ANALYSIS OF SHORT-CHANNEL MOSFET BEHAVIOR AFTER GATE OXIDE BREAKDOWN AND ITS IMPACT ON DIGITAL CIRCUIT RELIABILITY

摘要

In this paper we analyze the electrical performance of short channel MOSFET's after gate oxide breakdown. We show that dissimilar post-breakdown nMOSFET characteristics are consistently explained by the location of a constant-size breakdown path. For the simpler case of hard breakdown, device simulations with the breakdown path modeled as a narrow inclusion of highly doped n-type silicon well reproduce all post-breakdown nFET characteristics, including the substrate current behavior. An equivalent circuit model, describing the gate current in an nFET after gate-oxide breakdown is proposed. Also the model can be extended to the more complex but realistic case of soft breakdown. Then the influence of FET gate oxide breakdown on the performance of a ring oscillator circuit is studied using statistical tools, emission microscopy, and circuit analysis. It is demonstrated that many hard breakdowns can occur in this circuit without affecting its overall function. SPICE simulations of the ring oscillator with the affected FET confirm the measured influence of the breakdown on the circuit's frequency, the stand-by and the operating currents. It is concluded that if maintaining a digital circuit's logical functionality is the sufficient reliability criterion, a non-zero probability exists that the circuit will remain functional beyond the first gate oxide breakdown. Consequently, relaxation of the present reliability criterion in certain cases might be possible.