Effective electromechanical coupling coefficient (kt2) for fundamental mode of thickness extensional mode thin film bulk acoustic wave resonator fabricated by ZnO thin film

The effective electromechanical coupling coefficient has been studied for thin film bulk acoustic wave resonators (TBARs) with fundamental frequencies between 200 and 400 MHz. An increase in the effective electromechanical coupling coefficient (kt(TBAR)(2)) is observed with the increase in the ZnO thickness ratio: thickness of ZnO thin film and that of TBAR. In case where the ZnO thickness ratio is > 70%, kt(TBAR)(2) becomes a constant of 3.4%. The discrepancy between the experimental and the calculated kt(TBAR)(2) is remarkable when the membrane of the TBAR warps. It was found that the ZnO thickness ratio is a very important parameter for adjusting the TBAR characteristics to each application. (C) 2002 Elsevier Science Ltd. All rights reserved.