Yasushi Naito

Electrical and physical characteristics of thin nitrided oxides prepared by rapid thermal nitridation

Ultrathin oxides (5-12 nm) were nitrided by lamp-heated rapid thermal annealing in ammonia at temperatures of 900-1150°C for 5-300 s. Elemental depth profiles were measured by Auger electron spectroscopy (AES) and secondary ion mass spectroscopy (SIMS). Both the nitrogen concentration measured by AES and the hydrogen one measured by SIMS for a nitrided oxide are found to increase monotonically as nitridation proceeds. The AES depth profiles of oxygen show that the Si-SiO2interface does not move during nitridation. Dependences of midgap interface state density (D_{it}_{m}) and fixed charge density (Nf) on nitridation temperature and on oxide thickness were studied. For a given temperature, bothD_{it}_{m}and (Nf) are found to show turnarounds as nitridation time increases in a similar manner: at first both increase, reach respective maxima at a certain nitridation timet_{\max}, and then decrease gradually. The (D_{it}_{m}) and (Nf) increase more rapidly and thet_{\max}is shorter as the nitridation temperature is raised or the oxide film is thinner. The maximum ofD_{it}_{m}increases as the oxide film is thinner. A two-step model is newly proposed to explain the turn-around behaviors ofD_{it}_{m}and Nf: the first step is defect formation as a result of nitrogen incorporation and the second step is reduction of the defects by an annealing-type process. The simulation reproduces the turnaround behaviors very well.

Interface states and fixed charges in nanometer-range thin nitrided oxides prepared by rapid thermal annealing

Ultra-thin oxides (5-12 nm) were nitrided by lamp-heated rapid thermal annealing in an ammonia ambient at 900-1150°C for 5-300 s. Interface states and fixed charges in the nitrided oxides have been studied, and, for a given temperature, both are found to vary in a similar manner as nitridation time increases: at first both increase, reach respective maxima at a certain nitridation time, and then decrease gradually showing turnarounds. Interface state densities and fixed charge densities at the initial and the final nitridation stages are in the low 1010- cm-2/eV range and the low 1011-cm-2range, respectively, and are comparable with those of thermally grown oxides.

Effect of nitrogen distribution in nitrided oxide prepared by rapid thermal annealing on its electrical characteristics

Very thin oxides in nm range were nitrided by lamp heated rapid thermal annealing in an ammonia ambient. The nitrided oxides were studied using Auger electron spectroscopy, angular resolved x‐ray photoelectron spectroscopy, secondary ion mass spectroscopy, and electrical metal–insulator–semiconductor (MIS) characterization. It is found by Auger depth profiling that the thickness of the oxide film does not increase by nitridation. The hydrogen concentration increases monotonically as nitridation proceeds. In the early stages of nitridation, nitrogen piles up at SiOxNy/Si interface and in the final stages it distributes uniformly throughout the film. Fixed charge and interface state densities increase with nitrogen concentration at the interface up to a critical nitrogen concentration (about 9%). Beyond this value, they decrease as nitridation proceeds. Nitrided oxides of the same nitrogen distribution, that were obtained at different nitridation temperatures, show the same electrical MIS characteristics. T...

Effect of Bottom Oxide on the Integrity of Interpolysilicon Ultrathin ONO (Oxide/Nitride/Oxide) Films

The bottom oxide was found to play an important role on the integrity of interpolysilicon ultrathin oxide/nitride/oxide (ONO) films. Natural oxides grown on phosphorus‐doped polysilicon by diffusion from source contain defects and weak spots, which degrade the integrity of ONO films grown on these. On in situ phosphorus‐doped LPCVD polysilicon, defect‐free oxides can be obtained and introduce much better yield in ONO films. Dielectric strength measurement by voltage ramping method cannot separate this difference of integrity. We were able to discriminate this difference only by time dependent dielectric breakdown measurement in which negative voltage was applied to the upper polysilicon. The intrinsic wearout phenomena of ONO films, which have been observed on single‐crystal silicon surfaces, and depend only on the thicknesses of bottom oxide, nitride, and top oxide, were observed also on polysilicon if the bottom oxide has grown on in situ phosphorus‐doped LPCVD polysilicon.

Indirect trench sidewall doping by implantation of reflected ions

Ion implantation (i/i) technology is employed for silicon trench sidewall doping. The aspect ratio of trenches for high Mbit DRAM is very large (depth/width≥10), so that very small glancing‐angle i/i to sidewalls is necessary. In this case, reflected ions are large in number and are implanted to the opposite sidewall. It is very important to know the elemental depth profile in the opposite sidewall to understand the implantation mechanism. For the first time, we measured the depth profiles at several positions of the opposite trench sidewall by secondary‐ion mass spectroscopy for arsenic and boron ion implantations. It is found that reflected ions are distributed near the facing region of the directly implanted region with smaller energies than the primary energy. These findings are compared with the simulations based on the marlowe program.