Roger Jansson

Imaging ellipsometry revisited: Developments for visualization of thin transparent layers on silicon substrates

Imaging ellipsometry is presented as a technique for quantification and visualization of the lateral thickness distribution of thin (0–30 nm) transparent layers on solid substrates. The main advantage of imaging ellipsometry is that every point on a surface is measured at the same time with a high lateral resolution. The method is based on the use of combined null and off‐null ellipsometry at an incident angle close to the pseudo‐Brewster angle of a high index substrate such as silicon. In the present experimental setup, a xenon lamp, a collimator, and a wavelength‐selective filter provide an expanded collimated probe beam with a diameter of 25 mm. Other major components in the system are a polarizer, a compensator, and an analyzer. In this way, a 15×30 mm2 image of a sample surface can be focused onto a charge‐coupled‐device video camera and transferred to a computer for further evaluation by image processing. Thickness measurements are performed for calibration purposes with ordinary null ellipsometry. ...

Microstructural control of porous silicon by electrochemical etching in mixed HCl/HF solutions

Abstract The role of hydrochloric acid in the fabrication of porous silicon was investigated with atomic force microscopy (AFM) and spectroscopic ellipsometry. The study was focused on surface morphology, in terms of roughness and power spectral density (PSD), and layer microstructure and quality in terms of thickness, porosity, interface sharpness and layer homogeneity. The results show that HCl can be used as an additional component in the standard hydrofluoric acid-based etching solution in order to affect different properties of porous silicon layers such as volume porosity, thickness and pore size. In addition, HCl/HF mixtures can be used to obtain samples with sharper porous silicon/silicon interfaces and less in depth inhomogeneities as compared with porous silicon samples having similar volume porosities and thicknesses and manufactured in conventional solutions containing HF only. Whereas large variations in both thickness and volume porosity can be obtained by conventional solutions, the HCl/HF interplay yield large variations in one of the mentioned parameters only.

An optical gas sensor based on ellipsometric readout

A gas sensor system based on ellipsometric readout is presented. It includes a gas chamber and a compact null ellipsometer operated in off-null mode. Small, low-cost optical components are used to demonstrate that this advanced methodology can be implemented in simplified instrumentation. The off-null ellipsometric sensing principle and transducer mechanisms of the sensing layers, as well as the instrumentation, are described. The application of the sensor system is exampled with experimental results on low-concentration alcoholic gases (methanol, ethanol, and 2-propanol) using porous silicon as a sensing layer. Optimization of the optics of the sensor system, improvement of sensitivity or alteration of selectivity by modification of sensing layers, and multisensing by using several ellipsometric units in parallel are discussed.

Line-shape analysis of ellipsometric spectra on thin organic films.

A methodology for the line-shape analysis of ellipsometric spectra on thin (<200-A) organic films is presented. Four different line shapes are employed: Gaussian, Lorentzian, phase-relaxed Lorentzian, and a critical-point line shape. An analysis of analytic data addresses the problem of the modeling of unsymmetric absorption bands. The method is exemplified by an analysis of thin films of phthalocyanine and poly(3-hexylthiophene), and we show that the number and type of resonances in an absorption band can be obtained. The possibility of resolving the cause of a shift in the peak position of an absorption band is also demonstrated. In the case being studied the shift is due to the redistribution of the oscillator strengths between the individual resonances in the band and not to shifts in the energies of the resonances.

Optimization of azimuth angle settings in polarizer–compensator–sample–analyzer off-null ellipsometry

The dependence of the azimuth angle settings on the change in off-null intensity of a polarizer-compensator-sample-analyzer ellipsometer owing to changes in sample properties is studied. First, a closed-form expression for the relationship between azimuth angles that fulfill the null condition is presented. An approximation for the off-null light intensity near null that is valid for small changes of the p- and s-reflection coefficients of an isotropic sample is then derived. This approximation shows that the intensity change near the null can be described by changes in the ellipsometric parameters tan phi and A only. Expressions for finding the azimuth angle that gives the maximum possible intensity change for a given change in the sample parameters are also derived. The importance of optimization of azimuth angle settings for different samples is investigated and found to depend on tan psi. Numerical and experimental results chosen from the investigation of gas sensors based on porous silicon are included to verify the approximations as well as the optimization.

Line-shape analysis of ellipsometric spectra on thin conducting polymer films

Abstract Optical properties in terms of the complex dielectric function of thin films of poly(3-hexylthiophenes) on gold substrates have been measured with spectroscopic ellipsometry in the photon energy range 1.5–3.5 eV. With line-shape analysis using a modified Lorentzian model, the broad absorption band around 2.25 eV can be resolved in three resonances, and the changes in the line-shape parameters due to degradation, heat treatment and the dependence of film thickness were studied. Degradation is observed as a loss of total oscillator strength and broadening of the central resonance in the absorption band, while heat treatment mainly results in a redistribution of oscillator strengths. The energy difference between the resonances were found to be constant when the absorption coefficient is analysed, but not if the dielectric function is analysed. As the latter quantity is more directly related to the density of states distribution, this indicates that the previously suggested vibronic model for the absorption band is incomplete.

Selection of the physically correct solution in the n-media Bruggeman effective medium approximation

Abstract An algorithm is presented for selection of the physically correct solution when using the Bruggeman effective medium approximation for determination of the complex dielectric function of a multi-composite medium. The procedures, which are based on conformal mapping, are general and applicable to systems of any number of materials.

Optimization of off-null ellipsometry in sensor applications

The optimization of azimuth angle settings of a polarizer-compensator-sample-analyzer off-null ellipsometric sensor system to obtain maximum intensity changes with respect to changes in the properties of a sensing layer, with and without considering changes in s reflectance, is studied. Optimal conditions in the two cases are derived analytically under the assumption that linear relationships exist among the changes in the parameters of the sensing layer. The validity of these optimal conditions is verified by numerical examples. The advantage of using ellipsometry compared with reflectometry to readout sensing information for some sensing samples is also discussed.

Activation of hydrogen sensitive palladium-oxide-semiconductor structures studied with simultaneous ellipsometric and capacitance measurements

Abstract Ellipsometry and capacitance measurements are used to distinguish processes occurring on the metal surface and at the metal-insulator interface of a hydrogen sensitive Pd-MOS structure. It is shown that the inactivation of the hydrogen response is due to processes occurring on the (outer-) Pd surface. The ellipsometric data also suggest that the inactivation of the hydrogen response during storage in “inert” atmosphere can be explained by the formation of a thin layer with metallic-like optical properties on the Pd surface. During storage in oxygen, a more dielectric-like layer is formed on the Pd surface.

A quasi three-dimensional optical memory with n-bit memory cells based on the ellipsometric principle: concept and prototype devices

Abstract A concept of an optical memory with n -bit memory cells using ellipsometric read-out is presented. Prototypes with two-bit and three-bit read-only memory cells were fabricated by deposition of multilayers consisting of thin metal films and thin amorphous semiconductor films, respectively.