Researchers at Sandia National Laboratories have designed a method to fault-test integrated circuits quickly from the back using a laser. Light-induced voltage alteration scans a laser over a chip and watches for voltage changes. It improves both the speed and accuracy of back-of-the-chip testing. Focused ion beam cross section (above left) shows the short from metal 1 to metal 2. Thermally induced voltage alteration locates a metal short (above right, end of the dark vertical line). Photos courtesy of Edward Cole. The technique can analyze a chip's logic states and localize a defect to 500 nm, even up to the fifth layer of metal. Because chips typically are produced with only the back side open, a rapid, precise method of testing from the back helps avoid the problems of gaining access to the front of the chip. Edward Cole Jr., a technical staff member at Sandia, worked to develop the method, which scans a chip in less than 15 seconds. It can be used for failure analysis testing of ultrahigh-reliability microelectronics used in medicine, avionics and satellites. It is critical to know, he said, whether defects are a fluke or if they reflect manufacturing flaws that may turn up in products already in use. Voltage signals To test an integrated circuit, the power supply current to the chip is held constant while a scanning optical microscope powered by a 400-mW, 1064-nm Nd:YAG laser scans across the back side. The rate at which photons create electron-hole pairs in the silicon is different if the circuit is defective. Defective circuits respond to voltage signals much differently than satisfactory ones. These changes can be mapped with image processing software to point out the defects. Cole and his colleagues also have been developing a complementary method, known as thermally induced voltage alteration. This technique, which uses a 1300-nm Nd:YAG laser at 435 mW to heat the chip, also creates voltage changes. In empirical tests, he said, the method performs very quickly and can locate a short on an unknown device in less than a minute. The two techniques could be used together, Cole added, because things that show up in one test do not show up in the other.