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.
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.
The fab charges for test time; the prices are on the order of cents per second.
Chips are often designed with “testability features” such as "built-in self-test" to speed testing, and reduce test costs.