We propose a new experimental technique to study the transport properties of stress-induced leakage current (SILC). Based on the carrier separation measurement for p-channel MOSFETs, the quantum yield of impact ionization for electrons involved in the SILC process is evaluated directly from the change in the source and gate currents of p-MOSFETs before and after stressing. Since the relationship between the electron energy and the quantum yield is established for direct and FN tunneling currents, the electron energy of electrons involved in the SILC process can be determined from the quantum yield. The results reveal that the measured energy of electrons in the SILC process is lower roughly by 1.5 eV than the energy expected in the elastic tunneling process. Trap-assisted inelastic tunneling model is proposed as a conduction mechanism of SILC accompanied by energy relaxation. It is shown, through the evaluation of the substrate hole current in n-channel MOSFETs, that the contribution of trap-assisted valence electron tunneling, another possible mechanism to explain the energy relaxation, to SILC is small.

Experimental evidence of inelastic tunneling in stress-induced leakage current

A new I-V model to quantitatively represent stress-induced leakage current (SILC) is presented and compared with the experimental I-V characteristics. The trap-assisted tunneling model is modified so as to include the energy relaxation of tunneling electrons, which has been experimentally verified by applying the carrier separation technique to MOSFETs with the SILC component. The energy relaxation is treated in the new model as the change in the energy level of traps before and after the capture of electrons during two-step tunneling. It is demonstrated that this model successfully represents the experimental I-V characteristics of the SILC component and, particularly, the low apparent barrier height in the Fowler-Nordheim (FN) plot of the SILC component. The calculated low barrier height is attributed to the dominance of direct tunneling mechanism on both tunneling into traps and out of traps. The impact of the energy relaxation during tunneling, used in the present model, on the I-V characteristics is discussed in terms of the trap distribution inside the gate oxide, compared with conventional elastic tunneling model.

A new I-V model for stress-induced leakage current including inelastic tunneling

We investigate theoretically coherent electron tunneling through three-dimensional microscopic Si[100]/SiO2/Si[100] model junctions with oxide thicknesses between 0.4 and 4.6 nm. The transmission probabilities of these structures were calculated using a semiempirical tight-binding scattering method. Our calculations provide a basis for the microscopic understanding of the observed independence of tunneling transmission on the orientation of the bulk silicon and on the nature of inelastic defect-assisted tunneling. We document significant differences between transmission coefficients obtained with the present scheme and with the popular effective-mass-based approaches. The energy dependence of the effective tunneling mass in bulk silicon dioxide is predicted.

Tunneling through ultrathin SiO2 gate oxides from microscopic models

A detailed investigation of the steady-state and transient leakage currents in thin oxides is proposed. The experimental data are compared with numerical results obtained from a model based on an inelastic trap-assisted tunneling process, which includes both electron and hole contributions. In order to accurately reproduce the transient discharge currents, a continuous distribution of oxide traps was adopted. The energies of these levels can be either in correspondence of the conduction or valence band edges of the adjacent silicon/polysilicon layers. Both electrons and holes contribute to the transient stress-induced leakage current (SILC), but the extracted trap densities cannot account for the steady-state SILC. A different mechanism, involving trap levels with energy aligned to the energy gap of the silicon layers is proposed and is developed in the following paper. The model can be applied to any type of device and bias conditions and may be used to correctly recognize the role of electron and hole SILC and the spatial and energy distribution of defect states.

Modeling of SILC based on electron and hole tunneling. I. Transient effects

The excess leakage current across ultra-thin dielectrics has been studied for different ionizing radiation sources. Namely, X-rays, 8 MeV electrons, and three ion beams with different LETs values have been used on large area MOS capacitors with 4-nm thick oxides. Small oxide fields were applied during irradiation, reaching 3 MV/cm at most. For ionizing radiation with relatively low LET (<10 MeV cm/sup 2//mg), only Radiation Induced Leakage Current (RILC) was observed, due to the formation of neutral defects mediating electron tunneling via a single oxide trap. For high LET values, instead, the gate leakage current could be described by an empirical relation proper of Soft Breakdown (SB) phenomena detected after electrical stress. Moreover, the typical random telegraph signal noise feature of this Radiation induced Soft Breakdown (RSB) currents was observed during and after irradiation. RSB can be attributed to conduction through a multi-defect path across the oxide, produced by the residual damage of dense ion tracks. The oxide field applied during irradiation enhances the RSB intensity, but RSB can be achieved even for irradiation at zero field, being LET the main factor leading to RSB activation. The dose dependence of both RILC and QB have been investigated, showing a quasi linear kinetics with the cumulative dose. We have also studied the effect of modifying the angle of incidence of the ion beam on the intensity of the gate leakage current.

Low field leakage current and soft breakdown in ultra-thin gate oxides after heavy ions, electrons or X-ray irradiation

The origin of electron traps, which induce leakage currents in SiO/sub 2/ films, is predicted to be an oxygen (O) vacancy from first-principles total-energy calculations. Once a neutral O vacancy traps holes, it spontaneously changes to a large distorted structure. The distorted O vacancy can capture electrons, and remains as an electron trap in SiO/sub 2/ films. These results give a microscopic explanation for a trap model that is empirically deduced from experiments. The energy level of the generated electron trap exists near the band offset between the conduction band of SiO/sub 2/ and that of Si. From these results, the electron trap originating from a hole trapped O vacancy can be considered as one origin of the so-called stress-induced leakage currents in SiO/sub 2/ films.

A. Yokozawa

Papers

Oxygen vacancy with large lattice distortion as an origin of leakage currents in SiO/sub 2/