Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyFig. 1. Scanning imaging experimental set-upThe DUT is supplied by a generator whose frequency isused as reference for the lock-in amplifier which receives thedetector signal. This signal, as well as the threesynchronisation signals (pixel clock, line and frame signals),are recorded by a 12 bits 1.25MS/s data acquisition board.Figure 2 presents schematic representations of the twosamples we have studied: they are composed of a series of 9parallel strips resistors implemented on a Si substrate. Theresistors serve themselves as a heat source since, supplyingthem with a voltage, they are submitted to a temperaturevariation. The width of each resistor is 0.35 µm. They arecovered by a silicon oxide passivation layer which istransparent for visible wavelengths.The difference between both structures is the spacingbetween the resistors. In the first one noted sample A (figure2(a)), the distance between two consecutive resistors is 10 µmwhereas it is only 0.8 µm in the second one noted sample B(figure 2(b)). The value of each resistor is r=1845 Ω forsample A and 2934 Ω for sample B. The die wasimplemented using 0.35µm CMOS technology.(a)Fig. 3. (a) Optical image of sample A, (b) Cross section with suppression ofbackground signal.Using the experimental set-up presented in section 2, aR reflectivity image (figure 3(a)), i.e. an optical image, wasobtained for the sample A when not supplied. This image isobtained with a ×50 magnification objective (numericalaperture 0.55), the magnification being chosen so that wecould have an image of the whole sample. The image size is100×100 µm 2 constituted by 250×250 pixels (imageresolution). The pixel sampling frequency is 7kHz, thereforethe image is obtained in about 10 seconds. A cross section ofthe image is presented in figure 3(b). The illumination of thesample is not uniform, the left part of the image is brighterthan the right part. This is due to the fact that theillumination is not perfectly normal to the sample, and hencethe incident light flux is not uniform. It can be corrected butit is not a major point as the purpose is to measure ΔR/Rimages. Indeed, the incident light flux appears in ΔR and inR, and then disappears when measuring the ratio ΔR/R asexplained in reference [12]. Nevertheless, we easilydistinguish the 350 nm wide resistors, showing that thespatial resolution is clearly submicrometric. It is estimated toless than 600nm measuring the full width at half maximum.III. SCANNING THERMOREFLECTANCE TEMPERATUREVARIATION MEASUREMENTSThe source is a 1 mW He-Ne laser (λ=633 nm). The deviceis submitted to a f frequency positive cosine wave voltage:( 1 cos( 2πft))(b)V = V(1)0 +with V 0 the mean amplitude voltage. The power Pdissipated by Joule effect in each resistor is then given by:(a)Fig. 2. Samples constituted of nine 0.35 µm thin resistors with a distancebetween 2 resistors of: (a) 10 µm for sample A, (b) 0.8 µm for sample B.(b)2 2V V0⎛3 cos(2π2 ft)⎞P = = ⎜ + 2cos(2 π ft)+ ⎟r r ⎝2 2 ⎠Then, the resistor temperature variation is proportional tothe dissipated power P and each power frequency componentinduces a temperature variation on the resistor that spreadsall around and creates a temperature variation at each pointΔT(x,y). The corresponding reflectivity is then given by:(2)©EDA Publishing/THERMINIC 2008 184ISBN: 978-2-35500-008-9