Front-of-screen (FOS) display components are key to optimizing optical performance in displays as well as user experiences.
by Ion Bita, Marek Mienko, Rashmi Rao, George Mihalakis, Russel Martin, and George Valliath
A KEY ENABLER for designing and fabricating displays with the performance and functionality required by a target application is the ability to integrate the basic display panel with appropriately selected complementary components. Front-of-screen (FOS) display components, which are stacked on the viewer side of the display panel, play a major role in optimizing the display optical performance and user experience. Good FOS design enhances key image-quality metrics (FOS optical perfor-mance), surface robustness, and mechanical ruggedness. FOS architectures can also add functionalities such as touch sensing and illumination, to name just two. In this article, we survey a broad spectrum of FOS display components with an emphasis on highlighting the underlying technologies, the typical applications, and selection criteria where applicable.
FOS Components Overview
Generally speaking, front-of-screen refers to the components of a display module that are physically located between the display panel and the user, including all the layers above the display active area and the external surface of the display module (Fig. 1).
Fig. 1: The FOS stack, flat-panel display (FPD), and module interact with the ambient environment.
The roles played by FOS components span a wide spectrum, from improving the mechanical robustness, optical performance, or environmental robustness of a display module, to providing added functionalities required by the particular end-product application. Last, but not least, when mapping out the range of FOS components and treatments, it is important to also consider module assembly strategies that will optimize the FOS component functionality within the new system-level constraints (electrical, optical, and mechanical). Table 1 provides examples of various mainstream FOS components and their respective applications.
Giving a comprehensive description for each of the items identified in Table 1 is outside the scope of this short article. Instead, we will introduce a few widely used components that are of particular relevance relative to current trends in the display industry.
Mechanical Robustness Improvements
A common requirement of display products is the protection of the flat panel during normal device use or in accidental events. Particular attention goes toward specifying the top surface of the display module to ensure adequate levels of scratch resistance, chemical compatibility, cleanability, and tolerance to mechanical load and/or shock. A popular solution in many of today's portable devices is the use of a strengthened cover-glass (CG) substrate placed above the display, replacing traditional solutions based on hard-coated plastics.
While the use of strengthened glass substrates has a long history of rugged display assemblies for industrial and military applications, we have seen far higher use of this solution in the consumer application space in the past 5 years. The trend for reducing the thickness of portable devices (smartphones and tablets) coupled with the large business opportunity created by more than a billion units shipped yearly, greatly energized the development and manufacturing of strengthened CG solutions.
Compared to plastic – which was originally the first material choice for the cover lens of portable devices – the adoption of CG has been stimulated by advantages of superior hardness and scratch resistance, coupled with a higher perceived industrial design value. To increase the robustness of glass to required levels, a variety of strengthening methods are used. Chemical strengthening is the dominant approach, where a compressive surface layer is formed via a high-temperature ion-exchange process when sodium ions are replaced by larger volume ions such as potassium.1 Significant robustness gains are obtained by optimizing the built-in stress and thickness of this compressive layer (e.g., by ensuring it is thicker than typical surface microcracks that could otherwise become fracture initiation points during a mechanical event). The glass surface can become so strong that normal scribe-and-break singulation processes become ineffective, thus strengthening is typically done at panel level so that all exposed surfaces and edges are strengthened. One of the mechanical tests used for quantify-ing the benefits of using strengthened CG is the steel-ball drop test, where the impact energy for causing CG or display-panel damage is determined by varying the ball mass and drop height, as described in a number of reports.2
As shown in Fig. 2, the mechanical benefits of the CG are significantly enhanced in a bonded configuration free of air gaps between the individual layers of the display module. With additional optical performance gains resulting from removing air gaps in the FOS stack (see next section), a number of materials and lamination and bonding processes have been developed2,3 in order to enable an effective integration of the various FOS layers with the FPD panel.
Fig. 2: The chart depicts failure height and energy for 225-g ball-drop impact on 12.1-in. LCD modules protected by CG. Note the significant improvement when the CG is display bonded (DB) vs. separated by an air gap from the LCD, and the benefits of chemical strengthening (CS) when comparing 0.85-mm CS CG vs. 1.1-mm plain glass. (From Ref. 2.)
Display Optical-Performance Improvements
One of the consequences of including multiple layers between the FPD panel and the user is the impact on the display performance, i.e., the FOS optical performance (including brightness, contrast, color gamut, viewing angle, etc.). Key to minimizing the impact on the stand-alone FPD optical performance is to maximize the transmissivity through the entire FOS stack while minimizing stack reflections (transmission losses impact display brightness and luminous efficacy, while reflections cause degradation of the contrast and gamut in bright-ambient illumination conditions).
Two typical problems introduced by, and consequently addressed in the FOS architecture, are the front surface reflection (FSR) of the module and the reflections added by the interfaces between FOS layers. FSR proper-ties are optimized by using anti-reflection (AR) and/or anti-glare (AG) treatments as described below. Interface reflection solutions include the use of circular polarizers, originally popular with resistive touch panels and more recently in OLED displays,4 and the elimination of air gaps by lamination or bonding of the FOS components to the FPD panel.
Anti-Reflection and Anti-Glare Coatings
Reflections of ambient light at the front surface of any display can introduce a distracting glare to the user and degrade overall image quality. The significance of these reflections can vary based on the type of display and on the environment in which it is used.5 The optical performance of reflective displays is influenced directly by the magnitude of the FOS stack reflections. Emissive displays are typically only sensitive in high-brightness environments, but there the user experience can be severely limited and the impact on power consumption becomes significant in order to maintain a minimum contrast. A well understood solution addressing these issues is to apply AR coatings and/or AG coatings/ treatments on the front surface of the display. Figure 3describes the optical properties for both of these coatings.
Fig. 3: Optical properties of antireflection and antiglare coatings appear above. Inset pictures depict the impact of the AR and AG surface treatments on the reflected image of two fluorescent tubes.
AR coatings find widespread use in the display field, with multiple technologies and integration choices available for implementation. In the majority of cases, AR coatings are created by stacking subwavelength thin-film layers, which cause a destructive optical interference across the reflected visible spectrum and consequently reduce the typical 4–5% FSR to < 2% for a single-layered AR, or < 0.5% for multilayered coatings as shown in Fig. 4. Besides the magnitude of reflectance, when choosing between various coatings one also needs to consider the trade-offs related to the reflection color tint, the perceived color variations related to process control or viewing-angle changes, and cost. Examples of typical reflectance spectra from representative AR coating designs are shown in Fig. 4.
Fig. 4: Spectral reflection curves are shown for various types of AR coatings.
AR coatings can be applied to glass and plastic substrates by established thin-film deposition methods, including vacuum evaporation and sputtering, as well as low-cost wet-coating processes. Another approach for creating AR coatings is to fabricate a graded-index layer, where the refractive index is gradually reduced from that of the substrate to unity matching the air ambient. A sub-wavelength nanoscale surface structure commonly known as "moth-eye" produces this effect and can be embossed in plastics using standard roll-to-roll processes. The resulting performance can reach very low levels of photopic reflectance (< 0.1%) with a neutral color and an optical performance that is stable across a large viewing cone.6 Thus far, their use in displays has been limited by mass-production capacity, difficulty of surface cleaning, and reduced robustness compared to the more typical thin-film AR-coating stacks.
In all cases, AR coatings can either be applied directly to the cover glass and other FOS components or they can be laminated by using commercially available AR-coated plastic films. Given the display application and FOS requirements, suitable AR coating can be identified by careful consideration of all the above factors.
AG coatings are used to diffuse or spread out the specular reflections of ambient illumination sources by the display front surface.7 These specular reflections appear to the user as "glare." The surface roughness of this layer diffuses the reflected light into a wider angular cone, causing a mixing and redistribution of the reflected light. This reduces the specular reflection intensity of the front surface and diminishes its image forming qualities.
Figure 3 depicts examples of the appearance of two linear light sources (fluorescent tubes) reflected from AG and non-AG surfaces. The texture of AG coatings can be achieved by subtractive processes, such as chemical etching and sand blasting, by additive processes including spray and dip coating and by molding or embossing processes on plastic film substrates. AG roughness can be created by random undulation of the coating or dispersing filler particles of different sizes within the coating. In general, the key challenge for optimizing AG layers for display applications is the trade-off between the desired glare reduction and the degradation of light-transmission properties. The texture of an AG surface diffuses the reflected light but will also diffuse the light transmitted through that surface, which can add unwanted haze and potential blur of the displayed image. Another side effect of adding AG coating can be the appearance of sparkle due to a random modulation of the transmitted image pattern passing through the AG coating.8 This effect is related to the random nature of AG microstructure and strongly depends on the feature size and angular diffusion characteristics of its roughness. Optimized processes can tune these microstructures to yield desired AG properties with only minimal increase to haze and the perception of sparkle. Although visual perception properties of AG coatings are not typically quantified, quantitative metrics such as gloss, transmissive haze, and distinctness of image can be good indicators when evaluating and guiding the choice of AG solution for particular display applications.9 The process choice for producing the AG coating will also be set by the intended application and substrate type. For example, in large-sized consumer displays where CG is not required, the simple lamination of plastic AG films from a wide range of inexpensive film solutions is sufficient. In other commercial, automotive, or military displays where mechanical robustness dictates the use of an AG layer applied directly on the front surface of the CG, the number of off-the-shelf solutions is more limited, so achieving the desired AG performance may require some level of customization for each application.
As shown in Fig. 3, AG and AR coatings can be combined to achieve a cumulative effect of reduced glare and reflection. Both plastic films and glass substrates with AG layers can be used for this purpose, but applying AR coatings to a textured surface with good performance and uniformity can be challenging. The vacuum deposition of thin films lends itself better to this purpose but cost constraints can be a significant barrier for widespread implementation of these types of solutions.
Enhancing Environmental Robustness
Among the corresponding topics listed in Table 1, the topic of surface treatment for the reduction of contamination (fingerprints, smudges, etc.) is a good candidate for additional description given the CG, AR, and AG FOS treatments introduced earlier in this article. Two sets of approaches exist: in the primary one, the surface energy of the front surface is engineered to ensure that residues have a poor adhesion to the display surface and are thus easily cleanable, while in an alternate approach the surface optical properties are optimized to reduce the visible contrast difference between the clean and the contaminated areas.
While anti-fingerprint (AF) coatings cannot prevent fingerprint residue accumulation, they make the display surface readily cleanable by virtue of their oleophobic and hydrophobic surface properties. Fluorinated organic precursors are typical material choices, but the particular deposition process and film-thickness selection depends on the complete set of FOS requirements, such as compatibility with AR/AG, level of scratch resistance, etc. The AF coating thickness can range from a few molecular layers (no optical impact) to a few microns when AR/AG properties are desired, in which case it is possible to engineer the coating to provide multiple functionalities such as AF + AG + hard coating.10 Note that the combination of AF with AG further helps reduce the visibility of the fingerprint residue, which can be further optimized by specific designs of the AG texture.11
Enhancing Display Module Functionality
As shown in Table 1, a diverse array of FOS components has been developed over time to add the functionalities required by various display applications. We show two examples below to add more perspective to the importance of FOS components and reiterate the need for a system approach when designing the module structure and the assembly strategy that optimizes the performance of all functional components.
Touch sensing has taken the center stage as the preferred user input method for portable devices, as evidenced by the explosion in popularity of smartphones and tablets in the past 5 years. Not surprisingly, a number of touch technologies have been developed and multiple display integration strategies exist for each of them. Projected-capacitive touch is now the dominant technology in mobile devices, displacing the other candidates, notably the originally more common resistive touch sensors. Leaving aside its touch-performance advantages, another benefit of capacitive touch is a superior FOS performance compared to resistive touch, which due to its high back reflection significantly limits outdoor usability. Among the multiple possibilities for implementing capacitive touch,12 we are seeing configurations with better FOS performance becoming increasingly adopted (lower reflections, lower sensor pattern visibility, and reduced thickness) enabled by optimizations of the sensor design and architecture, material choices, and assembly strategies. Examples include replacing air gaps with bonding adhesives, elimination of the EMI shield layer between sensor and the FPD panel, preference for sensors with single ITO layers, and development of transparent conductive materials with low reflection (optically matched ITO,13 alternative transparent conductors,14 etc.).
As an example of display-technology specific FOS components, we examine the case of front-lighting reflective displays, currently better known as "e-Paper" displays. As reflective displays are increasingly used in mobile devices, frontlights (FLs) become a key enabler for optimizing the user experience across applications. The integration of a FL in the FOS stack enhances the display brightness in environments with insufficient ambient illumination, and thus introduces a hybrid reflective-emissive usage model that eliminates the traditional usability limitations of purely reflective e-Paper applications.15,16
FLs can share system characteristics with the much better known LCD backlights (BLs), such as the use of LED sources and lightguides; however, FL implementations are dramatically shaped by the requirements introduced when moving the illumination device from behind the screen to its front. Consider (arguably) the key FL requirement to be invisible in the off-state with minimal impact on image quality. While the same principles for optimizing FOS performance and mechanical and environmental robustness introduced above apply, it is important to account for the FOS requirements differences between emissive and reflective displays, given that reflective performance requires a roundtrip of the ambient illumination through the FOS stack (Fig. 5).
Fig. 5: Shown is a comparison of the FOS stack for (a) fully bonded frontlit reflective displays and (b) a backlit LCD, where the backlight stack has air gaps between all its components.
As a result of the stringent reflective-display FOS requirements, the emerging FL optomechanical system designs and manufacturing and integration strategies show a departure from traditional BL solutions in order to enable high emissive as well as reflective FOS performance.16,17
The rich ecosystem of FOS components is a key resource available to display module designers for meeting diverse and ever evolving application demands. We surveyed in this article a wide spectrum of FOS components and technologies and introduced in more detail a few select examples that are particularly relevant given current display trends. In particular, we showed through these examples the important role played by FOS in optimizing display performance (optical, mechanical, and environmental) and providing required functionalities (such as touch input or front lighting).
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