A Failure Mechanism of Semiconductor Devices and its Analysis

A group of high gain, small signal, NPN switching transistors were built to a high reliability specification which included pre-encapsulation processing controls and post-encapsulation screening. More than two years after fabrication an excessive number of failures (about 1%) was experienced. These failures were catastrophic in nature, permanently or intermittently open-circuited, and occurred both in fabricated, previously operational equipment, and in fresh, unused stock. Examination of opened units indicated that poor workmanship and quality control were responsible for about half the failures. In each of the remaining units, the fault was determined to be an open circuit in the base, and was localized by electrical microprobing to be in the metallization at the oxide step at the base contact window. However, inspection with optical microscopes at magnifications up to 1000 X did not disclose the exact location of the break in the metallization. (See Figures 1, 2, and 3.) Because of the possible latent nature of the failures, the vendor submitted newly fabricated units in which the metallization process was identical to that of the original process. No essential differences were detected between the older and newer devices. The Scanning Electron Microscope (SEM) was introduced to the problem and spectacularly demonstrated its power as a failure analysis tool. Characteristics of the metallization faults were clearly revealed. Figure 40shows that only about 1000 to 1500 Ã… of aluminum remains after etcIing (but before alloying) of the 8000 Ã… deposited. Some small void areas are also visible at the step.