Issues of time constant and aperture ratio

The additional CTEST directly increases the resistance-capacitance (RC) time constant TRC of the scan line and decreases the aperture ratio of the pixel, even though the CTEST can enhance the testability of TDV. Although CTEST is geometrically formed below the scan line, as shown in Fig. 7-1(b), the contact hole that connects CTEST to ITO occupies an extra area of 200 um², consequently, slightly decreases the aperture ratio from 43.1% to 41.4%.

In order to evaluate the TRC of the scan line, the parasitic resistance and capacitance of a scan line per pixel (Rscan, Cscan) are set to be 13.2Ω and 2.6fF, respectively for the pixel without CTEST. In the proposed pixel circuit, Cscan increases from 2.6 to 72.6fF as CTEST of 10, 30, 50, and 70fF are used. Besides, the frame rate of 30Hz is used to calculate the turn-on period of the scan line (Tscan-on) for the comparison between TRC and Tscan-on.

TRC is much smaller than Tscan-on for the conventional panel without CTEST. Even though in high resolution such as 1280xRGBx1024 (SXGA), TRC is merely 1.55% of Tscan-on, as shown in Fig. 7-9. However, TRC is increased when the proposed pixel is implemented into a panel. For example, in the resolution such as 480xRGBx360, CTEST of 70fF increases TRC to 1.98us, 28 times larger than that of conventional panel.

However, TRC to Tscan-on ratio of 2.15% is still smaller than 5% which is a generally acknowledged limitation in display panel design. In other words, from small to moderate resolution, CTEST can be useful for testability enhancement without seriously

impacting on the programming time of the signal. Unfortunately, if the dot resolution is from VGA to SXGA, TRC is dramatically increased by CTEST of 70fF from 3.53 to 14.13us, almost 5.1 to 43.4% of Tscan-on. The long TRC results in slow switching behavior of TSW as well as inaccurate data write-in. In order to reduce TRC, an intuitive approach is to use a small CTEST in the pixel circuit, however, the accuracy of charge detection is limited.

0.0 2.0x10⁵ 4.0x10⁵ 6.0x10⁵ 8.0x10⁵ 1.0x10⁶ 1.2x10⁶ 1.4x10⁶

Fig. 7-9. T

scan-on

and T

RC

/T

scan-on

versus dot resolution of display panel.

Using high conductivity material such as aluminum to fabricate the scan line instead of molybdenum is another approach to reduce TRC. The sheet resistance of aluminum is 70m Ω/square, merely 7% of that of molybdenum. In contrast with molybdenum, the scan line made by aluminum can achieve the lower Rscan of 0.924Ω in proposed dimension. TRC to Tscan-on ratio as a function of dot resolution with aluminum scan line is also plotted in Fig. 7-9. Evidentially, the scan line of aluminum dramatically reduces the TRC to Tscan-on ratio to 3% even in SXGA resolution.

Therefore, CTEST of up to 70fF can still be utilized to ensure the accuracy of TFT array

inspection.

7.5 Summary

An effective charge sensing scheme and corresponding pixel circuit were designed to enhance the testability of TFT array of AM-OLED. By using an additional CTEST, the function of all devices such as TFT and storage capacitance can be examined with electrical signal before the OLED process is performed. The proposed functional testing scheme can be performed without any mechanical motion during testing, and an in-situ measurement can be taken in real time with high stability. Two side effects: an increase of RC time constant can be restrained by using aluminum as bus line material; while the reduction of aperture ratio is within acceptable for operation. The simulated and calculated results presented herein including leakage, time constant, and threshold voltage, are useful in identifying the causes of array defects on the panels in situ. Furthermore, the array testing can be integrated into the in-line process as a batch job.

References

[1] R. L. Wisnieff, L. Jenkins, R. J. Polastre and R. R. Troutman, “In-process testing of thin-film transistor arrays,” in Symp. Dig. 1990 SID, 1990, p. 190.

[2] F. J. Henley, and G. Addiego, “In-line functional inspection and repair methodology during LCD panel fabrication,” in Symp. Dig. 1991 SID, 1991, p. 686.

[3] F. J. Henley, and H. J. Choi, “Test head design using electro-optic receivers and GaAs pin electronics for a gigahertz production test system,” in Proc. IEEE Int. Test Conf., 1988, p. 700.

[4] M. Brunner, R. Schmid, R. Schmitt, and D. Winkler, “In-process flat-panel-display testing with electron beams,” in Symp. Dig. 1994 SID, 1994, p. 755.

[5] H. P. Hall, and P. R. Pilotte, “Testing TFT-LCD substrates with a transfer admittance method,” in Symp. Dig. 1991 SID, 1991, p. 682.

[6] L. C. Jenkins, R. J. Polastr, R. R. Troutman, and R. L. Winsnieff, “Functional testing of TFT/LCD arrays,” in IBM J. Res. Develop., vol. 36, no. 1, p. 59, Jan. 1992.

Chapter 8

Conclusion

Active-matrix organic light emitting displays with integrated driver circuits using polysilicon thin-film transistor technology have demonstrated the capability for being the next generation display applications. In conjunction with the usage of thermal evaporation or ink-jet printing technologies to fabricate OLED pixel elements, it is possible to achieve thin, compact, lightweight, wide-viewing angle, fast response, flexible and yet low cost full color AM-OLED displays.

Unlike AM-LCDs, the organic EL elements for AM-OLEDs are current driven devices. Since the nature of the fabrication process of poly-Si TFT, variations of the TFT electrical characteristics, such as threshold voltage and mobility, over the entire substrate area is unavoidable. Besides, the native parasitic resistance of electrodes can result in a voltage drop as the OLED driving current passing through the addressing electrodes. Consequently, these characteristic and voltage variations cause a large steady state output current error and non-uniform pixel luminance. In this dissertation, we have successfully developed several key driving and testing schemes to improve the display image quality and the functionality evaluations. The characteristics of pixels and driving circuits were taken into consideration for optimizing the display performance, such as the charge injection, parasitic RC time delay and device degradation, etc. Additionally, from this thesis research, the fabrication processes of poly-Si TFT and OLED were utilized for realizing the AM-OLED panel with proposed electronic circuits and components. Most of all, these driving schemes and components greatly improve the display performance of AM-OLED displays and manufacturing yield control, thus, offering more appealing and competitive

AM-OLED’s.

8.1 AC driving scheme for voltage driven AM-OLED

A two-transistor pixel electrode circuit driven by voltage signal for AM-OLED’s is more attractive because of its high aperture ratio and compatibility with AM-LCD driver technology. Even though the parasitic resistance of addressing wire can lead to a voltage drop as the OLED current passing through it, our proposed voltage type AC driving scheme shows a significant improvement in the display luminance. By using an AC voltage at OLED cathode, the voltage drop at the addressing wire can be easily compensated without modifying the pixel circuit structure.

The normalized luminance of all measured regions of AM-OLED panel was well above 91.6%, for a various duty cycle of the flashing period from 20% to 80% shown experimentally. In contrast, the AM-OLED panel with conventional DC driving scheme shows lower luminance uniformity, due to luminance decays from surrounding to the central area. The lowest luminance measured at the central display region was of only 74.54% of the highest luminance at the surrounding area. Although a higher driving current is needed in the AC driving scheme, the treatment of reversed bias voltage can accelerate the recovery from degradation and lead to an improvement in the J-V characteristics and device lifetime of the OLED. In other words, the higher driving current may degrade OLED performance rapidly, however, the AC driving scheme with proper reversed bias voltage, can alleviate the OLED material degradation.

8.2 Current driven AM-OLED with fully integrated driver

In order to achieve multiple grayscales, circuit designers and process engineers

must address the issue of non-uniform pixel brightness over the display area caused by the non-uniform spatial distribution of threshold voltage in the driving transistor of pixel circuit. In this dissertation, we focus on certain methods and techniques used in designing poly-Si TFT pixel driver circuits to minimize the effect of threshold voltage variation on the display luminance. The key concern in design of current driven pixel circuit is to ensure that the OLED is driven with a specified input current not only during the OLED is addressed but also during the rest of the frame period as well when the OLED is not being addressed. Here we have designed, fabricated, and analyzed a current-driven four-transistor pixel electrode circuit based on poly-Si TFT technology for AM-OLED’s. The pixel circuit proposed here is able to reduce the effect of spatial variation of TFT threshold voltage in comparison with traditional voltage driven pixel circuits. Experimental results indicated that continuous pixel electrode excitation with constant current signal can be achieved during more than 12 hours Bias-Temperature-Stress experiment. The pixel electrode circuits showed an excellent electrical stability that no output current variation was observed even when a large TFT threshold voltage shift was measured.

For the sake of system integration, we also designed and implemented several key components of current driving circuits with poly-Si TFT technology, including current memory cell, reference current generator, and digital-to-current converter. The current memory circuit can operate with a high accuracy in a weak current range and the effect of the charge injection is reduced significantly by employing a cascade current mirror structure. The circuit operation does not need complex clock signals and large storage capacitance, thus, facilitating the circuit implementation and minimizing the required layout area. Furthermore, these improvements have been achieved without requiring any critical geometric matching of TFTs. Even though the

power consumption of the proposed current memory is inferior to those of the conventional current memory, the features, such as fast response time and high accuracy of output current, favor the applications of the proposed current memory in driver circuits of current driving AM-OLED’s.

In addition to the current memory circuit, the proposed DCC with reference current generator can prevent random characteristic variations in all the transistors of the current source cell by utilizing the external reference current signal and yield accuracy current signals under various driving conditions. By means of the adjustable TFT geometric size, the DCC permits us to flexibly and precisely design the required current signals corresponding to the gray scale of AM-OLED’s without using external current-type peripheral drivers, hence, resulting in notable reduction of input contact pins and fabrication cost. The proposed design methodology has been applied to the design and optimization of a 6-bit DCC integrated in an AM-OLED substrate. The opto-electrical characteristics of AM-OLED with above mentioned driver circuitries reveal that the current driving scheme is capable of enhancing the display luminance uniformity. Moreover, the fully integrated data driver circuits not only provide a digital interface which is compatible with AM-LCD but also reducing the usage of current-type external drivers, thus, possibly lower fabrication complexity and cost.

8.3 Current scaling pixel electrode circuit

Current driving scheme with four-transistor pixel electrode circuits can not only provide a continuous excitation to OLED, but also compensate for the TFT threshold voltage variation at the same time. Although the current driving scheme improves the display luminance uniformity, a large timing delay can be observed in low current driving condition. Due to combination of a high OLED efficiency and large

interconnect parasitic capacitance, a long data programming time is required as rendering a low gray-scale image. In this work, a new AM-OLED pixel electrode circuit employing a cascade capacitance structure to achieve current scaling function is proposed and verified by HSPICE simulation. This pixel design, which consists of four poly-Si TFTs and two capacitors, scales down the OLED current more effectively without sacrificing the pixel aperture ratio, so as to guarantee a speedy data programming time, compared with the conventional current mirror structure.

The modified pixel electrode circuit shows outstanding current scaling function in comparison with both the conventional current-driven and current-mirror pixels.

Although the current–mirror pixel is able to scale down the data current, the scale-down ratio is fixed. Besides, a large data current for high gray scales will result in high power consumption due to this fixed scale-down ratio. In addition, to achieve the current scaling function, a larger driving TFT needed in current-mirror pixel can substantially reduce the pixel electrode aperture ratio. From the simulated results, we can conclude that with the data current ranging from 0.1 to 10 µA, our proposed pixel circuit can achieve the widest OLED current ranging from 1 nA to 5 µA. By contrast, the conventional current-driven pixel and the current-mirror pixel can merely achieve the OLED current from 0.05 to 10 µA and from 0.01 to 2.5 µA, respectively.

Therefore, the proposed pixel circuit can yield not only a high data current and a high scaling ratio for the low gray scales, but also reasonable data current for a high gray scale to avoid large display power consumption. Furthermore, the proposed pixel circuit also compensates for the non-uniformity of electrical characteristics of poly-Si TFTs, such as the threshold voltage and mobility.

8.4 Functionality testing for AM-OLED

In the past few years, several AM-OLED pixel electrode circuits with complex driving schemes have been proposed to improve the luminance uniformity. However, there is no effective testing method proposed to evaluate the functionalities of the pixel circuits. In this work, the capacitor-on-gate structure has been proposed and applied to the AM-OLED pixel circuit to improve the testability. By utilizing the proposed charge sensing scheme, TFT array can be investigated and evaluated immediately after fabrication processes and again after the display has been deposited with OLED materials. It can also be used to verify array designs and perform failure analysis.

Among all kinds of OLED pixel circuits, the pixels are not complete before the OLED process is performed because the ITO anode of each pixel circuit is in a floating state. Hence the source voltage of driving TFT can not be confirmed and no path is available for conducting the driving current as well. The incomplete pixel circuits limit the testability of the pixel electrode circuits of TFT array. Our capacitor-on-gate structure can improve the testability of the AM-OLED pixel electrode circuits and ensures the circuit functions to prevent the floating status of anode before OLED material deposited onto the TFT array. This additional capacitor is below the scan line electrode, consequently, no additional pixel area is required.

Using the adequate control signals, an amount of testing charge can be supplied to each component in the pixel circuit including switching TFT, driving TFT, and then stored in the storage capacitor and additional testing capacitor. By sensing the testing charge, these components can be inspected completely so that the pixel circuit functionality can be evaluated. In addition to the pixel functionality, the characteristics of switching and driving TFTs, for example, threshold voltage, leakage current, and

sub-threshold slope, can be roughly measured. Although the scan line RC delay is increased due to the additional testing capacitor, the scan line of low resistance materials is beneficial to reduce the RC time constant. For instance, high conductivity material such as aluminum (R□=70m Ω/square) instead of molybdenum (R□=1 Ω/square) for the scan line is capable of significantly reducing the RC time delay of scan line. Therefore, the additional testing capacitor can still be utilized to ensure the TFT array inspection.

8.5 Future work

Conventional AM-LCD pixel circuit utilizes a voltage signal supplied from data line to control the LC cell for modulation of light transmission. OLED is a current-driven device, hence, the AM-OLED pixel circuit has to convert the voltage signal into current. Although the transmissive and the reflective structures can be integrated in AM-LCD pixel, a backlight is essential as a light source, yet at the expense of increase of the display module size and power consumption. In the case of AM-OLED’s, self-emitting characteristic exceptionally enhances the contrast ratio, viewing angle and the reduction of module thickness. Nevertheless, in the bright environment, a high driving current is required to maintain the AM-OLED visibility, consequently increasing the power consumption and degrading the reliability. In this dissertation, we propose a novel pixel circuit and structure by combining an OLED and a reflective LCD with active-matrix array to form a novel transflective display to improve the power consumption.

8.5.1 Hybrid AM-OLED with reflective LCD

The new hybrid pixel circuit is composed of a switching TFT TSW, driving TFT

TDV, and storage capacitor CST, and controlled by four signals: data, scan VAC, and Vdd. The symbols CLC and diode denote the LC cell and OLED, respectively, as shown in Fig. 8-1. Pixel circuit combined LC cell and OLED control function. (a)

schematic and (b) layout.. The difference of the proposed pixel circuit is that two

electrodes: Al(LC) and ITO(OLED) are used separately to control the LC and OLED.

The Al(LC) electrode which controls the LC cell is connected to storage node A and also works as a reflector. The other ITO(OLED) electrode functioning as OLED anode is formed at the source of TDV. The OLED common cathode is connected to an external driver by a switch SW. An AC power supply VAC is adjusted to certain voltage according to the corresponding display mode.

Fig. 8-1. Pixel circuit combined LC cell and OLED control function. (a) schematic and (b) layout.

In the AM-LCD mode, the function of voltage-to-current transformation should be disabled so that the equivalent pixel circuit is identical to the conventional

AM-LCD pixel. At this time, the SW disconnects the OLED cathode from ground and Vdd is set to 0 V. Therefore, the ineffective TDV becomes a small capacitance and can be neglected by comparison to large storage capacitance CST. An alternately inverse electric field which prevents LC cell from ionizing can be achieved by VAC. Consequently, the normal operation of AM-LCD can be achieved with the display data stored in CST regardless of TDV. In the AM-OLED mode, the SW connects the OLED cathode to the ground and Vdd is set to a high voltage. By means of normally white LC material, VAC changed to a high voltage can drive the LC cell into black state to decease the reflection. The voltage signal from data line is used to modulate the TDV to generate an adequate current for OLED.

Fig. 8-2. Cross-section view of proposed transflective AM-LCD+OLED pixel circuit.

In order to fabricate this LCD+OLED pixel circuit, some processes should be added or modified. First, the conventional amorphous TFT with bottom-gate stagger structure is fabricated on glass substrate, as shown in Fig. 8-2. Since the traditional organic planarization layer will be deposited after OLED process, silicon nitride or silicon oxide will be used as an insulation layer after pattering the data lines instead of the organic planarization material. The ITO anode is pattered following the insulation layer to form the light emitting area of OLED, then the OLED material and transparent common cathode are fabricated. It should be noted that the transparent common cathode might be deposited with shadow mask because the common cathode

is not allowed to contact the via which is reserved for Al(LC) electrode. Besides, the

is not allowed to contact the via which is reserved for Al(LC) electrode. Besides, the

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