Nitrogen-rich Silicon Nitride Films Research Articles

Ultra-high vacuum deposition and characterization of silicon nitride thin films

Silicon nitride thin films were deposited on (100) Si wafers in an ultra-high vacuum system using a Si effusion cell and reactive nitrogen from a radio-frequency plasma source. The films were characterized using infrared transmission spectroscopy, infrared reflectance, Rutherford backscattering spectrometry, spectroscopic ellipsometry, specular x-ray reflectivity, wet etching in a buffered-oxide etch solution, and the electrical characterization of metal-insulator-semiconductor capacitors. High-quality, stoichiometric silicon nitride films with a refractive index of 2.05 at 632.8 nm are produced when the deposition temperature is 750 °C. Lower deposition temperatures produce nitrogen-rich silicon nitride films with lower refractive index, lower density, greater tendency toward oxidation in ambient air, faster etching in a buffered oxide etch solution, and greater electrical leakage. A deposition model involving thermal evolution of weakly-bonded excess N is proposed to explain our observations.

Modeling the charge decay mechanism in nitrogen-rich silicon nitride films

The stability of negative charge in nitrogen-rich silicon nitride films deposited by plasma-enhanced chemical vapor deposition is investigated by analyzing the influence of storage temperature, postdeposition thermal annealing, and the presence of a tunnel oxide. The results are compared to a charge decay model. Comparison of experimental and modeled results indicates that (i) the tunnel oxide is almost entirely responsible for charge retention in samples with an oxide-nitride-oxide (ONO) structure, with the trap properties playing an insignificant role; (ii) thermionic emission over the tunnel oxide barrier is the limiting charge decay mechanism; and (iii) thermal annealing of the films at 800 °C leads to an increase in the oxide-nitride barrier height by ∼0.22 eV, which results in a significant increase in the charge stability. Annealed ONO samples are predicted to maintain a negative charge density of >5×1012 cm−2 for well in excess of 100 years at a storage temperature of 100 °C.

Characterization of nitrogen-rich silicon nitride films grown by the electron cyclotron resonance plasma technique

Amorphous hydrogenated silicon nitride films have been deposited by the electron cyclotron resonance plasma technique, using N2 and SiH4 as precursor gases. The gas flow ratio, deposition temperature and microwave power have been varied in order to study their effect on the properties of the films, which were characterized by Rutherford back-scattering spectrometry, elastic recoil detection analysis (ERDA), Fourier transform infrared spectroscopy and ellipsometry. All samples show N/Si ratios near or above the stoichiometric value (N/Si = 1.33). The hydrogen content determined from ERDA measurements is significantly higher than the amount detected by infrared spectroscopy, evidencing the presence of non-bonded H.As the N2/SiH4 gas flow ratio is increased (by decreasing the SiH4 partial pressure), the Si content decreases and the N–H concentration increases, while the N content remains constant, resulting in an increase of the N/Si ratio. The decrease of the Si content causes a decrease of the refractive index and the density of the film, while the growth ratio also decreases due to the limiting factor of the SiH4 partial pressure. The infrared Si–N stretching band shifts to higher wavenumbers as the N–H concentration increases.The increase of deposition temperature promotes the release of H, resulting in a higher incorporation of N and Si into the film and a decrease of the N/Si ratio. The effect of increasing the microwave power is analogous to increasing the N2/SiH4 ratio, due to the increase in the proportion of nitrogen activated species.

Preparation of SiN x film by pulsed laser ablation in nitrogen gas ambient

Silicon nitride films were synthesized by reactive pulsed laser ablation (PLA) of a Si target in N 2 gas atmosphere. At different laser fluences and N 2 gas pressures the infrared absorption peak attributed to Si–N bond was evaluated. The nitrogen concentration in the film increased with the increasing fluence. Nitrogen concentration depended also on N 2 gas pressure; it increased as N 2 pressure increase up to 10 Pa and then it decreased with further increasing N 2 gas pressure. These results indicate that decomposition of N 2 molecules and collisions of SiN x clusters with N 2 molecules are essential to prepare silicon nitride films by PLA method. The PLA is a promising method to fabricate nitrogen rich silicon nitride films without using poisonous gases such as silane and ammonia.

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.

Factors affecting interface-state density and stress of silicon nitride films deposited on Si by electron-cyclotron resonance chemical vapor deposition

The physical and electrical properties of nitrogen-rich silicon nitride films deposited by electron-cyclotron resonance chemical vapor deposition with silane and molecular nitrogen have been investigated for pressures below 0.4 Pa. No Si–Si bonding or oxygen has been observed in the nitride films by Auger spectroscopy, and no SiH or NH2 groups have been observed by Fourier transform infrared spectroscopy, showing that the films have the composition SiNy−z(NH)z. As the pressure was decreased by lowering the nitrogen flow, the stress in the films became more compressive while the amount of N–H bonding in the films increased. The electron temperature determined by Langmuir probe measurements increased at lower pressures as the plasma made a transition from overdense to underdense. Despite the increasing stress, electron temperature and NH concentration, capacitance–voltage (C–V) analysis of metal-nitride-Si〈100〉 diodes showed that the SiN/Si interface improved with decreasing pressure. Using Al gates and 5 Ω cm p-type Si〈100〉 substrates an interface state density of 5×1010 eV−1 cm−2 was determined by the high-low frequency C–V measurement method for 30 nm thick films deposited at a substrate temperature of 300 °C and the lowest pressure of 0.055 Pa. Bulk conduction by the Frenkel–Poole mechanism dominated the current–voltage characteristics for negative gate potentials and breakdown voltages >9 MV/cm were obtained at this pressure. In situ single-wavelength ellipsometry showed that the interface is formed predominantly by nitridation of the Si substrate, and thus its high quality must be attributed to this nitridation rather than the chemical vapor deposition process. The results are discussed in terms of recent models for defects at the SiN/Si interface.

Light-induced effects in hydrogenated amorphous nitrogen-rich silicon nitride films

We report on new photoconductivity fatigue effects and generation of positive fixed charges when nitrogen-rich gate-quality silicon nitride films are illuminated by sub-bandgap ultra-violet light. The photoconductivity fatigue is correlated with the density of neutral silicon dangling bonds measured by electron spin resonance. Spins and positive fixed charges are generated at the same time, but at different rates and in different amounts. Photon energy threshold for spins and positive fixed charges creation is around 3.4 eV. Annealing studies have also shown that a temperature of at least 350°C, and 400°C is needed to fully recover the initial (dark) spins and positive fixed charges densities, respectively, in the film deposited at 400°C.

Stability of electrical properties of nitrogen-rich, silicon-rich, and stoichiometric silicon nitride films

Metal-nitride-silicon capacitors were used to explore the stability of the electrical properties of silicon nitride films of various stoichiometries. Silicon-rich silicon nitride films were found to display a large and symmetric hysteresis loop in the capacitance-voltage curve, a large flat-band voltage shift approximately symmetric with respect to the polarity of the voltage under bias temperature stress, and a current density-voltage characteristic which is approximately symmetric with respect to polarity. It is argued that in these silicon-rich silicon nitride films the dominant defects are silicon dangling bonds which can trap either electrons or holes. A different class of behavior was observed, however, for nitrogen-rich silicon nitride films deposited by plasma enhanced chemical vapor deposition. These films have a much smaller and asymmetric hysteresis loop in the capacitance-voltage curve, an asymmetric flat-band voltage shift under bias temperature stress, and an asymmetric current density-voltage characteristic. In addition, the breakdown electric field was found to be higher for these films for positive than negative gate biases. Our proposed explanation for these observations is that silicon dangling bonds, which can trap either electrons or holes, are greatly reduced in nitrogen-rich films and the dominant residual traps are hole traps.

Investigation of the light-induced effects in nitrogen-rich silicon nitride films

We report the first observations of photoconductivity fatigue and generation of positive fixed charges under ultraviolet illumination in gate-quality nitrogen-rich silicon nitride films, using metal-nitride-silicon and metal-nitride-oxide-silicon structures. The photoconductivity fatigue is correlated with the density of neutral silicon dangling bonds measured by electron spin resonance. Spins and positive fixed charges are generated at the same time, but a different rates and in a different amount. Possible models are discussed to explain the observed results.

Electric conduction in nitrogen-rich silicon nitride films produced by SiH2Cl2 and NH3

The composition and conduction of silicon nitride film formed by the reaction of SiH2Cl2 and NH3 at a low pressure were studied. In the range investigated, this film is amorphous N-rich silicon nitride no matter what the growth conditions. Conduction at room temperature in a steady state was investigated by analyzing the charge distribution without using an average field approximation. Probable charge distribution in the nitride was derived from the thickness dependence of the applied voltage at a constant current and from the flat-band voltage shift giving the interface fields. When current density J = 10−5 A/cm−2, we obtained nt(0) = 3×1018 cm−3, interface fields Es = 4.0×106 V/cm, and the charge centroid ω = 210 Å. The dynamic dielectric constant is calculated as εd = 3.9 from the temperature dependence of conduction, using the average field approximation. The average field approximation is valid only at high temperatures.