The influence of film thickness on the magnitude and aging behavior of the transverse piezoelectric coefficient (d31) of PZT thin films

The wafer flexure technique has been developed for the rapid measurement (less than 10 minutes) of the d(31) coefficient of piezoelectric thin films. The technique is based upon the controlled bending of a clamped silicon substrate coated with a thin piezoelectric film. Flexure of the wafer results in the transfer of biaxial stress from the silicon to the film, and thus the production of an electric charge. The charge produced is used in conjunction with the applied principle stresses to determine the film's transverse piezoelectric coefficient (d(31)). For this study, the wafer flexure technique was modified from semi-ac operation to a mechanized ac measurement (i.e. electronic pressure oscillation and lock-in charge detection). Modifications made reduce electromagnetic noise and enhance both the resolution and precision of the device. The system was used to characterize the piezoelectric properties of lead zirconate titanate 52/48 thin films between 0.6 and 2.5 mu m thick synthesized using a modified sol-gel technique. The transverse piezoelectric constants (d(31)) of the PZT films were found to range from -60 to -90 pC/N for the 0.6 and 2.5 mu m films, respectively. Aging experiments of the d(31) coefficients were also conducted and results showed values to be on the order of 4 to 8% per decade.