Negative Thermal Expansion Thin Films

Thermal expansion must be considered in the design of many structures -- everything from bridges and buildings to micromachined mirrors. For lightweight compliant optics, mechanical aspects of the design must be considered to prevent deformation of the precision optical surface. In order to design a structure that does not deform it is useful to use materials with various coefficients of thermal expansion. Since most materials exhibit positive thermal expansion (the volume increases with increased temperature) it is useful to the optical designer to have low expansion or even negative expansion materials at their disposal. Unfortunately, many of the mechanical properties of optical films, including the expansion coefficient, are not well characterized.

Our group has deposited a negative expansion then film based on zirconium tungstate ZrW₂O₈ and performed preliminary characterization of the deposited material. Since negative expansion behavior is rather rare, a large aspect of the project has been determine the underlying mechanism that leads to the behavior. In the case of bulk ZrW₂O₈ the negative expansion behavior is thought to be a result of transverse motions in metal-oxygen-metal bonds, which is made possible by the crystal structure of the material. However, unlike the bulk material, the thin films deposited showed no crystal structure even though they exhibited negative expansion. Further investigations led to a hypothesis that the driving mechanism of the negative expansion in the deposited films is related to the density of the film.

Figure 1 - Micro test structure used to measure the coefficient of thermal expansion. The curvature of the devices was monitored interferometrically as the temperature was varied.

Figure 2 - Curvature of microstructure versus temperature. Analyzed data leads to a coefficient of thermal expansion of -12 ppm/K.