Determination Of Layer Thickness Research Articles

Real-time control of molecular beam epitaxy by optical-based flux monitoring

We have developed a real-time molecular beam epitaxy control system based on Al and Ga atomic beam resonant absorption of hollow cathode lamp emission. By continuously monitoring the absorption of Al and Ga beams during growth, this system accurately determines instantaneous growth rates and then integrates these over time to determine layer thickness. This information is used in real time for effusion cell shutter (and hence layer thickness) control. The accuracy and flexibility of this system is demonstrated here by growing AlAs/GaAs distributed Bragg reflectors with consistent layer thicknesses even though effusion cell temperatures were intentionally varied. In each instance the system automatically detected and compensated for the different growth rates, resulting in DBRs with center wavelengths controlled to within 0.3% of the target wavelength.

Determination of condensate layer thickness in the interaction of a high-temperature two-phase flow with a wall

Calculation algorithms are developed that permit a determination of the time variation for the position of the outer boundary of a condensate film, depositing on a barrier, based on its surface temperature measured experimentally.

Some laws governing the mechanical properties of laminar composition materials based on low chromium alloys

Laws governing the formation of mechanical characteristics and features of the failure of a laminar composition material (LCM) of a Cr-V system are investigated. The possibility of the broad regulation of the strength and impact strength of a composite as a result of altering the thickness of its layers is established. It is demonstrated that the law of additivity of ultimate LCM strength is not obeyed in the region of interlayer thicknesses investigated. A relationship is proposed for determination of the optimal layer thickness for which the maximum impact strength of the material is attained.

Optimum design of composite laminates with STROPT

AbstractThe design of a composite laminate involves the determination of layer thicknesses and orientation angles. The problem is posed as the optimum weight (or volume) design of composite laminates which withstand the specified loads without failure. The modified Hill‐Mises or Tsai‐Wu failure criterion is taken as a constraint, each layer being subjected to this criterion. The design variables considered are the layer thicknesses and orientation angles. Based on the finite‐element analysis capability of composite laminates provided by NISA II, a composite laminate optimization procedure has been developed. Nonlinear optimization techniques are used to approach the optimum design of the composite laminates. Efficient design sensitivity analysis procedures are implemented based on the direct differentiation and adjoint variable methods. Example problems illustrate that the program offers an efficient and practical design tool for composite laminates.

Characterization of GaAs/Ga 1− xAl xAs multilayer systems by infrared spectroscopy at normal incidence

We present experimental infrared reflectivity spectra, at normal incidence and room temperature of two thin-film heterostructures based on GaAs, AlAs and Ga 1− x Al x As. We have generated very satisfactory fits to these spectra by using a standard technique based on optical impedance mismatch. The fit yields both electrical and structual parameters. The latter include the composition concentration, x, and the layer thicknesses. It is demonstrated that the sensitivity of the layer thickness determination is greatly the measurements into the mid-infrared, even in the case of two comparably-sized and adjacent layers. For very thin layers (0.03μm) the thickness can be determined from the amplitude of the fringes in the mid-infrared spectrum.

In situ spectral ellipsometry for real-time thickness measurement: Etching multilayer stacks

A polarization modulated spectroscopic ellipsometer is used in situ to measure the thicknesses of films in real time during semiconductor plasma etch processing. Utilizing the speed of phase modulation multichannel detection, and digital signal processing techniques, this ellipsometer is capable of acquiring spectral data in less than 75 ms. Efficient algorithms were developed for determining layer thicknesses from the measured spectra of multilayer film stacks in real time with a typical solution time of a few seconds. The measured thicknesses and etch rates are used to anticipate interfaces in multiple layer stacks and control process end points. The ability of the spectral ellipsometer to measure multilayer stacks during etching is demonstrated by the etching of a stack consisting of silicon nitride, polycrystalline silicon, and silicon dioxide. Such stacks are commonly used as masks for field oxidation for electrical isolation in memory device fabrication. An isotropic plasma etch is used to remove this film in a single-wafer process environment.

Determination of critical layer thickness and strain tensor in InxGa1−xAs/GaAs quantum-well structures by x-ray diffraction

We have used the double-crystal x-ray rocking curve technique to determine lattice constant, strain relaxation, thickness, and critical thickness of a thin InxGa1−xAs layer embedded in GaAs. In this work we have measured and analyzed x-ray data over a wide scan angle (∼2.0°). This allows the simultaneous determination of buried layer thickness and strain. The measurement results were analyzed by the dynamical diffraction theory. The critical thickness for an InGaAs layer embedded in GaAs obtained from x-ray data is shown to be larger than that predicted by the force balance model. The strain tensors as a function of layer thickness are also analyzed for the buried InxGa1−xAs of different x values.

REMOTE THICKNESS MEASUREMENT OF OIL SLICKS ON WATER BY LASER-ULTRASONICS

ABSTRACT At the National Research Council of Canada Industrial Materials Institute, research is in progress on the application of laser-ultrasonics to remote measurement of the thickness of oil on water. Laser-ultrasonics is a novel technique developed for the nondestructive inspection of materials. It uses a short pulse laser for the generation of ultrasonic waves in the oil layer and a second laser, coupled to an optical interferometer, for the remote detection of the ultrasonic surface motion. Direct measurement of the time of flight of the ultrasonic wave provides the value of the thickness of the oil layer. Application of this technique to thickness measurement of oil on water has been studied in small and large scale laboratory tests. Small scale tests demonstrate the direct and unambiguous determination of the oil layer thickness. Accuracy is essentially limited by the knowledge of the acoustic properties of the oil. Large scale tests show that a distance of almost 37 meters does not severely impede the method, so airborne application appears possible. Surface motion such as that caused by sea waves does not reduce the accuracy of the thickness determination but does limit the measurement rate. Preliminary airborne tests with a single laser probe confirm that laser-ultrasonics monitoring of the thickness of an oil spill is feasible.

Total Reflection X-Ray Fluorescence Spectroscopy: Analysis Of Gaas And InGaAs

AbstractThis article explores applications of total reflection x-ray fluorescence (TRXRF) to GaAs processes. The applications include determination of surface contamination and InGaAs layer thicknesses. Surface contamination can deteriorate device performance and can occur in starting substrates and in subsequent processing. We demonstrate that TRXRF is a quick, nondestructive method for identifying sulfur contamination on incoming wafers and low levels of metallic impurities from device fabrication. Variable angle TRXRF has been used to determine heterostmeture film thickness, measuring films of InGaAs on GaAs as thin as 4 nm.

Criteria for the extraction of SIMOX material parameters from spectroscopic ellipsometry data

The feasibility of extracting quantitative information from spectroscopic ellipsometry (SE) experimental data is dependent on a number of factors. These include the issue of optical model sophistication and ellipsometric sensitivity to structural parameters. One area of increasing interest is the application of SE to the analysis of separation by implanted oxygen (SIMOX) substrates. For SIMOX layer thickness determination it has been demonstrated that a three-layer optical model is adequate to represent the SIMOX substrate. This paper will highlight critical constraints imposed by correlation between parameters in multi-parameter fitting of SIMOX spectral data. In the context of defect quantification a calculation of the theoretical sensitivity of the measured ellipsometric angles, ψ and Δ, to residual crystalline damage, via an effective-medium approximation, in the superficial silicon layer is presented and discussed.

Simultaneous Determination of Layer Thickness, Composition, and Mass Absorption by X-Ray Diffraction

AbstractThis paper describes a new method for the simultaneous determination of mineral composition, mass thickness and mass absorption coefficient of a thin layer of a crystalline substance deposited on a crystalline substrate.The samples were deposited on membrane disc filters, consisting of mixtures of cellulose acetate and cellulose nitrate. Quantitative results are achieved by measuring the diffraction intensity of the analyte and the attenuation of a reflection of the crystalline material supporting the deposited sample. The mean accuracy of the analysis was found to be: ≈ 3% for mass thickness, ≈ 1% for mass absorption coefficient and ≈ 4% for quantitative mineralogical determination.

Monitoring ion etching of GaAs/AlGaAs heterostructures by real time spectroscopic ellipsometry: Determination of layer thicknesses, compositions, and surface temperature

A real time spectroscopic ellipsometry (RTSE) study of the GaAs/AlGaAs/GaAs heterostructure, performed during Ar ion beam etching, provides the instantaneous GaAs and AlGaAs layer thicknesses and etch rates, the Al composition for the alloy layer, the evolution of the thickness and composition of the surface damage layer, and the near-surface temperature. In addition, combined effects of ion-induced atomic intermixing and etching inhomogeneity can be assessed as the interfaces are crossed. These results demonstrate the extensive capabilities of the RTSE technique for monitoring semiconductor structures during processing.

Dielectric functions and critical points of strained InxGa1−xAs on GaAs

Dielectric function spectra of strained InxGa1−xAs (x≤0.25) epilayers on GaAs are presented for the first time, together with spectra of relaxed layers of the same compositions. Critical point energies, obtained by line-shape fitting to second-derivative spectroscopic ellipsometry (SE) data, show an increase in the E1, E1+Δ1 splitting with strain, in agreement with theory using GaAs deformation potentials. SE is shown to be capable of determining layer thickness, composition, and strain in this alloy system.

Accurate determination of misfit strain, layer thickness, and critical layer thickness in ultrathin buried strained InGaAs/GaAs layer by x-ray diffraction

We have analyzed data obtained from double-crystal x-ray rocking curve measurements to determine the lattice constants, strain relaxation, thickness, and critical thickness of a thin InxGa1−xAs layer embedded in GaAs. The use of a very wide x-ray scan angle (∼2.0°) allows the simultaneous determination of buried layer thickness and strain with greater accuracy. The critical thicknesses so obtained for buried InGaAs layers are smaller than that predicted by the energy balance model and larger than that predicted by the force balance model.

Adsorbed polymer layer thickness determination at the solid—liquid interface by different techniques

Four techniques, (photon correlation spectroscopy (PCS), microelectrophoresis, sedimentation coefficient measurement and viscosimetry) were used to determine the thickness of a polyethylene glycol layer at the solid—liquid interface. A comparison of obtained results indicates that PCS is most recommended, then microelectrophoresis. The accuracies of the techniques for this particular application are compared, and the reasons for the differences are explained. The layer thicknesses are discussed in terms of their relationship to the radius of gyration of the polymer in bulk solution.

Metabolism of testosterone during in vitro transport across CACO-2 cell monolayers: evidence for beta-hydroxysteroid dehydrogenase activity in differentiated CACO-2 cells.

Testosterone has previously been used as a model compound for the determination of unstirred water layer thickness in the CACO-2 transport model. We have found, however, that testosterone is metabolized during in vitro transport across the CACO-2 cell monolayers. This suggests that testosterone is not an ideal model substance. Testosterone is metabolized to androstenedione, indicating the formation of 17-beta-hydroxysteroid dehydrogenase by differentiated CACO-2 cells. No reverse metabolism is observed, thus androstenedione is considered superior to testosterone for determination of unstirred water layer thickness in the CACO-2 system. Permeability coefficients for testosterone and androstenedione obtained under identical transport conditions were 66 (+/- 7) * 10(-6) (n = 26) and 84 (+/- 7) * 10(-6) (n = 9) cm/sec, respectively. The unstirred water layer thicknesses at different agitation rates are determined for the CACO-2 transport model used in our laboratory utilizing androstenedione as a model compound. The system is capable of controlling the water layer thickness from about 200 to 1000 microns.

Angular intensity modulation in angle‐resolved XPC and AES of non‐crystalline ultrathin surface layers: The phenomenon and its implications

AbstractThe well‐known phenomenon of forward (elastic) scattering of x‐ray photoelectrons and Auger electrons, which gives rise to prominent diffraction peaks for emission from monocrystalline materials, can lead to a much less prominent, yet significant, angular intensity modulation of emission from nominally non‐order materials in which stoichiometric or other constraints enforce some short to medium‐rang order. A seemingly small intensity modulation can have a drastic effect on the determination of layer thickness or of effective attenuation length from angle‐resolved measurements. This conclusion is arrived at by applying the known facts o forward scattering (summarized here) to the currently structural model of oxide layers on the semiconductors Si and GaAs (serving as examples). The predictions are compared with experimental data on carbonaceous contamination in the native oxide layers on Si (111) and GaAs (001), obtained by angle‐resolved XPS and AES and evaluated for angle‐dependent effective attenuation length. It is concluded that angular intensity modulation can be expected to be a frequently occurring phenomenon with serious implications for the study of ultrathin layers and ultraslow depth profiles by the electron spectroscopes XPS and AES.

Using Seismic Wavetests to Determine Velocity Structure

Seismic wavetests (or walkaway noise analyses) are routinely conducted prior to land CMP surveys in order to set acquisition parameters. Little use is made of the velocity information contained in the wavetest data despite its potential impact on successful acquisition. The probable reason for such neglect is the low S/N ratio inherent in single-fold data. However, S/N enhancement is possible due to the large effective aperture of wavetests. Through tau-p transformation, wavefront segments are compressed and cast in a coordinate frame which makes possible the direct determination of interval velocity and layer thickness. A modelling approach in the tau-p domain is derived which accommodates both vertical velocity gradients and jumps. This is inverted as a layer-stripping procedure analogous to the tausum method. Then, a reversed wavetest dataset from the Perth Basin is transformed to the tau-p domain. Semblance threshholding and multi-panel stacking are used to enhance events. Finally, forward modelling and inversion are used together to extract velocity structure from the wavetest.

A method for the determination of cellular permeability coefficients and aqueous boundary layer thickness in monolayers of intestinal epithelial ( Caco-2) cells grown in permeable filter chambers

A linear relationship between apparent permeability coefficients ( P app) and agitation rate was used to determine cell permeability coefficients ( P c) and the thickness of the aqueous boundary layer ( h aq) in monolayers of Caco-2 cells. Drugs with different physical/chemical properties were studied. P app values for ‘highly permeable drugs’ (testosterone, corticosterone, propranolol, metoprolol, warfarin) were dependent on the agitation rate, i.e. on the h aq. The permeation of testosterone was studied in detail. In the absence of agitation, the measured P app value of testosterone was 35.7 ± 3.3 × 10 −63 cm/s while the P c value was 137.4 ± 8.1 X 10 −6 cm/s, i.e. 3.8 times higher. P app values for ‘less permeable drugs’ (hydrocortisone, salicylic acid, sulphasalazine, mannitol) were not dependent on the agitation conditions. In the absence of agitation, h aq for testosterone was 1544 ± 142 μm and the contribution of the resistance of the aqueous boundary layer to the total diffusional resistance was 70%. h aq was reduced to 128 + 10 μm when the highest agitation rate was used. Under these conditions, the contributions of the resistances of the aqueous boundary layer and the Caco-2 cell monolayers were 16 and 73%, respectively. Similar results were obtained for the other highly permeable drugs. The results indicate that the P app values for highly permeable drugs are controlled not only by the aqueous boundary layer but also by the Caco-2 cell monolayers.

Hydrogen pile-up at interfaces between differently doped layers of amorphous silicon

Hydrogen profiles for layered a-Si:H structures are presented. Substantial accumulation of hydrogen at the p-i interface is found. Additional incorporation of hydrogen in p-layers is also observed for intermediate boron doping levels compared to intrinsic layers. Hydrogen profiles in single pin and in pinpin amorphous silicon structures enable determination of individual layer thickness.