Development of a flexible wearable piezoelectric tactile sensor based on AlN film

Piezoelectric microelectromechanical systems (MEMS) meet the growing demand for small size and low power consumption. They are used in a variety of fields from medical to healthcare for tactile applications and pressure measurement. In this context, flexible devices are important due to their high responsiveness and surface adaptability.

According to MEMS Consulting, researchers at the Italian Institute of Technology (Istuto Italiano di cnologia) have recently developed a miniaturized flexible piezoelectric device to increase the number of sensors that can be integrated with minimal crosstalk and to distinguish local pressure and contact per unit area. To this end, a series of aluminum nitride (AlN) piezoelectric sensors with different diameters (5 ~ 500 μm) were realized, and the electricity generated by the deformation of the sensor was amplified by a differential voltage circuit. By analyzing the shape of the piezoelectric signal that varies with the applied pressure, it was observed that the oscillation caused by the piezoelectric deformation was superimposed on the initial peak signal corresponding to the touch event. By calculating the integral of the electrical signal, the response of the sensor can be accurately described. The best responsivity was obtained in samples with diameters of 200 μm and 500 μm. In addition, the researchers found that the minimum distance between the edges of the sensor was about 500 μm when the crosstalk was below -20 dB. The relevant research results were published in the IEEE Senss Journal under the title "Fabrication, Charterization, and Signal Processing Optimization of Flexible and Wearable ezoelectric Tactile".

The developed piezoelectric sensor uses a sandwich structure based on AlN thin film embedded between two layers of molybdenum (Mo) grown on a PI flexible substrate. Among piezoelectric materials, AlN is selected for the manufacture of compact and efficient piezoelectric MEMS sensors due to its good performance and relatively low dielectric constant, despite its moderate piezoelectric coefficient. In addition, AlN is an environmentally friendly and non-toxic ceramic material. AlN exhibits high temperature and moisture resistance and can be directly deposited on soft substrates, making it conducive to its implementation in flexible wearable devices.

Fabrication design of AlN sandwich/Mo/AlN piezoelectric stack on PI 2555 substrate

Flexible piezoelectric samples featuring Mo/AlN/Mo thin film heterostructures were fabricated on PI substrates using reactive sputtering ultraviolet (UV) lithography and dry etching. The sputtered stack features a single-deposited AlN interlayer (120 nm) and a bottom Mo electrode (200 nm), a piezoelectric AlN (1.5 μm) layer, and a top Mo electrode (200 nm). The researchers fabricated a range of piezoelectric sensors with diameters ranging from 5 μm to 500 μm.

A range of piezoelectric sensors with different diameters

The electrical performance of the fabricated sensors was evaluated by measuring the impedance of each sensor before and after the release process. The researchers observed that sensors with diameters ranging from 20 to 500 μm exhibited electrical behavior consistent with expectations. Under a fixed pressure, the generated electrical signal was linearly related to the diameter of each sample. The calibration curve was obtained by analyzing the shape of the output electrical signal as a function of the applied pressure, using the numerical integration of the sensor output. For sensors with active area diameters of 200 μm and 500 μm, the responsivity values ​​were 0.001 Vs/kPa and 0.015 Vs/kPa, respectively, and the dynamic range values ​​were 314 kPa and 36.5 kPa, respectively. The active area of ​​these sensors is less than 0.2 mm², which improves the state of the art in device miniaturization. In the crosstalk analysis, the researchers found that the minimum distance between sensors with -20 dB crosstalk was about 0.5 mm, indicating that this is a safe distance for tactile applications.

Electrical properties before and after the release process

Crosstalk analysis

In summary, in this work, a flexible, wearable, and highly sensitive piezoelectric for tactile sensing was realized. It is capable of monitoring subtle grasping movements through a custom-implemented conditioning system. After characterizing the morphological and structural properties of the selected piezoelectric material (AlN), a series of flexible piezoelectric sensors with diameters ranging from 5 to 500 μm were realized, and the minimum sensor size (about 20 μm) that can maintain the functionality of the piezoelectric device was determined. Through the electrical characterization of these sensors, it was observed that sensors with diameters ranging from 20 to 500 μm exhibited electrical behavior consistent with expectations. According to the crosstalk analysis, the minimum distance between sensors with -20 dB crosstalk was found to be about 0.5 mm. In addition, the signal processing of the AlN piezoelectric film presented in this work is based on numerical integration during the calibration stage and allows to increase the dynamic range of miniaturized pressure sensors, which has not been exploited before. The improvements in responsiveness and dynamic range indicate that this integrated approach can be used to directly calibrate piezoelectric sensors and monitor electrical signals during subtle movements involved in grasping and manipulation activities.

Review editor: Liu Qing