Interfacial Chemistry of Langmuir-Blodgett Deposited Polyimide Films on Si (100)

Using the Langmuir-Blodgett (LB) technique, ultrathin films of the octadecylammonium salt of polyamic acid (PACS) on (100) oriented silicon wafers with one, three and five monolayers were prepared. The imidization of the films was investigated with x-ray photoelectron spectroscopy (XPS) during a stepwise heating procedure in vacuum. Significant differences in the XPS spectra indicate an incomplete polymerization of the films as a function of film thickness. It is believed that the chemical interaction at the interface between Si substrate and PACS is responsible for the incomplete polymerization of the LB film in direct contact with the substrate. From ellipsometric measurements the absolute thickness of a PACS and a polyimide layer has been determined to be 1.7 nm and 0.6nm, respectively. These measurements allow us to determine the electron mean free path for the Si2p electrons (E_k=1153 eV) of λ = 4.2±0.1 nm through these films.     

Dielectric behavior and magnetical response for porous BFO thin films with various thicknesses over Pt/Ti/SiO2/Si substrate

A novel sol–gel method has been developed to deposit multiferroic nanocrystalline bismuth ferrite (BFO) thin films over Pt/Ti/SiO₂/Si substrate by spin-coating technique with various thicknesses. It is found that the deposition parameters significantly influence the quality and the thickness of BiFeO₃ films. The films are all uniform and adherent to Pt/Ti/SiO₂/Si substrate. The spin-coated films are characterized by X-ray diffraction (XRD), Scanning electron microscope (SEM), Atomic force microscope (AFM), photoluminescence spectroscopy (PL) and Fourier transform infrared spectroscopy (FTIR). Rhombohedral structure of BFO is confirmed from the XRD and FT-IR studies. The SEM image shows a porous structure formation of BFO over Pt/Ti/SiO₂/Si substrate. The surface outgrowth for the films at various thicknesses is measured from root mean square (RMS) and surface roughness through AFM. The step height and the RMS are found to be high for the film at 500 nm in comparison with thickness of 200 nm. The influence of the dielectric properties of the porous BFO at different thicknesses is studied using LCRQ meter. Finally, the magnetic behavior of film is compared with M–H hysteresis loop and Magnetoresistance (MR) studies.     

Synthesis of MgO thin films grown by SILAR technique

Different thickness MgO thin films were grown on the glass substrate by successive ionic layer adsorption and reaction (SILAR) method as the first study in literature. X-ray diffraction (XRD) measurements demonstrate the cubic MgO structures and samples have (002), and (220) peaks. All film has nanoball structures observed from the scanning electron microscope (SEM) images. The band gap and transmittance values of MgO thin films decrease with increasing thickness. The photoluminescence (PL) spectrum demonstrates that samples have three visible emissions changing with thickness at 381?nm violet emission, 457?nm blue emission and 535?nm green emission. X-ray photoelectron spectroscopy (XPS) spectrum present confirms the elemental signals from carbon (C), oxygen (O) and magnesium (Mg) atoms in the sample. Both Moss and Herve and Vandamme relations refractive index values n, ε₀, and ε_∞ values and amount of oxygen increase with raising thickness of MgO thin films.     

Preparation and characterization of thin organosilicon films deposited on SPR chip

The paper reports on the preparation and characterization of organosilicon thin polymer films deposited on glass slides coated with 5 nm adhesion layer of titanium and 50 nm of gold. The polymer was obtained by the decomposition of 1,1,3,3-tetramethyldisiloxane precursor (TMDSO) premixed with oxygen induced in a N₂ plasma afterglow using remote plasma-enhanced chemical vapor deposition (PECVD) technique. The film thickness was controlled by laser interferometry and was 9 nm. The chemical stability of the gold substrate coated with the organosilicon polymer film (p-TMDSO) was studied in different acidic and basic solutions (pH 1-14). While the gold/polymer interface showed a high stability in acidic media, the film was almost completely removed in basic solutions. The resulting surfaces were characterized using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), water contact angle measurements, cyclic voltammetry, and surface plasmon resonance (SPR).     

Moisture absorption into ultrathin hydrophilic polymer films on different substrate surfaces

Moisture absorption into ultrathin poly(vinyl pyrrolidone) (PVP) films with varying thickness was examined using X-ray reflectivity (XR) and quartz crystal microbalance (QCM) measurements. Two different surfaces were used for the substrate: a hydrophilic silicon oxide (SiO_x) and a hydrophobic hexamethyldisilazane (HMDS) treated silicon oxide surface. The total equilibrium moisture absorption (solubility) was insensitive to the surface treatment in the thickest films (≈150 nm). However, strong reductions in the equilibrium uptake with decreasing PVP film thickness were observed on the HMDS surfaces, while the SiO_x surface exhibited thickness independent equilibrium absorption. The decreased absorption with decreasing film thickness is attributed a depletion layer of water near the polymer/HMDS interface, arising from hydrophobic interactions between the surface and water. The diffusivity of water decreased when the film thickness was less than 60 nm, independent of the surface treatment. Changes in the properties of ultrathin polymer films occur even in plasticized films containing nearly 50% water.     

Ultraviolet pulsed laser crystallization of Ba0.8Sr0.2TiO3 films on LaNiO3-coated silicon substrates

In this work, Ba_0.8Sr_0.2TiO₃ (BST) films on LaNiO₃-buffered SiO₂/Si (LNO/SiO₂/Si) substrates were crystallized by pulsed laser irradiation. Solution-derived amorphous barium–strontium–titanate precursor layers were crystallized with a KrF excimer laser in oxygen ambient at fluences ranging from 50 to 75 mJ cm⁻². With the substrate temperature set to 500 °C, the number of pulses and film thickness were varied until high-quality crystallinity could be achieved. It was found that films with a thickness of 40 nm are fully crystallized with a uniaxial {00l} orientation which is predetermined by the LaNiO₃ orientation. On the other hand, for 160 nm thick films, crystallization was observed after 12,000 pulses in the 70 nm close to the surface, while the rest of the film remained amorphous. The large temperature difference between the film surface and interface due to the low thermal conductivity of the amorphous BST is suggested as the origin of this behavior. Films thicker than 80 nm cracked on crystallization due to the stress caused by the different thermal expansion coefficients of film and substrate, as well as the large temperature variations within the BST film.     

Control of polymerization processes of 10,12-pentacosadiynoic acid LB films

Studies have been carried out on KrF excimer laser light (EX), X-ray or electron beam (EB) induced polymerization of 10,12-pentacosadiynoic acid (PDA) Langmuir-Blodgett (LB) films in relation to molecular density or molecular arrangement of the films using X-ray diffraction analysis, infrared (IR) spectroscopy and Raman spectroscopy. The molecular arrangement or density of the PDA LB films was controlled by subphase conditions when the films were built up, such as pH, temperature of a subphase or salt concentration in the subphase. Polymerization sensitivity of the PDA LB film was affected by the arrangement or molecular density. On low density (A type) films the polymerization occurred by irradiation with EX, X-ray or EB, but on high density (B type) films the polymerization occurred only when the irradiation was carried out by high energy beams such as X-ray or EB. Decomposition of polymerized films was observed further by excessive irradiation of EX or X-ray, but not on the B type films. It was revealed by X-ray diffraction analysis that in the A type film, the PDA molecules bent to a larger extent than those in the B type film and the polymerization proceeded topochemically, that is, the thickness decreased little after EB irradiation in a helium atmosphere. On the other hand, in the B type film, the thickness decreased by about 10% as a result of EB irradiation. By IR reflection-absorption (RA) and Raman measurements, it was confirmed that conjugated diacetylenic bonds disappeared and conjugated double and new conjugated triple bonds appeared after high energy beam irradiations. These results support the supposition that 1,4-polymerization, i.e. polydiacetylene type polymerization, occurs easily in the A type film and 1,2- or 3,4-polymerization, i.e. polyacetylene type polymerization, occurs in the B type film, and that the polymerized A type film was decomposed at the polydiacetylenic bond when the irradiation continued further. It was also shown that the polyacetylene type polymer was obtained only when the B type film was irradiated with the high energy beam.     

Possibility of graphene growth by close space sublimation

Carbon films on the Si/SiO₂ substrate are fabricated using modified method of close space sublimation at atmospheric pressure. The film properties have been characterized by micro-Raman and X-ray photoelectron spectroscopy and monochromatic ellipsometry methods. Ellipsometrical measurements demonstrated an increase of the silicon oxide film thickness in the course of manufacturing process. The XPS survey spectra of the as-prepared samples indicate that the main elements in the near-surface region are carbon, silicon, and oxygen. The narrow-scan spectra of C1s, Si2p, O1s regions indicate that silicon and oxygen are mainly in the SiO _x (x ≈ 2) oxide form, whereas the main component of C1s spectrum at 284.4 eV comes from the sp²-hybridized carbon phase. Micro-Raman spectra confirmed the formation of graphene films with the number of layers that depended on the distance between the graphite source and substrate.     

Effects of CCl4 concentration on nanocrystalline diamond film deposition in a hot-filament chemical vapor deposition reactor

The effect of CCl₄ concentration on the nanocrystalline diamond (NCD) films deposition has been investigated in a hot-filament chemical vapor deposition (HFCVD) reactor. NCD films with a thickness of few-hundred nanometers have been synthesized on Si substrates from 2.0% and 2.5% CCl₄/H₂ at a substrate temperature of 610 °C. Polycrystalline diamond films and nanowall-like films with higher formation rates than those of the NCD films were deposited from lower and higher CCl₄ concentrations, respectively. The grain sizes of the diamond film grown using 2.0% CCl₄ increased with film thickness while a diamond film with uniform nanocrystalline structure all over a thickness of 1 μm can be deposited in the case of 2.5% CCl₄. We suggest that both the primary nucleation and the secondary nucleation processes are crucial for the growth of the NCD films on Si substrates.     

Chemical solution deposition of pure and Gd-doped ceria thin films: Structural,morphological and optical properties

High quality smooth, uniform and crack-free ceria and gadolinium doped ceria (GDC) thin films were prepared on Si and Si/YSZ substrates by chemical solution deposition. The thermal behavior of Gd-Ce-O precursor was investigated by TG-DSC measurements. The phase purity and structure of deposited films were evaluated using X-ray diffraction (XRD) analysis and Raman spectroscopy. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were employed for the estimation of surface morphological features. Oxidation state of Ce ions in fabricated films was analyzed by X-ray photoelectron spectroscopy (XPS). Optical properties were evaluated by diffuse reflectance UV–vis spectrometry. Thickness of the films can be controlled by applying a certain number of spin coating cycles. A linear relation between the thickness of the films and the number of deposited layers was observed. The single-layer thickness was determined to be approximately 20 nm. The influence of annealing temperature and Gd content on the film structure, morphology and optical properties was studied and discussed. The dependence of an optical band gap as a function of grain size was demonstrated.     

Growth,structure, morphology,and magnetic properties of Ni ferrite films

The morphology, structure, and magnetic properties of nickel ferrite (NiFe₂O₄) films fabricated by radio frequency magnetron sputtering on Si(111) substrate have been investigated as functions of film thickness. Prepared films that have not undergone post-annealing show the better spinel crystal structure with increasing growth time. Meanwhile, the size of grain also increases, which induces the change of magnetic properties: saturation magnetization increased and coercivity increased at first and then decreased. Note that the sample of 10-nm thickness is the superparamagnetic property. Transmission electron microscopy displays that the film grew with a disorder structure at initial growth, then forms spinel crystal structure as its thickness increases, which is relative to lattice matching between substrate Si and NiFe₂O₄.     

Photoinduced electron transfer in nanostructures of ultrathin polyimide films containing porphyrin moieties

Photoinduced electron transfer between porphyrin moieties and pyromellitimide fragments has been investigated in multi-layered structures of ultrathin polyimide films prepared by the Langmuir-Blodgett (LB) technique. The LB films were composed of three kinds of polyimides, which contained zinc tetraphenylporphyrin (ZnTPP) unit as an electron donor (D-layer), no chromophoric groups (S-layer), and pyromellitimide fragments as an electron acceptor (A-layer). The layered structure and orientational distribution of porphyrin moieties in the LB films were evaluated by surface plasmon measurement and absorption dichroism measurement, respectively. The thickness of monolayer was estimated to be 0.9 nm for the polyamic acid films and 0.4 nm for the polyimide films. The molecular plane of porphyrin moieties was oriented in the direction parallel to the substrate plane. In the multi-layered structures of polyimide LB films, the efficiencies of photoinduced electron transfer from porphyrin moieties to pyromellitimide fragments varied sharply with the number of spacing layers, indicating that the short-range interactions such as electron transfer could be controlled by the fabric of ultrathin films. The rate of electron transfer observed by the fluorescence quenching measurements was numerically simulated for the nanostructure using the Monte Carlo method.     

Preparation of superconducting edge junctions using polymeric Langmuir–Blodgett films as barrier layers

Aromatic polyamic acid alkyl ammonium salt exhibits very good transfer behavior under suitable deposition conditions. A superconducting thin film of YBa₂Cu₃O_x (YBCO) on Langmuir–Blodgett film (LB) has been obtained when the substrate temperature is higher than 500°C. Superconducting edge junctions of (Pb—In)/LB/(Pb—In) and YBCO/LB/YBCO incorporated with polyimide LB films were fabricated. Their current–voltage characteristics are discussed in terms of the temperature stability of polyimde LB films. © 1994 John Wiley & Sons, Inc.     

Effect of Deposition Temperature on the Growth of Yttria-Stabilized Zirconia Thin Films on Si(111) by Chemical Vapor Deposition

An experimental study was made on the effect of deposition temperature on the growth of yttria-stabilized zirconia (YSZ) thin films in the chemical vapor deposition (CVD) process. The YSZ thin films were obtained in a temperature range of 650°–850°C, using β-diketone chelates and a Si(111) substrate. Dense and mirrorlike YSZ films with uniform thickness were prepared; the deposition rate was 12–20 nm/min at those temperatures. An examination of the crystalline structure of the YSZ films was made, and the appropriate temperature for the growth of c -axis-oriented YSZ thin films using a Si(111) substrate was determined. The quality of the YSZ films was strongly dependent on the deposition temperature. As the temperature increased, the film growth mechanism changed from being controlled by surface reaction to being controlled by gas-phase diffusion.     

Growth and characterization of fluorocarbon thin films grown from trifluoromethane (CHF3) using pulsed‐plasma enhanced CVD

Trifluoromethane (CHF₃) was used as a precursor gas in pulsed‐plasma enhanced CVD to deposit fluorocarbon films onto Si substrates. The film composition, as measured by X‐ray photoelectron spectroscopy (XPS) of the C1s peak, was observed to change as the plasma duty cycle was changed by varying the plasma off‐time; this offers a route to control the molecular architecture of deposited films. FTIR results indicate that the film is primarily composed of CF_x components, with little or no C H incorporation into the film. The rms roughness of the films is extremely low, approaching that of the Si substrate; the low growth rate and consequent high‐power input/thickness is believed to be partly responsible. CHF₃ produces films with higher % CF₂ compared to other hydrofluorocompound (HFC) monomers (CH₂F₂ and C₂H₂F₄). However, the deposition kinetics for all three HFC gases display similar trends. In particular, at a fixed on‐time of 10 ms, the deposition rate per pulse cycle reaches a maximum at an off‐time of approximately 100 ms. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 842–849, 2000     

Biaxial elastic modulus of very thin diamond-like carbon (DLC) films

The biaxial elastic modulus of very thin diamond-like carbon (DLC) films was measured by the recently suggested free overhang method. The DLC films of thickness ranging from 33 to 1100 nm were deposited on Si wafers by radio frequency plasma-assisted chemical vapor deposition (r.f.-PACVD) or by the filtered vacuum arc (FVA) process. Because the substrate was partially removed to obtain sinusoidal free overhang of the DLC film, this method has an advantage over other methods in that the measured value is not affected by the mechanical properties of the substrate. This advantage is more significant for a very thin film deposited on a substrate with a large difference in mechanical properties. The measured biaxial elastic moduli were reasonable values as can be judged from the plane strain modulus of thick films measured by nanoindentation. The biaxial elastic modulus of the film deposited by r.f.-PACVD was 90±3 GPa and that of the film deposited by FVA process was 600±50 GPa. While the biaxial elastic modulus of the film deposited by FVA is independent of the film thickness, the film deposited by r.f.-PACVD exhibited decreased elastic modulus with decreasing film thickness when the film is thinner than 500 nm. Although the reason for the different behavior could not be clarified at the present state, differences in structural evolution during the initial stage of film growth seem to be the reason.     

MOLECULAR STRUCTURE OF INTERFACES FORMED WITH PLASMA-POLYMERIZED SILICA-LIKE PRIMER FILMS: PART II. CHARACTERIZATION OF THE PRIMER/METAL INTERFACE USING X-RAY PHOTOELECTRON SPECTROSCOPY IN SITU

X-ray photoelectron spectroscopy (XPS) was performed in situ on plasma-polymerized silica-like films that were deposited onto metal substrates. Relatively thick films (∼8.0 nm) had spectra that were typical of bulk amorphous silicon dioxide (a-SiO 2 ). When thinner films were analyzed (∼2.4 nm), a Si(2p) peak emerged that was due to the formation of silicon suboxide at the interface. Changes in the metal and metal oxide peaks showed that oxidation of the substrates during plasma etching and deposition occurred. It was determined that during the initial stages of plasma deposition, metal atoms from the substrate migrated to the metal-oxide surface. This resulted in preferential oxidation of metal atoms with the formation of silicon suboxide at the film/metal interface. In addition, interfacial suboxide formation was shown to have a dependence upon the diffusivity of the metal substrate atoms through the surface oxide of the metal. As a result, more interfacial suboxide was observed to form for depositions on titanium substrates in comparison with depositions on aluminum substrates. A detailed analysis of the atomic species detected with in situ XPS enabled us to develop a model of the molecular structure at the a-SiO 2 /metal interface for plasma depositions on aluminum and titanium substrates. When the possible chemical reaction routes for film deposition were considered, the formation of primary Al-O-Si and Ti-O-Si bonds at the interface was proposed.     

The Role of Adhesion Promoters on the Molecular and Mesoscopic Structure of the Interphase

New experimental results are described from Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectroscopy, X-ray photoelectron spectroscopy (XPS) and Atomic Force Microscopy (AFM) on the molecular orientation, chemical composition, and topography of PMDA/ODA Polyimide (PI) deposited by the Langmuir-Blodgett (LB) technique onto Si (100) surfaces covered with a native oxide (SiO_x) and onto SiO_x substrates pre-treated with a variety of alkylsilanes. The alkylsilane monolayers used as adhesion promoters were found to be chemically and structurally stable during temperature treatments used to imidize the polymer precursor films. On both the oxidized silicon surface and on the silane-treated surfaces, we find a thickness-dependent, preferential orientation of the pyromellitimide unit in PI, but randomly-oriented oxydianiline fragments. We can not identify any specific chemical bonds between the polymer and the aminosilane film or, in the case of the untreated surface, the SiO_x substrate. However, a quantitative analysis of the NEXAFS data reveals a pronounced deficit in oxydianiline in the monolayer (ML) films, which decreases with film thickness, and a higher degree of imidization on the 3-aminopropyltrimethoxysilane (APS) surface indicating that imide linkages are formed between the polymer and the aminosilane. The AFM images show a heterogeneous surface topography with nucleation of PI polycondensates on both the SiO_x and aminosilane-treated surfaces. On the aminosilane surface additional nuclei are detected, believed to be polycondensates of aminosilane formed during imidization of the PI precursor. Nucleation of aminosilane polycondensates is not observed in the absence of polyamic acid. As suggested by a comparison of Lateral Force Microscopy (LFM) and Atomic Force Microscopy measurements, these aminosilane nuclei are covered by PI with increasing film thickness and, hence, provide mechanical connections between the polymer and substrate on a mesoscopic scale. Delamination experiments of 2 μm thick PI films spun onto ω-amino and methyl terminated alkylsilanes indicate that bond fracture always occurs in the aminosilane/PI and alkylsilane/PI and interphase. We conclude that the adhesion promotion effect of LB-deposited polyimide films by aminosilanes can be explained by chemical stabilization of the interface against decomposition and mechanical interlinking surface via polycondensates of aminosilane on a mesoscopic length scale. Our study is not conclusive as to whether covalent bonding of the polymer to the substrate via the silane is important for macroscopic adhesion.     

Residual stress relief of hard a-C films though buckling

To characterize the adhesive failure mode in amorphous-carbon (a-C) films, and to explore the effects of stress relief mechanism on the mechanical properties of the films, the microstructure and the morphologies of the buckled and peeled a-C films were characterized by various techniques, including transmission electron microscopy (TEM), scanning electron microscopy (SEM), Raman spectrum and X-ray photoelectron spectroscopy (XPS). Results indicated that there is obvious buckling between the stress relieved a-C films and Si substrates, and the development of the buckling blister was derived from the residual compressive stress. The as-deposited a-C films voluntarily buckled along the film growth direction above Si substrate when film thickness reached a certain size, and became more and more remarkable, resulting in eventual peeling. These buckling and peeling processes can relieve the residual stress of the a-C films by eliminating the mechanical restriction of Si substrates. The corresponding sp² hybridization transformation and the reconfiguring graphitic phase were detected in the stress relived a-C films, which can induce buckling and spalling in the a-C films.     

Ferroelectric properties of pure ZrO2 thin films by chemical solution deposition

Ferroelectricity in pure zirconia (ZrO₂) thin films, manufactured on Si (100) substrates via the chemical solution deposition method using all-inorganic aqueous salt precursor, has been demonstrated for the first time. The influence of thickness on the crystalline structure and ferroelectric properties of the thin films were measured and showed that they were strongly affected by the film thickness. The structural data indicated that as the film thickness increased from 30 nm to 50 nm, the m-phase fraction increased, and a phase transition from orthorhombic to cubic and then tetragonal occurred near the main diffraction peak of 30.7°. The lowest m-phase fraction of 15.4% was obtained in the pure ZrO₂ film with a thickness of 30 nm, and after 10³ field cycling, it exhibited the highest relative permittivity of 39.6 as well as the highest residual polarization of 8.5 μC/cm².