Abstract
Bringing bodies close together at sub-micron distances can drastically enhance radiative heat transfer, leading to heat fluxes greater than the blackbody limit set by Stefan–Boltzmann law. This effect, known as near-field radiative heat transfer (NFRHT), has wide implications for thermal management in microsystems, as well as technological applications such as direct heat to electricity conversion in thermophotovoltaic cells. Here, we demonstrate NFRHT from microfabricated hotplates made by surface micromachining of hbox {SiO}_2/hbox {SiN} thin films deposited on a sacrificial amorphous Si layer. The sacrificial layer is dry etched to form wide membranes ({100},upmu hbox {m} times {100},upmu hbox {m}) separated from the substrate by nanometric distances. Nickel traces allow both resistive heating and temperature measurement on the micro-hotplates. We report on two samples with measured gaps of {610},hbox {nm} and {280},hbox {nm}. The membranes can be heated up to {250},^{circ }hbox {C} under vacuum with no mechanical damage. At {120},^{circ }hbox {C} we observed a 6.4-fold enhancement of radiative heat transfer compared to far-field emission for the smallest gap and a 3.5-fold enhancement for the larger gap. Furthermore, the measured transmitted power exhibits an exponential dependence with respect to gap size, a clear signature of NFRHT. Calculations of photon transmission probabilities indicate that the observed increase in heat transfer can be attributed to near-field coupling by surface phonon-polaritons supported by the hbox {SiO}_2 films. The fabrication process presented here, relying solely on well-established surface micromachining technology, is a key step toward integration of NFRHT in industrial applications.
Highlights
Bringing bodies close together at sub-micron distances can drastically enhance radiative heat transfer, leading to heat fluxes greater than the blackbody limit set by Stefan–Boltzmann law
A plasma enhanced chemical vapor deposition (PECVD) amorphous silicon (a–Si) film acts as the sacrificial layer on which the SiO2/SiN /SiO2 membrane trilayer is deposited
We have demonstrated in this work evidence of near field thermal radiation on static semi-transparent membranes
Summary
Bringing bodies close together at sub-micron distances can drastically enhance radiative heat transfer, leading to heat fluxes greater than the blackbody limit set by Stefan–Boltzmann law. The sacrificial layer is dry etched to form wide membranes (1 00 μm × 100 μm ) separated from the substrate by nanometric distances Nickel traces allow both resistive heating and temperature measurement on the micro-hotplates. By approaching two materials which support similar SPhPs, a coupling of surface waves could drastically enhance the electromagnetic energy transfer, leading to N FRHT2 In this regime, radiative coupling from evanescent waves emitted by both bodies enhances transmitted power by up to a factor 100 at nanometer-scale gaps (5 0 nm ) compared to the blackbody limit of conventional far-field heat transfer[3,4]. To increase the heat transfer while minimizing conduction losses, large areas combined with good thermal insulation are required One such type of thermal MEMS is the micro-hotplate, a thin film membrane suspended by long thin legs with a resistive heater on top. Our study points to micro-hotplates as a viable option to leverage NFRHT in practical applications
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