Michael Andrae

Electron Transmission Coefficient for Tilted Specimens

Starting from a scattering model, the transmission coefficient of electrons for an oblique angle of incidence is derived. The descent of the transmitted fraction is described by the backscattering of electrons and the energy loss caused by the decreasing mean free path. The angular and energy distribution of electrons is replaced by the most probable angle and mean energy. A comparison with other models and measurements valid for normal incidence was made. The altered transmission characteristics by tilting the sample were verified by comparison with results from Monte Carlo simulations and results from Neubert and Rogaschewski, which were based on measurements. An analytical expression for the transmission law was obtained.

A New Technique for Standardless Analysis by EPMA-TWIX

A new standardless method (TWIX) for quantification of thin films and bulk materials has been developed. Based on a correction model for oblique angle of incidence the effect of attenuation of the detected x-ray line resulting from path length elongation in tilted samples has been used for thickness determination of thin films and composition of binary samples. It should be noted that this method is also applicable on WDS systems. Results with an average deviation of less than 10 per cent are obtained.

Thickness Determination of Thin Insulating Layers

Thin insulating films (Si3N4 on titanium-substrate) with various thicknesses (40–360 nm) are investigated with EPMA (electron probe micro analysis). A carbon- and, alternatively, gold-coating is applied to avoid charging phenomena and beam deflection.

Electron Transmission Coefficient for Oblique Angle of Incidence

Starting from a scattering model the transmission coefficient of electrons for oblique angle of incidence is derived. The decrease of the transmitted fraction is described by the backscattering of electrons and the energy loss caused by the decreasing mean free path. The angular- and energy-distribution of electrons is replaced by the most probable angle and mean energy. A comparison with other models and measurements valid for normal incidence is performed. The altered transmission characteristics by tilting the sample are verified by comparison with results from Monte Carlo simulations and results from Neubert and Rogaschewski, which are based on measurements. An analytical expression for the transmission law is obtained.

Efficiency Calibration of a Si(Li) Detector by EPMA

A φ(ρz)-model, well known for its high accuracy, is employed for determining the detection efficiency of a Si(Li) detector by EPMA. Problems and advantages of this approach are pointed out and possible errors in the resulting thicknesses of the beryllium window, the silicon dead layer, the gold contact layer and the ice build-up on the detector surface are estimated. Furthermore the method allows the calculation of the cross section for L 3 subshell ionization to be made with high reliability.

CALCULATION OF THE SURFACE IONISATION USING ANALYTICAL MODELS OF ELECTRON BACKSCATTERING

The value of the surface ionisation for oblique incidence is calculated by testing various new approaches. A model based on a multiple reflection theory developed at the Institute for Applied and Technical Physics at the Technical University of Vienna is extended to oblique incidence by the use of a new fit for the angular distribution of backscattered electrons (according to data published by Kanter) under non-normal incidence. Furthermore two analytically derived models (proposed by Werner and Niedrig) are tested for their applicability to calculate the surface ionisation by numerical integration of the double differential backscattering coefficient versus angle and energy. The combined model of Werner is extended to oblique incidence to make a direct comparison possible. The results are compared with expressions published by Pouchou, by Cazaux and by Merlet; reasons for discrepancies are discussed; commonly used simplifications are reviewed critically.

The Relative Intensity Factor for Lα Radiation Considering the Different Mass Absorption of Lα and Lβ Radiation

The mass attenuation coefficients of Lα and Lβ radiation can differ as much as a factor of five depending on the composition of the sample. A dependence of the relative intensity factor for Lα radiation on the incidence energy is evident. The relative intensity factors are very important not only for standardless EPMA but also for EPMA with standards when an EDS system is used and the whole L-intensity is measured. In this case calculating just Lα intensities leads to acceptable results only if the difference between the mass attenuation coefficients of Lα and Lβ radiation is rather small. If the difference is significant, a much too high mass attenuation coefficient for Lα is observed as can be shown mathematically. A comparison between theoretically calculated and measured intensity factors is carried out and their energy dependence is discussed.

Measurements of Ga1-xAlxAs Layers on GaAs with EDS

GaAlAs is a basic material in semiconductor technology and thus of great interest for industrial research. Several Ga1-xAlxAs bulk samples of predefined composition as well as layered samples of well defined thicknesses and composition were investigated. The experimental data are analyzed with a computer program, which has been developed at the Institute for Applied and Technical Physics at the Technical University of Vienna. Furthermore, analysis of the bulk samples using the program PAP of Pouchou and Pichoir is carried out. An excellent agreement of both correction procedures with the defined composition values is obtained. The determined layer thicknesses also correspond well with the defined data.