Determination Of Layer Thickness Research Articles

The determination of superheated layer thickness and wall superheat in vertical tube natural circulation reboiler

The characteristics of incipient boiling for vertical thermosiphon reboiler are studied in detail. The liquid film adjacent to the heating surface, the superheated layers thickness, δ*, must attain a threshold value so that the critical bubble nuclei with radius r c can further grow to the point of detachment. Thus, the relation between δ* and r c at the threshold of post-critical growth is of primary importance in determining the onset of nucleate boiling. In the present study a semi-empirical equation for wall superheat is proposed which includes the effect of submergence for the experimental data available in the literature. The data for water and organic fluids having wide range of thermophysical properties were correlated with a single correlation having mean absolute deviation of 14.73%. The value of δ*/ r c for each fluid and unified value have also been also proposed.

High-resolution X-ray diffractometry and X-ray reflectometry techniques for structural studies of complicated thin-layered heterostructures: Complementarity between Fourier Transform-based procedures and simulation softwares

Both x-ray reflectometry and high-resolution x-ray diffractometry techniques are used for the assessment of individual layer thicknesses and interfacial profiles inside complicated heterostructures, such as semi-conductor multilayers. In particular, the use of Fourier transform-based numerical treatments applied to both the reflectivity curves and the high-resolution diffraction profiles allows a fast and precise determination of the individual layer thicknesses. Moreover, we show the potentiality of this method by reporting x-ray reflectometry and diffractometry studies on waveguides structures and superlattices. Typical layer thicknesses from 0.5 nm to more than 1 μm are accessible with these methods. We show the complementarity of both x-ray reflectometry and high-resolution diffraction techniques. Finally, the reliability of using a simulation software for the assessment of complicated interfacial roughness, such as segregation profiles, is reported.

Transport and absorption in strain-compensated Si/Si 1−xGe x multiple quantum well and cascade structures deposited on Si 0.5Ge 0.5 pseudosubstrates

Various strain-compensated Si/SiGe quantum well structures with high Ge concentrations in the SiGe wells (70–85% Ge) have been successfully deposited by very low-temperature molecular beam epitaxy on thick strain-relaxed Si 0.5Ge 0.5 buffer layers. The samples have been subject to intensive and careful characterization of their structural, electronic and optical properties. X-ray diffraction and reflectivity measurements using synchrotron radiation allowed accurate determination of layer thicknesses and composition of the quantum well structures. The r.m.s. roughness at Si/SiGe interfaces is less than 0.4 nm I– V characteristics of resonant tunneling devices reveal strong negative differential resistance peaks for heavy hole tunneling but no trace for tunneling via light hole states. Two-dimensional hole gases with a mobility of up to 0.49 cm 2/Vs in 7 nm wide, modulation-doped Si 0.8Ge 0.2 quantum wells with Si barriers and relaxed Si 0.5Ge 0.5 spacer layers underline the high structural quality. The dependence of hole mobility on well width can be explained by an interface roughness of 0.4 nm, in accordance with the X-ray reflectivity measurements, and the associated fluctuation of the strain at the interfaces. Using the structural parameters determined by X-ray analysis as input parameters, the intersubband absorption spectra of strain-compensated quantum well structures have been calculated by the 6 band k· p method. The theoretical predicted spectra are in excellent agreement with those obtained by experiments. Our comprehensive study results in a set of very congruent data enabling to custom design complex Si/SiGe structures such as p-type quantum cascade emitters.

TEM and ellipsometry studies of nanolaminate oxide films prepared using atomic layer deposition

Nanolaminate oxide layers consisting of TiO 2 and Al 2O 3 have been deposited on silicon using atomic layer deposition (ALD). Characterisation of these films has been achieved by use of a range of modern transmission electron microscopy (TEM)-based techniques, including plasmon loss imaging, energy filtered imaging and scanning TEM (STEM) X-ray line profiling. These have shown that the target thickness of the individual layers in the nanolaminate structures (20 nm) has been met with a high degree of accuracy, that the layers are extremely flat and parallel and that the interfaces between the layers are compositionally abrupt. Localised crystallisation within the stacks, and responses to electron beam irradiation point to the presence of a stress gradient within the layers. The performance of ellipsometry in characterising multilayer stacks has been benchmarked against the TEM measurements. Errors in determination of individual layer thicknesses were found to increase with growing stack size, as expected given the increasing number of interfaces incorporated in each model. The most sophisticated model gave maximum deviations of ±4 nm from the TEM determined values for the 5- and 10-layer stacks.

In situ electrical characterization of DH4T field-effect transistors

We present an in situ electrical analysis of organic thin film transistors (OTFTs) during the deposition of the active layer of dihexylquaterthiophene (DH4T). At elevated temperatures (90 °C) DH4T films exhibit a smectic mesophase with extremely large 2D island formation. Upon cooling to room temperature they convert to extraordinary large crystalline domains ideal for transistor operation. The whole process is reflected in the in situ recorded transport properties. The data allow the determination of the transport layer thickness, which is given mainly by the first two monolayers (MLs). Furthermore, we find a threshold thickness of 1.5 nm for the onset of transistor operation. Thickness dependent mobility oscillations are reported for the first time, which correspond to the first two MLs. Upon cooling the transition from the smectic mesophase to crystalline phase increases the mobility by 45%. The morphology of the DH4T films shows 2D layer growth and close to the contacts an increased layer thickness.

Growth of two-dimensional arrays of silicon nanocrystals in thinSiO2 layers by low pressure chemical vapour deposition and high temperature annealing/oxidation.Investigation of their charging properties

Two-dimensional arrays of silicon nanocrystals embedded in ultrathinSiO2 layers for application in silicon nanocrystal memories were fabricated by athree-step process: (a) growth of a tunnelling silicon oxide, (b) low pressurechemical vapour deposition (LPCVD) of a thin layer of amorphous silicon(α-Si), and (c) solid phase crystallization of theα-Si layer in a high temperature furnace under nitrogen flow, followed by thermal oxidationin the same furnace. Transmission electron microscopy (TEM) was used for thestructural characterization of the three-layer structure and the determinationof layer thicknesses and silicon nanocrystal size, while capacitance–voltage(C–V) andcurrent–voltage (I–V) measurements were used to investigate the charging properties of the silicon nanocrystallayer. In an attempt to increase the silicon nanocrystal density, as suggested in theliterature, a dip of the oxidized wafer in diluted HF before LPCVD deposition was used,but this step was found to seriously affect the charging properties of the structure.

Quantitative thickness determination using x‐ray fluorescence: application to multiple layers

AbstractThe fundamental parameter method using x‐ray fluorescence was applied to the quantitative determination of single and multiple layer thicknesses using a compact experimental setup. Focused white‐beam radiation was obtained from a high‐voltage Mo x‐ray tube and the fluorescence spectra were acquired using a Peltier‐cooled Si solid‐state detector. The results show that in the case of single layers the accuracy of the thicknesses obtained is very good whereas for multiple layer structures the agreement is poorer, especially for the thinnest layers. This indicates a possible drawback of the standard‐free thickness determination scheme based on fundamental parameters for complex samples. Copyright © 2004 John Wiley & Sons, Ltd.

Determination of individual layer composition and thickness in multilayer HgCdTe structures

The reproducible molecular-beam epitaxy (MBE) growth of dual-band Hg1−xCdxTe (MCT) heterostructures requires routine post-growth wafer analysis for constituent layer thickness and alloy composition, therefore, demanding nondestructive characterization techniques that offer quick data feedback. This paper reports a multilayer structure model, which can be least-square fit directly to either Fourier transform infrared (FTIR) transmission or reflection spectra to provide individual layer thickness, alloy composition, and grading information for various complex structures. The model, we developed, is based on an accurate representation of both the real and imaginary parts of the MCT dielectric function across and above Eg as a function of alloy composition. The parametric, MCT optical-dielectric function for compositions varying between x=0.17 to x=0.5 was developed in the range from 400 cm−1 to 4,000 cm−1, based on a semi-empirical model for the absorption coefficient and extrapolation of the refractive index across Eg. The model parameters were refined through simultaneous fits to multiple reflection and transmission data sets from as-grown, double-layer planar heterostructure (DLPH) structures of variable thickness. The multilayer model was tested on a variety of simple DLPH structures with thick absorber layers (>8 µm) and was compared against traditional FTIR analysis and cross-section optical microscopy and showed good agreement in both composition and thickness. Model fits to dual-color MCT data and subsequent analysis of the internal parameter correlation have demonstrated that error bars on absorber layer composition and thickness could be as low as ∼0.0005 and ∼0.02 µm, correspondingly.

Thermal annealing effect on the photoluminescence properties of unstrained GaInAsSb/InP single quantum well grown by MOVPE

The unstrained and high quality GaInAsSb/InP single quantum well (SQW) is successfully grown by MOVPE on InP substrate. High resolution X-ray diffraction (HRXRD) and secondary ion mass spectrometry (SIMS) with Cs + ions as the primary beam are used for evaluation of material quality and determination of layer thickness. Low temperature (8 K) photoluminescence (PL) spectroscopy is used to characterize the SQW structure. Thermal annealing effect on the PL properties of GaInAsSb/InP SQW structure is performed and discussed in this paper. After 30 s annealing with temperatures 650, 750 and 850 °C, the PL peak intensity is not changed and the PL peak energy blue shifts. Furthermore, the increased PL peak full width at half maximum (FWHM) after annealing indicates that the inhomogeneous alloyed region increases.

Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions

In this paper, basic electrochemical processes (such as oxide film growth, anodic dissolution and oxygen liberation) on an aluminium anode in a model alkaline solution are considered under conditions of galvanostatic DC plasma electrolytic oxidation (PEO). The experiments performed include: (i) recording and analysis of the main electrical characteristics of the process; (ii) determination of the oxide layer thickness; (iii) anodic gas collection and composition analysis and (iv) electrolyte analysis to determine dissolved aluminium. Four different stages of the PEO process have been identified, characterised by various rate proportions of the partial anodic processes. Overall current efficiency of the oxide film formation has been estimated to be in the 10–30% range. The film growth rate decreases significantly with increasing electrolyte concentration from 0.5 to 2 g l −1 KOH, since the rate of anodic dissolution increases. Oxygen evolution is shown to be the main electrochemical process at the potentials corresponding to the plasma stages of the electrolysis. The overall rate of oxygen liberation at the anode exceeds the Faraday yield, which is probably due to the radiolytic effect of the plasma discharge on the adjacent electrolyte volume.

Characterization of multilayered materials for optoelectronic components by high-resolution X-ray diffractometry and reflectometry: contribution of numerical treatments

Both X-ray reflectometry and X-ray diffractometry techniques are used for the assessment of individual layer thicknesses inside complicated semi-conductor heterostructures, in particular for opto-electronic applications. The use of Fast Fourier transform-based numerical treatments applied to the reflectivity curve allows a fast determination of the individual layer thicknesses. We demonstrate the capability of this method by reporting X-ray reflectometry study on superlattices, multiple quantum wells, and other complicated structures. Typical layer thicknesses from 0.5 nm to 500 nm were successfully investigated. Fast Fourier transform-based procedure has also been employed successfully on high resolution X-ray diffraction spectra. We finally show the complementary of both techniques.

Non-Destructive and Non-Contact Determination of Layer Thickness and Thermal Properties of PVD and Sol-Gel Layers by Photothermal Methods

Thin layers in combination with metallic substrates are often used for moulding tools. The layer properties depend on the coating parameters and technology (e. g., sol-gel, PVD, CVD). This paper presents the photothermal measurement of layer thickness and thermal properties of PVD and sol-gel layers which are needed for the development of moulding tools. The influence of optical parameters and surface roughness on the results is discussed.

Growth mode and structural characterization of epitaxial TM/RE thin films

The study of growth conditions optimisation and detailed structural characterization of bilayer epitaxial thin films consisting of a transition metal (V, Cr, Mn) and a magnetic rare-earth (Gd) is presented. Due to the strong relation between the structure and magnetic properties in thin films, the structural characterisation was carried out by means of the following techniques: X-ray reflectometry, extended X-ray absorption fine structure, reflected high-energy electron diffraction and atomic force microscopy. Well defined epitaxial growth of Cr(110) and V(110) on Mo(110) was achieved. The distortion of the the V and Cr lattice was observed by reflected high-energy electron diffraction as a function of layer thickness. The distortion decreases when the thickness of the deposited layer increases approaching the bulk structure for the thickness of 10 ML. A very good agreement between measured and simulated reflectivity profiles was found that enabled determination of the exact constituent layer thickness and the interface roughness.

Photoemission Study of Onion Like Quantum Dot Quantum Well and Double Quantum Well Nanocrystals of CdS and HgS

Colloidally prepared CdS/HgS/CdS quantum dot quantum well nanocrystals and CdS/HgS/CdS/HgS/CdS double quantum well nanocrystals stabilized with polyphosphates have been investigated by photoelectron spectroscopy with tuneable synchrotron radiation. High-resolution spectra reveal in addition to a bulk species also a surface environment for Cd, whereas different S species could not be resolved. Although not expected for the ideal structure model, Hg occurs in two distinct environments. The total amount of Hg giving evidence for sample inhomogeneities is of the order of 10%. Furthermore, to some extent, the XPS experiments allowed characterization of the onion like structure of the quantum dot quantum well nanocrystals. A method for layer thickness determination previously developed for core shell nanocrystals is extended to the more complex case of quantum dot quantum well structures.

Characterization of High-k Dielectrics by Combined Spectroscopic Ellipsometry (SE) and X-Ray Reflectometry (XRR)

ABSTRACTAt present, new high-k dielectric materials are being intensively investigated to replace the silicon dioxide as gate dielectric for the next generation of electronic devices. Several candidate materials (such as ZrO2, HfO2, Al2O3) and deposition processes are currently under investigation. Because the layer thickness which is required in the next generations of devices is of the order of few nanometers, a precise determination and control of layer thickness will be mandatory. Although spectroscopic ellipsometry (SE) is well established non-contact, non-destructive and precise technique for determining thickness and optical properties of thin films, it becomes more difficult to obtain this information unambiguously and simultaneously for ultra-thin films with traditional SE alone because of possible high correlations between film structure and optical properties. The grazing x-ray reflectometry (XRR) is a complementary nondestructive optical technique and can be used to unambiguously determine ultra thin film thickness accurately. Combined with ellipsometry technique together, it will provide a promising way to characterize high-k gate dielectrics including thickness, roughness, interfacial layers and material composition information etc. In this paper, the principles for both SE and XRR will be briefly reviewed and limitation of each technique will be discussed. Following the high-k gate dielectric exploration and development, examples of using the combined SE/XRR techniques will be presented.

Precise Characterization of Silicon on Insulator (SOI) and Strained Silicon on Si1−xGex on Insulator (SSOI) Stacks with Spectroscopic Ellipsometry

ABSTRACTFurther improvements in CMOS circuit performance such as switching speed and power reduction will rely on the use of silicon on insulator (SOI) substrates with decreased functional layer thicknesses. According to the International Technology Roadmap for Semiconductors (ITRS), the silicon and buried SiO2 (BOX) layer thicknesses for a fully depleted device should be in the ranges of 10–16nm and 24 – 40nm by 2005, respectively. A key issue for fully-depleted CMOS transistors is control of such ultra-thin layer thicknesses and their uniformity along with other parameters such as surface and interface roughness. This poses a challenge to metrology, especially to conventional reflectometry technique because the layer thicknesses must be determined with angstrom precision for both silicon cap and SiO2 box layer.Spectroscopic ellipsometry (SE) is an optical and non destructive technique for determining thin film thickness and material optical properties. Because ellipsometry measures the change in the polarization state for both the amplitude ratio of the p to s polarizations, and in the phase retardation, it provides a precise way to characterize such ultra thin SOI stacks. Comprehensive characterization results for a number of thin and ultra thin SOI stacks with different thickness ranges will be presented together with measurement repeatability results relevant to the film thickness process tolerances. In addition, characterization results for advanced device applications such as strained silicon-on- Si1−xGex-on-insulator (SSOI) will also be shown, demonstrating the use and capability of spectroscopic ellipsometry for precise determination of layer thickness, material composition, interfacial layers, etc. Principles and advantages of the technique will also be discussed in the presentation.

X-Ray Combined QTA Using a CPS Applied to a Ferroelectric Ultrastructure

The new combined methodology for quantitative description of texture, structure and other microstructural parameters of thin layers using X-ray diffraction is presented and applied to the case of a ferroelectric thin film of Pb0.76Ca0.24TiO3 on a Pt/TiO2/SiO2/Si substrate. The approach allows the quantitative texture analysis of the ferroelectric thin film and the Pt electrode, refining simultaneously their structure, layers thickness, mean crystallite size and microstrain state. The layer thickness determination is estimated by the refinement of the thicknesses inserted in the absorption and volumic correction factors. The powerfulness of this methodology is discussed and compared with other approaches.

Effect of adhesive layer properties on stress distribution in composite restorations—a 3D finite element analysis

Objectives: Teeth, adhesively restored with resin-based materials, were modeled by 3D-finite elements analysis that showed a premature failure during polymerization shrinkage and occlusal loading. Methods: Simulation of Class II MOD composite restorations with a resin bonding system revealed a complex biomechanical behavior arising from the simultaneous effects of polymerization shrinkage, composite stiffness and adhesive interface strain. Due to a polymerization contraction, shrinkage stress increases with the rigidity of the composites utilised in the restoration, while the cusp movements under occlusal loading are inversely proportional to the rigidity of the composites. The adhesive layer's strain also plays a relevant role in the attenuation of the polymerization and occlusal loading stresses. Results: The choice of an appropriately compliant adhesive layer, able to partially absorb the composite deformation, limits the intensity of the stress transmitted to the remaining natural tooth tissues. For adhesives and composites of different rigidities, FEM analysis allows the determination of the optimal adhesive layer thickness leading to maximum stress release while preserving the interface integrity. Application of a thin layer of a more flexible adhesive (lower elastic modulus) leads to the same stress relief as thick layers of less flexible adhesive (higher elastic modulus).

Diffusion model validation and interpretation of stable isotopes in river and lake ice

AbstractThe stable isotope stratigraphy of river‐ and lake‐ice archives winter hydroclimatic conditions, and can potentially be used to identify changing water sources or to provide important insights into ice formation processes and growth rates. However, accurate interpretations rely on known isotopic fractionation during ice growth. A one‐dimensional diffusion model of the liquid boundary layer adjacent to an advancing solid interface, originally developed to simulate solute rejection by growing crystals, has been used without verification to describe non‐equilibrium fractionation during congelation ice growth. Results are not in agreement, suggesting the presence of important uncertainties. In this paper we seek validation of the diffusion model for this application using large‐scale laboratory experiments with controlled freezing rates and frequent sampling. We obtained consistent, almost constant, isotopic boundary layer thicknesses over a representative range of ice growth rates on both quiescent and well‐mixed water. With the 18O boundary layer thickness from the laboratory, the model successfully quantified reduced river‐ice growth rates relative to those of a nearby lake. These results were more representative and easier to obtain than those of a conventional thermal ice‐growth model. This diffusion model validation and boundary layer thickness determination provide a powerful tool for interpreting the stable isotope stratigraphy of floating ice. The laboratory experiment also replicated successive fractionation events in response to a freeze–thaw–refreeze cycle, providing a mechanism for apparent ice fractionation that exceeds equilibrium. Analysis of the composition of snow ice and frazil ice in river and lake cores indicated surprising similarities between these ice forms. Published in 2002 by John Wiley & Sons, Ltd.

Determination of interface layer thickness of a pseudo two-phasesystem by extension of the Debye equation

The Debye equation with slit-smeared small angle x-ray scattering(SAXS) data is extended from an ideal two-phase system to a pseudotwo-phase system with the presence of the interface layer, and a simpleaccurate solution is proposed to determine the average thickness of theinterface layer in porous materials. This method is tested by experimentalSAXS data, which were measured at 25 °C, of organo-modifiedmesoporous silica prepared by condensation of tetraethoxysilane (TEOS) andmethyltriethoxysilane (MTES) using non-ionic neutral surfactant as templateunder neutral condition.