OTFTs brings unique benefits to both product function and display manufacturing and is a key to unlocking the potential utility of wearables.
by Paul Cain
THE pace at which wearable electronics is evolving is now faster than ever before. In the last couple of years, “wearables” has become a recognized term in its own right. It refers to an increasing list of applications, including smart watches and smart bands, smart headgear and glasses, and now even smart jewelry and smart clothing.1
Wearable electronics is by no means a new market – we have been wearing items such as headphones, hearing aids, and digital watches for decades. However, most people now consider wearables to be those applications with electronics connected to the outside world – i.e., where they form part of the “Internet of Things.” In many cases (as in exercise monitors, for example), the wearable device is the bridge between the human body and the wider world. This intimate monitoring and interaction are much of what is driving excitement and opportunity in this market. In terms of technologies, low-power wireless communications and increasingly integrated MEMS packages have been key in achieving market success thus far.
Wearable electronics offers a huge amount of utility options to users – and the industry is presently in a phase of adapting and learning what end users really value. Undoubtedly, many of the innovations will be the output of agile and creative hardware startups with the vision and energy to bring something new to customers. We have already seen a wave of acquisitions of hardware companies in this area – Intel’s acquisition of health-tracking smart-watch company Basis for $100M, Facebook’s acquisition of Oculus VR for $2M, and Google’s acquisition of WIMM Labs (price unavailable), for example.
New hardware and device technology innovations are a key part of creating wearable product offerings that achieve mass consumer adoption. However, in order for wearable devices to achieve long-term engagement with consumers, wearable devices must be compelling enough to bring about long-term behavioral change.2 Plastic electronics will play a key role in providing compelling and unique functionality that unlocks the full potential in this new market.
As an example, a key challenge for many applications of wearable electronics is battery life, particularly for smart bracelets and smart bands, where there is little space for inclusion of a battery. One of the factors driving the use of plastic displays in smart bracelets, for example, is that their thinness leaves room for more battery capacity in a given form factor. Nevertheless, smart watches currently need charging after several days at best. Users have deeply ingrained behaviors and expectations when it comes to watches and power consumption, and these must be overcome if smart watches are to achieve mass adoption with sustained and long-term use.
When personal timepieces first became widely available (in the form of pocket watches in the sixteenth century), users adapted their behavior in order to wind their watches on a daily basis. The benefit of having, for the first time, an accurate measure of time on your person more than outweighed the need to wind the watch every day.
While modern digital watches can operate for years on a single battery, most smart watches last for a few days at best between charging. Improvements in battery capacity are, of course, a solution to this, and there are many companies working on higher capacity and even flexible batteries. There will be huge rewards for those that succeed. However, there is a second effective solution to this that should not be overlooked: Find a way to add enough utility to a smart watch to justify the sustained behavioral change needed to charge your watch on a daily basis. Mobile devices have become ubiquitous despite their battery life; indeed, many
of us complain about the life of our phone battery even while upgrading to a new phone with the same (or sometimes worse) battery life. People are prepared to change their behavior to use a device if it can bring them sufficient utility, and new hardware technologies play a big part in increasing the utility of wearable electronics.
It may sound obvious, but if you want to comfortably get a mobile-phone screen’s worth of information onto a smart bracelet display, you must have either a
flexible or a conformed curved display. Flexible displays can double or triple the amount of information that can be shown comfortably on your wrist – enabling you to do a lot more than is feasible with a planar display (Fig. 1).
Fig. 1: Flexible-display product concepts like this wrist phone show how new levels of utility might be unlocked for smart watches and other wearable devices.
Furthermore, interfacing electronics with humans presents its own specific challenges: adopting a one-size-fits-all approach for wearables is often not feasible when rigid components are involved; in particular, for those aspects of functionality that require continuous and intimate contact with the body (sensors that measure vital statistics for example). Similarly, wraparound displays in smart bracelets need to have an adjustable curvature in order to fit a range of wrist sizes.
It is no longer only about front-of-screen performance and power, but also and equally about ruggedness, flexibility, and conformability. These additional dimensions of performance offer industrial designers a entirely new playing field, and it will be a process of convergence to find the optimal use case for flexible OLED displays within the full performance envelope.
The OTFT Advantage
Recently, Plastic Logic has shown flexible active-matrix organic light-emitting-diode (AMOLED) displays made using organic thin-film-transistor (OTFT) backplanes, combined with an OLED frontplane material.3 This display (Fig. 2) was made using prototyping equipment, and thus contains some defects due to manual handling and cleanroom quality, but the toolkit of processes used to make the TFT array has already been proven to have a high yield, in a real manufacturing environment, for the production of flexible e-Paper displays.
Fig. 2: This demonstration of a wrist-wrappable flexible OLED display was made using an OTFT backplane printed on plastic.
These organic transistors are fabricated using solution-processed polymers (plastics) for semiconductor and dielectric layers in the stack and deposited through a combination of conventional printing and patterning techniques at low temperature, using standard flat-panel-display (FPD) equipment with low-temperature processing. A schematic cross section of the flexible array stack for a top-emission architecture is shown in Fig. 3, including flexible organic layers for semiconductors and dielectrics. The low-temperature process enables the stack to be fabricated directly onto a third-party barrier substrate material, thereby simplifying the stack and process. Further technical details can be found in the 2014 SID Symposium Digest paper, “Flexible AMOLED Display Driven by Organic TFTs on a Plastic Substrate.”4
Fig. 3: A schematic stack for the organic transistor array includes plastic (organic) semiconductors and dielectric layers and direct integration of the stack onto an integrated barrier substrate (directly enabled by the low-temperature process).
Such a demonstration would not have been possible 10 years ago because OTFT performance, in terms of mobility, stability, and uniformity, was not sufficient at the time. Since then, the mobility performance level of OTFT has risen by several orders of magnitude, to the point where the technology can now be used to drive OLED displays. And materials keep improving so that today’s organic materials are now very stable and show excellent uniformity across large areas. This progress has been driven by both improvements in materials sets and novel TFT architectures that increase current density for a given material set. Several materials suppliers have mobility roadmaps to 10 cm2/V-sec or more in the near future.
The materials stack used in the array is a combination of plastic/organic semiconductor and dielectric layers, combined with thin metal-alloy layers. A key differentiating benefit of OTFT arrays over other technologies that have been developed on glass is the innate flexibility of organic-based materials. At Display Week 2014, we showed that a TFT array can be curved at a radius of around 250 μm (i.e., a matchstick) with no change in transistor performance (Fig. 4).
Fig. 4: The OTFT-array electrical performance is unaffected during bend tests down to a radius of 250 μm.
All of the component materials are naturally flexible; therefore, the overall flexibility of the devices will continue to improve as interfaces are optimized, enabling tighter bend radii and even foldable form factors. Other TFT technologies have one or more inorganic/ ceramic layers within their stack, which fundamentally limits the flexibility because of cracking. In contrast, the crack onset strain for organic electronic materials tends to be several orders of magnitude higher than for their inorganic counterparts.
Technologists continue to develop strategies to fabricate ceramic-based TFTs on plastic films because of the prevalent use of these in today’s glass-based devices. For example, one way to increase the flexibility of a ceramic-based TFT on plastic is to position the most fragile layer at the neutral axis of a stack of
layers. In this way, when the device is curved, there is no strain for that layer (i.e., the one close to the middle of the stack) if the stack contains layers of materials of similar Young’s modulus. However, this is a significant design constraint on the final product, and, of course, not all of the fragile layers can be placed at the neutral axis.
In a truly flexible AMOLED display, it is not only the array that needs to be flexible, but the entire display stack, including the encapsulation layers. There are several companies developing increasingly flexible barrier films and barrier processes to allow a tighter radius of curvature. Figure 5 shows an example of an AMOLED-display prototype using an OTFT array, wrapped around a pencil during operation – equivalent to a radius of curvature of approximately 3 mm.
This display was made by using a PET-based barrier film directly as the substrate material – something that is only possible because of the low-temperature nature of the complete organic-transistor manufacturing process – in addition to OLED frontplane materials that were applied by evaporation.
Fig. 5: This flexible OLED display can be wrapped around a pencil while continuing to operate.
Manufacturing Options for OTFTs
Flexible displays offer benefits not only to product designers and end users but also to display makers. They represent an opportunity for makers to sell more display area not only because these displays will accelerate the development of new markets, but also because more display area (in some cases 2–3 times the area) can be added to a given device because the display can be curved.
The manufacturing process used for OTFT arrays has some significant and proven benefits for display makers as well. Plastic Logic has industrialized a complete toolkit of processes to manufacture flexible TFT arrays for displays and sensors. This array process enables existing display makers to bring true flexibility to
the display technology. The process is realized by using organic semiconductor and dielectric polymer solutions as well as a combination of standard FPD printing/ coating and patterning methods using equipment that can commonly be found in any LCD line, albeit running different processes.
From a manufacturing perspective, a key benefit of OTFTs for flexible-display production is processing temperature. The maximum process temperature used during the manufacture of our backplane is < 100°C, and this has several distinct benefits for processing transistors on plastic:
• Lower temperatures (< 100°C) mean almost any plastic substrate can be used, including lower-cost plastic substrates such as PET.
• Low temperature avoids thermal effects on the plastic, such as pitch variation in sheet-to-sheet processing (and provides a route to roll-to-roll).
• By avoiding the need for any high-temperature steps during the array process, handling of the plastic substrate is almost trivially simple (using a amount-demount process) compared to alternative approaches.
The third point, substrate handling, is particularly critical since it has such a significant effect on yield. For sheet-to-sheet processing of flexible backplanes, all approaches to handling the plastic substrate rely on somehow fixing the plastic layer to a piece of display mother glass (flat, inert, and compatible with standard FPD handling equipment), and then removing the plastic from the mother glass at the end of the fabrication process. There are radically different approaches that can be taken to achieve this, dictated by process temperature.
For example, one approach to handling plastic substrates is Electronics on Plastic by Laser Release (EPLaR) – a bond/de-bond approach in which a polyimide solution is coated onto a glass carrier and then cured at very high temperature (400–450°C). A variant of the TFT process is then performed (LTPS, oxide, or a:Si), and then the processed plastic substrate is removed from the mother glass at the end of the process by laser release. Such an approach allows the higher-temperature processes needed for inorganic TFTs to be carried out on a plastic substrate. However, the process itself is complex, requires custom equipment and processes, and can only be used with higher cost plastics that tolerate the very high temperatures. Crucially, the debond process (using a laser) occurs at the end of the process flow and thus its effect on the yielded bill-of-material cost is dramatic.
In contrast, for OTFT-array processing on plastic, a simple mount/demount process is employed in which a plastic substrate (typically PET, or virtually any other flat inert insulating film) is laminated onto a mother glass containing a thin adhesive layer. The stack is then taken through the array process, and, once complete, the
plastic substrate is removed by a thermal release process. This process is simple and, as a result, has already been shown to have near 100% yield in a factory setting. Additionally, the mother glass itself can be reused multiple times through the line. The relative simplicity of the mount and demount process is one example of how the process flow for OTFT is made simpler because high temperatures are unnecessary. The processes are compared in Fig. 6.
Fig. 6: (a) Processing LTPS arrays on a flexible substrate typically involves the addition of polyimide coating, high-temperature curing, and laser lift-off process steps, which is complex and has yield challenges. (b) By contrast, a simple high-yield mount-demount process was used for OTFTs and is enabled by the low-temperature process for OTFT-array processing.
These processing benefits, combined with the mechanical benefits and electrical maturity, give OTFTs a unique capability set for wearable and truly flexible OLED displays (Fig. 7).
Mobile phones and mobile devices may represent the biggest behavioral change of western society in the past 50 years. Wearable electronics have the potential to be the next wave, but need flexible electronics, in the form of flexible displays and sensors, to bring the level of utility necessary to unlock the full market potential. OTFTs offer unrivalled levels of flexibility for AMOLED displays and represent an opportunity for designers to bring completely new levels of user experience and utility to the world of wearable electronics.
Fig. 7: OTFTs have a uniquely enabling capability set for flexible AMOLED displays that can be used for wearable electronics.
4C. W. M. Harrison, D. K. Garden, I. P. Horne, “Flexible AMOLED Display Driven by Organic TFTs on a Plastic Substrate,” SID Symposium Digest of Technical Papers 45, 256–259 (2014). •