Modulated IR radiometry as a tool for the thickness control of coatings

The thickness of coatings can be determined using the data measured by Modulated IR Radiometry for sets of coatings, produced under specific controlled conditions: – Keeping constant all deposition parameters except the deposition time, coatings of approximately constant thermal transport properties, but different thickness are produced. The modulated IR phase lag signals measured for the coatings are calibrated with the help of signals obtained for homogeneous opaque reference samples of smooth surface. Quantitative results for the thermal transport properties are obtained using the inverse solution of the 2-layer thermal wave problem by which direct relations are established between the relative extrema of the inverse calibrated thermal wave phase signals measured as a function of the heating modulation frequency and the thermal coating parameters, the ratio of the effusivities coating-to-substrate, the coating's thermal diffusion time, and the coating thickness. The coating thickness values obtained by Modulated IR Radiometry are compared with the values measured by standard microscopic methods, and relative errors of 3 – 4% have been found for the coating thickness of a set of TiCO coatings on steel, presented here as an example.(undefined

Laser modulated optical reflectance of thin semiconductor films on glass

Semiconductor films, deposited by reactive magnetron sputtering on glass substrates have been analyzed with the help of laser-modulated optical reflectance. The results are discussed with respect to the thermal and charge carrier transport properties. Semiconductor properties have been identified both for micro-crystalline and amorphous film

Analysis of active semiconductor structures by combined SThM and SThEM

In this contribution we present a combined experimental set-up for the simultaneous detection of the thermal and thermoelastic signal of electrically heated semiconductor devices measured by scanning probe techniques. A commercial atomic force microscope has been operated using a wollaston wire thermal tip for temperature oscillations, while simultaneously the oscillations of the cantilever have been measured. This set-up has been applied to semiconductor structures, with the aim of characterizing temperature, heat source and current distribution. In addition, having available a very versatile tool for different thermal scanning application, the simultaneous imaging by scanning thermal microscopy and scanning thermoelastic microscopy can improve contrast and quantitative interpretation. In both cases, the measured signals only give an indirect information on the current and heat source distribution. Therefore, the data has to be interpreted by help of theoretical models based on thermal waves, either by analytical approach or by finite element simulations. Here, the combination of the two techniques, can be helpful to find a more precise set of parameters, that describes both the SThM and SThEM signal. Furthermore, the influence of non-thermal and non-thermoelastic contributions to the SThEM-signal will be analyzed and discussed. When performing simultaneous measurements, especially the influence of the tip expansion, either due to heat diffusion from the sample to the tip or due to the probe current in the tip has to be considered

Simulation of photothermal measurements on Cu-Carbon interface systems

Although the thermal depth profiles measured for Cu-C interface systems show similarities to the depth profiles of 2-layer systems with the effusivity of the first layer smaller than that of the substrate, large differences between the 2-layer model approximations and measured data are found at higher modulation frequencies. This is due to the fact, that the properties of the Cu films are not well described. If on the other hand, a 2-layer model with additional thermal contact resistance between the thin sputter-deposited Cu film and the C substrate is considered, a large variety of mathematically possible solutions can be found, which depend on parameter triplets consisting of the ratio of the effusivities film-to-substrate, the film’s thermal diffusion time, and the thermal contact resistance

High-temperature thermal wave measurements of Cu-C interface systems

Photothermal measurements on Cu-C interface systems before and after annealing, combined with mechanical measurements of the adhesion strength, have shown that both the depth profile of the effective thermal properties and the adhesion strength change considerably with high-temperature annealing processes. To study these effects in more detail, modulated IR radiometry has been applied as a function of temperature. Two effects are observed between 200°C and 250°C which are interpreted as re-crystallization of the RF sputter-deposited initially amorphous Cu films, accompanied by an increase of the thermal contact resistance between Cu film and carbon substrate. The irreversible strong changes of the effective thermal properties are confirmed by frequency-dependent measurements, run at constant temperatures before, during, and after repeated heating processes

Identification of efficient deposition conditions based on the determination of the effective thermal transport properties of Cu-C interface systems

For the photothermal characterization of Cu-C interface systems modulated IR radiometry has been applied. Based on two-layer model approximations, the measured effective thermal depth profiles are interpreted in the range of the intermediate modulation frequencies. The obtained thermal parameters are correlated with the mechanical adhesion strength between Cu film and C substrate, in order to identify film deposition conditions and substrate cleaning procedures contributing to good mechanical bonding and reduced thermal contact resistances

Laser modulated IR transmission of semiconductors

The influence of the charge carrier density on the IR properties of semiconductors is a well known effect, that can be exploited for the analysis of semiconductor materials. In this work, an experimental arrangement is presented, by which the IR transparency of thin semiconductor samples is modulated with the help of an Ar ion laser beam with photon energies h$\nu$ > E$_{\rm gap}$ and by which the oscillating transmitted IR radiation of an external IR source is measured. The transmittance signal (amplitude and phase) which only depends on the charge carrier density can be used for the characterization of semiconductors independent of thermal parameters

Evaluation of active semiconductor structures by combined scanning thermo-elastic microscopy and finite element simulations

In this contribution we report on combined investigations of hot areas in a high power high electron mobility transistor (HEMT) using a scanning thermo-elastic microscope and finite element simulations of the problem. The sample was a AlGaN/GaN-HEMT grown on sapphire substrate, with a gold coating for improved thermal management. The FE simulations were performed based on the ANSYS program version 5.7. The thermo-elastic response was detected with an Explorer AFM-head of Topometrix. To allow simultaneous detection of the topology and of the thermo-elastic expansion images, the explorer had been modified for AFM measurements in the DC mode and at the double frequency of the thermal sinus in AFM contact mode. The thermo-elastic image of the hot area of the HEMT recorded at 2f shows a bright line as the hot area which is located along the gate, between gate and drain. The absolute value of the vertical expansion has been calibrated from the measured diode signal by use of the microscope’s force-distance calibration curve. In order to obtain a reliable estimate of the maximum temperature on the hot line, the temperature image obtained by FE simulation is calibrated using the thermal expansion of the gold film of known thermal expansion coefficient

Heat treatment-induced bond layer diffusion and re-crystallization in copper carbon interface systems measured by modulated IR radiometry

Copper-carbon interface systems with additional Mo bond layers in the range of 25 nm to 200 nm have been analyzed with respect to their effective thermal depth profiles before and after heat treatment using modulated IR radiometry. Comparing the inverse calibrated modulated IR phase lags before and after heat treatment, several effects can be identified: – (1) The effusivity of the interface layer, which – due the contact resistance between the two elements copper and carbon – is rather low before heat treatment, increases considerably with heat treatment. – (2) This effect is accompanied by an increase of the thermal diffusion time of the interface layer, relying on the diffusion of Mo and Cu particles. – (3) The sputter-deposited copper films, which before heat treatment can be characterized as effective multi-layer structures, re-crystallize with heat treatment and show modulated IR phases, which are characteristic for thermally homogeneous thin films. – (4) The thermal diffusion times of the Cu films decrease considerably with heat treatment due to increased thermal diffusivities, and – (5) the thermal effusivities of the Cu films increase with heat treatment

Cu-C interface systems evaluated with the help of the thermal wave contrast

The effective thermal depth profiles of copper-carbon interface systems with and without additional metallic submicron bond layers have been measured by means of modulated IR radiometry and have been evaluated using a new quantity, the thermal (wave) contrast, defined with the help of the calibrated thermal wave amplitudes. Useful correlations between the thermal wave contrast and the mechanical adhesion strength between copper film and carbon substrate have been found. By comparing the measurements based on thermal wave excitation and contact-less IR detection with microscopic contact temperature measurements close to the copper-carbon interface, the similarities between the two experiments are discussed, and the advantages and problems related to the thermal wave contrast are analyzed