SILC

Abstract: Very thin thermal oxides are shown to exhibit a failure mode that is undetected by conventional breakdown tests. This failure mode appears in the form of excessive leakage current at low field and is induced by high-field stresses. The stress-induced oxide leakage is permanent and stable with time and thermal annealing. It becomes the dominant failure mode of thin oxides because it always precedes destructive breakdown. Experimental results and theoretical calculations show that the leakage current is not caused by positive charge generation and accumulation in the oxide. It is proposed that the oxide leakage originates from localized defect-related weak spots where the insulator has experienced significant deterioration from electrical stress. The leakage conduction mechanism appears to be thermally assisted tunneling through the locally reduced injection barrier, and the model seems to be consistent with both I-V measurements at temperatures from 77 K to 250 degrees C and theoretical calculations. >

Abstract: 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. >

Abstract: The effects of thinning the FLOTEX EEPROM tunnel oxide on its reliability are investigated using capacitors and cell structures with oxide thickness ranging from 47 to 100 AA. A low-electric-field oxide leakage current is induced by charge injection stressing, and it increases with decreasing oxide thickness. Its conduction mechanism is found to be different from that caused by positive charge accumulation in that it has the opposite thickness dependence. A corresponding increase of charge loss in a write/erase (W/E)-cycled EEPROM cell is observed with decreasing oxide thickness in a room-temperature retention test. When oxide thickness is decreased, the maximum number of W/E cycles to tunnel-oxide breakdown decreases with the decrease in charge to breakdown of the negatively biased gate. For scaling down the EEPROM tunnel oxide, the most serious limiting factor is oxide leakage current induced by W/E cycling stress, resulting in data-retention degradation. >

Abstract: Stress-induced leakage current (SILC) is examined both below and above the voltage at which the preexisting Fowler-Nordheim tunneling current dominates. Based on these results, it is argued that SILC is the result of inelastic rather than elastic trap-assisted tunneling. This clarification explains the well-known thickness dependence of the SILC at low fields that has identified it as a scaling limitation for nonvolatile memory tunnel oxide. It also explains a newly observed different thickness dependence at high fields and facilitates modeling of the electric field/voltage and trap density dependencies of the SILC.