Microscale and Nanoscale Thermal Characterization Techniques

In this paper the authors review various microscale and nanoscale thermal characterization techniques that could be applied to active and passive devices. Solid-state micro refrigerators on a chip can provide a uniform and localized temperature profile and they are used as a test vehicle in order to compare the resolution limits of various microscale techniques. After a brief introduction to conventional micro thermocouples and thermistor sensors, various contact and contactless techniques will be reviewed. Infrared microscopy is based on thermal emission and it is a convenient technique that could be used with features tens of microns in size. Resolution limits due to low emissivity and transparency of various materials and issues related to background radiation will be discussed. Liquid crystals that change color due to phase transition have been widely used for hot spot identification in integrated circuit chips. The main problems are related to calibration and aging of the material. Micro Raman is an optical method that can be used to measure absolute temperature. Micron spatial resolution with several degrees temperature resolution has been achieved. Thermoreflectance technique is based on the change of the sample reflection coefficient as a function of temperature. This small change in 10 ^-4 -10 ^-5 range per degree is typically detected using lock-in technique when the temperature of the device is cycled. Use of visible and near IR wavelength allows both top surface and through the substrate measurement. Both single point measurements using a scanning laser and imaging with CCD or specialized lock-in cameras have been demonstrated. For ultrafast thermal decay measurement, pump-probe technique using nanosecond or femtosecond lasers have been demonstrated. This is typically used to measure thin film thermal diffusivity and thermal interface resistance. The spatial resolution of various optical techniques can be improved with the use of tapered fibers and near field scanning microscopy. While sub diffraction limit structures have been detected, strong attenuation of the signal reduces the temperature resolution significantly. Scanning thermal microscopy which is based on nanoscale thermocouples at the tip of atomic force microscope has had success in ultra high spatial resolution thermal mapping. Issues related to thermal resistance between the tip and the sample and parasitic heat transfer paths will be discussed