Market Evolution and Demand for Optical Films
Over the years, thin films have contributed greatly to the evolution of LCD technology. With the current demand for 3-D and projective-capacitive touch, they continue to do so.
by John Schultz and Bret Haldin
SUPPLIERS have made phenomenal leaps in advancing materials so that electronics manufacturers can keep up with and, in some instances, surpass consumers' demands. Two such areas include the role of thin films in the evolution of LCD technology and the function of EMI management in optimizing performance as a result of the rapid growth in projective-capacitive (pro-cap) touch. This article will explore how these separate but interrelated aspects affect display design.
Evolution of LCD Technology
In the short, meteoric rise of liquid-crystal displays (LCDs), few technologies have had the impact-per-ounce that optical films have had. Early in the development of LCDs, these very thin polymeric sheets were able to increase light efficiency, thereby extending battery life so that laptops and other mobile displays became practical. Today, that same film-enabled efficiency means that carefully designed large-format televisions and monitors can become an acceptable choice for energy-conscious consumers. Optical films have also allowed the introduction of stunning new functional capabilities such as 3-D and touch sensitivity. As consumers have demanded sleeker and smaller devices, optical films have also enhanced both performance [by managing electromagnetic interference (EMI)] and the viewing experience. In addi-tion, films provide viewing privacy and protect display surfaces from the assaults of everyday use – from car keys rubbing across a smart-phone screen inside a purse to windblown sand striking your tablet as you read at the beach.
The contribution of films will continue for the foreseeable future. Work currently under way promises to give LCDs even greater functionality, better energy efficiency, and greater design flexibility. Autostereoscopic and EMI films, in particular, are illustrative of the recent past and future potential of thin films in LCD technology.
Autostereoscopic Films
Three-dimensional perception provides a level of sheer wonderment in viewers that is rivaled by few other advances in display technology. This has been true since the first full-length 3-D motion pictures were introduced in the last century and has become especially so with the advent of the latest generation of technologies, which include active-shutter and passive glasses. With regard to no-glasses (autostereoscopic) viewing, which is still under development for many commercial applications, the leading technologies are parallax barrier, lenticular, and directional backlight.
Films play a key role – indeed, an enabling role – in several of these technologies and particularly in the directional backlight solution. Recent advances in films for 3-D autostereoscopic displays demonstrate that, in combination with other technologies, such films can be instrumental in achieving the Holy Grail of handheld 3-D displays: "at-a-glance" 3-D perception on-axis along with clear off-axis 2-D perception, full-color fidelity, and full resolution – all without the use of glasses, which are generally unacceptable for mobile device use. (Off-axis 2-D means there is no risk of view reversal, as in parallax barrier or lenticular solutions.)
The key elements of one such possible system design are a 120-Hz LCD panel, a directional backlight unit (DBLU), and a specialized 3-D enhancement film. A schematic cross-section of this configuration is shown in Fig. 1.
Fig. 1: This autostereoscopic 3-D display uses a directional backlighting unit or DBLU. (Image is conceptual and not to scale.)
The DBLU includes a set of fast-response LEDs on the left and right side, which alternate off and on (with a brief dark interlude). When the left LEDs are lit, the unique shape of the film directs the left image to the left eye; when the right LEDs are lit, the film directs the right image to the right eye. This approach also incorporates a light guide and a reflective film below the light guide, a configuration that is similar to backlights in conventional mobile devices, although the light guide is designed with specific modifications that enable the 3-D enhancement film to achieve its highest performance
It is worth mentioning that the reflective film behind the light guide is an example of the early influence of film technology on LCDs. These films' high reflectivity and light weight – the result of hundreds of layers stacked into the thickness of a sandwich bag – were among the keys to the development of laptops and other mobile devices.
3-D enhancement film sets this display apart from other LCDs. It incorporates an innovative film design and a high degree of precision in film manufacturing, unlike other autostereoscopic designs; however, it does not require a precise alignment with LCD pixels or the light-guide features, which means it is easy to integrate into OEM systems.
To achieve its 3-D effect, the film uses lenticular features on the top surface and prism structures on the bottom surface, a design that has been informally dubbed "the ice cream cone," as shown in Fig. 2.
Fig. 2: The signature "ice cream cone" profile of 3M's 3-D film is apparent in this cross-section. This film features prism structures separated by a flat region that improves the film's cosmetic quality.
The top and bottom lenticular and prismatic features must be registered with micron-scale tolerances during manufacturing, and the exact size of the features can be adjusted to minimize moiré with the LCD. To provide autostereoscopic 3-D over a large display, the prism pitch is slightly larger than the lenticular pitch so that over the width of the panel the offset between each lenticular/prism pair increases with distance from the center of the display. This offset is a design parameter based on the desired viewing distance and the size of the panel. For example, the 3-D enhancement film shown in Fig. 2 has a 44.000-mm pitch per 1000 lenticular features and a 44.008-mm pitch per 1000 prismatic features; this difference in pitch produces an optimal viewing distance of 400 mm, although 3-D can be seen both closer and farther from the display.
The example in Fig. 2 introduces the latest refinement in 3-D enhancement films: in earlier generations, the prism structures butted directly against each other on the bottom surface; in this iteration, a flat region is introduced between prisms. This flat feature increases film reliability by reducing the tendency for stress fractures to form at the sharp inner groove. Cosmetic features of the film are also improved with the elimination of sharp peaks on the master tooling. Functional testing found no change in the light-output distribution when the flat regions were introduced between prisms.
Removing Barriers to Adoption
The integration of DBLUs and 3-D enhancement film resolves several of the barriers to a broader adoption of 3-D displays in handheld devices. Among those barriers are the aforementioned glasses. Another is viewer fatigue. There are multiple causes of this discomfort, but one frequent explanation is "reversed viewing" of images that are perceived when the viewer moves slightly off-axis while looking at autostereoscopic devices based on the parallax-barrier and lenticular-lens approaches. The DBLU 3-D enhancement-film approach is autostereoscopic 3-D on-axis and provides clear 2-D viewing when the viewer moves slightly off-axis. This approach also minimizes the "black bands" that can arise between left and right images with autostereoscopic systems.
As autostereoscopic films and their DBLU and fast LCD system components continue to evolve, the limitations of earlier generations of autostereoscopic displays are being resolved. The reduction of cross-talk and the elimination of image reversal – the disconcerting in-out or out-in of images and the "scratchy" visual appearance because of the half-resolu-tion display quality – are good examples of how films and other system components can help break down barriers to adoption. The combination of a 120-Hz LCD panel, the DBLU, and the 3-D enhancement film with appropriate selection of the LED sequencing relative to the LCD panel refresh rate has been effective at reducing cross-talk to visually acceptable levels in a full-resolution autostereoscopic display with at-a-glance viewing.
EMI Films and Touch Technology
Whether for entertainment, information, or work, consumers can not seem to get enough of smartphones, tablet computers, and other types of hand-held devices. State-of-the-art materials are needed to manage the electromagnetic interference (EMI) and touch-sensorperformance issues that can arise in these designs, while simultaneously enabling display engineers to keep up with five important trends:
• The rapidly increasing market penetration of projective-capacitive touch.
• Desire for devices that are increasingly thinner and lighter.
• Desire for increased functionality and capability in portable devices.
• Increased interest in larger display sizes for portable devices.
• Demand for state-of-the-art touch functionality
The above-mentioned demand has driven tremendous growth in the area of pro-cap touch sensing in particular. Pro-cap touch displays are currently incorporated into a significant portion of smartphones and are being integrated to support touch capability in larger devices, such as tablets and slates, as well. Pro-cap capability supports single- and multi-touch functionality that as a system integrates relatively well into the current form factors of interest for portable electronic devices.
However, along with these desirable attributes, display engineers and product designers interested in pro-cap systems for their devices face many integration challenges. The system's performance can be impacted by a number of issues, such as sensor design and composition, touch integrated-circuit (IC) capabilities, display characteristics, thickness of the optical stack and cover lens, and the presence or lack of an air gap between the pro-cap sensor and the display. In addition, the demand for multiple functionalities such as GPS, Bluetooth, and Wi-Fi, and the need for their associated antennas, can cause an increased amount of electromagnetic radiation to be present in and around consumer-electronic devices. It is important to note that this radiation, in this case potentially coming from within the device itself, can directly impact that same device's projected pro-cap touch-system performance.
In short, it is not easy to design and tune a pro-cap system for a portable electronic device, and there are many factors, such as the drive toward thinner devices with increased functionality and the demand for larger display sizes, that affect the overall system performance and associated level of touch capability. The combination of the above trends is causing significant focus on the area of EMI management. As alluded to above, pro-cap systems, due to their proximity to radiating surfaces within the device, and their use in a wide range of settings, can often be degraded due to EMI-related issues. These systems rely on the ability of a signal-processing algorithm to detect small changes in capacitance in a conductive circuit, and these changes can be masked by interference from surrounding electronic noise, either from outside the device or from the device's own display.
Materials designed to meet these challenges today are typically constructed using a base substrate such as polyethylene terephthalate (PET) ranging in thickness from 25 to nearly 200 μm, and a thin conductive layer that has either been coated or deposited on the substrate's surface. The conductive layer can be made of materials such as carbon nanotubes (CNTs), high-aspect-ratio structures such as silver nanowires, transparent conductive oxides such as indium tin oxide (ITO) or zinc oxide (ZnO), or conductive polymers such as poly 3,4-ethylenedioxylthiophene (PEDOT). All of these materials seek to minimize the inherent tradeoff that occurs with a transparent conductive material between optical quality – measured by criteria such as photopic transmission, haze, and color bias – and electrical conductivity. Figure 3 illustrates how these materials can be applied with a display stack construction, usually in conjunction with an optically clear adhesive (OCA), to minimize the impact of display-generated EMI on the performance of a pro-cap touch sensor.
Fig. 3: Film materials can be applied in a display stack construction.
There continues to be strong interest in the industry to reduce the width of or even eliminate the bezel around the display, and material suppliers are working to improve pro-cap touch sensors, particularly those on a film substrate, so their interconnects are as narrow as possible. 3M's Patterned Transparent Conductors (PTC) offer high-level performance in this area by utilizing a highly controllable patterning process to provide interconnect structures five to six times narrower than the industry norm. More specifically, PTC offers superior transparency, at resistances that are orders of magnitude lower than existing ITO products, on a polyester film substrate that is highly flexible. These benefits are based on the use of silver conductive traces that are 2–3 μm in width, configured in specific geometries to minimize the optical/electrical tradeoff. These materials offer excellent optics down to sheet resistances lower than 20 Ω/.
In Fig. 4, the narrow-bezel benefit of PTC is illustrated by comparing the screen-printed interconnect structure used in many ITO/PET pro-cap touch sensor designs with a PTC-based interconnect design. The structure at left in the figure measures 1.4 mm in width, while the structure at right is 250 μm across. It is worth noting that the interconnect structure on the left supports a 3-in. display size, while the PTC interconnect structure is for a 4.8-in. display. Despite having a higher number of interconnects, the PTC-based structure is almost six times narrower than the screen-printed design.
Fig. 4: On the left are micrographs at 50x magnification of a screen-printed interconnect structure and, at right, a 3M Patterned Transparent Conductor interconnect design for a pro-cap touch sensor.
Films and the Future of Display Design
Autostereoscopic and EMI-management films have done much to advance the performance and design of LCDs and, in turn, that of today's most popular consumer mobile devices, and they will continue to expand the potential of these devices. However, while using state-of-the-art materials is important, the most successful breakthroughs and designs that can move ahead of the trends will come through early collaboration between internal product design and engineering teams and their material suppliers. It is at this point that engineers will find these materials most valuable, as they continue to tackle the challenges presented by a fast-paced and ever-demanding market.
The films discussed in this article are but two examples of a new generation of films. Other new and transformational films, soon to be introduced, will remove long-standing barriers to improvements in the cost, performance, weight, thickness, and environmental profile of displays. Now more than ever, there are a number of diverse material sets for display engineers to consider that can meet a wide range of design goals while improving and optimizing performance. •