edfas.org 5 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 26 NO. 2 and as a heat flux meter. During tip-sample contact, the hot probe gives up a quantity of heat, Q, to the cold sample and cools down (Tp and Rp decrease). As Q depends on the thermal conductance of the system under study, SThM can be used to analyze the thermal properties of components at the subsurface of the system. Today, the active mode is preferred to the passive mode for sample surface temperature analysis. As with large-scale contact thermal sensors, measurement uncertainty is linked to the thermal resistance of the probe, tip-sample contact, and sample itself. The contact thermal resistance depends on many parameters, including topography, local variations in sample surface roughness and the sample’s thermal properties. These parameters can be determined in active mode using methods based on the principle of active probes with compensated thermal flux,[1] which is not possible in passive mode. For both passive and active modes, the sensor’s temperature can be determined from experimental raw data. Careful calibration of the probe and modeling of the probesample system are required to quantitatively analyze the temperature or thermal properties of a sample. The current state of the art is based on the post-processing of the probe’s electrothermal response with tridimensional models that reproduce the best experiments. This article does not fully address this aspect of the SThM technique but the reader can refer to references 1-3 for more details. DETECTION OF LOCALIZED INHOMOGENEITY The active-mode SThM technique can be useful for defect de- tection and coating/thin-film evaluation. The first experiment to demonstrate the technique for these applications involves an analysis of a polymer matrix filled with expanded graphite particles. As shown in Fig. 2, the contrast of the nanocomposite’s thermal image does not match the topography of the sample. SThM can therefore be used to locate particle agglomerations in the matrix.[4] The second experiment involved a sample consisting of nine silicon dioxide (SiO2) steps ranging in thickness from 3 to 1000 nm on a silicon (Si) substrate[3] (Fig. 3, left). The results presented in Fig. 3, right, show that the SThM technique can detect the variation in SiO2 thickness over the entire thickness range studied, remaining sensitive to the presence of the Si substrate up to a SiO2 layer thickness greater than 1 µm. Heat conduction in oxide layers, inhomogeneity in oxide thickness, layer/substrate thermal resistance as well as details on the sample’s surface can then be analyzed by means of SThM. DETECTION OF LOCALIZED HEATING SThM can also allow detecting and locating electronic component overheating and hot spots, which often reflect component malfunction or impending device failure. Fig. 1 Top, schematic of the SThM instruments; bottom SThM probe with a resistive metallic element deposited on the AFM tip. Fig. 2 Left, topography image; right, thermal image of 10% weight fraction of expanded graphite particle, high-density polyethylene sample.[4]
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