Overview: Low-Temperature Polysilicon

Low-temperature-polysilicon (LTPS) technology is an important part of the active-matrix-LCD manufacturing environment. Will its role expand or has it found its niche?

by Jenny Donelan

LOW-TEMPERATURE-POLYSILICON (LTPS) appeared on the manufacturing scene about 10 years ago with the promise of enabling new generations of faster, higher-resolution slimmer LCDs. Since that time, LTPS has not exactly taken over from its counterpart, amorphous-silicon (a-Si), but it has developed a substantial position in the overall LCD market. Here's a look at how LTPS works, what it's currently being used for, and how it fits into the general LCD landscape, now and in the future.

The Promise of LTPS

LTPS technology enables the manufacture of active-matrix LCDs (AMLCDs) that are faster than displays using a-Si. "Fast" refers to the addressing speed: how quickly can a cell be accessed and receive a charge, and then how quickly can the process be repeated with the next cell? All this affects the frame rate, resolution and drive-complexity factors in a panel design. LTPS has many additional advantages: integration, smaller transistors, lower power consumption, and ruggedness. However, it is important to note that all these benefits are available on a sliding scale of sorts: LTPS allows smaller pixels, or faster large pixels, or lower power consumption, but not necessarily all at the same time.

LTPS transistors can be fabricated by recrystallizing amorphous silicon (a-Si) using laser annealing. With LTPS, you can achieve either a much greater current density compared to a-Si for the same size transistors, or have much smaller transistors for the same current density. These features enable designs in which polysilicon drivers are integrated on the glass, unlike a-Si, which generally cannot achieve this integration. This added capability of LTPS arises from the fact that its electron mobility can be as much as 100 times higher than that of a-Si, and hole mobility can be much higher than a-Si as well. Polysilicon consists of many small crystallites of silicon, with the crystal structure enabling high mobility. In contrast, the silicon lattice in a-Si has no crystal structure, so mobility is reduced.

The following discussion of polysilicon is from the article "Flexible Transistor Arrays" by Peter Smith, David Allee, Curt Moyer, and Douglas Loy, in the June 2005 issue of Information Display: "Another advantage [of polysilicon] is that CMOS circuits can be made in poly-Si because it is possible to fabricate both N-and P-type TFTs. Since CMOS is the dominant technology in the integrated-circuit (IC) industry, there is substantial experience in designing sophisticated integrated circuitry in CMOS. In stark contrast, only N-type transistors are possible in a-Si, making circuit design more complicated. N-type-only circuits have not been widely built in the IC industry since the 1970s, although N-type active-matrix backplanes are the mainstay of the display industry." (Readers are strongly encouraged to review this and many other references for a complete technical summary of poly-Si technology).

Integration is key in that LTPS allows driver circuitry to be made part of the display. Because of its lower electron mobility, a-Si requires more extensive external driver circuitry that, in turn, must be made from conventional silicon. These driver types require external connections, although sometimes the driver chips can be put on glass without a separate PC board. Consequently, discrete logic chips must be mounted on printed circuit boards (PCBs) around the edge of the panel, a design that contributes to the overall bulk and expense of the display. The use of LTPS enables a smaller and more elegant panel layout (note the narrow rims on the displays shown in Fig. 1.)

Smaller transistors are possible for each subpixel due also to LTPS technology's higher electron mobility. The higher mobility reduces the size of the transistor for a given addressing time, and this feature can be used to increase the aperture ratio of the subpixel; i.e.,the ratio of transparent area to total area in each subpixel. This allows higher light-transmission efficiency that, in turn, enables brighter and higher-resolution displays (with lower power consumption).

Fig. 1: These LTPS TFT-LCDs for notebook PCs from Toshiba Mobile Display range in size from 8.9-in. WXGA to 13.3-in. WXGA. Image courtesy TMD.

The option of making a device rugged is one other distinct advantage of LTPS. Because the drive circuit can be integrated directly onto the glass surface, the number of potentially breakable connectors is decreased and the resulting LCD is vibration- and impact-resistant.

It is also important to note that when used as a backplane, LTPS is highly compatible with AMOLED displays. As Jennifer Colegrove, Director of Display Technology at DisplaySearch, puts it: "OLEDs are a current-driven type of display, and they need a lot of current." This is a requirement that is well-matched by the high electron mobility of LTPS.

High-Temperature and Low-Temperature Polysilicon

When display designers first began working with polycrystalline silicon, the standard methodology was to deposit a silicon layer on quartz, then heat it (using temperatures over 1000°C). The silcon would melt, and as it cooled, the crystals would reform in a uniform mode, creating a quartz semiconductor with much higher performance than a-Si. One of the drawbacks to this method, however, is that it is only possible to make a quartz substrate to a size of about 8 in., and it cannot be put on glass. High-temperature polysilicon (HTPS) has its place, however. Some manufacturers, such as Epson, use it for high-quality displays in projectors.

In the late 1990s, manufacturers began experimenting with using lasers in low-temperature annealing processes to replace the high-temperature annealing (the melting process described above). This allowed for the creation of larger displays that still had the basic electron mobility of the high-temperature process. And the low-temperature process had the added benefit of being usable with either glass or plastic. (While glass for displays can withstand about 500°C, plastic substrates are limited to about 200°C or less.)

LTPS products, most of them portable devices, began to be produced around 1998. In 1999, for example, Toshiba announced the world's first 4-in. VGA-resolution LTPS LCD. The "big three" manufacturers in LTPS today, according to Vinita Jakhanwal, Principal Analyst, Small/Medium Displays, with market-research-firm iSuppli Corp., are Sharp, Toshiba Mobile Display (TMD), and TPO Displays.

The Current State of LTPS

Jakhanwal notes that according to iSuppli, LTPS technology represents about 32% of the market share for active-matrix displays smaller than 10 in. (see Fig. 2).

Although a variety of large-screen LTPS TFTs have been produced, the technology seems to have found its main niche in portable active-matrix devices. These include notebooks, but also mobile phones, smartphones, and cameras (see Fig. 3). "In terms of overall volume for 2009," says Jakhanwal, "mobile phones represent 65%." The other main applications, she continues, are digital cameras (21%) and portable media players (11%).

The mobile-phone application is a logical one, as consumers are requiring devices to display an ever-wider range of content. "The mobile market is really being challenged to provide higher-resolution displays," says Steve Vrablik, Business Development Director, LCDs, with Toshiba America Electronic Components, Inc. As mentioned earlier, LTPS is very suitable for creating high-resolution displays. Vrablik notes that in 2008 TMD created 3.0-in.-diagonal WVGA displays that were used in many mobile phones in the Japanese market, and that this resolution trend is now catching on elsewhere. "This is very, very dense imagery." Not all LTPS displays are diminutive, however. In 2004, TMD came out with a 32-in. panel and currently focuses its larger-size production for notebook display applications in sizes up to 15.4 in.

Fig. 2: Through 2013, LTPS, along with a-Si, is predicted to gain in terms of overall LCD volumes shipped. Source: iSuppli Corp.

Fig. 3: The LTPS-LCD shipment market by application shows overall units increasing through 2013, and mobile handsets retaining the lion's share. Source: iSuppli Corp.

Drawbacks and Challenges

Although LTPS is highly useful technology, it does not appear poised to take over from a-Si TFTs. One reason for this is that "It is more expensive," says Colegrove, noting that the annealing process is expensive because the lasers themselves are costly, and the process itself takes extra time. And, as in all things, manufacturing, overhauling existing production lines, or creating entirely new fab lines to accommodate a different technology is no small matter. The question of scale comes into play – the more a company can produce, the more it stands to save from making such changes, but it has to have conviction in the long-term viability of the technology.

Toshiba's Vrablik says that TMD has been able to offset expenses incurred by LTPS through garnering savings in component costs. TMD built its AFPD, at the time the world's largest production plant for LTPS TFT-LCDs in Singapore in 2002. And in 2006, it announced plans to invest approximately $270 million in a new production line to produce LTPS LCD panels in its Ishikawa Works in Japan.

What's Ahead for LTPS

As mentioned earlier, another prominent and promising application for LTPS is as a backplane for AMOLED products. "Currently, about 100% of AMOLED products use LTPS," says Colegrove. Samsung is a major player in this area, notes Jakhanwal. In 2007, Samsung SDI introduced a 31-in. OLED panel with an LTPS backplane. In 2008, it showed a 40-in. model at FPD International, and in 2009 demonstrated a 21-in. OLED TV with a 1,000,000:1 contrast ratio.

Where will LTPS be in a few years? Jakhanwal points to its current market share: "LTPS is doing really well," she says. "There's been tremendous growth." Will LTPS eventually wrest the bulk of market share from a-Si? Probably not, according to Colegrove: "We would forecast that it will grow, but it will be limited. LTPS is only about 3% vs. a-Si of the total TFT substrate market now and over the next several years." For a technology that might take on a-Si at some point in the future, "Besides LTPS, I would look for some other technology, perhaps oxide-TFT," she says. •