Reduction Of Film Thickness Research Articles

CdTe thin film solar cells with reduced CdS film thickness

A study was performed to reduce the CdS film thickness in CdTe thin film solar cells to minimize losses in quantum efficiency. Using close space sublimation deposition for CdS and CdTe a maximum efficiency of ~ 9.5% was obtained with the standard CdS film thickness of ~ 160 nm. Reduction of the film CdS thickness to less than 100 nm leads to poor cell performance with ~ 5% efficiency, mainly due to a lower open circuit voltage. An alternative approach has been tested to reduce the CdS film thickness (~ 80 nm) by depositing a CdS double layer. The first CdS layer was deposited at high substrate temperature in the range of 520–540 °C and the second CdS layer was deposited at low substrate temperature of ~ 250 °C. The cell prepared using a CdS double layer show better performance with cell efficiency over 10%. Quantum efficiency measurement confirmed that the improvement in the device performance is due to the reduction in CdS film thickness. The effect of double layer structure on cell performance is also observed with chemical bath deposited CdS using fluorine doped SnO 2 as substrate.

X-Ray Diffraction Analysis of Residual Stress in Thin Polycrystalline Anatase Films and Elastic Anisotropy of Anatase

The importance of residual stress in anatase thin films for their photo-induced hydrophilicity was proved recently. Detailed X-ray diffraction (XRD) studies of residual stresses in titanium dioxide films are presented here. Measurements including multiple hkl reflections on several series of these films revealed the presence of tensile stresses in the films that were obtained by crystallization from amorphous state. Significant anisotropy of the strain was also found and compared with that of anatase, resulting from its theoretically calculated single-crystal elastic constants. The XRD data support the experimental evidence of the hypothesis that the [00l] axis is the elastically soft anatase direction, whereas the directions in the [h00] × [hk0] plane are elastically stiff. This is in agreement with the anisotropy predicted by single-crystal elastic constants that are obtained from ab-initio calculations. Residual stress analysis for materials with tetragonal symmetry is described and the theory is used to analyze the data. The anisotropy is very different from that for the rutile phase, and the experimental results agree well with the values calculated for anatase. A simplified method of XRD residual stress analysis in thin anatase films by total pattern fitting (TPF) is also presented. Tensile stresses are formed during the crystallization process and increase rapidly with reduced film thickness. They inhibit crystallization, which is then very slow in the thinnest films.

Adiabatic two-phase flow in rectangular microchannels with different aspect ratios: Part I – Flow pattern, pressure drop and void fraction

An aspect ratio is an important parameter for two-phase flow in a rectangular microchannel. To study the aspect ratio effect on the flow pattern, pressure drop and void fraction, experiments of adiabatic liquid water and nitrogen gas two-phase flow in rectangular microchannels were conducted. The widths and heights of rectangular microchannels are 510 μm × 470 μm, 608 μm × 410 μm, 501 μm × 237 μm and 503 μm × 85 μm. Therefore, the aspect ratios of the rectangular microchannels are 0.92, 0.67, 0.47 and 0.16; and the hydraulic diameters of the rectangular microchannels were 490, 490, 322 and 143 μm, respectively. Experimental ranges were liquid superficial velocities of 0.06–1.0 m/s and gas superficial velocities of 0.06–71 m/s. Visible rectangular microchannels were fabricated using a photosensitive glass. And pressure drop in microchannels was directly measured through embedded ports. The visualization of the flow pattern was carried out with a high-speed camera and a long distance microscope. Typical flow patterns in the rectangular microchannels observed in this study were bubble flow, transitional flow (multiple flow) and liquid ring flow. As the aspect ratio decreased, the bubble flow regime became dominant due to the confinement effect and the thickness of liquid film in corner was decreased. A void fraction in the rectangular microchannels has a linear relation with the volumetric quality. And the two-phase flow becomes homogeneous with decreasing aspect ratio owing to the reduction of the liquid film thickness. Like Zhang et al.’s [19] correlation, as the confinement number increased, the C-value in Lockhart and Martinelli correlation decreased. And a frictional pressure drop in the rectangular microchannels was highly related with the flow pattern.

Effects of atomic oxygen and ultraviolet in low earth orbit on low surface energy polymer film

AbstractEffects of atomic oxygen (AO) and ultraviolet (UV) on a polymer film with surface energy of 8.0 mJ m−2 derived from poly(1H,1H‐perfluorooctyl methylacrylate) were investigated by contact angle measurements, X‐ray photoelectron spectroscopy, and atomic force microscope. The film was exposed to AO with a flux of 6.73 × 1015 atoms cm−2 s−1 and UV with intensity of 15.8 mW cm−2 at wavelength of 200–450 nm, respectively. It is found that AO and UV irradiation resulted in the reduction of film thickness, change of wettability, and increase of surface energy, and AO exhibited more serious effects than UV on the fluorinated polymer film. Reduced rate of thickness of the film was almost proportional to the AO exposure time. After exposed to AO and UV irradiation, the surface energy of the film increased to 17.3 mJ m−2 and 11.0 mJ m−2, respectively. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Liquid Film Performance Analysis of the Mechanical Seals with a Laser-Textured Micro-Pore Face

A theoretical model has been developed for the mechanical seals with a laser-textured porous seal face. By means of variable dimensionless steps, parametric analysis has been performed to obtain dimensionless liquid film pressure by the finite difference method. Liquid film pressure profiles over the pore column have be achieved by the computer program MATLAB. It is found from calculating results that average liquid film pressure increases with increase of the rotational speed, liquid viscosity and reduction of liquid film thickness. Hydrodynamic effect produced by micro-pores decreases with increase of fluid pressure. In addition, the effect of the pore area density and depth over diameter ratio on the liquid film pressure is very significant. Optimum values of these parameters can maximize the average liquid film pressure.

크랭크축 경사도를 고려한 엔진 베어링의 유막거동에 관한 연구

The purpose of this paper is to analyze dynamic behaviors of the oil film thickness and engine bearings in both aligned and misaligned operation conditions of a crankshaft using computer simulation techniques. A crankshaft as an elastic body is modeled for a misaligned crankshaft which is very important design parameter of the film thickness and engine bearings. In this analysis, a dynamic characteristic of a minimum oil film is analyzed based on the elastohydrodynamic lubrication theory. The boundary conditions for analyzing the film behaviors include non-linear constraint forces and bending moments in engine bearings. The more expedient model of an engine bearing is extended to consider the effect of crankshaft misalignment. The computed results indicate that the minimum oil film thickness that causes a major influence on the performance of engine bearings has showed a decrease of 16% to 24% for the misaligned crankshaft compared with an aligned crankshaft. The computed results show that the misalignment of a crankshaft inevitably brings the reduction of minimum oil film thickness and this may increase the failure of a bearing. These results as design parameters are very useful for a bearing designer as a firm reference data of an automotive engine.

Photoresist modifications by plasma vacuum ultraviolet radiation: The role of polymer structure and plasma chemistry

While vacuum ultraviolet (VUV) photon irradiation has been shown to significantly contribute to material modifications of polymers during plasma exposures, the impact of radiation-induced material alterations on roughness development during plasma processing has remained unclear. The authors have studied the interaction of the radiation of Ar and C4F8/Ar plasma discharges with 193 and 248 nm advanced photoresists (PRs). Optical filters were used to vary the radiation exposure wavelength range in the ultraviolet (UV) and VUV emission spectra. This enables clarification of the respective roles of plasma photon radiation wavelength and PR polymer structure on the chemical and structural changes produced in the materials. Chemical changes in polymer composition at the film surface and in the material bulk were determined by vacuum transfer x-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy. Morphological changes, film thickness reduction, and changes in surface and pattern morphology were characterized by ellipsometry, scanning electron microscopy, and atomic force microscopy. The exposure of methacrylate based 193 nm PR to photon radiation in the UV/VUV spectral range (λ=112–143 nm) leads to detachment and removal of oxygen containing polymer pendant groups to a depth of about 200 nm. This causes changes in the polymer structure by chain scission, significant film thickness reduction, and reduced pattern critical dimensions and line edge roughness. Chain-scission reactions and residual detached polymer pendant groups are expected to effectively soften layers of 193 nm PR. In contrast to 193 nm PR, styrene based 248 nm PR was found to be significantly more stable under plasma-produced irradiation due to the low oxygen content, low ester linkage concentration, and absence of lactone. Small thickness reduction, reduced oxygen loss, and cross-linking were observed in the surface region of 248 nm PR. Radiation-induced material modifications of both PR materials decreased with increasing photon wavelength in Ar discharges. Increasing modification of 193 nm PR was observed for increasing photon flux at higher wavelengths (λ=143–300 nm) by the emission characteristic of fluorocarbon containing plasmas. In C4F8/Ar plasma, the authors observed strongly increased loss of oxygen at the film surface and in the COC and CO lactone bonds in the material bulk along with film thickness reduction compared to pure Ar discharges. These modifications are directly relevant to plasma processes used for pattern transfer, which often contain fluorocarbon species.

Mathematical Modeling and Characterization of Thin Film, Narrow Gap Sensor Array Units (SAU)

Design, modeling, testing and analysis multi-gap PbPc sensor array units for the purpose of gas detection are carried out. The tested devices showed reduction in conductance as a function of increasing gap width and reducing film thickness and an increase as a function of temperature up to 160°C. An observed morphological change in the sublimed film is realized at 190°C as the conductance of the sensor array started to drop. Analytical modeling using semi-infinite coplanar electrode arrangement supported the obtained testing results. Problem statement: Stability in gas detection and subsequent discrimination is closely related to sensor design and test parameter and its response to an applied set of chemicals. Approach: To enhance sensor performance and improve sensor designs, a suggested design with mathematical analysis and gas response analysis is carried out. Results: A solid analytical mathematical model is established with both electric field and conductance equations. Effect of deposited sensor film thickness and inter-electrode separation on its response is also established and proved through practical test data. Conclusion: The obtained experimental results agree with the derived mathematical solutions.

Investigation of the fatigue behavior of Al thin films with different microstructure

Cyclic compressive and tensile stresses occur in metallic films and interconnects applied in sensors and microelectronic devices when exposed to temperature changes. The stresses are induced by differences in the thermal expansion coefficients of the adjacent materials. Repeated cycling leads to damage evolution and, eventually, to failure. In this study we report on a successful strategy how to avoid thermal stress induced fatigue damage. We analysed the deformation structures of 0.2–2 μm thick Al films subjected to thermal cycling between 100 °C and 450 °C up to 10,000 times. The investigations reveal that a reduction in film thickness or controlling the Al texture and the Al/substrate interface structure can be used to prevent thermo-mechanical fatigue damage. The findings are explained by orientation dependent plasticity and differences in dislocation mechanisms for different interface structures, and less accumulated plastic strain for thinner films. The approach is expected to apply in general for metallic films on substrates.

Experimental Investigation of Elastohydrodynamic (EHD) Film Thickness Behavior at High Speeds

At very high speeds, elastohydrodynamic (EHD) films may be considerably thinner than is predicted by classical isothermal regression equations such as that due to Dowson and Hamrock. This may arise because of viscous dissipation, shear thinning, frictional heating or starvation. In this article, the contact between a steel ball and a glass disc over an entrainment speed ranging from 0.05 m s−1 to 20 m s−1 was studied. Two sets of tests were performed. In the preliminary testing, the disc was driven at speeds of up to 20 m s−1 and the ball was driven by tractive rolling against the disc, its speed being determined using a magnetic method. After all possible explanations for the reduction in film thickness at high speeds were considered, it was shown that the results, which fall well below classical predictions, are consistent with inlet shear heating at the observed sliding speeds. Another set of tests was then performed, with both disc and ball driven separately, so that the accuracy of the shear heating theory for different types of oils and at different sliding conditions could be assessed. It was found that the thermal correction factor predicts the trend of film thickness behavior well for the oils tested and is particularly accurate at certain slide–roll ratios (depending on the type of oil). Experimental data were also used to obtain improved coefficients for the correction factor for different types of oil to achieve better prediction of film thickness at high speed throughout the whole range of slide–roll ratios.

Inhomogeneous Magnetic Field Penetration in Superconducting Niobium Films

The slow increase of the external magnetic field typically results in a gradual entry of the magnetic induction into type II superconductor in the form of flux vortices, leading to a formation of a critical state characterized by a gradient of the vortex density, which corresponds to the critical current. However, in thin superconducting films under certain conditions the critical state becomes metastable due to local temperature fluctuations. This leads to the magnetic flux penetration in the form of flux avalanches which propagate much faster than the external magnetic field increases. Such instabilities of magnetic flux can result in the noisy behaviour of magnetization and they lead to the suppression of the apparent critical current density. In the present work the peculiarities of magnetic field penetration in the form of fingering or dendritic instabilities are studied by magneto-optical technique in the niobium films of different thickness. The distribution of flux vortices in the films is analysed in detail. It is observed that the reduction of Nb film thickness enhances instabilities and reduces the threshold field for instabilities, in agreement with the theoretical predictions [1]. Interestingly, it is shown that the Ag-layer deposited on the top of Nb film leads also to the surprising enhancement of instabilities.

Numerical simulation of the self-assembly of a polymer-polymer-solvent ternary system on a heterogeneously functionalized substrate

The spinodal decomposition of a polymer–polymer–solvent ternary blend spin coated on heterogeneously functionalized substrate is studied in a three-dimensional numerical model. The Cahn-Hilliard equation was used to describe the free energy profile of the domain. The mechanism of the morphology evolution was studied quantitatively. The well-established linear relationship of the characteristic length, R(t), with t1/3 can be observed in the simulation results. The functionalized substrate greatly affected the morphology evolution of blends with different solvent concentrations. The results indicated that a critical time can be observed, at which the evolution rate changes abruptly, also after which the compatibility of the surface morphology to the functionalized substrate pattern increases at a much lower pace in the diluted solution. In the condensed solution, the compatibility actually decreases beyond the critical time. The influence of solvent evaporation is investigated and a sharper interface was observed in the case with solvent evaporation and film thickness reduction. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers

Plasto-Elastohydrodynamic Lubrication (PEHL) in Point Contacts

Elastohydrodynamic lubrication (EHL) is an important branch of the lubrication theory, describing lubrication mechanisms in nonconformal contacts widely found in many mechanical components such as various gears, rolling bearings, cams and followers, metal-rolling tools, traction drives, and continuous variable transmissions. These components often transmit substantial power under heavy loading conditions. Also, the roughness of machined surfaces is usually of the same order of magnitude as, or greater than, the estimated average EHL film thickness. Consequently, most components operate in mixed lubrication regime with significant asperity contacts. Due to very high pressure concentrated in small areas, resulted from either heavy external loading or severe asperity contacts, or often a combination of both, subsurface stresses may exceed the material yield limit, causing considerable plastic deformation, which may not only permanently change the surface profiles and contact geometry but also alter material properties through work hardening as well. In the present study, a three-dimensional plasto-elastohydrodynamic lubrication (PEHL) model has been developed by taking into account plastic deformation and material work-hardening. The effects of surface/subsurface plastic deformation on lubricant film thickness, surface pressure distribution, and subsurface stress field have been investigated. This paper briefly describes the newly developed PEHL model and presents preliminary results and observed basic behavior of the PEHL in smooth-surface point contacts, in comparison with those from corresponding EHL solutions under the same conditions. The results indicate that plastic deformation may greatly affect contact and lubrication characteristics, resulting in significant reductions in lubricant film thickness, peak surface pressure and maximum subsurface stresses.

Water film thickness-dependent conformation and diffusion of single-strand DNA on poly(ethylene glycol)-silane surface

In this paper, we investigate, using molecular dynamics simulations, the conformation and diffusion of longer and shorter single-strand DNA (ssDNA) as a function of water film thickness. While the conformation of the shorter ssDNA is significantly affected and the diffusion is suppressed with reduction in water film thickness, the conformation and diffusion of the longer DNA is not influenced. We explain our observations by considering the competition between stacking interaction of bases and solvation tendency of ssDNA. This paper suggests an approach to control the surface motion of ssDNA in nanoscale water films using film thickness.

Amorphous-to-crystalline transition of silicon-incorporated anodic ZrO2 and improved dielectric properties

Sputter-deposited zirconium and Zr–16 at.% Si alloy have been anodized to various voltages at several formation voltages in 0.1 mol dm−3 ammonium pentaborate electrolyte at 298 K for 900 s. The resultant anodic films have been characterized using X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy, and electrochemical impedance spectroscopy. The anodic oxide films formed on Zr–16 at.% Si are amorphous up to 30 V, but the outer part of the anodic oxide films crystallizes at higher formation voltages. This is in contrast to the case of sputter-deposited zirconium, on which the crystalline anodic oxide films, composed mainly of monoclinic ZrO2, are developed even at low formation voltages. The outer crystalline layer on the Zr–16 at.% Si consists of a high-temperature stable tetragonal phase of ZrO2. Due to immobile nature of silicon species, silicon-free outermost layer is formed by simultaneous migrations of Zr4+ ions outwards and O2− ions inwards. An intermediate crystalline oxide layer, in which silicon content is lower in comparison with that in the innermost layer, is developed at the boundary of the crystalline layer and amorphous layer. Capacitances of the anodic zirconium oxide are highly enhanced by incorporation of silicon due to reduced film thickness, even though the permittivity of anodic oxide decreases with silicon incorporation.

New glass polyalkenoate temporary cement for cement-retained implant restoration: Evaluation of elevation and retentive strength

The purpose of this study was to evaluate four luting agents for cement-retained implant restorations in two aspects: post-cementation elevation of the superstructure and the retentive strength before and after thermocycling and mechanical cycling. The four evaluated luting agents were: new glass polyalkenoate hard-type (GH) and soft-type (GS) cements, polycarboxylate hard-type (HH) and soft-type (HS) cements. Elevation and retentive strength were measured using a CCD laser displacement sensor and a universal testing machine respectively. Elevation increased in this order: GH, GS, HH, and HS. Thermal and mechanical stresses significantly decreased retentive strength, and the mean values exhibited by GH were significantly higher than the other luting agents. The novel glass polyalkenoate cements (hard- and soft-type) experimentally prepared in the present study augured well as luting agents by virtue of their reduced film thickness and their excellent post-cementation retention and elevation.

Experimental Fabrication of High Efficiency Force Sensor by Cr-Si-C Composite Thin Film

Chromium silicon carbide (Cr-Si-C) composite thin film has been deposited on the stainless steel substrate by radio frequency (RF) sputtering and the possibility of the force sensor is discussed. The characteristic of the Cr-Si-C composite thin film force sensor is also described. Small temperature coefficient of resistance (TCR) dispersion has been obtained by a Cr-Si-C composite thin film force sensor, and the TCR was between -20 and 10 ppm/°C at the aspect ratio of 510 and the film thickness of 15 nm. The force sensor with hysteresis-free and good linearity has been prepared on the stainless steel substrate by using the Cr-Si-C composite film. The gauge factor of force sensor has increased according to the reduction of film thickness and the increase of sensor pattern aspect ratio. The gauge factor reached 14.2 at the aspect ratio of 510 and the film thickness of 15 nm, which is 7 times larger than that of Ni-Cr strain gauge.

Oxygen imaging in microfluidic devices with optical sensors applying color cameras

Luminescent sensors with improved performance for monitoring dissolved oxygen in microfluidic devices were developed. Our brightness enhanced sensing films with reduced film thickness showed higher signals than standard sensors and responded in real-time.Sensor layers were integrated into microfluidic chips and readout off-chip on a fluorescence microscope with lifetime imaging or ratiometric imaging using the color channels of a CCD camera. Cell respirometry of model culture systems was monitored with this sensor setup.

Thermoelastohydrodynamics of grease-lubricated concentrated point contacts

Isothermal and thermoelastohydrodynamic lubrication (TEHL) analyses of grease lubricated bearings are presented. A grease plug flow is formed in the conjunction that, with no shear at the boundaries with the solid surfaces, adheres to them in the region of high pressures under isothermal conditions. The elastohydrodynamic lubrication grease pressure distribution conforms fairly closely to that of its base oil alone, with the exception of inlet trail and pressure spike regions. The dependency of film thickness on speed (rolling viscosity) and load parameters for the base oil agrees with previously reported findings of the research community. For grease there are subtle differences with the base oil film thickness load and speed dependencies. However, it is clear that extrapolated oil film thickness formulae for oils can be used reasonably for the prediction of grease films, at least as a first approximation. The results presented agree well with optical interferometric measurements reported in the literature for grease-lubricated contacts at low temperatures and low surface velocities. TEHL analysis shows breakdown of the plug flow and significant reduction in film thickness, which can lead to changes in the regime of lubrication to mixed or boundary conditions.

Field-Induced Layer Thinning Transition on Free-Standing Smectic Films

Strongly anchored free-standing smectic films usually present a stepwise reduction of the number of layers when the temperature is raised above the smectic-isotropic bulk transition temperature. Here, we demonstrate that a field-induced layer thinning transition can take place in smectic films with a negative dielectric anisotropy even below the bulk transition temperature. Using an extended McMillan's model, we provide the phase diagram of this layering transition and show that, when the field is raised above the bulk transition field, the film thickness reduction is well described by a power law with an exponent that depends on the temperature and the aspect ratio of the liquid-crystal molecule.