Thermal conductivity of heavily doped LPCVD polysilicon

We report the first experimental values for the thermal conductivity κ
σ
of polycrystalline silicon, an important design parameter for polysilicon micromechanics applied to sensors [1,2]. Because conventional methods to measure κ
σ
[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 κ
σ
for heavily doped polysilicon range from 0.30 to 0.35 Wcm
^-1
K
^-1
, only 20 to 25% of the value for single-crystal silicon (1.41 Wcm
^-1
K
^-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.