The Science Behind the Screen: LCD TVs Keep Getting Better

January 23,2017

In the first piece of this series, Evolution of LCD, Part 1, we explained the importance of new LCD architectures in improving performance of thin film transistor (TFT) array technology and enabling curved LCD screens. That piece focused on technologies that incorporate black column spacers (BCS). In Part 2, we dive deeper into the benefits of the latest generation of TFT technology, paying attention to the impact organic insulator technology is making today.

Developments in TFT Materials
LCD TV technology has continuously evolved over the past two decades, driven by internal competition between LCD manufacturers and competition with rival technologies such as OLED TV. This technology evolution has progressed on two fronts: (1) Increasing the dynamic range of LCD pixels by harnessing a high-speed drive to process bright images first and dark images later to improve contrast, and (2) Improving the panel itself to support ever larger TVs with higher resolution. Recent developments in TFT technology are fueling the evolution of panel performance improvement.

TFTs for LCD panel manufacturers have historically deposited aluminum (Al) for the gate and source/drain electrodes. Growing demand for larger LCD panels with higher resolution requires electrodes with lower electrical resistance. To reduce electrode resistance for a given electrode material, designers can increase either pattern width or thickness, as resistance is inversely proportional to area. But increasing either of these dimensions is contrary to the desire to pack TFTs more densely to create smaller pixels and achieve higher resolution. Al electrodes can also cause a galvanic effect in heterogeneous systems like TFTs, generating pores in electrodes during etching. These pores increase resistance, hampering high resolution.

Copper (Cu) electrodes, which have lower resistance per unit area than Al, provide a route to reduce resistance while maintaining the dimensions of the layers. In addition, Cu electrodes can be micropatterned for more stable signal control and brightness, and they do not cause a galvanic effect.

Cu electrodes provide another important advantage over Al. Panels using Al electrodes require two display driver ICs for signal control, but those with Cu-based electrodes require only one, contributing to huge cost savings. Cu electrodes are therefore beneficial for ultra-high density (UHD) LCD panels for multiple reasons and can be considered essential to creating true RGB UHD panels.

Advantages of NPL Technology for Passivation
The passivation layer is another important factor affecting TFT performance. Standard TFT processes use silicon nitride passivation, applied via chemical vapor deposition (CVD). Beginning in the late 2000s, some LCD makers began to apply organic polyacrylate insulators instead of a standard passivation layer to simplify planarization and increase light transmission. Organic insulators may be either positive-type or negative-type. Japanese companies led early introduction of organic insulators with a positive-type material, but due to the drawbacks of long processing time for these materials, negative-type organic insulators (NPL) have become dominant in the market. In 2009, Dow became the first company worldwide to produce a commercial NPL passivation layer.

Early introduction of NPL drew attention to the reduction in processing cost inherent in replacing a CVD process with coating of an organic passivation layer. Beginning in 2010, the focus shifted toward improvement in panel performance as a result of adding NPL passivation. Organic insulators increase the aperture ratio, leading to greater brightness and a wider viewing angle, and this advantage is especially valuable for large, high-resolution TV panels.

As the display industry keeps evolving toward higher resolution with 4K and 8K UHD, panel makers need to pack many more RGB pixels into the same area. Higher pixel density leads to a dramatic decrease in light transmission and darker image quality, along with poorer contrast. Unlike OLED, LCD technology relies on backlighting to emit light. Therefore, one route to brighter images is to increase the level of LED backlighting to offset the reduced brightness from higher pixel density. This approach, however, increases cost and energy consumption, which raises concerns about the TV not being environmentally friendly.

Expanding pixel active area can be another way to increase resolution while preserving light transmission, but design constraints limit the ability of designers to expand the pixel area since there is no space available to widen the space between source bus lines (see Figure 1a). If the ITO gate electrode, which defines active pixel area, is widened, this will cause a narrow vertical spacing between the drain and gate electrodes near the edges of the ITO, leading to image distortion and other undesirable side effects.

In the first piece of this series, Evolution of LCD, Part 1, we explained the importance of new LCD architectures in improving performance of thin film transistor (TFT) array technology and enabling curved LCD screens. That piece focused on technologies that incorporate black column spacers (BCS). In Part 2, we dive deeper into the benefits of the latest generation of TFT technology, paying attention to the impact organic insulator technology is making today.

Developments in TFT Materials
LCD TV technology has continuously evolved over the past two decades, driven by internal competition between LCD manufacturers and competition with rival technologies such as OLED TV. This technology evolution has progressed on two fronts: (1) Increasing the dynamic range of LCD pixels by harnessing a high-speed drive to process bright images first and dark images later to improve contrast, and (2) Improving the panel itself to support ever larger TVs with higher resolution. Recent developments in TFT technology are fueling the evolution of panel performance improvement.

TFTs for LCD panel manufacturers have historically deposited aluminum (Al) for the gate and source/drain electrodes. Growing demand for larger LCD panels with higher resolution requires electrodes with lower electrical resistance. To reduce electrode resistance for a given electrode material, designers can increase either pattern width or thickness, as resistance is inversely proportional to area. But increasing either of these dimensions is contrary to the desire to pack TFTs more densely to create smaller pixels and achieve higher resolution. Al electrodes can also cause a galvanic effect in heterogeneous systems like TFTs, generating pores in electrodes during etching. These pores increase resistance, hampering high resolution.

Copper (Cu) electrodes, which have lower resistance per unit area than Al, provide a route to reduce resistance while maintaining the dimensions of the layers. In addition, Cu electrodes can be micropatterned for more stable signal control and brightness, and they do not cause a galvanic effect.

Cu electrodes provide another important advantage over Al. Panels using Al electrodes require two display driver ICs for signal control, but those with Cu-based electrodes require only one, contributing to huge cost savings. Cu electrodes are therefore beneficial for ultra-high density (UHD) LCD panels for multiple reasons and can be considered essential to creating true RGB UHD panels.

Advantages of NPL Technology for Passivation
The passivation layer is another important factor affecting TFT performance. Standard TFT processes use silicon nitride passivation, applied via chemical vapor deposition (CVD). Beginning in the late 2000s, some LCD makers began to apply organic polyacrylate insulators instead of a standard passivation layer to simplify planarization and increase light transmission. Organic insulators may be either positive-type or negative-type. Japanese companies led early introduction of organic insulators with a positive-type material, but due to the drawbacks of long processing time for these materials, negative-type organic insulators (NPL) have become dominant in the market. In 2009, Dow became the first company worldwide to produce a commercial NPL passivation layer.

Early introduction of NPL drew attention to the reduction in processing cost inherent in replacing a CVD process with coating of an organic passivation layer. Beginning in 2010, the focus shifted toward improvement in panel performance as a result of adding NPL passivation. Organic insulators increase the aperture ratio, leading to greater brightness and a wider viewing angle, and this advantage is especially valuable for large, high-resolution TV panels.

As the display industry keeps evolving toward higher resolution with 4K and 8K UHD, panel makers need to pack many more RGB pixels into the same area. Higher pixel density leads to a dramatic decrease in light transmission and darker image quality, along with poorer contrast. Unlike OLED, LCD technology relies on backlighting to emit light. Therefore, one route to brighter images is to increase the level of LED backlighting to offset the reduced brightness from higher pixel density. This approach, however, increases cost and energy consumption, which raises concerns about the TV not being environmentally friendly.

Expanding pixel active area can be another way to increase resolution while preserving light transmission, but design constraints limit the ability of designers to expand the pixel area since there is no space available to widen the space between source bus lines (see Figure 1a). If the ITO gate electrode, which defines active pixel area, is widened, this will cause a narrow vertical spacing between the drain and gate electrodes near the edges of the ITO, leading to image distortion and other undesirable side effects.

Figure 1. Diagram of aperture ratio increase with organic insulator application; Source: IHS Display Growth Opportunity, Dec. 2014

A new design based on organic insulators addresses this problem. By adding an organic passivation layer over the silicon nitride, the pixel area can extend over the source electrodes without causing performance issues. Figures 1b and 1c compare the old and new designs, illustrating how the increased aperture ratio improves light transmission through a given pixel. The organic insulator used in the new pixel design needs to adhere to the ITO electrode and meet stringent requirements in the areas of dielectric permittivity, thermal stability, planarization, hardness, chemical resistance, and transparency.

The Next Generation of NPL Technology
LG display introduced a pixel design technology called M+ at the Consumer Electronics Show in 2014, with the goal of addressing resolution and light transmission issues while increasing dynamic range and reducing power consumption for large displays. M+ technology adds white sub-pixels to the color filter of a conventional RGB panel and uses an organic insulator to maximize light transmission through the panel.

Figure 2. Comparison of M+ and standard RGB technologies; Source: LG display

The addition of the white sub-pixels increases photo processing cost and facility investment for panel manufacturers and also reduces productivity compared with the previous technology. To make an M+ panel, the RGB process is followed by depositing the white layer and then applying the organic insulator (see Figure 3a), but recent developments aim to combine the white layer process and the organic insulator process into a single step (see Figure 3b) in order to reduce equipment and processing costs. This streamlined process has been applied to products since early 2016, but the process has not yet been perfected.

Figure 3. Comparison of M+ COA structures: a) standard and b) single-step, after application of RGB, white, and organic insulator layers; Source: Dow

Existing organic insulators have trouble achieving sufficient planarization using the streamlined M+ process. Skipping the patterning of white pixels creates vacancies in the pixel array, resulting in a depression of about 5000 Å (0.5 µm) in the coating above each white pixel location. The color on array (COA) structure discussed in Part 1 requires an organic insulator with excellent planarization across the entire surface. Manufacturers are therefore motivated to switch to improved organic insulators that will be able to achieve sufficient planarization while taking advantage of higher-throughput processes. NPLs fill this need, enabling the next generation of displays and becoming indispensable for large, high-resolution, high-contrast LCD TVs that can remain competitive in the consumer market for 2017 and beyond.