F.K. Urban

Optical properties of cobalt oxide films deposited by spray pyrolysis

The optical constants of Co3O4 films were determined by analyzing variable angle of incidence spectroscopic ellipsometry data and normal incidence transmittance data, between 3500 and 17 000 A. Absorption was taken to be nonzero for the soda–lime–silica float glass substrate. The absorption of the soda–lime–silica float glass was fitted by an ensemble of four Gaussian oscillators over the entire measured spectral range. Surface roughness of the film was measured with atomic force microscopy and included in the optical model. The films were deposited by spray pyrolysis of cobalt acetylacetonate onto heated soda–lime–silica float glass and fused silica substrates. Fused silica was used to observe film absorption without the effect of substrate absorption. The oxide film phase was identified with thin‐film x‐ray diffraction analysis. Simultaneous fits of ellipsometry and transmittance data from coated samples were computed to obtain the film thickness, optical constants, and the amplitudes of the four Gaussi...

Development of artificial neural networks for real time, in situ ellipsometry data reduction

Development of real time in situ monitoring and control of thin film depositions using ellipsometry requires both data acquisition and processing to be rapid. Present speeds of measurement and computation of basic parameters, Ψ and Δ, are sufficient for data acquisition which is essentially real time. However, computation of film parameters, such as thickness and optical properties, generally cannot keep up with the incoming data and must be performed in a batch mode after the deposition. This work describes the development of enhanced, high speed data reduction algorithms using artificial neural networks (ANN). The networks are trained using computed data and subsequently give values of film parameters in the millisecond time regime. The ANN outputs are used as initial estimates in a variably damped least squares algorithm for accuracy improvement. The combination of these two algorithms provides very accurate solutions in 75 ms per point on a DEC VAX 8800 multiprocessor system running at a combined 12 Mips. This speed is suitable for real time film monitoring and control for growth rates up to 10 nm per second. Results for fixed angle of incidence, single wavelength, in situ data for Ni deposited on BK7 substrates are presented.

Nanophase films deposited from a high-rate, nanoparticle beam

The early efforts to form nanoparticles beams were characterized by unsuccessful nanoparticle formation, synthesis for only a few “high vapor pressure” solids, or prohibitively low deposition rates. A new type of deposition system described here overcomes these impediments and is demonstrated to produce at high rates nanostructured films of diverse materials. It employs ion sputtering of target materials and gas phase condensation into an inert gas atmosphere at pressures ⩽1 Torr. The condensed nanoparticles are then swept out of a 3 mm diameter converging–diverging exit nozzle, forming a low velocity beam. The beam is directed on a substrate on which a film is deposited. Nanostructured films were successfully grown using Al, Cr, Fe, Ni, Cu1−xCox, Cu, Zr, Mo, Ag, Ta, W, Pt, and Au targets at rates in the interval of 0.15–0.91 nms−1 with metal-atom deposition fluxes in the interval 1.45–11.9×1014 cm−2 s−1. Rutherford backscattering spectrometry revealed that metal-atom densities of the films ranged from 11...

Use of multiple analytical techniques to confirm improved optical modeling of SnO2:F films by atomic force microscopy and spectroscopic ellipsometry

Variable angle of incidence spectroscopic ellipsometry, reflectance, and transmittance techniques were used to determine the optical constants of a fluorine doped tin oxide film deposited by chemical vapor deposition onto a hot soda‐lime‐silica glass ribbon. To improve the optical characterization, an additional analytical technique, atomic force microscopy (AFM), was used to incorporate information about surface roughness into the optical model. Our earlier work demonstrated the necessity of including a surface roughness layer as six sublayers in the optical model. The present work further confirms the method and demonstrates its accuracy with additional analytical techniques. These include: (1) cross sectional in‐lens field emission scanning electron microscopy to measure total film thickness, determine presence of interface roughness and extent of surface roughness; (2) secondary ion mass spectrometry to give a first approximation of the film layer structure for optical modeling by depth profiling the ...

DETERMINING THE OPTICAL PROPERTIES OF A MIXED-METAL OXIDE FILM, CO3-X-YCRXFEYO4, WITH SPECTROSCOPIC ELLIPSOMETRY AND ATOMIC FORCE MICROSCOPY

The optical properties of a mixed-metal oxide thin film from the Co3−x−yCrxFeyO4 family has been determined from combined analysis of ellipsometry, atomic force microscopy, and transmittance measurements. These types of films are useful as solar absorbing films on a variety of float glass substrates to lower the solar heat gain admitted through a glass pane in a window, skylight, or door. A commercial product with a film composition in this family is sold under its registered trademark SOLARCOOL® glass. Each constituent metal oxide film was analyzed with variable angle of incidence spectroscopic ellipsometry and transmittance (T) measurements to determine their optical constants and film thickness. Surface roughness was measured with atomic force microscopy and included in the optical model as a known variable. The oxide films were optically modeled with a bulk layer and a known surface roughness layer. The mixed-metal oxide film was optically modeled with an effective medium approximation layer consistin...

Modelling of surface roughness in variable-angle spectroscopic ellipsometry, using numerical processing of atomic force microscopy images

Abstract Variable-angle spectroscopic ellipsometry is used for ex situ investigation of thin films deposited on substrates. Calculation of the thickness and optical properties of a number of films in a multilayer stack is possible, because a large amount of data is taken over a wide range of wavelengths and incidence angles. However, the authors here confirm that significant surface roughness of the outermost film must be taken into account for useful solutions. In our earlier work, the roughness was modelled as a single roughness layer at the film ambient interface. This layer was taken to be an effective medium, composed of film and voids. The thickness and void fraction were determined from atomic force microscopy (AFM) measurements. Improvement in ellipsometry solutions has been achieved using a new model of the roughness layer composed of six effective medium sublayers. Numerical processing of AFM data is described, considering suface scans ranging from 1 μ m × 1 μ m to 12 μ m × 12 μ m. The complete deposited film was best modelled as a rough surface layer (70 nm thick) over a continuous layer 350 nm thick.

Ellipsometry algorithm for absorbing films

Easy sample preparation and measurement makes ellipsometry attractive for surface and film investigations; however, specialized numerical methods are normally required to relate measurements to unknown physical attributes of reflecting surfaces. Although solution techniques have been developed, the problem of data analysis is by no means solved. This paper presents an alternative method for computation of the thickness and optical properties of an absorbing film overlying a known substrate from ellipsometer data obtained at two light incidence angles. Tests for selected cases required no a priori knowledge of the film, in contrast to other methods.Easy sample preparation and measurement makes ellipsometry attractive for surface and film investigations; however, specialized numerical methods are normally required to relate measurements to unknown physical attributes of reflecting surfaces. Although solution techniques have been developed, the problem of data analysis is by no means solved. This paper presents an alternative method for computation of the thickness and optical properties of an absorbing film overlying a known substrate from ellipsometer data obtained at two light incidence angles. Tests for selected cases required no a priori knowledge of the film, in contrast to other methods.

Numerical ellipsometry: enhancement of new algorithm for real-time, in situ film growth monitoring

Abstract The equations associated with the popular models of film deposition of a material on a substrate tend not to be invertible in practical situations. Thus, development of numerical techniques for obtaining approximate solutions to these equations has become necessary. The variably damped least squares (VDLS) algorithm of Levenberg and Marquardt is commonly used, although it requires accurate initial estimates and is typically to slow for real-time applications. Another algorithm combines the speed of an artificial neural network (ANN) for accurate initial estimates, and the real-time refinement capability of VDLS for real-time performance; however, substantial off-line processing (from hours to days) is necessary to train ANNs. A third algorithm (called 2C) has been developed, which is fast enough for reliable real-time solutions without requiring either accurate initial estimates or any off-line processing. The algorithm 2C employs the techniques of dynamic step adjustment, overrelaxation and reduction in dimensions, as well as curve following and curve intersection location techniques developed by the authors. The performance of the new 2C algorithm in terms of speed and convergence is comparable with the ANN-VDLS algorithms for Ni and BK7 glass.

Estimation of Maximal Cortisol Secretion Rate in Healthy Humans

CONTEXT Cortisol secretion is related to ACTH concentration by a sigmoidal dose-response curve, in which high ACTH concentrations drive maximal cortisol secretion rates (CSR(max)). OBJECTIVE We sought to estimate CSR(max) and free cortisol half-life in healthy humans (n = 21) using numerical methods applied to data acquired during cosyntropin (250 μg) stimulation. We also evaluated the effect of overnight dexamethasone (DEX; 1 mg) vs. placebo on estimates of CSR(max) and free cortisol half-life. DESIGN This study was a double-blind, placebo-controlled, randomized order of overnight DEX vs. placebo, cosyntropin (250 μg) stimulation with frequent serum cortisol sampling and computer-assisted numerical analysis. SETTING The study was conducted at a single academic medical center. PARTICIPANTS Twenty-one healthy adult subjects (15 females and six males), mean aged 46 yr, participated in the study. INTERVENTION Intervention in the study included DEX vs. placebo pretreatment, cosyntropin (250 μg) iv with frequent cortisol sampling. MAIN OUTCOME MEASURES CSR(max) and free cortisol half-life estimates, R² for goodness of fit, were measured. RESULTS Mean ± sd CSR(max) was 0.44 ± 0.13 nm/second, with free cortisol half-life of 2.2 ± 1.1 min. DEX did not significantly affect estimates of CSR(max) or free cortisol half-life. Our model accounts for most of the variability of measured cortisol concentrations (overall R² = 90.9 ±11.0%) and was more accurate (P = 0.004) during DEX suppression (R² = 94.6 ± 4.6%) compared with placebo (R² = 87.2 ± 8.7%). CONCLUSIONS Application of a mass-action model under conditions of cosyntropin stimulation provides a relatively simple method for estimation CSR(max) that accurately predicts measured cortisol concentrations. DEX administration did not significantly affect estimates of CSR(max) or free cortisol half-life.

Numerical techniques useful in the practice of ellipsometry

While well-established methods for sample preparation and measurement make ellipsometry attractive for surface and film investigations, development of reliable methods for computing unknown physical attributes of reflecting surfaces is just beginning. This situation has arisen because the equations of ellipsometry are transcendental, have not been inverted and thus require numerical techniques for solution in all but the simplest of reflecting surface configurations. The problem of data analysis is by no means solved and remains one of the key issues in ellipsometry. This paper presents a survey of numerical techniques and principles for approximating solutions to problems, and illustrates an application of these techniques in the development of a new algorithm to address the problem.