Sub-diffraction limit thermal imaging for HEMT devices

Thermal characterization of high-speed switching power transistors, such as high electron mobility transistors (HEMT), is critical for the evaluation of their performance as well as their long-term reliability. Unlike IR thermal imaging, thermoreflectance thermal imaging (TRI) uses LED lights in the visible range and therefore can be used to measure thermal response of these nanoscale devices under operating condition. However, TRI is also limited in terms of spatial resolution by optical diffraction as we reach device sizes on the order of hundreds of nanometer. We carried out a series of thermoreflectance thermal imaging experiments on the metal heater lines with widths ranging from 100 nm to 1 μm fabricated on InGaAs semiconductor film. Analytical and finite element numerical modeling are used to compare experimental data with theoretical temperature profiles. We demonstrate that thermoreflectance thermal imaging is capable of detecting temperature rise in devices with sub-diffraction feature sizes. We show that optical diffraction leads to underestimation of the magnitude of small scale hot spots. We also present a combined analytical-numerical model to reproduce the experimental results, and finally propose an approach that can be utilized to compensate for this diffraction artifact and acquire the correct temperature response from thermoreflectance thermal imaging results.