Silicon nitride films have been successfully deposited at a temperature as low as 300°C by chemical-vapor-deposition using tctra-silane (Si4 H10) and hydrogen azidc (HN3). Atomic ratio (N/Si) of the film deposited at 400°C was 1.47, i.e., the film was N-rich. Total hydrogen content was about 25atomic%. The breakdown-field strength was 6.5MV/cm at leakage-current density of 1μA/cm2, and the low-field resistivity was more than 1015 Ωcm. Similar electrical characteristics were obtained from films deposited at a temperature range between 300°C and 500°C. Amorphous silicon thin-film transistors equipped with this film as the gate dielectric showed good transfer characteristics.

Silicon nitride thin films were deposited on single crystalline silicon substrates at room temperature by ECR PECVD with SiH4 and N2 as source gases and the electrical properties were analyzed. The dominant conduction mechanism in a high field was Poole-Frenkel emission. A ledge in I-V curve was observed in the first voltage ramp and it was found to originate from the field reduction at the injecting electrode due to the charge trapped in deep traps in the film. It also turned out that the ledge is a characteristic of monopolar conduction. A new interpretation of the current at low field — tunneling into trap states — was proposed and the current variations according to the field and temperature could be well explained.

We report on the material properties of SiNx:H films deposited using a 2% SiH4/N2 mixture with additional N2 in an ECR reactor. Deposition rates, refractive index, and stoichiometry have been characterized using ellipsometry, Rutherford backscattering spectroscopy, and infrared spectroscopy. Reactor conditions of 2m Torr total pressure, 650W microwave power, and substrate temperature of 250°C result in high quality, stoichiometric silicon nitride. With a SiH4/N2 ratio = 0.003, hydrogen incorporation is approximately 1.5% and the refractive index is nr =2.0. Lower microwave power and a higher SiH4/N2 ratio result in slightly N-rich films which is attributable to increased H-incorporation. Higher total pressure results in significantly enhanced deposition rates, but with greatly increased H and O content.

ABSTRACT: Silicon nitride films (a-SixN1−x:H) have been prepared by rf reactive magnetron sputtering from a silicon target in a mixture gas of Ar, N2, and H2. The effects of the presence of hydrogen gas have been related to the refractive index, deposition rate, etch rate, and the Si-H and N-H bonding in the films. Hydrogen contents were measured by a quadrupole secondary ion mass spectrometer (SIMS) using deuterium implanted samples as reference standards. The deuterium implanted samples were annealed at 900°C for various periods of time to study the diffusion behavior of deuterium and hydrogen in a Si-rich and a nearly stoichiometric silicon nitride film.

Radiofrequency glow discharges, operating under various conditions, have been used to deposit hydrogenated and fluorinated silicon nitride (a-Si,N:H,F) from SiF4-N2-H2 gas mixtures. The effect of the feeding mixture composition has been investigated in order to establish the optimum deposition conditions for stable silicon nitride. High H2-dilution of the feeding mixture has been found to produce transparent (Eg>5.6eV) and stoichiometric (N/Si=1.3) films.

Conditions for depositing quasi-stoichiometric silicon nitride films by low-temperature, remote plasma-enhanced chemical-vapor deposition, RPECVD, have been identified using on-line Auger electron spectroscopy, AES, and off-line optical and infrared, IR, spectroscopies. Quasi-stoichiometric films, by the definition propose in this paper, do not display spectroscopic evidence for Si-Si bonds, but contain bonded-H in Si-H and Si-NH arrangements. Incorporation of RPECVD nitrides into transistor devices has demonstrated that electrical performance is optimized when the films are quasi-stoichiometric with relatively low Si-NH concentrations.

An original process has been developed enabling the fabrication of CVD insulating (Si3N4) thin films, by means of an in-situ activation of the reactions at T < 400°C and under P=1 to 2 torr. Mono-Si substrates were nitrided using a mixture of argon containing SiH4 and NH3·O2 has also been added to the reaction gases. The activation was performed by a DC electrical discharge. The substrate was not used as an electrode and was placed parallel to the discharge current. This configuration minimized the contamination of the films during their formation. The obtained layers have been analyzed using FT-IR and SIMS.

M.I.S. structures have been realized, and the flat-band shift ΔVFB and the interface state density Nit have been extracted from the high and low frequency C-V characteristics. The values of the flat-band shift depend on the discharge domain and decrease with temperature. Good electrical characteristics are obtained for thin films formed at low temperature.

A conventional parallel plate plasma enhanced chemical vapor deposition system using SiH4-NH3-N2 at 13.56MHz and 300°C was used to deposit SiN films on silicon substrates. Special care was used to assure an oxide free surface before deposition. The silane flow was varied to obtain stoichiometric Si3 N4 and silicon-rich films. Rapid thermal anneal (RTA) was performed for 30 sec at temperatures in the range 600°C — 900°C. Spectroscopic ellipsometry was used to characterize the films. Data was taken for 44 wavelengths (3200–7500Å range) at three angles of incidence on each sample. The structural and optical properties were obtained using several structural models of the film. The models assumed an effective medium approximation for the following three combinations of film constituents: Si3N4 and voids; Si3N4 and amorphous silicon (a-Si); Si3N4, voids and a-Si. In general, the three constituent model gave the best fits to the experimental data. In all films, we observed a decrease in the film thickness and void fraction as the RTA temperature increased. The a-Si percentage did not change vs. the RTA temperature. The reduction in void fraction for the silicon-rich samples was larger than for the Si3N4 film. The results are comparable to those measured by other methods on regular furnace annealed SiN films, i.e. annealing times of 30 minutes or more.

Rapid thermal nitridation of the polycrystalline silicon film prior to the deposition of the silicon nitride dielectric film has been shown to be very effective in improving the dielectric characteristics for thin films. The changes at the polysilicon-silicon nitride interface has been further investigated using an in-situ clean process. This pre-treatment reduces the oxygen levels at the interface and improves the time dependent dielectric breakdown. The leakage current increases slightly due to the thinning of the silicon dioxide film at the interface.

Interactions between Co and a-Si3N4 films during Rapid Thermal Anneal (RTA) in N2 and Ar ambients, and during vacuum anneal, have been studied using plan-view and cross-sectional Transmission Electron Microscopy (TEM). Annealing in the temperature range 600–1000°C reveals agglomeration of the 12.5 nm Co layer and partial exposure of the nitride layer to the surface. Morphological damage of the Co/Si3N4 interface, and diffusion of Co into the nitride layer and possibly to the Si substrate, were observed after high temperature anneals. Phases of Co and CoSi2 were identified from the plan-view electron diffaction patterns. The Co/Si3N4 interaction seems to occur at lower temperature when annealed in vacuum. The microstructures and morphology of the layers and interfaces are presented. The mechanism for the diffusion of Co into the nitride layer is proposed.

The paper reviews the electronic properties of defects in amorphous silicon nitride (a-Si3N4) and the hydrogenated alloys a-SiNx:H. The main defects in a-Si3N4 are the Si and N dangling bonds (DBs). The Si DB forms a sp3 state near midgap, while the N DB forms a highly localized pπ level just above the valence band edge. The behaviour of the alloys changes near x ≈ 1.1, the percolation threshold of Si-Si bonds. In the Si-rich alloys, both band edges are Si-like, only Si DBs are seen and their density is controlled by equilibration with weak Si-Si bonds. The x>1.1 alloys behave like silicon nitride; the valence band changes towards N pπ-like, both Si and N DBs can arise, the Si DB can have a high density and prefers to be in its charged diamagnetic configurations.

We present the changes upon isochronous annealing in the room temperature photoluminescence (PL) spectra of nearly stoichiometric silicon nitride. Samples are prepared by the 50kHz PECVD of a N2/SiH4/Helium gas mixture at 350°C. In the as-deposited films the hydrogen content was around 11% for a refractive index of 1.98. For a photoexcitation at 351 nm, the polarized PL spectrum of the as-deposited film is characterized by a main broad emission band in the visible region with a maximum at 2.55eV. A second narrow peak (FWHM = 55meV), at 3.02eV is observed. We study the evolution of the PL spectra with different isochronous anneals from 350°C to 1000°C. Even though the general shape of the PL spectra does not change, we observe an increase of quantum efficiency with maximum value upon 750°C annealing. Beyond this temperature the low energy part of the PL spectrum shows a slight increase. These variations are compared to those of the infrared absorption peaks measured on the same samples. Both the temperature dependence of the intensity of a well defined absorption peak in the Si-H stretching mode region and that of the PL features can be explained assuming that solid state chemical reactions involving hydrogen and Si-Si bonds occur in the bulk of the alloy.

We have used a fast analog technique (100 ps resolution) to study photoluminescence (PL) decay in a-SiNx:H films in nanosecond (0.1–50 ns) range. Films prepared using RF glow discharge of NF3 and pure SiH4 were used for these measurements. The PL decays in the films were studied as a function of nitrogen content for 0≤x≤0.13. When the PL decay measured at 25 K is fitted with I (t) = ao +Σai exp (-t/Ti), where ao, ai and ti, are fitting parameters, we get three distinct lifetimes with t1 = 0.8 ± 0.2 ns, T 2 = 3.5 ± 0.5 ns and T3 = 14.0 ± 3.0 ns. We find that these lifetimes do not change with nitrogen content but their relative contributions to the PL decay change with nitrogen content. We have also studied the effect of temperature and excitation energy on the PL decays at different emission energies. We suggest an excitonic origin to these three recombination processes.

We have measured the optical absorption properties of Plasma Enhanced Chemically Vapor Deposited (PECVD) films of hydrogenated amorphous silicon nitride (a-SiNx:H) over the energy range from 0.54 to 2 eV using Photothermal Deflection Spectroscopy (PDS). The near-infrared absorption properties of these films provide a quantitative estimate of the amount of H bonded to Si or N as well as a measure of the the density of electronic gap states due to species like Si dangling bonds. Typical PDS spectra obtained on as-deposited films show a broad absorption tail with a large number of narrow absorption peaks. Using the phase sensitive feature of this spectroscopy we are able to separate nitride-related absorption from that arising from the fused quartz substrates. In addition to the broad, featureless band tail, whose magnitude is quite sensitive to optical excitation with light above ̃3.9 eV, we observe a number of narrow absorption peaks which we identify as overtones of localized vibrational modes. Specifically, we see peaks at 0.82 and 1.20 eV which we identify as the 1st and second harmonics of N-H stretching vibrations, and several other peaks whose origins remain undetermined. Prolonged UV excitation produces no detectable change in these vibrational modes, although large changes are seen in the underlying absorption tail. This reinforces previous suggestions that photoinduced changes in these films do not arise from rearrangement of the bonded hydrogen.

The free energy model (FEM) for bonding in a-SixNyHz alloys has been extended to include the contributions of neutral and charged Si and N defects to the free energy of mixing of the amorphous alloy. The FEM predicts that the dominant defects in N-rich alloys are N2o, N2-, and either S3+ or N2+, in contrast to the results of experimental studies that find the dominant neutral, paramagnetic defect to be Si3o. It is concluded that either the observed Si3o defects are not in thermodynamic equilibrium with the amorphous network or the N2o defects have energy levels which lie much higher in the energy gap than currently believed.

The photocreation mechanisms and properties of nitrogen dangling bonds in amorphous hydrogenated silicon nitride (a-SiNx:H) thin films are investigated. We find that the creation kinetics are strongly dependent on the post-deposition anneal; this thermal process can be described by a simple exponential function which yields an activation energy of 0.8 eV. The compositional dependence of the nitrogen dangling bond center suggests that its energy level lies close to the valence band edge, in agreement with theoretical calculations. This energy level position can explain why a-SiNx:H films often become conducting following a high post-deposition anneal.

Thin amorphous SiN films grown on Si are studied with Near Edge X-Ray Absorption Fine Structure (NEXAFS). The NEXAFS spectra of N-rich films are characterized by a strong resonance line (RL) at the onset of the N K- edge. The intensity of the RL in N-rich films increases with the N/Si ratio, while the same RL is detected in the NEXAFS spectra of stoichiometric SiN films subjected to damage with Ar+ ion bombardment. The RL is attributed to a N-related defect which is strongly localized in a pure 2p orbital. The RL, which is identified as a bulk nitride property, can be annealed out at temperatures higher than the growth temperature. It is proposed that the annealing process of the RL, which is characterized by an activation energy of 0.87eV, is due to Si-N bond formation, where the Si atoms are provided by the stressed SiN/Si interface.

Silicon rich PECVD amorphous silicon nitride has been used as an inter-level metal dielectric for making laser programmable connections on restructurable VLSI. There is an apparent Schottky barrier characteristic that has a 0.11 eV lower barrier for Ti than for Al. The stoichiometry was analyzed using RBS and HFS, and found to contain approximately 55% Si, 25% N, and 20% H by atomic percentages. The optical bandgap is 2.01 eV as found by the Tauc method.

The insulating behavior depends on time in an anomalous manner at applied fields greater than about 0.2 MV/cm, whereby the current increases with time for several secondsuntil it reaches an equilibrium value. The current decays in a normal charging manner at lower fields and in samples with insulating sub-layers between the electrodes and thenitride. When used as a gate dielectric, there is a long-time charging behavior that shifts the flat band voltage in the opposite direction of the applied stress. This shift is indicative of polarization within the dielectric. This behavior is similar to that of a reverse biased a-Si:H p-i-n diode.

Stress in thin films is one of the mayor technological problems of silicon micromachining. Therefore stress reduction by a compensation method increases the technological use of a material. Three methods to obtain stress free films are investigated:

1. Stress in SiO2 and Si3N4 can be compensated by ion implantation.

2. Sandwich layers of SiO2 and Si3N4 can be made internally stress compensated by variation of the thickness ratio.

3. Polysilicon is stress reduced by appropriate doping and annealing.

The film stress is evaluated by the measurement of the bending of the substrate. Examples of processed membranes, bridges and cantilevers made of stress compensated films are given.

The investigations of thermal stability of a-SiNx:H films (x≤0.4) have xbeen performed bv both IR spectroscopy and DTA techniques. The properties of samples with x≤0.06 were attentively considered because in this case the nitrogen may be treated as an impurity in a-Si:H. It has been shown that controlled nitrogen doping (up to 0.5 Pa NH3) can be used as an effective technique for thermostabilization of a-Si:H properties.