Optimization of Film Stresses Utilized in Composite Piezoelectric Membrane Microgenerators
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OPTIMIZATION OF FILM STRESSES UTILIZED IN COMPOSITE PIEZOELECTRIC MEMBRANE MICROGENERATORS M.S. Kennedy1, M. Zosel1, C.D. Richards1, R.F. Richards1, D.F. Bahr1, K.W. Hipps2 and N.R. Moody3 1 School of Mechanical and Materials Eng., Washington State University, Pullman WA 99164 2 Department of Chemistry, Washington State University, Pullman WA 99164 3 Sandia National Laboratories, Livermore, CA 94550 ABSTRACT To improve the mechanical compliance of a piezoelectric generator membrane, alterations in PZT processing including variations in crystallization temperature, cooling rates and precursor chemistry were made to reduce the residual stress. In addition to changing the stresses, these treatments also altered the morphology of the PZT and its modulus. Nanoindentation of these films were made to determine the moduli of the PZT films, which varied from 70GPa to 90GPa. The residual stress of 40:60 2-MOE PZT taken by x-ray diffraction were shown to reduce in residual stress from 210 MPa to 154 MPa with annealing. The static pressure-deflection of membranes with these films showed correlating composite effective stresses of 110MPa and 83MPa. Adding PEG to the precursor solution also lowered the stress of the PZT films. Bulge testing showed that decreasing the crystallization temperature from 700˚C to 600˚C lowered residual stress from 210 MPa to 154 MPa. Raman spectroscopy showed differences in both stress and structure of PZT deposited on thin support structures (2µm) over bulk wafers (350 µm). The compliance of the generators was also increased by etching Pt in highly stressed regions. INTRODUCTION Micro-actuators and -generators can use the piezoelectric effect of lead- zirconatetitanate (PZT) films to provide relatively high power density in flexing systems. To improve generator performance by increasing the fatigue life and electrical characteristics, many groups have concentrated on producing high output PZT by altering the heat treatments [1, 2, 3] and solvents [4] for sol-gel deposition, chemistries [5] and substrates [6] of the PZT thin films. In addition to these film properties, both the PZT films and the generator systems will be optimized by understanding and reducing the residual stresses. These tensile stresses can affect both the ferroelectric and piezoelectric properties in thin film, in addition to lowering the compliance of the generator system. Stresses within a generator structure may be measured by a variety of methods including pneumatic testing, x-ray diffraction and Raman spectroscopy. Static pressure deflection testing is efficient for analysis of the effective stress of micromachined membranes. Bonnette et al. [7] showed that the pressure, P, was a function of the deflection Wo, biaxial modulus of the membrane EB, membrane thickness t, half of the membrane side length a, a geometric shape constant of the membrane α = 0.00126 and the residual stress σ0, and two unitless constants, c1=3.40 and c2=1.82, by P (Wo ) =
E B t 3Wo c1σ o tWo c 2 E B tWo + + 12αa 4 a2 a4
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