DISPLAY COMPATIBLE PMUT ARRAY FOR MID-AIR HAPTIC FEEDBACK Alexandre Halbach, Pieter Gijsenbergh, Yongbin Jeong, Wouter Devriese, Hang Gao, Margo Billen, Guilherme B. Torri, Christopher Chare, David Cheyns, Xavier Rottenberg, and Veronique Rochus IMEC, Kapeldreef 75, 3001 Leuven, Belgium alexandre.halbach@imec.be ABSTRACT This paper presents a Piezoelectric Micromachined Ultrasonic Transducer (PMUT) array targeting mid-air haptic feedback applications. Compared to existing bulk ultrasound transducer technologies, this array implements polymer-based micromachined ultrasound transducers fabricated in a display- compatible, large area technology. It is, thus, a good candidate for direct integration on top of large displays and allows to fabricate dense PMUT arrays for a fine non-contact haptic experience at interesting price point. BACKGROUND Contactless haptic feedback has already been demonstrated [1,2] and commercialized by the company Ultrahaptics [3] using phased arrays of discrete off-the-shelf transducers (about 1 cm diameter, 40 kHz resonance frequency). By focusing the emitted pressure of the transducers to a small spot in space and modulating the pressure at 200 Hz, the nano-receptors of the fingertip can be excited. It was determined by using an Ultrahaptics board that the pressure sensing threshold is 1 kPa [4]. As an alternative to the aforementioned array of discrete transducers, display-compatible polymer-based micromachined PMUTs, based on the technology already described in [5,6], can be used to get a high transducer density and a high resonance frequency, leading to much finer haptic patterns. At the same time, this technology is suited for direct integration on top of large displays. DESCRIPTION OF THE PMUTS In a PMUT, an electric field is applied across a piezoelectric layer to induce a membrane vibration and emit acoustic waves. Figure 1 illustrates the cross section of an individual PMUT fabricated with the aforementioned technology [5,6]. The membrane is made of a 15 µm thick polyimide layer on top of which a 500 nm thick piezoelectric PVDF layer is spin-coated. The PVDF layer is sandwiched between two electric traces for actuation. For the fabricated PMUTs, a diameter ranging from 100 µm to 1 mm provides any resonance frequency between 100 kHz and at least 2 MHz as shown with measurements matching simulations on figure 2. Figure. 1: Illustration of the cross-section of a PMUT. Figure. 2: Resonance frequency vs. cavity diameter for the three first vibration modes (measured and simulated). For this paper a 4 cm x 4 cm large 64 by 64 array of 480 µm diameter PMUTs was fabricated. A diameter of 480 µm was selected because it provides the highest pressure per PMUT area at 1 cm distance in air. The large 64 by 64 array consists of four 32 by 32 arrays as illustrated on figure 3 (top). The individual 32 by 32 arrays are actuated in a row- column fashion: the top electrodes of all PMUTs in a given row are electrically connected together while the bottom electrodes of all PMUTs in a given column are electrically connected together. This connection can be visualized on the microscope view of a 3 by 3 subarray on figure 3 (bottom). As can be seen, a circular shaped electrode is used to drive the PMUT at its first mode mechanical resonance (390 kHz). A 67% electrode coverage is used to maximize the first mode vibration [5]. 978-1-5386-8104-6/19/$31.00 ©2019 IEEE 158 T1C.003 Transducers 2019 - EUROSENSORS XXXIII Berlin, GERMANY, 23-27 June 2019