Kiyoteru Kobayashi

Origin of positive charge generated in thin SiO/sub 2/ films during high-field electrical stress

The characteristics of electron capture in a 131-/spl Aring/ silicon dioxide after hot-hole injection have been studied, which have been compared with those after high-field Fowler-Nordheim (FN) electron injection. After hole injection from the silicon substrate into the oxide, positive charges accumulated in the oxide and electrons could be captured even at low oxide fields only under the positive gate polarity. The charge centroid of the captured electrons was near the substrate-SiO/sub 2/ interface. The low-field electron capture can be explained based on the electron tunneling from the substrate into the positive charge and neutral trap centers created near the substrate-SiO/sub 2/ interface. In order to investigate the initial stage of the oxide degradation due to high-field FN stress, electrons were injected from the gate and the charge fluence was selected to be -1.0 C/cm/sup 2/. After the high-field stress, positive charges appeared in the oxide and electrons were captured only under the positive gate polarity by the positive charge and neutral trap centers, which were distributed near the interface. These facts are explained on the basis of the model describing that hole injection and trapping are the dominant causes for the generation of the positive charge centers during high-field FN stress.

Ultrathin Silicon Nitride Films Fabricated by Single‐Wafer Processing Using an SiH2Cl2 ‐ NH 3 ‐ H 2 System and In Situ H 2 Cleaning

We demonstrated the formation technique of highly reliable ultrathin oxidized silicon nitride film (34 A in oxide equivalent thickness) on three-dimensional cylindrical stacked capacitor cells. In situ H 2 cleaning and low-pressure chemical vapor deposition of silicon nitride using SiH 2 Cl 2 and NH 3 gases were successively carried out in a reactor, which can accommodate an 8 in. silicon wafer. The conduction current through the film was suppressed and the time-to-breakdown was substantially improved by the complete elimination of the bottom oxide. The intrinsic lifetime of the cylindrical stacked capacitors, which was comparable to that of the conventional stacked capacitors, was estimated to be long enough for use in 256 Mbit dynamic random access memory (DRAM). This result has revealed that the present single-wafer process is very effective and practical for the three-dimensional capacitor formation of the next generation DRAMs.

Stress-induced current in nitride and oxidized nitride thin films

We have studied the stress-induced current in nitride and oxidized nitride DRAM films using MOS capacitors and p-channel MOSFET transistors. When the gate current is measured as a function of electric field in MOS capacitors, gate current on oxidized nitride film is considerably lower than in nitride film. After subjecting both films to a constant current stress, however, measured gate current in oxidized nitride film becomes much greater than in nitride film. Using PMOSFET transistor, it is observed that holes dominate the current conduction both for nitride and oxidized nitride films. When the films are subjected to a constant current stress, both the hole and electron currents increased compared to those before stress. After the constant current stress. The electron current is increased more in the nitride film, while in oxidized nitride film, hole current increase is dominant. Hence it can be said that the current increase in nitride films is due to the stress-generated trap of electrons, while in oxidized nitride film, it is due to stress-generated hole traps in the top oxide. >

Excess currents induced by hot-hole injection and F-N stress in thin SiO/sub 2/ films [flash memories]

The behavior of excess currents induced by hot-hole injection and F-N stress is investigated in 60-/spl Aring/ oxides. The excess currents induced by the hot-hole injection and F-N stress are due to the filling of trap centers with electrons in addition to a leakage current through the oxides, and they decrease by annealing at 250/spl deg/C. The excess current induced by F-N stress is caused by the injected holes produced by high-energy-electrons. Dielectric breakdown caused by hot-hole injection has also been studied, and it is revealed that the total positive charge to breakdown (Q/sub p/) is independent of the oxide field. This supports previous measurement that the Q/sub p/ value is constant during F-N stress. The annealing-recovery mechanism in the dielectric breakdown is different from that in the excess current, though both are caused by the hot-hole injection.

High reliability of nanometer-range N/sub 2/O-nitrided oxides due to suppressing hole injection

Hole transport and trapping in N/sub 2/O-nitrided oxides have been studied. It is shown that N/sub 2/O-nitridation of oxides suppresses hole injection into the oxides. The suppression of hole injection is a mechanism leading to the enhancement of reliability of the nitrided oxides under channel hot-hole and F-N stresses.

Conduction and Charge‐Trapping Characteristics of MOS Capacitors with Oxidized Nitride Films of Different Nitride Thicknesses under Positive Stress Bias

Metal oxide semiconductor capacitors with oxidized thick and thin nitride films of two different nitride thicknesses were used to study the conduction and charge-trapping behavior under positive stress bias to the upper electrode. Although the top oxide thickness of the two groups of oxidized nitride films is the same, the charge-trapping characteristics are different. It was found that due to the positive stress to the upper electrode of oxidized thick nitride, the gate voltage is shifted to the negative direction, whereas in oxidized thin nitride, it is shifted to the positive direction, just like the nitride films. It was also observed that in nitride films, irrespective of nitride thickness, due to the positive stress bias, the gate voltage is shifted to the positive direction. From the experimental results of oxidized thick and thin nitride, it can be inferred that in oxidized thick nitride, due to electron injection from the lower electrode, holes are generated in the nitride film and are trapped near the interface. As a result of trapped holes near the interface of the top oxide and nitride, as well as a shallow electron trap centered below the conduction band of the nitride, a large increase in current is observed in oxidized thick nitride. On the other hand, in oxidized thin nitride films for the same positive stress bias, only a few holes are generated in the nitride film because most of the injected electrons from the lower electrode tunnel through the nitride and are trapped in the top oxide. Also shallow trap centers are generated below the conduction band of the thin nitride films. As a result of fewer trapped holes in the interface, the increase of current in oxidized thin nitride is less compared with those of oxidized thick nitride.

Electron traps and excess current induced by hot-hole injection into thin SiO/sub 2/ films

Electron capture and excess current after the substrate hot-hole injection into a 131 /spl Aring/ oxide have been studied. The gate current-gate voltage characteristics, drain current-gate voltage characteristics, and capacitance-voltage curves in p-channel MOSFETs were measured before and after the hole injection and after subsequent electron injection. Excess current obviously appeared under positive gate bias after the hole injection and disappeared after the electron injection from the substrate. For the oxide after the hole injection, the electron capture occurred even at low electric fields under positive gate bias. The trapping rate of the electrons injected from the substrate is much larger than that of the electrons injected from the gate. In order to explain the experimental results, we have proposed a model, including the tunneling of electrons from the substrate into the positive charge centers generated by the hole injection and into the neutral trap centers created during the electron injection. Dynamics of electron trapping in the oxide with the positive charge centers have been also studied. Analyzing the gate voltage shift under a constant gate current for the sample after the hole injection, the trapping parameters were obtained. The characteristic behavior of the excess current is explained sufficiently, taking into account the creation of the neutral trap centers-and filling of the positive charge and neutral trap centers. Finally, the effect of annealing on the positive charge centers and the electron capture is investigated. The annealing over 200/spl deg/C rapidly removes a large portion of the positive charges in the oxide. The density of electron trap centers is dependent on the density of the remaining positive charge centers.

Reliability evaluation of thin gate oxide using a flat capacitor test structure

A test structure with very low-level current measurement technique (minimum detectable current is 5/spl times/10/sup -17/ A) has been developed and is used for measuring very small change of leakage current caused by trapping and detrapping of electrons or holes. The present technique to measure very low level current of aA order is very useful for accurate evaluation of retention characteristics and stress induced degradation of gate oxide.