Thick Free-standing Films Research Articles

Enhanced microwave magnetic and attenuation properties of composites with free-standing spinel ferrite thick films as fillers

Free-standing thick films of spinel ferrite, Ni0.89−xCu0.11ZnxFe2O4 with x=0.55 and 0.60, were prepared as fillers to fabricate electromagnetic composites. Compared to those made with conventional spherical fillers, the composites made with thick film fillers showed enhanced static permeability (μ′0) and maximum imaginary permeability (μ″max). At the same time, complex permittivity (ε′ and ε″) were almost unchanged. A relative bandwidth WR of 7–8 was achieved, which is about 75% of the theoretical maximum relative bandwidth. These composites are potential candidates as electromagnetic attenuation materials with ultrabroad absorption bandwidth in L and S bands.

Structural and optical properties of thick freestanding AlN films prepared by hydride vapor phase epitaxy

The morphology, structural and optical properties of void-assisted freestanding HVPE-AlN films were investigated by a combination of non-destructive microscopic and spectroscopic techniques. The freestanding approximately 80μm thick clear film has a wurtzite crystalline structure with remarkable properties around the central film region. The E2(high)-phonon frequency coincides with reported stress-free film phonon frequency. The low temperature luminescence study of the growth and interface sides of the film is consistent with the incorporation of a high concentration of oxygen impurities. These results are promising as the growth method amenable to the production of freestanding stress-free large area substrates for epitaxial growth.

Structural and optical studies of thick freestanding GaN films deposited by Hydride vapor phase epitaxy

Thick freestanding films or bulk GaN substrates with very low background impurity levels (≤1×1015/cm3) and high crystalline quality are required for a number of electronic device applications. Low pressure chemical vapor and molecular beam epitaxy techniques can systematically deposit films with low residual impurity concentrations. However, their typical slow growth rate prevents their utilization for substrate growth. The hydride vapor phase epitaxy deposition technique can achieve hundreds of microns per hour growth rate, but these films have typically high free carrier concentration (≥3×1017/cm3). It is crucial to verify if this method can reproducibly deliver thick freestanding GaN films of high crystalline quality with exceptionally low free carrier concentration. Low temperature photoluminescence and room temperature Raman scattering experiments carried out on a number of samples indicate that they have high crystalline quality and uncommonly low donor background levels. Reduced concentration of uncompensated shallow donors verified by low temperature electron paramagnetic resonance was confirmed by detailed high sensitive SIMS analyses. In addition, it was verified by X-ray diffraction analysis that relatively low dislocation densities can be achieved.

Studies on poly(vinylidene fluoride-co-hexafluoropropylene) based gel electrolyte nanocomposite for sodium–sulfur batteries

Experimental investigations on a sodium ion conducting gel polymer electrolyte nanocomposite based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP), dispersed with silica nanoparticles are reported. The gel nanocomposites have been obtained in the form of dimensionally stable, transparent and free-standing thick films. Physical characterization by X-ray diffraction (XRD), Fourier transform Infra-red (FTIR) spectroscopy and Scanning electron microscopy (SEM) have been performed to study the structural changes and the ion–filler–polymer interactions due to the dispersion of SiO 2 nanoparticles in gel electrolytes. The highest ionic conductivity of the electrolyte has been observed to be 4.1 × 10 −3 S cm − 1 at room temperature with ~ 3 wt.% of SiO 2 particles. The temperature dependence of the ionic conductivity has been found to be consistent with Vogel-Tammen-Fulcher (VTF) relationship in the temperature range from 40 to 70 °C. The sodium ion conduction in the gel electrolyte film is confirmed from the cyclic voltammetry, impedance analysis and transport number measurements. The value of sodium ion transport number (t Na+) of the gel electrolyte is significantly enhanced to a maximum value of 0.52 on the 15 wt.% SiO 2 dispersion. The physical and electrochemical analyses indicate the suitability of the gel electrolyte films in the sodium batteries. A prototype sodium–sulfur battery, fabricated using optimized gel electrolyte, offers the first discharge capacity of ~165 mAh g − 1 of sulfur.

Field emission properties of as-grown multiwalled carbon nanotube films

Multiwalled carbon nanotubes have been produced by ethylene catalytic chemical vapor deposition and used to fabricate thick and dense freestanding films (“buckypapers”) by membrane filtering. Field emission properties of buckypapers have been locally studied by means of high vacuum atomic force microscopy with a standard metallic cantilever used as anode to collect electrons emitted from the sample. Buckypapers showed an interesting linear dependence in the Fowler–Nordheim plots demonstrating their suitability as emitters. By precisely tuning the tip-sample distance in the submicron region we found out that the field enhancement factor is not affected by distance variations up to 2 μm. Finally, the study of current stability showed that the field emission current with intensity of about 3.3 × 10 −5 A remains remarkably stable (within 5% fluctuations) for several hours.

Light scattering and diffuse light propagation in sintered porous silicon

The scattering coefficient and the refractive index of sintered porous silicon are deduced from measurements on 1–4μm thick freestanding films. Mie’s theory is applied to describe the light scattering by the spherical pores. Using a reduced effective refractive index for the host medium in Mie’s theory accounts for the close spacing of the pores and results in an agreement between the measured and calculated scattering coefficients. A coherent calculation for the specular nonscattered radiation is combined with a model that describes the propagation of the scattered diffuse light flux. For this diffuse model two approaches, the Kubelka Munk theory and a Lambertian model developed in this work, are compared. The combined model reproduces both, the specular as well as the diffuse component of the measured reflection and transmission.

Focused ion beam irradiation effects on nanoscale freestanding thin films

The focused ion beam (FIB) technique is a versatile tool for nanoscale manipulation, deposition and etching. However, degradation mechanisms which lead to residual stresses in materials exposed to high-energy ion beams are not well understood. In this study, we examine the evolution of residual stresses in 100 nm thick freestanding aluminum films subjected to typical ion beam exposures within a commercial FIB tool. Experimental results show that the magnitude of the residual stresses increase with cumulative ion beam exposure and that upper limits are attainable. Further investigation demonstrates that a decrease in ion beam current at constant acceleration-voltage augments the upper limits, which manifests itself in greater residual stresses. The stress gradients in thin films develop from surface modifications in the form of amorphous top layers, which are modeled as bilayer approximations. Experimental observations and analysis indicate that ion beam exposure effects on the mechanical properties of nanoscale thin films and nanostructures cannot be ignored.

Behaviour of Smectic Phases in Free Standing Film Geometry: Discontinuous Change in the Tilt Angle of Polar Smectic Phases at the Phase Transition Temperature

The temperature dependence of the thickness of thick freestanding Films (FSF) is studied using a novel high resolution film thickness measurement technique. A small discontinuity in the temperature dependence of the smectic tilt angle at every phase transition between the ferro-, ferri-, antiferroelectric phases is observed. An additional term in the total free energy which considers an interaction between the neighboring and the next neighboring smectic layers explains this discontinuity. A comparison of the experimental results with phenomenological theory suggests that the long-range interactions are inversely proportional to the square of the distance between the layers.

Microsystems elements based on free-standing thick-films made with a new sacrificial layer process

A new process derived from screen-printing technology and based on thick sacrificial layers has been developed in our laboratory for the fabrication of films partially released from the substrate. The sacrificial layer acts as a stable mechanical support during the firing of the active layer and is totally removed after the final thermal treatment of the sample. Fabrication of a copper electrothermal actuator was undertaken to demonstrate the efficiency of this simple, collective and low cost process. Passive components based on this new process include the fabrication of heating resistors, strain gauges and microchannels. Moreover, the process can also be used to facilitate the implementation of piezoelectric devices.

Modelling conduction in carbon nanotube networks with different thickness, chemical treatment and irradiation

We show how the main features of the electronic transport properties of single-wall carbon nanotube (SWNT) networks can be understood in terms of a simple model involving metallic conduction interrupted by thin tunnelling barriers, backscattering by zone-boundary phonons, and variable range hopping. Within this framework we examine the effect of reducing the thickness of the SWNT networks, chemical treatments, and ion irradiation. The conduction mechanism can be tuned from variable-range hopping between localized states (for the thinnest networks), to metallic conduction interrupted by thin barriers through which conduction is by tunnelling (for thick freestanding films). Chemical treatment of the thick films by different molecules leads to retention of metallic character but changes (increases or decreases) the charge carrier density. For ion-irradiated thick films we find two competing effects: a change to hopping-type conduction in the direct impact layer that lowers conductivity, and annealing effects extending deeper than the ion penetration depth that increase conductivity and lead to a peak in conductivity as a function of irradiation dose. We briefly discuss current–voltage characteristics and possible Luttinger liquid effects.

New Sacrificial Layer-Based Screen-Printing Process for Free-Standing Thick-Films Applied to MEMS

Thick-film sacrificial layers based on inorganic materials have been used for the fabrication of free-standing screen-printed films. A specific polymer ink based on a mixture of epoxy and strontium carbonate has been prepared for screen-printing of the sacrificial layer. After deposition and subsequent heat treatments of the structural and sacrificial layers, the latter is removed in an aqueous solution. The harmlessness and efficiency of the process have been demonstrated with regard to the structural layer(s). This new process, initially developed in our laboratory for the fabrication of a cantilever type beam, has been extended to the elaboration of an electrothermal actuator, constituted of two linked copper beams of different widths. The beams are partially suspended above the substrate to which they are anchored. Other developments include the fabrication of heating resistors and microchannels. Moreover, the process has been shown to facilitate the implementation of microassemblies such as piezoelectric transformers.

Discontinuous change in the smectic layer thickness in ferrielectric liquid crystals

The temperature dependence of the thickness of thick free-standing films is studied using a high-resolution film thickness measurement technique. A small discontinuity in the temperature dependence of the smectic layer thickness at every phase transition between ferro-, ferri-, and antiferroelectric phases is observed. We show that the major contribution to it arises from a change in the smectic tilt angle.

Properties of Fe-doped semi-insulating GaN substrates for high-frequency device fabrication

Thick freestanding GaN films were grown by hydride vapor-phase epitaxial method on both pattern-masked and unmasked GaAs substrates. Both approaches resulted on films characterized by a large excess of free carriers at room temperature. The use of a GaAs back surface NiTi substrate protective layer increased the concentration of incorporated iron impurities and yield semi-insulating films. The morphology and crystalline quality of these films show a strong dependence on nucleation layer growth approach and Fe doping concentration. Near infrared photoluminescence and Raman scattering verified the incorporation and activation of the Fe impurities, and its effect on the free carrier concentration.

An atomic force microscopy study of the effects of surface treatments of diamond films produced by chemical vapor deposition

Diamond films produced by chemical vapor deposition (CVD) technique must be polished and machined in order to be used for many applications (windows, heat spreaders, surface acoustic wave devices, etc). We present an Atomic Force Microscopy study of the effects of mechanical polishing, hydrogen plasma etching, and ion implantation followed by annealing and chemical etching, on the morphology of Microwave Plasma CVD polycrystalline diamond films. Both growth and nucleation sides of 0.3–0.5 mm thick free-standing films were characterized. We found that grain boundaries and anisotropy of mechanical properties (hardness and wear rate) mostly contribute to the overall roughness of Microwave Plasma CVD diamond films, which is below 0.2 nm at the single-grain scale. The further deposition of a diamond overlayer, even of small thickness, on highly polished polycrystalline diamond, results in a variety of growth modes and different growth rates depending on the grain orientation, leading to the roughness increase. Hydrogen plasma etching proved to be effective for inducing morphological and chemical changes on the surface, while an ion implantation/annealing/etching can be used for precise removal of thin diamond layers. The latter approach is promising for diamond lithography.

Test of clock model in ellipsometric study of thin and thick free-standing films of an antiferroelectric liquid crystal

The temperature dependences of the ellipsometric parameters in a weak dc external field are studied in thin and thick free-standing films of MHPOBC. The results for thin films consisting of two, three, and four layers are analyzed within the discrete phenomenological model. We find very good quantitative agreement between the theory and experiment, which indicates an odd-even effect. We find that the XY structures are stable for an odd number of layers, whereas planar, Ising-like structures are stable for an even number of layers. The experiments on thick (several tens of layers) films show a combination of bulklike and free-surface behavior. This is most pronounced at high temperatures, where the interior of the film is not tilted, whereas the layers at the air interfaces show qualitatively similar temperature dependance of the ellipsometric parameters as in the four-layer film.

Synthesis and characterization of nanoscale biomimetic polymer vesicles and polymermembranes for bioelectronic applications

An amphiphilic ABA triblock copolymer was synthesized using poly(2-ethyl-2-oxazoline)(PEtOz) as hydrophilic block [A] and poly(dimethylsiloxane) (PDMS) as hydrophobicblock [B]. The cationic ring-opening polymerization of 2-ethyl-2-oxazoline was initiated bybenzyl chloride in the presence of NaI. PEtOz–PDMS–PEtOz was characterized in aqueoussolution using transmission electron microscopy (TEM). The block copolymers formedvesicles with a [B] block hydrophobic component thickness of 4 nm, which is thinenough for successful reconstitution of proteins. The mean diameters of the vesicleswere measured to be in the range of 150–250 nm, with a narrow distribution. Theelectrochemical properties of planar PEtOz–PDMS–PEtOz membrane films spread across aTeflon aperture were investigated by electrochemical impedance spectroscopy(EIS). The impedance data showed an increase of planar membrane capacitance(CMEM) from2.58 × 10−7 to2.71 × 10−7 F cm−2 and a decrease ofmembrane resistance (RMEM) from 12 to 10.8 Ω cm2.The increase of CMEM over time in buffer solution can be explained by an increase of the dielectric constant as aresult of membrane electrolyte incorporation and/or by the formation and growth of defectin the free-standing films. In contrast to the formation of thin-walled vesicles, thespread triblock copolymer formed a free-standing membrane with a 9 nm thickness.Based on the two possible conformations (bridge midblock conformation and loopmidblock conformation) that the triblock copolymer can have, we can conclude thatPEtOz–PDMS–PEtOz formed 4 nm thick polymer vesicles with intercalated loop midblockstructure in aqueous solution, while 9 nm thick free-standing polymer films with bilayerloop midblock conformation or with bridge midblock conformation were formed by aperturespreading.

Dye incorporation in polyphosphate gels: synthesis and theoretical calculations

In this work we described theoretical calculations on the electronic structure and optical properties of the dyes crystal violet and malachite green based in semiempirical methods (Parametric Method 3 and Intermediate Neglect of Differential Overlap / Spectroscopic - Configuration Interaction) and the synthesis of a new hybrid material based upon the incorporation of these dyes in an aluminum polyphosphate gel network. The samples are nearly transparent, free-standing thick films. The optical properties of the entrapped dyes are sensitive to chemical changes within the matrix caused either by gel aging or external stimulli such as exposition to acidic and basic vapors that can percolate within the matrix. Our theoretical modeling is in good agreement with the experimental results for the dyes.

Mechanical behavior of 30–50 nm thick aluminum films under uniaxial tension

We present uniaxial tensile test results for 30–50 nm thick freestanding aluminum films. Young’s modulus and ductility were found to decrease monotonically with grain size. Reverse Hall–Petch behavior was observed with no appreciable room temperature creep. Non-linear elasticity with small irreversible deformation was observed for 50 nm thick specimens.

A transmission electron microscope study of microstructural development in magnetron-sputtered MoSi 2 thin films

Transmission electron microscopy has been used to reveal the microstructural development in magnetron-sputtered MoSi 2 films. Both 50 nm thick free-standing films and 1 μm thick films on (001)Si exhibited similar microstructures. The as-deposited films were amorphous. Annealing at 700 °C resulted in the crystallisation of heavily faulted, fine, untextured grains of the metastable hexagonal C40 MoSi 2 phase. Annealing above 800 °C led to the polymorphic transformation of the C40 phase to the equilibrium tetragonal C11 b MoSi 2 and D8 m Mo 5Si 3 phases. The C11 b grains contained high densities of coherent {112}<111> twins on {110}. It was deduced that the C11 b phase was produced from the C40 phase by a massive transformation with coherent growth twins formed by “double positioning” on {110} facets at the advancing transformation front.

Analytical model for intrinsic residual stress effects and out-of-plane deflections in free-standing thick films

An analytical model for the influence of residual stress on the out-of-plane deflection in a free-standing thick diamond film (the bowing phenomenon) is presented. The variation in residual stress with film thickness is usually believed to cause the bowing. In this study, the stress variation is assumed to be produced by a gradual increase in substrate deformation resulting from layer-by-layer deposition of the film. The model uses the infinitesimal plate-bending theory to describe the layer-by-layer film growth more exactly, considering the two deformation modes of contraction or expansion and bending. To verify the suggested model, thick diamond films were fabricated on Si, Mo, and W substrates of varying thicknesses by microwave plasma assisted chemical vapor deposition. The model’s predictions on bowing, based on the intrinsic stress value measured by the curvature method, were in good agreement with the bowing curvature of the as-released films measured by a profilometer. This confirms that the bowing of thick films depends on the intrinsic stress variation of the film associated with a gradual increase in substrate deformation. A method of eliminating bowing by depositing different layers with different intrinsic stresses is discussed.