Low-temperature polysilicon thin film transistors on polyimide substrates for electronics on plastic

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Low-temperature polysilicon thin film transistors on polyimide substrates for electronics on plastic
  2282 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 12, DECEMBER 1999 Low-Temperature Polysilicon Thin-FilmTransistor Driving with Integrated Driver forHigh-Resolution Light Emitting Polymer Display Mutsumi Kimura, Ichio Yudasaka, Sadao Kanbe, Hidekazu Kobayashi, Hiroshi Kiguchi,Shun-ichi Seki, Satoru Miyashita, Tatsuya Shimoda, Tokuro Ozawa, Kiyofumi Kitawada,Takashi Nakazawa, Wakao Miyazawa, and Hiroyuki Ohshima  Abstract— A high-resolution low-temperature polysilicon thin-film transistor drivenlight emitting polymer display (LT p-Si TFTLEPD) with integrated drivers has been developed. We adoptedconductance control of the TFT and optimized design and voltagein order to achieve good gray scale and simple pixel circuit. Ap-channel TFT is used in order to guarantee reliability in dcbias. An inter-layer reduces parasitic capacitance of bus lines.Because of the combination of the LT p-Si TFT and LEP, thedisplay is thin, compact, and lightweight, as well as having lowpower consumption, wide viewing angle, and fast response. I. I NTRODUCTION L OW-TEMPERATURE polysilicon thin-film transistors(LT p-Si TFT’s) have been utilized to drive liquidcrystal displays (LCD’s) [1]–[3]. There are many candidatesfor active matrix devices, i.e., single-crystal Si MOS FET,amorphous Si TFT, high-temperature p-Si TFT, LT p-Si TFT,other semiconductor devices, etc. Among the candidates, onlythe LT p-Si TFT has performance high enough to composeintegrated driver circuits and the capability of being fabricatedon a large transparent substrate simultaneously. Additionally,it has already been reported that the LT p-Si TFT can also befabricated on a plastic substrate [4]. These advantages of theLT p-Si TFT allow the present great successes to come true inLCD’s, not only in research and development, but also in themarket. However, the LT p-Si TFT is not only for LCD’s. TheLT p-Si TFT’s have great potential even for other displayswhich have integrated driver circuits and are large sizes [5].On the other hand, light emitting polymers (LEP’s) [6]–[8]promise to achieve thin, compact, lightweight, and inexpen-sive displays. Moreover, the display can have low powerconsumption, wide viewing angle, and fast response. Untilnow, for LEP displays (LEPD’s), mainly static and passivematrix driving methods have been utilized. However, for high-resolution displays consisting of many pixels, needless to say,the static method cannot drive the LEP. The passive matrix Manuscript received October 1, 1998; revised June 1, 1999. The review of this paper was arranged by Editor J. Hynecek.M. Kimura, I. Yudasaka, S. Kanbe, H. Kobayashi, H. Kiguchi, S.Seki, S. Miyashita, and T. Shimoda are with Base Technology ResearchCenter, Seiko Epson Corporation, Owa Suwa 392-8502, Japan (e-mail:kimura.mutsumi@exc.epson.co.jp).T. Ozawa, K. Kitawada, T. Nakazawa, W. Miyazawa, and H. Ohshima arewith L Project, Seiko Epson Corporation, Owa Suwa 392-8502, Japan.Publisher Item Identifier S 0018-9383(99)09015-2. method cannot drive the LEP, either [9], because the high-resolution display demands high voltage in the short scanningperiod in order to achieve the required average brightness, andthis high voltage results in a lower power efficiency of the lightemitting. Accordingly, instead of the static or passive matrixdriving method, an active matrix driving method is better forhigh-resolution display as the pixels may be driven close totheir best power efficiency point.Since the LEPD is not a cell structure, i.e., liquid layerand two sandwiching substrates, it does not need the secondsubstrate. Moreover, the LEPD does not need a backlight, lightguide, polarizer, diffuser, etc., which are used in the LCD.Therefore, the display consists of one substrate, peripheraldrivers, and many contacts between them. The next target is toeliminate the peripheral drivers and contacts. If the peripheraldrivers are replaced by monolithic drivers integrated on thesubstrate, not only can the peripheral drivers be eliminated,but the number of contacts can also be decreased. The displayis dramatically reduced to only one substrate. As a result, thedisplay will be exceedingly thin, compact, lightweight, andinexpensive.Because of the advantage of the wide viewing angle, theLEPD is suitable for direct view applications. Most applica-tions such as these are large size displays. In the case of thecurrent LEPD structure, since the polymers and cathode metalare serially stacked on the substrate and light emits throughthe substrate, the substrate must be transparent. Therefore, forthe device to drive the LEPD, the capability of fabrication ona large transparent, i.e., glass or plastic, substrate is needed.In conclusion, in order to drive the high-resolution LEPD,the active matrix device is needed and it must have enoughperformance to compose integrated drivers and have the ca-pability to be fabricated on the large transparent substrate,simultaneously. Only the LT p-Si TFT can satisfy thesedemands.Therefore, the objective of our development in this paperis to confirm how the LT p-Si TFT is suitable to drivethe high-resolution LEPD. A high-resolution LT p-Si TFTLEPD with integrated drivers is designed, fabricated, andevaluated [10]. We adopted conductance control of the TFTand optimized design and voltage in order to achieve goodgray scale and simple pixel circuit. A p-channel TFT is used inorder to guarantee reliability in dc bias. An inter-layer reduces 0018–9383/99$10.00  󰂩  1999 IEEE  KIMURA  et al .: HIGH-RESOLUTION LIGHT EMITTING POLYMER DISPLAY 2283 Fig. 1. Cross-sectional view of the LT p-Si TFT LEPD. LT p-Si TFT’s arefabricated the same as the TFT-LCD. Light comes through the glass substrate.Since the TFT-LEPD needs only some thin films on one substrate, very thin,compact, lightweight, and inexpensive displays can be achieved. The functionof the inter-layer is to distance the cathode and to reduce parasitic capacitanceof bus lines. parasitic capacitance of bus lines. The display is thin, compact,lightweight, low power consumption, wide viewing angle, andfast response.II. S TRUCTURE A cross-sectional view of the TFT-LEPD is shown inFig. 1. First, on a glass substrate, LT p-Si TFT’s, bus lines,and pixel electrode are fabricated the same as they are inthe TFT-LCD [1], [2]. A 50-nm a-Si is formed by LPCVDof Si H at 425 C. It is crystallized by multiple irradi-ation of 245 mJ/cm KrF excimer laser. Phosphorous ionsfor n-channel TFT’s and boron ions for p-channel TFT’sare implemented with a dose of the 10 cm order at anenergy of several ten keV. These impurities are activated at300 C–400 C for 4 h. The TFT characteristics are shownin Fig. 2. Mobility for n-channel TFT and p-channel TFT is120 cm /V s and 40 cm /V s, respectively. In the case of the LCD, the ITO pixel electrode is used in order to applyvoltage to the liquid crystal. On the other hand, in the caseof the LEPD, the ITO pixel electrode is used as an anode inorder to supply current to the LEP.Next, an adhesive layer, inter-layer are fabricated. Thefunction of the SiO adhesion layer is to improve adhesionbetween ITO and polyimide. The function of the polyimideinter-layer will be written in the following section. After thefabrication of the both layers, O plasma surface operation isdone in order to improve the wettability of the surface of thepolyimide and ITO.After that, LEP layer consisting of a conductive polymer anda light emission layer, and a cathode metal are fabricated insuccession [8]. First, polyethylene dioxythiophene/polystylenesulphonate (PEDOT/PSS) are dispersed in water and spin-coated. Since the surface of the substrate is wettable by Oplasma operation mentioned above, the spin-coated layer canbe very uniform. Then, the spin-coated layer is baked in orderto remove solvent and make a thin film. Next, precursor of poly(p-phenylene vinylene) (PPV) is deposited by spin-coatingwith a water/methanol mixture as solvent. Thermal conversionforms the precursor to the conjugated polymer, PPV. The (a)(b)Fig. 2. TFT characteristics. (a) Transfer characteristics and (b) output char-acteristics are shown. Mobility for n- and p-channel TFT is 120 cm   /V 1  sand 40 cm   /V 1  s, respectively. A TFT model is extracted from these char-acteristics. PEDOT/PSS and PPV are used as a conductive polymer anda light emission layer, respectively. After that, the aluminumwith lithium is sputtered and used as a cathode for the LEPD.Finally, a wire to supply current to the cathode is attachedby pasting. The whole cathode is encapsulated by epoxy resinin order to avoid the degradation of the LEP and the cathode.A flexible tape is heat-sealed to the contacts on the substrate.In the case of the TFT-LCD, an alignment layer, liquidcrystal, and opposite substrate are needed on the TFT arraysubstrate. On the opposite substrate, a black matrix, align-ment layer, and opposite electrode are necessary. Moreover,a backlight, light guide, polarizer, diffuser, and other opticalparts must be attached. On the other hand, in the case of the TFT-LEPD, whose structure is mentioned above, onlysome thin films on one substrate are needed. Therefore, verythin, compact, lightweight, and inexpensive displays can beachieved.III. N OVEL  T ECHNOLOGIES  A. Conductance Control We utilized conductance dependence of the driving TFTin order to control gray scale. A pixel equivalent circuit isshown in Fig. 3. A pixel is composed of two kinds of TFT,i.e., a switching and driving TFT, a storage capacitor, andan LEP diode. Actually, the switching TFT consists of threeTFT’s connected in series in order to decrease off current andreduce degradation caused by high electric fields around thedrain edges of their channels [11]. The driving TFT consists of three TFT’s connected in parallel in order to be cooled easily  2284 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 12, DECEMBER 1999 Fig. 3. Pixel circuit of the TFT-LEPD. A pixel is composed of a switchingand driving TFT, a storage capacitor, and an LEP diode. Conductancedependence of the driving TFT is used in order to control gray scale. Bythis method, the pixel circuit can be simple. and reduce degradation caused by self-heating [12], [13]. Thescan and signal drivers are integrated around the image areaon the substrate and their workings are the same as those usedin TFT-LCD’s.The mechanism for scanning, i.e., for transferring eachsignal voltage to the corresponding storage capacitor, is similarto that used in TFT-LCD’s. The only difference is that thesignal voltage stored in the capacitor does not have to beac, but can be dc, because the LEP diode is driven by dccurrent and the driving TFT should control the LEP diode bydc voltage. Therefore, it is possible to reduce the amplitude of the signal voltage and scan voltage.The signal voltage stored in the capacitor is also applied tothe gate terminal of the driving TFT. The gate voltage controlsconductance of the driving TFT and anode voltage depends onthe relationship between the resistance of the driving TFT andLEP diode. That is, by varying signal voltage, current throughthe LEP from the supply line via the anode to the cathode andlight emission can be modulated.In the bright state, since the resistance of the driving TFTis negligible compared to that of the LEP diode, there islittle voltage drop and wasted power consumption in thedriving TFT. The power reduction in the bright state is verymeaningful because the current is larger than other states.There are only two TFT’s in a pixel. This structure isconventional for current consuming devices, such as the LEPdiode, and there are some disadvantages, for example, nonuni-formity caused by variation of the characteristic between thedriving TFT’s. However, we chose this structure because thesimplicity is very practical when we avoid the yield rateproblem in mass production.Next, the design and voltage were optimized. The drivingTFT cannot work in the saturation region for all the gatevoltages even if its design parameter is varied. There are tworeasons. The first reason is low drain voltage. In order toreduce the power consumed in the driving TFT in the brightstate, the resistance of the driving TFT should be negligible Fig. 4. Operation point analysis of the driving TFT and LEP diode. Hori-zontal and vertical axis are voltage of the terminal between the driving TFTand the LEP diode and current through the driving TFT and the LEP diode.The characteristics of the driving TFT corresponding to each gate voltage andthe characteristic of the LEP diode are overlapped. The cross points of thecharacteristics mean operational points of the pixel equivalent circuit. compared to the resistance of the LEP diode. This means thatthe voltage drop between the drain and the source terminal, i.e.,drain voltage, is rather small. In addition, since the efficiencyof the light emission from the LEP becomes very high andits threshold voltage becomes very low, recently, only about5 V must be applied to the LEP diode for sufficient lightemission. The second reason is that, as shown in Fig. 2, theLT p-Si TFT has no saturation region defined clearly, i.e., aflat characteristic which is independent of the drain voltage,because of many defects in the channel [14].If a TFT worked in the saturation region, it would be easy tocalculate the current because the current would mainly dependonly on the gate voltage. However, since the TFT works inthe nonsaturation region, it is very difficult to calculate thecurrent by analytical calculation. The reason is as follows.The current depends on not only the gate voltage but also thedrain voltage. The drain voltage is decided by the relationshipof the resistance between the driving TFT and the LEP diode.This relationship is not decided until the current is decidedbecause both the TFT and the LEP diode are nonlinear electricdevices for applied voltages. Because of such a complicatedmechanism, operational point analysis or circuit simulation bya computer is needed to perform the design.Fig. 4 shows operational point analysis of the pixel equiva-lent circuit to achieve gray scale. The horizontal axis is voltageof the terminal between the driving TFT and the LEP diode,which is drain voltage of the driving TFT and anode voltageof the LEP diode, simultaneously. The vertical axis is draincurrent of the driving TFT, which is same as the currentthrough the LEP diode. The characteristics of the drivingTFT corresponding to each gate voltage, which is the signalvoltage stored in the storage capacitor, are overlapped. Thecharacteristic of the LEP diode is also overlapped. The crosspoints of the characteristics of the driving TFT and the LEPdiode mean operational points of the pixel equivalent circuitfor each gate voltage.Circuit simulation with a TFT and LEP model [15] wasdone in order to design the driving TFT and LEP diodecircuit including gray scale. These models are extracted fromthe measured data. Fig. 5 shows a simulated current–voltage  KIMURA  et al .: HIGH-RESOLUTION LIGHT EMITTING POLYMER DISPLAY 2285 Fig. 5. Simulated current–voltage (   0     ) characteristic of the driving TFTand LEP diode. Horizontal and vertical axis are signal voltage and currentthrough the LEP diode, respectively. Gray scale can be acquired by optimizingall the design parameters. Signal voltage can be adjusted within a range of less than 5 V.TABLE IS PECIFICATIONS AND  D ESIGN  P ARAMETERS OF THE  TFT-LEPD. D ESIGN P ARAMETERS  W ERE  O PTIMIZED BY  C IRCUIT  S IMULATIONS . A H IGH -R ESOLUTION LT p-Si TFT LEPD  WITH  I NTEGRATED  D RIVERS  H AS  B EEN  F ABRICATED ( ) characteristic of the equivalent circuit consisting of the driving TFT and LEP diode. The horizontal axis is signalvoltage, which is applied to the gate terminal of the drivingTFT. The vertical axis is current through the driving TFT andLEP diode. Gray scale from the bright state via the halftonestate to the dark state can be acquired.By such analyses and simulations, for the given area of theLEP, the design of the driving TFT, i.e., width and length, isoptimized. Signal voltage can be adjusted within a range of less than 5 V, which may achieve very low power consumedin video signal circuit in the peripheral controller. After that,the entire design of the TFT-LEPD, i.e., the switching TFT,storage capacitor, etc., is decided. All the optimized designparameters and specifications of the TFT-LEPD are shown inTable I.  B. P-Channel TFT for Reliability in DC  In order to ensure the reliability of the driving TFT evenwhen dc voltage is applied, a p-channel TFT is used. Fig. 6shows a comparison of the reliability by measurement betweenan n-channel and p-channel TFT. Initial transfer characteristicsand those after dc stress, i.e., gate voltage 20 V, drain voltage0 V, temperature 70 C, bias time 600 h, are overlaid. In thisstress condition, gate voltage and temperature is higher thanreal working conditions in the TFT-LEPD. This was done toaccelerate testing. It is clear that the p-channel TFT is muchmore reliable in dc bias than the n-channel TFT. The reason (a)(b)Fig. 6. Comparison of reliability between an (a) n-channel and (b) p-channelTFT. Initial transfer characteristics and those after dc stress, i.e., Vg 20 V,Vd 0 V, temperature 70    C, bias time 600 h, are overlaid. The TFT’s haveW/L of 100/12    m. It is clear that the p-channel TFT is much more reliablein dc bias. Thus, a p-channel TFT is used for the driving TFT. is not understandable now but we will research this importantphenomena from now on. C. Inter-Layer to Reduce Parasitic Capacitance As we can see in Fig. 1, an inter-layer made of insulatorcovers all areas except for the light emission area in the anode.The function of the inter-layer is to exist on the signal lines,to distance the cathode, and to reduce parasitic capacitance of the signal lines. The thickness of the inter-layer was decidedto be 1.0 m, while that of SiO adhesive layer, which existsbetween the inter-layer and the anode, is 0.1 m. From thesevalues, capacitance between the signal line and cathode is3.1 pF. By adding other capacitance, total capacitance of thesignal line is 10.5 pF. Resistance of the signal line itself is330 and the equivalent resistance of an LT p-Si TFT analogswitch on the edge of the signal line is on the order of 1 k at most. Therefore, the time constant of the signal line is onthe order of 10 ns. This panel is designed for the point-at-timedriving scheme [16], which means that during application of scan voltage, each signal voltage is applied sequentially. Theselecting time for one signal line is 240 ns. Since the timeconstant is enough smaller than the selecting time, correctsignal can be applied to the signal line.  D. Others The LEP layer, i.e., the conductive layer and the lightemitting layer, is not patterned. However, crosstalk of lightemission between pixels does not occur. The reason is as  2286 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 46, NO. 12, DECEMBER 1999 Fig. 7. View overlooking the TFT-LEPD. Because of no backlight, lightguide, polarizer, diffuser, peripheral drivers, etc., the TFT-LEPD can belightweight, thin, and compact.Fig. 8. Photographs of pixels and electroluminescence from the pixels. Sincethe light emission area ratio is at most 12%, reflection from cathode metalis reduced and contrast can be improved if the rest of the pixel is coveredwith a light shield layer. follows. Since the electric resistivity of the conductive layeris about 1 k cm and its thickness is very thin (10 nm), itssheet resistance is /sq and its resistance betweenpixels is . The resistance is much higher than theLEP diode resistance, the order of . Moreover, theelectric resistivity of the light emitting layer between pixelsis still higher than that of the conductive layer. As a result,the LEP diode can be supposed to be electrically separatedbetween each pixel.IV. R ESULTS A high-resolution LT p-Si TFT LEPD with a scan and signalintegrated driver has been fabricated. The specifications havealready been shown in Table I. A view overlooking the TFT-LEPD is shown in Fig. 7. No backlight, light guide, polarizer,diffuser, and peripheral drivers are needed. The number of contacts between the peripheral controller and the panel isreduced to only 27. Twenty-six of the contacts are througha flexible tape and one contact is through a wire pasted onthe cathode. Consequently, the TFT-LEPD can be exceedinglylightweight, thin, and compact, as shown in Table I. Here, thesecond glass substrate is used only for encapsulation of theLEP and supporter, which can be eliminated easily in the nearfuture. Fig. 9. Display image of the TFT-LEPD. Green monochrome display imageis acquired. Neither nonuniformity nor crosstalk occurs.Fig. 10. Measured and simulated gray scale. Brightness is normalized by themaximum value. The measured gray scale is similar to the simulated one. Itis found that good gray scale from the bright state via the halftone state tothe dark state can be acquired. Photographs of pixels and electroluminescence from thepixels are shown in Fig. 8. The light emission area ratio, i.e.,the ratio between light emission area and whole area in a pixel,is at most 12%. In spite of the small ratio, there is no seriousproblem because all light comes from the light emission area,no light loss occurs, and power is not wasted. On the contrary,the small ratio can reduce reflection from cathode metal. If the rest of the pixel is covered with a light shield layer, allreflection from the display can be reduced and contrast canbe improved.A display image of the TFT-LEPD is shown in Fig. 9. Here,a green monochrome display is acquired. Neither nonunifor-mity caused by the parasitic capacitance of the bus lines norcrosstalk between pixels occurs. Measured and simulated grayscale is shown in Fig. 10. Here, brightness is normalized bythe maximum value. The measured gray scale is similar to thesimulated one. It is found that good gray scale from the brightstate via the halftone state to the dark state can be acquired.The achieved power consumption with the voltage andcurrent are listed in Table II. The power consumed in theintegrated driver is 20 mW. In order to achieve brightnessof 100 Cd/m from the whole panel, only 5 V is needed tobe applied to the driving TFT and LEP, which leads to lessthan the current of 20 mA through the LEP and the powerconsumption of 100 mW. Therefore, total power consumptionof the TFT-OELD is 120 mW at most. When the displayed
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