Sub-Beam Size Temperature Measurement of Heavily Doped Silicon Heater Using Two-Wavelength Thermoreflectance Microscopy

This work describes a novel technique for simultaneously determining local temperature and thickness of a heavily doped Si heater having a submicron width by using two-wavelength thermoreflectance microscopy. The doped silicon line heater, whose thickness and width are, respectively, 480 nm and 900 nm, is fabricated by conventional microfabrication techniques on a fused silica wafer. The full width at half maximum (FWHM) of the focused laser beam is measured to be 2.00 μm and 2.28 μm for green (λ = 516 nm) and red (λ = 640 nm) lasers, respectively. Because the heater width is narrower than the focused laser beam size, the reflected beam contains background information (i.e., reflection from the fused silica substrate) in addition to the thermoreflectance signal from the doped silicon heater. With precise knowledge of the laser beam size, heater width, and exact location of the laser beam spot on the heater, one can quantitatively model the reflectance. In reality, however, due to the difficulty of aligning the laser beam with respect to the submicron-wide Si heater, precise determination of local temperature from thermoreflectance signal is not easily attained. In the present study, instead of aligning the laser beam to the center of the submicron silicon heater, the probe laser horizontally scans over a region of the heater. By taking into account the size of the focused laser beam and the width of the doped silicon heater, it is possible to determine the absolute temperature of a local region of the heater from the measured reflectance during the scanning, even though the width of the heater line is only 39% of the size of the laser beam.