Stable thermoreflectance thermal imaging microscopy with piezoelectric position control

Thermoreflectance thermal imaging microscopy is based on very small change in the surface reflection as a function of temperature. Image shift and instrument drift are limiting factors to obtain accurate and reproducible thermal images. Under large magnification and for devices with sizes on the order of hundreds of nanometers, image registration could significantly encumber accurate thermoreflectance measurements. Additionally, image blurring is an issue because of small sample movements during the measurement. The problem of image registration and defocusing is particularly important during the calibration process to extract the thermoreflectance coefficient of the materials under study. Calibration requires changing the sample temperature with an external stage, which causes significant movement due to heat expansion from the stage. In this work, we discuss how the image registration and defocusing affect accurate measurement and calibration in thermoreflectance thermal imaging. We also show that by incorporating and controlling the position of the sample under test with a closed-loop piezo-stage one can perform accurate and reproducible thermal measurement. Using this setup, we measured the coefficient of thermoreflectance (CTR) of gold at several wavelengths and under different magnifications. We present for the first time thermoreflectance calibration of feature sizes below the diffraction limit, on the order of 200nm.