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    The thermoreflectance (TR) technique for thermal imaging exploits the change in a material’s reflectivity due to a change in its temperature. This imaging technique uses a probing light source in the visible range to achieve sub-micron spatial resolution. Since the thermoreflectance technique is not based on blackbody radiation it is not subject to the same spatial resolution limitations as infrared (IR) imaging, which is in the order of 3 to 10 μm. Thermal images are typically obtained with illumination sources with wavelengths between 400 and 900 nm for a Si detector and 900 to 1700 nm for InGaAs. The probing light can also be pulsed to measure the temperature at specified time delays with respect to the device biasing pulse. The amount that the reflectivity coefficient changes with temperature is called the thermoreflectance coefficient. Since the thermoreflectance coefficient is very small for most materials of interest, a lock-in technique is employed to enhance the signal-to-noise ratio (SNR) to achieve good temperature resolution. Using a Near-IR illumination source, as with the Microsanj NT300A-series, enables thru-the-substrate imaging. This also enables the collection of emission images (EMMI) along with the thermoreflectance images.

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