D.J. Dumin

Correlation of stress-induced leakage current in thin oxides with trap generation inside the oxides

Increases in pre-tunneling leakage currents in thin oxides after the oxides are subjected to high voltage stresses are correlated with the number of traps generated inside of the oxides by the high-voltage stresses. The densities of the traps are calculated using the tunneling front model and analyzing the transient currents that flowed through the oxide after removal of the stress voltage pulses. It is found that the trap distributions are relatively uniform throughout the small portion of the oxide sampled by the transient currents. The trap densities increase as the cube root of the fluence of electrons that passes through the oxide during the stress, independent of the stress polarity. The voltage dependence of the low-level pretunneling current is dependent on the sequence in which the stress voltage polarities and the low-level current measurement polarities are applied. The portion of the low-level pre-tunneling current that is not dependent on the polarity sequence is best fitted by a voltage dependence consistent with Schottky emission. >

A model relating wearout to breakdown in thin oxides

A model has been developed relating wearout to breakdown in thin oxides. Wearout has been described in terms of trap generation inside of the oxide during high voltage stressing prior to breakdown. Breakdown occurred locally when the local density of traps exceeded a critical value and the product of the electric field and the higher leakage currents through the traps exceeded a critical energy density. The measurement techniques needed for determining the density of high-voltage stress generated traps have been described along with the method for coupling the wearout measurements to breakdown distributions. The average trap density immediately prior to breakdown was measured to be of the order of low-10/sup 19//cm/sup 3/ in 10 nm thick oxides fabricated on p-type substrates stressed with negative gate voltages. The model has been used to describe several effects observed during measurements of time-dependent-dielectric-breakdown distributions. The area dependence of breakdown distributions, the differences in the breakdown distributions during constant current and constant voltage stressing, and the multi-modal distributions often observed were simulated using the model. The model contained the provision for incorporation of weak spots in the oxide. >

High field related thin oxide wearout and breakdown

The high voltage wearout and breakdown of thin silicon oxides has been described in terms of traps generated inside of the oxide and at the interfaces by a high field emission process. The trap generation was accompanied by the motion of atoms which resulted in permanent traps fixed in space. Breakdown occurred when the local density of traps exceeded a critical density. The charge state of these traps could easily be changed by application of low voltages after the high voltage stresses. The energy levels of the traps varied depending on the probability of trap generation. This model has been applied to analyze the thickness, field, polarity, time, and temperature dependences of oxide wearout and breakdown observed in oxides thinner than 22 mn. It was concluded that the wearout process in oxides thinner than 22 nm was determined by the electric fields applied to the oxides and not by the passage of currents through the oxides. >

Thickness dependence of stress-induced leakage currents in silicon oxide

The thickness dependence of high-voltage stress-induced leakage currents (SILCs) has been measured in oxides with thicknesses between 5 and 11 nm. The SILCs were shown to be composed of two components: a transient component and a DC component. Both components were due to trap-assisted tunneling processes. The transient component was caused by the tunnel charging and discharging of the stress-generated traps near the two interfaces. The DC component was caused by trap-assisted tunneling completely through the oxide. The thickness, voltage, and trap density dependences of both of these components were measured. The SILCs will affect data retention in electrically erasable programmable read-only memories (EEPROMs) and the DC component was used to estimate to fundamental limitations on oxide thicknesses.

Properties of high-voltage stress generated traps in thin silicon oxide

It has previously been shown that trap generation inside thin oxides during high voltage stressing can be coupled to time-dependent-dielectric-breakdown distributions through the statistics linking wearout to breakdown. Since the stress-generated traps play a crucial role in the wearout/breakdown process, it is important to understand the properties of these traps. The properties of the traps in oxides with thicknesses between 2.5 nm and 22 nm have been studied, with emphasis on oxides in the 8.5-nm to 13-nm thickness range. The Coulombic scattering cross section of the traps responsible for the reduction in the tunneling current, an estimate of the spatial and energy distribution of the traps, and the charging/discharging properties of the traps have been measured. It will be shown that the measured properties of the high-voltage, stress-generated traps can be adequately described by the tunneling of electrons into and out of traps.

The charging and discharging of high-voltage stress-generated traps in thin silicon oxide

Excess high-voltage stress-generated low-level leakage currents through 10 nm silicon oxides, previously described as DC currents, are shown to decay to the limit of detection given adequate observation time and, thus, have no discernible component. A physical model is presented which describes the majority of the excess low-level leakage currents in terms of the charging and discharging of traps previously generated by the high voltage stress. Excess low-level leakage currents measured with voltage pulses with polarity opposite to that of the stress voltage are found to contain an additional current component, which is explained by the transient charging and discharging of certain traps inside the oxide. Evidence is presented which suggests that an oxide trap generated by the high-voltage stress can contain either a positive or a negative charge, in addition to being neutral and that the traps are located near both oxide interfaces. All of the trap charging and discharging currents can be explained by the flow of electrons into and out of traps generated by the high voltage stress, without resorting to the flow of holes in the oxide.

Low‐level leakage currents in thin silicon oxide films

The low‐level leakage currents in thin silicon oxide films were measured before and after the oxides had been stressed at high voltages. Four components of current were identified. These components were the tunneling current, the capacitive current associated with the measurement sweep rate, a negative differential current associated with the voltage sweep through the changing oxide C‐V characteristic, and an excess current that occurred after the high‐voltage stress. The excess current was due to the charging and discharging of traps generated inside of the oxide by the high‐voltage stress. The excess current was proportional to the number of traps generated in the oxide. The magnitude of the excess current could be changed by changes in the measurement procedures due to the charging and discharging of traps. A major portion of the stress‐generated excess low‐level leakage current may not be a current that flowed through the oxide, but may be a trap charging/discharging current. This paper will concentra...

High field emission related thin oxide wearout and breakdown

A model describing high voltage induced thin oxide wearout and breakdown in terms of traps generated inside of the oxide by high field emission has been developed. This model has been shown to be able to describe much of the thickness, field, polarity, time and temperature dependences observed during thin oxide wearout and breakdown.<<ETX>>

Bipolar stressing, breakdown, and trap generation in thin silicon oxides

Thin silicon oxide films were stressed with bipolar pulses in which the magnitudes of both the positive and negative pulses were independently varied, The time-to-breakdown, the charge-to-breakdown, and the number of traps generated inside of the oxides during the stresses were measured and compared with oxides that had been stressed with unipolar pulses or stressed with constant dc voltages. For the bipolar stresses it was found that the time-to-breakdown, the charge-to-breakdown, and the number of traps generated inside of the oxide all increased as the magnitude of the opposite polarity, nonstressing pulse was increased, until the opposite polarity pulse became large enough to become the stressing pulse. The time-to-breakdown reached a maximum when the magnitude of the stressing pulse was approximately 1 V larger than the magnitude of the nonstressing pulse. The model that was used to explain these increases involved generation of traps inside of the oxide and the lack of spatial correlation between the traps generated by injection from one interface with the traps generated by injection from the other interface. >

Nonuniqueness of time-dependent-dielectric-breakdown distributions

The time-dependent-dielectric-breakdown (TDDB) distributions measured on a series of identical oxides at the same voltages have been shown to depend on the resistance and capacitance of the measurement test equipment. The TDDB distributions were shifted to shorter times if the impedance of the test equipment was lowered and/or the capacitance of the test equipment was raised. The lower resistances and higher capacitances allowed the nonshorting early electric breakdowns to develop into shorting, thermal, dielectric breakdowns.