Thermal conductivity characterization of in-situ fabricated polysilicon nanowires for uncooled thermoelectric infrared detectors

Abstract

A microstructure along with a robust fabrication process is developed for measuring the thermal conductivity (K) of nanowires and thin films. The thermal conductivity of a thin-film material plays a significant role in the thermoelectric efficiency of the film and is usually considered the most difficult thermoelectric property to measure. The lower the K, the higher is the thermoelectric efficiency and hence a higher detectivity can be attained if utilized for infrared detection. We have previously shown high responsivity uncooled thermoelectric IR detectors [1] that utilize polysilicon as the thermoelectric material. To further improve the performance of these devices, it is required to understand how the wire dimensions and different deposition parameters affect the thermal conductivity of polysilicon.The nanowires of this work are formed by patterning a thin layer of low-pressure chemical vapor deposited polysilicon using e-beam lithography. Consequently, the common pick-an-place process followed by deposition of metallic contacts is avoided. As a result a significant source of error in calculating the thermal conductivity is eliminated. Additionally, several serpentine nanowires are fabricated between the two thermally-isolated membranes so that a greater amount of heat, comparable to heat loss through the arms, is transported through the nanowires for a more accurate measurement while the serpentine shape of the wires improves their structural integrity. The K of polysilicon nanowires are measured for the first time and it is shown that for nanowires with a cross section of ~60nmx100nm, the K is ~3.5 W/m.K (a 10X reduction compared to the bulk value of ~30W/m.K [2]).