenance, can reduce productive time another 5%. About 60% of the time a tester is not probing wafers is due to prober or tester-related situations; 40% is probe-card related. A company may only have five testers running a given device, each with a one-TD card. If one goes down, throughput drops 20%. Since cards are expensive, users hesitate before buying spares; thus, a system approach to reparability and supportability becomes important.

To address this, FormFactor (Livermore, Calif.) has introduced its Harmony probe card, a plug-and-probe design produced with four identical probe tiles (Figure ). If damage occurs, only the affected quadrant is changed, not the entire probe head. One spare tile replaces any of the quadrants. While a monolithic configuration is restricted by the surface of its layout, the quadrant tiles can be set with the plane of probe tips matching the ceramic across a localized area instead of a large one.

The Harmony bridge (yellow) is built into the test head and bridges the prober, tester and probe card. The force is routed and controlled through the system. The card is between the bridge and wafer, eliminating many test head and prober tolerances, leaving two planes to work with — the bridge plane and the wafer on the chuck. Since the card is tilted in relation to the wafer, the head remains the same. (Source: FormFactor)

A bridge has been developed for the Harmony card to supply a rigid reference surface, reduce soak time and provide automatic tilt correction. It can also be implemented on the tester or prober, reducing probe card components and providing lower system cost.

Probe cards can have thousands of resistors and capacitors. Additional components are necessary for one TD. The probe head takes up almost the whole wafer side of the probe card, leaving little space for components. The Harmony system approach, which considers the tester, prober, and probe card, creates more space by moving components off the probe card.

Most tester companies produce products to address specific issues, and must get the pin electronics into the test head. The probe card requires a good electrical connection to the wafer and tester. NAND flash test is somewhat easier on the system because each die is bigger and has fewer probes (11,000 pins may be required). DRAM has smaller die and may require up to 50,000 pins in the future, causing concerns about force. With 5 g/spring, 10,000 springs could exert 50 kg of force on a system; 40,000 springs could exert 200 kg. Chances for damage to the wire bond pads on the die increase with force.

Planarity also affects pad damage. Poor planarity results in greater probe card overdrive, which generates greater forces and larger marks on device bond pads. There are two components to planarity: the spring tips' natural planarity (affected by the distance across the wafer) and the correlation between the card and the customer's systems. The tilt comes to X slope, which gives Y distance. When extended to 300 mm, the slope remains the same but the distance doubles, as do planarity issues. Since each spring adds more force into the system, the part of the probe card closest to the wafer caused by tilt will overtravel a greater distance, creating larger scrub marks, while the part furthest away may have contact resistance issues and leave invisible marks. Considering the large 300 mm area and the need to make repeatable contacts across it, tilting the card to the system becomes advantageous.

Automatically tuned planarity minimizes the impact of greater forces and enables system-to-system portability. The Harmony bridge allows information gathered by the prober to be used to tilt the probe card into the optimum position, possibly in <3 min. Test systems working together result in less wasted time, force and human intervention, as well as better contact resistance and greater throughput — all enhancing one TD's value.

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