Silicon nitride films deposited from SiF4/NH3 gas mixtures

Silicon nitride films have been deposited from SiF₄/NH₃/H₂ gas mixtures. The deposition reaction at high pressure (52 torr), takes place only for temperatures above 800°C. In the temperature range 800–1000°C the reaction is controlled by a surface process. The increase in H₂ and SiF₄ partial pressures enhances the deposition rate. The SiF₄ molecules provide a high concentration of available silicon atoms, while the hydrogen molecules inhibit the etching effect of the free fluorine atoms. Finally, the effect of an r.f. plasma in the chemical vapour deposition reaction has been evaluated.     

Insight into excimer laser crystallization exploiting ellipsometry: Effect of silicon film precursor

The optical diagnostic of spectroscopic ellipsometry is shown to be an effective tool to investigate the mechanism of excimer laser crystallization (ELC) of silicon thin films. A detailed spectroscopic ellipsometric investigation of the microstructures of polycrystalline Si films obtained on SiO₂/Si wafers by ELC of a-Si:H and nc-Si films deposited, respectively, by SiH₄ plasma enhanced chemical vapor deposition (PECVD) and SiF₄-PECVD is presented. It is shown that ellipsometric spectra of the pseudodielectric function of polysilicon thin films allows to discern the three different ELC regimes of partial melting, super lateral growth and complete melting. Exploiting ellipsometry and atomic force microscopy, it is shown that ELC of nc-Si has very low energy density threshold of 95 mJ/cm² for complete melting, and that re-crystallization to large grains of ∼ 2 μm can be achieved by multi-shot irradiation at an energy density as low as 260 mJ/cm² when using nc-Si when compared to 340 mJ/cm² for the ELC of a-Si films.     

Nanostructuring Si-C alloys by plasma enhanced chemical vapor deposition: An ellipsometry and Raman spectroscopy investigation

Silicon nanocrystals embedded in a dielectric matrix are of considerable interest for Si-based optoelectronics and the third generation photovoltaics. This work discusses Si nanocrystals embedded in silicon-carbon, Si_1 − xC_x, thin films prepared by plasma-enhanced chemical vapor deposition (PECVD) using a non-conventional fluoride-based precursor mixture, i.e., SiF₄-CH₄-H₂-He plasmas. It is shown that the SiF₄/H₂ ratio and the He dilution are important parameters to control the volume fraction and the size of nc-Si, and the carbon content of the a-Si_1 − xC_x matrix. Films nanostructure and optical properties are studied by spectroscopic ellipsometry and Raman spectroscopy. The correlation existing between plasma processes and the film nanostructure and resulting optical properties is discussed.     

Role of argon in hot wire chemical vapor deposition of hydrogenated nanocrystalline silicon thin films

Structural, optical and electrical properties of hydrogenated nanocrystalline silicon (nc-Si:H) films, deposited from silane (SiH₄) and argon (Ar) gas mixture without hydrogen by hot wire chemical vapor deposition (HW-CVD) method were investigated. Film properties are carefully and systematically studied as a function of argon dilution of silane (R_Ar). We observed that the deposition rate is much higher (4-23 Å/s) compared to conventional plasma enhanced chemical vapor deposited nc-Si:H films using Ar dilution of silane (0.5-0.83 Å/s). Characterization of these films with Raman spectroscopy revealed that Ar dilution of silane in HW-CVD endorses the growth of crystallinity and structural order in the nc-Si:H films. The Fourier transform infrared spectroscopic analysis showed that with increasing Ar dilution, the hydrogen bonding in the films shifts from di-hydrogen (Si-H₂) and (Si-H₂)_n complexes to mono-hydrogen (Si-H) bounded species. The hydrogen content in the films increases with increasing Ar dilution and was found to be < 4 at.% over the entire range of Ar dilutions of silane studied. However, the band gap shows decreasing trend with increase in Ar dilution of silane and it has been attributed to the decrease in the percentage of the amorphous phase in the film. The microstructure parameter was found to be > 0.4 for the films deposited at low Ar dilution of silane and ~ 0.1 or even less for the films deposited at higher Ar dilution, suggesting that there is an enhancement of structural order and homogeneity in the film. From the present study it has been concluded that the Ar dilution of silane is a key process parameter to induce the crystallinity and to improve the structural ordering in the nc-Si:H films deposited by the HW-CVD method.     

Highly reflective nc-Si:H/a-CNx:H multilayer films prepared by r.f. PECVD technique

Multilayer thin films consisting of a-CN_x:H/nc-Si:H layers prepared by radio-frequency plasma enhanced chemical vapour (r.f. PECVD) deposition technique were studied. High optical reflectivity at a specific wavelength is one of major concern for its application. By using this technique, a-CN_x:H/nc-Si:H multilayered thin films (3-11 periods) were deposited on substrates of p-type (111) crystal silicon and quartz. These films were characterized using ultra-violet-visible-near infrared (UV-Vis-NIR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, field effect scanning electron microscopy (FESEM) and AUGER electron spectroscopy (AES). The multilayered films show high reflectivity and wide stop band width at a wavelength of approximately 650 ± 60 nm. The FTIR spectrum of this multilayered structure showed the formation of Si-H and Si-H₂ bonds in the nc-Si:H layer and CC and N-H bonds in a-CN_x:H layer. SEM image and AES reveal distinct formation of a-CN_x:H and nc-Si:H layers in the cross section image with a decrease in interlayer cross contamination with increasing number of periods.     

Characteristics of hydrogenated silicon thin film deposited by RF-PECVD using He–SiH4 mixture

Nanocrystalline silicon thin films (nc-Si:H) were deposited using He as the dilution gas instead of H₂ and the effect of the operating pressure and rf power on their characteristics was investigated. Especially, operating pressures higher than 4 Torr and a low SiH₄ containing gas mixture, that is, SiH₄(3 sccm)/He(500 sccm) were used to induce high pressure depletion (HPD) conditions. Increasing the operating pressure decreased the deposition rate, however at pressures higher than 6 Torr, crystallized silicon thin films could be obtained at an rf power of 100 W. The deposition of highly crystallized nc-Si:H thin film was related to the HPD conditions, where the damage is decreased through the decrease in the bombardment energy at the high pressure and the crystallization of the deposited silicon thin film is increased through the increased hydrogen content in the plasma caused by the depletion of SiH₄. When the rf power was set at a fixed operating pressure of 6 Torr, HPD conditions were obtained in the rf power range from 80 to 100 W, which was high enough to dissociate SiH₄ fully, but meantime low enough not to damage the surface by ion bombardment. At 6 Torr of operating pressure and 100 W of rf power, the nc-Si:H having the crystallization volume fraction of 67% could be obtained with the deposition rate of 0.28 nm/s.     

Characteristics of hydrogenated silicon thin film deposited by RF-PECVD using He-SiH4 mixture

Nanocrystalline silicon thin films (nc-Si:H) were deposited using He as the dilution gas instead of H₂ and the effect of the operating pressure and rf power on their characteristics was investigated. Especially, operating pressures higher than 4 Torr and a low SiH₄ containing gas mixture, that is, SiH₄(3 sccm)/He(500 sccm) were used to induce high pressure depletion (HPD) conditions. Increasing the operating pressure decreased the deposition rate, however at pressures higher than 6 Torr, crystallized silicon thin films could be obtained at an rf power of 100 W. The deposition of highly crystallized nc-Si:H thin film was related to the HPD conditions, where the damage is decreased through the decrease in the bombardment energy at the high pressure and the crystallization of the deposited silicon thin film is increased through the increased hydrogen content in the plasma caused by the depletion of SiH₄. When the rf power was set at a fixed operating pressure of 6 Torr, HPD conditions were obtained in the rf power range from 80 to 100 W, which was high enough to dissociate SiH₄ fully, but meantime low enough not to damage the surface by ion bombardment. At 6 Torr of operating pressure and 100 W of rf power, the nc-Si:H having the crystallization volume fraction of 67% could be obtained with the deposition rate of 0.28 nm/s.     

The analyses of an SiF4 plasma in an R.F. glow discharge for preparing fluorinated amorphous silicon thin films

The measurements of plasma parameters, such as an electron energy and an electron energy distribution function using a probe method, and of the optical emission spectroscopy for observing the chemical reactions in a plasma were studied for characterizing the glow discharge SiF₄ plasma. The probability of a basic dissociation process of the SiF₄ molecule by the electron impact was supported by the optical emission and also by the electron energy in the plasma. The discussions as to the electron energy distribution function in an r.f. glow discharge plasma suggest that a mean electron energy obtained by a double-probe method may not be very critical.The contribution of atomic hydrogen radicals and the probable reactions responsible for the deposition in the gas phase and/or on the substrate surface are also discussed.     

Preferred growth of nanocrystalline silicon in boron-doped nc-Si:H Films

Preferred growth of nanocrystalline silicon (nc-Si) was first found in boron-doped hydrogenated nanocrystalline (nc-Si:H) films prepared using plasma-enhanced chemical vapor deposition system. The films were characterized by high-resolution transmission electron microscope, X-ray diffraction (XRD) spectrum and Raman Scattering spectrum. The results showed that the diffraction peaks in XRD spectrum were at 2θ≈47° and the exponent of crystalline plane of nc-Si in the film was (2 2 0). A considerable reason was electric field derived from dc bias made the bonds of Si-Si array according to a certain orient. The size and crystalline volume fraction of nc-Si in boron-doped films were intensively depended on the deposited parameters: diborane (B₂H₆) doping ratio in silane (SiH₄), silane dilution ratio in hydrogen (H₂), rf power density, substrate's temperature and reactive pressure, respectively. But preferred growth of nc-Si in the boron-doped nc-Si:H films cannot be obtained by changing these parameters.     

Control of orientation from random to (220) or (400) in polycrystalline silicon films

The control of grain orientation in polycrystalline silicon thin films on glass substrates by low-temperature techniques was investigated. Either (220) or (400) preferential grain orientation could be attained by control of source gas ratio over substrate temperatures between 250°C and 360°C. A remote type plasma chemical vapor deposition system was used with source gas mixtures of SiF₄, H₂ and Ar. The (220) preferential films were obtained with Ar/H₂/SiF₄ gas flow rates of 60/15/30 sccm (respectively), while the (400) preferential oriented films were obtained at higher SiF₄/H₂ ratios (SiF₄/H₂=90/10 sccm). At the higher SiF₄/H₂ ratio during the crystal nucleation stage, either randomly oriented or (400) grains formed followed by the highly preferred deposition of (400) oriented crystallites. Raman scattering and ellipsometry spectra indicated that the (400) oriented films had a very smooth surface.     

Anisotropic etching of silicon in SF6 plasma

The reactive ion etching of silicon in SF₆ plasma is considered. During the experiment, silicon substrates are etched in SF₆ plasma at different pressures and energies of incident ions. High etching anisotropy is achieved decreasing the pressure in the reactor and increasing the energy of the bombarding ions. The obtained experimental measurements are compared with theoretical calculations. It is determined that the temperature of the sidewalls decreases with the decrease of concentration of F atoms due to suppressed plasmochemical etching of silicon. The etching anisotropy increases with the decrease of concentration of F atoms due to decreased desorption of SiF₄ molecules.     

The influence of argon addition on the deposition and properties of Si:H,Cl films prepared in a glow discharge

Thin silicon films produced in r.f. glow discharges fed with SiCl₄-H₂ mixtures are studied. The effect of argon addition to the feed is examined. Emission spectroscopy and laser interferometry were used to correlate some kinetic parameters, such as the deposition rate, with chemical, optical and structural properties of the deposited material. An overall film growth mechanism is suggested in which chemisorbed species and hydrogen atoms play an important role.     

Effects of applied power on hydrogenated amorphous carbon (a-C:H) film deposition by low frequency (60 Hz) plasma-enhanced chemical vapor deposition (PECVD)

Hydrogenated amorphous carbon (a-C:H) films were deposited onto glass substrates using low frequency (60 Hz) plasma-enhanced chemical vapor deposition and the effects of the applied power on a-C:H films deposition were investigated. During deposition, the electron temperature and the density of CH₄-H₂ plasma were 2.4-3.1 eV and about 10⁸ cm⁻³, respectively. The main optical emission peak of the carbon species observed in the CH₄-H₂ plasma is shown to be excited carbon CH* at 431 nm. The sp³/sp² ratio, band gap, hydrogen content, and refractive index of a-C:H films gradually increased up to a power of 25 W and then saturated at higher power. This tendency is similar to the variation of plasma parameters with varying applied power, thereby indicating that a strong relationship exists between the properties of the films and the plasma discharge.     

Optimization of n nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H₂ dilutions. High H₂ dilution resulted in n⁺ nanocrystalline silicon films (n⁺ nc-Si:H) with the lower resistivity (ρ ∼0.7 Ω cm) compared to that of doped amorphous silicon films (∼900 Ω cm) grown at low H₂ dilution. The change of the lateral ρ of n⁺ nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The ρ values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n⁺ nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (∼92 Ω cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiN_x:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH₃/SiH₄ gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiN_x:H film. Having lowest leakage currents, two a-SiN_x:H films with NH₃/SiH₄ ratios of ∼19 and ∼28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH₃/SiH₄∼19 ratio showed higher device performance than the TFT containing a-SiN_x:H with the NH₃/SiH₄∼28 ratio. This was correlated with the N−H/Si−H bond concentration ratio optimized for the TFT application.     

Structure of polycrystalline silicon films deposited at low temperature by plasma CVD on substrates exposed to different plasma

Polycrystalline silicon (poly-Si) films were deposited on glass substrates (corning 7059) at 300°C by a plasma enhanced chemical vapor deposition (PECVD) from a SiH₄/SiF₄ mixture. All poly-Si films were prepared under the same deposition conditions on the substrates subjected to nitrogen, hydrogen and/or CF₄ plasma with different gas pressures, just before deposition of the poly-Si films. Effects of such pretreatments for substrates on the structural properties of the resultant poly-Si films have been investigated. The Si film deposited on the substrates without any pretreatments was amorphous. However, formation of a strong 〈110〉 preferentially oriented poly-Si with improved crystallinity was obtained for the films deposited on the glass substrate after plasma pretreatments, which exhibit smoother surfaces. This result was interpreted in terms of a removal of weak Si–Si bonds during nucleation and the subsequent grain growth.     

On the performance of a class‐4 partial response digital communication system

Abstract

The dark conductivity of phosphorus‐doped amorphous‐silicon alloys (a‐Si:H:F) prepared by the RF plasma decomposition of a gaseous mixture of SiF₄, H₂ and diluted PH₃ is extremely high; it exceeds 10 (O‐cm)^‐1 with only a small amount of PH₃ (～500 ppm) added in the gas phase. These doping characteristics represent a significant improvement over the doping characteristics of a‐Si:H alloys prepared by a glow‐discharge of SiH₄. The improvement was found to be due to the fact that P‐doped a‐Si:H:F contains microcrystallites which are embedded in an amorphous network. The percolation process in these two‐phase systems gives rise to high conductivity. We have used transmission electron microscopy (TEM) and diffraction (TED) to determine the critical surface fraction, ρ_c, of crystallinity at the onset of extended conduction. The measured ρ_c is approximately 0.46. This percolation limit provides a basis for the analysis of the electrical properties of P‐doped a‐Si:H:F.     

A kinetic model for radio frequency plasma-activated chemical vapour deposition

A kinetic model for r.f. plasma-activated chemical vapour deposition is proposed. With this model we are able to explain the deposition profiles of silicon nitride films obtained from a gaseous mixture of SiH₄ and N₂. The plasma reactor is modelled in three zones: a central zone with electron impact dissociation, diffusion and convection and two lateral zones with only diffusion and convection. The mass transfer equation is solved for the case of convective diffusion (Péclet numbers from 0 to 2). The axial concentration of the activated species in the gas phase is correlated with the deposition profile. The model explains why the deposition profile is influenced by the r.f. power and the gas flow rate. The agreement between calculation and the Si₃N₄ mass profile is good for the deposition range investigated.     

Preparation of BN films by r.f. thermal plasma chemical vapour deposition

Boron nitride films were prepared at 1 atm by r.f. thermal plasma chemical vapour deposition from the gas systems of Ar-BF₃-N₂ (or NH₃, NF₃)-H₂, Ar-BCl₃-N₂ (or NH₃, NF₃)-H₂, and Ar-B₂H₆-N₂ (or NH₃)-H₂. The appearance and the deposition rate of the films changed drastically with the composition of the feed gas. Only from the Ar-BF₃-N₂(-NF₃) gas, were transparent and smooth films obtained, while from other gas systems, white flaky or powder-like deposits formed. The structure of these films was basically sp²-bonded turbostratic BN, and the formation of cubic BN was not confirmed.     

Nanocrystalline silicon fabrication by conventional plasma enhanced chemical vapor deposition for bottom gate thin film transistor

Nanocrystalline silicon was successfully fabricated using conventional plasma enhanced chemical vapor deposition (PECVD) for bottom gate thin film transistor. This was accomplished by promoting nucleation rate in the initial stage of silicon growth by H₂ or SF₆ plasma treatment of the surface of silicon nitride (SiN_x) films. Microstructure of hydrogenated nanocrystalline silicon (nc-Si:H) films confirmed the crystallization of silicon, and nanocrystalline silicon thin film transistor exhibited excellent stability.     

Characterization of polycrystalline SiC films grown by HW-CVD using silicon tetrafluoride

SiC films were synthesized by hot-wire chemical vapor deposition using a tungsten filament and a gas mixture of SiF₄ and CH₄. The etching of the substrate instead of the film growth occurred on the samples prepared using only source gases without H₂ dilution. The atomic or molecular hydrogen was believed to control the density of radicals containing F in a gas phase or on a growth surface. Polycrystalline 3C-SiC(111) films were successfully obtained at substrate temperatures lower than 500 °C by using H₂ dilution. The growth mode limited by source-gas supply was found to be important to obtain polycrystalline SiC films. The SiC film grown at higher deposition pressure was amorphous and contained no Si-H_x bonds but 6% fluorine. In SiF₄/CH₄/H₂ system, the radicals containing F are considered to play very important roles in the reactions both on a growth surface and in a gas-phase.