BORDER TRAPS - ISSUES FOR MOS RADIATION RESPONSE AND LONG-TERM RELIABILITY

We have performed an extensive study of the effects of border traps (n ear-interfacial oxide traps that can communicate with the underlying s i over a wide range of time scale) on the response of metal-oxide-semi conductor (MOS) devices to ionizing radiation. Estimates of border-tra p densities for several types of MOS devices are obtained by capacitan ce-voltage (C-V) hysteresis 1/integral noise, and combined C-V/thermal ly-stimulated-current methods. A new ''dual-transistor border-trap'' ( DTBT) technique is described in detail which combines conventional thr eshold-voltage and 1-MHz charge-pumping measurements on n- and p- chan nel MOS transistors to estimate radiation-induced oxide-, interface-, and border-trap charge densities. Estimates of border-trap charge dens ities obtained via the DTBT technique agree well with trap densities i nferred from other techniques. In some devices, border-trap charge den sities (which can be greater than 10(12) cm(-2) following ionizing rad iation exposure) can approach or exceed interface-trap charge densitie s, emphasizing the need to distinguish border-trap effects from interf ace-trap effects in models of MOS radiation response and long-term rel iability. This appears to be especially critical for MOS devices with ultrathin (less than similar to 6 nm) oxides, in which border traps an d interface traps likely will be the dominant defect types. Effects of border traps on MOS scattering rates, cryogenic applications, and lon g-term reliability assessment are also discussed.