Failure Analysis of ECL Memories by Means of Voltage Contrast Measurements and Advanced Preparation Techniques

After 4 - 6 weeks of testing, the computer installed ECL 100K memories showed defects such as loss of stored information. To analyse the failures, the ceramic housing of some samples were opened and the protection layers were removed. The preparation of the memories left any detectable characteristics uninfluenced. By means of SEM scanning and cross sectioning Al-fingers could be found which were located between the bit lines (2nd level metallization) and the collector contacts (lst level metallization). These irregularities appeared in nearly all cells, both the intact and defect cells. To perform voltage contrast measurements in the first level metallization, holes of about 5 ¿m in diameter were prepared into the isolation oxide. As a result of the voltage contrast measurements, the bit line voltage levels were reproducibly too low and the signals of the word lines and the collector contacts of defect and intact memory cells were very different. These results suggested failures in the isolation oxide, which led in conjunction with the Al-fingers to conduction bridges between the bit lines and the substrate or the collector contacts. In order to ensure this suggestion, some Al-fingers were eliminated by applying an etching process as well as soft Nd-YAG laser pulses. In both cases some defect memory cells could be "repaired".