Stoichiometric Silicon Nitride Research Articles

Effect of ammonia plasma pretreatment on the plasma enhanced chemical vapor deposited silicon nitride films

Silicon-rich, nearly stoichiometric and nitrogen-rich silicon nitride films have been prepared by the glow-discharge decomposition of silane and ammonia with nitrogen dilution, with and without ammonia plasma pre-treatment of the silicon substrate. A considerable improvement in the silicon nitride/silicon interface, as evident from the large reduction in the minimum interface state density ( D it) min, due to ammonia plasma pretreatment is observed.

Electrical conduction studies of plasma enhanced chemical vapor deposited silicon nitride films

Current conduction mechanisms have been studied for three representative films, namely, silicon-rich, nearly stoichiometric and nitrogen-rich silicon nitride films, prepared by rf glow-discharge decomposition of silane and ammonia with nitrogen dilution. Ohmic conduction has been observed for all the films at low electric fields. The dominance of Poole–Frenkel conduction at intermediate fields and Fowler–Nordheim conduction at high fields has been observed both for the nitrogen-rich and the nearly stoichiometric films. However, for the silicon-rich films, the Poole–Frenkel conduction mechanism dominates both for the intermediate as well as the higher fields. This study indicates that the silicon-rich films have the highest density of traps and the nitrogen-rich films have the lowest, which may be ascribed to the effect of nitrogen dilution.

Reaction-diffusion model for thermal growth of silicon nitride films on Si

Thermal growth of ultrathin silicon nitride films on Si in ${\mathrm{NH}}_{3}$ is modeled as a dynamic system governed by reaction-diffusion equations. Solution of the model yields profiles of the involved species consistent with experimental observations of a stoichiometric silicon nitride layer in the near-surface region and a subnitride layer of comparable thickness in the near-interface region. Self-limited growth kinetics are also obtained from the model equations in good agreement with experimental results, owing to a diffusion barrier layer to the nitridant species formed in the near-surface region by stoichiometric silicon nitride.

Highest-quality surface passivation of low-resistivity p-type silicon using stoichiometric PECVD silicon nitride

The surface passivation properties of silicon nitride (SiN) films fabricated by high-frequency direct plasma-enhanced chemical vapour deposition (PECVD) on low-resistivity (1 Ω cm) p-type silicon solar cell substrates have been investigated. The process gases used were ammonia and a mixture of silane and nitrogen. In order to find the optimum set of SiN deposition parameters, a large number of carrier lifetime test structures were prepared under different deposition conditions. The optimised deposition parameters resulted in outstandingly low surface recombination velocities (SRVs) below 10 cm/s. Interestingly, we find the lowest SRVs for stoichiometric SiN films, as indicated by a refractive index of 1.9. In former studies similarly low SRVs had only been obtained for silicon-rich SiN films. The fundamentally different passivation behaviour of our SiN films is attributed to the addition of nitrogen to the process gases.

Low-temperature anodic bonding to silicon nitride

Low-temperature anodic bonding to stoichiometric silicon nitride surfaces has been performed in the temperature range from 350°C to 400°C. It is shown that the bonding is improved considerably if the nitride surfaces are either oxidized or exposed to an oxygen plasma prior to the bonding. Both bulk and thin-film glasses were used in the bonding experiments. Bond quality was evaluated using a tensile test on structured dies. The effect of oxygen-based pre-treatments of the nitride surface on the bond quality has been evaluated. Bond strengths up to 35 N/mm 2 and yields up to 100% were obtained.

Silicon nitride film growth for advanced gate dielectric at low temperature employing high-density and low-energy ion bombardment

Direct nitridation of silicon surface can be realized at a temperature as low as 430 °C by using high-density plasma above 1012 cm−3 featuring low ion bombardment energy below 7 eV. This study shows that stoichiometric silicon nitride can be obtained for the first time at a temperature of 400 °C by precise control of the nitrogen partial pressure to generate N2+ in the plasma. Moreover, hysteresis in the capacitance–voltage data that has been attributed to charge traps in silicon nitride film can be reduced dramatically by adding hydrogen to Ar/N2 plasma for terminating dangling bonds with hydrogen. The dielectric strength of silicon nitride films is nearly equivalent to those of thermally grown silicon nitride and silicon oxide. The leakage current of silicon nitride film is dramatically reduced compared to that of thermally grown silicon oxide even if their equivalent thicknesses are equal. The silicon nitride films have almost no stress-induced leakage current and very little trap generation even in high-field stress.

Ellipsometric investigation of nucleation sites for chemical vapor deposition of Si on SiO2 and Si3N4 surfaces

The nucleation of polycrystalline silicon by chemical vapor deposition on stoichiometric and silicon-rich silicon dioxide and silicon nitride films was studied using real time ellipsometry. The time of the appearance of the first stable Si nuclei, the incubation time (tinc), was found to vary with the Si content within a film type (oxide or nitride) and also tinc varied for different film types with the shortest tinc for Si-rich silicon nitride films. Differences between Si bound (to O or N) and free Si (bound to Si) were observed with free Si being more effective at promoting nucleation. Anion screening effects might also yield second order reactivity differences.

Low-Temperature Formation of Silicon Nitride Film by Direct Nitridation Employing High-Density and Low-Energy Ion Bombardment

High-integrity silicon nitride films can be obtained at a temperature of 400°C using a newly developed high-density (>1012 cm-3) plasma characterized by low ion bombardment energies of below 7 eV. It was found for the first time that stoichiometric silicon nitride can be formed at a temperature of 400°C by precise control of nitrogen partial pressure to generate N2+ in plasma. The growth rate and the electrical properties of this silicon nitride are similar to those of thermally grown nitride. Moreover, hysteresis of the C–V curve attributed to charge traps in the silicon nitride film disappeared with the addition of hydrogen to Ar/N2 plasma, and the leakage current of the nitride film was decreased dramatically by irradiating in Ar/N2/H2 plasma after Ar/N2 plasma nitridation. These technologies are very promising for fabricating feature metal substrate silicon-on-insulator (SOI) devices and silicon nitride gate metal-insulator-semiconductor field effect transistors (MISFETs).

LPCVD deposition of silicon nitride assisted by high density plasmas

A low pressure chemical vapour deposition (LPCVD) reactor was transformed into an inductively coupled plasma like plasma enhanced chemical vapour deposition reactor by placing a coil at the inlet side of the tube and applying 13.56 MHz power to it. Silicon nitride films were deposited at 300 °C in the plasma region, or at 720 °C at the center of the tube, either without plasma or with a remote plasma. The films deposited in the plasma region have a high deposition rate, more than 2.5 times that of the deposition rate of films deposited without plasma. Their refractive index ranges from 1.5–1.8, increasing with the ammonium to dichlorosilane ratio, indicating a nitrogen rich film. Fourier Transform Infrared Spectroscopy shows that at the higher ammonium to dichlorosilane ratios, the number of Si H bonds is as low as for stoichiometric silicon nitride. Etching these films in 2% HF results in high etch rates, 2–30 nm/s. The wafers deposited at 720 °C with remote plasma show an increased deposition rate, compared to films deposited without plasma, and refractive index of approximately 2.00.

Silicon nitride thin films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition

Using an electron cyclotron resonance plasma compact source, we have studied the deposition of silicon nitride films at low deposition temperature (<300 °C) and low microwave power (<250 W). Nitrogen plasma and pure silane have been used as gas precursors. We report on the effect of the main process parameters on the composition and properties of the films. We show that each experimental parameter has an optimal range of values or a threshold value necessary to obtain films with high dielectric quality. For a deposition temperature of 300 °C, the best films exhibit a resistivity of 1015 Ω cm and a soft breakdown field (at 10−9 A cm−2) of 3 MV cm−1. The physicochemical properties of the films are close to those of stoichiometric silicon nitride: N/Si ratio of 1.33, optical index value of 2 at 3 eV and etch rate of 10 Å/min. Moreover, we observed strong correlations between the physicochemical and the electrical properties of the deposited films, over the entire range of process parameters.

Thin stoichiometric silicon nitride prepared by r.f. reactive sputtering

Thin silicon nitride films were prepared on silicon wafers at deposition rates of 4 nm/min to 56 nm/min by r.f. reactive sputtering. Various mechanical, chemical and optical properties were investigated as a function of r.f. power, gas composition and temperature by means of AFM, RBS, XPS and an ellipsometer. Chemical stoichiometric films, with N-to-Si atomic ratio of 4:3 and refractive index of 2.0 were achieved even at room temperature. An interference filter for a UV detector with central wavelength of 254 nm was manufactured based on sputtered nitride, aluminum and silicon dioxide.

Stress relaxation in Si-rich silicon nitride thin films

Si-rich silicon nitride thin films have been deposited by low pressure chemical vapor deposition, at 850 °C from mixtures of dichlorosilane and ammonia. The films’ elastic properties have been studied as a function of film composition. Fourier transform infrared spectroscopy and ellipsometric data indicate that the local atomic strain is a strong function of the calculated volume fraction of Si contained in the films. A relationship is observed that shows the strain to be inversely proportional to the cube root of the Si volume fraction. A model that accounts for distortion in Si–SixN4−x tetrahedra (x=0–4), upon substitution of silicon for nitrogen in the film is applied to the data. The model is shown to be consistent with measurements of intrinsic film stress across a compositional range from stoichiometric silicon nitride, Si3N4, to nitrogen-free amorphous silicon, a-Si.

Characterization of the oxidation rate of densified SiN thin films by Auger and infrared absorption spectroscopies

Non-stoichiometric silicon nitride (SiN) layers were grown on silicon by Plasma Enhanced Chemical Vapor Deposition (PECVD) at temperatures in the range from 300°C to 350°C. After a densification process, explained in the text, these layers were oxidized in a dry oxygen ambient at 1100°C for different periods. Then, a study of the rate of oxidation was made by means of the Auger spectroscopic analysis of oxidized films. It is shown that the SiN oxidation follows a parabolic law similar to the oxidation of silicon or stoichiometric silicon nitride (Si 3N 4) obtained by other methods. The quality of the SiN films was correlated to that of the silicon oxide obtained from them by determining the absorption coefficient in the infrared (1070 cm −1). It was observed that IR absorption due to oxidized SiN coincides with the one corresponding to the thermal silicon oxide with good dielectric quality.

Nitride formation and dangling-bond passivation on Si(111)-(7 × 7) with NH 3

Thermal nitridation of the Si(111)-(7 × 7) reconstructed surface with ammonia has been investigated using scanning tunneling microscopy (STM). True nitride formation in the form of ring-like structures as in stoichiometric silicon nitride (Si 3N 4) was observed at imperfections on the surface, which otherwise preserved the characteristics of the (7 × 7) reconstruction. However, the ratio of reacted adatoms in the reconstruction never exceeded ∼ 50%, indicative of a frustrated saturation behavior for the adatom dangling bonds in the Si(111)-(7 × 7)-NH 3 reaction system.

AES analysis of nitridation of Si(100) by 2–10 keV N +2 ion beams

Ion beam nitridation of Si(100) as a function of N + 2 ion energy in the range of 2–10 keV has been investigated by in-situ Auger electron spectroscopy (AES) analysis and Ar + depth profiling. The AES measurements show that the nitride films formed by 4–10 keV N + 2 ion bombardment are relatively uniform and have a composition of near stoichiometric silicon nitride (Si 3N 4), but that formed by 2 keV N + 2 ion bombardment is N-rich on the film surface. Formation of the surface N-rich film by 2 keV N + 2 ion bombardment can be attributed to radiation-enhanced diffusion of interstitial N atoms and a lower self-sputtering yield. AES depth profile measurements indicate that the thicknesses of nitride films appear to increase with ion energy in the range from 2 to 10 keV and the rate of increase of film thickness is most rapid in the 4–10 keV range. The nitridation reaction process which differs from that of low-energy (< 1 keV) N + 2 ion bombardment is explained in terms of ion implantation, physical sputtering, chemical reaction and radiation-enhanced diffusion of interstitial N atoms.

Nitrogen- and ammonia-plasma nitridation of hydrogenated amorphous silicon

Nitrogen- or ammonia-plasma nitridation of hydrogenated amorphous silicon (a-Si:H) was carried out. A nearly stoichiometric silicon nitride layer with a thickness of several tens Å is formed on a-Si:H. The nitride-layer thickness linearly increases with an increase in the nitridation time after a rapid increase in the early stage. The neutral-Si-dangling-bond (DB) density in the a-Si:H layer rapidly increases in the early stage of the nitridation, then it gradually decreases as the nitridation time increases. The larger increase in the neutral-Si-DB density in the early stage of the nitridation is observed in the nitrogen-plasma nitridation or at lower nitridation temperature. On the other hand, the sum of neutral- and charged-Si-DB densities in a-Si:H layer does not decrease with an increase in the nitridation time as the neutral-DB density does, suggesting that the charge injection from the nitride layer to the a-Si:H layer originating from the band bending occurs with the formation of the nitride layer on a-Si:H.

Molecular ion implantation in silicon

15N 2 + molecular ions were implanted with 10 keV (j = 10 μA/cm 2 ) under high vacuum conditions close to room temperature in silicon (c-Si) to study the 15 N depth distributions, particularly the dependence of peak concentration and dose on the ion fluence. The analysis were performed by the resonant nuclear reaction 15 N(p,xγ) 12 C(NRA). A maximum peak concentration of 65 at.% was measured. Thin stoichiometric silicon nitride layers with a thickness of approx. 20nm (15 at.% nitrogen at the specimen surface) were produced by this low-energy implantation of 15 N 2 - ions with an ion fluence of 1.5.10 17 ions/cm 2 . NRA analysis of 38 keV 15 N 2 and 19 keV 15 N + ion implantations were performed to compare the 15 N depth distributions. No significant changes in the depth distributions are measured, that means, the molecular 15 N 2 + ions are already disintegrated passing the very first atomic layers of the sample during implantation. Non-Rutherford RBS with 4 He + ions and 3.45 MeV was performed in order to confirm the results obtained by NRA.

Pulsed laser deposition of silicon nitride thin films by laser ablation of a Si target in low pressure ammonia

We report the deposition of Si-N films by multipulse excimer laser (λ = 308 nm, τFWHM = 30 ns) ablation of Si wafers placed in a slow flow of NH3 in the pressure range (1 μbar-1 mbar). The films are deposited on to a Si collector placed parallel to the Si target. We succeeded in depositing pure amorphous Si3N4 films at a pressure of 1 mbar of NH3. The deposition rate reached a maximum value of 0.2–0.3 nm per pulse. At lower pressures, the deposited films consist of a fine mixture of three amorphous phases (amorphous stoichiometric silicon nitride, amorphous non-stoichiometric silicon nitride and amorphous silicon). The amorphous silicon is prevalent in films deposited at a pressure of several to several tens of μbars. Droplets of polycrystalline α-Si are sometimes visible on the film surface. The experimental evidence, is analysed with a view to elucidating the participation in the chemical synthesis of the three main stages of the process: the substance expulsion from the target by laser ablation, the transition through the gas of the expulsed substance and it's final impact on the collector. We conclude that silicon nitride is mostly synthesized during the impact on the collector of the flow of the ablated substance.

Electrical conduction in silicon nitrides deposited by plasma enhanced chemical vapour deposition

Abstract Current conduction in Si-rich and stoichiometric silicon nitride as well as in oxynitride is studied in detail. Representative samples from each of the three categories exhibit Ohmic, Poole-Frenkel and Fowler-Nordheim tunnelling currents to differing extents. All the samples show an Ohmic region at low electric fields. The oxynitride sample has the largest Ohmic region, and the Si-rich sample the smallest. Under intermediate and high fields, the oxynitride sample shows only Fowler-Nordheim tunnelling, but the Si-rich sample is dominated by Poole-Frenkel emission. The stoichiometric sample shows a Poole-Frenkel region at intermediate fields and a Fowler-Nordheim region at high fields. These results indicate that the oxynitride sample has the lowest density of traps, and the Si-rich sample has the highest. Our observations are explained in terms of the flexibility of bonding configurations of N and O atoms, and the formation of Si[sbnd]Si bonds in Si-rich silicon nitride.

Electronic structure of nanometric Si/C, Si/N, and Si/C/N powders studied by both x-ray-photoelectron and soft-x-ray spectroscopies.

The electronic structure of nanometric Si/C, Si/N, and Si/C/N powders prepared by laser synthesis from appropriate gas mixtures has been investigated by using two complementary experimental methods. The total and partial Si 3p valence-band (VB) distributions were obtained from X-ray-photoelectron and soft-x-ray-emission spectra, respectively; the conduction-band (CB) Si p states were studied through the Si K x-ray-photoabsorption spectrum. For the binary compounds, the results confirm that the laser-synthesized (LS) powders are very similar to stoichiometric silicon carbide or silicon nitride. The valence states distributions are significantly different for the two compounds, due essentially to the presence of the lone pair N 2p\ensuremath{\pi} orbital at the top of the VB in silicon nitride. For ternary systems, differences are observed between the VB and CB distributions observed from two LS powders corresponding to C/N values 0.22 and 0.93. They are interpreted in terms of differences in local bonding of Si atoms. In one case, for C/N (atomic ratio)=0.93, both Si-${\mathrm{C}}_{4}$ and Si-${\mathrm{N}}_{4}$ groups associated, respectively, with silicon carbide and silicon nitride compounds are present in the network while for C/N=0.22, the results are consistent with the existence of a local Si environment in which both Si-C and Si-N are present around the same Si atom, as already proposed from previous x-ray-photoelectron and extended x-ray-absorption fine-structure studies of the same samples. \textcopyright{} 1996 The American Physical Society.