Simultaneous Multiwavelength Illumination for Thermoreflectance Sensitivity Enhancement

ThermoReflectance (TR) sensitivity is inherently limited by the material-dependent TR coefficient (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>TR</i></sub>), which varies with wavelength. This paper presents a novel approach to enhancing TR sensitivity by illuminating the inspected surface of the device under test with non-coherent light at multiple wavelengths simultaneously. By leveraging the superposition principle, the proposed method extracts an effective <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">C</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>TR</i></sub> from the combined wavelengths, using a simpler setup than mainstream approaches. A mathematical analysis investigates the influence of initial thermal conditions and camera multi-wavelength detection on temperature measurements, as well as the approach’s feasibility. Experimental assessment using a thermal test chip demonstrates that the multi-wavelength illumination strategy (at 430 and 546 nm) enhances in average, TR sensitivity by up to 280% compared to the single-wavelength one (at 546 nm). Consequently, the proposed solution achieves a TR accuracy of 4%, improves the signal-to-noise ratio by 11%, and the measurement time is halved compared to single-wavelength based mainstream strategies. Thus, this approach enables dynamic thermal measurements in microelectronics and other application fields requiring high lateral resolution.