B. K. Bein

Extremum method: Inverse solution of the two-layer thermal wave problem

An inverse solution of the two-layer thermal wave problem has been derived, which allows us to determine the relevant thermal transport parameters, the thermal diffusion time and the thermal reflection coefficient, respectively, the ratio of the effusivities of the two layers, deduced from the relative minimum or maximum of the calibrated phase lags measured between the periodically modulated excitation of the thermal wave and the detected thermal response. Applying a functional transformation by multiplying the calibrated phase lags with the variable (1∕f1∕2)q, where f is the modulation frequency of excitation and q a positive or negative real number close to zero, the inversion method is extended to other values of the calibrated phase lags measured in the neighborhood of the phase minimum or maximum. The application potential of these two solution methods is studied by analyzing the phase lags measured as a function of frequency for two-layer systems of technological importance, e.g., different plasma-...

Radiometric Analysis of Laser Modulated IR Properties of Semiconductors

A photothermal technique for the characterization of semiconductor materials is presented, in combination with the theoretical description of the signal generation process of the effects of the charge carrier density on the IR optical properties. It relies on the excitation of charge carrier density waves by modulated laser irradiation in the visible spectrum, leading to periodical variations of the IR optical properties. The detection is based on sensing the ir transmission of the semiconductor sample. The modulated laser irradiation in the visible simultaneously leads to small temperature variations and additional signal contributions due to the modulation of the internal IR radiation, which can be minimized and eliminated by appropriate focussing conditions. A principal understanding of the signal generation mechanism has been achieved by time-dependent measurements with a gradually increasing intensity of the external IR radiation source, while frequency-dependent measurements of the modulated IR transmission signal provide quantitative information on the semiconductor properties.

Photothermal characterization of amorphous thin films

Thin amorphous films deposited on steel substrates by physical vapour deposition, chemical vapour deposition and sputtering with thicknesses between 2 and 4 μm have been analysed with respect to their thermal properties by non-destructive evaluation based on infrared detection of thermal waves. Samples with and without exposure to friction wear have been compared. The main results are that the friction effects can be characterized by their effects on the thermal properties, the thermal diffusivity and effusivity of the coating. It has been found that it is mainly the thermal conductivity of the coating that is affected by friction wear and the induced stresses.

Photothermal characterization of thin-layer chromatography plates

The potential of photothermal beam deflection spectrometry (PTDS), photoacoustic spectroscopy (PAS) and photothermal radiometry (PTR) for the characterization of thin-layer chromatography (TLC) plates with respect to the surface and in-depth distribution of different compounds inside the sorbent was investigated. Photothermal measurements of TLC chromatograms of the Camag III test dye mixture demonstrate that accurate and comparable values for thermal diffusivity of spots in chromatograms and of the sorbent can be obtained by PTDS and PAS. PAS has been demonstrated to be the most reliable and least time-consuming among the three investigated techniques. Differences observed between the PAS signal and the phase-frequency scans obtained from the spots of the same compound on different TLC plates could be due to the irreproducibility of chromatogram development or the inhomogeneousness in different TLC plates.

Double modulated thermoreflectance microscopy of semiconductor devices

Photothermal microscopy based on combined optical and electrical excitation has been applied to insulating lines and conducting channels on SIMOX mesas prepared by focused ion beam implantation. The double excitation technique permits imaging of electrical properties of the implanted structures yielding complementary information achievable by a single excitation defect tracing. In addition, the contrast of the images of implanted structures can be increased considerably. The best contrast for the observation of insulating lines adjacent to a conducting channel has been achieved by recording the modulated reflectance signal at the fourth harmonic of the modulation frequency used for electrical and optical excitation. The contrast enhancement is found to be mainly due to thermal origins caused by the photoinduced currents, which can act as an additional heat source.

Analysis of multifunctional oxycarbide and oxynitride thin films by modulated IR radiometry

Multifunctional coatings consisting of transition metal oxycarbides and oxynitrides deposited by physical vapour deposition techniques on tool steel are analysed in this work by means of modulated IR radiometry (MIRR), a non-contact non-destructive thermal wave measurement technique, with respect to the thermal transport properties relevant for time-dependent surface heating processes of coating–substrate systems. In order to interpret the measured data quantitatively, an inverse solution of the two-layer thermal wave problem is applied, which relies on the thermal wave phase lag data measured as a function of modulation frequency of the periodically modulated laser beam heating intensity. Based on these measurements and their quantitative interpretation, correlations between the thermal transport properties of the coatings and their deposition conditions have been found, which can be used to monitor deposition processes. For a second objective of this work, namely to determine the film thickness by means of MIRR, different sets of thin films of approximately constant thermal transport properties, but differing film thickness, have been measured. To discuss the limitations and error limits of these non-contact non-destructive measurements of the coating thickness, the results obtained by MIRR are compared with the coating thickness determined by destructive measurements.

Photothermal characterization of sputtered thin films and substrate treatment

Photothermal radiometry has been applied to relatively thin TiN-based hard coatings on high speed steel. The main results are that the photothermal depth profiles of the samples are strongly influenced by the substrate preparation, that an empirical correlation between the measured photothermal depth profile and different coatings can easily be established, that however a quantitative determination of the thermal properties (thermal diffusivity, effusivity) based on unique solutions of the heat diffusion equation is difficult. This is due to three reasons: the coatings are relatively thin and transparent, and the substrates alone already exhibit strong gradients of the thermal properties.

Thermal wave characterization of plasma-facing materials by IR radiometry

An experiment based on the IR detection of thermal waves has been developed, which allows the measurement of periodic temperature oscillations of 3–7 mK at average sample temperatures of 450–1000 K. Measurements have been performed on various plasma-facing materials. The thermal diffusivity, effusivity and thermal depth profiles of graphite have been determined at different temperatures, thus demonstrating the potentiality of this method for remote in situ measurements of thermal properties and nondestructive evaluation of plasma surface modifications in nonaccessible hostile environments.

Interface resistance in copper coated carbon determined by frequency dependent photothermal radiometry

The heat transfer in copper-carbon flat model systems was studied by frequency dependent photothermal radiometry. A novel approach which relies on the frequency dependence of the photothermal signal phase and amplitude at intermediate frequencies was introduced to determine the thermal interface resistance between the Cu-film and the substrate. The frequency dependent amplitude and phase of the photothermal signals were analyzed in the frame of a model of a one- dimensional heat flow perpendicular to the film plane. The interface resistance of the investigated CuC-sample with a Ti-bonding layer was found to increase by a factor two on heat treatment.

Contrast enhancement of modulated optical reflectance microscopy of semiconductor devices

Photothermal microscopy based on combined optical and electrical excitation has been applied to insulating lines prepared by focused ion beam implentation on oxygen implanted mesas. The double excitation technique permits to image electrical properties of the implanted structures which are complementary to simple defect tracing when a single excitation is applied. In addition, the contrast of imaging of implanted structures can be increased considerably. Apart from combining modulated optical excitation with electrical direct current voltages, double modulation configurations can improve the contrast further, by detecting either on a higher harmonic of the single excitation frequency or on the sum or difference of two different excitation frequencies. The influence of different parameters characterizing the experimental configuration gives information on the physical processes of the signal generation, indicating that the observed contrast enhancement is mainly of thermal origin related to the photoinduced currents.