Thermal Contact Conductance of Micromachined Interfaces
The thermal contact conductance, G, is the most critical parameter in the study of heat transfer across interfaces. It is a measure of how much heat is able to flow across two surfaces in contact. For bulk materials (e.g. two metal rods stuck end-to-end) there are two primary paths for heat conduction between two interfaces at room temperature: one is solid-solid conduction through the contact points, and the other is conduction through an interstitial material, if there is one as shown in Figure 1. Measurements of these interfaces have shown that TCCs measured in vacuum are significantly lower than those measured with an interstitial gas, usually air. This behavior is typically attributed to the additional conduction paths offered by the air.
For micromachined devices, the roughness of interfaces is on the order of nanometers or angstroms, several orders of magnitude less than for bulk materials. We have done extensive testing of heat transfer in micromachined devices. A typical structure is shown in Figure 2 where we actuate a micromachined plate to the substrate and measure the heat dissipation across the interface. Our data shows suggests that trapped interstitial gas at nanometer roughness serves primarily to reduce the solid-solid contact area, overwhelming its traditional role in providing alternate conduction paths (see Figures 3 and 4).