Micromachined high-frequency ZnO ultrasonic linear arrays

ZnO thin film has been widely applied in high-frequency acoustic microscopy using single element transducer. It needs mechanical scanning which is much more time-consuming than electronic scanning with transducer array. However, one of the challenges in the implementation of ZnO transducer array is the patterning of small scale features in the array elements. In this paper, a controllable wet-chemical etching method is investigated to fabricate high-frequency ZnO-based ultrasonic transducer arrays. The wet-chemical etchant is NH₄Cl aqueous solution with a concentration of 10 wt% and the etching rate is 53 nm/min at room temperature. A ZnO array is achieved with a small ratio (0.25) of lateral etching to vertical etching. Finite element method is employed to calculate acoustic field, electrical impedance and crosstalk of the transducer. The characteristics of the transducer are measured and compared to the theoretical predictions. This etching method provides a possibility to acquire a pitch of a λ in a 300 MHz array. It indicates that the proposed wet etching is promising in the fabrication of high-frequency ZnO transducer arrays.