Pulsed Plasma-enhanced Chemical Vapor Deposition Research Articles

Thin film silicon nanowire photovoltaic devices produced with gold and tin catalysts

Silicon nanowires produced using pulsed plasma-enhanced chemical vapor deposition have been used as part of a thin film photovoltaic device. Nanowires of differing morphologies were produced by using both gold and tin thin films as a catalyst for growth. A prototype silicon nanowire-based thin-film photovoltaic device was produced by using doped silicon nanowires as the p-layer. Amorphous silicon was used as the intrinsic and n-layers of the device. The nanowires used in the photovoltaic devices had an average diameter of 420 nm after the deposition and coating of amorphous silicon intrinsic and n-layers. The nanowires were deposited in bulk as films of 3 to 42 μm in thickness. The resulting device, although of low efficiency, had a demonstrable photocurrent. Tin-catalyzed nanowires were found to produce a thin-film device with a measurable photocurrent whereas gold-catalyzed silicon nanowires did not. This was attributed to the length of the nanowires and thickness of the p-layer produced when using gold-catalyzed nanowires.

Pulsed Plasma Enhanced Chemical Vapor Deposition of Alumina Thin Films: Influence of the Duty Cycle on Structure and Elastic Properties

In this study, the duty cycle (Δ) has been systematically modified during the pulsed plasma enhanced chemical vapor deposition (PECVD) of alumina (Al2O3) coatings. This deposition method is prone to chlorine incorporation during growth as AlCl3 is used as precursor. Here, we show that the incorporation of chlorine can be reduced from 2.4 ± 0.1 to 1.2 ± 0.1 at.-% by increasing Δ from 62% to 91%. The film grown at Δ = 88% has a density of 3.85 ± 0.12 g cm−3 (96% of α-alumina bulk density) and an elastic modulus of 365 ± 50 GPa (∼83% of the bulk α-alumina value). Δ affects the normalized ion flux (NIF), which is the ratio between the flux of bombarding ions and the flux of deposited material. By increasing Δ, the formation of α-rich alumina is promoted at 560 °C, the chlorine content is reduced, and the film is densified. An increased Δ and hence NIF results in enhanced ion bombardment of the film surface during growth, promoting chlorine desorption. Furthermore, it is reasonable to assume that the deposition rate decrease measured is a consequence of increasing Δ and hence NIF. Both the density as well as the elastic modulus increase measured is consistent with the increased NIF caused by increasing the duty cycle.

Duty ratio impact on SiN films deposited in SiH 4-NH 3 plasma at room temperature

Silicon nitride (SiN) films were deposited by using pulsed plasma-enhanced chemical vapor deposition system. The experiments were conducted from SiH 4 and NH 3 plasma at room temperature. The duty ratio was varied in the range 40–90%. Duty ratio-induced ion energy diagnostics was conducted using a non-invasive ion energy analyzer. Ion energy variables collected include a high ion energy ( E h ), a low ion energy ( E l ), a high energy flux ( N h ), a low ion energy flux ( N l ). Their various ratios were calculated to examine relationships between ion energy and film properties, including deposition rate, refractive index, and surface roughness. It is noticeable that high deposition rate of more than 1150 Å/min could be achieved at 60%. The deposition rate and surface roughness highly correlated with N h . From the variation of refractive index, deposition mechanisms were inferred. The deposition rate, refractive index, and surface roughness were varied in a range of 1003–1148 Å/min, 1.64–1.96, and 1.37–3.07 nm, respectively.

Plasma deposited stability enhancement coating for amorphous ketoprofen

A hydrophobic fluorocarbon coating deposited onto amorphous ketoprofen via pulsed plasma-enhanced chemical vapor deposition (PPECVD) significantly prolonged the onset of recrystallization compared to uncoated drug. Rapid freezing (RF) employed to produce amorphous ketoprofen was followed by PPECVD of perfluorohexane. The effect of coating thickness on the recrystallization and dissolution behavior of ketoprofen was investigated. Samples were stored in open containers at 40 °C and 75% relative humidity, and the onset of recrystallization was monitored by DSC. An increase in coating thickness provided enhanced stability against recrystallization for up to 6 months at accelerated storage conditions (longest time of observation) when compared to three days for uncoated ketoprofen. Results from XPS analysis demonstrated that an increase in coating thickness was associated with improved surface coverage thus enabling superior protection. Dissolution testing showed that at least 80% of ketoprofen was released in buffer pH 6.8 from all coated samples. Overall, an increase in coating thickness resulted in a more complete drug release due to decreased adhesion of the coating to the substrate.

Self-limiting growth of anatase TiO 2: A comparison of two deposition techniques

Self-limiting deposition of titanium dioxide thin films was accomplished using pulsed plasma-enhanced chemical vapor deposition (PECVD) and plasma-enhanced atomic layer deposition (PEALD) at low temperatures ( T < 200 °C) using TiCl 4 and O 2. TiCl 4 is shown to be inert with molecular oxygen at process conditions, making it a suitable precursor for these processes. The deposition kinetics were examined as a function of TiCl 4 exposure and substrate temperature. The quality of the anatase films produced by the two techniques was nominally identical. The key distinctions are found in precursor utilization and conformality. Pulsed PECVD requires 20 times less TiCl 4, while PEALD must be used to uniformly coat complex topographies.

Characteristics of Ge-Sb-Te Films Prepared by Cyclic Pulsed Plasma-Enhanced Chemical Vapor Deposition

Ge-Sb-Te (GST) thin films were deposited on TiN, SiO2, and Si substrates by cyclic-pulsed plasma-enhanced chemical vapor deposition (PECVD) using Ge{N(CH3)(C2H5)}, Sb(C3H7)3, Te(C3H7)3 as precursors in a vertical flow reactor. Plasma activated H2 was used as the reducing agent. The growth behavior was strongly dependent on the type of substrate. GST grew as a continuous film on TiN regardless of the substrate temperature. However, GST formed only small crystalline aggregates on Si and SiO2 substrates, not a continuous film, at substrate temperatures > or = 200 degrees C. The effects of the deposition temperature on the surface morphology, roughness, resistivity, crystallinity, and composition of the GST films were examined.

Thin film composites of nanocrystalline ZrO2 and diamond-like carbon: Synthesis, structural properties and bone cell proliferation

We report on the synthesis of thin composites of diamond-like carbon (DLC) and nanocrystalline ZrO(2) deposited using pulsed direct current plasma-enhanced chemical vapor deposition at low temperatures (<120 degrees C). Films containing up to 21at.% Zr were prepared (hydrogen was not included in the calculation) and their structural and surface properties were determined using a number of spectroscopic methods and contact angle measurements. Bone cell adhesion to the films was studied using a 3 day cell culture with osteoblasts. These nanocomposites (DLC-ZrO(2)) consist of tetragonal ZrO(2) nanocrystals with an average size of 2-5 nm embedded in an amorphous matrix consisting predominantly of DLC. The surface water contact angle of the films increased from approximately 60 degrees to 80 degrees as the Zr content increased from 0 to 21at.%. The cell culture study revealed that although the cell counts were not significantly different, the morphology of the osteoblasts growing on the DLC-ZrO(2) nanocomposites was markedly different from that of cells growing on DLC alone. Cells growing on the DLC-ZrO(2) surfaces were less spread out and had a smaller cell area in comparison with those growing on DLC surfaces. In some areas on the DLC-ZrO(2) surfaces, large numbers of cells appeared to coalesce. It is postulated that the difference in cell morphology between osteoblasts on DLC-ZrO(2) surfaces and DLC surfaces is related to the presence of very small tetragonal nanocrystals of ZrO(2) in the composite film.

Deposition of thick and adherent Teflon-like coating on industrial scale stainless steel shell using pulsed dc and RF PECVD

A unique combination of pulsed dc and radio frequency (RF) discharge deposition was used to deposit thick (∼5 μm) and adherent (2–4 MPa) Teflon-like coatings on a stainless steel (SS) shell of 2 m diameter size, through plasma enhanced chemical vapor deposition (PECVD). The details of deposition on such a big industrial scale component are reported for the first time. In this method, highly adherent thin interface layers were grown on SS shell that was electrically grounded, using pulsed dc discharge, followed by RF discharge deposition to build up the required coating thickness. The fluorocarbon precursor molecules, required for the deposition of Teflon-like coating, are generated indigenously by pyrolyzing the Teflon powder. The deposited coating was studied for its chemical bond state, surface roughness (Ra), morphology, thickness, and adhesive strength. These studies were carried out by using XPS, AFM, SEM, etc. The adhesive strength of the coating was measured by pin-pull test as per ASTM D4541 standard test. The coatings deposited with pulsed dc discharge were observed to have higher adhesive strength when compared with those deposited with RF discharge.

Low-Temperature Pulsed-PECVD ZnO Thin-Film Transistors

We report high-quality ZnO thin films deposited at low temperature (200°C) by pulsed plasma-enhanced chemical vapor deposition (pulsed PECVD). Process byproducts are purged by weak oxidants N2O or CO2 to minimize parasitic CVD deposition, resulting in high-refractive-index thin films. Pulsed-PECVD-deposited ZnO thin-film transistors were fabricated on plasma-enhanced atomic layer deposition (PEALD) Al2O3 dielectric and have a field-effect mobility of 15 cm2/V s, subthreshold slope of 370 mV/dec, threshold voltage of 6.6 V, and current on/off ratio of 108. Thin-film transistors (TFTs) on thermal SiO2 dielectric have a field-effect mobility of 7.5 cm2/V s and threshold voltage of 14 V. For these devices, performance may be limited by the interface between the ZnO and the dielectric.

Pulsed plasma-enhanced chemical vapor deposition of Al 2O 3–TiO 2 nanolaminates

Self-limiting synthesis of alumina–titania nanolaminates (ATO, Al 2O 3/TiO 2) was accomplished via pulsed plasma-enhanced chemical vapor deposition. At the synthesis temperature of 150 °C the alumina layers were amorphous, while TiO 2 layers displayed a polycrystalline anatase structure. Digital control over nanolaminate structure was demonstrated through elemental analysis and transmission electron microscopy imaging. The dielectric performance of the ATO structures was examined as a function of composition and bilayer thickness. Capacitance–voltage measurements showed that the effective dielectric constant was consistent with treating the nanolaminates as individual capacitors in series. Current–voltage measurements showed that leakage current deteriorated with TiO 2 content, though low leakage was restored through interfacial engineering.

Digital Control of SiO2−TiO2 Mixed-Metal Oxides by Pulsed PECVD

Pulsed plasma enhanced chemical vapor deposition (PECVD) was used to deliver digital control of SiO(2), TiO(2), and SiO(2)-TiO(2) composites at room temperature. Alloy formation was investigated by maintaining constant delivery of TiCl(4) while varying the SiCl(4) flow. Film composition was assessed by spectroscopic ellipsometry, XPS, and FTIR. It is shown that the alloy composition and refractive index can be tuned continuously over a broad range using pulsed PECVD. The two precursors were found to be highly compatible, with the alloy growth rate simply reflecting the sum of the contributions from the two individual precursors. Digital control over both thickness and composition was demonstrated through the production of antireflection (AR) coatings for crystalline silicon. AR coatings were synthesized on the basis of optimized designs, and in each case the measured optical performance was found to be in excellent agreement with model predictions. The average reflectance across the visible spectrum was reduced from 39% for uncoated wafers to 2.5% for the three-layer AR coating.

Self-limiting deposition of semiconducting ZnO by pulsed plasma-enhanced chemical vapor deposition

Self-limiting growth of zinc oxide was accomplished over a temperature range from 25to155°C by pulsed plasma-enhanced chemical vapor deposition using dimethyl zinc [Zn(CH3)2] as the metal precursor. The deposition rate was independent of plasma exposure (1–5s) but was found to increase from 1.4to6.0Å∕cycle as a function of temperature. Over the narrow range explored, substrate temperature had a dramatic impact on the film structure and properties. Amorphous films were obtained at room temperature, while a polycrystalline morphology with a preferred (100) orientation developed as the temperature increased. The electrical resistivity decreased linearly with temperature from 45to∼2Ωcm. Spectroscopic characterization showed that films deposited at room temperature were contaminated by carbon and hydroxyl impurities; however, these defects were attenuated with temperature and were not detected in films deposited above 64°C. Room temperature photoluminescence was dominated by defect emission in most films; however, this signal was attenuated, and a strong band edge emission was observed for films deposited at temperatures &amp;gt;135°C. Film quality was comparable to material grown by plasma-enhanced atomic layer deposition in the same reactor; however, precursor requirements and net deposition rates were improved by an order of magnitude.

Digital Control of SiO2 Film Deposition at Room Temperature

In this Letter, we propose and demonstrate a robust process for digital control of high quality SiO2 thin films at room temperature using pulsed plasma-enhanced chemical vapor deposition (PECVD). Plasma activation of the SiCl4 precursor is critical, as atomic layer deposition does not occur under these conditions. Sub-angstrom control over deposition rate was obtained by adjusting the density of SiCl4 present at plasma ignition. The intrinsic refractive index of 1.46 was obtained at rates between 0.8 and 1.6 Å/pulse. No impurities were detected by either X-ray photoelectron spectroscopy (XPS) or Fourier transform infrared (FTIR) in films deposited under optimal conditions.

Molecular structure of SiO x-incorporated diamond-like carbon films; evidence for phase segregation

Silicon-oxide incorporated amorphous hydrogenated diamond-like carbon films (SiO x –DLC, 1 ≤ x ≤ 1.5) containing up to 24 at.% of Si (H is excluded from the atomic percentage calculations reported here) were prepared using pulsed direct current plasma-enhanced chemical vapour deposition (DC-PECVD). Molecular structure, optical properties and mechanical properties of these films were assessed as a function of Si concentration. The spectroscopic results indicated two structural regimes. First, for Si contents up to ~ 13 at.%, SiO x –DLC is formed as a single phase with siloxane, O–Si–C 2, bonding networks. Second, for films with Si concentrations greater than 13 at.%, SiO x –DLC with siloxane bonding and SiO x deposit simultaneously as segregated phases. The variations in mechanical properties and optical properties as a function of Si content are consistent with the above changes in the film composition.

Self-Limiting Deposition of Anatase TiO[sub 2] at Low Temperature by Pulsed PECVD

The self-limiting deposition of anatase TiO 2 was accomplished by pulsed plasma-enhanced chemical vapor deposition (PECVD) using a simultaneous delivery of TiCl 4 and O 2 . The amorphous to anatase phase transition was examined as a function of temperature, plasma power, and film thickness. Films deposited at T = 120°C, with the plasma power set at 100 W were amorphous, containing residual amounts of chlorine. At 200 W, the films displayed an anatase structure, and no chlorine was detected by X-ray photoelectron spectroscopy. Spectroscopic ellipsometry, Fourier transform infrared, and X-ray diffraction measurements concur that a minimum film thickness of ∼25 nm is required for the formation of the anatase phase.

Enhancement of metal oxide deposition rate and quality using pulsed plasma-enhanced chemical vapor deposition at low frequency

The deposition rate and quality of alumina thin films fabricated by plasma-enhanced chemical vapor deposition (PECVD) increased significantly when square wave power modulation was applied at low frequency (∼1Hz). The pulsed PECVD rate was enhanced by a factor of ∼3 relative to continuous wave operation, and the quantity of impurities was dramatically attenuated. Deposition experiments on trenches with aspect ratios ranging from 4 to infinity demonstrated that the technique achieves a high degree of conformality. Important reactor design and operating considerations are described. Pulsed PECVD produced similar quality improvements for Ta2O5, TiO2, and ZnO, suggesting that the approach has widespread potential for metal oxide synthesis.

Self-limiting deposition of aluminum oxide thin films by pulsed plasma-enhanced chemical vapor deposition

Self-limiting deposition of aluminum oxide (Al2O3) thin films was accomplished by pulsed plasma-enhanced chemical vapor deposition using a continuous delivery of trimethyl aluminum (TMA) and O2. Film characterization included spectroscopic ellipsometry and Fourier transform infrared (FTIR) spectroscopy. Deposition rates scaled with TMA exposure and could be controlled over a large range of 1–20Å∕pulse. For fixed conditions, digital control over film thickness is demonstrated. Deposition rates initially decreased with substrate temperature before becoming constant for Ts&amp;gt;100°C. Higher growth rates at low temperature are attributed to the thermal reaction between H2O, produced during the plasma on step, with TMA during the plasma off step. Gas-phase analysis confirms the coexistence of these species, and their degree of overlap is a strong function of the chamber wall temperature. With both the substrate and chamber wall temperature elevated, impurities related to carbon and hydroxyl groups are attenuated below the detection limit of FTIR.

Comparison of Electrolyte Performance for Ta2O5 Thin Films Produced by Pulsed and Continuous Wave PECVD

Tantalum oxide films were deposited by pulsed and continuous wave (CW) plasma-enhanced chemical vapor deposition (PECVD). The pulsed films were stoichiometric and free of impurities as measured by X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. CW films contained significant amounts of hydroxyl impurities, resulting in a nonstoichiometric composition with an O/Ta ratio of . Impedance spectroscopy was used to quantify ion transport through electrochromic half-cells formed by depositing tantalum oxide on both tungsten and vanadium oxides. Logarithmic plots of the imaginary component of impedance vs frequency were employed to extract equivalent circuit parameters. Despite the differences in composition the pulsed and CW films displayed similar ionic conductivities, with values of and for and , respectively. However, the pulsed PECVD films displayed dramatically reduced electrical leakage. The ratio of ion/electron conductivity exceeded 100 for pulsed PECVD films, while was in CW material.

Synthesis and characterization of super hard, self-lubricating Ti–Si–C–N nanocomposite coatings

Quaternary super hard Ti–Si–C–N coatings with different carbon contents were deposited on high-speed steel substrates by pulsed direct current plasma-enhanced chemical vapor deposition (PECVD) technology, using a gaseous mixture of TiCl 4/SiCl 4/N 2/H 2/CH 4/Ar. A variety of technologies have been employed to characterize the coatings, including X-ray diffraction, scanning and transmission electron microcopies, X-ray photoelectron spectroscopy, energy dispersive X-ray analysis, automated load–depth sensing and pin-on-disc. The super hard Ti–Si–C–N coatings were found to have unique nanocomposite structures composed of nanocrystallite and amorphous nc-Ti(C,N)/a-Si 3N 4/a-C and/or nc-Ti(C,N)/nc-TiSi 2/nc-Si/a-Si 3N 4/a-C, depending on the carbon contents in the coatings. The friction coefficient of the Ti–Si–C–N coatings with a higher carbon content (nc-Ti(C,N)/a-Si 3N 4/a-C nanocomposites) were found to be much lower than those of the Ti–Si–N coatings both at room and elevated temperatures, suggesting the formation of a graphite-like lubricious phase of amorphous carbon. However, they are still super hard (32–48 GPa) in spite of the carbon incorporation. This is due to a strong, thermodynamically driven and diffusion-rate-controlled (spinodal) phase segregation that leads to the formation of a stable nanostructure by self-organization. The energy difference between the grain boundary and the crystallite/amorphous phase interface hinders grain boundary mobility, leading to a gradual decrease in the grain size of the nanocrystallites. As a result, nanocomposite Ti–Si–C–N coatings with high hardness and a low friction coefficient can be produced. The coatings are foreseen to have high potential in dry and high-speed cutting tool applications, thus providing for cleaner, healthier and more pleasant machining conditions.

Effect of wall conditions on the self-limiting deposition of metal oxides by pulsed plasma-enhanced chemical vapor deposition

Pulsed plasma-enhanced chemical vapor deposition has been engineered to deliver self-limiting growth (i.e., ∼Å∕pulse) of metal oxides such as Ta2O5 and Al2O3. In this process the reactor walls are alternately exposed to atomic oxygen and metal precursors. The degree of adsorption in the latter step can dramatically influence both deposition rates and film quality. The impact of precursor adsorption on the plasma and gas-phase composition in these systems was quantified using optical emission spectroscopy and quadrupole mass spectrometry, respectively. It is shown that the time scale for a complete adsorption on the chamber walls is much greater than gas-phase residence times. Adsorbed compounds significantly alter the reactor composition, particularly at the initiation of each plasma pulse. As a consequence, careful attention must be paid to reactor design and operation to control deposition rates and maintain film quality.