Tactile multisensing on flexible aluminum nitride

Abstract

The integration of a polycrystalline material such as aluminum nitride (AlN) on a flexible substrate allows the realization of elastic tactile sensors showing both piezoelectricity and significant capacitive variation under normal stress. The application of a normal stress on AlN generates deformation of the flexible substrate on which AlN is grown, which results in strain gradient of the polycrystalline layer. The strain gradient is responsible for an additional polarization described in the literature as the flexoelectric effect, leading to an enhancement of the transduction properties of the material. The flexible AlN is synthesized by sputtering deposition on kapton HN (poly 4,4'-oxydiphenyl pyromellitimide) in a highly oriented crystal structure. High orientation is demonstrated by X-ray diffraction spectra (FWHM = 0.55° of AlN (0002)) and HRTEM. The piezoelectric coefficient d(33) and stress sensitive capacitance are 4.7 ± 0.5 pm V(-1) and 4 × 10(-3) pF kPa(-1), respectively. The parallel plate capacitors realized for tactile sensing present a typical dome shape, very elastic under applied stress and sensitive in the pressure range of interest for robotic applications (10 kPa to 1 MPa). The flexibility of the device finalized for tactile applications is assessed by measuring the sensor capacitance before and after shaping the sensing foil on curved surfaces for 1 hour. Bending does not affect sensor's operation, which exhibits an electrical Q factor as high as 210, regardless of the bending, and a maximum capacitance shift of 0.02%.

Flexible Piezoelectric Energy-Harvesting Exploiting Biocompatible AlN Thin Films Grown onto Spin-Coated Polyimide Layers

“…This value is higher than that reported for AlN films grown on polymeric substrates 30,31 and in good agreement with our previous values of AlN layer on Kapton© foils. 6 Representative flexible devices undergoing folding/unfolding states have been characterized to assess the voltage/current generation properties providing a mechanical stimulus by a bending/unbending setup using a linear motor (Figure S3 in the Supporting Information).…”

“…On the other side, wurtzite films present moderate piezoelectric properties, which are commonly used in CMOS technology and show biocompatibility . Additionally, being non-ferroelectric materials, they can exhibit a permanent polarization along the c -axis crystallographic direction depending on the growth conditions without need of poling/annealing processes, thus enabling the direct deposition of high quality piezoelectric films on soft substrates , and the exploitation of MEMS technologies for device fabrication. Several works reported on flexible piezoelectric energy harvesters exploiting both Pb­(Zr,Ti)­O 3 (PZT)-based layers even for implantable applications ,− and lead-free biocompatible BaTiO 3 or (Na,K)­NbO 3 perovskite thin films.…”

Flexible AlN flags for efficient wind energy harvesting at ultralow cut-in wind speed

“…The process is simple and compatible with common microfabrication tools used in semiconductor foundries. 20,21 Aer resist removal the foil is released by the support on which it was previously laminated by dipping the sample in isopropanol at room temperature for about 2 hours. Isopropanol is able to swell the PDMS layer under the Kapton foil and to detach the ag without any crack.…”

Integrated Piezoelectric AlN Thin Film with SU-8/PDMS Supporting Layer for Flexible Sensor Array

“…Based on the transition and Curie temperatures, the working temperature of PVDF film is limited to between −20 and 70 • C [28,29]. Additionally, in terms of tactile sensing for multiple stimuli, undesirable coupling of soft piezoelectric polymers between neighbor sensors can occur through high mechanical transmission, and may be a critical issue for tactile sensor arrays with respect to detecting multiple stimuli [30]. Unlike PVDF film, AlN thin films show a wide working temperature range, from −196 to 1150 • C, with a high electromechanical coupling coefficient due to the low dielectric permittivity and relevant piezoelectric coefficient [29].…”