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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.
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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
Yield Management Solutions
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