Design of a Reverse-Connected Module for Nanostructure Removal From Display Color Filters

3388 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 57, NO. 12, DECEMBER 2010

Design of a Reverse-Connected Modulefor Nanostructure Removal From

Display Color FiltersP. S. Pa

Abstract—This paper proposes a new design module forreverse-connected microelectrochemical machining (μ-ECM),which is based on a novel electrochemical reaction that offers thefast removal of indium-tin-oxide (ITO) nanostructures from thesurfaces of color filters in displays. Unlike general μ-ECM models,the reverse-connected μ-ECM model is implemented by connect-ing a workpiece to the cathode and an electrode to the anode of adc power supply. During the course of machining, electrons travelfrom the cathode to the anode, so that the ITO film is lifted andremoved at high speed by an electric field. In general μ-ECMmodels, a workpiece is connected to the anode, an electrode isconnected to the cathode, and the material removal is achievedby dissolution. In contrast, the reverse-connected μ-ECM modelprovides higher removal efficiency and higher surface qualityof color filters in displays during machining processes, therebybreaking new ground in superfine machining for applications inthe photoelectric semiconductor industry.

Index Terms—Displays, microelectrochemical machining(μ-ECM), module design, nanoscale, nanostructures, opticaldevices, reverse connected.

I. INTRODUCTION

THE DEVELOPMENT of flat-panel displays (FPDs), par-ticularly thin-film transistor liquid crystal displays (TFT-

LCDs), has made the TFT-LCD industry one of the fastestgrowing industries in the world, following the semiconductorindustry [1]. The purpose of a display monitor is to recreatethe real world in front of our eyes so that we can enjoy avisual experience with the best possible quality and the mostaccurate representation of information. Displays must havecolor to achieve this ideal. Flat-panel LCD displays are, withouta doubt, the way of the future, and their rapid development inrecent years has made them one of the most important fieldsin high technology today [2], [3]. The method for creatingthe color filter begins with depositing chromium oxide onthe glass substrate, forming a chromium metal-based black-matrix photomask layer. The openings in the matrix are then

Manuscript received May 1, 2010; revised June 29, 2010; acceptedSeptember 12, 2010. Date of publication October 11, 2010; date of currentversion November 19, 2010. This work was supported by the National ScienceCouncil under Contract 98-2221-E-152-001 and Contract 99-2221-E-152-001.The review of this paper was arranged by Editor A. M. Ionescu.

The author is with the Department of Digital Content Design and theGraduate School of Toy and Game Design, National Taipei University ofEducation, Taipei City 106, Taiwan (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TED.2010.2078820

overlaid with the three primary colors, i.e., red (R), green (G),and blue (B), to create a color layer. This is then given anovercoat (O/C) of indium tin oxide (ITO) to create a transparentelectrode. There are a variety of methods used to achievethe above result, such as dyeing, pigment dispersion, printing,and electrodeposition. Resin is used in the pigment dispersionmethod of manufacturing the photomask for the color resist.The procedure is as follows: First, the resin photomask layeron the glass substrate is subjected to the patterning process,and then, the three colors R, G, and B are deposited usingthe patterning process. If necessary, a protective O/C or silicondioxide (SiO2) layer is applied. When the transparent electrodelayer of the ITO film is applied, the procedure is complete.To increase the production capacity for LCD panels, the sizeof the glass substrate must increase. To achieve this, severallarge glass substrates with preformed color filters are matchedto glass substrates with identical pixel electrode arrays. Liquidcrystal is then injected to complete the assembly process [4],[5]. Glass plated with ITO has good electric conductivity and iscommonly used for the electrodes in LCDs [6].

The color filter is the main component within the TFT panel.Sixty percent of its costs are material related, with the glass sub-strate having the biggest share. In a 15-in TFT panel, the colorfilter represents 24% of the overall costs. This is higher than thepercent of the overall cost for the backlighting module (17%) orthe driver IC (17%), making the color filter the most expensivecomponent to produce. The trend is for glass substrates tobecome lighter (lower density), larger, and thinner in responseto environmental factors. Color filter growth is closely linked topanel trends. As panels have become cheaper, the demand forNotebook (NB) and LCD monitors has continued to grow. Thegrowth has been particularly strong in LCD monitors. As paneldimensions continue to increase, it is becoming increasinglydifficult to transport color filters. Manufacturers will eventuallyadopt internal color filter production lines. This is expected toincrease the proportion of color filters manufactured in-house.The substrates from the fifth generation onward are over a meterlong and less than 0.7 mm thick. Thus, the technical difficultyof production and the risk of breakage during transportationhave both greatly increased [7], [8]. Production is a completelyautomated process and is carried out in a clean room. The returnon investment from LCD production is determined by the yieldrate. As such, LCD manufacturers’ return on investment hasa major influence on the price of the LCD. For example, ifthe glass substrate on the production line has the dimensions

0018-9383/$26.00 © 2010 IEEE

PA: DESIGN OF A MODULE FOR NANOSTRUCTURE REMOVAL FROM DISPLAY COLOR FILTERS 3389

Fig. 1. Experimental setup of the reverse-connected model.

500 × 650 mm, then they can each produce four sets of 15-inLCD modules. If the yield rate is 100%, then 2500 color filtersof the same size are required. If the yield rate is only 50%, then5000 color filters are actually needed [9].

Addressing all of the aforementioned problems is extremelydifficult. To reduce the production costs of color filters, defec-tive color filter substrates must be recovered so that they canbe reused. Defective color filters cannot be recovered using thepreviously mentioned recovery technology. No matter how theissue is approached, the disposal of large numbers of costlyglass substrates with color resin, or resin photomask layersalready formed, is a major environmental and economic prob-lem. Therefore, the primary objective of this paper is to ensurethat, during the recovery process, the defective glass substrateis not corroded by other substances [10]. Fierce competitionbetween panel makers means that, apart from striving for betterquality, continuing to reduce costs is a critical issue as well.Panel makers that are aggressively controlling costs wouldbenefit from a means to lower costs through further integrationwhile maintaining a stable output. The fifth-generation productlines continue to experience high defect rates in color filterglass substrates. As the industry becomes more competitive,manufacturers must focus on reducing costs and improvingtheir technology [11].

Color filter recycle technology and applications have there-fore become the latest hot topic in the optoelectronic man-ufacturing industry. This system has great potential in thedomestic market with widespread applications in optoelectronicand semiconductor-related industries, particularly for new orexisting manufacturing facilities that deal with nanoscale mi-croelectromechanics. The production cost of a TFT-LCD panelis affected by the following factors: 1) material cost; 2) size ofthe panel; and 3) yield rate. The yield rate can be affected bymany factors, such as size, purpose, material, and equipment.The primary cause of reduced yield rate in LCD productionis “dust.” When dust particles attach to the LCD substrate,they impair its function, causing breaks in the circuit, shortcircuits, or poor performance [9], [12]. In this paper, we focuson the method and technology required for the recovery andreuse of defective color filters. This method has applications forcolor LCD monitors used in notebook computers, workstations,instrumentation used in cars, planes, and ships, as well as LCDtelevisions. This technology offers a method of recovery and

reuse of defective color filters discovered during the produc-tion of color displays. Through the treatment proposed in thispaper, it is possible to efficiently strip away the ITO layer,the color resist layer, and the O/C layer, leaving a color filterglass substrate that retains its resin photomask or chromiumphotomask layer. This can then be recovered and fed back intothe color filter production line, where the color resist can beapplied again, resulting in significant savings during the massproduction of color filters. In addition, increasingly stringentenvironmental protection requirements mean that this will alsoresult in a reduction of glass and heavy metal waste, which isa serious pollution problem. Therefore, this technology offersmany benefits [13]–[16].

The electrolyte between the workpiece is dissolved andtherefore removed through an electrochemical reaction dur-ing electrochemical machining (ECM) [17]. The micro-ECM(μ-ECM) process, which is also known as the electrochemicalmicromachining process, has the advantage that a microma-chined metal workpiece will have any residual stress remainingon its surface and the tool electrode will not break. The electricfield distribution of the μ-ECM process improves the geometricprofile and fabrication precision of microholes [18], [19]. Datahave shown that the gap width between the electrode and theworkpiece directly influences the current condition and thedischarge dreg of the electrolyte in the electrochemical process[20]. Low yield during production is easily seen. A futurechallenge facing the display industry is how to bring downproduction costs and improve yield as much as possible [21].

In this paper, the reverse-connected model of μ-ECM, withthe workpiece (ITO) connected to the cathode, is applied to thenanoscale precision recovery process of display color filters,and an empirical approach is adopted to compare the reverse-connected μ-ECM model with the general μ-ECM models.

II. EXPERIMENTS AND PARAMETERS

The experimental setup of reverse-connected μ-ECM isschematically illustrated in Fig. 1. The configuration of theelectrodes and the workpiece of the general μ-ECM moduleis shown in Fig. 2(a); the rotational anode is closer to theworkpiece (ITO) surface, and another rotational cathode is keptat a set distance from the ITO surface. The configuration of theelectrodes and the workpiece of reverse-connected μ-ECM is


Fig. 2. (a) Configuration of the electrodes and the workpiece of the general μ-ECM model. (b) Configuration of the electrodes and the workpiece of the reverse-connected μ-ECM model.

shown in Fig. 2(b), where the rotational cathode is closer to theITO surface and delivers the negative charge to the ITO. Therotational anode was kept a distance s from the ITO surface.The workpiece used in this paper was a sixth-generation LCDpanel (1300 × 1100 mm; 0.7 mm). The amount of ITO mate-rial removed from the surface of the display color filter afterμ-ECM was 150 nm (the average thickness of the ITO was150 nm in this paper). The electrolyte was 10 wt.% H3PO4

(phosphoric acid) and 10 wt.% NaNO3. The temperature ofthe electrolyte was 50 ◦C. The flow rate of the electrolyte was20 L/min. The rotational speed of the electrodes was 600 r/min.The electric voltages were 50, 70, 90, and 110 V. The feedrate of the workpiece (display color filter) ranged from 25 to500 mm/min. The distances between the anode and the cathodewere 3, 6, and 9 mm. All workpieces were cleaned with waterafter the recycling process and dried by air. These recycled ITOnanostructures were measured at more than two locations bythe NanoSpec Film Thickness Measurement System (NanospecFilm Analyzer 3000). The chromaticity of the color filters wasmeasured using a Display Color Analyzer (Model 7121). Thelight transmittance of the display’s color filters was measuredusing spectrometers (PerkinElmer LAMBDA 900).

III. RESULTS AND DISCUSSION

According to the formula for the theoretical removal rate ona purity metal derived from Faraday’s law [17]

W =(I)(T )(M)

(F )(n)(1)

where I is the current, t is time, F is the Faraday constant, andni is the atom number.

For an alloy consideration

W =(I)(T )

(F )(

nA

MAaA + nB

MBaB + · · ·

) (2)

where ai is the proportion of composition, and Mi is the atomicvalance.

Let E = W/ρAT

E =(V )(σ)

(F )(A)(ρ)(

nA

MAaA + nB

MBaB + · · ·

) (3)

where V is the electric voltage, σ is the reciprocal resistance ofthe electrolyte, A is the ECM area, ρ is the workpiece density,and E is the removal rate in the normal direction. From thepreceding equations, the theoretical feed rate of the workpiecethat gives the same material removal rate can be calculated.The parameters F and A are regarded as constants of thematerial.

From Fig. 2(a), one assumes

K = Rc + h = Ra + s + h (4)

where s is the distance between the anode and the cathode(ITO surface), h is the isotropic reactions of the removal


Fig. 3. Comparison of theoretical predictions and experimental results atdifferent feed rates of the workpiece using different current ratings (H3PO4 of10 wt.% and NaNO3 of 10 wt.%, 50 ◦C, 20 L/min, continuous dc, electrodes600 r/min).

depth of μ-ECM, and Ra is the rotational radius of the anode.Moreover

cos φ =K − h

K=

RC

RC + h=

Ra + s

Ra + s + hp (5)

(Em) sin φ =E. (6)

Squaring and simplifying (5) and (6), one obtains

h =(RC)E2

2 (E2m − E2)

=(Ra + s)E2

2 (E2m − E2)

(7)

where Em is the feeding velocity of the workpiece, and E is theremoval rate in the normal direction. From (7), one obtains

h=

(Ra+s)(V 2)(σ2)

[1

(F )2(A)2(ρ)2(

nAMA

aA+nBMB

aB+···)2

]

2 (E2m)−2(V )2(σ)2

[1

(F )2(A)2(ρ)2(

nAMA

aA+nBMB

aB+···)2

] .

(8)

The theoretical feed rate of the workpiece Em that gives thesame material removal rate can be calculated. The parametersF and A are regarded as constants of the material.

Fig. 3 shows that the ITO nanostructures will be graduallydissolved at the nanoscale in the μ-ECM model when a higherfeed rate of the display color filter is combined with enoughelectric voltage. At a constant electric voltage, a display colorfilter has an optimal feed rate for the removal process. Afaster feed rate reduces the power delivered per unit area ofthe plane surface, whereas a slower feed rate increases it. Toremove 150 nm of an ITO thin film, the following combina-tions of parameter values are suggested for traditional μ-ECM:50 V and 300 mm/min, 70 V and 350 mm/min, 90 V and400 mm/min, and 110 V and 450 mm/min. From (8), theisotropic reactions of the removal depth h of the theoreticalpredictions of μ-ECM agree well with the experimental results

Fig. 4. Comparison of removal speeds from reverse-connected μ-ECM andgeneral μ-ECM (H3PO4 of 10 wt.% and NaNO3 of 10 wt.%, 50 ◦C, 20 L/min,continuous dc, 90 V, electrodes 600 r/min).

(see Fig. 3). According to the experimental results, the removaldepth h is directly proportional to the electric voltage V andis inversely proportional to the feed rate of the workpiece Em.This agrees well with the theoretical prediction (see Fig. 3).

For the purpose of demonstrating the practical performanceof reverse-connected μ-ECM, as shown in Fig. 4, the ITO thinfilm on the surface of the display color filter was completelylifted and removed at high speed by the reverse-connectedmodel of μ-ECM. This method is different from generalμ-ECM, in which the ITO thin film is gradually removed bydissolving the module (4). The average ITO film removingspeed of reverse-connected μ-ECM is higher than that of gen-eral μ-ECM under the same ECM conditions (Fig. 4). There isa distance b between the cathode and the workpiece surface thatallows the ITO thin film to be lifted [Fig. 2(b)].

Thus, one assumes

K = Rc + h = Ra + (s + b) + h. (9)

Following the deducing process contrasts, as in (5)–(7), oneobtains

h=

[(Ra+(s+b)](V 2)(σ2)

[1

(F )2(A)2(ρ)2(

nAMA

aA+nBMB

aB+···)2

]

2(E2m)−2(V )2(σ)2

[1

(F )2(A)2(ρ)2(

nAMA

aA+nBMB

aB+···)2

] .

(10)

From [10], one obtains

Em =

⎡⎢⎣ 1

(F )2(A)2(ρ)2(

nA

MAaA + nB

MBaB + · · ·

)2

⎤⎥⎦

12

×[Ra + (s + b)(V )2(σ)2 + 2h

2h

] 12

. (11)


Fig. 5. (a) ITO beginning to lift by reverse-connected μ-ECM (H3PO4 of10 wt.% and NaNO3 of 10 wt.%, 50 ◦C, 20 L/min, continuous dc, 90 V,electrodes 600 r/min). (b) ITO removed and completely lifted by reverse-connected μ-ECM (H3PO4 of 10 wt.% and NaNO3 of 10 wt.%, 50 ◦C,20 L/min, continuous dc, 90 V, feed rate 475 mm/min of the display color filter,electrodes 600 r/min).

The experimental results (Fig. 4) for ITO film removal usingthe reverse-connected model of μ-ECM are directly propor-tional to the theoretical prediction Em, as shown in (11).Once the rotational cathode was connected closer to the ITOsurface and has delivered the negative charge to the ITO bythe reverse-connected μ-ECM module, it appears that the ITOis completely lifted (Fig. 5) and removed at the feed rate(500 mm/min) of the display color filter compared with thefeed rate of 375 mm/min by being completely dissolved in thegeneral μ-ECM module (Fig. 4). Thus, the ITO removal speedfor reverse-connected μ-ECM is 1.33 times that of generalμ-ECM.

Fig. 6 shows the effects of varying the distance s betweenthe anode and the cathode. The results illustrate that a smalldistance s results in less time required for removing the sameamount of ITO nanostructure, because the effect of reverse-connected μ-ECM is easily developed to supply sufficientelectrochemical power. Compared with the experimental resultsand (11), a small distance s accompanied by a large electricpower and a fast feed rate of the workpiece (display color

Fig. 6. Removal speed at different feed rates using different distances sbetween the anode and the cathode (H3PO4 of 10 wt.% and NaNO3 of10 wt.%, 50 ◦C, 20 L/min, continuous dc, 90 V, electrodes 600 r/min).

filter) reduced the etching time. To facilitate effective removalof the lifted ITO film from the machining gap, the anodeand the cathode are preferably spaced at a distance greaterthan 3 mm. The anode and the cathode are of a rotary type(600 r/min), and the electrolyte is fed along the direc-tion in which the anode and the cathode rotate. The sur-face roughness is also maintained at the same value whenthe ITO film is removed through reverse-connected μ-ECM[Fig. 7(a)]; the average surface roughness of the display’scolor filters, i.e., Ra, is around 0.32 μm before ITO evap-oration. The reverse-connected μ-ECM module is a way ofcompletely lifting the ITO film on the surface of the dis-play color filter during the removal process, decreasing thescoring of the color filter surface and easily maintaining thequality of the color filter surface [Fig. 7(b)]. The averagechromaticity and the average light transmittance of the threeprimary colors (R, G, and B) are also maintained after ex-ecuting reverse-connected μ-ECM to remove the ITO film(Table I; Fig. 8) compared with without coating the ITO.The defective ITO layer can be removed, and the defectivecolor filter can be put back into the production line. Selec-tively removing the ITO film from color filter substrates byreverse-connected μ-ECM is an effective way to maintain thehigh quality of workpiece surfaces while effectively reducingproduction costs.

IV. CONCLUSION

Establishing a new design module for a reverse connection,using μ-ECM, that removes the ITO thin film from FPDcolor filters and reclaims the filters for production helps theoptoelectronic semiconductor industry and provides an efficientand clean process. During the removal process, the ITO thinfilm on the surface of the display color filter is completelylifted and removed at high speeds using the reverse-connectedmodel of μ-ECM. The average ITO film removing speed forreverse-connected μ-ECM is at least 1.27 times higher thanthat of general μ-ECM. A small distance (3 mm) betweenthe anode and the cathode results in less time required forremoving the same amount of ITO nanostructure, because


Fig. 7. Surface condition of the display’s color filters after the electroremoval process (H3PO4 of 10 wt.% and NaNO3 of 10 wt.%, 60 ◦C, 25 L/min, continuousdc, 75 V, electrodes 600 r/min). (a) Surface roughness. (b) Surface condition.

TABLE ICOMPARISON OF CHROMATICITY OF THE DISPLAY’S COLOR FILTERS

Fig. 8. Comparison of light transmittance of the color filters (H3PO4 of 10 wt.% and NaNO3 of 10 wt.%, 60 ◦C, 25 L/min, continuous dc, 75 V,electrodes 600 r/min).

the effect of reverse-connected μ-ECM is easily developed tosupply sufficient electrochemical power. When applied to thenanoscale precision recovery process for display color filters or

electronic paper, the method proposed in this paper proves to bean excellent engineering solution capable of lowering costs andreducing the fraction of defective devices.


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P. S. Pa received the Ph.D. degree from National Tsing Hua University,Hsinchu, Taiwan.

He is currently a Professor with the Department of Digital Content De-sign and the Graduate School of Toy and Game Design, National TaipeiUniversity of Education, Taipei, Taiwan. His research interests are innovativedesign and manufacturing processes. His current research interests include flat-panel displays, electrochemical machining processes, precision manufacturingprocesses, toy and game design, robots, and semiconductor and electroopticalengineering.

Design of a Reverse-Connected Module for Nanostructure Removal From Display Color Filters

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