Nitrided Oxide Films Research Articles

An improvement of hot-carrier reliability in the stacked nitride-oxide gate n- and p-MISFET's

Thin LPCVD stacked nitride-oxide gate MISFET's offer the potential for high current drive as a consequence of the high permittivity of the nitride. However, thin nitride-oxide films have yet to be used in actual MISFET's for LSI products because thin nitride films have poor masking ability during re-oxidation and low reliability under hot-carrier stress. We have investigated improvements to thin LPCVD stacked nitride-oxide films as regards masking ability during re-oxidation and hot-carrier reliability. The method proposed to improve film quality is densification through high-temperature rapid thermal processes and its effectiveness has been tested. Simple rapid thermal annealing (RTA) did not improve the film quality at all. On the other hand, rapid thermal nitridation (RTN) on the deposited nitride film improved it considerably. The reason for the improvement by RTN was investigated.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characteristics of the NO dielectric film with low pressure chemical vapor deposition in-situ nitridation

We investigated the formation of the thin NO dielectric films by in-situ nitridation of native oxide, and subsequent deposition of silicon nitride in the low pressure chemical vapor deposition systems for the application to the capacitors in high density dynamic random access memory. The native oxide was nitrided at elevated temperatures of 690 or 780°C in the flowing ammonia gas atmosphere, and nitride was deposited by flowing silane gas additionally immediately after the nitridation process. By in-situ nitridation process, we could obtaine 5 and 4.5 nm thick (equivalent oxide thickness) nitride/oxide (NO) dielectric films. These films were characterized to be electrically more reliable than the conventional oxide/nitride/oxide (ONO) films of the same equivalent oxide thickness. The nitrided NO films also showed lower leakage current and higher breakdown voltage than conventional ONO films. We obtained electrically most reliable NO films by loading the wafer at 400°C and nitriding the native oxide at 780°C.

Comparison of defect structure in N2O- and NH3-nitrided oxide dielectrics

Electron spin resonance spectroscopy is used to identify and compare point defects in N2O-nitrided, NH3-nitrided, and conventional SiO2 films. We detect only three types of defects in these dielectrics. Pb centers, the primary source of interface states in Si/SiO2 systems under all technologically significant circumstances, appear in all three dielectrics. Both N2O and NH3 nitridation result in higher as-processed Pb interface defect densities, but lower radiation-induced Pb defect generation. Thus N2O nitridation appears capable, as does NH3 nitridation, of providing reduced radiation-induced interface state generation. In addition, both nitridations appear capable of lowering the number of radiation-induced E′ centers, the dominant hole trap in conventional thermal oxides. NH3 nitridation, however, appears to offer greater resistance to radiation-induced generation of these traps. NH3 nitridation also results in a large number of bridging nitrogen centers, and strong evidence indicates that the bridging nitrogen centers are the dominant electron trap in NH3-nitrided and -reoxidized nitrided oxide films. These defects are absent in N2O-nitrided films, which are known to exhibit reduced levels of electron trapping.

Study of process-dependent electron-trapping characteristics of thin nitrided oxides

A detailed investigation on the process dependent characteristics of electronic traps in thin nitrided oxide films is presented. By monitoring the field shift in the Fowler-Nordheim (FN) plots and the flatband voltage of metal-insulator-semiconductor (MIS) structures, the trap centroid of the nitrided oxide film was found to be process dependent. Turnaround behaviour in the location of the trapped charge centroid ( X 0, measured from the oxide/silicon interface) and trap density were found. The centroid moves to the surface for light nitridation and then moves as close as X 0/ L = 0.48 ( L is the thickness of the dielectric) to the silicon/oxide interface. On the other hand, the current conduction of the nitrided film is found to be enhanced remarkably by shallow traps (∼ 1.0 eV) for electric field strengths less than 8.5 MV/cm. A shallow trap-assisted electronic conduction mechanism in the nitrided oxides is proposed. For electric fields in the range of 8.5–11 MV/cm, a quasi-saturation region due to electron-trapping is observed. In the high electric field region (> 11 MV/cm), the conduction current is governed mainly by the FN mechanism. Detrapping, either by two-step tunnelling or Schottky emission, is also obvious in the high field region.

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Growth Rate and Characterization of Silicon Oxide Films Grown in N 2 O Atmosphere in a Rapid Thermal Processor

A rapid thermal oxidation process of silicon in ambient was investigated using a commercially available reactor with a fixed wafer position and a gas flow parallel to the wafer surface. For such a configuration, thickness uniformities in the 2% range were obtained for the first time. The oxidation rate as a function of process temperature and time was investigated. A retardation in the oxidation rate as compared to the oxidation in ambient is explained by the formation of a nitrided interfacial layer. A comparison of experimental results with an oxidation model calculation shows that this interface can affect either the oxidant diffusivity through the oxide or the reaction rate at the silicon surface. Infrared spectra of nitrided oxide films reveal a regular arrangement in the Si‐O network, similar to that of high quality thermally grown oxides. A vibrational contribution to that spectrum from a Si‐O‐N subnetwork is displayed. The accumulated charge to breakdown on metal‐oxide‐semiconductor capacitors as a function of the injected current density revealed different slopes for oxides either thermally grown or grown in . Hence, the projected lifetime for devices with grown oxide under low operating fields is extended by one order of magnitude in comparison with the thermal oxide.

Bridging nitrogen dangling bond centers and electron trapping in amorphous NH3-nitrided and reoxidized nitrided oxide films

We present evidence supporting the designation of the bridging nitrogen center as the dominant electron trap in NH3-nitrided and reoxidized nitrided oxide dielectric films. This defect, an unpaired electron highly localized in a nearly pure 2p-orbital of a nitrogen atom bonded two silicon atoms, is also present in many other silicon oxynitride dielectrics. We provide evidence suggesting the precursor is of the form (≡Si)2NH.

Simplified closed-form trap-assisted tunneling model applied to nitrided oxide dielectric capacitors

Conduction has been studied in ultrathin nitrided oxide, re-oxidized nitrided oxide, and nitrogen-annealed nitrided oxide film capacitors in which the nitridation step was performed by a low-partial-pressure nitridation technique. Results indicate that, as well as some degree of barrier lowering due to the build-up of nitrogen at the injecting interface, a trap-assisted mechanism could be responsible for the enhanced conduction exhibited by the nitrided oxide devices. A simplified closed-form trap-assisted tunneling model is employed that produces a fit to the data with a trap depth of 2.1 eV. The difference between this trap model and a model requiring numerical integration was negligibly small (∼2%).

Metal alkoxides and β‐diketonates as precursors for oxide and non‐oxide thin films

AbstractSingle‐components or multicomponent oxide thin films are of interest for electronic and opto‐electronic devices, optical applications, catalysis, corrosion protection etc. Their preparation by chemical routes is based on the hydrolytic (sol‐gel process) or pyrolytic (MOCVD) conversion of precursors. Derivatives having MO bonds, namely metal alkoxides, carboxylates or β‐diketonates, are the most common sources of metal oxides. The properties of alkoxides are appropriate for sol‐gel as well as MOCVD applications, whilst the limited hydrolytic susceptibility but good volatility of β‐diketonates is most convenient for MOCVD purposes. The low temperature and flexibility of sol‐gel routes, and the presence of residual OH groups in the final films, are favorable for the encapsulation of organic or organometallic derivatives, the anchoring of enzymes and in general for the development of functional and composite coatings. The facile formation of heterometallic alkoxides is also attractive for the development of coatings based on multimetallic formulations. MOCVD is favorable for the buildup of heterostructures and epitaxial layers. Although metal alkoxides and β‐diketonates are usually oxide precursors, nitride or sulfide films can be obtained by reacting with the appropriate reagents. Fluorinated ligands enhance volatility but often result in the formation of metal fluorides.

Electron spin resonance study of radiation-induced points defects in nitrided and reoxidized nitrided oxides

X-ray irradiated oxide, nitrided oxide, and reoxidized nitrided oxide films were studied with electron spin resonance. Nitridation of oxide films reduces the density of radiation-induced E' centers and creates bridging nitrogen center precursors, while reoxidation of nitrided oxides increases the number of radiation-induced E' centers, reduces the number of bridging nitrogen center precursors, and introduces overcoordinated nitrogen center precursors. Nitridation and reoxidation of these films also change the distribution of the radiation-induced defects. The present evidence supports earlier findings that the bridging nitrogen defect plays an important role in electron trapping in nitrided oxides and that the dominant hole trap in reoxidized nitrided oxides is not the E' center. Exposure to molecular hydrogen at room temperature annihilates paramagnetism of the nitrogen-associated centers and reduces that of the E' centers. >

Optical Effects in IR Spectroscopy: Thickness-Dependent Positions of Absorbance Lines in Spectra of Thin Films

Optical effects give rise to thickness-dependent positions of absorbance lines ( v0) in the normal-incidence transmission spectra of thin films. Two reasons can be distinguished. Simple Fresnel refraction on the boundaries of the film shifts the line toward higher wavenumbers, with the maximum increment at small film thicknesses ( d). The second effect is due to multiple reflectance. The interference fringes cause the oscillations of the absorbance line around the mean value, which is defined by the effect of the boundaries. Both effects are more pronounced for broad lines. Whereas for the strong bands the first effect dominates, for the weaker ones both can be seen. The effective dielectric constant of the medium increases the first effect and diminishes the second one. Theoretically predicted vo ( d) curves compare well with absorption line positions in silicon nitride and silicon oxide films deposited on silicon wafers.

Ion and Electron Beam Processing of Condensed Molecular Solids to Form Thin Films

ABSTRACTElectron and ion beams can be used to deposit thin films and etch surfaces using gas phase precursors. However, the generation of undesirable gas phase products and the diffusion of the reactive species beyond the region irradiated by the electron or ion beam can limit selectivity. Tn this paper, the feasibility of processing condensed precursors such as diborane, tri-methyl aluminum, ammonia and water at 78 K with low energy (100–1000 eV) electron and ion beams (Ar+, N2+ and H2+) ranging in current density from 50 nA to several μ a per cm2 is examined. It was found that boron, boron nitride and stoichiometric aluminum oxide films could be deposited from the condensed volatile species using charged particle beams and some of the physical and chemical aspects and limitations of this new technique are discussed.

Electrical characteristics of ferroelectric PZT thin films for DRAM applications

Ferroelectric lead zirconate titanate (PZT) films with as much as 2.5 times the storage capacity of the best reported silicon oxide/nitride/oxide (ONO) stacked dielectrics have been fabricated. A 2000-AA film with an effective SiO/sub 2/ thickness of 10 AA is demonstrated. Because of the extremely high dielectric constant ( epsilon /sub r/>or approximately=>1000), even larger storage capacities can be obtained by scaling the ferroelectric film thickness, whereas the thickness of ONO films is limited by direct tunneling through the film. Electrical conduction in the PZT films studied is ohmic at electric fields below 250 kV/cm and follows an exponential field dependence at higher fields, which is shown to be consistent with a simple model for electronic hopping through the film. Leakage current as low as 9*10/sup -8/ A/cm/sup 2/ at 2.5 V for a 4000-AA film is obtained with the addition of La and Fe to compensate for Pb and O vacancies in the film. Further improvement in both leakage current and time-dependent dielectric breakdown characteristics are necessary to ensure reliable DRAM operation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of wet oxidation on the electrical properties of sub-10 nm thick silicon nitride films

Capacitors with sub- 10 nm thick oxidized nitride layers formed by wet oxidation of 8 nm thick silicon nitride films deposited on a polysilicon substrate were prepared. The effects of the wet oxidation time on the electrical properties of the silicon nitride film itself have been investigated with oxide/nitride composite film and oxidized nitride film by removing the top oxide from the oxide/nitride composite film. The capacitance of the capacitors consisting of oxide/nitride composite film decreases sharply and their breakdwon fields increase with increasing thickness of the top oxide layer. On the contrary, the capacitance and breakdown fields of silicon nitride layer, which was created by removing the top oxide layers grown thermally on silicon nitride, increase with increasing wet oxidation time. The reduction of thickness and the improved quality of silicon nitride are responsible for the improved capacitance and increased breakdown fields of nitride layer, respectively. In addition, wet oxidation improves the intrinsic TDDB characteristics and the early breakdown failure rate of silicon nitride film itself, probably due to the reduction of defects in the silicon nitride. Consequently, silicon nitride film created by removing top oxide is suitable for dynamic memories as a thin dielectric film.

The oxide nitride oxide film deposition on the tunnel-structured polycrystalline silicon (polysilicon) electrodes for high-density dRAMs

The uniform deposition of the oxide nitride oxide (ONO) film on the tunnel-structured polysilicon electrodes was examined by measuring the electrical characteristics of the capacitor and by using transmission electron microscopy. The conventional low pressure chemical vapor deposition method was adopted for the deposition of the nitride film of the ONO. The tunnel length is varied from 1.6 to 15.6 μm. The ONO film deposited over the entire tunnel of 15.6 μm showed good electrical characteristics, and the thickness uniformity as well. In addition to the maximum tunnel length which can accommodate the uniform ONO deposition, the growth mechanism of the ONO film inside of the tunnel is discussed.

Charge trapping and retention in ultra-thin oxide-nitride-oxide structures

This paper describes charge trapping, detrapping and memory cycling and retention in ultra-thin oxide-nitride-oxide (ONO) structures in submicron FET devices with polysilicon gates, also known as silicon-oxide-nitride-oxide-silicon (SONOS) structures. The ONO films had various thicknesses (4.9–9.0 nm) and their top oxide was obtained either by reoxidation of nitride or by in situ deposition of oxide by chemical-vapor-deposition (CVD). Memory characteristics are found to be critically dependent on details of processing and film thicknesses. The reoxidized ONO films show larger threshold shifts than the CVD films. A surprising result is that the “write” threshold shift could be “erased” only in one of the reoxidized structures.

Impurity barrier properties of reoxidized nitrided oxide films for use with P/sup +/-doped polysilicon gates

The ability of thin reoxidized nitrided oxide (ONO) gate dielectrics formed by rapid thermal processing to act as a barrier to boron penetration resulting from p/sup +/ poly gate processing are investigated. Measurements comparing the threshold voltage instability of capacitors fabricated with BF/sub 2/ implanted poly gates subjected to various postgate thermal cycles have been performed. The ONO gate dielectrics are found to be an excellent impurity barrier to boron diffusion, even in the presence of fluorine. The extent of the nitridation is also found to affect the diffusion barrier properties, with the highest temperature nitridations forming the best barriers. Reoxidation of the nitrided films reduces the barrier properties somewhat, but improvement is still observed over SiO/sub 2/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

High-field electron trapping and detrapping characteristics in thin SiO xN y films

The electron trapping at high-field and detrapping in thin thermally nitrided silicon oxide (SiOxNy) films were studied in the present work. For lightly nitrided oxide, the effective electron trap concentration was found to increase rapidly with the nitridation duration. In addition, the detrapping ratio of trapped electrons is almost zero and independent of the detrapping time and the applied field strength up to 8 MV/cm. This observation indicates that the increased traps have deeper energy levels. For heavily-nitrided sample, the surface trap concentration decreases and the energy levels of these traps become shallower. The detrapping ratio therefore increases and is an exponential function of the field strength and detrapping time. The detrapping ratio remains unchanged if no external field is applied.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

STUDY ON HIGH-FIELD ELECTRON TRAPPING AND DETRAPPING PROPERTY IN THIN SiOx Ny FILMS

The electron trapping at high-field and detrapping in thin thermally nitrided silicon oxide (SiOxNy) films are studied in the present work. It is observed that when the nitridation degree is the lighter, with the increasing of the nirridation time the effective electron trap surface concentration in the films increases rapidly, and the detrapping ratio of trapped electrons is almost zero and independent of the shorting time for samples and the strength (until 8MV/cm) and duration of application of external reverse field, and the depth of the electron trap level is the deeper; when the nitridation degree is the heavier with the further increasing of the nitridation time the effective electron trap surface concentration decreases and the depth of electron trap level becomes shallow gradually, and the detrapping ratio of trapped electrons gradually increases and is approximately dependent exponentially on the strength and duration of time application of external reverse field and is almost independent of the shorting time for the samples.

Ultrathin oxide-nitride-oxide films

It is demonstrated that the thickness limit of a thin nitride film which can withstand reoxidation is reduced to about 3.5 nm when it is deposited in situ on a thin-deposited oxide film. The deposited oxide apparently provides a better surface for nitride nucleation and initial growth. Using this finding an oxide-nitride-oxide (ONO) film as thin as 4.6 nm was fabricated and shown to have good electrical properties and low defect density. The current leakage through the film was close to the acceptable limit in dynamic-random-access-memory technology. It was also found that electron trapping is substantially higher in ONO films produced by reoxidation than in films having a top deposited oxide.