Thermal conductivity of heavily doped LPCVD polysilicon

We report the first experimental values for the thermal conductivity κσof polycrystalline silicon, an important design parameter for polysilicon micromechanics applied to sensors [1,2]. Because conventional methods to measure κσ[3] are inappropriate for thin-film materials, we introduce a new technique that makes use of an inhomogeneously doped polysilicon microbridge. A bias is applied across the bridge and current-voltage characteristics are measured when the bridge is in vacuum and again when the bridge is immersed in heated silicone oil. The high-vacuum and oil-bath data are plotted on the same axes. Intercepts in the resulting pair of curve families provide a means to determine a power dissipation versus temperature plot. A thermal analysis relates the oil-bath temperature to the bridge temperature. The thermal conductivity is obtained from these data. Experiments have been repeated several times using various sizes for the polysilicon bridges. Values obtained for κσfor heavily doped polysilicon range from 0.30 to 0.35 Wcm^-1K^-1, only 20 to 25% of the value for single-crystal silicon (1.41 Wcm^-1K^-1) [4]. This research provides a basis for thermal modeling of polycrystalline silicon structures and offers a technique for studying possible effects on a mechanical polysilicon property of variations in the process technology of the material. The technique can be used for other conducting materials that can be processed into small bridge structures.