Inspecting the wafer test
Inspections performed after wafer test can analyze the marks left by probe cards to ensure that the test process itself did not compromise wafer quality.
by Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 5/20/2008 12:22:00 PM
Wafer inspection has become a critical part of the semiconductor manufacturing process. Inspections performed after wafer test can analyze the marks left by probe cards to ensure that the test process itself did not compromise wafer quality. But damage to a particular die on a wafer can be remedied only by designating that die for rejection later on. As manufacturers are pressured to increase throughput, increase yields, and reduce costs, they need to find ways to detect process-related damage quickly and, as much as possible, prevent process-related damage from occurring at all.
Nathan Little, executive VP for Rudolph Technologies, contends that examining the marks left by wafer-probe cards reveals much about the quality of the test. "Traditionally, we looked at individual probe marks on the wafer for a go/no-go decision," he said. "The marks may have occurred in the functional area of the die, for example, or they may be positioned too close to the edge. You can correlate those marks to infer the 'goodness' of the prober and the probe card. A good probe card ensures a more accurate test and minimizes the chance of damage to the die."
Little continued, "For example, a mark's size and geometry can tell you a lot about the probing operation. Depending on the type of probe card used, a geometrically perfect probe leaves only a pin prick or an oval mark elongated along the scrub axis on the test pad on the wafer surface. If the probe is squished or bent or touches down on the pad with excess force, it leaves a distinctive trace that shows some horizontal as well as vertical movement. A bent probe tip will scratch the wafer surface as it moves across the pad."
Little explained that you may need to inspect multiple probe marks in order to get a complete picture. He said, "A fairly clear mark on one side of the wafer coupled with a faint mark on the other side may indicate that the probe card is tilted. The probes do not make contact with the wafer at the same time or with the same force. You also want the probe to come down in the middle of the test pad. If on one side of the wafer the mark is skewed to the southeast and on the other side it is skewed to the northwest, the card may be rotated slightly from its ideal position. In each case, software that analyzes these pieces of information can issue alerts that allow making appropriate corrections before the process actually produces any out-of-spec die."
According to Little, another approach takes one step back from the wafer-probing operation itself. Instead of looking down at the wafer, this inspection step looks up at the probe card with a probe-card analyzer.
"We inspect all three dimensions of the probe tip under load and no-load," he said. "Inspecting multiple tips on the probe card simultaneously, we can assess its planarity. We also look for worn tips, as well as electrical characteristics such as capacitance and electrical planarity [constant electrical contact]. If you change probe cards on a tester, this type of inspection step ensures that when the prober places the needles on the wafer they hit the center of the pads.
"Wherever possible, we isolate the inspection information down to the particular tip that is causing the problem. The customer can either repair that tip or replace it, depending on circumstances. Probe-card manufacturers have different strategies as to how much they want customers to perform such repairs. In some cases, especially with the most complex cards, the customer returns them to the manufacturer for repair. Customers may be able to repair less complex cards themselves, or—if they are inexpensive enough—can scrap them altogether."
Little concluded, "In the past, wafer manufacturers would typically perform probe testing for a predetermined time, then remove the probe card for inspection. By putting multiple probe-mark inspection capability in-line, they can now inspect quickly and more frequently. Predicting when the prober should be pulled from the line for further inspection and repair dramatically reduces the cost of preventive maintenance as well as the likelihood of damage to the wafer. With the size, complexity, and cost of today's probe cards, manufacturers need to ensure maximum uptime and maximum productivity. We see this type of multiple probe-mark inspection becoming increasingly common."
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