The Need for Extensive IC Testing
Due to the complexity of the manufacturing processes used in IC production, 100% of all ICs, both logic and memory, are tested to eliminate less-than-acceptable performing units. Defects can have many causes thanks to the numerous steps required to build a finished IC from a bare wafer of semiconducting material. One example is the possibility of gaps in the resistive layers produced via photolithography or electron beam lithography that may create electrical short circuits in the device’s circuitry. Likewise, errors due to dust or debris on an in-process wafer may prevent a trace from being completed during the deposition of conductive materials using vacuum sputtering or electrochemical deposition (ECD) processes.
A dedicated industry has emerged to make the complex equipment needed to test these ICs at high speeds and throughput. The equipment consists of test tools called “probers” and the “probe cards” that reside within them. Probers are machines which transfer individual ICs, or full wafers with many ICs into the test fixture. The IC is known in the industry as the “device under test” (DUT). The prober contains sophisticated electronics and programming to power the ICs, deliver input signals, and collect output data with the goal of testing the proper function of each DUT. This communication is performed through the probe cards, which contain an array of test needles. The test needles, commonly called “probes”, make contact with pads on the DUT. All the input power and data required to perform the test is transmitted through the probes, and feedback is similarly routed through the probe card back into the prober electronics. Since the probes operate in very close proximity, the bulk of the probe length is typically coated with an insulating polymer layer to avoid shorting through adjacent channels.
Application-specific test algorithms identify and record defective devices and note the actual portion of the ICs that are malfunctioning. They also record the location on the wafer where the defective IC originated. The data collected helps troubleshoot and correct potential process issues. All this is done in a very short time, necessitated by the high production rate of these devices.
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