Device Test
Once the front-end process has been completed, the semiconductor devices are subjected to a variety of electrical tests to determine if they function properly. The proportion of devices on the wafer found to perform properly is referred to as the yield. Manufacturers are typically secretive about their yields, but it can be as low as 30%.
The fab tests the chips on the wafer with an electronic tester that presses tiny probes against the chip. The machine marks each bad chip with a drop of dye. Currently, electronic dye marking is possible if wafer test data is logged into a central computer database and chips are "binned" (i.e. sorted into virtual bins) according to predetermined test limits. The resulting binning data can be graphed, or logged, on a wafer map to trace manufacturing defects and mark bad chips. This map can be also used during wafer assembly and packaging.
Chips are also tested again after packaging, as the bond wires may be missing, or analog performance may be altered by the package. This is referred to as "final test".
Usually, the fab charges for test time, with prices in the order of cents per second. Test times vary from a few milliseconds to a couple of seconds, and the test software is optimized for reduced test time. Multiple chip (multi-site) testing is also possible, since many testers have the resources to perform most or all of the tests in parallel.
Chips are often designed with "testability features" such as scan chains and "built-in self-test" to speed testing, and reduce test costs. In certain designs that use specialized analog fab processes, wafers are also laser-trimmed during test, to achieve tightly-distributed resistance values as specified by the design.
Good designs try to test and statistically manage corners: extremes of silicon behavior caused by operating temperature combined with the extremes of fab processing steps. Most designs cope with more than 64 corners.
Read more about this topic: Semiconductor Growth
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