New Materials and Tools for High-Resolution and More Energy Efficient Displays
Kerry L Cunningham
The screens on today’s smartphones and tablet PCs may be stunning, but they are just a preview of what’s yet to come in the world of displays. New manufacturing technologies for the two main types of flat screens—liquid crystal (LCD) and organic light-emitting diode (OLED) displays—are set to deliver unparalleled color, clarity and brightness in larger, thinner, lighter, higher-resolution and more energy efficient displays.
These advances are based on the use of new materials to make the arrays of thin-film transistors (TFTs) that control the screen’s color pixels (see figure 1).
TFTs traditionally have been manufactured using amorphous silicon (a-Si) films, but when a-Si TFTs are scaled smaller to accommodate more pixels for higher-resolution displays, picture quality (i.e., brightness, as measured by aperture ratio) suffers and the devices draw more battery power. Consequently, a-Si cannot enable higher-resolution displays with faster switching speeds and high energy efficiency.
To get around this, materials with higher electron mobility are required. Electron mobility is a measure of how fast electrons can move through a semiconductor, which translates into how quickly and efficiently a transistor made from the material can switch on and off.
All else being equal, TFTs made from materials with higher electron mobility than a-Si will operate more quickly and at lower power at smaller sizes. Therefore, new materials are now the drivers of display performance.
Two materials that are beginning to replace a-Si for LCD and OLED displays are low-temperature polysilicon (LTPS) and metal oxide (MO or MOx). LTPS is in commercial production right now while MO is moving more slowly from R&D to production.
LTPS has higher electron mobility than either a-Si (up to 100 times greater) or MO but each material is well suited to specific requirements (see table 1).
LTPS-based LCD screens are capturing an increasing share of the mobile phone display market, where higher resolution and lower power consumption are both key requirements. LTPS is expected to dominate capacity starts for mobile phone applications in the near future because of its proven manufacturing processes. It can command higher prices than a-Si LCD displays, and is absolutely required for smartphone screens with a pixel density ≥300 pixels per inch (ppi).
Figure 1. Higher-resolution displays require new materials for the TFTs that control the screen’s color pixels. Source: Applied Materials AKT Display Group.
Table 1. Comparison of the properties of materials used to make TFTs for flat-screen displays. Source: Applied Materials AKT Display Group.
OLED displays that can be made today by LTPS film also meet the demand for thinner and lighter displays with more vivid, higher-contrast images. Used primarily for the smaller screens of mobile devices, OLED technology at this point is not cost-effective for larger screen sizes. MO technology, meanwhile, delivers the fast refresh rates needed for large-screen LCD TVs and tablets at a lower cost than LTPS film.
LTPS is currently the material of choice for most of the high-end LCD and active-matrix OLED displays found in today’s smartphones. It delivers up to 10 times the mobility of today’s MO-based screens, along with much higher resolution and pixel density. LTPS is in fact the only technology that really works for active-matrix OLED displays at present, because of the higher currents this type of display technology requires.
For all these reasons, as smartphones begin to move beyond 400ppi, LTPS is likely to remain a key technology for the foreseeable future. The technology is similar to a-Si but a key difference is that higher process temperatures (>400ºC) are used, which results in improved electrical properties. However, LTPS transistor manufacturing is also more complex than a-Si and MO transistor fabrication, requiring additional steps and processes that increase costs.
Among the issues impacting LTPS films is that they are prone to significant process variation as a result of recrystallization techniques. The film nonuniformities are visible as mura, a background with clouds of dark and bright areas in the display.
Another issue is that LTPS photomasking processes are more complicated. While the standard a-Si TFT process requires 4 or 5 photomask steps, LTPS TFTs may require up to 12, which increases the capital investment required and the difficulty of achieving high yield rates.
A highly uniform and stable distribution of threshold voltage (Vth) across the backplane of the display is also critical with LTPS films, for both OLED (current-driven TFTs) and LCD (voltage-driven TFTs) displays.
LTPS backplanes have shown better long-term stability compared to a-Si backplanes, but the required high Vth uniformity leads to the demand for LTPS panels with a high degree of crystal homogeneity. Shifts in Vth that arise from polysilicon nonuniformities also can cause mura defects.
Figure 2 shows that while LTPS backplanes have the highest electron mobility among these materials, as well as good on/off ratios, they suffer from higher off-currents compared to a-Si and MO technology. Lower off-current is a significant advantage of devices made from MO materials such as indium gallium zinc oxide (IGZO) because it enables faster refresh rates and lower power consumption in mobile devices.
Figure 2. Schematic comparison of TFT I-V transfer curves for a-Si, MO (indium gallium zinc oxide, or IGZO) and LTPS, the three main materials used to make TFTs for flat-screen displays.
SCALING LTPS FILMS TO LARGER SUBSTRATES
Applied Materials’ introduction of AKT-PiVot DT physical vapor deposition (PVD) equipment for fabricating LTPS backplanes (see figure 3) enables panel makers to produce the next generation of ultrahigh-resolution products. It also allows them to scale LTPS technology to larger substrates and drive down costs by adopting rotary target technology.
The new AKT-PiVot DT PVD systems are a case in point. They enable manufacturing of LCD and OLED flat panel displays by supporting TFT array processes such as gate and source/drain metallization, and the fabrication of color pixels and MO active layers. They also provide for a wider process window with greater control variables and options, higher productivity with faster total average cycle time (TACT) and longer preventive maintenance (PM) cycles with high target utilization.
Figure 3. Photo of an Applied Materials AKT-PiVot DT PVD system for manufacturing large-area, ultrahigh-definition LTPS films for LCD and OLED displays. The equipment is aimed at helping meet consumer demand for greater screen performance, clarity, color and brightness.
Using Applied’s proprietary rotary cathode array technology, AKT-PiVot DT PVD systems deposit highly uniform, homogeneous and low-defect materials such as ITO, Ti, TiN, Mo and Al for use as interconnects, pixel electrodes, and integrated passivation layers with LTPS films on larger substrates.
The Applied AKT-PiVot 55K DT PVD is for glass substrates as large as 2200mm x 2500mm, while the Applied AKT-PiVot 25K DT PVD is for substrates up to 1500mm x 1850mm.
The impact of uniformity and particles on yield is significantly magnified as TFTs get smaller and substrates get larger (see figure 4). Particles that were not problems before may now become “killer defects” in smaller TFTs, as they are relatively larger.
Equipment manufacturers must reduce both the number (density) and size of particles when scaling to larger substrates and/or when enabling customers to achieve higher display resolutions. The rotary target array in Applied’s AKT-PiVot tools does this by providing for uniform gas distribution and less material redeposition and nodule formation, leading to fewer and smaller particles.
Figure 4. Optical microscope images showing the potential of particles to become “killer defects” as TFTs shrink in size.
Compared to conventional planar cathode systems, Applied’s AKT-PiVot rotary cathode arrays self-clean, reduce particle defects and enhance device performance, yield and product value. The rotary target array with sophisticated magnet motion improves film uniformity, enabling design rule improvement, better glass edge utilization and mura-free devices (see figure 5). It also affects the post-etching process of pixel indium tin oxide (ITO), resulting in superior etching residue performance.
Figure 5. Photograph of an LTPS film showing that the high film uniformity achievable with Applied Materials’ new AKT-PiVot DT PVD systems on large substrates eliminates visible mura defects. The photo shows that <10% resistance (RS) uniformity is achievable with a thickness (THK) uniformity of <7%,along with a high deposition rate and low, homogeneous stress levels.
The AKT-Pivot DT PVD systems also feature a new dual-track, single-chamber design that reduces overall footprint, and allows for increased throughput with two independent tracks for ITO, MO or IGZO single-layer deposition.
Through precision materials engineering and productivity innovations, the Applied AKT-PiVot DT PVD systems provide an optimized, cost-effective solution for volume production of future ultrahigh-resolution displays. They support multiple technology roadmaps for manufacturing large-area TVs and energy-efficient screens for mobile devices.
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