P.K. de Bokx

Characterization of titanium nitride layers by grazing-emission X-ray fluorescence spectrometry

Abstract Grazing-emission X-ray fluorescence spectrometry is a new development in X-ray metrology instrumentation. The combination of wavelength-dispersive detection with a total-reflection geometry in the detection path allows thin layer characterization also for light elements. The technique was applied to analyze a series of titanium nitride layers, reactively sputtered using different Ar N 2 flow ratios of the working gas. Composition, thickness and density of the layers result from fitting the experimental data to model calculations. It was found that above a critical flow value, the samples are slightly over-stoichiometric (with respect to nitrogen) with a considerably reduced density. The GEXRF method has potential both for complete layer characterization and for process control with layer density as the control parameter.

Laboratory grazing‐emission x‐ray fluorescence spectrometer

We have developed a laboratory grazing‐emission x‐ray fluorescence (GEXRF) spectrometer. The instrument utilizes in vacuo wavelength‐dispersive detection. Due to the high resolution of a crystal monochromator at long wavelengths, the range of applicability of grazing x‐ray techniques is substantially extended to the longer wavelengths as compared to techniques used presently. It is demonstrated that interference fringes in the take‐off angle dependence of the fluorescent x‐ray intensity of a layered sample can be resolved using a conventional x‐ray tube.

Grazing-emission X-ray fluorescence spectrometry : principles and applications

In grazing-emission X-ray fluorescence spectrometry (GEXRF), the sample is irradiated at approximately normal incidence, and only that part of the fluorescence radiation is detected that is emitted at grazing angles. This configuration allows the use of wavelength-dispersive detection. This type of detection has the advantages of substantially better energy resolution at longer wavelengths (light elements, L and M lines of heavier elements) and a much larger dynamic range than the energy-dispersive detectors currently used in grazing X-ray techniques. Typical examples are presented of applications that are made possible by this new technique.

Quantification in grazing-emission X-ray fluorescence spectrometry☆

Abstract In grazing-emission X-ray fluorescence (GEXRF) spectrometry wavelength-dispersive detection can be applied. Much softer radiation and hence lighter elements than in total-reflection X-ray (TXRF) spectrometry can thus be detected. We used simulations to investigate methods of quantification of GEXRF results involving soft characteristic radiation. From these studies, it is concluded that for ultra-thin layers, e.g. the sub-monolayer amounts encountered in semiconductor contamination analysis, calibration plots are linear. For thicker layers, quantification should be performed very carefully because of deviations from linearity due to absorption of radiation and to oscillations in the calibration curve. These oscillations are caused by interference of fluorescence radiation emitted directly towards the detector and radiation reflected at the sample–substrate interface. Suggestions for a judicious choice of measurement conditions are made and the benefits of internal standardisation are discussed.

Characterization of silicon oxynitride films by grazing-emission X-ray fluorescence spectrometry

Abstract Silicon oxynitride films, prepared by post-annealing of pre-oxidized silicon wafers in nitric oxide (NO), were characterized by grazing-emission X-ray fluorescence spectrometry (GEXRF). GEXRF is a new method of instrumental analysis that not only allows determination of the film composition, but also provides information on depth distributions and densities of films. Experiments were carried out using a prototype laboratory GEXRF instrument. It was found that nitrogen is predominantly incorporated in the ultra-thin oxide films (~3.5 nm) at the SiO 2 /Si interface. The measured total amount of nitrogen in the films was around 10 15 atoms/cm 2 and increased with annealing time. A slight decrease in the amount of oxygen with annealing time was observed. Oxygen could be determined with a relative precision of 3%, whereas instrumental improvements were found to be required to determine nitrogen with comparable precision. On selected samples, cross-analysis with Rutherford backscattering (RBS) and Auger electron spectroscopy (AES) was performed.

X-ray irradiation effect on the reliability of ultrathin gate oxides and oxynitrides

Abstract During X-ray irradiation of MOS devices, interface states as well as bulk electron and hole traps are generated at the Si SiO 2 interface and in the SiO 2 layer respectively. In this paper, we investigate the effect of X-ray irradiation on the electrical characteristics of 4.2 nm oxide and 4.4 nm oxynitride layers. It is shown that the bulk electron traps generated during irradiation lead to an increase of the gate current in the low gate voltage region, behaviour which can be attributed to trap assisted tunneling. From the analysis of time-dependent dielectric breakdown characteristics, it is demonstrated that the irradiation-induced bulk traps participate in the formation of the breakdown (percolation) path in the gate oxide layer.

Modelling of displacement chromatography using non-ideal isotherms

Abstract The chromatographic transport problem in the displacement separation of multi-component mixtures leads to a set of coupled, non-linear partial differential equations. Neglecting axial dispersion, the set of equations can be solved using a transformation of variables ( h or ω transformation) for the case that the multi-component distribution is considered to be ideal (Langmuirean). If intermolecular interactions (non-idealities) are taken into account in the description of the multi-component distribution equilibrium, such a transformation of variables is no longer possible. The displacement separation of binary mixtures was analysed by applying shock-wave theory to the problem of non-ideal distribution isotherms. A description of the separation as a function of time and distance was obtained (development graph). Experimental results pertaining to the separation of Na-K mixtures using Li as the carrier and Rb as the displacer are presented. Quantitative agreement with predicted development graphs was obtained if non-idealities were taken into account, whereas the use of multi-component Langmuir isotherms yielded agreement with experimental results over only a limited composition range.

Effect of x-ray irradiation on the electrical characteristics of ultra-thin gate oxides

The effect of x-ray-induced damage on the electrical characteristics of metal-oxide-semiconductor capacitors with ultrathin gate oxide and oxynitride layers is investigated. The tunnelling current of irradiated capacitors is observed to be enhanced in the low gate voltage range, a behaviour quite similar to the increased leakage current observed in electrically stressed devices. On the other hand, the slopes of the time-to-breakdown distributions decrease after x-ray irradiation, indicating that the critical density of traps needed to trigger breakdown in the gate oxide is lower in irradiated capacitors. In addition, the probability of observing soft breakdown of the SiO2 layer increases after the irradiation. These results suggest that the radiation-induced traps participate in the formation of the percolation path formed in the gate insulator at breakdown or soft breakdown.

Fluorescence detector cell for use in an integrated electrically driven separation system

Abstract An integrated design for electrically driven separations is presented. The injector, column and detector are all located within the same cartridge, allowing for integral thermostating and short column lengths. Results on the performance of the proposed fluorescence detector are reported. Molar amounts as low as 2·10 −19 mol of fluorescein (100-pl detector cell volume) can be detected. Short column lengths permit very fast separations, as is demonstrated by the separation of four laser dyes within 35 s.

On the number of shocks in chromatography

Displacement chromatography can be modelled mathematically by interacting Riemann problems for a system of conservation laws. The dependence of the number of shocks on the isotherms used to model the exchange process is investigated theoretically. For a particular separation process, the number of shocks predicted by the Langmuir isotherm and by a recently introduced non-ideal isotherm are compared with experiment. The conclusion is that the Langmuir isotherm predicts too few shocks.