The present invention is directed generally to circuit testing and, more particularly, to the testing of circuits constructed using solid state fabrication techniques.
After the fabrication of a chip containing one or more solid state circuits, it is common in the industry to require that the chip pass certain tests before being identified as a good part. For example, after the fabrication of a memory device, the memory device is connected to a tester which automatically performs a series of preprogrammed tests on the part. See, for example, U.S. Pat. No. 6,483,333 entitled Automated Multi-Chip Module Handler and Testing System.
Often during the fabrication of parts, particularly new parts, the signals available at the output pins of the part are insufficient to provide the designer with the information necessary to understand how the part is performing. In those situations, diagnostic systems are available such as the system disclosed in U.S. Pat. No. 6,841,991. In such diagnostic systems, probes are brought into contact with various nodes on the circuit to sample and analyze the signals available at those nodes. For that to be performed, the nodes of the circuit must be available to the probe of the diagnostic system. Thus, the part must be tested before fabrication is complete at which time the circuits of the part are accessible only through the part's output pins.
There is a need to be able to access various nodes within a circuit even after a device has been completely fabricated.