Zirconium tungstate (ZrW₂O₈)-based micromachined negative thermal-expansion thin films

Negative expansion materials are relatively rare but promise to be particularly useful in designing thermally sensitive mechanical devices. Although negative thermal expansion (NTE) in bulk materials such as ZrW₂O₈ has been extensively studied, this paper reports the first deposition of a NTE material thin film. ZrW_xO_y films were deposited by electron beam evaporation and reactive cosputtering. The films were processed and patterned for various microstructures. The coefficients of thermal expansion of the deposited thin films were determined by measuring the change in curvature with temperature. It was found that evaporated films but not sputtered films, which were denser than the evaporated films, exhibited NTE. It was also found that NTE behavior occurred across a variety of stoichiometries. Since crystalline ZrW₂O₈ and thin film ZrW_xO_y both have low densities and show negative expansion, it is speculated that similar physical mechanisms, as discussed in the text, are at work. Further, since the deposition conditions of a thin film can often be changed to control density, it is speculated that a wider variety of thin films than bulk crystals might be made to have NTE.