Degradation Mechanisms of Silicon Dioxide in MOS Devices: the Roles of Hydrogen and Charge Carriers

Destructive breakdown of the gate oxides in metal -oxide-semiconductor (MOS) devices has been extensively investigated for improving device reliability. In particular, understanding and control of "interface-trap formation" has been at the core of MOS reliability research, and represents a critical step in the fabrication of very large scale integrated (VLSI) circuits. We have investigated the degradation mechanism of SiO_2 in MOS devices in view of the effects of hydrogen and charge carriers involved in the stressing. We propose a unified interface -trap generation mechanism which can accurately predict the course of interface degradation. The unified model predicts that at fluences below 0.001 C/cm^2 , hole trapping near the cathode-SiO_2 interface is induced via impact ionization and dominates interface trap generation; beyond 0.001 C/cm ^2, trap creation due to hydrogen becomes the dominant degradation mechanism until destructive breakdown occurs. In addition, we report new reversible interface traps, and show that these are generated directly due to the presence of trapped holes in MOS devices. We do not believe the origin of these interface traps to be P_{b} centers, nor recombination centers in general. Finally, we propose a new model for the formation of anomalous positive charge (APC) in MOS devices wherein the interaction of hydrogen with a trapped hole produces H^+ ions which interact with the SiO_2 lattice near the Si-SiO_2 interface to produce APC. We have determined that the APC center is also formed via release of hydrogen from the anode interface and diffusion to the cathode interface where it reacts to form APC. The latter is similar to the mechanism of interface-trap generation. These contributions not only broaden our understanding regarding the degradation of SiO_2 in MOS devices, but also deepen our knowledge concerning the specific role of hydrogen and charge carriers involved in the stressing. The current work will fortify understanding and control of destructive breakdown of SiO_2, and shall contribute to the fabrication of reliable integrated circuits at the submicron level.