High-resolution display analysis

The flat-panel display industry is experiencing its most significant technological change in 20 years, but it may not be immediately apparent to the average consumer. The changes are all related to advances in transistor technology, the tiny electronic switches that control the display screen and produce crisp, clear images.

One transistor corresponds to one display pixel on the LCD panel, and the brightness of the LCD is controlled by switching the transistor on and off. The array of transistors corresponding to all pixels of the display is also called the backplane, as shown in the figure. Obviously, the performance of the backplane directly affects the quality of the display, that is, your TV screen, mobile phone screen, tablet computer, etc. The speed of pixel switching is related to the refresh rate, and the total number of all pixels in the display is called resolution.

Today, there are three main types of backplane technologies: amorphous silicon (a-Si), low-temperature polycrystalline silicon (LTPS), and metal oxide (MO). If you are considering buying a flat-screen TV, do you pay attention to the tiny transistor technology in the TV screen?

Before introducing backplane technology, let's first define some display terms. Most consumers are very interested in image clarity, color brightness, viewing angle, and afterimages of moving images.

Resolution is an important display indicator that everyone is familiar with.

High-definition resolution (HD) is the current standard configuration for televisions, which refers to the number of pixels on the display screen. The HD TV specification refers to 1080 rows x 1920 columns, a total of 2,073,600 pixels (in fact, each pixel is composed of three RGB sub-pixels, and the display unit in terms of sub-pixels is three times the number of pixels mentioned above). However, even for a 55-inch flat-panel TV with HD resolution, its pixel density is only 40ppi. Smartphones and tablet screens have a higher ppi index because they are viewed at a closer distance. For example, Apple's third-generation iPad has an astonishing pixel density of 264ppi. It seems that as TV sizes continue to increase, ultra-high-definition resolution will become the next milestone in the development of TVs towards higher resolutions.

Another key parameter is the refresh rate. 60 Hz, that is, a picture is refreshed 60 times per second, is the industry standard. When it is necessary to display faster-moving images, in order to make LCD images clearer, many manufacturers have begun to use 120 Hz and 240 Hz refresh rates; whether the increase in frequency is worth the extra price seems to depend more on the degree to which each consumer recognizes the difference in image quality.

3-D display content requires a refresh rate of no less than 120Hz. Currently, most 3D displays use two images with different visual depths to create a 3D image effect. 3D glasses help us see only a single image at a time, and then our brain combines the two images at different times into a complete image with three-dimensional depth of field.

High definition, faster refresh rates and 3D display content have gradually exceeded the driving capabilities of current amorphous silicon transistors (a-Si TFTs) and cannot meet the requirements of the above higher indicators.

Looking back at the development of flat panel displays over the past years, a-Si has always been the main technical solution for TFT backplanes. A-Si TFT technology has low manufacturing costs, high yields, and relatively simple substrate manufacturing upgrades. Currently, it can be mass-produced using ultra-large 9-square-meter glass substrates. The cost reduction brought about by this generational upgrade has increasingly met consumers' price expectations, making a-Si TFT-LCD products the mainstream products in the display industry.

The function of the transistor is to charge the pixel, quickly reach the operating voltage, and maintain the charge on the pixel until the next refresh signal. For transistors, a key performance is the mobility of the carriers, that is, the speed at which the electrons move. Low electron mobility means slow response. A-Si's carrier mobility will not be able to meet the requirements of high-resolution, high-response speed displays, such as the latest high-end smartphones and tablets.

The technologies that can replace a-Si currently include LTPS (low temperature polysilicon) and MO (metal oxide). The best alternative material should have higher carrier mobility, low manufacturing cost, high mass production yield, and the manufacturing process can be upgraded to large size (while maintaining the uniformity of the film), further leveraging the advantages of lower manufacturing cost per unit area brought about by generational upgrades.

Relatively speaking, LTPS has the highest carrier mobility, but the manufacturing cost is also the highest, because LTPS requires more process steps. Moreover, the LTPS process also faces great challenges in obtaining good film uniformity and high yield. In addition, LTPS is subject to certain limitations in generational upgrades. The main bottleneck is currently in the laser annealing equipment, which can only correspond to medium substrate sizes (less than 3 square meters).

MO technology, using metal oxides with indium gallium zinc oxygen components, has become the industry's most promising alternative technology, capable of achieving the required high mobility and high uniformity on large-size glass substrates. (Today, Applied Materials officially released a new PECVD technology for making insulating films for MO TFTs, which can accommodate substrates as large as 9 square meters, such as the size mentioned in the previous a-Si. See the Electronic Engineering Times report: Applied Materials' new PECVD film technology for the next generation of higher-resolution displays) Compared with LTPS, the MO process requires fewer mask steps and has the advantage of depositing thin films at room temperature, which means that the same film formation process can be performed on future flexible display substrates.

If the display industry can successfully achieve this leap in new backplane technology, the cost of the change will still be very economical, and the details of the change will be relatively clear and visible to consumers.