Virtual separation approach to study porous ultra-thin films by combined spectroscopic ellipsometry and quartz crystal microbalance methods

Spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation (QCM-D) techniques have been extensively used as independent surface characterization tools to monitor in-situ thin film formation. They provide different information for ultra-thin films because QCM-D is sensitive to the solvent content while SE is not. For using these two techniques in tandem, we present a virtual separation approach to enable the determination of both ultra-thin film thickness and porosity. Assumptions for the intrinsic molecular polarizability (index of refraction n_o) and density (ρ_o) of the organic adsorbent must be made, and the consequences for these parameters' values are discussed.     

Detection of deposition rate of plasma-polymerized acrylate and methacrylate derivatives using quartz crystal microbalance

The deposition rates of plasma-polymerized (pp-)films of 12 acrylate derivatives (CH₂CHCOOR: substitution R is H, CH₃, CHCH₂, CH₂CH₃, CH₂CH₂CH₃, (CH₂)₃CH₃, C(CH₃)₃, CH₂CH(CH₃)₂, CH₂(CH₂)₄CH₃, CH₂CF₃, CH₂CF₂CF₃, CH₂(CF₂)₂CF₃), and 12 methacrylate derivatives (CH₂CCH₃COOR′: substitution R′ is H, CH₃, CHCH₂, CH₂CH₃, CH₂CH₂CH₃, (CH₂)₃CH₃, C(CH₃)₃, CH₂CH(CH₃)₂, CH₂(CH₂)₄CH₃, CH₂CF₃, CH₂CF₂CF₃, CH₂(CF₂)₂CF₃) are determined by the quartz crystal microbalance technique. Using the same polymerization conditions (100 W RF and 100 Pa vapor pressure) for the various monomers, it is found that the deposition rates were proportional to the polymerization time. The average deposition rate of pp-acrylate derivatives is larger than that of pp-methacrylate derivatives without pp-hexylacrylate. The average deposition rate of pp-fluoroalkylacrylates was higher than 7-25 times that of pp-alkylacrylate, and the average deposition rate of pp-fluoroalkylmethacrylates was higher than 10-31 times that of pp-alkylmethacrylates, respectively. The average deposition rate of pp-film depends on the chemical structure of the monomer suggesting different mechanisms under plasma-polymerization.     

Application of plasma polymerized siloxane films for the corrosion protection of titanium alloy

Organosilicon film and SiO_x-like film are deposited on titanium alloy (Ti6Al4V) surfaces by atmospheric pressure (~ 10⁵ Pa) dielectric barrier discharge to improve its corrosion resistance in Hanks solution. Hexamethyldisiloxane (HMDSO) is used to be the chemical precursor. The organosilicon film deposited in Ar/HMDSO system has high growth rate (75 nm/min) and low surface roughness (3 nm), while the SiO_x-like film deposited in Ar/O₂/HMDSO system has lower growth rate (35 nm/min) and slightly higher surface roughness (9 nm). The potentiodynamic polarization tests show that both the two siloxane films coated Ti6Al4V samples have more positive corrosion potential and one order of magnitude lower corrosion current density than the substrate, indicating the corrosion resistance of Ti6Al4V can be improved by depositing siloxane film on its surface. In particular, as the surface is more compact and cross-linked, the SiO_x-like film has better corrosion resistance than the organosilicon film.     

Heat treatment and aging effect on the structural and optical properties of plasma polymerized 2,6-diethylaniline thin films

The monomer, 2,6-diethylaniline has been used to deposit plasma polymerized 2,6-diethylaniline (PPDEA) thin films at room temperature on to glass substrates by a capacitively coupled parallel plate glow discharge reactor. A comparative analysis on the changes of morphological, structural and optical properties of as-deposited, heat treated and aged PPDEA thin films is ascertained. Scanning electron microscopy shows uniform and pinhole free surface of PPDEA thin films and no significant difference in the surface morphology is observed due to heat treatment. Electron dispersive X-ray and Fourier transform infrared spectroscopic investigations indicate some structural rearrangement in PPDEA thin films due to heat treatment. Differential thermal analysis, thermogravimetric analysis and differential thermogravimetric analysis suggest that the PPDEA is thermally stable up to about 580 K. The study on the optical absorption spectra of as-deposited, heat treated and aged PPDEA thin films of different thicknesses lead to the determination of the allowed direct and indirect transition energies ranging from 3.63 to 2.73 and 2.38 to 1.26 eV respectively. Urbach energy, steepness parameter and extinction coefficient are also assessed. It is observed that the optical parameters of as-deposited PPDEA thin films change due to heat treatment and do not change appreciably due to aging.     

Micelle-assisted bilayer formation of cetyltrimethylammonium bromide thin films studied with combinatorial spectroscopic ellipsometry and quartz crystal microbalance techniques

We report on a combinatorial approach to study the formation of ultra-thin organic films using in-situ spectroscopic ellipsometry and quartz crystal microbalance methods. In contrast to the quartz crystal microbalance, which is sensitive to the total mass attached to the surface, including coupled and entrapped solvent, spectroscopic ellipsometry only measures the amount of adsorbent on the surface. By using these two techniques in tandem, we define and determine the solvent fraction of cetyltrimethylammonium bromide thin films adsorbed onto a gold-coated quartz crystal. Cetyltrimethylammonium bromide thin films grown from aqueous solutions above the critical micelle concentration reveal critical phases in thickness and porosity evolution. We relate these effects to the mechanisms of formation and removal and the structure of cetyltrimethylammonium bromide films, which we determine to have systemic defects due to the presence of micelles.     

Formation of microcrystalline SiC films by chemical transport with a high-pressure glow plasma of pure hydrogen

The formation of microcrystalline 3C-SiC films on Si substrates by the plasma-enhanced chemical transport method was investigated using a pure hydrogen glow plasma at 0.027 MPa. In this method, no source gas was necessary. Instead, the erosion products of a sintered 3C-SiC plate in a hydrogen plasma were used as the deposition source. By Fourier transform infrared (FT-IR) absorption gas analysis, the species generated by the hydrogen etching of sintered SiC were found to be SiH₄ and CH₄, which can serve as precursors for SiC film formation. The etch rate of sintered SiC by hydrogen plasma decreased with increasing source temperature. The maximum etch rate of the sintered SiC was 450 nm/min at an input power of 47 W/cm². Films prepared by this method at substrate temperatures (T_sub) of 600 and 1073 K were analyzed by FT-IR absorption spectroscopy. An absorption peak at 800 cm^- 1 related to Si-C bonds was clearly observed, but no significant hydrogen-related absorption peaks, such as C-H and Si-H, were observed in the prepared films. The deposition rate of SiC was about 8 nm/min, independent of T_sub. The SiC films had a columnar structure, and their surface morphologies revealed faceted growth. With decreasing T_sub, the lateral grain size became large. The current-voltage characteristics of a prepared SiC/Si heterojunction np diode showed rectifying behavior, demonstrating that the doping of an SiC film can be achieved without a doping gas source. The dopant distribution near the SiC/Si interface deduced from capacitance-voltage measurements suggests that the precise control of the initial growth stage is important to obtain a good SiC/Si interface.     

Deposition of silicon doped and pure hydrogenated amorphous carbon coatings on quartz crystal microbalance sensors for protein adsorption studies

In this study hydrogenated amorphous carbon films (a-C:H) and silicon doped hydrogenated amorphous carbon films (a-C:H:Si) with different hydrogen and silicon contents were deposited onto sensors of a quartz crystal microbalance with dissipation detection (QCM-D). The resulting films were investigated with regard to their structural and elemental compositions using Raman spectroscopy, elastic recoil detection analysis and Rutherford backscattering spectroscopy. Furthermore the surface free energy (SFE) of the films was determined using contact angle measurements. The polar part of SFE of the a-C:H:Si films was found to be adjustable by the silicon content in these films and increased by increasing amounts of silicon. Carbon films with a broad range of chemical composition showed similar structure and properties when deposited on QCM-D sensors as compared with the deposition on silicon wafers. Subsequently, the amorphous carbon coated QCM-D sensors were used to study the adsorption of human serum albumin. These QCM-D results were related to the surface properties of the films.     

Dense plasma formation on the surface of a ferroelectric cathode

An enhanced electron emission mode of the ferroelectric plasma cathode operation is reported. The enhanced emission is achieved due to the generation of dense plasma (10¹⁹-10²⁰ m⁻³). This plasma is formed by a flashover which is initiated by charged particles. These particles are attracted to the ferroelectric surface by a driving electric field and are released during its decay. Generation of an electron beam with current amplitude ?2.5 kA is demonstrated in a diode under an accelerating voltage of 150-300 kV and pulse duration of 300 ns.     

PbSe单晶薄膜的分子束外延及其表面微结构

用分子束外延技术在BaF2（111）衬底上生长了PbSe单晶薄膜,观测了表面形貌和微结构．结果表明,在高Se／PbSe束流比（≥0．4）条件下,PbSe按照二维层状模式生长,外延层中的应力通过位错滑移发生塑性形变而获得充分弛豫,获得的PbSe薄膜是具有单原子层平整度的表面台阶和螺旋结构;降低Se／PbSe束流比至0．2,首次在PbSe样品表面观察到规则的三角形纳米孔状结构;当Se／PbSe束流比为0时,PbSe薄膜表面出现三维岛状结构,应力只能得到部分弛豫．在BaF2（111）衬底上分子束外延生长PbSe单晶薄膜的最佳温度为450℃．     

Production of high density and large volume plasma by using a polyphase AC voltage source

Production of high density and large volume plasma for a plasma source ion implantation has been studied. Plasma is produced by a polyphase AC voltage source with maximum voltage and power of 2 kV and 12 kVA, respectively. Produced N₂ plasma had a small ripple frequency of 720 Hz and a ripple ratio of 20% though frequency of the AC voltage source was 60 Hz. At output voltage of 600 V and N₂ gas pressure of 5 Pa, plasma density in 6-phase operation and 12-phase operation were 8×10⁸ cm⁻³ and 4×10⁹ cm⁻³, respectively. The plasma density in 12-phase operation was about five times larger than that in 6-phase operation. Electron temperature of 0.9 eV was obtained in both operations. The uniform plasma was produced in the inner region of the ring basket electrode which had a radius of 20 cm and length 80 cm at 12-phase operation. The plasma density was increased in proportion to the N₂ gas pressure and output voltage of the AC voltage source. When line-cusp magnets were attached to the process chamber wall, the plasma density increased about 2.5 times.     

Impact of oxygen and argon plasma exposure on the roughness of gold film surfaces

The impact of oxygen and argon plasma exposure on the roughness of gold film Quartz Crystal Microbalance (QCM) electrodes is reported here, employing low levels of gas uptake and scanning tunneling microscope measurements to probe the post-exposure surface morphology. For equal exposure times, argon plasma bombardment is observed to produce both greater material removal and greater change in surface roughness. A possible explanation for this is that the oxygen plasma produces a protective gold oxide layer, which may remove the contaminants from the surface without creating defects in the gold surface. The result is also consistent with prior reports of chemical cleaning of the surface by reactive oxygen ions. Pentane gas adsorption on the argon bombarded QCM surfaces was, moreover, observed to occur at pressures that are several orders of magnitude lower than that for an unbombarded surface.     

A sensor of alcohol vapours based on thin polyaniline base film and quartz crystal microbalance

Thin films of polyaniline base, emeraldine base (EB), coating on the quartz crystal microbalance (QCM) electrode were used as a sensitive layer for the detection of a number of primary aliphatic alcohols such as ethanol, methanol, 2-propanol and 1-propanol vapours. The frequency shifts (Δf) of the QCM were increased due to the vapour adsorption into the EB film. Δf were found to be linearly correlated with the concentrations of alcohols vapour in part per million (ppm). The sensitivity of the sensor was found to be governed by the chemical structure of the alcohol. The sensor shows a good reproducibility and reversibility. The diffusions of different alcohols vapour were studied and the diffusion coefficients (D) were calculated. It is concluded that the diffusion of the vapours into the EB film follows Fickian kinetics.     

Study of the effect of process parameters for n-heptylamine plasma polymerization on final layer properties

Plasma polymerization processes are widely used to chemically functionalize surfaces, which properties can be tuned by different operating variables. In this study, thin amine-containing polymer layers were produced on solid substrates in a custom-made cylindrical plasma polymerization reactor by radio frequency glow discharges of n-heptylamine vapours. Carefully planned experiments were conducted to evaluate the importance of four different process parameters on the chemical composition and thickness of the resulting films. The parameters investigated were: 1) deposition time, 2) power of the glow discharge, 3) distance between the electrodes, and 4) monomer pressure. Possible interactions between these variables were investigated through the use of statistical analyses (i.e., factorial design). This study reveals that n-heptylamine plasma polymer (HApp) layer thickness is influenced by the power of the glow discharge and the deposition time, as assessed by surface plasmon resonance and atomic force microscopy step height measurements. Also, the atomic ratio of nitrogen to carbon atoms on the treated surfaces is mainly influenced by the power of the glow discharge, as revealed by X-ray photoelectron spectroscopy. Quartz crystal microbalance analysis also confirmed that HApp layers are stable when immersed in aqueous solution.     

Synthesis and characterization of plasma-polymerized tert-butylacrylate films

Plasma polymerized tert-butylacrylate (pp-t-BA) film was prepared using tert-butylacrylate monomer under 100 Pa of vapor pressure with varying RF power of 10-250 W and continuous wave RF power of 13.56 MHz. The deposition rates of pp-t-BA films were determined using quartz crystal microbalance (QCM) method. The chemical structure of pp-t-BA films was characterized using FT-IR, contact angle and XPS techniques. The gas sorption properties of pp-t-BA were measured using a QCM sensor array. Results showed that deposition rates of pp-t-BA film were proportional to the polymerization time at 100 Pa of monomer pressure under the same RF power. The deposition amount of pp-t-BA films increased gradually with increasing RF power of 30-150 W. Increasing the RF power on plasma polymerization decreased the amount of ester group in pp-t-BA films. Sensitivity of gas sorption on pp-t-BA films is related to the RF power of polymerization.     

The effects of plasma pre-treatment and post-treatment on diamond-like carbon films synthesized by RF plasma enhanced chemical vapor deposition

Diamond-like carbon (DLC) films were synthesized by RF plasma enhanced chemical vapor deposition and the effects of plasma pre-treatment and post-treatment on the DLC films were investigated. Experimental results show that the surface roughness of the substrate, ranging from 0.2 to 1.2 nm, created by the plasma pre-treatment, will affect the surface roughness of the DLC films deposited using methane as the carbon source. However, the film surface roughness (0.1-0.4 nm) is much smaller than that of the substrate. Raman analysis and hardness measurement by nanoindentation indicate that the structure and the hardness of the DLC films are relatively unchanged for the film surface roughness investigated. For the argon or hydrogen plasma post-treatment of the DLC films deposited using acetylene as the carbon source, it is found that surface roughness decreases with the post-treatment time. Although the hardness decreases after post-treatment, it remains relatively constant with increasing post-treatment time.     

A combined sensor for the diagnostics of plasma and film properties in magnetron sputtering processes

A combined sensor for the simultaneous measurement of plasma and deposition parameters has been designed and built. It comprises (i) a quartz crystal microbalance, (ii) a planar Langmuir probe and (iii) a calorimetric (Gardon) probe, which allows to measure the deposition rate, typical plasma parameters (plasma density and electron temperature) and the total energy input into a growing film. The combined sensor is electrically insulated against ground, allowing these measurements also for floating or substrate-bias conditions. These parameters are measured (nearly) simultaneously, controlled by a specific measurement and analysis program. The operation of this combined sensor is demonstrated for the deposition of copper and tungsten films with a 2 inch planar magnetron.     

Hot-wire chemical vapor deposition and characterization of polycrystalline silicon thin films using a two-step growth method

A two-step growth method was proposed to reduce the amorphous incubation layer in the initial growth of polycrystalline silicon (poly-Si) films prepared by hot-wire chemical vapor deposition (HWCVD). In the two-step growth process, a thin seed layer was first grown on the glass substrate under high hydrogen dilution ratios (φ ≥ 0.9), and then a thick overlayer was subsequently deposited upon the seed layer at a lower φ value. The effect of various deposition parameters on the structural properties of poly-Si films was investigated by Raman spectroscopy and transmission electron microscopy. Moreover, the electrical properties, such as dark and photo conductivities, of poly-Si films were also measured. It was found that the Si incubation layer could be suppressed greatly in the initial growth of poly-Si with the two-step growth method. In the subsequent poly-Si film thickening, a lower φ value of the reactant gases can be applied to enhance the deposition rate. Therefore, a high-quality poly-Si film can be fabricated via a two-step growth method with a sufficient growth rate using HWCVD.     

Preparation and characterization of germanium oxysulfide glassy films for optics

Homogeneous amorphous films in the GeS₂-GeO₂ system have been deposited by a rf sputtering technique. Optical characterizations have shown that the cut-off wavelength and the linear indices increase with an increase in the S/O ratio. Raman spectroscopy indicates the presence of new modes that can be assigned to intermediate germanium oxysulfide structural units. Photo-sensitivity of the oxysulfide films has been demonstrated for irradiation near the band-gap. Diffraction gratings inscribed using 488 nm exposure displayed a limited diffraction efficiency (≤3%) that weakens with a corresponding decrease in the glass S/O ratio.     

Langmuir Blodgett films of arachidic acid and a nematic liquid crystal: Characterization and use in homeotropic alignment

We study the behavior of a mixed Langmuir monolayer consisting of a fatty acid and a nematic liquid crystal. We demonstrate that the mixed monolayer successfully transfers as a Langmuir-Blodgett film and characterize the transferred film using UV spectroscopy. An important application of Langmuir-Blodgett films is in the alignment of liquid crystals for electro-optical applications, such as displays. We show that including the liquid crystal in the Langmuir-Blodgett film produces homeotropic alignment for a system which fails to align by other standard techniques.     

Preparation and characterization of new hybrid nanostructured thin films for biosensors design

The present paper reports on an innovative route for the preparation of new hybrid nanostructured thin films based on hydroxyapatite and functionalized polyurethane. Hybrid nanopowders based on hydroxyapatite and functionalized polyurethane have been synthesized by a hydrothermal method with high pressure and low temperature conditions and further used for spin coating deposition. Biocompatible thin films with a thickness of about 50 nm have been deposited onto Si/SiO₂/Ti/Au substrates and their properties recommend them suitable as possible electrodes for the fabrication of impedance biosensors. Hybrid materials with improved properties are obtained, combining the mechanical properties of polyurethane with biocompatible properties of hydroxyapatite (bioactivity and osteoconductivity). The presence of functional groups in polyurethane structure ensures the existence of strong interactions between components and an increased affinity of the thin films for further protein bonding in biosensor design. Hybrid nanostructured thin films based on hydrothermally synthesized hydroxyapatite-polyurethane nanopowders could enhance the amount of immobilized biomolecules in the construction of an impedance biosensor for diagnosis and therapy of bone diseases.