Scholarly article on topic 'A Flexible Polymer Sensor for Light Point Localization'

A Flexible Polymer Sensor for Light Point Localization Academic research paper on "Materials engineering"

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Procedia Engineering
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{"Light point localization" / "optothermal sensor" / "large area sensor" / "flexible electronics" / polymer / PVDF / pyroelectric}

Abstract of research paper on Materials engineering, author of scientific article — G. Buchberger, P. Bartu, R. Schwödiauer, B. Jakoby, W. Hilber, et al.

Abstract We present a flexible polymer sensor for light point localization based on the pyroelectric polymer poly(vinylidene fluoride) (PVDF) in combination with large area resistive electrodes. This large area sensor is simple in design; a division of the sensor surface into an array of individual sensor elements as used in active matrix technology is avoided. Electronic circuitry is placed only at the edges of the device. We have fabricated a flexible sensor strip with a length of 12cm and a width of 1.6cm according to the proposed concept. The position sensitive output signals of the sensor strip have been measured in response to the intensity-modulated light of a red laser diode. Two model-based normalization procedures have been developed in order to increase the quality of the measurement signals. The results indicate that the sensor strip can be treated mathematically analogous to a transmission line. Calculations based on the solutions of the telegrapher's equations under the appropriate boundary conditions are in good agreement with the experimental results, proving the suitability of the proposed concept for position sensitive detection.

Academic research paper on topic "A Flexible Polymer Sensor for Light Point Localization"

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Procedía Engineering

ELSEVIER

Procedía Engineering 47 (2012) 795 - 800

www.elsevier.com/locate/proeedia

Proc. Eurosensors XXVI, September 9-12, 2012, Krakow, Poland

A Flexible Polymer Sensor for Light Point Localization

G. Buchbergera' *, P. Bartub, R. Schwödiauerb, B. Jakobya, W. Hilbera, and

S. Bauerb

aInstitute for Microelectronics and Microsensors, Johannes Kepler University Linz, 4040 Linz, Austria bDepartment of Soft Matter Physics, Johannes Kepler University Linz, 4040 Linz, Austria

Abstract

We present a flexible polymer sensor for light point localization based on the pyroelectric polymer poly(vinylidene fluoride) (PVDF) in combination with large area resistive electrodes. This large area sensor is simple in design; a division of the sensor surface into an array of individual sensor elements as used in active matrix technology is avoided. Electronic circuitry is placed only at the edges of the device. We have fabricated a flexible sensor strip with a length of 12 cm and a width of 1.6 cm according to the proposed concept. The position sensitive output signals of the sensor strip have been measured in response to the intensity-modulated light of a red laser diode. Two modelbased normalization procedures have been developed in order to increase the quality of the measurement signals. The results indicate that the sensor strip can be treated mathematically analogous to a transmission line. Calculations based on the solutions of the telegrapher's equations under the appropriate boundary conditions are in good agreement with the experimental results, proving the suitability of the proposed concept for position sensitive detection.

© 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp.z.o.o.

Light point localization, optothermal sensor, large area sensor, flexible electronics, polymer, PVDF, pyroelectric

1. Introduction

Whereas the technology of microelectronics has been advancing by miniaturization, in parallel the technology of macroelectronics has been developed with the objective to enlarge the size of electronic surfaces [1]. Flexible or even stretchable, large-area position sensors reacting to pressure, touch, temperature changes or incident light have been demonstrated [2-11]. Most sensors employ active matrix

* Corresponding author. Tel.: +43 732 2468 6260 ; fax: +43 732 2468-6252 . E-mail address: gerda.buchberger@jku.at.

1877-7058 © 2012 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the Symposium Cracoviense Sp. z.o.o.

doi: 10. 1016/j .proeng.2012.09.267

cells to locate the stimulus on the sensor surface [2-6], whereas a few concepts avoid the division of the sensor surface into a large number of individual sensing elements [7-11]. In [7] a flexible, large-area ferroelectret sensor for location sensitive touchpads without active matrix technology was fabricated and characterized experimentally. In [9-11] similar configurations are used to determine the position of a light point and of an applied force on a large area organic photodiode or on stretchable waveguides. The base polymers for the waveguides in [9-11] comply well with the requirements of low cost and simple fabrication technologies in macroelectronics. The pyroelectric properties of another polymer, namely poly(vinylidene fluoride) (PVDF), are for example used for an active matrix sensor network in a touchless control interface [4,6]. PVDF in the ^-phase is a ferroelectric polymer with pyroelectric and piezoelectric coefficients of p3 « -25 |aC/(m2 K) and d33 « 20 pC/N, which compare well to those of the more familiar inorganic ferroelectrics [12-14]. Ferroelectric PVDF is available as highly flexible and transparent films, which have been used for transducers [15,16].

In this contribution we propose a concept of flexible polymer sensors for light point localization without active matrix technology based on pyroelectric PVDF films together with large area electrodes. According to the proposed concept, we have fabricated a flexible sensor strip with a length of 12 cm and a width of 1.6 cm from a 110 |am thick PVDF film. The fabricated PVDF sensor strip has been characterized experimentally by the position-sensitive output signals of the sensor in response to the intensity-modulated light of a red laser diode. Furthermore, a description of the sensor strip by the solutions of the telegrapher's equations under the appropriate boundary conditions has been developed. We introduce two model-based normalization procedures by which the quality of the measurement signals can be enhanced significantly. The measurement data and the theoretical results are in good agreement.

2. Proposed Concept of a Flexible Sensor Strip for Light Point Localization

We propose a concept of flexible, large area polymer sensors for light point localization without matrix technology. The design of a polymer sensor strip according to the proposed concept is depicted in Fig. 1(a). The sensor strip consists of a film of the pyroelectric polymer PVDF, which is covered by large area electrodes. The top electrode is highly conductive and grounded, whereas the bottom electrode is highly resistive. Electrical ports are fixed to both ends of the sensor, which are connected to the input channels of measurement devices.

m-t1.0

u(i2,t)

polyfvinylidene fluoride)

high resistance electrode

I—| 1 cm

/,(-/,,o w/,

1 1 IT---1

C'dz R'dz I R dz

— -y *m " -T-

/,(-/,.0 /,+ctf, /,(0.0 yo.f)

Z, M-M) 1,+dl, /,(0,0 I'" 1 /2(/2,() Z2

equivalent circuit diagram

Fig. 1. (a) Scheme of the flexible polymer sensor for light point localization. (b) Equivalent circuit diagram of the transducer (as resistances and capacitances dominate, inductances and conductances are not shown).

If incident light causes temperature changes in the PVDF film, electrical charges will be generated on the electrodes by the pyroelectric effect. The corresponding electrical signals are measured at both ends of the device. The signals have different absolute values depending on the distances between the position of the stimulus and the electrical ports. Therefore the stimulus can be localized on the sensor surface. For a more defined localization procedure, the beam of the incident light is intensity-modulated. If the system has reached the steady state regime, the signals at the ends of the device will oscillate with the reference frequency; the oscillating signals can be measured by phase sensitive detectors.

2. Theoretical: Modeling of the Pyroelectric Sensor Strip for Light Point Localization

The proposed pyroelectric sensor strip serves as a one dimensional model system for sheet-type large area sensors (see Fig. 1(a)). The sensor strip is treated mathematically analogous to a two wire transmission line, where the two large area electrodes on the PVDF film correspond to the wires of the transmission line and the polymer film to the dielectric medium which separates the wires. Considering an elementary small section dz of the line, a quasi-stationary treatment will be sufficient. The transmission line is modeled as an infinite series of elementary sections (see Fig.1(b)). Per unit length the sensor strip has an equivalent inductance L', a capacitance C', a total equivalent resistance R', as well as an equivalent leakage conductance between the electrodes G'. Since C and R' are dominant in the fabricated device, the other quantities are not shown in Fig. 1(b). The stimulus i.e. the incident light is modeled as a current source /m (t). According to the transmission line theory [17] two coupled partial differential equations, the so-called telegrapher's equations, describe the voltage and current distributions V(z,t) and I(z,t) along the sensor strip with regard to the distance z and to the time t

-8V(z,t)/fe = L'dI(z,t)/dt + R'I(z,t) a -81(z,t)/dz = C'dV(z,t)/dt + GV(z,t). (1) These coupled differential equations can be solved by transforming them into the frequency domain

d2Va{z)/dz2-r*Va(z) = 0 A a2i„(z)/&2-xX(z) = 0. (2;

In (2) the voltage and current distributions along the sensor strip in the frequency domain are V (z) and Ia (z). The propagation constant ym in (2) is defined as ya =^J(R' + iaL') (G' + irnC') . The distances

between the stimulus and the left and the right end of the sensor strip are denoted by A and l2, respectively. Since we measure the absolute values of the voltage signals at both ends of the sensor strip, the solution of (2) under the appropriate boundary conditions has to be evaluated at l1 and l2 in order to compare the theory with the experiment

V^K Hlm,.;W2){e-l>(1 - - W"2))/(1 - W^22)] 3

V%m{l2 ) = (! m,^/2).[e--l2(1 - - W") " s^W"2)]- (4)

In (3) and (4) the reflection coefficients zwim for i = 1, 2 and the characteristic wave impedance Z0 a are gven by z^ =(Zojffl - Zt )j(Z0a + Z,) and Zo,M =J(R' + imL'^(G' + '') . The light of the incident laser beam is intensity-modulated. Therefore it heats up the pyroelectric PVDF film with the

frequency a>0. As a consequence the pyroelectric current source in the steady state regime is given by 7m = /m>OTo • exp(/®0 t). Transforming (3) and (4) back into the time domain results in

for the measured voltage signals at the ends of the sensor strip. The absolute values of the voltage signals V1>ffl0 and V2,ot0 in (5) and (6) are proportional to the magnitude of the pyroelectric current 7m,OT0.

3. Experimental: Materials, Fabrication, Measurement Setup and Comparison between the Measurement Data and the Theoretical Results

In order to proof the proposed concept of a flexible large area sensor for light point localization, we have fabricated a sensor strip with a length of 12 cm, a width of 1.6 cm and a thickness of around 6.1 mm (see Fig. 1(a)). A strip of a 110 |am thick PVDF film was covered by large electrodes, one of which is highly resistive, while the other one is conductive and grounded. An approximately 100 nm thick aluminum layer serves as conductive electrode; this electrode is prepared by evaporation. An approximately 6 mm thick antistatic mat forms the highly resistive electrode; it is glued onto the PVDF film by an epoxy adhesive. At both ends of the sensor strip the resistive electrode is connected to a lock-in amplifier. The input impedances of the lock-in amplifiers Z1 and Z2 are parallel circuits with resistances Ruj = Rli,2 = 10 MO and capacitances CLI,i = 30 pF and CLI,2 = 25 pF, respectively.

The sensor strip is fixed on an x-y translation stage. An intensity-modulated beam of a red laser diode is used for the thermal excitation of the film. The laser beam is positioned in steps of one millimeter distance along the axis of the sensor strip. The heating of the PVDF film by the diode with intensity modulated light causes pyroelectric signals of the frequency a>0 as shown in Fig. 2(a). At each point the absolute values of the corresponding output voltages at the ends of the strip are measured by the lock-in amplifiers. Fig. 2(a) shows the measured absolute voltage values. The measurement data is fitted according to (5) and (6). As fit parameters m0 = 45 Hz and 7in,OT0 = 25 ^A are used. The characteristic parameters of the sensors strip are C'PVDF = 13 nF/m, L'PVDF = 8.7 nH/m, G'PVDF « 0, R'aluminum = 17 Q/m, and R'antistatic mat = 2.8 GQ/m. At low frequencies the inductance per unit length can be neglected compared to the other quantities, which can be seen from the above expressions for the propagation constant and the characteristic wave impedance. By normalization procedures the influence of the space dependent inhomogeneity of the generated pyroelectric current 7m,ra0 can be reduced. This inhomogeneity is mainly due to the variations in the reflectivity of the aluminum electrode, but also due to the inhomogeneous polarization of the PVDF film. Two different model-based normalization procedures have been developed, by which the pyroelectric current can be canceled out of the voltage signals in (5) and (6). The normalized values are depicted in Fig. 2(b) and Fig. 2(c). The normalized data shows that position sensitive detection of the light beam is possible by measuring the pyroelectric response without knowing the intensity of the stimulus.

As the graphs demonstrate, the quality of the measurement signals is significantly increased by the introduced normalization procedures. The first normalization procedure involves the division of the measured absolute value at each end of the sensor strip by the sum of the absolute values at both ends of the sensor strip (see Fig. 2(b))

^ (-/1 )= (-/1 VfK, H )|+ K, (¡2 )|)a )= K, VfK, ("l1 )|+ K, (¡2 )|). ^

VU ("¡1. t )= ^ tZ0„/2 / - ^^ ^ )(l - ^^ ^ -11 - ^^ V^ (¡2,t) = 4,0 0 /2 {-l (1 - zw20 - e1H - ■

e"2 ')]

ÇT 150

2 4 6 8 Position (cm)

20 • 10

0.5 0.2 0.1

■ v :

4 6 Position (cm)

0 2 4 6

Position (cm)

Fig. 2. (a) Measured and calculated absolute voltage values at the left end of the sensor strip (red) and at the right end of the sensor strip (blue) versus position of the light point (&>0 = 45 Hz and /m,®0 = 25 pA). (b) By normalization procedures the influence of the space-dependent inhomogeneity of the generated pyroelectric current /m,®0 can be reduced. This inhomogeneity is due to the variations in the reflectivity of the aluminum electrode and in the polarization of the PVDF film. (c) If the right normalization procedure is applied, the measurement values will depend nearly linearly on the position of the light point on the sensor surface.

The second normalization procedure involves the division of the measured absolute value at each end of the sensor strip by the absolute value at the other end of the sensor strip (see Fig. 2(c))

¿L,nH)=R(4)| A (4)=K^ (4R)|. 8

With the help of the second normalization procedure the normalized absolute voltage values depend approximately linearly on the position of the light point on the sensor surface.

4. Conclusions and Outlook

We have proposed a concept of a flexible, large area sensor for light point localization without the division of the sensor surface into individual sensing elements. To this end, we use the pyroelectric properties of the soft polymer PVDF together with flexible large area resistive electrodes. We have fabricated a sensor strip according to the proposed concept. This sensor strip serves as a one dimensional model system and is studied both theoretically and experimentally. The theoretical results and the measurement data are in good agreement. With the help of the developed theoretical description, modelbased normalization procedures could be found which significantly enhance the quality of the measurement signals. The fabricated device allows for flexible, large area sensor surfaces for interactive human machine interfaces. Due to the used materials these interfaces are low-cost and light-weight. As a next step we want to fabricate flexible as well as transparent sensors based on the proposed concept. Therefore we will use PVDF films combined with polymer electrodes. Since PVDF films can be as thin as

around 5 |am, ultrathin, sheet-type sensors for light point localization can be fabricated. These sensors can be placed on top of flexible organic displays for interactive screens for presentations.

Acknowledgements

The research was funded partially by the Austrian Research Promotion Agency (FFG) under the contract number 825348/K-Licht.

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