Infrared Quantum Dots for Liquid-Phase Thermometry in Silicon Microchannels

The exponential growth in the component density of integrated circuits has created huge thermal management challenges for next-generation microelectronics, with projected local heat fluxes approaching 1 kW/cm² within five years. There is thus an urgent need to develop compact, high heat flux cooling methodologies. Developing and evaluating effective microelectronic single-phase liquid cooling systems, however, also requires measuring liquid-phase and wall surface temperatures to determine the local convective heat transfer coefficient in complex piping systems with dimensions comparable to the diameter of a human hair. Yet there are few if any practical thermometry techniques that can measure temperatures in such small geometries without disturbing the coolant flow and hence affecting cooling performance. Most optically based non-intrusive liquid-thermometry techniques exploit the temperature-sensitive spectral characteristics such as emission intensity and lifetime of the photoluminescence from various tracers at visible wavelengths. Applying such techniques to measure temperatures in silicon (Si) devices, however, has been hampered by a lack of suitable temperature indicators because Si is opaque at visible wavelengths. Silicon is, however, partially transparent in the near-infrared (IR), with absorption coefficients as high as 15 cm⁻¹ at wavelengths of 1.2–1.6 μm. We have therefore investigated the temperature sensitivity of the photoluminescent emission from oleate-capped lead sulfide (PbS) quantum dots (QD) suspended in toluene that emit at a wavelength of 1.55 μm. These QD are found to have an emission intensity that decreases by as much as 0.5% per K for temperatures ranging from 293 K to 333 K. Results are presented for temperature measurements through Si surfaces using PbS QD. The accuracy and reproducibility of these temperature measurements are discussed.