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Wafer Test Yield Monitoring by Iain Gardner, Software Engineer
When wafers have low yield, the challenge is to find out if the problem was due to the manufacturing or the test process. Comparing wafer yields against a norm can show that intervention is required, and monitoring probe card performance prevents yield loss. Both probe card performance and yield may be checked after testing of a wafer, but by then it is often too late to fix the problem. Real time yield monitors examine the test results “on the fly” — as a wafer is probed — and look for specific patterns indicating that something has gone wrong. Such monitoring has greater value as wafer size or selling price increases. Consecutive fail monitor
The most effective yield monitor for examining test results is the consecutive fail monitor. If a certain number of failures happen in a row, a failure event is triggered. For example, assume a limit of four consecutive failures where P is a passing die and F is a failure:
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The arrows indicate the direction of probing for figure 1. In this case, the outlined die show a sequence of four consecutive failures, triggering the yield monitor. Consecutive fail triggering is more challenging when using a multi-die probe card. Figure 2 shows a wafer being probed by a four die diagonal probe card: P
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Figure 2. With a multiple-die probe card.
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Figure 3. Failures can go unnoticed.
In this case, the per site yield triggers would see the pattern PFFP and would not fail. A comprehensive consecutive fail trigger watches for both types of failure, the per site consecutive fail and the per test result consecutive fail. Die at the edge of a wafer are often unreliable because they have not been correctly processed. To reduce false triggering, starting and ending rows can be established. Bin count monitor
Figure 1. Example of consecutive fail monitor.
P
The test results, as reported by the tester for the last 16 die are PFPP-PFPP-PFPP-PFPP. If the consecutive fail trigger only observes test results as they happen, the consecutive fail on the second probe card site would not be found. Therefore, it is important that a consecutive fail trigger observe test results for each probe card site independently. There may arise a situation where all probe card sites fail simultaneously. This mode of failure may not be caught by a per-site trigger (figure 3).
Over time, it is possible to examine a product’s yield history and predict the number of times a bin code will occur on a wafer. If the frequency is higher than expected, the increase may indicate a test setup or manufacturing process problem. When a bin code limit is exceeded, the monitor takes action automatically, e.g., to confirm that the failing bin code is genuine by retesting both good devices and devices with that bin code. The monitor could even clean the probe card or have the prober confirm that the probe card is correctly aligned with the wafer.
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A bin count trigger can also be used to guarantee a minimum amount of good die. In this case, the trigger ensures that there are at least as many good die as were Summer 1998
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specified in the bin limit file. A more sophisticated version of this type of trigger would examine a running percentage of good die to die tested and only be able to take action after a predefined number of die had been tested. This allows for earlier detection of low yielding wafers. Delta yield monitor
The delta yield trigger catches problems with multi-die probe cards. Generally, good die on a wafer can be randomly distributed across all probe card sites. However, if there was a problem with a particular site, such as dirt on a probe card needle, it would yield lower than the other sites. The delta yield trigger catches this type of problem. At each prober move, there is no guarantee that all sites of the probe card will make contact with die to be tested. This means that the total number of die tested for each site
will differ as the wafer is probed. For this reason, the sites must be compared by the difference in their yields. The delta yield monitor should only be enabled when a predefined number of die have been tested. Where the probe card only makes contact a few times per wafer, it may be better to specify an expected yield for each probe card site. Yield monitor actions
• Retest a good die. If it is still good, continue probing. If it is bad, take action. • Inspect the probe marks on the failing die with the prober. • Using the prober, put the wafer under test back in the cassette, load a correlation wafer, test a few known good die and if the correlation wafer yields as expected, resume testing the wafer under test.
Once a yield monitor has detected a failure, appropriate action must be taken. The simplest thing to do is to halt probing and let a technician determine the cause of the problem. However, more sophisticated actions are possible, including:
Any combination of these and other actions may be used, depending on the most likely failure mechanism. A yield monitor should fix a testing problem or confirm the problem before stopping the test process and alerting a technician.
• Unload the wafer but continue testing the lot. This avoids wasting time by testing the rest of a low yielding wafer.
Use of test floor automation software alerts the user when the yield is less than expected or the probe card has a problem. Reprinted with permission from Test magazine.
For more information call 800.450.5308
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Summer 1998
Yield Management Solutions
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