Measurement of piezoelectric strength of ZnO thin films for MEMS applications

A new method of measuring the thin-film piezoelectric parameter (s(11)(E) + S-12(E))(-1)d(31), which determines the actuating strength of the film, has been devised. Dubbed the 'plank method', it allows the magnitude and sign of the parameter to be measured accurately. Finite-element analysis was used to aid development of the method by modeling static behavior of the test device. Experimental dynamic analysis was also used to verify the method. Plank-method measurements, together with bulge-test measurements, were used to determine d(31), the biaxial modulus of elasticity, and the residual stress of polycrystalline ZnO films deposited by rf magnetron sputtering. Values varied strongly with position in the deposition chamber, and d(31) of both signs was clearly observed. The characterization techniques presented are applicable to other piezoelectric film materials as well.