Effect of electrodes on GHZ ZnO thin film bulk-acoustic-wave resonator

Thin film bulk acoustic resonators (FBARs) were prepared with ZnO films using Al and Cr/Au electrodes. ZnO films were deposited by RF sputtering. Al and Cr/Au electrodes were prepared by DC sputtering and thermal evaporation respectively. S parameters were measured with a network analyzer. From TEM images, it is apparent that there are some defects on ZnO with Al electrode while no defects are detected on ZnO with Cr/Au. Also, an intermediate layer is observed between ZnO and Al but nothing between ZnO and Au. As a result of those differences, the resonant frequency, the width between series and parallel frequency and the value of S₁₁ between peak and valley, are different between the FBAR samples with Al or Cr/Au even though both FBAR samples have the same thickness of ZnO.