Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyWe present in figure 6(a) the ΔR/R amplitude signalmeasured on a section of the central part of figure 5(a) (themean signal over several tens of lines) between points A andB. We can individually distinguish 7 of the 9 resistors andguess the 2 last ones on the left. It is not an optical resolutionissue as we have already seen that 350 nm wide resistors areclearly observed in figure 3(b). It can be explained by thepixel size (200×200nm 2 ), each resistor width being less than2 pixels, but also by a thermal limitation as the diffusionlength is much higher than the distance between twoconsecutive resistors. Hence, the heat generated by oneresistor spreads on the surrounding ones. We then note aglobal parabolic shaped reflectivity variation (low spatialfrequency signal) on which are superimposed local resistorreflectivity variations (high frequency signal). We checkedthat using a spatial high pass filter, we only keep the spatialhigh frequency signal and we then distinguish the 9 resistors.To analyze the sensitivity limit of our set-up, we havereduced the supplying voltage down to 0.75 V (V 0 =0.375 V),corresponding to a 96 µW dissipated power. The image size is75×75 µm 2 , hence the pixel size is now 150×150 nm 2 . Wepresent in figure 6(b) the mean ΔR/R amplitude signalmeasured on several sections. The thermal signal is stillvisible and the ΔR/R sensitivity limit (noise level) has beenestimated to 2×10 -5 for an accumulation of 10 images.IV. SCANNING INTERFEROMETRY SURFACEDISPLACEMENT MEASUREMENTSTo perform these measurements, we use the set-updescribed in section II but the source is now a commercialBMI SH-140 heterodyne interferometer probe in which thephotodetector is included (figure 7). It is constituted by a lasersource and a Mach-Zender interferometer: the laser beam issplit in two beams, a reference one at frequency f l and a probeone modulated by an acousto-optic modulator (frequency f b )and reflected at the surface of the device under test. Thedevice is still submitted to a f frequency positive cosinevoltage, and the temperature variation at f frequency creates acosine surface displacement at the same frequency:d ( t)= D cos(2π ft + ψ ) . (6)(a)Fig. 6. Thermoreflectance ΔR/R amplitude signal sections of sample B for a(a) 4.5V and (b) 0.75V positive cosine supplying voltage.(b)Fig. 7. Heterodyne interferometric probe.The probe beam is then submitted to a f b frequency shiftdue to the acousto-optic modulator and to a f frequency shiftdue to the surface displacement. Both beams are thenrecombined and the intensity detected by the photodetectorcan be written:[I( t)= I 0 cos(2πfb t + ϕ)+ kD cos(2π( f b + f ) t + ϕ + ψ )(7)− kD cos(2π( f − f ) t + ϕ −ψbwhere k=2π/λ and φ represents the phase differencebetween reference and probe beam. The signal spectrum isthen constituted by three components: a central one atfrequency f b , and two symmetrical lateral ones at frequenciesf b +f and f b -f. The detection system is included and the ratiobetween the amplitudes of the central component and of oneof the lateral components directly gives kD, hence thedisplacement amplitude D.In classical Michelson interferometry[16], the source is alaser, for instance an He-Ne laser, and the sample is displacedusing a translation stage. One measurement is made for eachposition of the translation stage. The use of the translationstage demands at least an acquisition time of 1s by point. Thislimitation will disappear with the use of fast galvanometricmirrors which displace the laser source instead of displacingthe sample with the translation stage. Moreover, to reach anoptimal sensitivity, we must ensure that the phase differenceφ between both beams remains constant and multiple of π/2for each measurement point. It demands the use of an activestabilization that makes the method time consuming (about 1minute by point). This limitation disappears with theheterodyne probe as we see that possible fluctuations of φwill affect equally the central and lateral components.Therefore, this source coupled to the use of the fastgalvanometric mirror system makes it a fast technique.All the images presented in this section have been madewith a pixel clock frequency at 2 kHz and an image size of100×100 µm 2 .A. Scanning Interferometry on Sample AWe present in Fig. 8 the image obtained on a 100×100µm 2area corresponding to 250 lines of 250 points each withV 0 =2.5 V and f=50 kHz. The acquisition time, with 10images accumulated, is then about 5 minutes. We clearlydetect the displacement on the resistors due to the Jouleeffect.)]©EDA Publishing/THERMINIC 2008 186ISBN: 978-2-35500-008-9