Heat loss through surrounding air has important thermal effect on microfabricated structures. This effect is generally modeled as a natural convection boundary condition. However, how to determine the convective coefficient (h) at microscales is a debate. In this paper, a micro heater is fabricated on a suspended thin film membrane. The natural convection is investigated using the 3-omega measurements and complex analytical modeling. It is found that h seems larger than that at larger scales; however, it is also proved that the increased h is actually contributed by heat conduction instead of heat convection. A method of determining the phenomenal h that can be used for microfabricated structures is proposed by using the heat conduction shape factor.

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