C.H. Huang, L. Demi, S. Mao, Y. Jeong, D. Cheyns, X. Rottenberg, V. Rochus
Keywords: pMUT, gesture recognition, display, ultrasound
Summary:1. BACKGROUND Recently, piezoelectric micromachined ultrasound transducers (pMUT) have proved their potential to make a reliable 2D ultrasound array. As shown in Fig. 1, the displacement of piezoelectric material layer is galvanised by electrical signals/acoustic waves. Besides, a structural layer is also included to modulate its frequency response. In this research, we demonstrated a pMUT design compatible with a flat panel display fabrication process as well as the investigation to achieve gesture recognition with this technology. By placing the pMUT array on top of the display, the efficiency of transmission and receiving of acoustic signals is expected to be significantly improved. The detailed design parameters are indicated in Fig. 2, the ratio of top and bottom electrodes is 67% based on previous researches and our own simulations. For the first prototype, most of the materials are transparent except the Al electrodes which can be replaced by transparent ITO layers in the future. 2. DEVELOPMENT A. FEM model and Characterization of pMUT Fig. 3 indicates the fabricated pMUT array. A FEM model is built using COMSOL for evaluating the dynamic characteristics and the profile of the first mode of pMUTs with different radii. In this research, Polytec MSA-500 laser Doppler vibrometer is used to measure the dynamic profiles of pMUTs with different designs in the air and vacuum. An optical white light interferometer is used to measure the static profiles. B. Acoustic Simulation for Mid Air Ultrasound Gesture Recognization The target application was to recognize gesture of a hand counting from 0 to 5. Acoustic simulations using kwave software are used to select the optimum device in terms of diameter and resonance frequency. The dynamic measurements described above are used to compute the acoustic properties of the pMUT. First, the peak displacement and resonance frequency of the membrane are used to calculate the acoustic intensity of the pMUT. After it, the bandwidth is taken into account to determine the axial resolution of ultrasound imaging and beam steering. Besides, the pitch of the pMUT array is also considered defining the lateral resolution. 3. RESULTS The fabricated pMUTs have resonance frequency from 1.16 MHz to 120 kHz. The measurement results of static surface profile are presented in Fig. 4. The larger membrane are collapsed due to the pressure difference between air and vacuum and FEM model accurately predict the collapsing depth. In Fig. 5, measured resonance frequencies in the air and vacuum are fitted with FEM model. Besides, the dynamic profile of a 560um pMUT is shown in Fig.6. As indicated in Fig. 7, the goal was to resolve a hand counting from 0 to 5. For that application, a lateral resolution of 3 cm is required and the 248 kHz pMUT seems to be the best option. In Fig. 8, acoustic beam plots in different depth for 248kHz pMUT array with 2 transmitters and 14 receivers were also investigated. The pMUT shows promising performance as a display fabrication compatible ultrasound unit for gesture recognition applications on smart phones or monitors.