Nitridation Of Si Research Articles

Multiple adsorption configurations ofNH3molecules on the Si(001) surface

Initial adsorption configurations of $\mathrm{N}{\mathrm{H}}_{3}$ molecules on the Si(001) surface are studied by using scanning tunneling microscopy (STM) and ab initio pseudopotential calculations. In contrast to the previous consensus of single configuration, we demonstrate that there exist two distinct STM features. One is found to be the existing model of on-dimer configuration, that is characterized by the previously unreported ``U-shape'' empty-state feature. The other is identified to be $\mathrm{N}{\mathrm{H}}_{2}$ and H adsorbed at the same side of two adjacent dimers along the dimer row. The identification of all the adsorption configurations would be useful for the quantitative analysis of Si nitridation.

Oxidation of Nitrided Si(100) by Gaseous Atomic and Molecular Oxygen

The nitridation of Si(100) by ammonia and the subsequent oxidation of the nitrided surface by both gaseous atomic and molecular oxygen was investigated under ultrahigh vacuum (UHV) conditions using X-ray photoelectron spectroscopy (XPS). Nitridation of Si(100) by the thermal decomposition of NH3 results in the formation of a subsurface nitride and a decrease in the concentration of surface dangling bond sites. On the basis of changes in the N1s spectra obtained after NH3 adsorption and decomposition, we estimate that the nitride resides about four to five layers below the vacuum-solid interface and that the concentration of surface dangling bonds after nitridation is only 59% of its value on Si(100)-(2 x 1). Oxidation of the nitrided surface is found to produce an oxide phase that remains in the outer layers of the solid and interacts only weakly with the underlying nitride for oxygen coverages up to 2.5 ML. Slight changes in the N1s spectra observed after oxidizing at 300 K are suggested to arise primarily from the introduction of strain within the nitride, and by the formation of a small amount of Si2=N-O species near the nitride-oxide interface. The nitrogen bonding environment changes negligibly after oxidizing at 800 K, which is indicative of greater phase separation at elevated surface temperature. Nitridation is also found to significantly reduce the reactivity of the Si(100) surface toward both atomic and molecular oxygen. A comparison of the oxygen uptake on the clean and nitrided surfaces shows quantitatively that the decrease in dangling bond concentration is responsible for the reduced activity of the nitrided surface toward oxidation, and therefore that dangling bonds are the initial adsorption site for both gaseous oxygen atoms and molecules. Increasing the surface temperature is found to promote the uptake of oxygen when O2 is used as the oxidant, but brings about only a small enhancement in the uptake of gaseous O-atoms. The different effects of surface temperature on the uptake of O versus O2 are interpreted in terms of the efficiency at which dangling bond pairs are regenerated on the surface at elevated temperature and the different site requirements for the adsorption of O and O2.

In situ Monitoring of Surface Processes in Plasma by Coaxial Impact-Collision Ion Scattering Spectroscopy

We demonstrated in situ monitoring of surface processes in plasma, using coaxial impact-collision ion scattering spectroscopy (CAICISS) operable in a gas atmosphere, taking the plasma nitridation processes of Si surfaces with (111) and (001) faces as examples. From the results of real-time monitoring of CAICISS spectra, it was found that (1) the top layer of the Si(111) surface is readily nitrided even at room temperature, while the Si(001) surface requires thermal activation for its nitridation and that (2) at a high temperature, the nitridation of Si(001) proceeds to layers deeper than in the case of Si(111).

NH 3-RTP grown ultra-thin oxynitride layers for MOS gate applications

Ultra-thin oxynitrides can serve as gate dielectrics for the technology nodes 100 nm and below. In this work, we present the properties of ultra-thin oxynitride gate dielectrics prepared by RTP nitridation of Si in NH 3 followed by post-grown oxidation in O 2 or in steam (post nitridation anneal, PNA). The layers show excellent barrier properties including significantly lower leakage current compared to SiO 2 of identical equivalent oxide thickness (EOT). For the same EOT, the tunnel current density of the RTP oxynitride layers were about four orders of magnitude lower compared to SiO 2. With optimised, the interface state density ( D it) of the RTP-grown oxynitride layer is in the region of a good SiO 2 layer ( D it ∼ 1E11 eV −1 cm 2). X-ray photoelectron spectroscopy (XPS) data of selected oxynitride layers indicate that a nitrogen concentration of as high as 31% can be achieved by RTP process. RTP grown oxynitride layers were applied to NMOS transistors as gate dielectrics and their device performances were compared with those prepared by RF plasma nitridation (RF-PN). Transistors with RTP-grown oxynitride gate show a significantly better uniformity in threshold voltage on 200 mm wafers than those oxynitride layers grown by RF-PN. It was also found that the leakage currents of the RTP and RF-PN gate oxynitrides obey the same trend from the 1.5 nm EOT regime down to the 1.0 nm EOT regime. This observation indicates that the leakage current barrier quality of the RTP oxynitride is at least as good as the RF-PN oxynitrides.

Effect of diluents and NH 4F additive on the combustion synthesis of Yb α-SiAlON

In this paper, Yb α-SiAlON crystalline powders with elongated grains were prepared by combustion synthesis. The effects of diluents and NH 4F additive on the phase composition and microstructure of combustion products were discussed in detail. It is found that proper diluents are helpful to weaken the agglomeration, improve the infiltration of N 2, and hence facilitate the nitridation reaction. Single-phase α-SiAlON with elongated grains could be fabricated with advisable addition of diluents. Furthermore, adding proper amounts of NH 4F additive was also beneficial to the nitridation of Si and the development of elongated grains. However, excessive addition of α-Si 3N 4 diluents or NH 4F additive will result in β-SiAlON in final phase composition.

Characterization of Interfacial Oxide Layers in Heterostructures of Hafnium Oxides Formed on NH3-Nitrided Si(100)

In the stack structures fabricated by NH3 nitridation of Si(100) at 650–700°C and subsequent electron-beam evaporation of HfO2, the blocking properties of ultrathin SiNx (x=∼1.3) upon oxidation in dry O2 ambient in the temperature range of 300–600°C have been studied by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared attenuated total reflection (FT-IR-ATR). Although the oxidation of Si(100) during O2 anneal is retarded using a ∼1-nm-thick SiNx layer grown by 700°C nitridation in NH3 ambient prior to the HfO2 evaporation, the surface oxidation of the ultrathin SiNx layer proceeds, accompanied by the movement of nitrogen atoms from the oxidized SiNx surface to the Si surface. As a result, the interfacial layer thickness is increased with no significant changes in nitrogen bonding features at the interface. From the temperature dependence of the interfacial oxide formation in the stack structure of HfO2 on thermally nitrided Si(100), we suggest that the control of O2 partial pressure is practically inevitable in suppressing completely the interfacial oxide growth at a thermal budget higher than 350°C.

Post-densification behavior of reaction-bonded silicon nitride (RBSN): Effect of various characteristics of RBSN

The effects of the various characteristics of reaction-bonded silicon nitride (RBSN), such as crystalline secondary phases, residual Si, pore size distribution, and agglomerates developed during nitridation on the post-densification behavior, were investigated in detail. A model experiment was carried out to study the effect of crystalline secondary phases in RBSN on the post-densification. The result clearly indicates that the pre-formation of crystalline secondary phases does not influence the sintering of Si3N4-powder compacts, which is further demonstrated in the post-densification of RBSN, because the formation of a liquid, including its composition and properties, is dependent on the composition of the system and sintering temperature, but independent of the presence of crystalline secondary phases. In particular, the complete melt of crystalline secondary phases is not related to their melting points and only relies on the composition of the system. The residual Si and pore size distribution in RBSN show no significant effect on the post-densification. Some large pores of ∼5 μm in size could be eliminated by liquid filling and grain growth during post-sintering. The residual Si in RBSN could change into β-Si3N4 by nitridation of liquid Si and contribute to the weight gain during the early stage of post-sintering. However, it is shown that the agglomerates in RBSN play an important role in the post-densification. The effect of agglomerates on the post-densification strongly depends on their size and amounts in RBSN. Nearly complete densification of RBSN could be reached through lowering of the nitriding temperature or adding Si3N4 powder to green bodies, which leads to the formation of more uniform microstructure during nitridation. A mechanism that explains how the agglomerates affect the post-densification of RBSN was discussed based on the theories of liquid phase sintering.

Si(100) Nitridation by Remote Plasmas for Enhanced High-κ Gate Performance

ABSTRACTHafnium oxide is the leading high-κ candidate for next generation CMOS devices, however, it is plagued by problems such as a propensity to react with the Si(100) substrate, to crystallize, and to have fixed and trapped charges and low transistor mobilities. A remote N2/He plasma was used as a reactant during MOCVD with Hf (IV) t-butoxide which incorporates 6 at.% nitrogen located at the film-substrate interface and reduces the interdiffusion problem upon anneal [1]. A new process for pretreating the silicon wafer with a N2/He plasma was developed to improve nitrogen concentrations at the interface and reduce interdiffusion further. Films deposited with N2/He plasma and the pretreatment method were compared to O2/He plasma deposited films and N2/He plasma deposited films without the nitridation step. It is shown that a 16Å interface SiNx layer is sufficient to prevent reaction during a 1000°C Ar/O2 anneal at atmosphere and an intermediate annealing step is crucial for desired reduction in interdiffion. Thick films show some crystalline peaks by XRD which are suppressed using the N2 process. Electrical measurements on thick films show the pretreatment process results in the lowest leakage current density and the highest dielectric constant of 21.5.

Study of plasma-nitriding process in Si with in situ spectroscopic ellipsometry

In situ spectroscopic ellipsometry was applied to plasma-nitriding process in single crystalline Si. It was shown that plasma nitridation of Si takes place even at room temperature and the plasma-damaged layer is recovered to single crystalline layer in heating-up process to 1300 K. The disappearance of oxide layer from Si surface during heating-up process at ∼1200 K was observed by in situ model analysis, which was used to determine the start timing of plasma nitridation.

Dynamics of the ion beam induced nitridation of silicon

High-resolution Rutherford backscattering and channeling has been used to study the energy and angular dependence of the ion beam induced nitridation of Si in a secondary ion mass spectrometry system. The nitridation of Si is characterized by two critical angles θc1 and θc2, corresponding to the formation of stoichiometric and overstoichiometric Si-nitride layers, respectively. For the N2+ bombardment in the 10 to 13.5 keV range, θc1 changes from 40° to 45°, while θc2 changes from 28° to 30°. Further, strong oscillations in the secondary ion signal, observed for angles of incidence below θc2, are directly related to charging of the Si-nitride surface. We demonstrate that the response of the Si-nitride layer under ion bombardment during the transient stage of nitridation can be described by a second order differential equation.

Growth Processes and Electrical Characteristics of Silicon Nitride Films Formed on Si(100) by Radical Nitrogen

We have investigated nitridation processes on clean Si(100)-2×1 surfaces and the electrical characteristics of metal-insulator-semiconductor (MIS) capacitors with nitride films formed by radical nitrogen. The radical nitrogen exposure to Si surfaces at 500°C initially causes both nitridation of Si and local detachment reactions on the surface. It is found from scanning tunneling microscopy and scanning tunneling spectroscopy that the nitridation proceeds in a layer-by-layer manner in which a continuous film is formed by the coalescence of two-dimensional islands. At nitride thicknesses above 0.6 nm, the nitride film is homogeneously formed over the entire surface. The MIS capacitor with a 5.4-nm-thick nitride film formed by radical nitrogen shows low hysteresis of less than 0.1 V and low leakage current density. These electrical properties indicate that the nitride films formed by radical nitrogen are applicable to gate insulators.

Monolayer Nitridation of Si(001) Surfaces

Well-ordered nitrogen-covered Si(001) surfaces were formed for the first time by thermal nitridation using an N2/H2 gas mixture. The scanning tunneling microscope images showed clear Si(001)-2×2 reconstruction over the entire surface. The ordered structure of the nitrided surfaces can be explained by a model in which the surface nitridation by N2 and the termination of the remaining Si dangling bonds by hydrogen atoms occur simultaneously to minimize the surface free energy during the nitridation reaction.

Nitridation of Si(100)-( 2x1) surface by NH3: a quantum chemical cluster model study.

Based on density functional cluster model calculations, we present the first detailed mechanisms for the complete decomposition of NH3 to NHx(a) (x = 0-2) on the Si(100)-(2x1) surface. Three kinds of elementary processes, namely, N-H bond cleavage, NHx(a) insertion into the Si-Si surface dimer bond or backbond, and H2 libration, are investigated. A plausible microscopic mechanism for the nitridation of Si(100)-(2x1) surface by NH3 is proposed.

Thermal effects in atomic-order nitridation of Si by a nitrogen plasma

Thermal effects in atomic-order nitridation of Si(100) by an electron-cyclotron-resonance nitrogen plasma were investigated by controlling the Si surface temperature. It is found that the N atom concentration increases linearly and then tends to saturate with increasing plasma exposure time. In the radical reaction limited nitridation, the N atom concentration on the Si surface is normalized by the radical density in the nitrogen plasma, and the nitridation rate does not depend on the Si surface temperature above about −25 °C and decreases with decreasing the temperature below about −25 °C. The saturated N atom concentration is a single atomic layer and more than double atomic layers on the Si surface of about −80 °C and above about −25 °C, respectively. On the other hand, by the contribution of the incident ion, the nitridation of the deeper Si atoms below the surface is enhanced, and the saturated N atom concentration is lower at higher pressure and lower Si surface temperature. Consequently, it is sugg...

Atomic-order thermal nitridation of Si(100) and subsequent growth of Si

Atomic-order nitridation by NH3 on Si(100) and subsequent Si growth by SiH4 were investigated using an ultraclean low-pressure chemical vapor deposition system with a Xe flash lamp. In thermal nitridation on Si(100), it is found that nitridation occurs even at 260 °C with flash heating, and the N atom concentration tends to saturate to about 2.4×1015 cm−2. In Si deposition on the ultrathin Si nitride, it is found that N desorption from the Si nitride films hardly occurs, and Si grew on Si nitride at 385 °C in an SiH4 environment with and without the flash lamp light irradiation. An incubation period of Si growth is observed and increases with increasing N atom concentration of the Si nitride film. On Si with N atom concentration of about 2×1014 cm−2, the incubation period is hardly observed and the reflection high energy electron diffraction patterns indicates that Si epitaxially grew. Layer-by-layer growth control of Si nitride is proposed by combining atomic-order nitridation on Si and atomic-layer growth of Si on the Si nitride.

Influence of substrate nitridation before growth on initial growth process of GaN heteroepitaxial layers grown on Si(0 0 1) and Si(1 1 1) substrates by ECR-MBE

We have investigated influence of substrate nitridation before growth on initial growth process of GaN heteroepitaxial layers grown on Si(0 0 1) and Si(1 1 1) substrates by electron cyclotron resonance assisted molecular-beam epitaxy. The substrate nitridation of Si(1 1 1) suppresses the inclusion of zincblende-GaN (β-GaN) crystal grains and facilitates the epitaxial growth of wurzite-GaN (α-GaN) accompanying the improvement of crystalline quality. On the other hand, the nitridation of Si(0 0 1) does not suppress the inclusion of β-GaN and degrades crystalline quality of α-GaN.

Oxynitridation of silicon with nitrogen plasma for flash memory applications characterized by high frequency capacitance–voltage measurements

Si(100) wafers were oxynitrided by nitrogen plasma exposure under different conditions and thermally oxidized in dry O2 without the use of toxic or global warming gases. The atomic concentration ratio of N/Si was 0.075 at a plasma discharge power of 0.5 kW and an exposure time of 1 min and 0.176 at a plasma power of 2.0 kW and an exposure time of 3 min, as determined from X-ray photoelectron spectroscopy measurements. A progressive reduction in the oxidation rate with increasing N concentration, corresponding to an increase in N plasma power, was observed. The uniformity of film thickness over a 4-inch wafer was improved after nitrogen plasma exposure and rapid thermal oxidation, with a thickness variation of less than 2.6% compared with 13.2% for the N2O thermal oxidation process. The advantages of exposing Si to N plasma for application in silicon devices was investigated by high-frequency capacitance–voltage measurements. A distortion in the capacitance–voltage curve was observed for samples that did not undergo nitridation, a distortion that was not observed in the nitrided samples. Capacitance–voltage curve distortion was found to be significantly reduced by Si nitridation under low plasma power.

Isotope-beam modification of materials at eV energies

We developed a low energy ion beam deposition system for isotope-selective modification of materials. It consists of a conventional ion implanter (HVEE 500 kV) and an attachable deceleration system. 29(N 2) + ion beams were used for the nitridation of Si(0 0 1) and the resulting 15N retained doses and profiles were determined by narrow nuclear resonance profiling. 29Si was deposited on amorphous carbon films on Si(0 0 1) and the doses evaluated by channeled α particle beams with detection of scattered α at grazing angles. 29Si was also deposited on Si(0 0 1) and the resulting profiles determined by narrow nuclear resonance.

Growth and Characterization of SiC / SiNx / Si Structures

Silicon carbide (SiC) films were grown on structures. The layers were fabricated on Si surface with different thickness by either the nitridation of Si(111) surface with or the deposition of thick films by rf magnetron sputtering technique. The grown films were a single-crystalline 3C-SiC(111) which follows the crystal orientation of Si substrate. The thickness of SiC films grown on substrates decreased with the increase of the nitridation time as well as the thickness. The formation of voids was significantly suppressed from SiC/Si interface by the nitridation of Si surface. It was observed that SiC films grown on 100, 300, and 500 nm thick substrates consisted of columnar grains of SiC crystal. This work suggests that the fabrication of SiC devices can be realized in the structure. © 2001 The Electrochemical Society. All rights reserved.

Ammonia as a precursor in electron-enhanced nitridation of Si(100)

Electron beam-enhanced nitridation of Si(100) using ammonia as a precursor at 110 K was studied with electron-stimulated desorption, XPS, AES and high-resolution electron energy-loss spectroscopy (HREELS). Hydrogen ion kinetic energy distributions from adsorbed ammonia exhibited a component from NH 3 (a) at 7.8 eV, from NH 2 (a) at 5.4 eV and from H(a) at 4 eV. Formation of the nitride following electron beam irradiation of adsorbed ammonia was shown by both N 1s at 398 eV and Si 2p at 102 eV in XPS spectra. From HREELS spectra following electron beam irradiation, we are able to show that the electron beam is highly effective in removal of hydrogen from NH x (a) (x = 3,2) and from the silicon surface. Electron-stimulated desorption of adsorbed hydrogen and dissociation of adsorbed NH x (x = 3,2) are believed to be responsible for enhanced nitridation. Nitride can also form on the surface by irradiating the surface in an ammonia environment at 110 K. A nitride growth rate of 1 A min -1 was obtained from the bulk Si LVV AES signal intensity attenuation.