IEICE TRANS. ELECTRON., VOL.E90–C, NO.11 NOVEMBER 20072105

INVITED PAPER Special Section on Electronic Displays

“Front Drive” Display Structure for Color Electronic Paper UsingFully Transparent Amorphous Oxide TFT Array

Manabu ITO†a), Masato KON†, Chihiro MIYAZAKI†, Noriaki IKEDA†, Mamoru ISHIZAKI†,Yoshiko UGAJIN†, and Norimasa SEKINE†, Nonmembers

SUMMARY We demonstrate a novel display structure for color elec-tronic paper for the first time. Fully transparent amorphous oxide TFTarray is directly deposited onto color filter array and combined with E InkImaging Film. Taking advantage of the transparent property of the oxideTFT, the color filter and TFT array are positioned at the viewing side of thedisplay. This novel “Front Drive” display structure facilitates the alignmentof the color filter and TFT dramatically.key words: transparent amorphous oxide semiconductor, a-InGaZnO, EInk display, color filter array, thin film transistor, front drive structure

1. Introduction

Recently, transparent oxide semiconductors have been at-tracted tremendous attention as a novel candidate for thinfilm transistor (TFT) materials [1], [2]. Various kindsof oxide semiconductors have been reported so far, suchas ZnO [3]–[9], MgxZn1−xO [10], [11], Zn-Sn-O [12]–[15], Zn-In-O [16]–[18], SnO2 [19], [20], Ga2O3 [21],In-Ga-O [22], In2O3 [23], TiO2 [24], single crystallineInGaO3(ZnO)5 superlattice [25]–[27] and amorphous In-Ga-Zn-O (a-InGaZnO) [28]–[31]. Among these recentlyexplored oxide semiconductors, a-InGaZnO is superior inits high mobility (∼10 cm2/Vs), high on/off current ratios(∼106), low process temperature and long-term chemicalstability [28]–[31]. Despite its amorphous structure, rela-tively high mobility of amorphous oxide TFT is explainedby the electronic structure (n − 1)d10ns0 (n ≥ 4) of the in-corporated heavy-metal cations [32], [33]. These ns orbitalshave large radii, so that there is a large overlap between theadjacent orbitals, which leads to insensitiveness to the dis-torted metal-oxygen-metal chemical bonds.

Unique features of transparent amorphous oxide semi-conductor compared with conventional amorphous Si (a-Si)semiconductor can be summarized as follows.

• High performance• Low process temperature• Transparency

High performance of oxide TFT seems quite attractivefor current-demanding applications, such as organic lightemitting diode (OLED). Until now, a-Si [34], low tempera-

Manuscript received March 1, 2007.Manuscript revised May 10, 2007.†The authors are with Technical Research Institute, Toppan

Printing Co., Ltd., Saitama-ken, 345-8508 Japan.a) E-mail: manabu.ito@toppan.co.jp

DOI: 10.1093/ietele/e90–c.11.2105

ture polycrystalline Si (LTPS) [35]–[37] and organic semi-conductors [38] have been examined as active layer materi-als for OLED. Although a-Si enjoys uniformity over largearea and maturity of the fabrication process, its field-effectmobility is limited to less than 1 cm2/Vs, which is not sat-isfactory for OLED applications. Degradation of a-Si byelectrical bias stress also causes a serious problem [39].LTPS has been extensively studied as an alternative to a-Si,due to its high mobility of more than 100 cm2/Vs [40], [41].However, LTPS TFT suffers from its poor homogeneity ow-ing to the grain boundary. High process cost also hindersthe widespread use of LTPS TFT. As for organic TFT, or-ganic semiconductor will not be in the market in the nearfuture due to its poor mobility (< 1 cm2/Vs) and low stabil-ity. On the other hand, oxide TFT shows high mobility (>10 cm2/Vs) and high on/off ratio (> 106), which are suitablefor OLED applications. Moreover, oxide TFT can be de-posited by standard sputtering technique, so that large areadevices can be easily manufactured. Lee et al., reported thatthey have successfully driven 3.5-inch OLED display by a-IGZO TFT array [42]. This result strongly suggests thatthe oxide TFT is one of the most promising candidates as abackplane for OLED device.

Second feature of transparent amorphous oxide TFT isthe applicability of “low process temperature.” Above men-tioned performance can be realized even at room temper-ature process. Low process temperature is fully compati-ble with plastic substrate, which leads to “flexible display.”Recently, “flexible display” becomes the “buzzword” of thedisplay industry [43]–[48]. Flexible display or display fabri-cated onto plastic substrate is superior to existing rigid dis-plays on glass in light-weight, thin and non-fragile prop-erty. Moreover, flexible displays have the ability or possi-bility of using roll-to-roll manufacturing process, which candramatically reduce the process cost. In our earlier report,we have fabricated a-IGZO TFT array onto poly-ethylene-naphthalate (PEN) film at room temperature and success-fully demonstrated a flexible active-matrix, electronic paperfor the first time [49]. The display can be bent without anyperformance loss. Oxide TFT seems most plausible materi-als as an active channel layer for flexible TFT.

Then, what about the third feature, “transparency”? Isthere any good thing about the transparent TFT? Here, inthis study, we propose a novel display structure for colorelectronic paper taking advantage of the “transparent” prop-erty of the oxide semiconductor. Fully transparent amor-

Copyright c© 2007 The Institute of Electronics, Information and Communication Engineers

2106IEICE TRANS. ELECTRON., VOL.E90–C, NO.11 NOVEMBER 2007

phous oxide TFT array is fabricated onto color filter arrayand combined with E Ink Imaging film.

2. Experimental

2.1 E Ink Imaging Film

As a front plane of the display, we employed “E Ink Imag-ing Film,” which is developed by E Ink Corporation. E InkImaging Film consists of polyethylene terephthalate (PET),ITO-layer, ink-layer, adhesive and release sheet, as shown inFig. 1 [50]–[52]. Ink-layer is composed of millions of tinymicrocapsules, which contains positively charged white pig-ment particles and negatively charged black particles sus-pended in a clear fluid. When a negative electric field isapplied, the white particles move to the top of the microcap-sule where they become visible to the user. This makes thesurface appear white on that spot. By reversing this process,black appearance can be seen.

E Ink display can be realized by removing the releasesheet and laminating the E Ink Imaging Film onto TFT ar-ray. It should be noted that the resolution of the displayis determined not by the size of the microcapsules, but bythe size of the pixel electrodes on backplane. Complicatedalignment between E Ink Imaging Film and TFT array is notnecessary.

The mechanism of displaying image in E Ink displayis reflective, which is similar to that of ink on paper, so thatE Ink display enjoys high reflectance, good contrast ratio,wide viewing angle, and low power consumption due to itsimage stability. Using color filter array, color E Ink displaycan be realized [53].

2.2 Structure of the Display

Conventional structure of the color E Ink display is shownin Fig. 2(a) [53]. In this structure, color filter array is fab-ricated onto glass substrate and subsequently microcapsulesare coated on it. TFT array is fabricated onto another sub-strate and then combined with color filter array. In this pro-cess, the integration of the display involved precise align-ment of the color filter array and TFT array in both the hor-izontal and vertical axis. In E Ink display, the alignmentbetween upper and lower substrate is much more difficultthan that of liquid crystal display (LCD) due to the follow-ing reasons.

First, in LCD case, color filter array and TFT array can

Fig. 1 Schematic view of the E Ink Imaging Film.

be aligned through 4 to 6 µm height spacer. As for E Ink dis-play, however, upper and lower substrate should be alignedthrough 40–50 µm thick ink layer. Thickness of inner layeris different by one order of magnitude between LCD and EInk display. This difference makes manufacturing processof color E Ink display extremely difficult.

Secondly, color filter array and TFT array are lami-nated through adhesive in E Ink display. Once two sub-strates are adhered, position adjustment is practically im-possible. These troubles of alignment leads to low processyield and high cost of color E Ink display.

In order to address above mentioned problems, we pro-pose a novel display structure taking advantage of the trans-parent property of the oxide TFT. In this structure, transpar-ent TFT array is directly deposited onto color filter array.Furthermore, this TFT array on color filter is positioned atthe viewing side of the display, as shown in Fig. 2(b). AsTFT array is fabricated directly onto color filter array, align-ment between color filter array and TFT array can be easilyrealized. It is expected that fully transparent TFT array doesnot affect the visibility of the display. Moreover, amorphousoxide TFT can be deposited at low temperature. Therefore,heat damage to color filter during deposition of TFT canbe avoided. Here, we name this display structure as “FrontDrive” structure, because the display is driven from the frontside of the display.

2.3 Fabrication Process of Color Filter Array and Trans-parent TFT Array

As a substrate, Corning 1737 non-alkali glass with 0.7 mm-thick was used. The color filter array was prepared bystandard photo-lithography process using pigment disper-

(a)

(b)

Fig. 2 (a) Structure of conventional color electronic paper, (b) our novel“front drive” structure.

ITO et al.: “FRONT DRIVE” DISPLAY STRUCTURE FOR COLOR ELECTRONIC PAPER2107

Table 1 Deposition conditions.

Fig. 3 Schematic view of “Front Drive” E Ink display and TFT structure.

sion color resists with a staggered red, green, blue and white(RGBW) square subpixel layout. White subpixel was addedin order to enhance the brightness and dynamic range of thedisplay [53]. RGBW four subpixels constitute one pixel,whose size is 250 µm × 250 µm. After fabrication of RGBWsubpixels, transparent overcoat layer was formed in order tosmooth the surface roughness of the color filter array. Sub-sequently, TFT array was processed directly onto color filterarray. In order to realize fully transparent TFT, only trans-parent materials are chosen as components of the TFT lay-ers. We employed bottom gate and top contact type struc-ture for transparent oxide TFT array. All films were de-posited at room temperature by standard magnetron sput-tering technique. As for gate, capacitor, source and drainelectrodes, 100 nm-thick indium tin oxide (ITO) was used.The ITO layer was deposited by DC magnetron sputteringtechnique using In2O3 - 10% SnO2 ceramic target. Siliconoxynitride (SiON) layer of 330 nm-thick serves as a gate in-sulator. The SiON layers were deposited by RF-magnetronsputtering with Si3N4 target using Ar and O2 gas. As a chan-nel layer, we employed 30 nm-thick a-IGZO. AmorphousIGZO films were deposited by conventional RF-magnetronsputtering apparatus equipped with InGaZnO4 ceramic tar-get in Ar and O2 gas ambient. The deposition conditionof the a-IGZO layer was optimized by carefully choosing

the oxygen flow rate during deposition. In order to enhancethe transparency of the TFT array, post deposition annealingwas performed twice at 200◦C for 1 hour after depositionof ITO. Each layer was defined by standard photolithogra-phy and lift-off technique, with the channel length (L) of20 µm and channel width (W) of 5 µm. After fabricationof TFT structure, a-IGZO channel layer was covered by50 nm-thick SiON film by RF magnetron sputtering tech-nique. This SiON film serves as a passivation layer for a-IGZO channel. Deposition conditions are summarized inTable 1. Schematic view of our “Front Drive” display struc-ture and a magnified view of TFT structure are depicted inFig. 3. In our TFT array layout, aperture ratio of the eachpixel is estimated to approximately 49%. Size of the TFTarray is 1-inch diagonal. After TFT array was processed, EInk Imaging Film was laminated onto TFT array to completethe color E Ink display. Any type alignment is not necessaryin laminating E Ink Imaging Film.

Electrical characterization was carried out with a semi-conductor parameter analyzer (model Keithley 4200) atroom temperature in the atmospheric ambient. In order toevaluate the surface profile of the color filter array, pro-filometer, Dektak 6M, was employed. Transmittance ofthe each pixel was measured by microspectroscope (Olym-pus, OSP-SP200) in the wavelength range from 380 nm to760 nm. Effective area of transmittance measurement is ap-proximately 100 µm in diameter, which is slightly smallerthan the size of subpixel with 125 µm × 125 µm square.

3. Results and Discussions

Top view micrograph of color filter array with a staggeredRGBW square subpixel layout is shown in Fig. 4(a). In or-der to avoid disconnection of TFT array, overcoat layer wasformed to smooth the surface roughness of the color filter ar-ray. Surface profile of the RGBW subpixel without overcoatand with overcoat is depicted in Figs. 5(a) and (b), respec-tively. Scan is made in the transversal direction as depictedin the Fig. 5(a) from R, G, B and to W subpixels.

It is evident that the surface roughness is dramaticallyreduced by overcoat layer. Sharp valley between each sub-pixel was almost evened out. Although there is 200–250 nmdifference of height between subpixel B and subpixel W,slope is so slow that either disconnection or peeling of TFTarray were not observed. Micrograph of TFT array fabri-cated onto color filter array is shown in Fig. 4(b). Transpar-ent TFT can be seen through color filter array. Each compo-nents of TFT are pointed out in Fig. 4(b). After TFT arraywas processed, E Ink Imaging film was laminated onto TFTarray. Figure 4(c) shows the E Ink Imaging Film throughtransparent amorphous oxide TFT and color filter array. Mi-crocapsules of E Ink Imaging Film are observed throughtransparent TFT and color filter array.

In order to evaluate the presence of transparent TFTon color filter, transmittance spectra were measured in eachsubpixel (R, G, B, W) with transparent TFT and withoutTFT. The reference was measured by glass substrate. The

2108IEICE TRANS. ELECTRON., VOL.E90–C, NO.11 NOVEMBER 2007

(a)

(b)

(c)

Fig. 4 (a) Top view micrograph of color filter array, (b) fully transparenta-IGZO TFT array on color filter array, and (c) E Ink Imaging Film throughtransparent TFT and color filter array.

spectra are shown in Fig. 6. Although slight decrease oftransmittance is observed, transmittance spectra of each sub-pixel with transparent TFT exhibit more than 80% transmit-tance of each color filter. Decrease of transmittance is at-tributed not only to the absorption of light in ITO layers butalso to the reflection of light in the short wavelength andlong wavelength range. Further optimization of optical de-sign in each layer will increase the transmittance. Theseresults suggest us that the presence of fully transparent ox-ide TFT array is not supposed to have a major impact on theappearance of the color E Ink display.

In Fig. 7, transfer characteristic (gate-to-source volt-age, VGS versus drain-to-source current, IDS ) of fully trans-

Fig. 5 Surface profile of without (a) and with (b) overcoat of color filterarray.

Fig. 6 Transmission spectra of each subpixel with and withouttransparent TFT.

Fig. 7 Characteristic of a-IGZO TFT on color filter array.

ITO et al.: “FRONT DRIVE” DISPLAY STRUCTURE FOR COLOR ELECTRONIC PAPER2109

Fig. 8 Color E Ink display driven by “front drive” structure.

Table 2 Specification of color E Ink display driven by “front drive”structure.

parent a-IGZO TFT fabricated on color filter array withdrain voltage ranging from 5 V to 15 V. The off current isless than 10−10 A and on off ratio is more than 106. Thresh-old voltage, Vth and field effect mobility, µFE in the satu-ration region are evaluated from the intercept and the slopeof the VGS -(IDS )1/2 curve, respectively, according to the fol-lowing equation [54]:

IDS =

(CiµFEW2L

)(VGS − Vth)2 (1)

where Ci the capacitance per unit area of the gate insula-tor. The a-IGZO TFT on color filter exhibits normally-offcharacteristics with the µFE of about 6.1 cm2/Vs and Vth of1.0 V, which indicates the a-IGZO TFT operates in the en-hancement mode.

Image of color E Ink display driven by “Front Drive”structure is shown in Fig. 8. We have successfully driven theE Ink display by our novel “Front Drive” structure for thefirst time. Blue Letter “T” can be seen in the various colorcharts. Specifications of “Front Drive” color E Ink displayare summarized in Table 2. It seems that fully transparentamorphous oxide TFT array dose not affect the visibility ofthe color E Ink display. Although it is just a primitive result,however, we see a big potential for further development. Weare confident that our front drive structure will increase theyield and reduce the cost of the display dramatically, due tothe simplicity and easy-to-manufacture process.

It should be pointed out that influence of the transmit-ted light to TFT operation was not observed. In our “Front

Drive” structure, transmitted light to TFT was not shieldedeither by black matrix of the color filter or by metal gateelectrode as in the case of LCD. Therefore, influence of thetransmitted light to TFT should be carefully investigated.Influence of the light irradiation to oxide TFT has been re-ported [7], [17]. Any detectable photoresponse was not ob-served in our TFT structure under indoor lighting condi-tions. However, further systematic study is needed to under-stand the effect of the light illumination to fully transparentoxide TFT.

4. Conclusions

We propose a novel “Front Drive” display structure for colorelectronic paper. Taking advantage of the transparent prop-erty of the oxide TFT array, fully transparent amorphous ox-ide TFT array is successfully fabricated directly onto colorfilter array and positioned at the viewing side of the display.Amorphous IGZO TFT array fabricated on somewhat roughsurface of color filter array exhibit neither disconnection norfilm peeling. Amorphous IGZO TFT on color filter showsexcellent performance of µFE: 6.1 cm2/Vs and on/off ratio> 106. E Ink Imaging Film is laminated on oxide TFT ar-ray and 1-inch diagonal color E Ink display was successfullydriven by our “front drive” structure for the first time. Fullytransparent amorphous oxide TFT array dose not affect thevisibility of the color E Ink display. “Front Drive” structurefacilitates the alignment between color filter and TFT arrayremarkably, which will reduce manufacturing cost consider-ably.

We believe that our idea of “Front Drive” structure,where display is driven from the viewing side of the display,can be applied to not only to color E Ink Display but also tocolor LCD, OLED and other types of color electronic paper.

Acknowledgments

We would like to express our deepest appreciation to Profes-sor Hosono of Tokyo Institute of Technology for providingvaluable suggestions. We are thankful to E Ink Corp. forgraciously supplying us with E Ink Imaging Film for thisexperiment. We would like to thank to Mr. T. Nishimoto,Mr. T. Okubo, Mr. O. Kina, Mr. R. Matsubara, Mr. K. Hatta,Mr. K. Imayoshi, M. Tamakoshi, Mr. H. Yamada and Mr. Y.Takashima of Toppan Printing. Co., Ltd. for offering benefi-cial suggestions.

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Manabu Ito received the B.E. and M.E. de-grees in Material Engineering from Universityof Tokyo in 1993 and 1995, respectively. Hejoined Toppan Printing Co., Ltd. in 1995. Dur-ing 1997–2000, he worked as a guest scientistat Institute of Physical Electronics of StuttgartUniversity in Germany. He got Doctor of Engi-neering from Tokyo Institute of Technology in2005. He has been focusing on the research anddevelopment of oxide based TFT since 2005. Heis now a research leader of Technical Research

Institute of Toppan Printing Co., Ltd.

Masato Kon received the doctorate in inor-ganic chemistry from Aoyama-Gakuin Univer-sity in 2003. His thesis is focused on the highrate depositions of oxide films by reactive mag-netron sputtering. In the summer of 2001, heworked as a Guest Scientist at the FraunhoferInstitute in Dresden. He is now a research sci-entist in the Technical Research Institute at Top-pan Printing Co., Ltd. His current interests arein electronic oxide films and its sputter deposi-tion.

Chihiro Miyazaki received the B.S. andM.S. degrees in Science and Technology fromKeio University in 2004 and 2006, respectively.During 2004–2006, she studied microstructurecontrol in textured bismuth layer-structured fer-roelectrics in Ceramic Science Laboratory. In2006, she joined Toppan Printing Co., Ltd. Sheis now research scientist of Technical ResearchInstitute. She is involved in the research and de-velopment oxide thin film transistors since 2006.

Noriaki Ikeda received the B.S. andM.S. degrees in Chemistry from Departmentof Chemistry of Toho University in 2004 and2006, respectively. He studied self-assembledmonolayers (SAMs) of C60-derivatives and goldnanoparticles (AuNPs). Now he belongs to Top-pan Printing Co., Ltd. Technical Research Insti-tute, and he is involved in the research and de-velopment of electric paper.

Mamoru Ishizaki received the B.E., M.E.and Dr. degrees in Engineering from Tokyo In-stitute of Technology in 1986, 1988 and 1991,respectively. He joined Toppan Printing Co.,Ltd. in 1991. He has been focusing on researchand development of flexible TFT since 2004. Heis now a senior researcher of Technical ResearchInstitute at Toppan Printing Co., Ltd.

Yoshiko Ugajin received B.S. and M.S. inChemistry from Tokyo University of Science in1985 and 1987, respectively. She joined ToppanPrinting Co., Ltd. in 1987. She has worked inthe fields of recording media. She has been fo-cusing on the research and development of flex-ible display since 2005. She is now the teamleader of Technical Research Institute at ToppanPrinting Co., Ltd.

Norimasa Sekine received B.E. and M.E. inOrganic Material Science from Tokyo Instituteof Technology in 1982 and 1984, respectively.Since 1984, he has been with Toppan PrintingCo., Ltd. He has worked in the fields of record-ing media and packaging material. He has beenfocusing on the research and development of or-ganic TFT and flexible display since 1999. Heis now the team leader of Technical Research In-stitute at Toppan Printing Co., Ltd.