Design Of The Structure For On-Line Determination Of Thermal Diffusivity Of Polysilicon Thin Films

The dimension of MEMS devices ranges from several microns to a few hundred microns. The properties of the material used are very different from the bulk material. And Polysilicon thin films are widely used in MEMS devices and ICs, which are important constituents of such micromechanical elements. The diffusivity of it has close relationship with the dynamic response of thermal CMOS devices. The thermal parameters of the Polysilicon thin films, such as thermal conductivity, thermal diffusivity, and temperature coefficient of resistance, depend strongly on the fabrication process, production environment, the concentration and type of dopant atoms and so on. Hence, special test structures should be designed to achieve the properties of microstructures with different sizes and shapes. In this paper, we present the structure for on-line determining the thermal diffusivity of thin films.The key rules for design of an on-line test structure for determining diffusivity of thin films mainly rely on these points as follows. (1). The processes for the test structure and the materials to be tested should be identical and no special processes for the test structure are permitted. (2). The test structure should be able to provide compatible electrical output interface for post data disposal modules. (3). The structure should be as easy as possible and its calculation model should be able to reflect the actual situation of the test structure under the precondition that the accuracy can be assured. (4). The requirements for the test circumstance and equipments can not be too high. There are already several test structures to measure the diffusivity of thin films, however, they can not meet all the requirements mentioned above.We proposed, fabricated and tested novel characterization structures and measurement method for on-line testing the thermal conductivity of surface-micro-machined Polysilicon thin films in this paper. The features of the measurement structure are analyzed, the analytical model and measurement method are also given. The effects of all heat exchange by convection, radiation and the heat transfer through the air gap and into the substrate are considered in our electrothermal model, and all measurements can be carried out in free. Finite element software ANSYS is used to verify the design and the electrothermal model. And the experiment using QFI InfraScopeⅡshows that thermal diffusivity of Polysilicon thin film which is gained from test is close to the exact value.In this paper, the theoretical model using numerical arithmetic to measure the thermal diffusivity is also proposed. This method might gain thermal parameters simultaneously by debugging program. It's convenient for on-line test.