Current-Based Testing for Deep-Submicron VLSIs

tioned the ability to carry out effective currentbased testing (IDDX) for deep-micron VLSIs.1-5 Yet, current-based test methods for such devices are more relevant than ever. The probability of a defect occurring increases exponentially as its size decreases. As the technology scales, even smaller defects may become potential threats to yield. Furthermore, ensuring gate oxide quality and reliability for a multimillion-transistor device under test (DUT) solely through voltage may become unrealistic. Other techniques, such as burn-in, although particularly successful for memories, might not be economically viable for most commercial digital products. Several recent studies have raised concerns about new failure mechanisms in scaled geometries that may be more difficult to detect with conventional means. Nigh et al. reported the existence of many timing-only failures. These failures did not influence the circuit logic functionality; hence, slow-speed SA-based (stuck-at fault) or functional tests did not detect them.6 Similarly, for Intel’s manufacturing processes, Needham et al. reported an increasing shift toward “soft defects” as technology migrated from 0.35 to 0.25 microns.7 These defects do not always cause failures at all conditions of temperature and voltage. According to Needham et al., defects correlate with longterm device reliability. These defects may be due to resistive vias, highly resistive bridging defects, and so on. IDDQ testing can detect some defects, provided background leakages are under control and circuits are designed to make them IDDQ testable. Traditionally, voltage testing and IDDQ testing have had complementary objectives. In logic testing, the stress is on DUT logic correctness, performance evaluation, and detection of catastrophic faults such as stuck-at faults. In IDDQ testing, on the other hand, the focus is on detecting subtle manufacturing-process defects and reliability failures. As the technology scales, the roles for these two types of testing will diverge further. Therefore, effective deepmicron current-based testing can play an important role not only in ensuring VLSI quality and reliability but also in arresting the already escalated costs of VLSI testing.