CHAMPAIGN, Ill. - By applying a mechanical bending stress to offset the effects of residual stress in a piezoelectric ceramic thin film, researchers at the University of Illinois have found a way to significantly enhance the film's performance.
"Understanding the effects of residual stress in piezoelectric ceramic thin films is critical for their design and optimization as smart materials," said Nancy Sottos, a professor of theoretical and applied mechanics at the UI. "Not only can we greatly improve their performance as tiny sensors and actuators in microelectromechanical (MEMS) devices, we can also put the effects of residual stress to work in a unique patterning process to better incorporate these materials on electronic chips."
In previous work, Sottos and graduate research assistant Lei Lian found that as the ceramic films became thinner, the desired piezoelectric response also became smaller. Stresses within the films were thought to be primarily responsible.
Significant stresses build up in piezoelectric thin-film structures during the fabrication process, Sottos said. "There are intrinsic stresses caused by shrinkage and densification during the drying and firing stages, and there are extrinsic stresses that are induced upon cooling due to the mismatch between the thermoelastic properties of the film and substrate. As the films become thinner and thinner, the residual stress affects the piezoelectric properties more and more."
To further explore the connection between residual stress and piezoelectric response, Sottos and Lian exposed lead-zirconate-titanate thin films to varying amounts of mechanical stress. By applying a small mechanical load in the opposite direction to the tensile stress, they could relieve some of the residual stress in the film. The film's piezoelectric response was then recorded with a high-resolution, laser Doppler heterodyne interferometric measuring technique.
"The film's response increased significantly with the application of a compressive bending stress," Sottos said. "A 10-pecent reduction in the residual stress netted a 30-percent increase in displacement." In practice, it may be possible to compensate for the residual stress and recover film response by changing stress states during processing or by applying a mechanical deformation, Sottos said. It's also possible to put the residual stress to work in patterning the films for use on integrated circuits. "It is difficult to selectively etch a ceramic, so standard subtractive chip processing techniques won't work well for some smart materials," Sottos said. "But, methods to first pattern a substrate with a special polymeric monolayer and then lay down the ceramic film have recently been developed at Illinois. The film will adhere to the exposed substrate, but not to the monolayer. Residual stress induced in the film during drying will cause it to crack off the monolayer with extremely clean edges."
The researchers presented their latest findings at the International Congress of Theoretical and Applied Mechanics, held Aug. 27 to Sept. 2 in Chicago.