The breakdown of thin oxides (7.9-32 nm) subjected to high-field current injection is investigated in this study. The physical mechanism of breakdown is found to be localized field enhancement at the cathode interface due to hole trapping. The source of this hole trapping is believed to be impact ionization in the SiO 2 . A quantitative model for oxide breakdown based on impact ionization and hole trapping at the cathode is presented and shown to agree well with the experimental J - t and time-to-breakdown, (t BD ) results. We observe that log t BD varies linearly with 1/ E rather than with E as commonly assumed. The field acceleration factor, i.e., the slope of the log t BD versus 1/ E plot, is approximately 140 decades per centimeter per megavolt for the 7.9 nm oxide, with approximately 25 percent of this coming from the field dependence of the impact ionization coefficient and the remainder from the Fowler-Nordheim current dependence on 1/ E . Based on this model, oxide wearout performance might be improved by process changes that reduce interface hole trapping, such as radiation-hard processing, in addition to the reduction of particulate contamination and crystal defects.

Electrical breakdown in thin gate and tunneling oxides

The breakdown of thin oxides (7.9-32 nm) subjected to high-field current injection is investigated in this study. The physical mechanism of breakdown is found to be localized field enhancement at the cathode interface due to hole trapping. The source of this hole trapping is believed to be impact ionization in the SiO/sub 2/. A quantitative model for oxide breakdown based on impact ionization and hole trapping at the cathode is presented and shown to agree well with the experimental J - t and time-to-breakdown, (t/sub BD/) results. We observe that log t/sub BD/ varies linearly with 1/E rather than with E as commonly assumed. The field acceleration factor, i.e., the slope of the log t/sub BD/ versus 1/E plot, is approximately 140 decades per centimeter per megavolt for the 7.9 nm oxide, with approximately 25 percent of this coming from the field dependence of the impact ionization coefficient and the remainder from the Fowler-Nordheim current dependence on 1/E. Based on this model, oxide wearout performance might be improved by process changes that reduce interface hole trapping, such as radiation-hard processing, in addition to the reduction of particulate contamination and crystal defects.

Electrical Breakdown in Thin Gate and Tunneling Oxides

Electrical breakdown of thin (32-nm) SiO 2 films subjected to constant-current stressing is studied. By studying the effects of reversing the polarity of the constant-current bias and the effects of thermal annealing on the charge-to-breakdown it is determined that electrical breakdown of SiO 2 is not caused by the widely-cited accumulation of trapped electrons. Rather it is caused by the buildup of positive charges near the cathode at localized areas. The positive charges are not mobile ions but exhibit many characteristics of trapped holes. We conclude that electrical breakdown in SiO 2 is caused by the accumulation of holes, generated by impact ionization in the oxide.

On physical models for gate oxide breakdown

Studies of (100) silicon oxidation in dry oxygen at 800° C and 20 atmospheres showed the data to be modelled accurately over a wide thickness range by the simple equationx =a(t g +t o ) b , wherex is the final measured thickness,a andb are constants,t g is the time for growth measured in an oxidation experiment, andt o is the time to grow an oxide of thickness,x o , already present on the silicon surface, and/or formed as a consequence of furnace ramp-up and ramp-down sequences. The values fort o , andx o are determined readily fromx-t data. Employing this power law model, there is no evidence of an “anomalous” oxidation region at small thicknesses, or of a linear oxidation region in the earlier stages of oxidation. These surprising results led to an analysis of most of the published silicon “dry” oxidation data from 1965 to the present time to determine whether this power law model was general, or representative only of the data obtained in the present study. It was found that all of the published data, including that published by Deal and Grove in 1965, could be mathematically modelled precisely and simply in all of the thickness ranges reported, and at partial pressures of oxygen from less than 1 × 10−5 up to 20 atm. The critical parameterb was found to range from less than 0.25 to about 0.8, the value decreasing with decreasing partial or total pressure of oxygen. The constanta represents the value of oxide thickness at unit time, and for constant values ofb andt o , causes a shift of thex-t curves along the thickness axis. The physical origin(s) for the model is not known. However, as the data appear to suggest a diffusion limited oxidation is in play over the entire thickness regime, it is likely that the diffusion coefficient for the process varies with oxidation time, therefore oxide thickness.

The modelling of Silicon oxidation from 1 x 10 –5 to 20 atmospheres

The generation and annealing of fixed positive and fixed negative charges are discussed in the context of neutral hole and neutral electron traps. The analyses are based on the results of experimental studies involving hydrogen ambient atmosphere annealing and/or electron injection of gate insulators in insulated gate field‐effect transistors which have been damaged by 1.5‐keV x rays. It is found, for example, that annihilation of fixed positive charge either by hydrogen annealing, or by electron injection results in indistinguishable ‘‘repair’’ of this defect, indicating that such repair probably involves loss of an electron by the hydrogen to an E’γ center. It is postulated that a new electron spin resonance (ESR) center representing fixed negative charge, Nn, which is derived from a large cross‐section neutral electron trap might be detected under the proper conditions. Since large cross‐section neutral traps (10−15 cm2) are known to exist in significant concentrations in insulators damaged with ionizi...

A. Reisman

Papers

The effects of pressure, temperature, and time on the annealing of ionizing radiation induced insulator damage in N-channel IGFET's