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24-26 September 2008, Rome, Italy100pm0A’ B'Fig. 8. Interferometric expansion amplitude image, for a 5V positivecosine supplying voltage, f=50 kHz (sample A).Fig. 10. Interferometric expansion amplitude section signal for a 1.4Vpositive cosine supplying voltage, f=50 kHz (sample A).100displacement(pm)8060402000 10 20 30 40interferometryposition (µm)thermoreflectanceFig. 9. Interferometric amplitude section and ΔR/R amplitude sectionsignals, for a 5V positive cosine supplying voltage, f=50 kHz (sample A).Fig. 9 corresponds to the mean amplitude along sectionsbetween points A’ and B’ on the 250 lines of Fig. 8 (blacksolid line). On this curve, we have superimposed (grey dottedline) the thermoreflectance ΔR/R amplitude mean signalmeasured in the same experimental conditions (in arbitraryunits). As expected, the location of the displacementcorresponds to the maximal relative reflectivity variationareas. But in the interferometric displacement signal, there isnot only a 80 pm displacement of the resistors but also aglobal displacement between the resistors whose amplitude isabout 40 pm. In the thermoreflectance signal, the signal ismaximum on the resistors but very low between the resistors.This can be explained by a lower temperature variation butalso by a lower κ thermoreflectance coefficient[7].We have then reduced the supplying voltage to 1.4V(V 0 =0.7V), which corresponds to a 500µW dissipated power.Fig. 10 corresponds to the mean amplitude along sections onthe 250 lines of the displacement image. We note the samebehavior as in figure 9 with maxima located at 10µm onefrom each other. Even for a 500µW dissipated power, we arestill able to locate the resistors where the maximaldisplacement is 12 to 13pm. The noise level is lower than10pm.(a)Fig. 11. Interferometric expansion (a)amplitude image and (b)amplitudemean section signal, for a 4.5V positive cosine voltage, f=50kHz (sample B).B. Scanning Interferometry on Sample BFinally, we have measured the displacement image onsample B when supplied by a 4.5V cosine positive voltage(V 0 =2.25V) at f=50kHz. The image resolution is this time500×500 pixels. Figure 11(a) presents the displacementamplitude detected and figure 11(b) the mean amplitude alongsections on the 500 lines. The behavior is very similar to theone observed for temperature variations (Fig.6(a)) in the sameexperimental conditions.(b)V. SCANNING TECHNIQUES PERFORMANCESThe performances of the scanning system presented in thispaper are summarized in table 1. As an optical system, thespatial resolution is limited by diffraction and is better than600 nm. The sensitivity and acquisition time are acompromise between point measurement and CCD imagingsystem performances. To compare the various methods, wepresent the acquisition time by pixel.For the ΔR/R and displacement sensitivities, we haveadded the time (in minutes) necessary to obtain a 250×250pixels image with these sensitivities. With the CCD imagingsystem, we reach the lower acquisition time but thethermoreflectance and displacement sensitivities are not verygood. Knowing that the thermoreflectance coefficient of most©EDA Publishing/THERMINIC 2008 187ISBN: 978-2-35500-008-9