fabrication and characterization of piezoelectric paper based device for touch and force sensing applications

Available online at www.sciencedirect.com ScienceDirect Procedia Engineering 168 (2016) 688 – 691 30th Eurosensors Conference, EUROSENSORS 2016 Fabrication and Characterization of Piezoelectric Paper Based Device for Touch and Force Sensing Applications Sepehr Emamiana,*, Binu B Narakathua, Amer A Chlaihawia, Massood Z Atashbara a Western Michigan University, Electrical and Computer Engineering Department, Kalamazoo, MI 49008, USA Abstract A piezoelectric based touch sensitive device has been successfully fabricated by screen printing silver (Ag) and polyvinylidene fluoride (PVDF) inks as conductive and piezoelectric materials, respectively, on paper substrate X-ray diffraction (XRD) analysis was performed to verify the formation of β-phase crystals in the PVDF layer during the curing process Voltages, as high as 0.22 V, were observed for human finger touch tests and a sensitivity of 0.3 V/N was obtained when varying forces ranging from 0.2 N to 1.4 N, in steps of 0.2 N, were applied on the printed device The piezoelectric-voltage analysis demonstrated that the printed device can be used for both touch and force sensing applications © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license © 2016 The Authors Published by Elsevier Ltd (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Screen printing; Polyvinylidene flouride (PVDF); Piezoelectric; Touch sensors; Force sensors; Paper Introduction User interface of electronic devices have been transitioning from old-fashioned button-type controls to embedded touch pads during the last few decades This trend of employing touch pads as part of the built environment has resulted in an inevitable demand for the fabrication of flexible touch sensitive devices Touch sensors are typically manufactured using conventional CMOS processes which are often expensive and fabricated on rigid substrates [15] Moreover, most of the sensor configurations used not provide the high flexibility and conformability, required for various touch sensing applications, thereby limiting their applications area A promising approach to overcome the drawbacks associated with conventional touch sensors is to use continuous layer-on-layer deposition of electronically functional inks onto flexible substrate materials * Corresponding author Tel.: +1-269-370-1788 E-mail address: sepehr.emamian@wmich.edu 1877-7058 © 2016 The Authors Published by Elsevier Ltd This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference doi:10.1016/j.proeng.2016.11.248 Sepehr Emamian et al / Procedia Engineering 168 (2016) 688 – 691 Printed electronic (PE) technologies have demonstrated the ability to produce flexible and mechanically stable circuits and sensors using screen, inkjet, gravure and flexographic printing processes [6-10] The main advantage of PE technology is that it is an additive process where functional materials are deposited selectively, thereby eliminating the need for masking and etching The additive nature of this process results in less usage of materials, faster fabrication process and, consequently, more cost-efficient production These techniques offer other advantages such as low manufacturing temperatures, mechanical flexibility, and lightweight products The use of printing technologies thus overcomes some of the drawbacks associate with conventional silicon technology Researchers have employed different transduction mechanisms such as piezoelectric [11], capacitive [12], resistive [13] and optical [14] in touch sensing devices The main disadvantage of capacitive based touch sensors is that their accuracy depends on the capacitance of the person who touches them Resistive based touch sensors suffer from poor durability when compared to other technologies Optical based sensors have lower resolutions than other technologies and surface obstruction can cause false positives Among these, the piezoelectric based transduction mechanism has generated significant interest due to its accuracy and capability to measure dynamic events, irrespective of the material of the object that comes in contact with the sensor This mechanism has also been widely used especially for applications in sensors, actuators and energy harvesters [15-17] Therefore, the development of piezoelectric based touch sensors using printing processes, on flexible substrates, is important In this work, a layer-on-layer piezoelectric based touch sensor was screen printed on a paper substrate Bottom and top electrodes were fabricated using silver (Ag) flake ink Polyvinylidene fluoride (PVDF) ink was used for screen printing of the piezoelectric layer The different layers of the fabricated touch sensor were characterized The printed PVDF was polarized by applying an electric field of 80 V/μm across the layer, while heat was applied to the touch sensor Varying forces were applied to the touch sensor and its voltage response was investigated Experimental A schematic of the sensor is shown in Fig 1(a) The device consists of a 4×4 sensor array, with each sensor having a piezoelectric layer sandwiched between top and bottom electrodes Each sensor has top and bottom electrodes with a dimension of 4×4 mm2 and piezoelectric layer with dimension of 6×6 mm2, thereby resulting in a capacitive area of 4×4 mm2 The sensors have a spacing of mm with 0.5 mm wide interconnecting lines The sensor was fabricated using coated paper (NB-RC3GR120) from Mitsubishi as the substrate, conductive Ag flake ink (Electrodag 479SS) from Henkel for the metallization layers and PVDF ink (SOLVENE™) for the piezoelectric layer Initially, a 4×4 array of bottom electrodes were screen printed using the Ag flake ink on paper The printed layers were then cured for 20 minutes at 120 °C Then, two piezoelectric PVDF layers were screen printed onto the deposited bottom electrode Each printed piezoelectric layer was cured at 130 °C for hours Finally, a 4×4 array of the top electrodes was screen printed using Ag flake ink The printed Ag was cured for 20 minutes in the oven at 120 °C The fabricated touch sensor is shown in Fig 1(b) The printed touch sensor was polarized by applying an electric field of 80 V/μm for hour (a) (b) Fig (a) Schematic of the touch sensor and (b) screen printed touch sensitive device on paper substrate 689 690 Sepehr Emamian et al / Procedia Engineering 168 (2016) 688 – 691 X-ray diffraction (XRD) measurement was performed to characterize the crystalline contents of the printed PVDF film, both before and after the curing process XRD patterns were obtained on a Bruker D8 DaVinci diffractometer equipped with Cu X-ray radiation (λ = 1.5406 Å) operating at 40 kV and 40 mA Peak intensities were obtained by counting with the Lynxeye detector, for every 0.02° at sweep rates of 0.5° 2θ/min The XRD spectra of the bare paper (green), shows that the peaks at 2θ = 12.6° and Fig XRD spectra of substrate, uncured PVDF, cured PVDF at 2θ = 26° are related to the paper substrate The peak at 90°C and 130°C (Inset: XRD spectra in the range of 18.5° - 21.5°) 19.9° is assigned to the β-phase crystallinity of PVDF The inset shows the XRD spectra of uncured PVDF, cured PVDF at 90 °C and 130 °C, at 2θ = 19.9° It was observed that the higher curing temperature of 130 °C resulted in a higher intensity β-phase formation, when compared with the 90 °C curing process The XRD based results thus confirm that the printed PVDF layer have a predominantly β-phase structure A Bruker vertical scanning interferometer microscope (CounterGT) was used to characterize the thickness and roughness of the conductive bottom electrode as well as the piezoelectric layer An average thickness of 9.3 μm was measured for both the Ag electrode and PVDF layer The root mean square (rms) roughness of the Ag electrode and PVDF layer were measured to be 0.9 μm and 0.6 μm, respectively The touch sensor was placed on the base compression plate of a motorized test stand (Mark-10 ESM 301) A digital force gauge (Mark-10 M5-200), with a vertically movable rubber-tip attachment, was used to apply varying forces ranging from 0.2 N to 1.4 N, in steps of 0.2 N The force gauge data was collected using the MESUR™ gauge software An oscilloscope was used for acquiring and for post-processing of the voltage response Results and Discussion Fig 3(a) shows the response of a single sensor in the 4×4 array while it was repeatedly touched by a human finger Voltage peaks, as high as 0.22 V, were observed due to the forces applied Variations in the voltage peak amplitudes were observed due to inconsistency in the forces applied by the finger However, it is worth noting that the voltage response of the fabricated sensor is comparable to touch sensors fabricated using conventional silicon technology on non-flexible substrates [15, 18] The response of the printed touch sensor was also tested by applying varying force loads, ranging from 0.2 N to 1.4 N, in steps of 0.2 N Fig 6(b) shows the voltage response of the sensor for the different applied forces It was (a) (b) Fig (a) Voltage response of the touch sensing device towards repeated finger touches and (b) Piezoelectric based voltage response of the paper based printed device towards varying applied forces Sepehr Emamian et al / Procedia Engineering 168 (2016) 688 – 691 observed that the paper based touch sensor response was linear with a sensitivity and correlation coefficient of 0.3 V/N and 0.9859, respectively Even though the responses obtained for both the simulation and practical measurements were linear in nature, the difference in the voltage amplitudes can be attributed to the different structural characteristics used in the simulated and fabricated devices The results obtained thus demonstrate that the printed sensor can be used not only as a touch sensor but also as a force sensor Conclusion In this work, a screen printed piezoelectric based touch sensor on paper substrate was successfully fabricated The device was fabricated using Ag and PVDF inks as the metallization and piezoelectric layers, respectively Characterization of the various printed layers of the touch sensor was performed The proper formation of the βphase crystals in the cured PVDF film was investigated using XRD measurements A sensitivity of 0.3 V/N, with correlation coefficient of 0.9859, was obtained for the printed touch sensor Piezoelectric-voltage analysis demonstrated that the printed sensor can be used as both touch and force sensors The advantage of fabricating touch sensors on flexible substrates is the ability to fold and place the sensors on nearly any platform or to conform 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sensor on paper substrate was successfully fabricated The device was fabricated using Ag and PVDF inks as the metallization and piezoelectric layers, respectively Characterization