Laser testing of integrated circuits

Comprehensive testing is one of the major impediments to the production of cheap LSI. This problem is caused by the small number of circuit nodes which are directly accessible and the time required for exhaustive testing. A technique is presented in which data are injected directly into the internal circuitry of an integrated circuit by a finely focused laser beam, at rates over 1 MHz, using an electrooptic modulator. A laser power of below 15 μW is needed for MOS circuitry, but larger powers are required for other logic families. In addition to the provision of an input to key nodes, two important semiconductor parameters can be measured by using the laser probe. Large savings in the time taken and ease of both fault detection and diagnosis are envisaged by the use of this test method (which involves little or no modifications to the circuit design of layout) when used in conjunction with conventional test equipment.