A Process for Using Oxide TFTs over LTPS TFTs for OLED-TV Manufacturing
Technical solutions for the fabrication of large-scale OLED displays include new manufacturing equipment and processes developed by Applied Materials.
by Kerry L. Cunningham
OLED TVs are compelling because they are lightweight and thin, requiring no backlight, and produce vivid color and true-black contrast ratios. Low manufacturing yields, however, have thus far held them back. According to Jae-Hak Choi, Senior Analyst for FPD Manufacturing at NPD DisplaySearch, “OLED TV has experienced various technical hurdles and roadblocks. The primary reason for the high cost of OLED TV is its low yield ratio. The organic-material evaporation process is a bottleneck for OLED-TV panels made using the RGB method, and oxide-TFT yields have not met expectations. NPD DisplaySearch’s recent AMOLED Process Roadmap Report indicates that the manufacturing cost for a 55-in. OLED-TV panel is up to 10 times more than the manufacturing cost of that of a 55-in. LCD panel (Fig. 1).
Key opportunities for equipment manufacturers to reduce large-scale manufacturing costs for OLED-TV applications include (1) scaling processes to Gen 8.5 while maintaining high yields to reduce the manufacturing cost per area, (2) high material utilization, and (3) implementation of materials engineering to improve overall device performance. This article will discuss equipment and process solutions that Applied Materials has developed to reduce these costs.
Fig. 1: The relative manufacturing costs of technologies for 55-in. TV panels are compared for LCDs (left) and two types of OLED panels, IGZO (middle) and LTPS (right). Source: IHS Technology, AMOLED Process Roadmap Report – July 2015 update.
TFT and Backplane Performance
The OLED-display structure consists of a TFT backplane to control pixel driving, AMOLED layers to emit light, and encapsulation to protect the highly sensitive organic materials from exposure to moisture and air. The primary goal of many manufacturing technologies is to achieve uniformity and performance stability for both the backplane and OLED emission layers. The introduction of thin-film encapsulation (TFE) processes aims to support flexible OLED displays while providing key barrier protection. High electron mobility is also critical to backplane performance reliability and AMOLED display performance stability. AMOLED displays utilize a current-driving method so backplanes have to maintain a stable voltage. The use of LTPS TFTs is currently the best solution for driving the AMOLED device because of their high electron mobility. However, the LTPS-TFT manufacturing process is more complicated and therefore more costly than the a-Si or MOx TFT processes.
In order to drive an AMOLED pixel in a basic OLED circuit, each subpixel requires a minimum of two TFTs and a capacitor circuit to provide the current for the OLED light-emitting material. The first TFT controls the flow of electricity stimulating the OLED emitting material, while the second TFT controls the writing of the signal. LTPS is the best material to drive the device due to its high electron mobility (~50–100 cm2/V-sec). However, because the semiconductor layer has uniformity and reliability issues, stability is insufficient without additional compensation circuits. Furthermore, the additional TFTs and capacitors increase the overall cost and have a negative impact on aperture ratio and brightness. More power is also required to maintain picture quality. The OLED pixel luminance is directly charged by the current; subtle variations in the TFT current result in brightness differences from pixel to pixel.
As a result, even the slightest non-uniformity in TFT performance can affect image quality. The inherent instability of the device is measured in variances in Vth, so a small variance of 0.1 V can cause image-quality problems, including mura. The large number of transistors required per pixel also has a big impact on manufacturing yield because a failure of any one transistor results in a complete malfunction of that pixel. The use of MOx over LTPS (Fig. 2) has the potential to help reduce manufacturing costs by enabling simpler pixel-circuit designs, fewer masking steps, less capital-equipment intensity, and overall improved TFT uniformity.
The adoption of oxide TFTs in an AMOLED backplane requires high mobility and uniform TFT specifications. Electron mobility depends on oxide deposition using physical-vapor-deposition (PVD) technology. To efficiently drive OLED TV at >240 Hz, the mobility specification should be >30 cm²/V-sec. The Vth shift (~2 V) is currently higher than that for LTPS. The target is 0.1 V or 1.0 V with compensation. To improve gate insulation, high-quality SiOx reduces interface trapping and minimizes hydrogen content and boosts the etch-stopper layer performance that protects the IGZO layer during subsequent integration processes (source/drain, etch). Changing the passivation material to Al2O3 also improves the moisture barrier. Another issue is that submicron-sized particles can strongly affect the oxide-TFT yield, and some particles that are <1 µm create defects as black spots.
Fig. 2: This metal-oxide (MOx) device structure uses a six-mask process compared to the 10 masks typically needed for LTPS, thus reducing costs. Source: Display Business Group, Applied Materials.
Scaling LTPS Films to Larger Substrates
The Applied Materials AKT– PiVot DT™ PVD system for fabricating a-Si, LTPS, or MOx backplanes addresses the challenges outlined above, allowing panel makers to produce next-generation ultra-high-resolution displays and scale to larger substrates at lower costs than were previously possible (Fig. 3).
Fig. 3: The AKT-PiVot 55K DT PVD system is designed for manufacturing large-area ultra-high-definition LCD and OLED panels.
Using proprietary rotary target technology, the system deposits highly uniform homo-geneous and low-defect materials such as ITO, IZO, TiNx, Ti, MoW, Mo, and Al for use as interconnects and pixel electrodes and supports integrated passivation layers with LTPS films, IGZO, or Al2O3 for the active layer and passivation of MOx on larger substrates. The system ultimately provides a wider process window with greater control of layer properties, higher productivity with faster TACT, and longer PM cycles with high target utilization. The equipment supports TFT-LCD and OLED flat-panel-display manufacturing, including TFT-array processes such as gate and source/drain metallization and the fabrication of color pixels and MOx active layers.
To ensure a robust display product based on a-IGZO TFTs, it is essential to evaluate electrical stability. The reason is that OLED displays are current driven. If the current is changing, the amount of light generated in the OLED pixel is also changing. It is therefore essential that the current flowing through the TFT is stable over time. The threshold-voltage stability is a good measure for the electrical stability of a TFT. Typically, the TFT stability is characterized at elevated temperatures by applying a voltage for a long time and by recording the response of the TFT. A bias-temperature stress (BTS) study (Fig. 4) shows that the a-IGZO TFT has stable electrical properties with a threshold-voltage shift (ΔVth) much smaller (0.2 V) than the ΔVth for an average a-Si:H TFT (>1.8 V) under similar AMOLED stress conditions.
Fig. 4: Stress-test results show the TFT transfer curves after operating at elevated temperature and high humidity, with negative and positive bias stress. Humidity and negative voltage stress do show a very small influence on Vth (ΔVth < 0.2 V), while positive voltage leads to a drift of Vth on the order of 1.2 V.
The impact of uniformity and particles on yield is significantly magnified as TFTs get smaller and devices get larger (Fig. 5). Particles that were not problematic before can now become “killer defects” in smaller TFTs because they are relatively larger.
Fig. 5: These optical microscope images show the potential of particles to become “killer defects” as TFTs shrink in size.
Equipment manufacturers must therefore reduce both the number (density) and size of particles when scaling to higher resolution and larger displays. The rotary target array in the AKT-PiVot system accomplishes this by providing less material re-deposition and nodule formation, which results in fewer and smaller particles, thereby enhancing device performance, yield, and product value.
This is accomplished by using a sophisticated magnet motion in the rotary target array to improve film uniformity, design rule, and glass-edge utilization, and also to deliver mura-free devices (Fig. 6). In addition, the rotary target-array configuration enhances the post-etching process of pixel ITO, resulting in superior etching residue performance.
Fig. 6: This photograph of an LTPS film shows the high film uniformity achievable with the AKT-PiVot DT™ PVD systems on large substrates. Visible mura defects are eliminated. The surface plot shows that <10% sheet resistance (RS) uniformity is achievable with a thickness (THK) uniformity of <7%, along with a high deposition rate and low homogeneous stress levels.
PECVD Technology: Improving Dielectrics and Barrier-Film Protection
Newly available plasma-enhanced chemical vapor deposition (PECVD) films provide an excellent dielectric-layer interface for MOx transistors with exceptionally high-quality insulating films that minimize hydrogen impurities to deliver optimized performance. Applied Materials’ AKT-55KS PECVD system can deposit high-quality silicon-oxide (SiOx) films with precise uniformity on sheets of glass up to 9 m2 in size – a capability that is critical in achieving high production yields and low manufacturing costs. MOx TFT requires low defect and hydrogen-free SiOx dielectric material. The contact between the active IGZO layer and gate insulator is key to a stable TFT and requires a low defect hydrogen-free interface. Passivation is also required to protect against moisture and air, and process control is critical to ensuring superior film quality. Essential feature improvements have been applied to achieve these requirements in the process chamber of the system.
A closer look at the cross-view inside the process chamber (Fig. 7) shows the new features for IGZO-ready PECVD films, which provide advantages for both uniformity and low defects. The enhanced hollow-cathode gradient diffuser shapes diffuser holes to the plasma to improve deposition uniformity across the entire substrate. The gas deflector pre-distributes gas before it goes through the diffuser, and the enhanced center support diffuser provides improved flatness to the diffuser. Together they provide superior SiOx film uniformity for MOx applications. The top-down remote plasma source clean based on NF3 plasma cleaning provides an efficient chamber clean resulting in fewer defect particles and disassociation of greenhouse gasses.
Fig. 7: Key process improvement features of the AKT-55KS PECVD (below) system that achieve IGZO-ready CVD are shown in the top image and include an enhanced hollow-cathode gradient (HCG) diffuser, (zero-field feed through (ZFFT), enhanced physical offset feed (POF), gas deflector, and enhanced center support diffuser (CSD).
Thin-Film Encapsulation
Effective encapsulation is critical to preventing degradation of AMOLED materials by moisture and particles. Applied Materials’ new AKT 40K™ TFE PECVD system deposits diffusion barrier films with very low water and oxygen penetration at low temperatures of <100°C. Encapsulation performance directly affects the lifespan and lighting performance of the AMOLED device. Frit encapsulation is currently the most common approach to enabling stronger, lighter, and more flexible AMOLED displays. The current standard for encapsulation lifetime (frit method) is approximately 2500 hours.
OLED devices are vulnerable to environmental factors such as moisture and oxygen. Particles are another major problem because they lead to dark spots and delamination issues (Fig. 8). High-quality encapsulation is required for lifetime and lighting performance and to enable the roadmap for flexible OLED displays.
Fig. 8: OLED device failures occur due to environmental factors including edge growth and dark spots.
The thin-film method depicted in (Fig. 9) uses an alternating deposition of organic and inorganic materials. By alternating barrier SiN layers with SiCN inorganic buffer layers (for OLED TV applications), the multilayer concept reduces water permeation by decoupling defect sites in the barrier films and increasing the permeation channel length. The barrier-layer functions as a barrier to water and oxygen permeation, and the buffer layer releases stack film stress and covers unavoidable particles in upstream processes.
Fig. 9: The multilayer thin-film encapsulation concept is shown at left with a 7-layer SiN/SiCN cross-section shown on the right.
The SiCN buffer layer demonstrated high optical transmittance (>90% at 400 nm and above) and low stress. The samples passed a device lifetime test (SiN/SiCN, 7 layers) and passed a 100,000-cycle 1-in.-diameter bending test (Fig. 10). Without high stress points, this buffer layer provides excellent particle coverage without leaving any voids or diffusion channels. The architecture of the TFE tool is based on a cluster-tool design to facilitate high-throughput multilayer deposition without breaking vacuum.
Fig. 10: Film performance for PECVD SiN barrier (top) and SiCN (lower) buffer layers appears above. Results show promising WVTR barrier performance with high optical transmittance (>90% at 400 nm and above) at an excellent deposition rate. Buffer results show low stress in the stack and complete coverage without voids for good particle performance.
Toward More Affordable OLED TVs
Applied Materials offers the backplane TFT and TFE manufacturing solutions described in this article to enable affordable OLED TVs. Specifically, MOx backplanes have the potential to simplify pixel-circuit design, reduce masking steps and capital equipment intensity, and improve overall TFT uniformity. Film uniformity and particle control are where equipment manufacturers have the most impact in driving cost-effective manufacturing and increasing overall yield and device stability. Not only does TFE improve the lifetime of OLED TVs, it also supports the roadmap to flexible OLED displays, provides barrier protection, and reduces cost with the potential for eliminating glass encapsulation. These display-manufacturing solutions help make possible necessary cost reductions to boost OLED-TV adoption rates. •