Touch-Fingerprint Display for Supporting Identity Sensing

Pranav Koundinya+,Xi Zhao†,Tao Feng†,Sandhya Theril+ and Weidong Shi†Department of Computer Science†,Department of Electrical and Computer Engineering+

University of Houston, 4800 Calhoun Road, Houston, TX 77004, U.S.A

ABSTRACT

The need for touch based authentication in smart devicessuch as smartphones and tablets are increasing day by day.Historically, fingerprint has been one of the most effective waysfor establishing the uniqueness of an individual’s identity. Mo-tivated by the fact that both touch sensing and fingerprint scancan be based on the same capacitive sensing principle, onecan design a single unified sensing device that can supportboth multi-touch interactions and identity sensing based onfingerprint biometrics. Such design will not only enable userfriendly mobile identity solutions, but also support more ele-gant and compact physical design of future smart mobile de-vices. In this paper, we present and explore three approachesfor creating such a device. They are, transparent fingerprintreaders on-top of a touch panel - fingerprint reader on-panel,unified touch-fingerprint panel, and touch-fingerprint in-cellTFT-LCD. Further, we discuss the challenges faced by eachapproach, compare the pros and cons of each approach, andpropose solutions accordingly. Finally, we conduct evaluationsusing state-of-the-art modeling tools and perform quantitativestudies.

I. OBJECTIVE AND BACKGROUND

The wide adoption of smartphones creates strong demandsfor touch based user identity verification that can enhance usersecurity, support user friendly access control and carry outsmoother and safer online activities (e.g. that provide passwordfree access for web sites from a phone). Many approaches havebeen proposed for smartphone based user identity managementby leveraging the smartphone sensors such as mobile camerasfor user identity recognition. Vendors also have integrated dis-crete optical fingerprint sensors with the smartphone models(e.g., iPhone 5S and Samsung S5). However, there is a new,potentially revolutionary approach, that takes advantage of thefact that both human touch interactions and fingerprint sensingcan be captured using capacitive sensing techniques. The ob-jective of this paper is to examine the different design optionsfor a unified touch-display that can function both as a touchsensing device and a fingerprint scanner. Such device will leadto cleaner smartphone physical design (e.g., eliminate opticalfingerprint home button), and smoother user interactions withbuild-in support for identity protection.

Figure 1 illustrates three approaches for integrating finger-print scanning capability with a touch-display. Firstly, one canleverage the recent advances in design and fabrication of trans-parent electronics and thin-film transistors [11, 14] to designoptically transparent fingerprint readers [3] (e.g., transparentcapacitive TFT fingerprint reader). It is then possible to inte-grate such transparent fingerprint readers with a touch-display1. The second approach is to design a single transparent sens-ing device that supports both multi-touch detection and finger-print scan in two operational modes. Such a unified touch-fingerprint sensing device can be either used in conjunction

1To avoid potential sensing conflict, touch sensing and fingerprint scancan be based on different techniques (e.g., capacitive fingerprint readers plusacoustic wave multi-touch).

A. Fingerprint Sensors over Touch Panel B. Touch-Fingerprint Sensors in Conjunction

with Touch Panel

D. Incell Touch-Fingerprint SensingC. Single Touch-Fingerprint Sensor

Fig. 1. Illustration of different approaches for supporting unified touchsensing and fingerprint scan. A. Optically transparent fingerprint readerson-top of a touch panel (fingerprint reader on-panel); B. Touch-fingerprintpanels with dual operational mode support used in conjunction withconventional touch panel; C. Single touch-fingerprint panel with dualoperational mode as touch panel replacement; and D. Touch-fingerprintin-cell TFT LCD.

with conventional touch panels (Fig. 1B), or be applied to re-place the current touch sensing only device (Fig. 1C). Thethird option is to exploit the capacitive in-cell TFT-LCDs (e.g.,[9, 13]) for both touch sensing and fingerprint scan by increas-ing the sensor resolution (Fig. 1D).

There are technical challenges that one has to overcome foreach of these approaches. For transparent touch-fingerprintpanels, the large panel size and high sensor density will cer-tainly causes challenges in touch sensing speed (preferably lessthan 10ms) and fingerprint scan latency. For capacitive in-cellTFT-LCDs, it is infeasible to increase sensor density arbitrar-ily to support fingerprint scan. One has to balance the impactof increased number of capacitive in-cell sensors on displayaperture ratio and the minimal sensing resolution required forscanning fingerprint images with acceptable quality. Here inthis paper, we explore and evaluate these approaches in termsof technical feasibility, challenges, design choices, and solu-tions.

II. DESIGN AND EVALUATION

A. Transparent Fingerprint Reader

High performance transparent thin-film transistors (TTFT)have been demonstrated and fabricated in room-temperatureusing transparent amorphous oxide materials such as SnO2,ZnO, In-Ga-Zn-O (e.g., [2, 6, 10]). The achieved mobilityis from 10 to 50 cm2s−1v−1 as reported in various studies.Typically, for TTFT, ITO (Indium tin oxide) is often appliedas the gate, source and drain electrode for optical transparency.

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Fig. 2. Circuit model of capacitive sensing array using transparent oxidesemiconductors. All components are made from transparent materials (e.g.,ITO, transparent TFT) and glass substrate.

A capacitive touch panel can be created using only ITO andglass. However, for scanning fingerprint based on the princi-ple of capacitance sensing, thin-film transistors are necessary.Figure 2 shows an abstract circuit model of transparent TFTbased touch-fingerprint sensing array. All the components aremade from transparent materials (ITO, glass, TTFT). The arrayworks on the principle of capacitive sensing. Each sensing cellconsists of a transparent capacitive sensing electrode (ITO),connected to two transparent TFTâAZs.

In microscopic scale, the surface of the fingerprint has adeep part called the valley or elevated part called the ridge.When the ridge of a fingerprint lies directly over the electrode,a capacitor is formed between the electrode and the finger.The capacitor between the sensor plate and the finger surfaceis charged by switching on the transparent charging transistorwith the discharging transistor kept off, and then switching offthe charging transistor. The capacitance charge depend on thedistance between the sensor plate and the finger surface. If avalley in the fingerprint lies over the electrode, then change incapacitance is much smaller, and a negligible charge is saved.The stored charge is later transferred onto a data line electrodeby switching on the discharging transistor when the next col-umn is activated. The charge is then amplified by a chargeamplifier and processed by an external readout circuit.B. Unified Touch Sensing and Fingerprint Scan

The sensor capacitance is at its maximum value when a ridgehas contact with the capacitive sensing electrode. As the dis-tance between the chip surface and the fingerâAZs skin in-creases, the capacitance becomes smaller. The same princi-ple is also applicable to touched versus untouched area by afinger. Based on Comsol Multiphysics simulation, beyond cer-tain distance threshold, the capacitance value becomes almostconstant. This suggests that when a proper threshold is cho-sen, a capacitive sensor cell can be configured for both touchdetection and ridge/valey sensing. Our system comprises of acapacitive sensor array that consists of many sensor cells fordetecting the capacitances and can be used for both touch de-tection and fingerprint scan, row and column driving circuitsfor sequentially charging and discharging the sensing cells inrows and columns, and signal processing readout circuit.Multi-resolution Sensor Sampling The capacitive touch-fingerprint panel supports dual operational modes for bothtouch sensing and fingerprint scan [8]. This is achieved us-ing multi-resolution row and column drivers that support highresolution sampling of sensor cells for fingerprint scan, andlow resolution sensor cell sampling for touch detection. Therow driver consists of a multi-resolution shift register. A rowdecoder can decode a given row address and store the decodedline in the shift register. In shift mode, the shift register canselect rows sequentially with different row stride distances de-pending whether the system is set to detect touches or scanfingerprints.

For touch detection, the sensing system outputs a patternimage with reduced resolution. By subjecting the pattern im-age to image processing, the touch location can be determined.

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Fig. 3. Dual operational mode with multi-resolution sampling support. Therow and column drivers support different sensor cell sampling resolutions fortouch detection and fingerprint scan. A. Touch detection mode: scanlines andcolumns in yellow color are these activated in touch detection. B. Fingerprintscan mode: scanlines and columns surrounding a touch location are selected(in yellow color).

For fingerprint scan, sensing cells will be sampled at the max-imum resolution. However, not all the rows and columns areneeded for fingerprint scan. For efficient and fast fingerprintscan, the sensing system first determines the location of the fin-ger using touch detection mode. Then when fingerprint scan isneeded, the sensing system only selects the rows and columnssurrounding a finger touch location. This is achieved by send-ing a row scan address to the row driver. For example, assumethat in the touch detection mode, a central touch location isdetected at row X. Then in the fingerprint scan mode, the rowdriver can start selection of the rows from row X - d where dis determined based on the size of finger tip. There is a rowcounter that stores the number of rows that need to be scannedbefore it rotates to the beginning row, X-d. Similar design alsoapplies to the columns. The sensing system can select columnoutputs surrounding a touch location.

Evaluation We implemented the designed touch-fingerprintsensing system using Silvaco CAD tools. Simulation of thecapacitive sensing array was carried out in SmartSpice andbased on transparent TFT model module using Hoffman’sclosed form approach written in Verilog-A. We implementedthe TTFT model in Verilog-A because it provides a mean ofmodeling devices at a wide range of levels of abstraction andat the same time integrates the implemented model with com-puter aided design tools. Simulation of the designed touch-fingerprint sensing system was conducted using Smartspiceand Silvaco TFT EDA tools. We validated our Verilog-Amodel by comparing the output characteristics of our Verilog-A TTFT module with measured data from fabricated trans-parent amorphous oxide TFT [2]. The simulated DC trans-fer characteristic between Id-Vds under 10V Vgs is on aver-age 94.26% accurate when compared with the measured re-sult. Under 15V Vgs, the simulated DC transfer characteristicbetween Id-Vds is on average 97.09% accurate. Readout cir-cuits are designed using CMOS or Polycrystalline Silicon (e.g.,[4, 5]). The readout circuits don’t need to be transparent.

The driving circuits have a clock speed of 333.33KHz andsupply voltage of 5V. The charging and discharging transistorsare transparent SnO2 TTFT with a turn on voltage of 2.7V. Thechannel mobility is 17.4cm2s−1v−1. The capacitor electrodes,scanlines, column lines, and data lines are all made from ITO.For supporting touch-fingerprint dual sensing modes, the rowdriver uses multi-resolution shift register with shift stride dis-tance of 1 or 8 corresponding to fingerprint and touch sensingrespectively.

C. Touch-Fingerprint Display with Incell TechnologyIn-cell capacitive TFT-LCD (e.g., [15]) integrates touch sen-

sors with the LCD module itself. Unlike out-cell or on-celltouch sensing techniques, in-cell LCD is more compact be-cause capacitive touch sensors are embedded inside the LCDcells. In the recently years, several capacitive sensing basedin-cell TFT-LCD prototypes have been fabricated and demon-strated (e.g., [9, 13]).Fingerprint Scan with In-cell Sensors Generally, the fin-gerprint pattern has line width and space in the range of200 µm to 400 µm. To achieve high resolution for acquiredfingerprint image, the size of unit sensor cell should haveenough granularity. For in-cell TFT-LCD, capacitive sensorsare grouped together with display subpixels. As demonstratedin [13], a density of capacitive sensors at pitch of 600 µm issufficient for supporting touch detection. However, for finger-print imaging, more sensors are required. The sensor densitycan be represented by the sensor pitch. A pitch value smallerthan 100 µm is infeasible because of its impact on aperture ra-tio. For sensor pitch of 150 µm or larger, effects on apertureratio can be controlled at a rate less than 25%. Since low res-olution fingerprint images are taken at such sensor pitch level,one has to apply super-resolution approach to enhance the ob-tained sensor images for fingerprint verification.Design of Incell Touch-Fingerprint Display Figure 4shows a block diagram of the touch-fingerprint TFT-LCD. Asdemonstrated by the figure, a row of capacitive sensors areplaced between rows of LCD display pixels. The display pixelsare controlled by a display driver. A capacitive sensor drivercontrols the pre-charge gates of the capacitive sensors using amulti-resolution shift register to support either touch detectionor fingerprint scan. In touch detection, capacitive sensors aresampled at lower resolution. For fingerprint scan, capacitivesensors are sampled at the highest sensor resolution. Outputsfrom the sensors are processed by a readout circuit. Similarto fabricated in-cell TFT-LCDs (e.g., [13]), the sensor and thedisplay driver circuits are integrated on the glass substrate. Allthe LTPS-TFTs used in the sensors, the pixels, and the drivercircuits are PMOS type devices, which simplifies the TFT sub-strate fabrication process.

The capacitive in-cell TFT-LCD supports dual operationalmodes for both touch sensing and fingerprint scan. A row de-coder can decode a given row address and store the decodedline in a shift register. In shift mode, the shift register can

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Fig. 4. Block diagram of touch-fingerprint in-cell TFT-LCD. A row ofcapacitive sensors are placed between rows of LCD display pixels. Thedisplay pixels are controlled by the display driver. The capacitive sensordriver controls the pre-charge gates of the capacitive sensors using amulti-resolution shift register. The touch-fingerprint in-cell TFT-LCDsupports two sensing mode, touch detection and fingerprint scan. The readoutcircuit can be used for either locating touch locations or scanning fingerprintimages.

TABLE ISPECIFICATIONS OF THE CAPACITIVE IN-CELL TFT-LCD

Display size 4.5 inch diagonalDisplay resolution 960 x 640Display pixel pitch 0.075mmNumber of sensor rows 320Sensor pitch 0.15mm x 0.05mmTouch sampling resolution 80 rowsTouch sampling delay 8.32msFingerprint sampling period 17.9msFrame rate at highest sensing resolution 55.8Hz

select capacitive sensor rows sequentially with different rowstride distances depending on whether the system is set to de-tect touches or scan fingerprints. For fingerprint scan, the rowswill be selected one after the other at the maximum resolution.For touch detection, the rows will be selected with reduced res-olution (e.g., one row out of four sensor rows). For column se-lection, dual operational modes for fingerprint and touch sens-ing are supported by selecting sensor dataline outputs at dif-ferent resolutions. For touch detection, fewer columns are se-lected and whose signals are converted into digital data by thereadout circuit. For fingerprint scan, the datalines surroundinga target touch location are sampled at the maximum resolution.Super Resolution Due to under-sampling caused by the rel-atively large sensor pitch, fingerprint images captured by in-cell TFT-LCD have lower resolution than typically required forfingerprint verification. One solution to this problem is to ap-ply super resolution (SR) processing. The goal of SR is to ex-tract the independent information from a set of fingerprint im-ages taken by the in-cell TFT-LCD and combine the informa-tion into a single high resolution (HR) fingerprint image. SRapproach (e.g., [12]) based on POCS (Projection onto ConvexSets) is ideally suited for such a purpose as POCS is designedto account for both the blurring introduced by the sensors aswell as the effects of under-sampling.Evaluation Based on the documented designs of capacitivein-cell TFT-LCDs (e.g., [9, 13]), we evaluated our touch-fingerprint in-cell LCD using circuit simulation and measuredin-cell TFT-LCD characteristics reported in the literature. Thetouch-fingerprint sensing system is implemented using SilvacoCAD tools. Table I summarizes performance numbers of thein-cell LTPS TFT-LCD validated by circuit simulation andmeasured results from fabricated in-cell TFT-LCDs.

To evaluate the performance of the in-cell TFT-LCDs onverifying users, we used simulated capacitive sensors at dif-ferent sensor pitches, 0.1mm, 0.15mm, and 0.2mm. The mod-eled sensor has signal-to-noise ratio (SNR) value 5, which iscomparable with fabricated in-cell LTPS TFT-LCD [13]. Fin-gerprint images are produced with the modeled in-cell deviceusing fingerprint database of ten actual users. To compensatefor the large sensor pitch and insufficient sampling, we haveapplied a SR algorithm to increase the resolution from a num-ber of consecutive fingerprint images. Features are extractedfrom the result HR (high resolution) fingerprint images andthen subjected to user verification. Table II shows the verifica-tion results at various conditions. Both false reject rate (FRR)and false acceptance rate (FAR) increase when sensor pitchgrows from 0.1mm to 0.15mm and 0.2mm (leading to reducedresolution). The results indicate that sensor pitch at 0.1mm issufficient for fingerprint verification. However, this setting isimpracticable due to its impact on aperture ratio. After apply-ing the SR algorithm, the verification performance at 0.15mmsensor pitch has gained significant improvements in terms ofboth FAR and FRR. Unfortunately, for 0.2mm sensor pitch, SRis no longer effective to restore the features. The results sug-gest that with image super resolution, it is feasible to supportfingerprint based verification on capacitive in-cell TFT-LCD

TABLE IITHE RESULTS OF FINGERPRINT VERIFICATION. SR MEANS RESULTS

USING HR FINGERPRINT IMAGES AFTER APPLYING SUPER RESOLUTION.

Pitch 0.1mm 0.15mm 0.15mmSR 0.2mm 0.2mmSRFAR (%) 0.0 0.0 0.0 2.5 4.0FRR (%) 5.0 42.5 12.5 41.5 37.5

with around 0.15mm sensor pitch.

III. COMPARISON

Among the three approaches (transparent fingerprint read-ers on-top of a touch panel - fingerprint reader on-panel, uni-fied touch-fingerprint panel, and touch-fingerprint in-cell TFT-LCD), unified touch-fingerprint panel can be integrated withthe existing display techniques in the same way as a multi-touch panel. Issues of dual mode support (touch detection andfingerprint scan) and sensing speed can be addressed by us-ing a variety of techniques such as multi-resolution driver forsampling capacitive sensing cells at different scales and par-allel analog-to-digital comparators. Impact of unified touch-fingerprint panel to display brightness is similar to today’stouch panel because highly transparent TTFTs have beendemonstrated recently (e.g., [2, 6]). Touch-fingerprint in-cellTFT-LCD has the advantages of being compatible with the ex-isting capacitive in-cell TFT-LCD techniques. However, itsmain challenge is the required minimal sensor density for fin-gerprint scan and its impact on display aperture ratio. Imagesuper resolution provides a potential solution to support finger-print scan at sparse level of sensor cells.

IV. IMPACTS

Multi-users and Identity Based Phone User InterfacesSmartphone platforms have been designed to serve a singlephysical user. However, many people lend their mobile phonesto someone else; a friend, a coworker, or even a stranger, inwhich case the owner would prefer to disallow access to per-sonal information (e.g., contacts, pictures, personal applica-tions etc.) that is stored on the phone (e.g., [7]).Lost Smartphones Corporations, universities, and govern-ment institutes have increasingly handed out mobile comput-ing systems and applications allowing their employees to workremotely while continuously staying connected to the organi-zation’s infrastructure. As such, sensitive information (con-fidential documents, trade secrets, credentials) is frequentlystored or accessed from these systems. A recent study of 439companies and over 3 million smartphones shows that the per-centage of unrecoverable smartphone loss is 8.76% [1].Online Transactions Smartphone enables its users to carryout effective online transactions. To ensure security, the cur-rent practice is to download a “Digital Certificate” from an IDprovider that resides in the particular terminal application onthe smartphone. The digital certificate, when coupled with aPIN or a user defined password serves to prove the identity ofthe user. It becomes necessary for the user to remember thePIN/password in order to prove his/her identity every time theapplication needs to be used. This, burdensome task, however,can be avoided with our system.

V. ACKNOWLEDGEMENT

This material is based upon work supported by the U.S.Department of Homeland Security under Award NumberN66001-13-C-3002. The views and conclusions contained inthis document are those of the authors and should not be in-terpreted as representing the opinions or policies of the U.S.Government. Mention of trade names or commercial productsdoes not constitute their endorsement by the U.S. Government.

VI. CONCLUSIONSThis paper explores and compares different design ap-

proaches for creating a unified touch sensing and fingerprintscan device to enable burden free and user transparent iden-tity sensing capability. Such capability will result in new userexperiences that combine identity sensing with touch interac-tions. Integrating fingerprint based identity sensing with touch-display will further lead to more elegant and compact physicaldesign of smartphones. The paper evaluates the different ap-proaches in terms of feasibility, challenges, and technical solu-tions.

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