Surface wetting to enhance thermoreflectance characterization of integrated circuits

Thermoreflectance (TR) imaging enables non-contact thermal imaging of devices and integrated circuits (ICs) with sub-µm spatial resolution. TR coefficient of most metals and semiconductors in visible wavelengths is in the 10^-5 to 10^-3 K^-1 range, which gives a temperature resolution of 0.1-0.5 °C with a few minutes of averaging. Here, we show that surface wetting with various solvents, such as water, methanol, as well as Fluorinert, which is a commonly used coolant for high-power ICs, can enhance the TR coefficient by up to 19 times. Systematic characterizations as a function of the heating power, illumination-wavelength, liquid layer thickness, sample's tilt, and objective lens's numerical aperture are presented. TR images are distorted due to interference in the liquid layer, but this technique could be used for fast detection of small temperature variations and hot spots in ICs.