Thermal Characterization of Thin Films for MEMS Applications

Army Research Laboratory

Monday, December 01 2008

This method could also be easily modified, using material stacks, for use with conducting materials, such as semiconductors used in MEMS processes.

Dielectric thin films play a very important role in the development of microelectromechanical systems (MEMS). These dielectric materials often are used as insulating layers in devices, major components in MEMS structures, or even as materials strictly used for fabrication processes in a clean-room environment. In these applications, heat is often a crucial factor. Whether it be heat transfer through a device to stimulate operation, a device being exposed to certain temperatures during fabrication, or any other manner of heat transfer, these thermal processes are critical to device operation in MEMS. As such, thermal properties of these thin film dielectrics, especially thermal conductivity, are very important parameters to insure proper device operation.

Silicon dioxide and photoresist are two dielectrics that are commonly used in MEMS processes and in a cleanroom environment. SiO₂ is often used as an insulating layer between a silicon substrate and device stacks on top of the substrate, as well as between conducting layers within the actual device. Photoresist is used in the photolithography process to pattern device layers. It can also be cured using heat and ultraviolet light to be incorporated permanently in devices as an insulator.

At present, thermal conductivity data of silicon dioxide and photoresist at the scale to be studied is very limited. The majority of known values for these materials are for bulk thicknesses.

In comparing known bulk values to those of thinner films, it has been found that a decrease in thermal conductivity results when the size of the test specimen goes from bulk to these thinner films. Also, process conditions between fabrication environments can vary greatly. For example, PECVD SiO₂ may be slightly silicon-rich or –poor, depending on the recipe. An increase in silicon content would result in an increase in the thermal conductivity of the dielectric layer. Thus, data from a silicon-rich PECVD would differ from that of a stochiometric PECVD.

The method upon which this work is based involves a micromesa test structure, seen in Figure 1. The geometry that these mesa structures employ limits the heat path to conduction perpendicular to the surface of the substrate with very little excess heat lost from the sides or top of the structures. This form of testing and measurement constitutes a “direct” measurement of thermal conductivity as DC values are collected from the devices themselves. In this approach, data is collected at “steady state,” which is more representative of the application environment for these materials. Thus, the testing conducted on these structures results in data that relies solely on this vertical conductivity, which is ideal for the purposes of this experiment.