12.1-in. WXGA AMOLED display driven by InGaZnO thin-film transistors

Jae Kyeong Jeong (SID Member)Jong Han JeongHui Won YangTae Kyung AhnMinkyu KimKwang Suk KimBon Seog GuHyun-Joong ChungJin–Seong ParkYeon-Gon MoHye Dong KimHo Kyoon Chung

Abstract — A full-color 12.1-in. WXGA active-matrix organic-light-emitting-diode (AMOLED) displaywas, for the first time, demonstrated using indium-gallium-zinc oxide (IGZO) thin-film transistors(TFTs) as an active-matrix backplane. It was found that the fabricated AMOLED display did not sufferfrom the well-known pixel non-uniformity in luminance, even though the simple structure consistingof two transistors and one capacitor was adopted as the unit pixel circuit, which was attributed to theamorphous nature of IGZO semiconductors. The n-channel a-IGZO TFTs exhibited a field-effectmobility of 17 cm2/V-sec, threshold voltage of 1.1 V, on/off ratio >109, and subthreshold gate swingof 0.28 V/dec. The AMOLED display with a-IGZO TFT array is promising for large-sized applicationssuch as notebook PCs and HDTVs because the a-IGZO semiconductor can be deposited on large glasssubstrates (larger than Gen 7) using the conventional sputtering system.

Keywords — Oxide TFT.

DOI # 10.1889/JSID17.2.95

1 IntroductionActive-matrix organic-light-emitting diodes (AMOLEDs)provide the best solution to achieving the “ultimate display”due to their fast motion-picture response time, vivid color,high contrast, and super-slim lightweight nature. Recently,the AMOLED display has been burgeoning for mid-to-large-sized applications, such as notebook personal computers(NPCs), monitors, and high-definition television (HDTV),which makes up most of the flat-panel-display market. Themost important point for AMOLED NPCs and/or HDTV ishow to find a way to mass produce it in large quantities at anaffordable price. The best strategy is to increase the mother-glass size up to Gen 6, which corresponds to the most cost-effective backplane size for NPC applications. However,scaling-up of the production line causes several technologi-cal challenges. First, amorphous-Si (a-Si) TFTs are well-es-tablished and is now a proven technology in the LCDindustry. This technology has the advantage of good scalabil-ity up to Gen 8 and a low-cost process because it does notrequire crystallization and the ion-doping process. In addi-tion, the excellent uniformity in device performance,including the mobility and threshold voltage, can be assureddue to its amorphous nature. However, the mobility of a-SiTFTs are quite low (~1 cm2/V-sec), which may not be highenough to drive large-sized and high-resolution AMOLEDdisplays.1 Furthermore, the device instability has been along-time issue.2 For example, the threshold voltage of a-SiTFTs is seriously shifted under constant current stress dueto either the charge trapping into the underlying gate dielec-tric or weak-bonding break-up of silicon and hydrogen ina-Si thin films, leading to the image burn-out or seriousimage sticking (short lifetime) in AMOLED displays. This is

the reason why a-Si TFTs are rarely used as an actual back-plane in AMOLED displays. On the other hand, low-tem-perature polycrystalline-Si (LTPS) TFTs have high mobilityand excellent stability, which is in contrast to their a-Si TFTcounterpart. The key process in fabricating LTPS TFTs isthe crystallization methods that convert a-Si into polycrys-talline-Si (poly-Si). These methods can be classified to non-laser crystallization and laser annealing. Among the non-lasercrystallization processes, the simplest method is solid-phasecrystallization (SPC). But SPC requires 600°C annealing fortens of hours, which makes it unsuitable for large-area glasssubstrates. Other non-laser methods employ metal seeds forcrystallization, which may cause large leakage current in thechannel area. Among the laser methods, excimer-laserannealing (ELA) has been the most widely used due to itsexcellent crystallinity, fast crystallization speed, and highmobility. In addition, ELA is already employed in mass-pro-duction; thus, well-developed apparatus are commerciallyavailable. However, ELA suffers from a narrow process win-dow, high initial investment, and maintenance costs. More-over, the limitation in laser-beam length and instability arethe major obstacles in using ELA on large-sized glass: thelargest equipment available is applicable to Gen 4 (mother-glass size of 730 × 920 mm). Finally, all LTPS TFTs, includ-ing the ELA technique, suffer from non-uniformity becauseof the existence of grain boundaries, which requires the useof a complicated compensation unit pixel circuit such as afive transistor + two capacitor circuit, leading to loss indevice yield.3,4

Therefore, an obvious question arises: “Is there anynew TFT that both demonstrates high mobility and excel-lent uniformity that is suitable for large-sized AMOLEDdisplays?” Amorphous-oxide TFTs can be an attractive alter-

Extended revised version of a paper presented at Display Week 2008 (SID ‘08) held May 20–23, 2008 in Los Angeles, California.

The authors are with Samsung SDI Co., Ltd., Corporate R&D Center, 428-5 Gongse-dong, Kiheung-gu, Yongin-si, Gyeonggi-do 449-902, Korea;telephone +82-31-288-4734, fax –4482, e-mail: ygmo@samsung.com.

© Copyright 2009 Society for Information Display 1071-0922/09/1702-0095$1.00

Journal of the SID 17/2, 2009 95

native solution to this question. Amorphous-oxide semicon-ductors (AOSs) provide unique properties that combine theadvantages of a-Si and LTPS TFTs.5–8 For example, amor-phous-oxide TFTs are free of the non-uniformity in mobilityand threshold voltage, yet exhibit large carrier mobility (~10cm2/V-sec) and excellent subthreshold gate swing (down to0.20 V/dec). Moreover, the channel layer can be fabricatedby using a simple sputtering process. Therefore, large-sizedfabrication can be easily implemented up to Gen 8 withoutusing expensive laser apparatus. The process route is essen-tially the same as that for a-Si TFTs so that the existing pro-duction line can be used without significant change. Inaddition, oxide TFTs can be deposited at room temperature,which, in principle, makes it possible for the mass-produc-tion of AMOLEDs on flexible plastic substrates or cheapsoda-lime glass. The technological comparisons among a-Si,poly-Si, and oxide TFTs are summarized in Table 1.

We have developed a bottom-gate a-IGZO TFT struc-ture that has excellent device uniformities in terms of field-effect mobility, subthreshold gate swing, and thresholdvoltage. Moreover, we successfully demonstrated a 12.1-in.WXGA AMOLED display with a bottom-emission mode,which was driven by a-IGZO TFTs array.

2 ExperimentalLithographically patterned Mo (200 nm) on a SiO2/glasssubstrate with a surface area of 370 × 400 mm2 was used asthe gate electrode. SiNx (120 nm) film, as the gate dielectriclayer, was deposited by plasma-enhanced chemical vapordeposition (PECVD) at a substrate temperature of 390°C.The a-IGZO film was grown by sputtering on the SiO2/glasssubstrate using a polycrystalline In2Ga2ZnO7 target at roomtemperature. The sputtering was carried out at a gas mixingratio of Ar/O2 = 72/28 and an input power of 1200 W. Afterdefining the a-IGZO channel using photolithography andwet-etching, a SiOx etch stopper layer (ESL) was depositedby PECVD and then patterned. The device with photo-acryl(PA) material as the ESL was also fabricated; the PA layer

was formed by using the photo-resist coater and developedto define the source/drain (S/D) contact region. As a S/Delectrode, either Mo or Ti/Al/Ti material was formed bysputtering and defined by photolithography and then pat-terned by dry-etching. Finally, the sample was subjected tothermal annealing at 250°C for 1 hour. The device charac-teristics of the a-IGZO TFTs were measured at room tem-perature with an Agilent 4156C precision semiconductorparameter analyzer.

3 Oxide TFT technology

3.1 TFT structureFigure 1 shows a schematic cross section of the IGZO TFTs,which have an inverted-staggered bottom-gate architecturewith an ESL. For an a-IGZO TFT without an ESL, severedegradation of the subthreshold gate swing and the uni-formity of threshold voltage were observed. This is why wechose an ESL-type structure rather than a back-channeletch structure, which is generally adopted for LCDs.

3.2 Device characteristicsIt was found that the etch stopper material and process isone of the key factors for high-performance transistors witha bottom-gate architecture. Figure 2 compares the repre-sentative transfer characteristics of the a-IGZO TFTs withPA material and SiOx thin film used as the passivation layer.Table 1 lists the principal device characteristics of botha-IGZO TFTs. The threshold voltage (Vth,sat) was definedby the gate voltage, which induces a drain current of L/W ×10 nA at a VDS of 5.1 V. It is noted that the roughly 1 µA and1 nA are needed to embody the full-on and black gray scalefor OLED devices, respectively. Although this type of Vth,satdefinition does not have a unique physical meaning, the gatevoltage to induce a drain current of 10 nA can be a usefulguideline for the panel design as well as the process control.That is why Vth,sat is defined in this way. The apparent field-effect mobility induced by the transconductance at a lowdrain voltage (VDS ≤ 1 V) is determined by

TABLE 1 — Comparison of TFT technologies including a-Si, poly-Si, andoxide TFTs.

FIGURE 1 — The schematic cross section of an a-IGZO TFT with aninverted-staggered architecture.

96 Jeong et al. / A 12.1-in. WXGA AMOLED display driven by InGaZnO TFTs

(1)

where Ci and gm are the gate capacitance per unit area andthe transconductance, respectively. From the transfer char-acteristics, the subthreshold gate swing (S) can be extractedusing the equation

(2)

It can be seen from Table 1 that the µFE (8.4 cm2/V-sec) and Ion/off ratio (>108) for the organic passivated device(device A) are comparable to those (10.8 cm2/V-sec, >108)for the inorganic passivated device (device B). However,there is a dramatic difference in the S value of the twodevices. The S value of device B (0.62 V/decade) is superiorto that (0.90 V/decade) of device A, indicating that theorganic coating and curing on the IGZO back surface led tothe creation of an appreciable number of interfacial traps ator near the IGZO/PA. A similar deterioration of the S valuewas reported for a-Si TFTs with an organic passivation layer,which was attributed to the presence of more fixed chargesand a higher density of states in the back-channel interface.9

Thus, we believe that the interface between the IGZO filmand organic PA material is more susceptible to the forma-tion of water- or carbon-related traps compared to theSiOx/IGZO interface, as will be discussed later. It should be

noted that the overall device performances of amorphous-IGZO TFTs are better than those of microcrystalline-Si anda-Si TFTs.

Next, we investigated the effect of ESL material onthe bias stability of the resulting a-IGZO TFTs. The devicewas stressed under the following conditions: the IDS was setto 10 µA and the VDS was fixed at 5.1 V. The maximum stressduration was 36,000 sec. The switching and driving transis-tor in the basic unit pixel circuit of AMOLED displays actas a switcher for the data signal and analog driver (constantcurrent source) for the OLED diode, respectively.10 There-fore, the driving transistor in an actual pixel is under condi-tions of more severe stress. This is the reason why theconstant IDS stress condition was chosen. Considering thatan IDS of approximately 1 µA is required to manifest thefull-white gray color in an AMOLED device,10 the appliedstress current (10 µA) corresponds to very severe test con-ditions. Figures 2(a) and 2(b) show the evolution of thetransfer curves as a function of the applied stress time fordevices A and B, respectively. It can be seen that whiledevice A shows a change in the S value as well as a largepositive Vth shift during bias stress, a parallel Vth shift to ahigher gate voltage with increasing stress time without anysignificant change in the field-effect mobility, S, or Ion/offratio was observed for device B. Although Vth for device Awas strongly shifted by approximately 8.1 V, from 5.5 to 13.6V, the positive Vth shift in device B was very small (~2.0 V)after a constant drain current stress of 36,000 sec.

The positive Vth shift of oxide TFTs under positive-gate-voltage stress has been explained by two models:charge trapping or defect creation.11–14 While the parallelshift in Vth without significant change in the S value duringstress is attributed to simple charge trapping in the gatedielectric and/or at the channel/dielectric interface, thepositive shift in Vth accompanying the change in S is a resultfrom the creation of defects within the oxide-semiconductorchannel material. In addition, it has been claimed that therecovery behavior of Vth without any thermal annealingexcludes the possibility of charge injection into the gatedielectric.11 In this study, because devices A and B have anidentical gate dielectric and IGZO semiconductor, thecharge trapping at or near the channel/dielectric interfaceduring gate-voltage stress would be expected to be similarto each other. However, the positive Vth shifts for devices Aand B were 8.1 and 2.0 V, respectively. Therefore, the insta-bility of the Vth shift cannot be explained by only the charge-trapping model. We believe that this instability comes fromthe environmental influence (O2, H2O, etc.) rather thanchare trapping at/near the interface between the gatedielectric and a-IGZO semiconductor, which will be pub-lished elsewhere because its detailed mechanism is beyondthe scope of this paper. Therefore, it is suggested that a suit-able ESL of the active back channel for a bottom-gate struc-ture is believed to be one of a key factors in achieving a goodsubthreshold property and excellent device reliability.

mFEm

i DS

LgWC V

= ,

SdV

d IGS

DS=

(log ).

FIGURE 2 — The representative transfer curve of an a-IGZO TFT withan W/L = 25/10 µm with an ESL. The S value and Vth were very sensitivelyaffected by the material of the ESL.

TABLE 2 — Comparisons of the extracted parameters forn-channel a-IGZO TFTs with (a) photo-acryl and (b) SiO2 thinfilm as the ESL.

Journal of the SID 17/2, 2009 97

3.3 Uniformity of IGZO TFT performanceBecause the luminance of an OLED device is determinedby the drain current of the driving transistor, which is seri-ally connected to an OLED diode, the long-range uniform-ity of TFTs is of importance compared to its TFT-LCDcounterpart. The standard deviation of the electricalparameter for the transistor performance was determinedfrom the evaluation of nine points on one panel.

Figure 4 shows the representative transfer charac-teristics of optimized IGZO TFTs with W/L = 25/10 µm,which was taken for a full-array panel device rather than anindividual test device without any suitable ESL or passiva-tion layer. The enhancement in the performance of theIGZO TFT was related to the structural properties of

the semiconductor material and the deposition condition ofthe ESL. In particular, the densification of IGZO thin filmvia reducing the chamber pressure during channel deposi-tion was found to be an effective way to improve the field-effect mobility and S value of the oxide device. We notedthat the film density (6.2–6.3 g/cm3) of IGZO active filmsused in this transistor larger than that compared to that(5.7–5.9 g/cm3) of the devices shown in Fig. 2. Also, thedevice performance was greatly dependent on the deposi-tion temperature of the SiOx ESL. The S value increased byincreasing the deposition temperature, and the ESL of theoptimized device was deposited by PECVD at 200°C. Thus,the high µFE of 17 cm2/V-sec, an excellent SS of 0.28 V/dec-ade, and a good Ion/off ratio of >109 was achieved, which isthe state-of-the-art characteristics for ZnO-based oxideTFTs. We previously reported that the denser IGZO filmhas a lower interfacial trap density (Dit) between the gatedielectric and active channel layer, leading to an improve-ment in the SS value.15 Therefore, the improvement indevice performance is attributed to the lower Dit due to theelectronic high quality of the denser active film. The long-range standard deviations for threshold voltage and subthre-shold gate swing for oxide TFTs were 0.1 and 0.02 V/decade,respectively, indicating that the excellent statistics can beguaranteed because of the amorphous phase nature of theIGZO channel layer.

Next, we investigated the short-range uniformity(SRU) of an oxide TFT: The adjacent distance between tran-sistors was approximately 150 µm, which is quite similar tothe pixel-to-pixel distance. The standard deviation of thefield-effect mobility for nine-point evaluation was 0.055cm2/V-sec. Surprisingly, the SRU of the threshold voltagewas dramatically improved, which is essential for high-image

FIGURE 3 — (a) The evolution of the transfer curve of an a-IGZO TFTwith (a) photo-acryl and (b) SiOx during severe drain current stress of thea-IGZO. The VDS was fixed at 5.1 V and the stressed VGS voltage wasset to the gate voltage, which induces a drain current of 10 µA.

FIGURE 4 — The collected transfer curves of an a-IGZO TFT with W/L =25/10 µm with an ESL. SRU data from a nine-points measurement isincluded in the inset of Fig. 4, where the adjacent distance betweentransistors was approximately 150 µm, which is quite similar to thepixel-to-pixel distance.

98 Jeong et al. / A 12.1-in. WXGA AMOLED display driven by InGaZnO TFTs

quality displays without pixel non-uniformity. The achievedstandard deviation of the SRU was less than 0.01 V as shownin Fig. 4. The simple calculation predicts the non-uniform-ity in luminance to be less than 2%. This result suggests theuse of an ultra-simple pixel circuit of 2Tr + 1Cap can be usedfor the design of AMOLED displays, which will impact theresulting device yield and cost very positively.

4 12.1-in. AMOLED protopyteIn this work, we have successfully developed a full-color12.1-in. WXGA AMOLED display, which was driven by ana-IGZO TFT backplane. The specifications of an 12.1-in.AMOLED display is summarized in Table 3. The display hasa pixel count of 1280 × RGB × 768 with a resolution of 123ppi. Its subpixel pitch is 69 × 207 µm2 and the pixel elementis 2Tr + 1Cap. The channel length for the driving transistorwas designed to be 10 µm, but the kink effect, whichappears in poly-Si TFTs with the same channel length, wasnot observed in the output characteristics. The scan driverwas integrated on the panel and its functionality was suc-cessfully demonstrated. The bottom-emission structure wasadopted. The structure has a transparent anode, organic lay-ers, and a cathode on the TFT backplane. The OLEDdevice structure consisted of a hole-injection layer (HIL),hole-transport layer (HTL), RGB-emitting layer (EML),electron-transfer layer (ETL), electron-injection layer(EIL), and cathode. The phosphorescent red and fluores-cent green and blue were used as emitting materials. Figure5 shows an image of a 12.1-in. WXGA AMOLED display forNPC application. The luminance and contrast ratio were>300 nit and 20,000:1, respectively. And the NTSC of thefabricated panel was more than 80%. These features arecomparable to the specifications of a NPC attainable usingTFT-LCDs. Recently, LG Electronic reported a 3.5-in.

QCIF+ AMOLED prototype, which was driven by ana-IGZO TFT backplane.15 We have also reported a 4.1-in.transparent QCIF AMOLED display using a-IGZO TFTs atIMID 2007.16 Thus far, the research and development fora-IGZO TFTs has been focused on the demonstration ofrather small-sized (<5 in.) AMOLED displays. It should beemphasized that our prototype is the largest and highest-resolution AMOLED displays that uses a a-IGZO-TFTbackplane.

5 ConclusionIn summary, our fabricated IGZO TFT exhibited a high µFEof 17 cm2/V-sec, an excellent SS of 0.28 V/decade, and agood Ion/off ratio of >109. Also, the impact of ESL on thestability of IGZO TFTs was investigated in detail. It wasshown that the device with SiOx ESL had better stabilitythan that with PA ESL. Furthermore, the full-color 12.1-in.WXGA AMOLED display was demonstrated using a-IGZOTFT backplane. Considering that a-IGZO TFTs can be eas-ily expanded into a large-sized substrate (greater thanGen. 7) because an a-IGZO semiconductor is deposited bya conventional sputtering technique, our demonstration of a12.1-in. WXGA indicates that a-IGZO backplane technol-ogy is an ideal candidate for large-sized AMOLED displays.

AcknowledgmentsThe authors appreciate K. N. Kim who partially designedthe layout and simulated circuit for the 12.1-in. panel. Theauthor would like to thank all the engineers related to theoxide TFT project at Samsung SDI.

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FIGURE 5 — The display image of a 12.1-in. WXGA AMOLED display,driven by an IGZO TFT array.

TABLE 3 — The specifications of a 12.1-in. WXGA AMOLEDdisplay.

Journal of the SID 17/2, 2009 99

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