Mobile-Display Evolution: More at Your Fingertips

Mobile-display developers have been improving the quality and lowering the cost of their products as the demands on mobile devices increase.

by Paul Semenza

OVER THE LAST FEW YEARS, the handheld-device market has been transformed by waves of innovation in hardware and services – enabling mobile music, mobile video, and all forms of mobile information, ranging from Internet access to e-mail to navigation. While Moore's Law, and the digitization of all forms of content, have enabled incredibly compact devices, the requirements for displays have actually gone in the other direction – toward larger and brighter units capable of showing more information at higher quality. A complementary development has been the increasing integration of touch technology into mobile devices, thus increasing the area available for displays.

The technology and manufacturing of mobile displays have evolved to meet these needs. Specifically, advanced-generation factories for large-area TFT-LCD applications, such as TVs, have enabled panel makers to devote increasingly larger fabs to the production of mobile displays, increasing output and lowering costs, while display technologies such as low-tempera-ture-polysilicon (LTPS) TFT-LCD and organic light-emitting-diode (OLED) technology, as well as newer technologies such as reflective displays and embedded projectors, have focused on mobile applications by offering improvements in display performance, thickness, weight, and power consumption.

Handsets Get Bigger

For the display market, the term "mobile devices" really refers to mobile phones. As a portion of all small–to–medium-sized displays (those less than 10 in. in diagonal), mobile phones exceeded a 50% share in 2008 and are approaching 60%. The next two largest mobile applications – digital still cameras and portable media players – make up 10% of small-to-medium revenues. As the mobile phone becomes the primary device for mobile communications, gaming, video, and Internet usage, the display has become increasingly important.

In 2008, 1.4 billion displays were shipped for mobile phones, not including secondary displays used on clamshell-type handsets. More than two-thirds of these displays, or nearly a billion units, were TFT-LCD based, using either amorphous-silicon (a-Si) or LTPS TFTs (Fig. 1). Due to a decreasing price premium and the growing requirement for graphics and video, active-matrix technologies (including AMOLED technology) will continue to grow as a share of the mobile-phone display market, and passive-matrix displays will remain only in the least-expensive phones.

 

Fig_1

Fig. 1: Active-matrix displays – TFT-LCDs, LTPS-LCDs, and AMOLEDs – are increasing as a share of the mobile market. Passive-matrix forms such as MSTN and CSTN (monochrome- and color super-twisted nematic) LCDs and PMOLEDs (passive-matrix OLEDs) are declining. Source: DisplaySearch, Q2 '09 Quarterly Mobile Phone Shipment and Forecast Report.

 

TFT-LCD panels for mobile phones are mainly made in Gen 4 or smaller fabs. For the most common screen sizes (between 1.8 and 3.5 in), these fabs can make anywhere from a few dozen to more than 300 displays on each input sheet. As of 2008, 60% of mobile-phone main displays were 2 in. or smaller in diagonal (Fig. 2). Handsets are rapidly moving toward larger screen sizes, however, and by 2012, the same share will be taken up by displays from 2.2 to 3 in. Screen sizes larger than 3 in. are growing very rapidly as well, but the 2.2—3.0-in. range will continue to increase its share of the market because this size range can provide reasonably high information content while keeping the overall handset size reasonable.

A key challenge for mobile displays is to provide a high level of information (ideally, the same amount as on a desktop monitor, notebook PC, or TV) in a very small area. This requires high levels of pixel density, a requirement that has been a key driver for the adoption of LTPS technology, which allows for a smaller switching transistor in the pixel region than a-Si. The smaller transistor means less dead area in the pixel and, consequently, the display pixels can be smaller overall.

The move toward larger screen sizes is driven by users' requirements for higher levels of information content. The QVGA (320 x 240 pixels) format became the mainstream size in 2008 and will continue to grow for the next few years, as shown in Fig. 3. For the smart-phone segment of the market, HVGA (480 x 320 pixels) and VGA (640 x 480 pixels) are becoming the standard formats. New formats that are fractions of HD (1920 x 1080 pixels) are also growing, includ-ing nHD (1/9th of HD) and qHD (1/4th of HD).

Multimedia Demands Colorful, Bright Displays

The growth in mobile video, through streaming content and downloads, is creating a need for high levels of color reproduction. Color gamut is influenced by the gray scales provided by the driver IC, as well as the color-filter and backlight design. In 2008, 256K color displays (this is 6 bits per color; the potential number of colors produced is 262,144) made up half the mobile-phone market. Full-color (16.7M colors) displays recently entered the market and are likely to remain a small portion of it for the next few years. While white LEDs have become the mainstream approach, some panel makers have developed RGB LED backlights for mobile displays, which offer purer primary colors than filtered white, enabling a wider color gamut. An intermediate solution has been to enhance the red and green output of white LEDs through the addition of special phosphors. The typical mobile display will have a color gamut that approaches the NTSC standard used for TV.

 

(a)Fig_2a   (b)Fig_2b

Fig. 2: From 2008 (left) to 2016 (right), mobile-phone display screen sizes will continue to increase in terms of market share. Source: DisplaySearch, Q2 '09 Quarterly Mobile Phone Shipment and Forecast Report.

 

Fig_3

Fig. 3: The mainstream QVGA format will continue to gain market share for the next few years. Note: QVGA = 320 x 240 pixels; HVGA = 320 x (>320) pixels; VGA = 480 x (640+) pixels. Source: DisplaySearch, Q2 '09 Quarterly Mobile Phone Shipment and Forecast Report.

 

In order to provide good video quality, especially in bright ambient lighting, displays need high brightness. This is another area of performance that is impacted by the display and lighting design: LTPS panels typically have higher aperture ratios, allowing more light through, and color-filter designs such as RGBW also enable higher brightness. Samsung is pursuing RGBW displays using the PenTile technology it acquired through Clairvoyante. Of course, the display could also use more (or higher power) LEDs, but this would have a direct impact on battery life. Most mobile displays currently have a brightness in the 300–400-nit range (with portable navigation devices and automotive displays significantly higher), and over the next few years displays in excess of 500 nits are likely.

Another requirement introduced by the growth of mobile video is wide viewing angle. Whereas most mobile information is viewed by one person holding the display at a normal angle, video users often have the desire to share with others, meaning that some viewing will be at off-normal angles. Thus, mobile displays now implement wide-viewing-angle technologies such as MVA and IPS, as well asdifferent liquid-crystal modes, such as optically compensated bend (OCB) and special polarizer and optical compensation films. These approaches typically introduce additional trade-offs because they can result in lower brightness.

Keeping Power and Weight to a Minimum

Improving visual performance through high brightness, broad color gamut, and wide viewing angle increases power consumption. The engineering challenge for mobile-display designers has been to increase performance while at the same time reduce power consumption. As mentioned previously, RGBW and similar pixel architectures enable optimization of color gamut and brightness while holding power consumption constant. Transflective design – using partial mirrors behind each pixel to reflect ambient light – also has resulted in power savings. Another approach is to manage the backlight brightness by using two forms of control. Content-adaptive backlight control (CABC) analyzes the content of the frames written to the display and modifies backlight output, dimming the LEDs when there are dark frames. Another form of backlight control uses an ambient-light sensor to determine the lighting conditions in which the display is being viewed, dimming the LEDs when ambient-light levels decrease. Because LEDs are the primary source of power consumption in mobile displays, these two forms of backlight control can have significant impact on power consumption.

As mobile displays get larger and the devices that contain them get smaller, a premium is placed on thin, lightweight displays. The primary approach to achieving these is to thin the glass substrates, which account for most of the thickness and mass. While it is technically possible to produce glass substrates thinner than the typical 0.5-mm thickness used in Gen 4 and smaller fabs, the higher cost of production and handling has been prohibitive. The most common approach is to thin the substrates after the liquid-crystal cell has been produced, either by mechanical (grinding, lapping, polishing) or chemical (etching) means. Typical thickness reduction is 0.1–0.2 mm on each substrate, resulting in a significant reduction of material. In some cases, mechanical strength is achieved through the use of an additional, chemically treated cover glass.

Mobile applications are very challenging for display reliability due to the level of shock, vibration, and other physical stresses that mobile devices typically encounter. As a result, one of the main failure mechanisms is damaged connections, particularly between the driver IC and display. Thus, integrating the driver functions has reliability benefits, another strength of LTPS technology. However, amorphous-silicon TFT-LCD designs have integrated driver functions as well. Developed by Samsung, an amorphous-silicon gate (ASG) incorporates the gate-driver function directly on the surface of the glass panel, reducing the number of components and connections, which can increase reliability and decrease cost. This technology is an indication that amorphous-silicon will continue to compete with LTPS.

Touch Adds Functionality and Performance

The most notable new feature in mobile displays has been the expansion of touch screens. Technologies such as projected capacitive, as well as new designs and user interfaces, particularly the multi-touch-based user interface developed by Apple for its iPhone, have created increased momentum of touch in mobile devices. DisplaySearch estimates that the penetration of touch screens into mobile phones will pass 20% in 2009, up from less than 16% in 2008. By 2015, touch screens are expected to be used in nearly 40% of mobile phones (Fig. 4).

Driven by the simplicity of manufacturing and low cost, resistive has been the most widely used touch-screen technology. However, the relatively poor optical transmissivity and durability of resistive have led to adoption of other technologies, particularly projected capacitive. This technology, while costing more, enables excellent optical performance, as well as the ability to design-in chemically treated cover glass, which protects the display and touch screen, and, as initiated by the iPhone, can become the entire front face of the phone. Perhaps the most disruptive touch-screen technology eliminates the touch screen itself in favor of embedding it into the TFT-LCD. So-called in-cell touch technology utilizes photosensors or voltage sensors in the TFT array, or capacitive sensing on the color-filter plate. As such, it eliminates any films or glass layers required by external touch technologies, reducing the size and number of components, while at the same time increasing reliability. At present, the main challenge is the increased cost that the additional elements impose on the display by reducing manufacturing yield, but it is expected that as more TFT-LCD makers focus on this technology, they will bring down the cost.

 

Fig_4

Fig. 4: Touch screens, whether in-cell, projected capacitive, or resistive, will penetrate more and more of the mobile-phone market through 2015. Source: DisplaySearch, 2009 Touch Panel Market Analysis Report.

 

New Functionality through 3-D and Flex

By utilizing fast-switching LC modes, optical films, or other approaches, TFT-LCD makers have developed mobile displays with 3-D functionality. The key challenges are to produce displays that have both 3-D and 2-D capability, with minimal performance or cost impact. At the same time, there is limited content (mostly gaming) that would create the demand for 3-D on mobile devices. There is some movement toward such functionality in cameras and digital photo frames.

While it will be a long time before fully functional mobile devices are mass-produced in flexible form factors, there are benefits to using flexible displays. When constructed with rigid plastic, plastic film, or metal foil, displays can be made more rugged and lightweight. Current flexible-display technology is not mature enough to be full color and video capable, but simpler displays (such as e-paper devices) are in production. Flexible-display technology was used, albeit in a rigid form, in the Motorola Motofone, but that product was removed from the market due to lower resolution than required to use the phone for text messaging. However, flexible-display technology is being utilized in other ways, such as the Samsung Alias 2, which utilizes E Ink's electrophoretic technology to enable a dynamic keypad that changes depending on whether the phone is used in portrait or landscape mode (Fig. 5).

Future Technologies

While TFT-LCD technology is increasing its dominance in the mobile-display market, the demand for lighter, lower-power, higher-performance displays continues to draw in new technologies. In addition to OLED and electrophoretic technologies, several companies are developing alternatives to LCDs. One class of devices utilizes microelectro-mechanical systems (MEMS) to selectively reflect colors from an external light source or to "shutter" light from LEDs. These devices have simpler architectures because they do not incorporate the liquid crystal and associated optical films. However, as the long effort to bring OLED technology into mobile devices indicates, simplicity does not necessarily mean that other manufacturing or materials challenges do not exist.

Societal trends mean that workers and consumers alike are more mobile in their technology usage. The significant upgrades in wireless technology around the world, including 3G and higher cellular technology and WiFi networks, is bringing the notion of "access anywhere" closer to reality. This means that our mobile devices will become increasingly important. The demand for mobile displays – and the demands on them – will both continue to increase. •

 

Fig_5

Fig. 5: Samsung's Alias 2 mobile phone uses display keys based on E Ink technology.

 


Paul Semenza is Senior Vice President, DisplaySearch. He can be reached at paul_semenza@displaysearch.com.