This article is a summary of some of the market and technology analysis from the IDTechEx research report Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts.
There are a number of significant trends that have been driving technological innovation in the display industry since its early days. These trends include image quality, portability, screen size and form factor. While these trends are still strong undercurrents, new drivers are being introduced that will play a dominant role in shaping the industry from now on. These new drivers open up new frontiers both on the market and the technology side. They allow displays to both expand their existing markets and to diversify into new spaces.
These drivers will change the display technology and landscape and include flexibility, product differentiation, transparency, 3D, power savings, system-on-panel etc. These trends can result in significant changes in technology, which would create opportunities and risks to all those exposed to the value chain of the display industry. The article will assess how these market trends impact the choice of backplane technology and which backplane technology is likely to emerge as the winner.
Adoption of Backplane Technologies
There are already several different backplane technologies that are mature, available and are emerging fast. These include nanocrystalline silicon, amorphous silicon, low-temperature poly-silicon, evaporated or solution-processed or evaporated organic semiconductor and various metal oxide thin film transistor technologies.
The table below outlines key parameters of several thin film deposition techniques. They are assessed on parameters such as mobility, deposition technique, temporal stability and spatial uniformity. These attributes will determine how each technology will ultimately fit within the changing display industry.
Table 1. Key parameters of thin-film deposition techniques
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Source: IDTechEx report Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts
With regards to large-sized LCDs, amorphous silicon backplane is a winner. It is amorphous hence spatial uniformity is not an issue. The industry also has considerable experience in using PECVDs for large-area deposition. The synergy with the a-Si photovoltaic technology also helps. The material can also be made thin and can also be transferred off glass and onto flexible substrate, introducing flexibility. When it comes to 3D the technology fails, higher on-board processing power, and large-sized OLEDs. The limiting factor in all these aspects is the limited field-effect mobility.
Polycrystalline silicon is a winner in several small size displays. Its position is not challenged in small to medium-sized OLED displays. It is also ideal in offering 3D, CMOS capability and high on-board processing power. It however falls short on increasing display size and increasing flexibility.
Organic thin film transistors are strong on flexibility, but fail on other parameters such as 3D, resolution, size, etc. We note that they perform better on size when it comes to LCDs because spatial uniformity requirements are not as stringent. This assessment is maintained despite the fact that ultra-large size printing was a major driver for investment in organics. Present technological status and recent lab-to-fab transfer experiences suggest that reproducibility and spatial uniformity are critical issues.
The Role of Oxides in this Emerging Landscape
Oxide semiconductor TFT technology is evolving and has high mobility making them suited for 3D, OLED and on-board processing. They have wide bandgaps therefore they can also be transparent. Their high mobility enables smaller pixels and lower aspect ratio and hence higher resolution and lower consumption.
They can be processed at low temperatures making them amenable to flexible substrates, although metal oxides (similar to ITO) are not expected to be fully flexible and their properties are likely to change if bent too much and/or too many times. They are amorphous hence they can be put down over large areas. Their key deposition technique is sputtering though there has been considerable progress in printing it. It is easy to achieve high-throughput using sputtering.
This is a leading technology that can step in to enable large-sized OLED or/3D displays. The challenges that must be overcome include stability under illumination and large-area manufacturing using sputtering. The former challenge is key in transparent displays, whereas the latter is critical if oxide TFTs are to be used for large-sized displays. Table 2 outlines the merits of oxide thin film transistors for enabling different market drivers in the display industry.
Table 2. Assessing the merits of oxide thin film transistors for enabling different market drivers in the display industry and comparing with its closest rival technology
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Source: IDTechEx report Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts
A Key Report by IDTechEx
Metal Oxide TFT Backplanes for Displays 2013-2018: Analysis, Trends, Forecasts evaluates the key drivers that are shaping the display sector, examines the trends and assesses how these trends create new functional needs on the technology side. It offers an in-depth review of existing and emerging thin film transistor solutions and critically assesses the pros and cons of each.
In terms of metal oxides, it evaluates the different material systems available (IGZO, HIZO, IZO, ZNO, TZO, ZnO, etc) and the merits of each. In doing so, it outlines and discusses the leading research frontiers. The report also discusses several requirements of dielectrics for emerging displays and explores the material options for use as dielectrics on wide-bandgap metal-oxide semiconductors.
The report links material properties of all thin film transistor technologies to device figures-of-merit, including sub-threshold voltage, mobility, threshold voltage, stability, contact resistance, etc. These figures-of-merit are then connected to backplane attributes and thereby to the emerging functional needs of the display industry as a whole.
The report also provides a detailed outline of activities in the OLED display segment, including:
- An analysis of announced production capacity
- Number of units sold by manufacturer
- Which backplane technologies are used by which manufacturers
- A timeline of venture/partnerships activities taking place across the world in the OLED space.
- Product development cycle for oxide semiconductors
Conclusion
It is important to note that here will be no one-size-fits-all solution. The key market drivers are modifying the requirements on the backplane and in doing so they will fragment the market along technical capabilities. This implies that each technology whether emerging or existing will occupy different sections of the market and will address different needs.
The makes the R&D and investment decisions of the future challenging as all efforts cannot be consolidated on one technology for all markets. This fragmented picture will persist until one technology sufficiently improves to outperform all others in every aspect.
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