Evolution of LCDs, Part 1: BCS Technology

Answer: Seok Han

At the turn of the century, liquid crystal display (LCD) television (TV) completely replaced cathode ray tube TV. The conventional thinking that the “TV is a thick and heavy product” was long gone. This led to dramatic growth of the flat-panel market. As many more companies entered the LCD TV market, product prices rapidly dropped, and upgrades to product performance, like viewing angle, and reduced thickness, evolved just as quickly. However, the emergence of organic light emitting diode (OLED) TVs in 2013, equipped with a high color gamut and fast response time, forced LCD TV manufacturers to look beyond the current technology. LCD TV manufacturers had to leverage their price advantage and urgently secure technology to counter rival products.

First, LCD TV manufacturers made improvements to the LCD’s response time. The response time represents a structural drawback of LCD alone, which is not present in OLED TVs. All LCD panel makers have made efforts to lower the response time. The response time stood at the 40ms range in the early 2000s, but improved to 25, 16 and then to 8ms, and now it’s below 5ms—a great achievement in resolving the response time barrier.

The second target was to improve the scanning speed, which eases eye fatigue by providing softer screen images. It began with 60Hz and was improved to 120, 180 and then to 240Hz, making it more competitive with OLEDs due to response time.

Another consideration is screen resolution, which is closely associated with broadcasting and content, so it parallels the progress in broadcasting equipment technology. The overall shift has taken place from standard definition to high definition (HD) to full HD and to ultra HD, and considering the price advantage, this is a significant area in which LCD TV is superior to OLED TV.

Answer: Seok Han

LCD TV makers have continued efforts to match performance with rival products, but they are still struggling with a free-form design. For example, OLED TV emits light on its own and makes a flat screen curved, which leads to curved displays. In contrast, LCD TV can realize colors with white-light-emitted thin film transistors (TFT), and liquid crystal and color filter back-lighting. These multiple film layers should be aligned for color creation so that any warp in the layers will disrupt the alignment, also causing a color shift. In addition, the smaller the radius of curvature, the more serious the distortion. This structural limit poses a problem for LCD manufacturers.

The curved display panels available today are made with heat-treated transformation after complete processing. It is critical in this process to ensure there is no change in display performance caused by heat. Since LCD TV has an alignment issue, panel makers need to minimize it to realize curved TV.

This is realized by a technology called color on array (COA) or color on TFT (COT), which the curved LCD TV first launched in 2014.

The image below is a simple description of conventional LCD structure and COA structure, a new architecture where black column spacer (BCS) is applied.

The COA or COT structure represents a new LCD architecture that requires direct formulation of the color filter previously located on the upper part of the conventional LCD structure on top of the thin film transistor (TFT) layer at the lower part. Due to this technology challenge for the new LCD architecture, the application varies by company.

The COA/COT technology was initially developed to reduce the costs that resulted from the use of extra materials (overcoat, resin black matrix (BM)) in the upper glass panel, additional color filter process and other processing costs, but the performance benefits of this technology are also substantial. The technology realized process simplification with the application of black column spacer (BCS) that integrates BM and column spacer (CS) processes by moving the color filter and all other layers at the upper part of a large TV panel to the lower part of the TFT, while increasing the final brightness and offering an even wider viewing angle with a 15-20% improvement in the opening rate compared with previous panels. In addition, by minimizing the intra-layer alignment, the LCD TV evolved from its flat form into the first successful production of a curved LCD TV model.

Answer: Ho-Suk Song

BCS technology is an important element in achieving performance improvements for the new TFT array technology. I will explain BCS, focusing on benefits of its use.

As previously mentioned, BCS, the ultimate approach to realize COA/COT, has been studied and applied in a varying manner by Korean LCD panel makers. Recently, panel makers in Greater China have also looked to develop the technology, but it is expected to take about two years to be applied sequentially.

So why is BCS beneficial in terms of panel cost and performance? As seen in the image below, BM and CS processes are combined into one BCS. Combining two processes into one shortens the overall processing time, and leads to improvements in productivity and cost savings. In addition, the opening rate is increased when the BM area, which was previously located in the upper part of the structure, is pulled down to the TFT substrate, making it possible to realize bigger pixels. This means more light can be transmitted, contributing to increased TV brightness and sharper contrast, leading to crisper and clearer image quality, as well as the completion of the LCD structure optimized to curved TV without misalignment.

BCS is a material that requires “ambidextrous performance” on three aspects. First, as above, BCS is required to realize high-end products based on LCD manufacturing technology; and at the same time, it can contribute to cost savings for customers. Second, in terms of performance, it’s required to realize the two roles of CS and BM used in the previous LCD process. Finally, the material can realize the color (black) and half-tone multipatterning lithography.

Dow’s BCS design concept is the world’s first commercial technology development that resolves historic challenges with this technology. To underscore the black properties, an increase in optical density is required. However, previous technology increased the dielectric constant caused by an increase in carbon black content when optical density increases. For this reason, Dow used organic black pigment technology to resolve the issue and to enable mass production. This design concept allows customers to secure sufficient black properties without a fatal trade-off in resistivity.

Dow’s BCS’ system-level design is characterized by the development of a formulation that can realize a pattern of structures with more than two different heights (tones) in a single exposure process. The realization of two tones (main and sub patterns) was achieved with the previous technology to some degree, but the realization of three tones (main, sub1, sub2 patterns) in the single exposure process is available only from Dow’s BCS. For example, when a customer designs a three-tone BCS structure for image quality improvement, previous materials cannot realize the structure in the single-exposure process, but Dow BCS makes it possible.

Combined with design performance in the advanced concept level, black materials with high optical density can realize the three-tone BCS structure through the single exposure process, contributing to image quality improvement and process simplification.