Texture, morphology, and stress in PZT thin films
EMSL Project ID
2605
Abstract
WSU is developing the P3 microengine, a MEMS device that utilizes a composite flexing membrane to generate power by converting mechanical energy to electrical energy. The membrane is made by lead- zirconate- titanate (PZT), onto a boron doped silicon wafer. In order to optimize the electrical output and mechanical reliability of piezoelectric thin films for MEMS application, crystal orientation, grain structure and chemistry of PZT thin films must be controlled. The PZT films are deposited using solution deposition and subsequent heat treatments, with up to 30 layers of PZT being deposited to generate the device thickness required for producing electrical power. Previous studies at EMSL demonstrated that the layers can be resolved using electron microscopy, even after heat treatments which should homogenize the structure are performed. Altering the ratio of Zr to Ti allows control of PZT crystal structure, from rhombohedral to tetragonal films. It is expected that the 52:48 solution would yield a film with a mixture of rhombohedral and tetragonal crystals while the 40:60 solution would yield purely tetragonal crystals. To obtain the highest electrical output per given pressure flexing the PZT membrane and the highest coupling coefficient, the residual stress should be within the membrane when fabrication is complete should be minimized. Additionally, texture development in the thin film may lead to increased output, as films which are columnar and highly oriented will be able to approach the piezoelectric coefficient of single crystals (which is generally higher than in bulk polycrystalline samples). Three experimental apparatus available through EMSL would allow us to study the effects of texture, morphology, and residual stress on the performance of the PZT film in the MEMS device. First the Four-Circle X-ray Diffractometer would provide relationships to be drawn between crystal orientation, initial solution stoichiometry and electrical output. Since many of these films have a thickness less than 2 ??m, the Four-Circle XRD would permit analysis of the film without interference of the substrate. Additionally, the stress in the films can be determined using the Four-Circle XRD system. Secondly, the LEO 982 Field Emission Scanning Electron Microscope (FESEM) can provide thickness measurements of all the layers within the generators. Since surface grain diameters between 37-125 nm have been found in similar samples via AFM, the LEO 982 FESEM would provide the necessary resolution to determine if these grains are columnar or equiaxed along the cross-section. It was previously found on the LEO 982 FESEM located at EMSL that banding occurred with each PZT crystallization. Finally, the use of the PHI 680 Auger Electron Spectrometer, could reveal local chemical differences to determine if the previously observed layers and banding was a result of segregation of constituents to the PZT/PZT interface or contamination. Also, as the thickness of the films is increased to improve electrical output, a larger amount of Pb may be lost with more exposure to high temperatures. Loss of Pb during processing has been found to decrease piezoelectric properties. Therefore, with investigation of the chemistry, it can be determined if variation in electrical output in thicker membranes is due to less deflection for a given pressure or the loss of Pb. The following experiments are proposed: Varying the Zr:Ti ratio from 60:40 to 30:70 using the 2MOE deposition route to determine the texture, grain size, local chemistry, and stress in the resulting films (4 samples, all three instruments). Varying the precursor chemistry and annealing treatments to reduce the organic component of the solution, and monitoring residual stress and resulting grain structure (6 samples, x-ray and SEM only). All samples will be fabricated at WSU, and the piezoelectric and mechanical properties will be determined using bulge testing at WSU.
Project Details
Project type
Exploratory Research
Start Date
2002-09-16
End Date
2005-09-18
Status
Closed
Released Data Link
Team
Principal Investigator
Related Publications
Influence of structure and chemistry on piezoelectric properties of lead zirconate titanate in a microelectromechanical systems power generation application
Microstructural characterization and mechanical reliability of interfaces in piezoelectric based microelectromechanical systems