Technological Considerations for Manufacturing Flexible Active-Matrix OLED Displays

AMOLEDs hold great promise for use in flexible displays. LG Display has showcased a full-color 4-in. flexible AMOLED prototype on an 80-μm-thick stainless-steel foil substrate, achieving a curvature of 5-cm bending radius. This article discusses the challenges ahead, including transporting the flexible backplane substrate and obtaining reliable TFT characteristics in order to achieve brightness and uniformity suitable to commercialize this technology.

by Juhn S. Yoo, Nackbong Choi, Yong-Chul Kim, In-Hwan Kim, Seung-Chan Byun, Sang-Hoon Jung, Jong-Moo Kim, Soo-Young Yoon, Chang-Dong Kim, In-Byeong Kang, and In-Jae Chung

THE display market of the future demands ubiquitous devices that are more portable, fashionable, and environmentally friendly.¹Display manufacturers need to advance their technologies to build lighter, slimmer, more rugged devices that consume low amounts of power while at the same time improve the picture quality. The emerging technology of flexible active-matrix displays is being developed in order to fulfill these needs. Currently, there are active research projects in reflective-type flexible liquid-crystal displays (LCDs),² flexible electrophoretic displays (EPDs),³ and emissive-type flexible organic-light-emitting-diode (OLED) displays.⁴ Today, EPD technology is considered the most desirable flexible-display technology because of its simple fabrication process and very low power consumption. An active-matrix OLED (AMOLED), on the other hand, is an emissive-type display device that promises better picture quality – including brightness, color, contrast ratio, viewing angle, and response time – compared to active-matrix liquid-crystal displays (AMLCDs). An OLED is a thin-film solid-state device, which makes it easier to apply to flexible displays because of its relatively simple fabrication process and reduced distortion according to the geometric form of display. However, in general, AMOLED displays have yet to overcome numerous technological obstacles for mass production.⁵ When implementing AMOLEDs into flexible displays, even more technological issues need to be considered.⁶

Fig. 1: A cross-sectional schematic of the developed flexible AMOLED display. A low-temperature a-Si TFT array and top-emission OLED is integrated on a thin stainless-steel substrate. The display device is encapsulated with thin-film passivation and hard-coat lamination.

This article examines the technological considerations for transferring flexible AMOLED display manufacturing to existing AMLCD product lines based on LG Display's recent development of a 4-in. prototype using amorphous-silicon thin-film transistors (a-Si TFTs) on stainless-steel foil. The structure and fabrication process of the flexible backplane, issues concerning TFT performance, and suggestions on process improvements will be discussed. The structure and design of the display panel is also presented, as are the issues of our OLED driving circuits and possible solutions.

Flexible Backplane Fabrication

Compared to plastic substrates, metal foil has excellent dimensional stability under relatively high process temperature. Until the thermal instability issue of plastic substrates is resolved, stainless-steel substrates remain the most promising candidate for the manufacture of flexible displays.

Our first consideration for using stainless-steel foil is that it is opaque, meaning it is limited to either reflective or emissive display devices. Therefore, we designed the TFT backplane to be compatible with a top-emission OLED structure, of which the cross-sectional schematic is shown in Fig. 1. Our second consideration is the method of transporting the substrates to prevent sagging while being transferred in and out of the manufacturing equipment. Our third consideration is the surface-planarization method to reduce the roughness of the steel plate, where protrusions and dents can cause line-open defects in the display array. Our fourth consideration is the insulation of the conductive substrate from the TFT array, where capacitive coupling causes signal line delay and crosstalk. Our fifth and most important consideration is the material and process integration needed to build a reliable TFT.

We fabricated a back-channel etch-structure a-Si TFT array using a typical five-mask process. In order to prevent substrate sagging, we fixed a set of metal foils to a glass carrier with adhesive. We developed a set of epoxy adhesive films to endure thermal stress as high as 200°C without failure. The stainless-steel foil was mechanically and chemically polished to reduce the surface roughness and minimize the height and depth of protrusions and dents. In order to further planarize the metal surface, a multi-barrier structure was prepared by coating 3-μm-thick polymer resin and depositing 0.4-μm-thick plasma-enhanced chemical-vapor-deposition (PECVD) silicon nitride. Figure 2 shows the structure of the prepared substrate and Fig. 3 compares the atomic-force microscopy (AFM) images of bare stainless steel and the planarized substrate surface. The thick insulating layer not only reduces the capacitive coupling between the conducting substrate and the TFT array, but also protects the metal foil from chemical attack. The RMS value of the substrate surface roughness decreased from 1000 to 50 Å.⁷ The characteristics of fabricated a-Si TFT are shown in Fig. 4.

Despite the comparable characteristics of the a-Si TFT on a flexible substrate to a glass counterpart, the threshold-voltage shift under bias temperature stress is drastic, as shown in Fig. 5. The process temperature of the PECVD deposition of the gate insulator and active layers is limited to 150°C, thus preventing debonding of the adhesive film. The temporal variation of threshold voltage creates image sticking, as shown in Fig. 6, where a ghost image of the previous picture is observed. Therefore, it is not encouraging to use the bonding–debonding method for transporting the flexible substrate. One way to increase the process temperature by removing the thermally unstable bonding process is to adopt the method used in the glass-thinning process.

Fig. 2: A cross-sectional schematic of the thin metal substrate bonded on a glass carrier and coated with planarization layers.

 (a) Bare stainless steel (RMS ~1000 Å)

 (b) Planarized substrate (RMS ~50 Å)

Fig. 3: Images of (a) AFM surface-roughness measurement of bare stainless steel and (b) the planarized substrate.

Top-Emission OLED

Considering the opaqueness of the metal substrate, a conventional bottom-emission OLED structure is not applicable. Hence, a top-emission OLED structure was integrated on the flexible TFT backplane. Considering the process-temperature limitation, a-Si TFTs are our preferred choice over polycrystalline-silicon TFTs, of which the typical maximum process temperature is more than 350°C. Because a-Si TFTs have very low transconductance, a highly efficient electro-optical characteristic of the OLED is required. We employed a top-emission OLED structure with a highly efficient phosphorescent emissive layer and highly transparent cathode material. To secure the stability of the OLED under bending stress, organic-bank and thin-film passivation structures are recommended. We employed acrylic resin for the bank layer and multiple layers of organic and inorganic films for passivation structures. The OLED frontplane was finally encapsulated by a hard-coat lamination to protect from moisture permeation and scratching. In order to further enhance the luminous efficiency of the top-emission OLED, development of a reflective anode combined with a well-designed microcavity structure is also considered requisite.

Fig. 4: Measured transfer characteristics of the a-Si TFT with channel dimensions of W/L = 30 μm/5 μm fabricated on metal foil at a maximum process temperature of 150°C. Field-effect mobility and threshold voltage are 0.35 cm²/V-sec and 1.47 V, respectively, of which the values are comparable to a typical a-Si TFT on glass.

Fig. 5: Measured transfer characteristics of the fabricated a-Si TFT before and after bias temperature stress applied for 1000 sec under 60¼C with V_gs = 30 V and V_ds = 0.1 V. The threshold voltage shifted 2.6 V.

 (a)   (b)

Panel Design

Even if the substrate of the display panel is flexible, it would be difficult to curve the display module with a low-enough bending radius if rigid driver electronics are attached. It would be preferable to have just one or two flexible connectors tabbed on the panel, but this would also require a chip-on-glass (COG) type drive IC. As illustrated in Fig. 7, this module configuration is applicable for a curved display with a fixed bending radius, but not for real-flexible or rugged display applications because the rigid drive IC could easily de-bond when twisting or wiggling the substrate. We used a tape-carrier package (TCP) type source drive IC assembled on one side of the panel.

It is also highly imperative to integrate the gate driver circuit on the panel using a-Si TFTs to reduce the number of IC components, thus increasing the flexibility of the display module. As a result, we were able to operate the display in curvature with a bending radius of less than 5 cm along the vertical axis. In order to increase the flexibility along the horizontal axis of the panel, the control printed-circuit board (PCB) made of rigid plastic should be replaced with a flexible printed circuit (FPC). Bending demonstrations of our recently developed 4-in. flexible AMOLED display module with three drive ICs, one integrated gate driver, and one control PCB, along with the previously developed display module with six drive ICs and three control PCBs, are shown in Fig. 8.

 (a) COG-type display module

 (b) TCP-type display module

Fig. 7: Schematic diagram of the suggested flexible-display-module configuration using TCP-type drive ICs (b). A COG-type module (a) is more compact but limited in flexibility, whereas a TCP-type module is more rugged due to better flexibility.

 (a) Flexible display module with six ICs and three PCBs

 (b) Flexible display module with three ICs and one PCB

Fig. 8: Bending demonstration of the 4-in. flexible AMOLED panels displayed in curvature. (a) The previously developed display panel is limited in bending because it is surrounded by three control PCBs. (b) The bending radius of our recently developed display panel is less than 5 cm due to gate-driver integration and one control PCB assembly on one side of the panel.

Conclusion

Preparing the metal-foil substrate for manufacturing flexible AMOLED displays is a demanding process, which involves coating of a thick planarization layer to reduce the surface roughness and capacitive coupling between the conductive substrate and TFTs. Due to the process temperature limitation of a carrier glass bonding method for substrate transport, the reliability of a-Si TFTs fabricated below 150°C exhibits rather poor device stability under bias temperature stress. To increase the process temperature and thus achieve sufficiently reliable TFT backplanes, new planarization and transporting methods for stainless-steel substrates is under development. We employed a highly efficient top-emission OLED structure with a phosphorescent emissive layer integrated with organic-bank and multilayer thin-film encapsulation to secure flexibility. The development of a reflective anode and microcavity structure is considered a key technology in enhancing the luminous efficiency of the display. To realize a truly flexible or rugged display module, it is essential to have a flexible interface of driver electronics assembled on one side of the panel. It is also imperative to reduce the number of drive-IC components by integration of gate drivers using TFT devices and replacing the rigid plastic control PCBs with flexible printed circuits.

We have demonstrated 4-in. flexible AMOLED displays in curvature having a bending radius of less than 5 cm along one axis. In order to manufacture reliable flexible AMOLED displays with good picture quality, device process and OLED-driving technologies need to be improved in terms of the above-mentioned considerations.

Acknowledgment

This work is a joint development project with Universal Display Corp.

References

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³C-D. Kim, I-B. Kang, and I-J. Chung, "TFT Technology for Flexible Displays," SID Symposium Digest Tech Papers 38, 1669 (2007).

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