Thermal waves can reveal thermal properties of different layers forming a multilayer structure. If the thickness of each layer is known, specific ranges of thermal wave frequencies can be implemented to study the thermal response of a specific number of layers and eventually extract the thermal properties of individual layers. As a first approach this idea can be simplified by means of the thermal penetration depth parameter, δ. The thermal penetration depth is defined as, $δ=k/πCf$, where k and C are respectively the thermal conductivity and volumetric heat capacity of the material carrying the thermal wave and f is the frequency of the thermal wave. From this expression it can be seen how it is possible to constrain the material thermal response to a desired depth by controlling the frequency. Thus, using high enough frequencies, the top layer properties would be measured first. Decreasing the thermal wave frequency by an appropriate amount would include the next layer in the thermal response. Since the properties of the first layer are now known, it would be possible to extract the properties of the current layer. The measurement would continue in a similar fashion for the remaining layers. Frequency domain thermoreflectance (FDTR) can be used to generate thermal waves. In this technique, a periodically modulated continuous wave laser (red pump beam) provides the periodic heat flux input into the material while a second laser (green probe beam) monitors the surface temperature through a proportional change of the surface reflectivity. The measured value is the phase lag (degrees) between the incoming thermal wave and the surface temperature response. In this study, an FDTR system was used in conjunction with a piezo stage to obtain thermal images of two different multilayer structures. The first one consisted of a CPU chip formed mainly by layers of SiO2 and Cu. The second case consisted of a TFT LCD screen from a mobile device. Regarding the CPU chip, the low frequency thermal wave travelled well past the second layer of Cu wires and provided thermal information about the bottom layers of the chip. In contrast, the high frequency wave could not penetrate through the second layer, which resulted in a more sensitive response upon the Cu wires close to the surface. A similar phenomenon occurred with the LCD screen. In this case the top layer was a glass layer used to sandwich the liquid crystal and the second layer is composed of the ITO electrodes that provide the electric field. It can be observed how the high frequency wave did not penetrate through the top glass layer providing no thermal information about the bottom layer as opposed to the low frequency wave, which clearly shows the ITO electrodes. The estimated thermal penetration depths displayed on top of each image were calculated using the equation provided before with known thermal properties of SiO2, Cu and ITO.

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