Substrate Optical Constants Research Articles

Measurements and modeling of infrared reflectance signatures of microparticle traces on surfaces

Standoff detection of hazardous materials using infrared backscattering spectroscopy shows promise due to its speed of detection, sensitivity, chemical specificity, eye safety, and the ability to perform detection in a stealthy manner. However, infrared diffuse reflectance spectra of trace particles on substrates exhibit a strong dependence on substrate type, particle size, and mass loading. This has a negative impact on the performance of detection algorithms and false alarm rates when using commercially available spectral databases. We present two models that are quickly computed yet capture most of the observed spectral features. Model I is best suited for calculating the diffuse reflectance spectra of trace amounts of particles on relatively smooth substrates, while model II extends the applicability of model I to particles on very rough substrates. The models can be used for algorithm development and training. The main inputs to the models are the analyte and substrate optical constants as a function of wavelength, and the particle size distribution and average mass loading. The accuracy of the models was checked by comparing to experimentally measured diffuse reflectance spectra of several carefully prepared samples.

Simultaneous measurement of film and substrate optical parameters from multiple sample single wavelength ellipsometric data

A procedure has been developed for the accurate measurement of film and substrate optical parameters from the multiple sample single-wavelength ellipsometric data. The dimensional reduction of the unknowns from newly formulated ellipsometric functions, the root selection and the thickness-dependent integer deduction enhance the rapidity of finding solutions and the convergence from a wide range of initial guesses while avoiding undesirable solutions. An error analysis carried out shows that the procedure is very resistant to the propagation of angular errors and allows the estimation of optimum film thickness ranges under which the parameters can be accurately found. The standard SiO2/Si structure is particularly studied using the procedure that is further illustrated with the experimental data on Ni/BK7-glass structures. The SiO2 film refractive index and thickness are thus shown to be accurately determined when sought along with the substrate optical constants. Moreover, the film and substrate real indexes are not altered in the presence of an interface layer between the film and the substrate while its existence is indicated by a systematic lowering of the Si substrate extinction coefficient. The procedure can be efficiently used in the continuous real-time optical characterization of films growing on substrates.

Ellipsometry of Al2O3 thin films deposited on Si and InP

thin films, possessing potential for MIS electronic applications, have been deposited on clean Si and InP surfaces by evaporating powder from a graphite cell heated by electron bombardment. The build up of the films was monitored by Auger electron spectroscopy (AES). Continuous, amorphous films were also prepared on microgrids, for TEM studies. The film thickness was measured using multiple angle of incidence (66 - ) and angle ellipsometry using a 633 nm laser line. Calibration curves have been calculated for n = 1.61 - 1.77 limits for and various substrates. They were considerably affected by the substrate optical constants. Evaluation of ellipsometric data resulted in typical thickness values of 5 - 10 nm as confirmed with cross-sectional TEM.

Studies on MgO-stabilized zirconia thin films in the UV-visible region

ZrO 2-MgO optical coatings were deposited by reactive thermal coevaporation. The nickel substrate optical constants were determined by ellipsometry. Film reflectance measurements at normal incidence were made in the region 350 nm–800 nm in order to determine the Fresnel reflection coefficient. The film's refractive index was computed using the Fresnel formalism. It was found that it varies with the amount of MgO in the alloy. The maximum values were 2.29 ± 0.12, 2.18 ± 0.10 and 2.06 ± 0.07 respectively for λ = 0.37 μm, λ = 0.63 μm and for λ = 0.75 μm. These maxima were obtained from the sample containing 15 mol.% MgO in the alloy. At molar percentages of MgO greater than 20% a monotonic decrease of index is noted. The investigated samples displayed a large dispersion of film refractive index. We found a film packing density ranging from 0.75 to 0.98. As a function of MgO mole fraction, this important factor related to film structure determines the refractive index behaviour. These investigations aimed to produce thin optical coatings with variable optical constants for optimizing the performance of an optical device for a certain wavelength or wavelength region.

Determination of the optical constants of substrates by angle-of-incidence derivative ellipsometry in the presence of unknown surface overlayers

Regions of precise measurement of surface complex optical dielectric constants are found in the presence of unknown optically thin overlayers by computer simulation of angle-of-incidence derivative ellipsometry experiments. The immersion method is considered as a tool for expanding the regions of satisfactory accuracy. The values of errors in the determination of substrate optical constants in the intervals −10 < ∊1 < 10 and 0 < ∊2 < 20 are shown.

Film Thickness Determination of PMMA on InP by Ellipsometry

The thickness of polymethylmethacrylate (PMMA) films deposited in solution on is measured by ellipsometry to determine its dependence on terminal spin speed. The PMMA solution behaves as a steady‐state Newtonian fluid. Film thickness in excess of the periodic thickness is measured by combining data taken at three angles of incidence; a procedure for applying this technique is discussed. The main feature consists of assigning the film index from data taken at the incident angle where the ellipsometer readings are most sensitive to film index, and using this index in determining the film thickness at the other two angles. The basis supporting this procedure is presented, and the dependence of the ellipsometric model on uncertainties in the assignment of film and substrate optical constants is discussed specifically in terms of its effect on the three‐angle technique.

Ellipsometric investigation of the intercalation threshold in the graphite and bromine system

The adsorption of bromine onto the basal (0001) surface of highly oriented pyrolytic graphite is studied as a function of pressure at 25°C by an ellipsometric technique. Below the intercalation threshold, the extend of adsorption is given directly by variations in Δ, in tan ψ exp(iΔ) = r p/ r s, while ψ remains constant. At the threshold (20 torr Br 2) ψ falls steeply because intercalation here causes a discontinuous change in the substrate optical constants n and k. Reasons are given for supposing the adsorbed bromine to be predominantly ionic (Br − 2, Σ u) at low coverages, but 1 3 ionic in the completed monolayer.

Application of classical oscillator functions to the simultaneous determination of substrate optical constants and film thickness from ellipsometric measurements

A method is described whereby substrate optical constants, film refractive index and film thickness are simultaneously obtained from one set of ellipsometric measurements at various wavelengths of light performed on a film-covered metal surface. In this method the metal substrate's optical constants are represented by one of three functional forms: a modified free electron function, a free electron plus classical oscillator, and a sum of classical oscillators. Values of metal optical constants, film refractive index and film thickness so obtained agree well with independent determinations of these quantities.

Effect of Substrate on Radiative Properties of Thin Films

An expression for the directional transmittance of an absorbing thin film on anabsorbing substrate was developed by using the classical electromagnetic theory. This expression when combined with the wellstudied reflectance can yield absorptance values that are of utmost interest in heat transfer studies. The behavior of the normal and hemispherical reflectance, absorptance, and transmittance is exhibited for some selected values of film and substrate optical constants and is presented graphically as a function of film optical thickness. The results indicate that the imaginary part of the complex index of refraction for both film and substrate should be accounted for when calculating radiative transfer and properties of such films.

Ellipsometric Technique for Obtaining Substrate Optical Constants

A method for determining substrate optical constants, surface film thickness, and surface film optical constants has been devised by measuring the ellipsometer angles Δ and ψ at several angles of incidence. This method, previously reported as impossible, solves in a simple fashion the ellipsometry equations for one consistent values of surface film thickness in association with compatible values of substrate optical constants. The technique is advantageous in that vacuum ellipsometry is eliminated and previously unmeasured materials such as alloy structures may be studied readily. Data on aluminum, molybdenum, and silicon are presented illustrating the technique.

Note on the constant - reflectivity method of determining substrate optical constants

The “constant-reflectivity” method for determining all the optical parameters of a system of an isotropic nonabsorbing film on an isotropic absorbing substrate using ellipsometry is examined. It is shown that this technique can only be considered for either very high or very low ratios of substrate extinction coefficient to substrate refractive index and even then caution must be exercised. This infers that the calculational method is very sensitive to the material involved.