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Sensors and Actuators A 165 (2011) 2–7

Contents lists available at ScienceDirect

Sensors and Actuators A: Physical

journa l homepage: www.e lsev ier .com/ locate /sna

apacitive tactile sensor array for touch screen application

ong-Ki Kim, Seunggun Lee, Kwang-Seok Yun ∗

epartment of Electronic Engineering, Sogang University, 1 Shinsu-dong, Mapo-gu, Seoul 121-742, Republic of Korea

r t i c l e i n f o

rticle history:vailable online 13 January 2010

a b s t r a c t

In this paper, we propose and demonstrate a transparent and flexible capacitive tactile sensor which is

eywords:actile sensorlexible deviceouch screenulti-touch

apacitive sensor

designed for multi-touch screen application with force sensing. A sensor module is composed of 2D arraytactile cells with a spatial resolution of 2 mm to measure the touch force at multiple positions. The deviceis fabricated by using transparent materials on a transparent plastic substrate. The optical transmittanceof the fabricated tactile sensor is approximately 86% in the visible wavelength region, and the maximumbending radius is approximately 30 mm. The cell size is 1 mm × 1 mm, and the initial capacitance of eachcell is approximately 900 fF. The tactile response of a cell is measured with a commercial force gaugehaving a resolution of 1 mN. The sensitivity of a cell is 4%/mN within the full scale range of 0.3 N.

© 2010 Elsevier B.V. All rights reserved.

. Introduction

A touch screen is a display that can detect the presence and loca-ion of a touch on a display area. Currently, touch screens, becausehey provide very intuitive user interfaces, are widely used notnly in computer systems in the industry but also in hand-heldevices such as mobile phones, PDAs, and car navigation systems.he important characteristics of a touch screen that is used as aisplay include transmittance, resolution, resistance to surface con-amination, durability (lifetime), multi-touch recognition, displayize, and force sensing. Among these characteristics, multi-touchecognition, which has recently been incorporated in several mod-ls of mobile phones and portable electronic devices, enables aser to interact with a system by simultaneously using multiplengers. As will be discussed briefly in this paper, it has been diffi-ult to apply multi-touch recognition to most classical touch screenechnologies.

Various sensing technologies have been developed using diversepproaches, and they are widely used in commercial productssing touch screens. Resistive [1], capacitive [2], optical using

nfrared (IR) [3], and acoustic using surface acoustic wave (SAW)4] detection methods have been used in most conventional touchcreens. However, these types of touch screens recognize only

single touch point. There are several technologies for multi-ouch recognition. The patterned capacitive-type touch screenonsists of transparent row and column electrode arrays embeddedithin some insulating material [5,6]. This arrangement moni-

∗ Corresponding author. Tel.: +82 2 705 8915; fax: +82 2 705 8915.E-mail address: ksyun@sogang.ac.kr (K.-S. Yun).

924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.sna.2009.12.031

tors the change in capacitance that occurs at the point on thescreen where a finger is placed. Han reported multi-touch sens-ing on rear-projected interactive screens based on the frustratedtotal internal reflection technique, which required a video cam-era to monitor the finger locations [7]. The above-mentioned touchscreen technologies are well-adopted to a flat panel display. How-ever, nowadays, many studies have reported on flexible displays,because the flat panel display using a glass substrate is fragileand difficult to carry [8]. To be utilized in a flexible display, thetactile sensor for a touch screen should also exhibit flexibility.Therefore, in this work, a transparent and flexible tactile sensorhas been designed for a multi-touch screen application. In addi-tion, we are aiming at developing a touch sensor capable of forcesensing in order to discriminate among different levels of touchstrength.

In fact, touch sensors with force sensing have been researchedfor the last few years as tactile sensors mainly for artificialskin for robot applications [9,10], minimally invasive surgery[11,12], wearable computers [13], and mobile or desktop hap-tic devices [14]. Four popular pressure-sensing mechanismsfor tactile sensors have been reported: resistive, piezoresistive,piezoelectric, and capacitive-sensing mechanisms. In resistive sen-sors, a resistance change induced from the resistive materialsqueezed between electrodes is measured [15]. A piezoresis-tive sensing mechanism uses a strain gauge to measure thedeformation of a tactile cell [16]. A piezoelectric mechanism

measures the accumulation of charges and the resulting volt-age buildup as a membrane is forced. However, a piezoelectricsensor cannot detect static force [17]. A capacitive-sensingmechanism measures the capacitance change induced by thechange in the gap between the electrodes [9]. However, most

nd Actuators A 165 (2011) 2–7 3

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f these devices are not suitable for touch screen display sys-ems because of the non-transparency of the materials they are

ade of. In order to meet the requirement for tactile sensorsor multi-touch screens for flexible display applications, we haventroduced a capacitive tactile sensor array constructed witholycarbonate (PC) films and indium–zinc-oxide (IZO) electrodesor flexibility and transparency. In this paper, we present theoncept, fabrication, and experimental results of our sensor inetail.

. Design

Fig. 1 shows the cross-sectional view and the dimension of anit cell of the proposed tactile sensor array. The upper and bottomubstrates are transparent PC films with a thickness of 120 �m. Ahin transparent IZO layer was used as the electrodes and the signalines. The two electrodes formed a capacitor separated by a distancef 13 �m by SU-8 spacers. The cell size and electrode size weremm × 2 mm and 1 mm × 1 mm, respectively. The capacitance of aell can be expressed as

= 1(t /ε A) + (t /ε ε A)

, (1)

a 0 d d 0

here ε0 is the permittivity in free space, εd is the relative per-ittivity of the SU-8 insulation layer, ta is the air-gap distance,

d is the thickness of the SU-8 insulator layer, and A is the elec-rode area. The initial capacitance of a cell was estimated to be

Fig. 2. Center deflection (solid line: calculated, dashed line: simulati

Fig. 1. Cross-sectional view of a tactile cell and its dimensions.

926 fF using Eq. (1) assuming that the relative permittivity of SU-8 was 3.2. When a touch pressure was applied on the surface ofthe upper plate, the gap between the two plates decreased and thecapacitance increased until the gap was closed. By measuring thecapacitance for all the capacitive array cells, we could determinethe touch position and the applied force on multiple locations.

The membrane deflection and resultant capacitance change asthe touch force applied must be considered for a capacitive celldesign. These factors were examined by the finite element method(FEM) simulation for a capacitive cell with the dimensions given

on) and capacitance (dash–dot line) for various applied forces.

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Fig. 3. Fabrication processes of the proposed tactile sensor: (a) electrode layer for-mation, (b) spacer formation on top plate, (c) electrode layer formation on bottomplate, (d) insulation layer coating, and (e) the completed device after bonding pro-cess.

Fig. 4. Fabricated tactile sensor: (a) flexibility and (b) magnified view of touch sen-sor.

uators A 165 (2011) 2–7

in Fig. 1 using a COMSOL multiphysics simulator (COMSOL Inc.).Fig. 2(a) shows two examples of simulation results: the initial sta-tus with zero touch force (left) and with the touch force of 68 mNat a point where the upper plate just begins to touch the bottomplate (right). Fig. 2(b) shows the center deflection and the resultingcapacitance for various applied forces. The solid line is the centerdeflection versus the applied force, and the dashed line is the capac-itance change. The initial capacitance was estimated to be 938 fF,which was close to the calculated value of 926 fF. The upper platebegan to touch the bottom plate when the applied force was 68 mN;the capacitance at this point was approximately 3.4 pF. Anotherimportant factor that must be considered is mechanical responsetime of the cell membrane because slow response time will result inafterimage lag on display. The calculated and simulated resonancefrequency of the designed membrane is about 21.5 kHz which isfast enough comparing with 60 Hz, a general refresh time of displaypixel.

3. Fabrication

In our design, we used transparent PC films as structural mate-rials, SU-8 (Microchem Co.) as spacers and an insulator, and an IZOthin film as electrodes. The fabrication process is shown in Fig. 3.Each layer was processed separately and bonded together usingSU-8 as the bonding material, which was also used as an insula-

tor and a spacer. We used an IZO-coated PC film. The thickness ofthe PC film and the IZO thin film was 120 �m and 130 nm, respec-tively. For photolithography, the films were mounted on a siliconwafer. For the top plate, first, the IZO layer was patterned (Fig. 3(a))using general photolithography and wet etching. The solution with

Fig. 5. (a) Optical transmittance of tactile sensor measured with spectrophotometerand (b) tactile sensor on LCD display of mobile phone.

nd Actuators A 165 (2011) 2–7 5

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decrease in visibility.We set up custom-made equipment for touch force char-

acterization. Fig. 6(a) displays our setup for the contact force

H.-K. Kim et al. / Sensors a

ydrochloric acid and nitric acid (HCl:HNO3 = 3:1) was used as anZO wet etchant, and the etch time was approximately 5 s at roomemperature. Next, SU-8 2007 (Microchem Co.) was spin-coated toave a thickness of 13 �m and patterned to form spacers on theop substrate (Fig. 3(b)). As the bottom plate, the IZO layer was pat-erned again for the bottom electrode (Fig. 3(c)). Then, a thin SU-8005 was spin-coated to have a thickness of 5 �m, forming an insu-

ation layer between the top and the bottom electrodes (Fig. 3(d)).ext, the top substrate was aligned with the bottom substrate, andressure was applied at room temperature (Fig. 3(e)). Then, the twoubstrates that were bonded together were heated on a hot platet 95 ◦C for 1 min to cure the thin SU-8 layer. Finally, the SU-8 wasardened after UV exposure and post-exposure bake at 95 ◦C formin.

Fig. 4 shows the fabricated tactile sensor. The initial deviceas designed to have 20 × 20 capacitive cells, and the size of

he entire sensor module was 6 cm × 6 cm, including the intercon-ection pads. The fabricated sensor exhibited good flexibility, ashown in Fig. 4(a). Fig. 4(b) shows the magnified view of the fab-icated tactile sensor. The overlap area of each capacitive cell wasmm × 1 mm, and the diameter of SU-8 spacer was 200 �m.

. Experimental results

The transparency of the fabricated tactile sensor was measuredsing a UV/Visible spectrophotometer (SCINCO). The average trans-ittance was approximately 86% in a visible light range from

80 nm to 770 nm, as shown in Fig. 5(a). We placed the tactile sen-

ig. 6. (a) Measurement setup for single tactile cell characterization and (b)chematic representation of readout circuits for the fabricated sensor module.

Fig. 7. Measured response (solid line) and simulation result (dashed line) of thefabricated cell for various touch forces.

sor on top of the LCD display of a commercial mobile phone to testthe visibility, and as seen in Fig. 5(b), there was no interference or

measurement. A force gauge with a tip was used to precisely

Fig. 8. (a) Photograph of rubber stamps and their touch images captured by thefabricated tactile sensor module and (b) multi-touch tactile images captured fromthe fabricated sensor. Areas of two neighboring unit cells are designated with redand blue dashed squares. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of the article.)

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pply pressure on a specific capacitive cell. The end of the tipas treated to have a flat rectangular shape with the dimension

f 1 mm × 1 mm, and the force gauge from AIKOH Engineeringo. had a force resolution of 1 mN. Fig. 6(b) shows the custom-esigned readout circuitry. First, each tactile cell was selected by aow decoder and reset. Then, it was charged to Vstep. When theell was selected by a column decoder, the stored charge wasransferred to the feedback capacitance (Cf) and generated out-ut voltage, as given in the equation. The signal was processedy a custom-designed field programmable gate array (FPGA) chip,nd the final image was displayed by LabVIEW (NI). In order toemove the offsets from the circuit, we designed the circuit toead a single cell twice with and without resetting the feedbackapacitance.

Fig. 7 shows the measured response of the fabricated cell for var-ous touch forces. Further, the FEM simulation result is depicted asdashed line in this figure for the sake of comparison. A single cellas pressed by using a micro-force gauge with a tip, as shown in

ig. 6(a). All experimental data are the averaged value of 10 mea-urements on different cells and standard deviation is less than.7%. The initial capacitance of a cell was measured to be approx-

mately 900 fF, which was close to the theoretical value of 926 fFbtained from Eq. (1) and the simulated value of 938 fF. The capac-

tance increased linearly with the applied force before 0.1 N andaturated after that pressure, which implied that both the uppernd the bottom electrodes were in contact with the insulation layeretween them. Moreover, we can see that the experimental results

ig. 9. (a) Tactile sensor attached on cylindrical structure with radius of curvaturef 30 mm and (b) movement of touch points on screen according to finger motion.

uators A 165 (2011) 2–7

adequately followed the simulation result before saturation.Multi-touch tactile images captured from the fabricated sen-

sor are shown in Fig. 8. Pressure was applied by using a rubberstamp with the letter “T” on it, and the corresponding imagewas clearly captured in Fig. 8(a). Further, several point imagesaccording to various touch pressures are seen in Fig. 8(b) andthe areas of two neighboring units cells in post processing dis-play program are designated with the red and blue dashed squaresin this figure. The program was designed to change the both ofdarkness and size of color in a cell area. In this experiment, wefirst applied pressures on different locations at the same timeusing several tips and capacitance values of each cell are mem-orized. To find the pressure values on each cell giving recordedcapacitance values, we applied pressure on each cell using forcegauge with sharp tip. We can clearly see that the brightness andsize of the point images increased in proportion to the touchpressure.

Sliding experiments on a curved surface were also performed,and their results are shown in Fig. 9. The tactile sensor was attachedon a cylindrical structure with the radius of curvature of 30 mm, asshown in Fig. 9(a). Two fingers touched two different points onthe tactile sensors and moved on the surface with slight pressure.The sliding speed of the fingers was approximately 2 cm/s. Fig. 9(b)shows that the touch points on the screen satisfactorily follow thesliding of the fingers on the curved tactile sensor.

5. Conclusions

In this study, a new flexible and fully transparent tactile sen-sor for touch screen applications was proposed and successfullydemonstrated. A sensor module consisted of a 20 × 20 tactile cellarray with a spatial resolution of 2 mm. The fabricated tactile sen-sor module exhibited good flexibility with a maximum radius ofcurvature of 30 mm and captured multi-touch images. The cell tocell variation of capacitive response was measure as 6.7%. Eventhough the proposed tactile sensor modules need more optimiza-tion in the design and fabrication to increase the uniformity, theycan be a good candidate for touch screens for flexible display in thefuture with their flexibility, transparency, and capability for forcesensing.

Acknowledgements

This work was supported by the IT R&D program of MKE[2009-F-024-02, Development of Mobile Flexible IOP Platform], theNational Research Foundation of Korea (NRF) grant funded by theKorea government (MEST) [2009-0076641] and a research grantfrom Sogang University in 2008.

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[3] R.W. Doering, Infrared touch panel, US Patent 4868912 (1989).[4] R. Adler, P.J. Desmares, An economical touch panel using SAW absorption, IEEE

Trans. Ultrason. Ferroelectr. Freq. Control 34 (1987) 195–201.[5] H. Philipp, Capacitive sensor and array, US Patent 6452514 (2000).[6] S. Hotelling, J.A. Strickon, B.Q. Huppi, Multipoint touchscreen, US Patent

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tion, UIST’05, October 23–26, 2005, pp. 115–118.[8] P. Mach, S.J. Rodriguez, R. Nortrup, P. Wiltzius, J.A. Rogers, Monolithically

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Biographies

Hong-Ki Kim received his BS degree in Physics from Chungnam National Universityin 2007, and his MS degree in Electronic Engineering from Sogang University in2009. He joined the LATTRON Co., Ltd, Korea in 2009. His research area includesBio-MEMS and Ceramic Device.

Seunggun Lee received his BS degrees in Electronic Engineering from Sogang Uni-versity in 2009. He is currently pursuing his MS degree in Electronic Engineeringfrom Sogang University. His research area includes Touch Sensors.

Kwang-Seok Yun received his BS degree in Electronics Engineering from Kyung-pook National University in 1996, MS and PhD degrees in Electrical Engineering

and Computer Science from Korea Advanced Institute of Science and Technology(KAIST) in 1997 and 2002, respectively. He was a post-doctorial researcher at Uni-versity of California, Los Angeles from 2005 to 2007. He joined the Department ofelectronic Engineering at Sogang University, Korea in 2007, where he is now anAssistant Professor. His current research area includes micro total analysis systems,Lab-on-a-chip, MEMS, and micro sensors and actuators.