Capacitive tactile sensor array for touch screen application

Sensors and Actuators A 165 (2011) 2–7

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Sens Sensor orss and and Actua ctuato tors rs A: Phys Physic ical al j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / s n a

Capacitive tactile sensor array for touch screen application Hong-Ki Kim, Seunggun Lee, Kwang-Seok Yun ∗ Department of Electronic Engineering, Sogang University, 1 Shinsu-dong, Mapo-gu, Seoul 121-742, Republic of Korea

a r t i c l e

i n f o

Article history: Available online 13 January 2010 Keywords: Tactile sensor Flexible device Touch screen Multi-touch Capacitive sensor

a b s t r a c t

In this paper, we propose and demonstrate a transparent and flexible capacitive tactile sensor which is design designed ed formulti-to formulti-touchscree uchscreen n applic applicati ation on with with force force sensin sensing. g. A sensormodul sensormodule e is compos composed ed of 2D array array tactil tactile e cells cells with with a spatia spatiall resolu resolutio tion n of 2 mm to measur measure e thetouchforce at multip multiple le positi positions ons.. Thedevice Thedevice is fabricated fabricated by using transparen transparentt materials materials on a transpare transparent nt plastic plastic substrate substrate.. The optical optical transmitta transmittance nce of the fabric fabricate ated d tactil tactile e sensor sensor is approx approxima imatel tely y 86%in thevisible thevisible wavele wavelengt ngth h region region,, and and the maximu maximum m bendin bending g radius radius is approx approxima imatel tely y 30mm. Thecell size size is 1 mm × 1 mm,and the initia initiall capaci capacitan tance ce of each each cell is approxima approximately tely 900 fF. The tactile tactile response of a cell is measured measured with a commercial commercial force gauge having 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.

1. Introducti Introduction on

A touch touch screenis screenis a displa display y that that candetect candetect thepresenceand thepresenceand localocation of a touch on a display area. Currently, touch screens, because they provide very intuitive user interfaces, are widely used not only in computer systems in the industry but also in hand-held devices such as mobile phones, PDAs, and car navigation systems. The important characteristics of a touch screen that is used as a display display include include transmitt transmittance,resoluti ance,resolution, on, resistanceto resistanceto surface surface contamination, durability (lifetime), multi-touch recognition, display size, and force sensing. Among these characteristics, multi-touch recogn recogniti ition, on, which which has recent recently ly been been incorp incorpora orated ted in severa severall modmodels of mobile phones and portable electronic devices, enables a user to interact with a system by simultaneously using multiple fingers. As will be discussed briefly in this paper, it has been difficult cult to apply apply multimulti-tou touch ch recogn recogniti ition on to mos mostt classi classicaltouchscreen caltouchscreen technologies. Various Various sensing sensing technologieshave technologieshave beendevelopedusing diverse diverse approaches, approaches, and they are widely widely used in commercia commerciall products products using touch screens. screens. Resistive Resistive [1] [1],, capacitive [2] [2],, optical optical using infrared (IR) [3] (IR) [3],, and acoustic using surface acoustic wave (SAW) [4] d [4] detection etection methods have been used in most conventional touch screens. screens. However, these types of touch screens screens recognize recognize only a single single touch point. There are several several technologies technologies for multitouch recognitio recognition. n. The patterned patterned capacitive-typ capacitive-type e touch screen consists consists of transparen transparentt row and column column electrode electrode arrays arrays embedded embedded within some insulating insulating material material [5,6]. [5,6]. This arrangemen arrangementt monimoni-

∗

Corresponding Corresponding author. Tel.: +82 2 705 8915; fax: +82 2 705 8915. E-mail address: [email protected] (K.-S. [email protected] (K.-S. Yun).

0924-4247/$ 0924-4247/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2009.12.031 doi:10.1016/j.sna.2009.12.031

tors the change in capacitance that occurs at the point on the screen where a finger is placed. Han reported multi-touch sensing on rear-proje rear-projected cted interactive interactive screens based on the frustrated frustrated total internal reflection technique, which required a video camera to mon monito itorr the finger finger locati locations ons [7] [7].. The above-ment above-mentioned ioned touch screen technologies are well-adopted to a flat panel display. However, nowadays, many studies have reported on flexible displays, because the flat panel display using a glass substrate is fragile and difficult to carry [8] [8].. To be utilized in a flexible display, the tactile tactile senso sensorr for a touch screen should also exhibit flexibility. flexibility. Therefore, in this work, a transparent and flexible tactile sensor has been designed for a multi-touch screen application. In addition, we are aiming at developing a touch sensor capable of force sensing in order to discriminate among different levels of touch strength. In fact, touch sensors with force sensing have been researched for the last last few years as tactil tactile e sen sensor sorss mainly mainly for artific artificial ial skin for robot robot applicati applications ons [9,10], [9,10], minimally minimally invasive invasive surgery surgery [11,12],, wearable computers [13], [11,12] [13], and mobile or desktop haptic device devicess [14]. [14]. Four popular popular pressure-s pressure-sensing ensing mechanisms mechanisms for tactile tactile senso sensors rs have been reported: resistive, resistive, piezoresist piezoresistive, ive, piezoelect piezoelectric, ric, and capacitive capacitive-sensi -sensing ng mechanisms mechanisms.. In resistive resistive sensors, sors, a resist resistanc ance e change change induce induced d from from the resist resistive ive materi material al squeezed squeezed between between electrodes electrodes is measured measured [15]. [15]. A piezoresispiezoresistive tive sen sensin sing g mechan mechanism ism uses a strain strain gauge gauge to measur measure e the deform deformati ation on of a tactil tactile e cell cell [16]. [16]. A piezoelect piezoelectric ric mechanism mechanism measures measures the accumulat accumulation ion of charges charges and the resulting resulting voltage buildup as a membrane is forced. However, a piezoelectric sensor sen sor cannot cannot detect detect static static force force [17]. [17]. A capacitive capacitive-sensi -sensing ng mechan mechanism ism measur measures es the capaci capacitan tance ce change change induce induced d by the change change in the gap betwee between n the electr electrode odess [9] [9].. However, most

H.-K. Kim et al. / Sensors and Actuators A 165 (2011) 2–7

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of these devices are not suitable for touch screen display systems because of the non-transparency of the materials they are made of. In order to meet the requirement for tactile sensors for multi-touch screens for flexible display applications, we have introduced a capacitive tactile sensor array constructed with polycarbonate (PC) films and indium–zinc-oxide (IZO) electrodes for flexibility and transparency. In this paper, we present the concept, fabrication, and experimental results of our sensor in detail. 2. Design

Fig. 1 shows the cross-sectional view and the dimension of a unit cell of theproposed tactile sensor array. The upper andbottom substrates are transparent PC films with a thickness of 120 m. A thin transparent IZOlayerwas used as theelectrodes andthe signal lines.The twoelectrodesformeda capacitorseparated by a distance of 13 m by SU-8 spacers. The cell size and electrode size were 2 mm × 2mmand1mm × 1 mm, respectively. The capacitance of a cell can be expressed as

C =

1 , (t a /ε0 A) + (t d /εd ε0 A)

(1)

where ε 0 is the permittivity in free space, ε d is the relative permittivity of the SU-8 insulation layer, t a is the air-gap distance, t d is the thickness of the SU-8 insulator layer, and A is the electrode area. The initial capacitance of a cell was estimated to be

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

926fF using Eq. (1) assuming that the relative permittivity of SU8 was 3.2. When a touch pressure was applied on the surface of the upper plate, the gap between the two plates decreased and the capacitance increased until the gap was closed. By measuring the capacitance for all the capacitive array cells, we could determine the touch position and the applied force on multiple locations. The membrane deflection and resultant capacitance change as the touch force applied must be considered for a capacitive cell design. These factors were examined by the finite element method (FEM) simulation for a capacitive cell with the dimensions given

Fig. 2. Center deflection (solid line: calculated, dashed line: simulation) 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 formation, (b) spacer formation on top plate, (c) electrode layer formation on bottom plate, (d) insulation layer coating, and (e) the completed device after bonding process.

in Fig. 1 using a COMSOL multiphysics simulator (COMSOL Inc.). Fig. 2(a) shows two examples of simulation results: the initial status with zero touch force (left) and with the touch force of 68 mN at a point where the upper plate just begins to touch the bottom plate (right). Fig. 2(b) shows the center deflection and the resulting capacitance for various applied forces. The solid line is the center deflectionversus theappliedforce, andthe dashedlineis thecapacitance change. The initial capacitance was estimated to be 938fF, which was close to the calculated value of 926fF. The upper plate beganto touch thebottomplatewhenthe applied force was68 mN; the capacitance at this point was approximately 3.4 pF. Another important factor that must be considered is mechanical response time of thecellmembranebecauseslow response time will resultin afterimage lag on display. The calculated and simulated resonance frequency of the designed membrane is about 21.5 kHz which is fast enoughcomparing with 60Hz, a general refresh time of display pixel. 3. Fabrication

In our design, we used transparent PC films as structural materials, SU-8 (Microchem Co.) as spacers and an insulator, and an IZO thin film as electrodes. The fabrication process is shown in Fig. 3. Each layer was processed separately and bonded together using SU-8 as the bonding material, which was also used as an insulator and a spacer. We used an IZO-coated PC film. The thickness of the PC film and the IZO thin film was 120 m and 130 nm, respectively. For photolithography, the films were mounted on a silicon wafer. For the top plate, first, the IZO layer was patterned (Fig. 3(a)) using general photolithography and wet etching. The solution with

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

Fig.5. (a)Opticaltransmittance of tactilesensormeasured withspectrophotometer and (b) tactile sensor on LCD display of mobile phone.


hydrochloric acid and nitric acid (HCl:HNO3 = 3:1) was used as an IZO wet etchant, and the etch time was approximately 5 s at room temperature. Next, SU-8 2007 (Microchem Co.) was spin-coated to have a thickness of 13 m and patterned to form spacers on the topsubstrate (Fig. 3(b)). As the bottom plate, the IZO layer was patterned again for the bottom electrode (Fig. 3(c)). Then, a thin SU-8 2005 wasspin-coated to have a thickness of 5 m, forming an insulation layer between the top and the bottom electrodes (Fig. 3(d)). Next, the top substrate was aligned with the bottomsubstrate, and pressure wasapplied at room temperature (Fig.3(e)).Then,the two substrates that were bonded together were heated on a hot plate at 95 ◦ C for 1 min to cure the thin SU-8 layer. Finally, the SU-8 was hardened after UV exposure and post-exposure bake at 95 ◦ C for 1 min. Fig. 4 shows the fabricated tactile sensor. The initial device was designed to have 20 × 20 capacitive cells, and the size of the entire sensor module was 6 cm × 6 cm, including the interconnection pads. The fabricated sensor exhibited good flexibility, as shown in Fig. 4(a). Fig. 4(b) shows the magnified view of the fabricated tactile sensor. The overlap area of each capacitive cell was 1 mm × 1 mm, and the diameter of SU-8 spacer was 200 m. 4. Experimental results

The transparency of the fabricated tactile sensor was measured usinga UV/Visiblespectrophotometer(SCINCO).The average transmittance was approximately 86% in a visible light range from 380nm to 770 nm, as shown in Fig. 5(a). We placed the tactile sen-

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

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

sor on top of the LCD display of a commercial mobile phone to test the visibility, and as seen in Fig. 5(b), there was no interference or decrease in visibility. We set up custom-made equipment for touch force characterization. Fig. 6(a) displays our setup for the contact force measurement. A force gauge with a tip was used to precisely

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

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apply pressure on a specific capacitive cell. The end of the tip was treated to have a flat rectangular shape with the dimension of 1mm × 1 mm, and the force gauge from AIKOH Engineering Co. had a force resolution of 1mN. Fig. 6(b) shows the customdesigned readout circuitry. First, each tactile cell was selected by a row decoder and reset. Then, it was charged to V step. When the cell was selected by a column decoder, the stored charge was transferred to the feedback capacitance (C f ) and generated output voltage, as given in the equation. The signal was processed by a custom-designed field programmable gate array (FPGA) chip, and the final image was displayed by LabVIEW (NI). In order to remove the offsets from the circuit, we designed the circuit to read a single cell twice with and without resetting the feedback capacitance. Fig.7 shows themeasured response of thefabricatedcell forvarious touch forces. Further, the FEM simulation result is depicted as a dashed line in this figure for the sake of comparison. A single cell was pressed by using a micro-force gauge with a tip, as shown in Fig. 6(a). All experimental data are the averaged value of 10 measurements on different cells and standard deviation is less than 6.7%. The initial capacitance of a cell was measured to be approximately 900 fF, which was close to the theoretical value of 926 fF obtained from Eq. (1) and the simulated value of 938 fF. The capacitance increased linearly with the applied force before 0.1 N and saturated after that pressure, which implied that both the upper andthe bottomelectrodes were in contact with theinsulation layer between them. Moreover, we can see that the experimental results

adequately followed the simulation result before saturation. Multi-touch tactile images captured from the fabricated sensor are shown in Fig. 8. Pressure was applied by using a rubber stamp with the letter “T” on it, and the corresponding image was clearly captured in Fig. 8(a). Further, several point images according to various touch pressures are seen in Fig. 8(b) and the areas of two neighboring units cells in post processing display program are designated with the red and blue dashed squares in this figure. The program was designed to change the both of darkness and size of color in a cell area. In this experiment, we first applied pressures on different locations at the same time using several tips and capacitance values of each cell are memorized. To find the pressure values on each cell giving recorded capacitance values, we applied pressure on each cell using force gauge with sharp tip. We can clearly see that the brightness and size of the point images increased in proportion to the touch pressure. Sliding experiments on a curved surface were also performed, andtheirresults areshownin Fig.9. The tactile sensor wasattached on a cylindrical structure with the radius of curvature of 30mm,as shown in Fig. 9(a). Two fingers touched two different points on the 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 the sliding of the fingers on the curved tactile sensor. 5. Conclusions

In this study, a new flexible and fully transparent tactile sensor for touch screen applications was proposed and successfully demonstrated. A sensor module consisted of a 20 × 20 tactile cell array with a spatial resolution of 2mm. The fabricated tactile sensor module exhibited good flexibility with a maximum radius of curvature of 30 mm and captured multi-touch images. The cell to cell variation of capacitive response was measure as 6.7%. Even though the proposed tactile sensor modules need more optimization in the design and fabrication to increase the uniformity, they can be a good candidate for touch screens for flexible display in the future with their flexibility, transparency, and capability for force sensing. Acknowledgements

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

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

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Biographies Hong-Ki Kim receivedhis BS degree in Physics fromChungnam NationalUniversity in 2007, and his MS degree in Electronic Engineering from Sogang University in 2009. He joined the LATTRON Co., Ltd, Korea in 2009. His research area includes Bio-MEMS and Ceramic Device. Seunggun Lee received his BS degrees in Electronic Engineering from Sogang University in 2009. He is currently pursuing his MS degree in Electronic Engineering from Sogang University. His research area includes Touch Sensors. Kwang-Seok Yun received his BS degree in Electronics Engineering from Kyungpook 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 University of California, Los Angeles from 2005 to 2007. He joined the Department of electronic Engineering at Sogang University, Korea in 2007, where he is now an Assistant Professor. His current researcharea includes micrototal analysissystems, Lab-on-a-chip, MEMS, and micro sensors and actuators.

Capacitive tactile sensor array for touch screen application

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