Plasma-enhanced Chemical Vapor Deposition System Research Articles

Use of a neural network to characterize the charge density of PECVD-silicon nitride films

Silicon nitride (SiN) films were deposited using a plasma-enhanced chemical vapor deposition system (PECVD). The charge density of the SiN films was modeled using a generalized regression neural network (GRNN). The PECVD process was characterized by means of a face-centered Box Wilson experiment. The prediction performance of the GRNN model was optimized using a genetic algorithm (GA). The GA-GRNN model significantly improved the GRNN prediction performance by more than 55%. The optimized GA-GRNN model was used to investigate the effects of various parameters on the charge density. A higher charge density was obtained at higher temperatures (i.e. a lower H concentration). Increasing the pressure increased the charge density at all temperatures levels with a much stronger impact at a lower H concentration. The effects of the SiH4 and N2 (or NH3) flow rates on the charge density were similar in that a higher charge density was achieved at a lower Si−N ratio (N-rich films). A considerable increase in the charge density with a radio frequency power at a lower NH3 flow rate was attributed to the generation of more Si−H than N−H bonds.

Optical and electrical properties of p-type ZnO fabricated by NH 3 plasma post-treated ZnO thin films

In this paper, a simple method is reported to obtain nitrogen-doped p-ZnO film. In this method NH 3 plasma, generated in a plasma-enhanced chemical vapor deposition system, was employed to treat ZnO thin film. By Hall-effect measurement, a p-type conductivity was observed for the treated film with the hole density of 2.2 × 10 16 cm −3. X-ray photoelectron spectroscopy (XPS) results confirmed that nitrogen was incorporated into ZnO film during the treatment process to occupy the oxygen positions. In low temperature photoluminescence spectra, an emission peak corresponding to acceptor–donor pair was observed. From this emission peak we calculated the N-related acceptor binding energy to be 130 meV.

Radio-frequency plasma system to selectively grow vertical field-aligned carbon nanofibers from a solid carbon source

Vertical field-aligned carbon nanofibers (CNFs), exhibiting a “herring-bone” and a “bamboo-like” structure, were grown at 560 °C using nickel (Ni) as a catalyst and an innovative radio-frequency (RF) plasma-enhanced chemical vapor deposition system. To limit the carbon supply, thereby providing a highly selective growth process with no detrimental parasitic carbon layer formation, a solid graphite sample-holder, RF-polarized, was used as a single carbon source in combination with a pure H 2 feed gas. The morphology and the dimensions of the obtained CNFs are investigated with respect to the growth duration. High-resolution transmission electron microscopy analyses typically display a Ni particle at the fiber tip, but this particle is not encapsulated by graphene layers, allowing its easy removal with a chemical acid treatment. Moreover, the particle’s upper surface consists of a peculiar polycrystalline area, assumed to be essential for the growth mechanisms and possibly made of nickel carbide. The crucial role played by the average vertical electric field, naturally created in the plasma sheath and responsible for sample-holder and substrate bombardment by cationic species, is highlighted to understand the growth mechanisms of these as-grown oriented CNFs and their progressive base destruction by etching phenomena.

Investigation of Initial and Steady-State Sliding Behavior of a Nearly Frictionless Carbon Film by Imaging 2- and 3-D TOF-SIMS

Using imaging time-of-flight secondary ion mass spectrometry (TOF-SIMS), we investigated the initial and steady-state sliding behavior of a nearly frictionless carbon (NFC) film. Specifically, TOF-SIMS images (both 2-D and 3-D) of these surfaces were constructed to highlight the spatial distributions of ionized and molecular species that were present on as-received and friction-tested NFC surfaces and as a function of depth. As a complementary technique, we used X-ray photoelectron spectroscopy (XPS) to gain further insight into the chemical nature of the sliding surfaces. The NFC films were produced on Si wafers and steel substrates in a gas discharge plasma that consisted of 25 vol.% methane and 75 vol.% hydrogen using a plasma-enhanced chemical vapor deposition (PECVD) system. They were then subjected to sliding friction and wear experiments in a pin-on-disk machine under 5- and 10-N loads and at sliding velocities of 0.2–0.5 m/s in dry nitrogen. The initial friction coefficients of the NFC films were in the range of 0.05–0.1, but decreased rapidly to values less than 0.01 at steady state. Positive and negative TOF-SIMS spectra and 2- and 3-D images reconstructed from selected masses revealed that the elemental distribution of certain chemical species differs substantially between undisturbed and tribo-tested areas of the NFC films. Specifically, the tribo-tested areas are essentially made up of carbon and hydrogen, while undisturbed or as-received areas are covered by a layer that is rich in oxygen and other species. These findings correlate well with the initial and steady-state friction coefficients of these films and help further explain their superlubricity in inert test environments.

Optical Characteristics of Hydrogenated Amorphous Silicon Carbide Films Prepared at Various Gas Flow Rate Ratios

Hydrogenated amorphous silicon carbide (a-SiC:H) films were prepared using a home-built plasma-enhanced chemical vapor deposition (PECVD) system with different flow rate ratios of methane (CH4) and silane (SiH4) gases. Fourier-transform infrared (FTIR) spectra indicate multiple bonding configurations consisting of wagging, bending and stretching modes of silicon, hydrogen and carbon atoms with a steady depletion of Si–H wagging and stretching modes as the gas flow rate ratio increases. Micro-Raman spectra show evidence of amorphous silicon structure in all the films. Only the a-SiC:H film prepared at the highest CH4 to SiH4 gas flow rate ratio shows the existence of the Si–C vibrational mode. All the samples prepared show room-temperature luminescence with two peaks centered at 467 and 698 nm. The photoluminescence (PL) intensity increases as the CH4 to SiH4 gas flow rate ratio increases but a reduction in intensity is observed for a high CH4 to SiH4 gas flow rate ratio. a-SiC:H films with higher optical energy gaps were obtained by allowing the gases to flow at higher CH4 to SiH4 gas flow rate ratios.

High Photo-to-Dark-Current Ratio in SiGe/Si Schottky-Barrier Photodetectors by Using an a-Si:H Cap Layer

SiGe/Si heterojunction infrared Schottky-barrier photodetectors (PDs) with different thicknesses (30-60 nm) of amorphous silicon (a-Si:H) cap layers were successfully fabricated. The SiGe and a-Si:H layers were deposited by using ultrahigh-vacuum chemical vapor deposition and plasma-enhanced chemical vapor deposition systems, respectively. In the fabricated PDs, it was observed that the a-Si:H cap layer can effectively suppress a dark current and that the thicker a-Si:H cap can obtain lower dark current. The lowest dark current was observed in the 60-nm a-Si:H capped PDs. When compared with a capless a-Si:H device, the dark current in the 60-nm a-Si:H capped PD was reduced by a magnitude of 317 at 4-V reverse-bias voltage. However, by increasing the a-Si:H thickness, the photocurrent was reduced. Therefore, a compromise in a-Si:H thickness should be chosen. We discovered that the photo-to-dark-current ratio was enhanced by a factor of 850 for 30-nm a-Si:H capped PDs as compared to that of capless a-Si:H devices. Thus, a PD with a higher noise-rejection ability was achieved by merely inserting a thin a-Si:H layer. Possible mechanisms are discussed in detail by the use of spectrum and band diagrams.

Plasma and gas-phase characterization of a pulsed plasma-enhanced chemical vapor deposition system engineered for self-limiting growth of aluminum oxide thin films

Aluminum oxide thin films were fabricated by pulsed plasma-enhanced chemical vapor deposition (PECVD) with continuous delivery of both O 2 and trimethyl aluminum (TMA). By appropriately controlling the gas phase environment, self-limiting growth kinetics are obtained. Diagnostic measurements of the plasma and gas-phase composition were performed using emission spectroscopy and mass spectrometry. Emission spectroscopy shows that the plasma achieves steady state within a couple of seconds. Addition of TMA to an O 2/Ar plasma causes the O atom density to drop ∼ 50%, while emission peaks due to TMA decomposition products scale with TMA partial pressure. Mass spectrometry confirms that TMA is unreactive with O 2. Upon plasma ignition TMA is immediately consumed, forming the combustion products CO, CO 2, H 2O, and H 2. Self-limiting film growth is demonstrated at room temperature.

Enhancement of sensitivity in gas chemiresistors based on carbon nanotube surface functionalized with noble metal (Au, Pt) nanoclusters

Multiwalled carbon nanotube (MWCNT) films have been fabricated by using plasma-enhanced chemical vapor deposition system onto Cr–Au patterned alumina substrates, provided with 3nm thick Fe growth catalyst, for NO2 and NH3 gas sensing applications, at sensor temperature in the range of 100–250°C. Nanoclusters of noble metal surface catalysts (Au and Pt) have been sputtered on the surface of MWCNTs to enhance the gas sensitivity with respect to unfunctionalized carbon nanotube films. It was found that the gas sensitivity of Pt- and Au-functionalized MWCNT gas sensors significantly improved by a factor up to an order of magnitude through a spillover effect for NH3 and NO2 gas detections, respectively. The metal-functionalized MWCNT sensors exhibit very high gas sensitivity, fast response, reversibility, good repeatability, and sub-ppm range detection limit with the sensing properties of the MWCNT films tailored by surface catalyst used to functionalize the MWCNT sensors.

Passivation layer on polyimide deposited by combined plasma immersion ion implantation and deposition and cathodic vacuum arc technique

A thin passivation layer of aluminum oxide was deposited on polyimide by using the combined plasma immersion ion implantation and deposition (PIII&D) and cathodic vacuum arc technique. X-ray photoelectron spectroscopy C 1s spectra showed that the carbonyl bond (CO) and ether group (C–O–C and C–N–C) presented in pristine polyimide were damaged by implantation of aluminum ions and deposition of an aluminum oxide passivation layer. O 1s and Al 2p spectra confirmed the formation of a thin aluminum oxide passivation layer. This passivation layer can be implemented in aerospace engineering where polyimide may suffer degradation from fast atomic oxygen in the low-earth-orbit environment. To test the protection of this passivation layer to energetic oxygen ions, a plasma-enhanced chemical vapor deposition system was used to simulate the oxygen-ion irradiation, and the results showed that a higher weight occurred for passivated samples compared to pristine ones. X-ray diffraction showed that Al peaks were presented on the surface region, but no aluminum oxide peak was detected. The authors then concluded that Al clusters were formed in polyimide besides aluminum oxide, which was in an x-ray amorphous state. Furthermore, contact-angle measurements showed a reduced contact angle for passivated polyimide from a pristine value of 78° to 20° by using deionized water. Several discussions have been made on the surface chemical and structural property changes by using the combined PIII&D and cathodic vacuum arc technique.

PECVD‐based nanocrystalline‐silicon TFT backplanes for large‐sized AMOLED displays

Abstract— A 14.1‐in. AMOLED display using nanocrystalline silicon (nc‐Si) TFTs has been developed. Nanocrystalline silicon was deposited using conventional 13.56‐MHz plasma‐enhanced chemical vapor deposition (PECVD). Detailed thin‐film characterization of nc‐Si films was followed by development of nc‐Si TFTs, which demonstrate a field‐effect mobility of about 0.6–1.0 cm2/V‐sec. The nc‐Si TFTs show no significant shift in threshold voltage when over 700 hours of constant current stress is applied, indicating a stable TFT backplane. The nc‐Si TFTs were successfully integrated into a 14.1‐in. AMOLED display. The display shows no significant current decrease in the driving TFT of the 2T‐1cap circuit because the TFTs are highly stable. In addition to the improved lifetime of AMOLED displays, the development of nc‐Si TFTs using a conventional 13.56‐MHz PECVD system offers considerable cost advantages over other laser and non‐laser polysilicon‐TFT technologies for large‐sized AMOLEDs.

Deposition of Ge-doped silica thin films for an integrated optic application using a matrix distributed electron cyclotron resonance PECVD reactor

Optical quality Ge-doped SiO2 thin films, suitable for an integrated optic version of a gain equalizer for erbium-doped fibre amplifiers (EDFAs), have been deposited using a matrix distributed electron cyclotron resonance plasma-enhanced chemical vapour deposition (MDECR-PECVD) system. Using spectroscopic ellipsometry and infrared transmission spectroscopy, the optical constants and hydroxyl content of the films were calculated. Losses due to the hydroxyl overtone at 1.37μm are found to be approximately 0.251dB/cm. An RBS analysis determined the germanium content of the films to be in the vicinity of 4 at.%. A comparison of the atomic percentage of germanium in the films and their corresponding refractive indices with values obtained using other deposition methods is also discussed.

Effect of chemical bonds on the properties of SiN in thin film transistor liquid crystal display

The hydrogenated amorphous silicon nitride (a-SiNx:H) thin films were produced in a radio-frequency plasma-enhanced chemical vapor deposition system using NH3∕SiH4∕N2 mixture source gases at 330°C. In this study the authors determine the structural properties of a-SiNx:H thin films with Fourier transform infrared measurement and relate these to both the processing parameters and their physical/optical properties. The Si–H and N–H bond densities are affected by the processing parameters. They find that the plasma power density and chamber pressure play minor roles compared to the gas flow rate of SiH4, and the role of the electrode spacing is indistinctive. The Si–H, N–H, and Si–N bond densities are important parameters affecting the physical/optical properties of low deposition rate thin film (the thin films in the proximity of a-SiNx:H∕a-Si interface) and high deposition rate (HDR) thin film (bottom gate insulator thin films). The optical band gap (E04) could be tuned by the N radical density. The N radical density increases with increasing N–H and Si–N bond densities and decreasing Si–H bond density. The Si–N bond density and the Si and N dangling bonds enhance the dielectric constant, but the Si–H and N–H bond densities have different effects. The dielectric constant increases with increasing Si–H bond density for the HDR thin films, because the high plasma power density (>38.5W∕cm2) could break more Si–N bonds. Finally, process parameters are obtained for optimized performance of the thin film transistors.

Substrate Effect on Plasma Clean Efficiency in Plasma Enhanced Chemical Vapor Deposition System

The plasma clean in a plasma-enhanced chemical vapor deposition (PECVD) system plays an important role to ensure the same chamber condition after numerous film depositions. The periodic and applicable plasma clean in deposition chamber also increases wafer yield due to less defect produced during the deposition process. In this study, the plasma clean rate (PCR) of silicon oxide is investigated after the silicon nitride deposited on Cu and silicon oxide substrates by remote plasma system (RPS), respectively. The experimental results show that the PCR drastically decreases with Cu substrate compared to that with silicon oxide substrate after numerous silicon nitride depositions. To understand the substrate effect on PCR, the surface element analysis and bonding configuration are executed by X-ray photoelectron spectroscopy (XPS). The high resolution inductively coupled plasma mass spectrometer (HR-ICP-MS) is used to analyze microelement of metal ions on the surface of shower head in the PECVD chamber. According to Cu substrate, the results show that micro Cu ion and theCuOxbonding can be detected on the surface of shower head. The Cu ion contamination might grab the fluorine radicals produced byNF3ddissociation in the RPS and that induces the drastic decrease on PCR.

Synthesis of Vertically Aligned Carbon Nanotubes by dc PECVD

Chemical vapor deposition (CVD) is one of the various synthesis methods that have been employed for CNT growth. In particular, Ren et al reported that large areas of vertically aligned multi-wall carbon nanotubes could be grown using plasma enhanced chemical vapor deposition (PECVD). In the present study, we synthesized aligned CNT arrays using a direct current (dc) PECVD system. The synthesis of CNTs requires a metal catalyst layer, etchant gas, and a carbon source. In this study, the substrate consisted of Si wafers with 10, 30, and 50 nm Ni-sputtered film. Ammonia (NH3) and acetylene (C2H2) were used as the etchant gases and carbon source, respectively. NH3 pretreatment was processed using a flow rate of 180 sccm for 10 min. CNTs were grown on pretreated substrates at 30% C2H2:NH3 flow ratios for 10 min. Carbon nanotubes with diameters ranging from 60 to 80 nanometers and lengths of about 2.7 μm were obtained. Vertical alignment of the carbon nanotubes was observed by FE-SEM.

Raman spectra of intrinsic and doped hydrogenated nanocrystalline silicon films

Raman scattering characteristics of intrinsic and doped hydrogenated nanocrystalline silicon films which prepared by a plasma-enhanced chemical vapor deposition system are investigated. Results indicate that Raman spectra depend intensively on microstructure and impurity in the films. Taking into account phonon confinement effect and tensile strain effect in Si nanocrystals, peak redshift of measured transverse optical modes in Raman spectra of intrinsic films can be well interpreted. With respect to Raman scattering from doped samples, besides phonon confinement effect, the peak of experimental transverse optical mode further downshifts with heightening doping level, which can be primarily assigned to impurity effect from doping. In addition, the increase in relative integral intensity ratio of transverse acoustic branch to transverse optical mode and that of longitudinal acoustic branch to transverse optical mode with decreasing mean dimension of nanocrystals and heightening doping ratio, respectively, can be ascribed to disorder. Furthermore, at the same doping level, incorporation of boron can induce higher disorder than incorporation of phosphorus in nc-Si:H films.

Pulsed radio frequency plasma deposition of a-SiNx:H alloys: Film properties, growth mechanism, and applications

In this work, we propose a fabrication process of a-SiNx:H alloys by pulsing the radio frequency (rf) signal in a low pressure plasma-enhanced chemical vapor deposition (PECVD) system. The characteristics of the films can be controlled simply by adjusting the duty cycle of the pulsed rf power, while keeping the N2∕SiH4 gas mixture constant. Spectroscopic ellipsometry analysis in the ultraviolet-visible-near infrared and far infrared ranges, atomic force microscopy, and elastic recoil detection reveal strong variations in the optical properties (1.88⩽n⩽2.75, 10−4⩽k⩽5×10−2 at 550nm), optical gap (4.01eV⩽Eg⩽1.95eV), microstructural characteristics (1.3nm⩽surfaceroughness⩽8.3nm), and chemical composition (0.47⩽x⩽1.35) of the coatings as a function of duty cycle. This behavior is interpreted in terms of radical concentration changes in the gas phase, as well as variation in the average ion bombardment energy at the film surface, leading to modifications of both chemical and physical mechanisms that sustain the film growth. Using the control of duty cycle, we fabricated two types of a-SiNx:H-based thin film devices, namely, (i) a model Fabry-Perot optical filter deposited on plastic substrate and (ii) a superlattice structure displaying a photoluminescence signal four times higher than the reference single layer. These two examples of applications point out the main advantages of this pulsed rf PECVD process, in particular, low deposition temperature, reproducibility, versatility, and ease of use.

Thermal conductivity of diamond films deposited at low surface temperatures

Polycrystalline diamond films are deposited on p-type Si (100) and n-type SiC (6H) substrates at the low surface deposition temperatures of 370 °C–530 °C using a microwave plasma-enhanced chemical vapor deposition system in which the surface temperature during deposition is monitored and controlled. A very high growth rate up to 1.3 μm/h on SiC substrate at 530 °C surface temperature is obtained. The room temperature in-plane thermal conductivity of the low-surface-temperature–deposited thin films is measured by a traveling wave method. The diamond films of grain sizes between 3 and 7 μm and deposited at 370 °C showed a high thermal conductivity value of ∼6.5 W/cm-K, which is much higher than the single crystal SiC thermal conductivity value at room temperature. Diamond films deposited on Si and SiC single crystals at higher temperatures showed even higher thermal conductivities of 11–17 W/cm-K. The structure and microstructure of these films are characterized by x-ray diffraction, scanning electron microscopy, and Raman spectroscopy, and are related to measured thermal conductivities.

Plasma-enhanced chemical vapor deposition of TiO2 thin films for dielectric applications

Amorphous TiO2 thin films were formed by plasma-enhanced chemical vapor deposition (PECVD) from mixtures of titanium IV isopropoxide (Ti(O-i-C3H7)4) and oxygen. The deposition rate was found to be weakly activated, with an apparent activation energy of 4.5 kJ/mol. The deposition rate increased with equivalence ratio and decreased with plasma power. This dependence on atomic oxygen density was consistent with behavior observed in other metal oxide PECVD systems. Metal-insulator-silicon devices were fabricated, and characterized using capacitance–voltage measurements. The apparent dielectric constant of the TiO2 thin films increased from 15 to 82 with film thickness. The observed variations were consistent with the formation of an interfacial SiO2 layer. Assuming that a TiO2/SiO2 bilayer behaves as two capacitors in series, an intrinsic TiO2 dielectric constant of 82±10 and an interfacial SiO2 layer thickness of 3±1 nm were extracted from electrical measurements.

Microcrystalline Silicon Solar Cells Fabricated using Array-Antenna-Type Very High Frequency Plasma-Enhanced Chemical Vapor Deposition System

An array-antenna-type very high frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) system based on a multisubstrate deposition technique has been developed for the large area (>1 m2), high-throughput and low-cost production of thin-film silicon solar cells. A hydrogenated microcrystalline silicon ( µc-Si:H) solar cell fabricated at around 20 Pa showed an initial efficiency of 7.44% for a deposition rate of 0.32 nm/s with evaporated Ag as the back contact. The uniform formation of each layer was observed, and the quantum efficiency spectrum after exposure to air indicates high-stability high-quality film formation. The advantage of the novel array antenna method is discussed in terms of the material property as well as production cost.

Spatial distribution of electron characteristic in argon rf glow discharges

The spatial distributions of the electron density and the mean electron energy of argon radio frequency (rf) glow discharge plasma in a plasma-enhanced chemical vapour deposition (PECVD) system have been investigated using an established movable Langmuir probe. The results indicate that in the axial direction the electron density tends to peak at midway between the two electrodes while the axial variation trend of mean electron energy is different from that of the electron density, the mean electron energy is high near the electrodes. And the mean electron energy near the cathode is much higher than that near the anode. This article focuses on the radial distribution of electron density and mean electron energy. A proposed theoretical model distribution agrees well with the experimental one: the electron density and the mean electron energy both increase from the centre of the glow to the edge of electrodes. This is useful for better understanding the discharge mechanism and searching for a better deposition condition to improve thin film quality.